311 213 PDF

Riedel, M., Collett, T.S., Malone, M.J.
, and the Expedition 311 Scientists

Proceedings of the Integrated Ocean Drilling Program, Volume 311
Expedition 311 synthesis: scientific findings1

M. Riedel,2 T.S. Collett,3 and M. Malone4
Chapter contents Abstract

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Integrated Ocean Drilling Program Expedition 311 was conducted
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
to study gas hydrate occurrences and their evolution along a tran-
sect spanning the entire northern Cascadia accretionary margin.
Constraints on the vertical extent of the gas
hydrate stability zone and the occurrence A transect of four research sites (U1325, U1326, U1327, and
of gas hydrate. . . . . . . . . . . . . . . . . . . . . . . . . 3 U1329) was established over a distance of 32 km, extending from
Gas hydrate concentration estimates . . . . . . . . 5 Site U1326 near the deformation front to Site U1329 at the east-
Lithologic controls on gas hydrate ern limit of the inferred gas hydrate occurrence zone. In addition
distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 to the transect, a fifth site (U1328) was established at a cold vent
Gas hydrate formation—from in situ methane setting with active fluid and gas expulsion, which provided an op-
production or deeper methane sources? . . . . 6 portunity to compare regional pervasive fluid-flow regimes to a
A modified model of fluid expulsion and site of focused fluid advection. In this synthesis, a revised gas hy-
gas hydrate formation on the northern drate formation model is proposed based on a combination of
Cascadia margin . . . . . . . . . . . . . . . . . . . . . . 10 geophysical, geochemical, and sedimentological data acquired
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 during and after Expedition 311 and from previous studies. The
Acknowledgments. . . . . . . . . . . . . . . . . . . . . . 12 main elements of this revised model are as follows:
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1. Fluid expulsion by tectonic compression of accreted sediments
Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 at nonuniform expulsion rates along the transect results in the
Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 evolution of variable pore water regimes across the margin.
Sites closer to the deformation front are characterized by pore
fluids enriched in dissolved salts at depth, where zeolite
formation from ash diagenesis is dominant. In contrast, the
landward portion of the margin shows a freshening of pore
fluids with depth as a result of the progressive overprinting of
diagenetic salt generation with freshwater generation from the
smectite-to-illite transition at greater depth.
2. In situ methane produced by microbial CO2 reduction within
the gas hydrate stability zone is the prevalent gas source for gas
hydrate formation.
1
Riedel, M., Collett, T.S., and Malone, M., 2010. 3. Some minor methane advection from depth is required overall
Expedition 311 synthesis: scientific findings. In to explain the occurrence of gas hydrate (and the associated
Riedel, M., Collett, T.S., Malone, M.J., and the downhole isotopic signatures of CH4 and CO2) within the
Expedition 311 Scientists, Proc. IODP, 311:
Washington, DC (Integrated Ocean Drilling
sediments of the accretionary prism and the absence of gas hy-
Program Management International, Inc.). drate within the abyssal plain sediments. In contrast, methane
doi:10.2204/iodp.proc.311.213.2010 migrating from depth is a dominant source for gas hydrate for-
2 Natural Resources Canada, Geological Survey of
mation at the cold vent Site U1328 (Bullseye vent).
Canada, 9860 West Saanich Road, Sidney BC V8L 4. Gas hydrate preferentially forms in coarser grained sandy/silt
4B2, Canada. mriedel@nrcan.gc.ca
3 U.S. Geological Survey, Denver Federal Center, turbidites, resulting in very high local gas hydrate concentra-
Denver CO 80225, USA. tions. Typically, gas hydrate occupies <5% of the pore space
4 Integrated Ocean Drilling Program, 1000 throughout the gas hydrate stability zone. Higher gas hydrate
Discovery Drive, College Station TX 77845, USA. saturations were observed in intervals with abundant coarse-
Proc. IODP | Volume 311 doi:10.2204/iodp.proc.311.213.2010

M. Riedel et al. Expedition 311 synthesis
grained sand layers and within fault-controlled (2) determine the mechanisms that control the na-
fluid and gas migration conduits at the cold vent ture, magnitude, and distribution of the gas hydrate
Site U1328. occurrence; (3) find the pathways of upward fluid
migration required to form large concentrations of
gas hydrate; (4) examine the effect of gas hydrate on
Introduction the physical properties of the host sediment; and (5)
investigate the microbiology and geochemistry asso-
Integrated Ocean Drilling Program (IODP) Expedi-
ciated with the gas hydrate occurrence.
tion 311 was conducted on the northern Cascadia
margin (Fig. F1) during September and October 2005 Prior to Expedition 311, a general model for gas hy-
(see the “Expedition 311 summary” chapter). Expe- drate formation by removal of methane from up-
dition 311 was the third deep drilling expedition on wardly expelled fluids was generally accepted for the
the Cascadia margin with a major gas hydrate re- Cascadia margin (Hyndman and Davis, 1992). In
search objective. The first drilling on the Cascadia this model, dissolved microbial methane, inferred to
margin was conducted in 1992, when Ocean Drilling be produced over a thick sediment section, migrates
Program (ODP) Leg 146 (Westbrook, Carson, Mus- vertically and forms gas hydrate when it enters the
grave, et al., 1994) established three drill sites on the stability field. Gas hydrate concentration is predicted
northern Cascadia margin (Sites 888, 889, and 890) to be greatest just above the base of gas hydrate sta-
as well as Site 892 on Northern Hydrate Ridge, off- bility, seismically defined by the bottom-simulating
shore of Oregon (southern Cascadia margin). In reflector (BSR). A model was also proposed for how
2002, ODP Leg 204 (Tréhu, Bohrmann, Rack, Torres, free gas and the resulting BSR are formed as the base
et al., 2003) followed with a dedicated study of of gas hydrate stability moves upward because of
Southern Hydrate Ridge (SHR). During this expedi- post-Pleistocene seafloor warming, uplift, and sedi-
tion, nine closely spaced drill sites were established ment deposition (e.g., Paull and Ussler, 1997; von
in a short transect across the ridge to study the Huene and Pecher, 1998). In addition, physical and
summit of SHR, where seafloor gas hydrate had been mathematical models have been developed for the
discovered (e.g., Suess et al., 1999). formation of gas hydrate from upward methane ad-
vection and diffusion (e.g., Xu and Ruppel, 1999).
Expedition 311 was designed to study gas hydrate
occurrences along a transect across the northern Cas- Evidence that focused fluid/gas flow and associated
cadia margin, and thus the expedition augmented gas hydrate formation contrasts with the diffuse, re-
the geographical range of previous gas hydrate stud- gional fluid-flow regime has been identified on the
ies on the Cascadia margin. A transect of four sites northern Cascadia margin at various locations. The
(U1325, U1326, U1327, and U1329) was established most studied site, referred to as Bullseye vent, is an
to characterize the different geologic and tectonic active cold vent field associated with near-surface gas
settings of gas hydrate occurrence across the margin, hydrate occurrences and was drilled as Site U1328
spanning a distance of 32 km from Site U1326 near during Expedition 311. These vents were shown to
the deformation front to the landward Site U1329 at be associated with fault-related conduits for focused
the eastern limit of the previously inferred gas hy- fluid and/or gas migration and with massive gas hy-
drate occurrence. These drill sites roughly follow drate formation within the fault zone (Riedel et al.,
multichannel seismic (MCS) Line 89-08, which was 2002, 2006).
acquired in 1989 as part of the presite survey for Leg In this synthesis, we first report integrated findings
146 (Fig. F2). Projecting the previously established on the distribution and magnitude (concentration,
deep basin Site 888 onto the end of the new transect Sh) of gas hydrate, as determined from logging data
extended the profile to >40 km. In addition to the and core-derived pore water freshening (Chen et al.;
transect sites, a fifth site (U1328) was established at a Chen, 2006; Malinverno et al., 2008; Ellis et al.,
cold vent with active fluid and gas expulsion. This 2008). We then show the results of sedimentological
provided an opportunity to compare regional perva- and geochemical core studies as well as infrared (IR)
sive fluid-flow regimes to a site of focused fluid flow. imaging of the recovered core, demonstrating the
The main goal of Expedition 311 was to test geologic lithological control on gas hydrate occurrences on
gas hydrate formation models and associated model the northern Cascadia margin (Torres et al., 2008;
parameters in subduction zone accretionary prisms, Hashimoto and Minamizawa; Wang, 2006). These
especially those that account for the formation of results are the basis for discussion of the underlying
concentrated gas hydrate occurrences driven by fluid-flow model along the Expedition 311 transect.
upward fluid and methane transport. The detailed We combine results from downhole logging data and
objectives of the expedition were to (1) study the for- detailed pore fluid and gas chemistry data (Torres
mation of natural gas hydrate in marine sediments; and Kastner; Pohlman et al., 2009; Lu et al., 2008;
Proc. IODP | Volume 311 2

Kim and Lee) to develop an updated model that can salinities from the onboard analysis of core samples
explain the occurrences and distribution of gas hy- were used in the calculations. Furthermore, the pore
drates along the Cascadia margin. pressure gradient was assumed to be hydrostatic
(9.795 kPa/m) and the geothermal gradients linear.
Results of all regression analyses at all sites are sum-
Constraints on the vertical marized in Table T1.
extent of the gas hydrate One of the more important methods used to identify
and quantify the occurrence and extent of gas
stability zone and hydrate is pressure core degassing. Pressure cores re-
the occurrence of gas hydrate trieved at in situ pressure conditions were used to
determine gas hydrate quantity (see also below) us-
A primary objective of Expedition 311 was to define
ing mass balance calculations (e.g., Dickens et al.,
the vertical extent of the gas hydrate stability zone
1997; Milkov et al., 2004). Pressure cores were also
(GHSZ) and the occurrence of gas hydrate. The depth
used to investigate gas hydrate distribution using
of the base of gas hydrate stability zone (BGHSZ) was
nondestructive physical property measurements of
estimated before drilling by assuming that the seis-
the cores at in situ pressures. Pressure coring is cru-
mically observed BSR represents the BGHSZ. Among
cial for understanding the concentrations of gas hy-
the proxies used to determine the depth of the
drate and free methane gas in marine sediments,
BGHSZ and also to define the occurrence of gas
their nature and distribution, and their effect on the
hydrate and to compare these occurrences to the pre-
intrinsic properties of the sediment. Pressure cores
dicted depth of the BSR are
were collected using the IODP Pressure Core Sampler
• Downhole temperature measurements, (PCS), the Fugro Percussion Corer (FPC), and the HY-
• Well-log measurements of P-wave velocity and ACE Rotary Corer (HRC). Combined results from all
electrical resistivity, degassing experiments at the five sites visited are
• Degassing experiments on pressure cores, shown in Figure F4.
• Pore water chlorinity,
• IR imaging, and Top of gas hydrate occurrence
• Hydrocarbon gas ratios (e.g., C1/C2 and i-C4/n-C4)
Gas hydrate occurrence, as inferred from downhole
of the void gas.
logs, IR core images, pore water chlorinity freshen-
There are notable differences between the individual ing, and physical recovery in cores, was much
techniques because each is highly dependent on shallower than expected based on previous studies at
how the measurement is conducted and the resolu- this margin. At Site U1326, gas hydrate was first re-
tion or sensitivity of the particular measurement and covered at ~47 meters below seafloor (mbsf), and, at
because the results are biased by core recovery (IR Site U1325, gas hydrate was inferred from the electri-
imaging, pore water chemistry, and gas chemistry) cal resistivity logs to occur as shallow as 73 mbsf. In
and sampling density (e.g., the frequency of temper- contrast, at Site U1327, gas hydrate was first recov-
ature tool deployments and linear regression analy- ered at 111 mbsf. Site U1329 did not show any sig-
sis). The results of the various techniques used to nificant evidence of gas hydrate content; however, a
estimate the BGHSZ and predict the occurrence of small pore water freshening trend at ~123 mbsf may
gas hydrate at all sites are shown in Table T1. indicate low concentrations of gas hydrate just
The expedition included 36 temperature tool deploy- above the BSR. The apparent progressive decrease in
ments to characterize the thermal regime of all five the top depth of gas hydrate occurrence along the
drilled sites. Three standard IODP temperature tools transect was investigated by Malinverno et al. (2008)
were deployed, including the Advanced Piston Corer and Torres and Kastner, who used one-dimensional
Temperature Tool (APCT, as well as the newer version geochemical diffusion modeling to explain the ob-
[APCT-3; Heesemann et al.], 20 times), the Davis- served landward deepening of the top of gas hydrate
Villinger Temperature Probe (DVTP, 11 times), and occurrence on a site-by-site basis. The model com-
the Davis-Villinger Temperature-Pressure Probe putes methane concentration in the pore fluid for a
(DVTPP, 5 times). A compilation of the in situ tem- given in situ bacterial methane production rate, sedi-
perature estimates from Expedition 311 is compared mentation rate, and fluid advection velocity. Model-
to previous results from Site 889 (Fig. F3). The tem- ing shows that lower rates of sedimentation or fluid
perature data acquired were used to estimate the advection result in lower methane concentrations
depth of the BGHSZ at each site. A pure methane gas with depth and thus a deepening of the first occur-
chemistry was assumed for the in situ hydrate (con- rence of gas hydrate. Sedimentation rates decrease
firmed by shipboard analysis), and interstitial water landward along the coring transect (Akiba et al.),

corresponding to an increase in depth to the top of northern Cascadia margin is <1% of the pore space
the first gas hydrate occurrence. Fluid advection (e.g., Hyndman et al., 2001; Yuan et al., 1996, 1999)
rates, due to dewatering of the accretionary wedge, and that the free gas zone is relatively thin (<10 m;
are expected to first increase with distance from the Chapman et al., 2002).
deformation front, to a maximum of ~15 km into In order to trap some free gas at the BGHSZ, a tempo-
the accretionary prism (Hyndman and Davis, 1992), ral permeability barrier of some sort most likely ex-
and then progressively decrease farther landward. A ists. At all sites investigated during Expedition 311,
combination of these two mechanisms can explain the the gas hydrate concentration is <5% (of the pore
deepening of the top of the gas hydrate occurrence space) on average; additionally, no evidence was
along the transect. However, considerable variability found for a systematic and regionally distributed in-
along the margin is induced by locally variable sedi- crease in gas hydrate concentration just above the
mentation rates and erosion along uplifted ridges. BSR, as was previously suggested (e.g., Hyndman et
Also, because gas hydrate forms predominantly in al., 2001).
sandy turbidite sediments (see next section), the
It is also important to note that the upward move-
presence of an appropriate host strata further affects
ment of gas bubbles in porous sediments can be
gas hydrate occurrence.
restricted by capillary forces. For free gas to pass
Fluid advection rates required for modeling the through the pore throat of sediment, the gas pres-
thickness of the gas hydrate occurrence zone may be sure inside the bubble must overcome the capillary
constrained by values of sulfate (and thus methane) pressure. Vertical migration of bubbles requires an
flux obtained from reactions of sulfate and methane interconnected gas phase and a gas column thick
at the sulfate–methane transition zone (e.g., enough that the pressure difference between gas
Borowski et al., 1996). However, there is a complex, bubbles and pore water (due to different densities)
uncertain correlation between the depth of the sul- can overcome the capillary pressure in the pore
fate–methane transition zone and the thickness of throats (Schowalter, 1979). Gas bubbles beneath the
the gas hydrate occurrence when data from various BGHSZ can also be stuck or trapped in the pore space
sites of gas hydrate are compared globally (Kastner et of a fine-grained sediment with a gas saturation of
al., 2008; Dickens and Snyder, 2009; Torres and only a few percent (as estimated at the northern Cas-
Kastner). cadia margin).
The coupled microbial reactions of anaerobic oxida- Thus, as little as a few percent gas hydrate in a clay-
tion of methane (AOM) are also a sink for sulfate, rich, low-permeability sediment could already be an
and the presence of AOM can best be determined effective barrier or at least a boundary that impedes
from δ13C isotopic data. Evidence for AOM is present gas and water from flowing across this horizon and
at all sites studied during Expedition 311, and results results in a concentration gradient across the bound-
from the northern Cascadia margin compare well ary with free gas being trapped.
with observations made at SHR during Leg 204
As described by Haacke et al. (2007), gas hydrate re-
(Claypool et al., 2006).
cycling at the BGHSZ is a common phenomenon at
convergent margins. The continued growth of the
The base of gas hydrate stability zone and accretionary prism (tectonic uplift) combined with
the nature of the bottom-simulating sedimentation processes (erosional and depositional)
reflector results in continuous changes in the BGHSZ and
A regional BSR was observed along the entire thus the dissociation of gas hydrate and the release
northern Cascadia margin. This BSR is believed to of free gas and water. The resulting free gas zone be-
represent the base of gas hydrate stability and thus low the BGHSZ is also much thinner compared to
to mark the transition from gas hydrate–bearing sed- passive margins where the free gas zone is generally
iments and sediments containing some free gas. As- thicker (Haacke et al., 2007). It is possible that the
sociated changes in the physical properties of the free gas migrates back into the GHSZ (if buoyant
sediment at the BSR from (possibly) higher to lower enough, or, for example, by processes such as the
P-wave velocity are the cause of this prominent seis- self-generated permeability described by Flemings et
mic reflection. Combined geophysical data analyses al., 2003); however, in most cases, a residual amount
have shown that the amount of free gas required to (concentrations of a few percent or less) of free gas is
yield the BSR reflection strength observed on the trapped by capillary forces in the pore space.

Gas hydrate concentration the highest gas hydrate concentrations are not found
near the BGHSZ (i.e., just above the BSR), as was pre-
estimates dicted by the preexpedition Hyndman and Davis
Gas hydrate concentrations were determined during (1992) pore fluid expulsion model.
Expedition 311 using mainly downhole logging- The two key variables for estimating concentrations
while-drilling (LWD) electrical resistivity data as well of in situ pore fluid constituents (i.e., not settings
as wireline electrical resistivity log data (see the with gas hydrate in fractures) using the Archie
“Expedition 311 summary” chapter). Procedures calculations (in addition to porosity) are pore fluid
followed standard Archie analyses (Archie, 1942; salinity and geothermal gradient. Geothermal gradi-
Collett and Ladd, 2000), and a detailed description ents were successfully determined from individual
of the methods and assumptions involved can be downhole temperature probe deployments at all
found in the “Methods” chapter. More detailed sites except Site U1326 (where only one downhole
analyses of gas hydrate concentrations from the deployment succeeded in a reliable measurement).
downhole resistivity log data were conducted after Detailed results of each deployment can be found in
the expedition by Malinverno et al. (2008) and Chen the individual site chapters of this volume. Pore fluid
et al. Malinverno et al. (2008) presented a method to salinities and chlorinity were carefully determined
calculate gas hydrate concentrations from the direct onboard to (a) establish the background trend in
comparison of core-derived salinity and downhole pore fluid salinity and (b) capture any gas hydrate
log data from Site U1325 that honors the spatial un- present and determine the local gas hydrate concen-
certainty in the measurements from different bore- tration from the pore fluid freshening relative to the
holes located ~25 m apart. The technique was also background trend.
applied to all other sites drilled along the transect.
Figure F6 shows the entire data set available for pore
Chen et al. analyzed in detail the effect of porosity
water chlorinity analyses at all sites. Sites U1325 and
uncertainty on gas hydrate concentration estimates
U1326 both exhibit increasing pore water chlorinity
by comparing results obtained from density and
with depth, as opposed to the other three sites
neutron porosity calculations for all transect sites.
(U1327, U1328, and U1329), which all decrease in
Seismically derived velocities (mainly P-wave veloc- pore water chlorinity with depth. Previously, the en-
ity, VP) can also be used to calculate gas hydrate tire freshening trend observed at Site 889 was attrib-
concentrations. Two studies were carried out by uted to the dissociation of gas hydrate upon recovery
Goldberg et al. (2008) and Chen (2006) using wire- (Hyndman et al., 1999). In contrast, Kastner et al.
line and LWD data to determine velocity log–based (1995a) suggested some component of mixing with a
gas hydrate concentrations. Although both methods deeper, fresher fluid source to account for the com-
show generally comparable results, gas hydrate con- bined observations of the pore fluid geochemical
centrations calculated from acoustic velocities are profiles, thus resulting in reduced gas hydrate con-
slightly higher than resistivity-based estimates. For centrations compared to the study by Hyndman et
the purpose of understanding geologic controls on al. (1999). However, more recent data from Expedi-
the occurrence of gas hydrates, these differences tion 311 clearly show that only the discrete outliers
were disregarded. can be attributed to gas hydrate recovered in the core
Additional constraints on gas hydrate concentra- because they were coincident with IR-inferred gas
tions were obtained from pressure core degassing ex- hydrate occurrences. In addition to the IR data ac-
periments, as originally outlined by Dickens et al. quired from cores within the first few minutes after
(1997). A total of 16 PCS cores were recovered under recovery, a second IR camera was used in the
in situ pressure conditions and yielded estimates of onboard geochemistry laboratory to allow further
gas hydrate concentrations (Fig. F4). All of the results detailed discrimination of gas hydrate in the recov-
of the PCS degassing experiments are superimposed ered core (Fig. F5; see discussion in the next section
on the logging-derived results shown in Figure F5. for further details).
All data from Expedition 311 confirm that gas hy- Despite evidence from IR imaging, some doubts re-
drate concentrations at the drill sites along the tran- mained about the cause of the freshening trend, as
sect are generally relatively low (<5% of the pore originally argued by Hyndman et al. (1999). There-
space) but locally can exceed 50% of the sediment fore, Chen et al. carried out the same analyses
pore space (especially at Site U1326, 50–120 mbsf, originally conducted by Hyndman et al. (1999) to si-
and Site U1327, 120–140 mbsf). A summary of resis- multaneously solve for the in situ pore water salini-
tivity data and derived gas hydrate concentrations ties as well as gas hydrate concentrations. The new
(Sh) from all five sites drilled during the expedition application of the Archie analyses by Chen et al.
are shown in Figure F5. The results demonstrate that using LWD and wireline data from Expedition 311

verified that the measured pore water salinities gen- linkages to gas hydrate occurrences. Wang (2006)
erally follow the assumed background freshening also used soupy and mousselike sediment textures to
trend. further infer the presence of gas hydrate. A good cor-
relation was found between the occurrence of in-
ferred gas hydrate and the coarser grain fraction.
Lithologic controls on gas An interesting general question arises from observa-
hydrate distribution tion of the strong lithologic control on gas hydrate
occurring mainly in sands and the apparent deepen-
During onboard analyses of the recovered core, it being of the top of gas hydrate occurrence landward
came evident that most of the gas hydrate recovered from the deformation front (as modeled by Malin-
occurred in the coarser grained sandy turbidite sec- verno et al., 2008): What defines the shallowest oc-
tions (see the “Expedition 311 summary” chapter). currence of gas hydrate? Is it the combination of in
Other forms of gas hydrate were recovered within situ methane production, sedimentation, and meth-
veins and fractures, especially at the cold vent Site ane advection rates or simply the limit in sand
U1328, and will be discussed later in this report. Al- occurrence at a given depth?
though previously observed on other expeditions
Recently, Malinverno (2010) showed that if gas hy-
(e.g., Ginsberg et al., 2000; Weinberger et al., 2005),
drate formation is inhibited in the small pores of
and thus not fully unexpected, direct observation
fine-grained marine muds, microbial methane gener-
(visually and through IR imaging) of gas hydrate in
ated in these mud layers will stay in solution. This
sands on the Cascadia margin was systematically
dissolved methane will instead be transported by dif-
documented for the first time during Expedition
fusion into the coarser grained sand or silt layers,
311. Furthermore, the relatively high amount of
where it forms concentrated gas hydrate. As an ex-
sand recovered at all sites (but especially Sites U1325
ample, sediments recovered at Site U1325 show a
and U1326) was surprising compared to results from
large abundance of sand and silt layers across the
Leg 146.
entire 300 m cored interval. An interval with a high
The strongest evidence for the occurrence of gas hy- abundance of sand layers occurs between 57 and
drate within coarser grained sediment comes from IR 67 mbsf, where ~40 individual sand layers were iden-
imaging of recovered core segments, as described by tified (see “Lithostratigraphy” in the “Site U1325”
Torres et al. (2008). Typically, 10–30 cm long whole- chapter). However, the shallowest gas hydrate at Site
round segments were cut from the core and taken to U1325 was inferred from the resistivity logs to be at
the geochemistry laboratory for further analyses. Al- ~73 mbsf. The first sand layer with gas hydrate was
though historically the entire core section would identified from IR images and pore water chlorinity
have been mechanically squeezed after cleaning to freshening at ~80 mbsf. Combining these observa-
collect pore water samples, the core section was first tions with the models by Malinverno (2010) and
laid out and reimaged with an IR camera (Fig. F7). Malinverno et al. (2008) shows that the top of gas
Cores with IR anomalies were further sampled for in- hydrate is a factor of overall in situ methane produc-
dividual squeezing and pore water analyses. These tion and sedimentation rates and is not necessarily
analyses documented the preference for gas hydrate related to the amount of sand present (or lacking) in
to be present in coarser grained sandy-to-silty turbi- the system. Advection is not necessarily a require-
dites, with only a very minor fraction present in ment to form the observed concentrations of gas
fine-grained sediments. At Site U1325, the gas hy- hydrate. However, if methane advection is added to
drate concentration is directly correlated to the sand the scenario, the top of gas hydrate remains at the
content of the host sediment (Torres et al., 2008). same depth if in situ methane production is reduced
However, Site U1326 samples contained a lower av- proportionally.
erage gas hydrate concentration than what would be
expected based on the amount of sand present in the
sediments. This was interpreted by Torres et al. Gas hydrate formation—from
(2008) as indicative of an insufficient availability of
methane to fully “charge” the sand to the maximum
in situ methane production
possible gas hydrate concentration (e.g., from re- or deeper methane sources?
duced local in situ methane production rates).
A general model for deep-sea gas hydrate formation
In addition to the study by Torres et al. (2008), two by removal of methane from upwardly expelled
other studies were conducted by Hashimoto and fluids was developed by Hyndman and Davis (1992).
Minamizawa and Wang (2006) to analyze grain-size In this model, mostly microbial methane produced
distribution at the Expedition 311 drill sites with over a thick sediment section pervasively migrates

upward to form gas hydrate as it enters the stability the Expedition 311 transect (Westbrook, Carson,
zone. The gas hydrate concentration is therein pre- Musgrave, et al., 1994). However, note that Site
dicted to be greatest just above the BSR associated U1329 is dominated by a marked unconformity at
with the BGHSZ. ~136 mbsf, where ~5 m.y. of sediments were eroded
With the five sites established during Expedition 311 (a jump from 1.6 to 6.4 Ma was observed across the
and combinations of observations made during Leg unconformity; Akiba et al.).
204, there are now several data sets available to help The presence of an advecting pore fluid from greater
modify and further develop the pore fluid expulsion depth is also required to explain other observed inor-
model. These data sets include ganic pore water constituents, including iodine, bro-
mine, and ammonia (Lu et al., 2008). The advection
• δ13C isotopic composition of methane and carbon
rates calculated by Lu et al. (2008) have very similar
dioxide gases from sediment core samples and gas
values to those modeled by Malinverno et al. (2008)
from void spaces in the recovered cores (Pohlman
and range from 0.015 cm/y at Site U1325 to 0.06 cm/y
et al., 2009), gas analyses from pressure core sam-
at Site U1326. All other sites have values of ~0.03 cm/y.
ples, and gas hydrate samples;
Additional modeling to achieve a better fit between
• δ13C isotope composition of the dissolved inor- the predicted and observed depth profiles of pore
ganic carbon (DIC) (Torres and Kastner); water halogen and ammonia constituents was also
• Pore fluid chlorinity and related fluid advection conducted by Lu et al. (2008) by incorporating lat-
modeling (see the “Expedition 311 summary” eral advection through fractures/faults.
chapter; Malinverno et al., 2008; Wortmann et al. Sites U1325 and U1326 show increasing pore water
2008); chlorinity/salinity with depth, whereas Sites U1327,
• Strontium (Sr) and lithium (Li) components of the U1328, and U1329 show (to various degrees) pore
pore water following earlier results from Legs 146 water freshening with depth. The observed increase
and 204 (Teichert et al., 2005; Kastner et al., in pore water chlorinity/salinity is attributed to dia-
1995b); and genetic processes (e.g., through low-temperature
reactions where volcanic ash is transformed to zeo-
• Iodine, bromine, and ammonium pore fluid con- lite, releasing salt). Given that no information exists
stituents (Lu et al., 2008). about the exact type zeolite involved in the actual
reaction at depth and the possibilities of having gen-
Inorganic pore water constituents and erated zeolites from very low temperatures (as low as
upward fluid migration constraints 5°C in the case of phillipsite), it is difficult to deter-
mine an exact depth range for this reaction. For
Downhole trends in inorganic pore water constitu-
example, diagenetic alteration of ash to zeolite was
ents (e.g., chlorinity) suggest a strong component of
reported to occur from 200 m to 11 km below sea-
pore water migration from below with distinctly dif-
floor by Iijima and Utada (1966), but it likely occurs
ferent source compositions (Fig. F6). This inferred
as soon as sediment is deposited (i.e., diagenetic
upward pore water migration is a critical component
alteration and related salt production happens
of the dewatering process in the Cascadia accretion-
throughout the entire sediment column).
ary prism, where the incoming ascending sediment
section is deformed and squeezed by the tectonic The pore water freshening with depth was expected
processes that form the accretionary wedge. Modeling to be a margin-wide phenomenon and simply a
conducted by Malinverno et al. (2008) assumed rela- function of distance from the deformation front
tively low advection rates (a maximum of 0.017 cm/y based on results from Leg 204 (Kastner et al., 1995a;
was inferred), which is about an order of magnitude Torres et al., 2004). The source of freshwater was at-
lower than rates used in previous studies (Bekins and tributed to low-temperature clay dehydration pro-
Dreiss, 1992; Wang et al., 1993; Hyndman and Davis, cesses (e.g., smectite-to-illite transformation) that
1992). Fluid advection modeling is also strongly typically occur over a temperature range of 60°–
dependent on accurate sedimentation rates. Sedi- 160°C (e.g., Bekins et al., 1994). With a temperature
mentation rates at the Expedition 311 transect sites gradient of ~60°C/km at Site U1327, the source of
were estimated from diatom biostratigraphy (see the this freshwater pool is at a depth of ~1000–
“Expedition 311 summary” chapter; Akiba et al.) 2750 mbsf.
and decrease markedly from Sites U1325 and U1326 The clear separation between salt and freshwater
(>400 m/m.y.) to Site U1329 (<100 m/m.y.). Sedi- sources along the margin remains a puzzle because
mentation rates in the Cascadia Basin seaward of the two assumed diagenetic reactions could occur
the deformation front are even higher, reaching over the same depth range. Furthermore, the distri-
~1000 m/m.y. at Site 888, located ~60 km south of bution of volcanic ash and smectite should not be

drastically different across the accretionary prism. fluid at temperatures >70°C. At the thermal gradients
Bartier et al. indicate only modest variation in the measured during Expedition 311 (~60°C/km), in-
clay mineralogy among all of the sites, and ash layers ferred in situ lithium leaching mainly occurs at
have been reported at all sites, including Site 888 depths >1 km (see also Kastner et al., 1995b). Thus,
(Westbrook, Carson, Musgrave, et al., 1994; see the higher-than-seawater lithium concentrations found
“Expedition 311 summary” chapter). at shallower depths are indicative of an upward-
However, the apparent complex segregation of the migrating deep fluid source. Pore water lithium con-
margin from more saline to fresher formation waters centrations increase with depth at all sites (especially
can be explained by a relatively simple model that Sites U1327 and U1329) and with distance away
incorporates the change in fluid expulsion rates from the deformation front along the transect (Fig.
along the margin, as modeled by Hyndman and Da- F9A). A shift to elevated lithium concentrations even
vis (1992) (Fig. F8). The expulsion rate is expected to at shallow depths of <50 mbsf was observed at Sites
be at maximum ~15 km landward of the deforma- U1327 and U1329, which is in very good agreement
tion front, and little fluid is expelled along the with the previously stated evolution of fluid expul-
transect until a distance of ~10 km. The pore water sion rates along the drilling transect. However, the
freshening reported in Figure F8 for Leg 204 and Ex- cold vent Site U1328 surprisingly does not show
pedition 311 sites was determined at a constant much evidence for equivalent deeper fluid advec-
depth of 200 mbsf and is all relative to seawater. Pore tion, although it is only 3.5 km south and about the
water freshening is seen only at sites ~15 km east of same distance from the deformation front as Site
the deformation front, and the freshening quickly U1327.
increases landward. The slowdown of the modeled From strontium concentration and isotopic ratio
expulsion rate is reflected in the decrease in freshen- data (Fig. F9B–F9C) it is apparent that the values at
ing between Sites U1327 (Site 889) and U1329; how- Site U1329 below the unconformity are the only
ever, the erosion and associated drastic difference in anomalous values found along the Expedition 311
sedimentology at Site U1329 may also be a factor. transect. It is also evident that the entire set of values
from all sites is overprinted by diagenesis from car-
Furthermore, at Sites U1325 and U1326, little water
bonate precipitation (Fig. F9D), especially for Site
has been expelled from depth, and, because the
U1327—a finding that was already noted by Kastner
source of any freshwater must come from greater
et al., (1995b) for nearby Site 889. Similar observa-
depth (~1000 mbsf), there is little mixing of the pore
tions were made by Teichert et al. (2005) for data
waters whose salt contents increased from the ash-
from Leg 204. Teichert et al. (2005) also proposed a
to-zeolite transformation. As soon as freshwater is
mixing line (dotted black line in Fig. F9D) for the
expelled in large quantities, freshening overprints
strontium isotope and concentration data for the
salt formation and the pore waters show an overall
southern Cascadia margin using seawater and bot-
reduction in chlorinity. Additionally, the accretion-
tom fluids collected from ODP Sites 1027 and 1026,
ary prism is relatively thin near the deformation
similar to the previous mixing line proposed by Kast-
front, which reduces the possible region for
ner et al. (1995b).
freshwater generation (which requires higher tem-
peratures—i.e., a thicker sediment column). Thus, The strontium concentrations and isotopic ratios at
there is a relatively lesser amount of source material all sites from all previous drilling (including Expedi-
beneath Sites U1325 and U1326 compared to the tion 311, with the exception of Site U1329), can eas-
more mature, thicker prism farther east beneath Site ily be explained by the mixing of seawater and the
U1327 (Site 889) and others. deep fluid source measured at Sites 1026 and 1027.
The apparent end-member inferred for Site U1329
Additional constraints on the deep fluid sources
could be different (but basaltic in nature) from the
come from lithium and strontium data (e.g., Teichert
incoming plate (M. Torres, pers. comm., 2008). On
et al., 2005; Kastner et al., 1995b). Here we use the
the other hand, the observation based on Leg 146
measured concentration-depth profiles of lithium
that the sites drilled from Northern Hydrate Ridge to
and strontium as well as 87Sr/86Sr ratios (Fig. F9) to offshore Vancouver Island communicate with the
discern types of fluid sources that may have contrib- same deep fluid is striking (Kastner et al., 1995b),
uted to the pore water sampled along the Expedition and it would make sense if Site U1329 also commu-
311 transect (M. Kastner and M. Torres, unpubl. nicates with the same deep fluid. Thus, the observed
data). local deviations at Site U1329 may indicate some
The mobility of lithium is temperature dependent. mixing with pore fluids influenced by in situ reac-
At lower temperatures, lithium is partitioned into tions with certain more radiogenic detrital phases,
clay minerals, whereas it is leached into the pore though this is not necessarily the case for regional

variations. This is further emphasized by the fact ment/water interface to a maximum of ~100 µM at
that the anomalous values were obtained only for 250 mbsf. However, Site U1328 deviates from this re-
samples from depths below the unconformity at Site lationship, where maximum values reach almost 800
U1329, where sediments older than 6.0 Ma were re- µM at 250 mbsf. The δ13C values of acetate range
covered, in contrast to all other sediments of mainly from –48‰ to –8‰. However, the largest enrich-
younger than 1.0 Ma age (Akiba et al.). ment of δ13C acetate (about –10‰) occurs over the
depth range from 130 to 220 mbsf at Site U1327
Methane production from CO2 reduction and (Heuer et al., 2007, 2009).
dissolved inorganic carbon isotopic As noted by Pohlman et al. (2009), although the CO2
composition and methane profiles are influenced by closed-
The δ13C values of methane range from a minimum system KIE, additional sources and sinks also affect
of –82.2‰ near the deformation front at Site the mass balance of CO2 and methane. A completely
U1326 to a maximum of –39.5‰ at the most land- closed system would result in δ13C-CO2 values that
ward location Site U1329 (Fig. F10A). The δ13C continually increase with depth, whereas the δ13C-
enrichment of methane has classically been inter- CO2 profiles at each site approach constant values
preted as evidence for the transition from microbial with increasing depth. Constant δ13C values with in-
to thermogenic methane. However, the associated creasing depth for CO2 and methane may be
CO2 sampled during Expedition 311 exhibits a simi- explained by mixing with a deeper isotopically
lar trend of δ13C enrichment, with values ranging uniform gas reservoir (Paull et al., 2000) or contribu-
from –22.5‰ to +25.7‰ (Fig. F10A). The magni- tions from organic matter fermentation that balance
tude of the carbon isotope separation between losses to carbonate reduction (Claypool et al., 1985).
methane and CO2 is consistent with the kinetic iso- As mentioned before, the carbon isotopic composi-
tope effect (KIE) that occurs during microbially tion of the residual DIC shows a general progressive
mediated carbonate reduction. Furthermore, the enrichment with distance from the deformation
gaseous hydrocarbon content is composed of front as well as downcore for all sites along the Expe-
>99.8% (by volume) methane and the methane has dition 311 transect, which can be explained by a KIE
uniform δDCH4 values (–172‰ ± 8‰) that are also from preferential consumption of the lighter 12CO2
consistent with carbonate reduction. Little evidence during methanogenesis. However, Site U1326 shows
was found of any thermogenic gas source along the a distinctively different depth profile compared to
transect, which is distinct from the thermogenic the other sites in that there are two intervals where
methane signatures seen at Barkley Canyon located δ13C values are 13C depleted (Fig. F11). The upper
~60 km southeast of the Expedition 311 transect anomalous interval (minimum δ13C is around –4‰
sites (Pohlman et al., 2005). These combined results Peedee belemnite [PDB]) extends from ~40 to
suggest that microbial CO2 reduction is the pre- ~140 mbsf, with the top of the gas hydrate reported
dominant source of the methane. The increase in at ~47 mbsf. The maximum δ13C value of +11‰ PDB
δ13C with sediment depth at each site is a closed- is reached at ~155 mbsf before values decrease again
system KIE from preferential consumption of 12CO2 to around +4‰ PDB. This distinct pattern toward
during methanogenesis, which drives the residual lighter carbon isotopic composition is a possible in-
CO2 and accumulated methane toward more en- dicator of the influx of nondepleted pore water from
riched 13C values. Similar observations were made depth.
at Blake Ridge (Leg 164; Paull et al., 2000) and SHR
(Leg 204; Claypool et al., 2006). The trend of 13C Regional in situ methane production
enrichment in DIC (Torres and Kastner) is similar and vent-associated gas migration
to that of CO2, with the difference being related to
The δ13C composition of the methane recovered
equilibrium isotope effects that occur during the
from gas hydrate samples from 44 and 53 mbsf at
degassing of CO2 from the dissolved phase (Emrich
Site U1326 is identical to the composition of core
et al., 1970). The δ13C-DIC values are near –5‰ at
void and sediment methane gas, which suggests that
Cascadia Basin Site 888 and near +32‰ for Site
methane in the gas hydrate samples was formed in
U1329 (Fig. F10B).
situ near the site of gas hydrate nucleation and was
An additional explanation for the downcore increase not derived from a deeper source that migrated into
in δ13C composition is the possibility of acetoclastic the GHSZ (Pohlman et al., 2006). This contrasts with
methanogenesis at Sites U1327 and U1329 (Heuer et observations made at the cold vent at Site U1328.
al., 2007, 2009). Most often, acetate concentrations The cold vent is characterized by a cap of gas hy-
increase in the sediments from <5 µM at the sedi- drate–rich sediments that extends to ~40 mbsf (see

the “Site U1328” chapter). The depth profiles of methane production rates were only seen in accreted
δ13C composition of core void and sediment meth- sediments below 100 mbsf).
ane gas at Site U1328 are similar to those seen at the
other sites along the transect. Values gradually de-
crease from around –62‰ PDB at 300 mbsf to about A modified model of fluid
–72‰ PDB at the seafloor. However, within the up- expulsion and gas hydrate
permost 50 mbsf there is a secondary trend of in-
creasing δ13C composition to values similar to those formation on the northern
observed at the bottom of the hole. Gas hydrate sam- Cascadia margin
ples recovered within the depth interval where the
The combined scientific results from Expedition 311
core gas methane was 13C enriched have similar val-
allow us to modify the earlier fluid expulsion model
ues, suggesting that the gas hydrate–bound gas and
proposed by Hyndman and Davis (1992). A concep-
dissolved gas in that interval are derived from a
tual diagram incorporating the main features of this
deep-migrated source (Pohlman et al., 2006).
revised model is shown in Figure F12. As in the ear-
The main source of methane in gas hydrates sampled lier model, pervasive fluid expulsion as a result of
during Expedition 311 is CO2 reduction, which raises tectonic thickening and shortening of the sediment
a key question: What are the production rates? Sev- package within the accreted wedge is required to ex-
eral microbiological experiments were conducted to plain the observations. The margin is characterized
determine the rate of methane production from Ex- by nonuniform fluid expulsion rates, which result in
pedition 311 sediments (Yoshioka et al., 2010) and the evolution of different fluid sources in the upper-
Leg 204 (Colwell et al., 2008). However, accurate in most few hundred meters below seafloor along the
situ microbial methane production rates are difficult margin. At Sites U1326 and U1325 (within the first
to determine using incubation experiments, which 10 km from the deformation front), salt generation
often result in unrealistically high production rates from the ash-to-zeolite transformation dominates. In
(Colwell et al., 2008). Nevertheless, methane produc- contrast, fresher pore fluids are observed farther
tion rates at Sites U1327 and U1329 were determined from the deformation front (Sites U1327 and U1328)
to be on average ~1 pmol/cm3/day, with highly vari- as a result of deeper rooted smectite-to-illite transfor-
able rates with depth (Yoshioka et al., 2010). The mation and freshwater expulsion that becomes more
value of ~1 pmol/cm3/day is comparable to values prevalent landward (especially 15–20 km away from
obtained by Colwell et al. (2008), who reported a the deformation front). However, farther landward,
rate of 0.017 fmol/cell/day using an average cell the fluid expulsion rate decreases and the amount of
count of ~1000 cells/g and a density of ~1.7 g/cm3. freshening in the pore fluid is reduced (as seen at Site
Another independent estimate of methane produc- U1329). Data from Site U1329 may also indicate
tion can be obtained from the progressive enrich- communication with a different (and likely more
ment of the δ13C composition of residual DIC (Torres landward) fluid source of basaltic origin (other than
et al., 2007) which yields results similar to estimates the downgoing crust). However, the data are non-
made by Colwell et al. (2008). conclusive and the issue is further complicated by
the presence of a pronounced unconformity, where a
Yoshioka et al. (2010) determined that methane pro-
5 m.y. long record of sediments is missing.
duction at Site U1327 is higher within the depth in-
terval where gas hydrate is inferred to occur than in With the exception of the cold seep Site U1328,
the near-surface, gas hydrate–free sediments or the methane is produced predominantly in situ within
sediments below the BSR. They also show that at Site the GHSZ. Microbes utilize organic matter (deposited
U1327 methanogens are more abundant in the gas either by pelagic or turbidite sedimentation) to pro-
hydrate–bearing sediments than in sediments at duce methane that is consequently incorporated
other depths. In contrast, at Site U1329 there is no into gas hydrate. Continuous sedimentation and
evidence for a substantial amount of gas hydrate in associated burial, as well as lateral transport during
the sediments. Methane production rates at this site accretionary prism formation, result in the enrich-
are also relatively high in the sediments between 70 ment of the δ13C composition of CH4 with distance
and 140 mbsf (comparable to those at Site U1327). from the deformation front and with depth at each
This suggests that the enhancement of methane pro- site along the Expedition 311 transect as the organic
duction rates in the hydrate-bearing interval at Site matter is progressively degraded landward as well as
U1327 is likely unrelated to the actual occurrence with depth.
and distribution of gas hydrate but instead may be The preference for gas hydrate occurrence in coarser
related to the location of the site within the grained sediments has been observed at many ma-
accretionary prism or sediment composition (high rine (Collett et al., 2008; Weinberger et al., 2005;

Kimura, Silver, Blum, et al., 1997; Ginsberg et al., and pressure, local rates in fluid advection and sedi-
2000) and terrestrial (Dallimore et al., 1999; Boswell mentation, and the abundance of organic matter
et al., 2008) locations and in laboratory experiments and microbes, as well as by appropriate host sedi-
(e.g., Tohidi et al., 2001; Uchida et al., 1999). The ment (sand). Thus, a rather heterogeneous picture of
separation of high gas hydrate concentrations within gas hydrate occurrence is derived with a high degree
the sandier turbidite sequences and low to absent gas of variability on the kilometer scale from site to site
hydrate concentrations within the finer grained sedi- along the Expedition 311 transect, on the 10–100
ments without any evidence for an underlying gas meter scale between adjacent holes at one site, and
migration component can be explained by the on the meter to submeter scale vertically within each
model of Malinverno (2010), in which gas is micro- hole. Focused fluid flow along faults can generate
bially generated in the finer grained sediments and cold vents with massive gas hydrate formation near
then transported by diffusion into the sandier sedi- the seafloor, as seen at Site U1328. Methane in the
ments, where it accumulates to saturations in excess shallow (<40 mbsf) gas hydrate accumulation at the
of local solubility. cold vent is transported from greater depth. A model
Although these models and associated geochemical of cold vent–related fluid flow and gas hydrate for-
and isotopic data show that in situ methane produc- mation was previously published by Riedel et al.
tion from carbonate reduction is the main source of (2006). In this model, methane-rich pore fluid and/
methane and is sufficient to explain all observations or free methane gas is passed through the sediment
of gas hydrate distribution, concentration, and mode column along a series of filamentous fractures. As
of occurrence along the Expedition 311 transect, the methane solubility drastically decreases near the sea-
advection of methane is still a requirement in the floor, massive gas hydrate is formed and excess
overall formation of gas hydrate within the accre- methane is vented into the overlying ocean. The re-
tionary prism. If in situ methane production (peak- sult is the formation of a massive gas hydrate cap
ing near the sulfate–methane transition zone and and widespread seafloor carbonate formations and
then exponentially decreasing with depth) is the sole chemosynthetic communities.
source for gas hydrate formation, gas hydrate should The BGHSZ (seismically defined as a BSR) is a tempo-
also be abundant in the sediments of the abyssal ral trap for some amounts of free gas below, which
plain as the total organic carbon content at Site 888 gives rise to the seismic velocity decrease across the
(average = 0.54 wt%; Cragg et al., 1996) is relatively interface. Because there appears to be only little mi-
similar to that at all other sites along the Expedition gration of free gas from greater depths on a regional
311 transect (U1325 = 0.52 wt%, U1326 = 0.42 wt%, scale, the free gas trapped below the BGHSZ could
U1327 = 0.7 wt%, U1329 = 0.61 wt%, and U1328 = best be explained as being derived from gas hydrate
0.5 wt%; data from Kim and Lee). However, logging recycling. The combination of continuous sedimen-
and coring at Site 888 did not show any evidence of tation processes and tectonic uplift with relatively
gas hydrate. rapid fluid migration (subsaturated in methane) re-
Fluid advection is observed and constrained by con- sults in the recycling of gas hydrate at the BGHSZ
centration gradients of multiple pore water constitu- and the accumulation of a thin free gas zone as de-
ents (e.g., Cl, Br, Li, and Sr), and fluid sources can be scribed by Haacke et al. (2007).
relatively deep (~1 km). Dissolved methane can also
be transported by these advecting fluids; however,
the depth of the methane does not necessarily have
Summary
to be deep, and migration pathways can be relatively The combined results of studies carried out during
short (i.e., entirely within the gas hydrate occurrence and in conjunction with Expedition 311 reveal a
zone). Continuous recycling of gas hydrate and free very complex sedimentological, geochemical, and
gas is occurring at the BGHSZ because of the active geophysical regime that controls the formation and
tectonic deformation of the accretionary prism, distribution of gas hydrate on the northern Cascadia
combined with rapid sedimentation and mass wast- margin. Results from Expedition 311 have signifi-
ing processes. The characteristic downhole isotopic cantly augmented our understanding of the geologic
signature of the dissolved methane can result from controls on the occurrence of gas hydrate. The main
the mixing of “freshly” produced methane by in situ conclusions that can be drawn from the studies asso-
bacterial processes and some “older” methane ad- ciated with Expedition 311 are as follows:
vected from below. 1. Methane within sediment and recovered gas
The vertical and lateral extent of the local gas hy- hydrate along the expedition transect were pro-
drate occurrence is governed through temperature duced primarily by microbial CO2 reduction and

secondarily by acetate fermentation; all hydro- as previously predicted (Hyndman and Davis,
carbon sources are microbial, and no thermo- 1992; Hyndman et al., 2001).
genic gases were detected. 12.The top of gas hydrate occurrence along the ex-
2. Isotope fractionation during microbial genera- pedition transect (and the overall thickness of gas
tion of methane results in a progressive isotopic hydrate occurrence) deepens with distance from
enrichment of the carbon in the methane, CO2, the deformation front, likely as a result of a pro-
and DIC with increasing sediment depth and dis- gressive decrease in pore fluid advection and/or
tance from the deformation front. decreasing organic matter quality.
3. Pore fluid composition (e.g., chlorinity) demon-
strates a significant component of pervasive up-
ward fluid migration across the accretionary Acknowledgments
complex. Samples and data were provided by the Integrated
4. Fluid expulsion is nonuniform across the margin, Ocean Drilling Program (IODP), which is funded by
with expulsion rates being highest ~15 km away the U.S. National Science Foundation and participat-
from the deformation front. The systematic ing countries under management of IODP Manage-
change in expulsion rates is the main cause of the ment International, Inc. We thank the captain and
apparent separation into saltier and fresher pore crew of the R/V JOIDES Resolution and the technical
fluids at depth because it controls the amount of staff for their support at sea. Additional thanks goes
upward-migrating fresher pore fluids that mix to the onboard and postcruise scientific community,
with those pore waters affected by salt-generating whose research results were synthesized in this pa-
diagenetic processes. per. Other data collected from IODP Expedition 311
5. Diffuse upward pore fluid migration is likely samples but not reviewed in this synthesis can be
overprinted by flow through conduits such as found in data reports by Bahr et al., Hester et al.,
fractures, faults, or permeable strata. Wortman, Blanc-Valleron et al., and Pierre et al.
6. Evidence for a deep-migrated source of methane
was observed in shallow (<50 mbsf) gas hydrate
accumulations at the cold vent Site U1328 (Bulls- References
eye vent), where near-vertical fracture systems
delivered methane from a deep source to the Archie, G.E., 1942. The electrical resistivity log as an aid in
determining some reservoir characteristics. Trans. Am.
surface.
Inst. Min., Metall. Pet. Eng., 146:54–62.
7. Some component of regional dissolved methane
Bekins, B., McCaffrey, A.M., and Dreiss, S.J., 1994. Influ-
advection through the GHSZ is required to ex-
ence of kinetics on the smectite to illite transition in the
plain the presence of gas hydrate occurrence Barbados accretionary prism. J. Geophys. Res., [Solid
within the accretionary prism and not within the Earth], 99(B9):18147–18158. doi:10.1029/94JB01187
abyssal plain sediments. This advection, in com- Bekins, B.A., and Dreiss, S.J., 1992. A simplified analysis of
bination with methane production and sedimen- parameters controlling dewatering in accretionary
tation rates, defines the overall thickness of the prisms. Earth Planet. Sci. Lett., 10(3–4)9:275–287.
gas hydrate occurrence zone. doi:10.1016/0012-821X(92)90092-A
8. Gas hydrate occurs preferentially in coarser Borowski, W.S., Paull, C.K., and Ussler, W., III, 1996.
grained sediments (sandy/silty turbidites) and is Marine pore-water sulfate profiles indicate in situ meth-
mainly formed from methane produced in situ. ane flux from underlying gas hydrate. Geology,
9. Gas hydrate concentrations in the pore space of 24(7):655–658. doi:10.1130/0091-
the sediments are low (<5%) when averaged over 7613(1996)024<0655:MPWSPI>2.3.CO;2
the entire gas hydrate occurrence zone. However, Boswell, R., Hunter, R., Collett, T.S., Digert, S., Hancock,
locally, gas hydrate concentrations within sand S.H., Weeks, M., and the Mount Elbert Science Team,
layers can be as high as 50% of the pore space. 2008. Investigation of gas hydrate–bearing sandstone
reservoirs at the “Mount Elbert” stratigraphic test well,
10.Gas hydrate distribution is highly heterogeneous
Milne Point, Alaska. Proc. Int. Conf. Gas Hydrates, 6.
across the margin at all scales between each site
https://circle.ubc.ca/handle/2429/1167
visited as well at each site between adjacent bore-
Chapman, N.R., Gettrust, J.F., Walia, R., Hannay, D.,
holes, making remote gas hydrate detection and Spence, G.D., Wood, W.T., and Hyndman, R.D., 2002.
quantification challenging. High-resolution, deep-towed, multichannel seismic sur-
11.Gas hydrate occurs anywhere within the gas hy- vey of deep-sea gas hydrates off western Canada. Geo-
drate stability zone where favorable conditions physics, 67(4):1038–1047. doi:10.1190/1.1500364
occur (sufficient gas concentration above local Chen, M.-A.P., 2006. Northern Cascadia marine gas
solubility and the presence of coarse-grained sed- hydrate: constraints from resistivity, velocity, and AVO
iments) and not preferably right above the BSR, [M.S. thesis]. Univ. Victoria, British Columbia.

Claypool, G.E., Milkov, A.V., Lee, Y.-J., Torres, M.E., carbonate. Earth Planet. Sci. Lett., 8(5):363–371.
Borowski, W.S., and Tomaru, H., 2006. Microbial meth- doi:10.1016/0012-821X(70)90109-3
ane generation and gas transport in shallow sediments Flemings, P.B., Liu, X., and Winters, W.J., 2003. Critical
of an accretionary complex, southern Hydrate Ridge pressure and multiphase flow in Blake Ridge gas
(ODP Leg 204), offshore Oregon, USA. In Tréhu, A.M., hydrates. Geology, 31(12):1057–1060. doi:10.1130/
Bohrmann, G., Torres, M.E., and Colwell, F.S. (Eds.), G19863.1
Proc. ODP, Sci. Results, 204: College Station, TX (Ocean Ginsburg, G., Soloviev, V., Matveeva, T., and Andreeva, I.,
Drilling Program), 1–52. doi:10.2973/ 2000. Sediment grain-size control on gas hydrate pres-
odp.proc.sr.204.113.2006 ence, Sites 994, 995, and 997. In Paull, C.K., Matsu-
Claypool, G.E., Threlkeld, C.N., Mankiewicz, P.N., Arthur, moto, R., Wallace, P.J., and Dillon, W.P. (Eds.), Proc.
M.A., and Anderson, F.T., 1985. Isotopic composition of ODP, Sci. Results, 164: College Station, TX (Ocean Drill-
interstitial fluids and origin of methane in slope sedi- ing Program), 237–245. doi:10.2973/
ment of the Middle America Trench, Deep Sea Drilling odp.proc.sr.164.236.2000
Project Leg 84. In von Huene, R., Aubouin, J., et al., Init. Goldberg, D., Guerin, G., Malinverno, A., and Cook, A.,
Repts. DSDP, 84: Washington (U.S. Govt. Printing 2008. Velocity analysis of LWD and wireline sonic data
Office), 683–691. doi:10.2973/dsdp.proc.84.124.1985 in hydrate-bearing sediments on the Cascadia margin.
Collett, T.S., and Ladd, J., 2000. Detection of gas hydrate Proc. Int. Conf. Gas Hydrates, 6. https://circle.ubc.ca/
with downhole logs and assessment of gas hydrate con- handle/2429/1619
centrations (saturations) and gas volumes on the Blake Haacke, R.R., Westbrook, G.K., and Hyndman, R.D., 2007.
Ridge with electrical resistivity log data. In Paull, C.K., Gas hydrate, fluid flow and free gas: formation of the
Matsumoto, R., Wallace, P.J., and Dillon, W.P. (Eds.), bottom-simulating reflector. Earth Planet. Sci. Lett.,
Proc. ODP, Sci. Results, 164: College Station, TX (Ocean 261(3–4):407–420. doi:10.1016/j.epsl.2007.07.008
Drilling Program), 179–191. doi:10.2973/ Heuer, V., Pohlman, J., Elvert, M., and Hinrichs, K.-U.,
odp.proc.sr.164.219.2000 2007. Carbon isotopic compositions of acetate as prox-
Collett, T.S., Riedel, M., Cochran, J., Boswell, R., Presley, J., ies for biogeochemical processes in gas hydrate bearing
Kumar, P., Sathe, A.V., Sethi, A., Lall, M., Sibal, V., and sediments. Abstracts of the 17th Annual V. M. Gold-
the NGHP Expedition 01 Scientists, 2008. Indian schmidt Conference. Geochim. Cosmochim. Acta,
National Gas Hydrate Program (NGHP) Expedition 01, Ini- 71(15S):A402. http://goldschmidt.info/2007/
tial Report: India (DGH, Ministry of Petroleum and Nat- abstracts/A402.pdf
ural Gas). Heuer, V.B., Pohlman, J.W., Torres, M.E., Elvert M., and
Colwell, F.S., Boyd, S., Delwiche, M.E., Reed, D.W., Phelps, Hinrichs, K.-U., 2009. The stable carbon isotope biogeo-
T.J., and Newby, D.T., 2008. Estimates of biogenic meth- chemistry of acetate and other dissolved carbon species
ane production rates in deep marine sediments at in deep subseafloor sediments at the northern Cascadia
Hydrate Ridge, Cascadia margin. Appl. Environ. Micro- margin. Geochim. Cosmochim. Acta, 73:3323–3336.
biol., 74(11):3444–3452. doi:10.1128/AEM.02114-07 doi:10.1016/j.gca.2009.03.001
Cragg, B.A., Parkes, R.J., Fry, J.C., Weightman, A.J., Hyndman, R.D., and Davis, E.E., 1992. A mechanism for
Rochelle, P.A., and Maxwell, J.R., 1996. Bacterial popu- the formation of methane hydrate and seafloor bottom-
lations and processes in sediments containing gas simulating reflectors by vertical fluid expulsion. J. Geo-
hydrates (ODP Leg 146: Cascadia margin). Earth Planet. phys. Res., [Solid Earth], 97(B5):7025–7041. doi:10.1029/
Sci. Lett., 139(3–4):497–507. doi:10.1016/0012- 91JB03061
821X(95)00246-9 Hyndman, R.D., Spence, G.D., Chapman, R., Riedel, M.,
and Edwards, R.N., 2001. Geophysical studies of marine
Dallimore, S.R., Uchida, T., and Collett, T.S. (Eds.), 1999.
gas hydrate in northern Cascadia. In Paull, C.K., and
Scientific Results from JAPEX/JNOC/GSC Mallik 2L-38
Dillon, W.P. (Eds.), Natural Gas Hydrates: Occurrence, Dis-
Gas Hydrate Research Well, MacKenzie Delta, North-
tribution, and Detection. Geophys. Monogr., 124:273–
west Territories, Canada. Bull.—Geol. Surv. Can, 544.
295.
Dickens, G.R., Paull, C.K., and Wallace, P., 1997. Direct
Hyndman, R.D., Yuan, T., and Moran, K., 1999. The con-
measurement of in situ methane quantities in a large
centration of deep sea gas hydrates from downhole elec-
gas-hydrate reservoir. Nature (London, U. K.),
trical resistivity logs and laboratory data. Earth Planet.
385(6615):426–428. doi:10.1038/385426a0
Sci. Lett., 172(1–2):167–177. doi:10.1016/S0012-
Dickens, G.R., and Snyder, G.T., 2009. Interpreting upward 821X(99)00192-2
methane flux from marine pore water profiles. Fire Ice, Iijima, A., and Utada, M., 1966. Zeolites in sedimentary
Winter 2009:7–10. rocks, with references to the depositional environments
Ellis, M., Minshull, T., Sinha, M., and Best, A., 2008. Joint and zonal distribution. Sedimentology, 7(4):327–357.
electrical and seismic interpretation of gas hydrate bear- doi:10.1111/j.1365-3091.1966.tb01299.x
ing sediments from the Cascadia margin. Eos, Trans. Kastner, M., Kvenvolden, K.A., Whiticar, M.J., Camer-
Am. Geophys. Union, 89(53)(Suppl.):OS43F-02. (Abstract) lenghi, A., and Lorenson, T.D., 1995a. Relation between
http://www.agu.org/meetings/fm08/waisfm08.html pore fluid chemistry and gas hydrates associated with
Emrich, K., Ehhalt, D., and Vogel, J.C., 1970. Carbon iso- bottom-simulating reflectors at the Cascadia margin,
tope fractionation during the precipitation of calcium Sites 889 and 892. In Carson, B., Westbrook, G.K., Mus-

grave, R.J., and Suess, E. (Eds.), Proc. ODP, Sci. Results, Geochem., 36(5):703–716. doi:10.1016/j.orggeo-
146 (Pt. 1): College Station, TX (Ocean Drilling Pro- chem.2005.01.011
gram), 175–187. doi:10.2973/odp.proc.sr.146- Pohlman, J.W., Kaneko, M., Heuer. V., Plummer, R., Coffin,
1.213.1995 R.B., and the IODP Expedition 311 Scientific Party,
Kastner, M., Sample, J.C., Whiticar, M.J., Hovland, M., 2006. Molecular and isotopic characterization of gases
Cragg, B.A., and Parkes, J.R., 1995b. Geochemical evi- from IODP Expedition 311: source and gas hydrate
dence for fluid flow and diagenesis at the Cascadia con- related controls. Eos, Trans. Am. Geophys. Union,
vergent margin. In Carson, B., Westbrook, G.K., 87(52)(Suppl.):OS11E-06. (Abstract). http://
Musgrave, R.J., and Suess, E. (Eds.), Proc. ODP, Sci. www.agu.org/meetings/fm06/waisfm06.html
Results, 146 (Pt. 1): College Station, TX (Ocean Drilling Pohlman, J.W., Kaneko, M., Heuer, V.B., Coffin, R.B., and
Program), 375–384. doi:10.2973/odp.proc.sr.146- Whiticar, M., 2009. Methane sources and production in
1.243.1995 the northern Cascadia margin gas hydrate system. Earth
Kastner, M., Torres, M., Solomon, E., and Spivack, A.J., Planet. Sci. Lett., 287(3–4):504–512. doi:10.1016/
2008. Marine pore fluid profiles of dissolved sulfate; do j.epsl.2009.08.037
they reflect in situ methane fluxes? Fire Ice, Summer Riedel, M., Spence, G.D., Chapman, N.R., and Hyndman,
2008:6–8. R.D., 2002. Seismic investigations of a vent field associ-
Kimura, G., Silver, E.A., Blum, P., et al., 1997. Proc. ODP, ated with gas hydrates, offshore Vancouver Island. J.
Init. Repts., 170: College Station, TX (Ocean Drilling Pro- Geophys. Res., 107(B9):2200. doi:10.1029/
gram). doi:10.2973/odp.proc.ir.170.1997 2001JB000269
Riedel, M., Novosel, I., Spence, G.D., Hyndman, R.D.,
Lu, Z., Hensen, C., Fehn, U., and Wallmann, K., 2008. Hal-
Chapman, N.R., Solem, R.C., and Lewis, T., 2006. Geo-
ogen and 129I systematics in gas hydrate fields at the
physical and geochemical signatures associated with gas
northern Cascadia margin (IODP Expedition 311):
hydrate–related venting in the northern Cascadia mar-
insights from numerical modeling. Geochem., Geophys.,
gin. Geol. Soc. Am. Bull., 118(1):23–38. doi:10.1130/
Geosyst., 9(10):Q10006. doi:10.1029/2008GC002156
B25720.1
Malinverno, A., 2010. Marine gas hydrates in thin sand
Schowalter, T.T., 1979. Mechanics of secondary hydrocar-
layers that soak up microbial methane. Earth Planet. Sci.
bon migration and entrapment. AAPG Bull., 63(5):723–
Lett., 292(3–4):399–408. doi:10.1016/
760.
j.epsl.2010.02.008
Suess, E., Torres, M.E., Bohrmann, G., Collier, R.W., Grein-
Malinverno, A., Kastner, M., Torres, M.E., and Wortmann, ert, J., Linke, P., Rehder, G., Tréhu, A., Wallmann, K.,
U.G., 2008. Gas hydrate occurrence from pore water Winckler, G., and Zuleger, E., 1999. Gas hydrate destabi-
chlorinity and downhole logs in a transect across the lization: enhanced dewatering, benthic material turn-
northern Cascadia margin (Integrated Ocean Drilling over and large methane plumes at the Cascadia
Program Expedition 311). J. Geophys. Res., [Solid Earth], convergent margin. Earth Planet. Sci. Lett.,170(1–2):1–
113(B8):B08103. doi:10.1029/2008JB005702 15. doi:10.1016/S0012-821X(99)00092-8
Milkov, A.V., Dickens, G.R., Claypool, G.E., Lee, Y-J., Teichert, B.M.A., Torres, M.E., Bohrmann, G., and Eisen-
Borowski, W.S., Torres, M.E., Xu, W., Tomaru, H., Tréhu, hauer, A., 2005. Fluid sources, fluid pathways and diage-
A.M., and Schultheiss, P., 2004. Co-existence of gas netic reactions across an accretionary prism revealed by
hydrate, free gas, and brine within the regional gas Sr and B geochemistry. Earth Planet. Sci. Lett., 239(1–
hydrate stability zone at Hydrate Ridge (Oregon mar- 2):106–121. doi:10.1016/j.epsl.2005.08.002
gin): evidence from prolonged degassing of a pressur- Tohidi, B., Anderson, R., Clennell, M.B., Burgass, R.W., and
ized core. Earth Planet. Sci. Lett., 222(3–4):829–843. Biderkab, A.B., 2001. Visual observation of gas-hydrate
doi:10.1016/j.epsl.2004.03.028 formation and dissociation in synthetic porous media
Paull, C.K., Lorensen, T.D., Borowski, W.S., Ussler, W., III, by means of glass micromodels. Geology, 29(9):867–870.
Olsen, K., and Rodriguez, N.M., 2000. Isotopic composi- doi:10.1130/0091-
tion of CH4, CO2 species, and sedimentary organic mat- 7613(2001)029<0867:VOOGHF>2.0.CO;2
ter within samples from the Blake Ridge: gas source Torres, M.E., Kastner, M., Wortmann, U.G., Colwell, F., and
implications. In Paull, C.K., Matsumoto, R., Wallace, Kim, J., 2007. Estimates of methane production rates
P.J., and Dillon, W.P. (Eds.), Proc. ODP, Sci. Results, 164: based on δ13C of the residual DIC in pore fluids from
College Station, TX (Ocean Drilling Program), 67–78. the Cascadia margin. Eos, Trans. Am. Geophys. Union,
doi:10.2973/odp.proc.sr.164.207.2000 88(52)(Suppl.):GC14A-04. (Abstract) http://
Paull, C.K., and Ussler, W., III, 1997. Are low salinity www.agu.org/meetings/fm07/waisfm07.html
anomalies below BSRs a consequence of interstitial gas Torres, M.E., Teichert, B.M.A., Tréhu, A.M., Borowski, W.,
bubble barriers? Eos, Trans. Am. Geophys. Union, and Tomaru, H., 2004. Relationship of pore water fresh-
78(46):F339. (Abstract) ening to accretionary processes in the Cascadia margin:
Pohlman, J.W., Canuel, E.A., Chapman, N.R., Spence, fluid sources and gas hydrate abundance. Geophys. Res.
G.D., Whiticar, M.J., and Coffin, R.B., 2005. The origin Lett., 31:L22305. doi:10.1029/2004GL021219
of thermogenic gas hydrates on the northern Cascadia Torres, M.E., Tréhu, A.M., Cespedes, N., Kastner, M., Wort-
margin as inferred from isotopic (13C/12C and D/H) and mann, U.G., Kim, J.-H., Long, P., Malinverno, A., Pohl-
molecular composition of hydrate and vent gas. Org. man, J.W., Riedel, M., and Collett, T., 2008. Methane

hydrate formation in turbidite sediments of northern Wortmann, U.G., Chernyavsky, B.M., Torres, M.E., and
Cascadia, IODP Expedition 311. Earth Planet. Sci. Lett., Kastner, M., 2008. 34S/32S and 18O/16O ratios of dissolved
271(1–4):170–180. doi:10.1016/j.epsl.2008.03.061 sulfate from interstitial water samples above gas hydrate
Tréhu, A.M., Bohrmann, G., Rack, F.R., Torres, M.E., et al., bearing sediments of IODP Expedition 311, Cascadia.
2003. Proc. ODP, Init. Repts., 204: College Station, TX Eos, Trans. Am. Geophys. Union, 89(53)(Suppl.):OS33A-
(Ocean Drilling Program). doi:10.2973/ 1308. (Abstract) http://www.agu.org/meetings/fm08/
odp.proc.ir.204.2003 waisfm08.html
Uchida, T., Ebinuma, T., and Ishizaki, T., 1999. Dissocia- Xu, W., and Ruppel, C., 1999. Predicting the occurrence,
tion condition measurements of methane hydrate in distribution, and evolution of methane gas hydrate in
confined small pores of porous glass. J. Phys. Chem. B, porous marine sediments. J. Geophys. Res., [Solid Earth],
103(18):3659–3662. doi:10.1021/jp984559l 104(B3):5081–5096. doi:10.1029/1998JB900092
Yoshioka, H., Maruyama, A., Nakamura, T., Higashi, Y.,
von Huene, R., and Pecher, I.A., 1998. Vertical tectonics
Fuse, H., Sakata, S., and Bartlett, D.H., 2010. Activities
and the origins of BSRs along the Peru margin. Earth
and distribution of methanogenic and methane-oxidiz-
Planet Sci. Lett., 166(1–2):47–55. doi:10.1016/S0012-
ing microbes in marine sediments from the Cascadia
821X(98)00274-X
margin. Geobiology, 8(3):223–233. doi:10.1111/j.1472-
Wang, J., 2006. The lithological constraint to gas hydrate 4669.2009.00231.x
formation: evidence of grain size of sediments from Yuan, T., Hyndman, R.D., Spence, G.D., and Desmons, B.,
IODP 311 on Cascadia margin. Eos, Trans. Am. Geophys. 1996. Seismic velocity increase and deep-sea gas hydrate
Union, 87(52)(Suppl.):OS33B-1708 (Abstract). http:// concentration above a bottom-simulating reflector on
www.agu.org/meetings/fm06/waisfm06.html the northern Cascadia continental slope. J. Geophys.
Wang, K., Hyndman, R.D., and Davis, E.E., 1993. Thermal Res., [Solid Earth], 101(B6):13655–13671. doi:10.1029/
effects of sediment thickening and fluid expulsion in 96JB00102
accretionary prisms: model and parameter analysis. J. Yuan, T., Spence, G.D., Hyndman, R.D., Minshull, T.A.,
Geophys. Res., [Solid Earth], 98(B6):9975–9984. and Singh, S.C., 1999. Seismic velocity studies of a gas
doi:10.1029/93JB00506 hydrate bottom-simulating reflector on the northern
Weinberger, J.L., Brown, K.M., and Long, P.E., 2005. Paint- Cascadia continental margin: amplitude modeling and
ing a picture of gas hydrate distribution with thermal full waveform inversion. J. Geophys. Res., [Solid Earth],
images. Geophys. Res. Lett., 32(4):L04609. doi:10.1029/ 104(B1):1179–1192. doi:10.1029/1998JB900020
2004GL021437
Westbrook, G.K., Carson, B., Musgrave, R.J., et al., 1994. Initial receipt: 23 August 2009
Proc. ODP, Init. Repts., 146 (Pt. 1): College Station, TX Acceptance: 11 May 2010
(Ocean Drilling Program). doi:10.2973/ Publication: 9 July 2010
odp.proc.ir.146-1.1994 MS 311-213

Figure F1. Map of the Cascadia continental margin. Shown are the research sites established by Ocean Drilling
Program (ODP) Legs 146 and 204 and Integrated Ocean Drilling Program (IODP) Expedition 311. The inset
shows the inferred regional distribution of gas hydrate along the northern Cascadia margin (shaded area) from
the occurrence of a bottom-simulating reflector as well as the general plate-tectonic regime of the northern Cas-
cadia subduction zone (after Hyndman et al., 2001).
52°
N
British Columbia
50°
IODP Exp. 311 Sites 889 and 890

48° transect Site 888
Washington
Juan de
46°
Fuca plate
Site 891 Site 892

ODP Leg 204
44° Southern Hydrate Ridge
Oregon
km
0 100 200
42°
130°W 128° 126° 124° 122° 120°
52°
North American
Co
plate
nt
in
Va
en
nco
ta
uv
l
Canada
sh
er
Isl
el
an
fe
Pacific d U.S.
dg
plate 48°
e
Juan de
Fuca plate
49°
N
IODP Exp. 311
transect Area of regional
gas hydrate
occurence
D
ef
or
m
at
Continental shelf
io
n
Cascadia Basin <500 m depth

fro
nt
~2600 m depth
48°
127°W 126°

Figure F2. Seismic Line 89-08 from the ODP Leg 146 presite survey covering the Expedition 311 drill sites
(U1326–U1329). ODP Site 888 is projected onto this line but is located ~55 km to the southeast. Note that Site
CAS-04B has not yet been established but is proposed as part of IODP Proposal 553-Full2. BSR = bottom-
simulating reflector.
Site U1329
1.0
Site U1327
Site U1326
2.0
Two-way traveltime (s)
Proposed Site U1325

Site CAS-04B
(equiv. to Site 888)
3.0
BSR
4.0
5.0
10 km
Oceanic crust

Figure F3. Compilation of all downhole temperature measurements conducted during IODP Expedition 311
compared to results obtained from ODP Leg 146, Site 889.
20
Site U1325
Site U1326
Site U1327
15
Site 889
Site U1328
Temperature (°C)
Site U1329
10
0
0 50 100 150 200 250 300
Depth (mbsf)

Figure F4. Compilation of all data points of methane concentration derived from pressure core degassing ex-
periments.
Methane (mM)
10 100 1000 104
0
Dissolved methane
50 and methane hydrate
100
150
Depth (mbsf)
Dissolved
methane
200
250
Site U1325
Site U1326
Site U1327
300
Site U1328
Dissolved methane
Site U1329
and methane gas
350

Figure F5. Comparison of pore water saturations (Sh) calculated from logging-while-drilling (LWD) data. Shown for all sites are the main lithologic
Proc. IODP | Volume 311
M. Riedel et al.
units (see “Lithostratigraphy” in all site chapters for definition of units), resistivity-at-the-bit (RAB) resistivity data, and Sh estimates from Archie
analyses. BSR = bottom-simulating reflector, PCS = Pressure Core Sampler.
Site U1326 Site U1325 Site U1327 Site U1328 Site U1329
Lith. Sh Lith. Sh Lith. Sh Lith. Sh Lith. Sh
unit 1 0 unit 1 0 unit 1 0 unit 1 0 unit 1 0
I IA
I
IB
I
50
I
II II
II
100
BSR
II
Depth (mbsf)
150 III
II
III
200
BSR
BSR
III
BSR III
III
250 BSR
IV
Expedition 311 synthesis

300
Low High
Deep RAB resistivity
PCS (average)
20
Figure F6. Comparison of logging-while-drilling (LWD) data with core-derived pore water chlorinity values. Shown for all sites are the main lith-
M. Riedel et al.
ologic units, color-coded resistivity-at-the-bit (RAB) resistivity data, and pore water chlorinity from the recovered cores (blue) and pressure cores
(red) as well as results from ODP Leg 146, Sites 889 and 890 (gray). BSR = bottom-simulating reflector, PCS = Pressure Core Sampler.
Lith. CI (mM) Lith. CI (mM) Lith. CI (mM) Lith. CI (mM) Lith. CI (mM)
unit 300 500 unit 300 500 unit 300 500 unit 300 600 unit 300 500
I IA
I
IB
I
50
I
II II
II
100
Depth (mbsf)
II BSR
150 III
II
III
200
III BSR
III BSR
250 BSR III

BSR
IV

300 Low High
Deep RAB resistivity
PCS (average)
21
Figure F7. Evidence for grain-size control on gas hydrate occurrence. A. Photograph of a recovered core at Site
U1326 (311-U1326C-6X-4, 83–96 cm [44.85 mbsf]). B. Infrared (IR) image made in the geochemistry laboratory
(modified after Torres et al., 2008). The IR temperature color scale is given on the right-hand side of the image.
The sandy portion of the entire sample shown contains ~70% gas hydrate in the pore space, as defined from
pore water freshening compared to the assumed background, whereas the mud portion contains no gas hydrate
(within the uncertainty of knowing the background salinity).
A B
15
Mud Mud
Temperature (°C)
~10 cm
10
Gas hydrate
~70%
4
Sand Sand

Figure F8. Comparison of fluid expulsion rates and total fluid expelled (as modeled by Hyndman and Davis,
1992) with observed total pore water freshening at a common depth of 200 mbsf observed at drill sites from
ODP Legs 146 and 204 and IODP Expedition 311. SW = seawater.
1000 100
Cumulative
fluid expelled
50
800 Site U1325
Site U1326 3.0

Sites 1247 and 1248
0
at 200 mbsf relative to SW

Chlorinity difference (mM)
Site 888
Expulsion rate (mm/y)

Fluid expelled (m3/m2)
Site
600
1245
Site U1328
Smoothed Site
2.0 -50
expulsion rate 1252 Site 1244
Site U1329
400
Site 1251 (47 km)
-100
1.0
200
-150
Sites U1327 and 889
-200
0 10 20 30
Distance landward from deformation front (km)

Figure F9. Depth profiles of lithium and strontium pore water data from the Expedition 311 transect. A.
Lithium. B. Isotopic ratio of 87Sr/86Sr. C. Strontium normalized to chlorinity. D. Mixing diagram of strontium
data from Expedition 311 compared to results from ODP Leg 204 by Teichert et al. (2005). Unconf. = uncon-
formity.
87Sr/86Sr
A Lithium (µM) B
0 50 100 150 0.705 0.706 0.707 0.708 0.709 0.710
50 50
100 100
Unconf. Site U1329 Unconf. Site U1329
Depth (mbsf)
Depth (mbsf)
150 150
200 200
250 250
300 300
C Sr/Cl D 1/Sr
0.1 0.2 0.3 0.4 0.5 0.6 0.005 0.015 0.025 0.035
Seawater
0.709
50
5)
l. (200
Teiche g line by
r t et a
100
Mixin
Unconf. Site U1329 0.708

Depth (mbsf)
87Sr/86Sr
Carbonate
150
precipitation shift
Site 1027
0.707
200
Site U1325
250
Site U1326
0.706 ?
Site U1327
Site U1328
Site U1329
300

Figure F10. A. Depth profiles of the δ13C isotopic composition of methane and CO2 from all transect sites. Data
from the vent Site U1328 are not included (from Pohlman et al., 2009). Also shown for reference are data from
ODP Leg 146 Sites 888 and 889. B. Increase of the δ13C signature of dissolved inorganic carbon with distance
from the deformation front. Data from ODP Legs 146 (Site 888) and 204 (Sites 1244, 1252, and 1251), and IDOP
Expedition 311 (Sites U1326, U1325, U1327, and U1329) are combined (from Torres and Kastner). PDB =
Peedee belemnite.
A δ13C (‰)
-100 -80 -60 -40 -20 0 20 40
CH4 CO2
50
100
Depth (mbsf)
150
200
250
Site U1325
Site U1326
300
Site U1327
Site 888 Site U1329
-50‰ to -57‰ Site 888
350-600 mbsf Site 889
350
B 40
30 Site U1329
Site U1327
δ13C (‰ PDB)
20
Site 1252 Site 1251
Site U1326 Site 1244

10
Site U1325
Site 888
-10
0 10 20 30 40 50
Distance landward from deformation front (km)

Figure F11. Downcore trend in the δ13C isotope composition of dissolved inorganic carbon (DIC) at Sites U1326
and U1325. Modified after Torres and Kastner. PDB = Peedee belemnite.
δ13C (‰ PDB) DIC

-40 -30 -20 -10 0 10 20
0
50
Lateral
flow?
100
Depth (mbsf)
150
200
250
Site U1325
Site U1326
300

Figure F12. Schematic cartoon (not to scale) of the fluid flow and gas hydrate formation model for the northern
Cascadia margin. See text for details. DF = deformation front, BGHS = base of gas hydrate stability, BSR =
bottom-simulating reflector, GH = gas hydrate, BGHSZ = base of gas hydrate stability zone.
Pelagic organic input Turbiditic organic input Site U1329
Site U1327 Site U1328
Cold
Site U1326 Box 2 vents
In situ
Sedimentation R
and Site CH4
S/BS
burial U1325 production
BGH
Deformation Enrichment
front of δ13C of CO2 pool
Site 888 (with depth and distance from DF) Focused fluid flow
along faults
Box 1
BGHS/BSR Free gas
Diffuse fluid expulsion

Subd and advection of freshwater
u cting
ocea
nic p
la te
Box 1 Mud
Sand
GH
Mud Continued sediment

Small and tectonic uplift
fractures
Sand
Few GH
(<5%) Dissolved CH4 BGHSZ
advection BSR
Box 2
GH Sand
Organics In situ
GH and and CH4 Mud
Free gas free gas recycling bacteria production GH Sand

Table T1. Calculated depth of the base of gas hydrate occurrence (in meters below seafloor) from various geophysical and geochemical techniques.
M. Riedel et al.
(See table notes.)
Technique U1325A U1325C U1326A U1326D U1327A U1327D U1327E U1328A U1328C U1329A U1329C U1329D
Precruise seismic BSR* 230 234 223 219 125

Depth below seafloor (s) 0.2817 0.2883 0.272 0.2673 0.154
LWD resistivity 240 x 260 x 230 x x 219 x ND x x
CWL computed velocity x DNRB x 260 x x 228 x 220 x x 125
CWL resistivity x DNRB x 260 x 235 235 x 220 x x 125
Deepest Cl– and IR anomaly x 238.9 x 264 x 222 x x 220 x 128 x
C1/C2 x 243 x x x 225 x x 214 x 119 x
i-C4/n-C4 or C3 x x x x x 225 x x 219 x x x
VSP x x x x x 245 x x ND x x x
Downhole thermal measurements 250–300 250–270 225–250 220–245 127–129
Best estimate of BGHSZ: 240.5 264 230 219 124
Notes: * = velocity used for BSR depth conversion from two-way traveltime (1636 m/s = maximum average velocity from seafloor to BSR; 1619 m/s = minimum average from seafloor to BSR);
reported depth is the average of minimum and maximum estimates (note that velocity was determined from VSP at ODP Site 889). BSR = bottom-simulating reflector, LWD = logging-
while-drilling, CWL = coring-while-logging, IR = infrared, VSP = vertical seismic profile, BGHSZ = base of gas hydrate stability zone, ND = not detected, DNRB = wireline logging tools did
not reach BSR depth. Deepest chloride and IR anomaly: Hole U1326D: salinity of sample at 263.63 mbsf is 32 (baseline = ~33.5); the next shallowest anomaly (salinity = 23) is at
253.61 mbsf (no indication of s-II hydrate); Hole U1327D: salinity of two samples at 221.96 mbsf is 3.7 and 18, respectively (background = 22); Hole U1328C: salinity of sample at
216.5 mbsf is 28 (background = ~31); Hole U1329C = freshening spike in chlorinity (not salinity) between 119 and 128 mbsf (502–508 mM over a background of ~520 mM). Break in
slope of C1/C2 ratios: Hole U1325C: below 243.85 mbsf (C1/C2 > 60,000 above; C1/C2 < 20,000 below); Hole U1327D: in void gas at ~225 mbsf; Hole U1328C: between 209.3 (C1/C2 >
10,000) and 217.7 mbsf (C1/C2 < 1,500); Hole U1329C: at 119.3 mbsf. Break in slope of i-C4/n-C4 at 224.62 mbsf in Hole U1327D. i-C4 enriched samples at 207.1 and 209.3 mbsf and C3
enriched samples at 217.1 and 218.8 mbsf in Hole U1328C; values are much lower below. Best estimate of BGHSZ: Sites U1325 and U1327–U1329: simple average value, rounded to near-
est 0.5 m. Site U1326: deepest chlorinity anomaly is deeper than all other proxies, but there is no indication of s-II gas hydrate.

28

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Riedel, M., Collett, T.S., Malone, M.J.

, and the Expedition 311 Scientists

Expedition 311 synthesis: scientific findings1

Chapter contents Abstract

Proc. IODP | Volume 311 doi:10.2204/iodp.proc.311.213.2010

Proc. IODP | Volume 311 2

Proc. IODP | Volume 311 3

Proc. IODP | Volume 311 4

Proc. IODP | Volume 311 5

Proc. IODP | Volume 311 6

Proc. IODP | Volume 311 7

Proc. IODP | Volume 311 8

Proc. IODP | Volume 311 9

Proc. IODP | Volume 311 10

Proc. IODP | Volume 311 11

Proc. IODP | Volume 311 12

Proc. IODP | Volume 311 13

Proc. IODP | Volume 311 14

Proc. IODP | Volume 311 15

IODP Exp. 311 Sites 889 and 890

Site 891 Site 892

Cascadia Basin <500 m depth

Proc. IODP | Volume 311 16

Proposed Site U1325

Proc. IODP | Volume 311 17

Proc. IODP | Volume 311 18

Proc. IODP | Volume 311 19

Expedition 311 synthesis

Deep RAB resistivity

250 BSR III

Expedition 311 synthesis

Deep RAB resistivity

Proc. IODP | Volume 311 22

Site U1326 3.0

at 200 mbsf relative to SW

Expulsion rate (mm/y)

Proc. IODP | Volume 311 23

Unconf. Site U1329 0.708

Proc. IODP | Volume 311 24

Site 1252 Site 1251

Site U1326 Site 1244

Proc. IODP | Volume 311 25

δ13C (‰ PDB) DIC

Proc. IODP | Volume 311 26

Pelagic organic input Turbiditic organic input Site U1329

Site U1327 Site U1328

BGHS/BSR Free gas

Diffuse fluid expulsion

Mud Continued sediment

Proc. IODP | Volume 311 27

Precruise seismic BSR* 230 234 223 219 125

Expedition 311 synthesis

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