The removal of exchangeable oxygen from diatom opal prior to δ18Odiatom analysis is a crucial first step before analyzing frustule oxygen isotopes for paleoceanographic applications. We present the results of experiments that quantify the temperature-dependent reactivity of biogenic silica with water under laboratory conditions. We demonstrate that controlled exchange between rinse water and diatom opal at room temperature results in predictable alteration of δ18Odiatom, after vacuum dehydroxylation. Diatom samples equilibrated with an 18O-enriched δ18Oequil. water solution of +94.4‰ at ~21 °C for 70 hr prior to dehydroxylation yield δ18Odiatom data that can be directly interpreted with existing empirical δ18Odiatom versus temperature relationships. We compare sediment trap-based δ18Odiatom temperature data with modern δ18Oforam temperatures. Finally, we present an ~220 year record of δ18Odiatom data from an eastern Pacific Guaymas Basin boxcore, analyzed using the microfluorination technique, that indicates diatom chlorophyll maximum (Chlmax) temperatures have shifted from a period during the late eighteenth and nineteenth centuries that was 8 °C cooler during the fall bloom relative to mean annual alkenone Uk′37 sea surface temperature, to the late twentieth century when we observe no difference between alkenone sea surface temperature and diatom Chlmax. These data suggest a twentieth century seasonal shift in the timing of fall upwelling and Guaymas Basin stratification breakdown that could be due to reduced upwelling efficiency and/or increased summer mixed layer thermal stratification.

Compilations of paleoceanographic observations for the deep sea now contain a few hundred points along the oceanic margins, mid-ocean ridges, and bathymetric highs, where seawater conditions are indirectly recorded in the chemistry of buried benthic foraminiferal shells. Here we design an idealized experiment to test our predictive ability to reconstruct modern-day seawater properties by considering paleoceanographic-like data. We attempt to reconstruct the known, modern-day global distributions by using a state estimation method that combines a kinematic tracer transport model with observations that have paleoceanographic characteristics. When a modern-like suite of observations (Θ, practical salinity, seawater δ18O, δ13CDIC, PO4, NO3, and O2) is used from the sparse paleolocations, the state estimate is consistent with the withheld data at all depths below 1500 m, suggesting that the observational sparsity can be overcome. Physical features, such as the interbasin gradients in deep δ13CDIC and the vertical structure of Atlantic δ13CDIC, are accurately reconstructed. The state estimation method extracts useful information from the pointwise observations to infer distributions at the largest oceanic scales (at least 10,000 km horizontally and 1500 m vertically) and outperforms a standard optimal interpolation technique even though neither dynamical constraints nor constraints from surface boundary fluxes are used. When the sparse observations are more realistically restricted to the paleoceanographic proxy observations of δ13C, δ18O, and Cd/Ca, however, the large-scale property distributions are no longer recovered coherently. At least three more water mass tracers are likely needed at the core sites in order to accurately reconstruct the large-scale property distributions of the Last Glacial Maximum.

Small, disc-shaped shell beads are recorded as mortuary offerings in many Neolithic and Bronze Age burials in Southeast Asia. Yet the provenance of these artefacts is often obscure, as production processes involve the removal of diagnostic morphological features, negating taxonomic classification. Here, the authors report on the combined isotopic and morphological analysis of a subset of shell beads from the site of Ban Non Wat in north-east Thailand. In addition to identifying freshwater sources for nearly all the beads, the results suggest the presence of multiple shell production centres - each with access to distinct aqueous environments - and widespread exchange in the Bronze Age.

Understanding the coupling of oxygen, carbon, and sulfur cycles in the past is critical for reconstructing the history of biogeochemical cycles, paleoclimatic variations, and oceanic chemistry. The abundance of sulfur isotopes (δ34S) in sulfate from ancient marine carbonates, or carbonate-associated sulfate (CAS), is commonly used, along with other archives (mainly evaporites and barite), to estimate the δ34S of seawater throughout Earth history. Analyses of CAS from hand-picked foraminifera are potentially valuable because this group of organisms is used in numerous paleoceanographic studies. They could provide coupled, high-resolution records of δ13C, δ18O, and δ34S isotopic changes directly linked to orbitally tuned records of climate change through the Cenozoic. Such measurements have not previously been possible due to limitations of sensitivity in conventional IRMS-based techniques. However, the recent development of CAS analysis by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) now allows us to work on samples containing just a few nmol of sulfur with accuracy for δ34S values approaching 0.1‰ and, consequently, to analyze hand-picked samples of foraminifera shells. Here we report the results of culture experiments with the planktonic species Orbulina universa, that establish a shell:seawater δ34S calibration for future applications to the fossil record. Our new method uses <650 μg of carbonate (∼15 shells) per analysis. The results show that S isotopes are fractionated consistently by -1‰ between seawater and O. universa tests. We also demonstrate that O. universa faithfully records the [SO42-]/[Ca2+] ratio of the seawater in which it grew. © 2014. American Geophysical Union. All Rights Reserved.

Geochemical observations indicate that planktic foraminifer test Mg/Ca is heterogeneous in many species, thereby challenging its use as a paleotemperature proxy for paleoceanographic reconstructions. We present Mg/Ca and Ba/Ca data collected by laser ablation ICP-MS from the shells of Orbulina universa cultured in controlled laboratory experiments. Test calcite was labeled with Ba-spiked seawater for 12 h day or night calcification periods to quantify the timing of intratest Mg-banding across multiple diurnal cycles. Results demonstrate that high Mg bands are precipitated during the night whereas low Mg bands are precipitated during the day. Data obtained from specimens growing at 20 °C and 25 °C show that Mg/Ca ratios in both high and low Mg bands increase with temperature, and average test Mg/Ca ratios are in excellent agreement with previously published empirical calibrations based on bulk solution ICP-MS analyses. In general, Mg band concentrations decrease with increasing pH and/or [CO2-3] but this effect decreases as experimental temperatures increase from 20 °C to 25 °C. We suggest that mitochondrial uptake of Mg2+ from the thin calcifying fluid beneath streaming rhizopodial filaments may provide the primary locus for Mg2+ removal during test calcification, and that diurnal variations in either mitochondrial density or activity produce Mg banding. These results demonstrate that Mg banding is an inherent component of test biomineralization in O. universa and show that the Mg/Ca paleothermometer remains a fundamental tool for reconstructing past ocean temperatures from fossil foraminifers.

A paleo-data compilation with 492 δ13C and δ18O observations provides the opportunity to better sample the Last Glacial Maximum (LGM) and infer its global properties, such as the mean δ13C of dissolved inorganic carbon. Here, the paleo-compilation is used to reconstruct a steady-state water-mass distribution for the LGM, that in turn is used to map the data onto a 3D global grid. A global-mean marine δ13C value and a self-consistent uncertainty estimate are derived using the framework of state estimation (i.e., combining a numerical model and observations). The LGM global-mean δ13C is estimated to be 0.14‰ ± 0.20‰ at the two standard error level, giving a glacial-to-modern change of 0.32‰ ± 0.20‰. The magnitude of the error bar is attributed to the uncertain glacial ocean circulation and the lack of observational constraints in the Pacific, Indian, and Southern Oceans. To halve the error bar, roughly four times more observations are needed, although strategic sampling may reduce this number. If dynamical constraints can be used to better characterize the LGM circulation, the error bar can also be reduced to 0.05 to 0.1‰, emphasizing that knowledge of the circulation is vital to accurately map δ13C in three dimensions.

The discovery of an intact valve of the fossil bivalve Chesapecten jeffersonius and shells of three tropical snail species in a c. 1610 James Fort well speaks to the curiosity that European colonists brought to the New World. While implementing the Virginia Company of London's mandate to identify and secure profitable natural resources, the Jamestown, Virginia, colonists apparendy also gathered interesting natural objects. The shells may have been collected either as personal souvenirs, much like modern-day tourists, or as curios destined for the lucrative European conchology market. Chesapecten jeffersonius, Virginias state fossil, was collected locally as representatives can still be found in James River Pliocene deposits near Jamestown. In contrast, the tropical shells were likely brought to Jamestown in May 1610 by survivors of an English shipwreck on Bermuda. The shells from both Virginia and Bermuda were discarded in the forts well by June 1610 as the settlers hastily prepared to permanendy abandon Jamestown.

In this study, we present experimental results from the planktic foraminifer Orbulina universa, cultured in the laboratory. We demonstrate that the δ13C of shell calcite precipitated in 13C-labeled seawater for 24h can be resolved and accurately measured using Secondary Ion Mass Spectrometry (SIMS). Specimens maintained at 20°C were transferred from ambient seawater (δ13CDIC=+1.3‰) into seawater with δ13CDIC=+51.5‰ and enriched [Ba2+] for 24h. Specimens were then transferred into ambient seawater with elevated [87Sr] for 6-9h of calcification, followed by a transfer back into unlabeled ambient seawater until gametogenesis. This technique produced O. universa shells with calcite layers of distinct geochemical signatures. We quantify the spatial positions of trace element labels in the shells using laser ablation ICP-MS depth profiling. Using fragments from the same shells, we quantify intrashell δ13Ccalcite using SIMS with a 6 or 8μm spot (2 SD range ±0.5‰ to 1.7‰). Measured δ13Ccalcite values in O. universa shell layers precipitated in ambient seawater are within 2‰ of predicted δ13Ccalcite values. In 13C-labeled bands of calcite, 6μm SIMS spot measurements are within 2‰ of predicted δ13Ccalcite values, whereas 8μm SIMS spots yield intermediate, mixed values. The spatial agreement between trace element and carbon isotope data suggests that 13C and cation tracers are synchronously incorporated into shell calcite. These results demonstrate the ability of SIMS δ13C measurements to resolve ~10μm features in foraminifer shell calcite using a 6μm spot, and highlight the potential of this technique for addressing questions about ecology, biomineralization, and paleoceanography. © 2014 .

Biometric characteristics of Orbulina universa (d'Orbigny) were used to differentiate two morphotypes present in sediment trap samples collected from the Cariaco Basin, Venezuela. Specifically, wall thickness and weight-area relationships were used to separate shells into thin (Mthin) and thick (Mthick) morphotypes. Mthick (mean thickness=19-41μm) comprises 75% of the total O. universa in these samples and has morphometric characteristics similar to that of the previously described Type I Caribbean genotype, whereas Mthin (mean thickness=6-22μm) is comparable to the Type III Mediterranean genotype. The flux of Mthick increases during periods of upwelling, whereas Mthin flux shows no systematic relationship with changing hydrographic regimes in the basin. The δ18O and δ13C of Mthick are on average 0.34% higher and 0.38% lower, respectively, than those of Mthin, suggesting that they calcify their final spherical chamber at different depths in the water column and/or differ in their vital effects on shell geochemistry. Additionally, the absolute offset in the stable isotopic compositions of the two morphotypes varies as a function of surface ocean stratification. During periods of upwelling, the δ18O and δ13C absolute offsets between Mthin and Mthick are on average -0.15 and 0.25%, whereas average δ18O and δ13C absolute offsets between these two morphotypes increase to -0.54 and 0.41% during non-upwelling periods. We attribute the higher δ18O and δ13C offsets during non-upwelling periods to a difference in final chamber calcification depth and the combined effects of temperature (δ18O), δ13CDIC (δ13C), irradiance (δ13C) and [CO32-] (δ18O and δ13C). These data provide field evidence that thin and thick morphotypes of O. universa likely experience different environmental conditions during the formation of their final chamber and, therefore, should not be combined in geochemical analyses for reconstructing past surface ocean conditions. Finally, we introduce a new proxy for reconstructing past surface ocean stratification changes via the use of Mthin and Mthick δ18O and δ13C offsets.