Tagus River

The megalithic burial monuments of the interior of the Iberian Peninsula, together with those of Brittany, are amongst the oldest in Europe. They are exceptional
because they are so clearly interwoven with early farming settlements, the latter providing occupational sequences that are topographically and ideologically linked to those of the megalith builders. The use of stonework and clay,
the system of building burial mounds in concentric circles, and even the spatulae idols of the San Martin-El Miradero type are clear antecedents of classical Millaran
forms. The megalithic tradition of the Iberian interior is a fundamental component of the Iberian megalithic tradition more generally. It occupied a key geographical
position in relation to the rest of Europe, and will yield greater insights as research projects continue.

The distributions of branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are transported from the soils where they are predominantly produced to marine sediments via rivers, have been applied in reconstructing mean annual air temperature (MAT) and pH of soils. However, paleoclimate reconstructions using sedimentary brGDGTs have proven difficult in arid regions, including the Iberian Peninsula. Recently, six novel 6-methyl brGDGTs have been described using new analytical methods (in addition to the nine 5-methyl brGDGTs previously used for climate reconstructions), and so new pH and MAT calibrations have been developed that were shown to improve the accuracy of reconstructions in a set of global soil samples, especially in arid regions. Because of this we decided to apply the new method to separate the 5-and 6-methyl isomers along with the novel calibrations to a sample set from the Iberian Peninsula to determine whether it improves paleoclimate reconstructions in this area. This set includes samples that run in a transect from source to sink along the Tagus River and out to the deep ocean off the Portuguese margin spanning the last 6000 years. We found that although pH reconstructions in the soils were improved using the new calibration, MAT reconstructions were not much better even with the separation of the 5-and 6-methyl brGDGTs. This confirmed the conclusion of previous studies that the amount of aquatically produced brGDGTs is overwhelming the soil-derived ones in marine sediments and complicating MAT reconstructions in the region. Additionally, the new separation revealed a strong and until now unseen relationship between the new degree of cyclization (DC') of the brGDGTs and MAT that could be making temperature reconstructions in this and other arid regions difficult.

Abstract (Geochronology of Middle and Late Pleistocene terraces from Tagus river in Toledo, Spain): We present the results of the quartz-OSL and post-IR IRSL dating of different terrace levels located in the vicinity of Toledo (Spain). The results obtained for the upper terraces have not been accurate due to the high environmental radiation and saturation of the samples. However, those obtained from +25-30 m terrace and lower terrace levels, indicate their development between the Marine isotopic stages OIS 9 to OIS 2. Fluvial terrace sedimentation took mainly place during warm isotopic stages (MIS 9, MIS7, MIS5). On the contrary a large phase of valley infilling by lateral alluvial deposits is identified to the end of the MIS5 extending during the following MIS 4 and MIS 3. During the MIS 2 reworking of aeolian deposits presumably developed during the Last Glacial Maximum in the area (c. 22 -21 kyr) fossilize most terrace surfaces down to +30 m.

Palabras clave: Luminiscencia Opticamente Estimulada (OSL), Geocronología, Terrazas fluviales, Tajo, España Central

Keywords: Optically Stimulated Luminiscence (OSL), Geochronology, Fluvial terraces, Tagus River, Central Spain.

RESUMEN
Se presentan los primeros resultados de los proyectos de investigación dirigidos al análisis cronoestratigráfico de las terrazas del río Tajo en el entorno de Toledo. El análisis se ha centrado en niveles fluviales del Pleistoceno Medio y Superior con yacimientos paleontológicos y achelenses con valoración tecnológica general similar al conjunto lítico de Pinedo (colección Máximo Martín Aguado). En el presente trabajo se incluyen las dataciones por ISRL y AAR de los niveles de terraza de +25-30 m que contienen los yacimientos achelenses de Pinedo y Cien Fanegas. Las edades ISRL obtenidas (>280 ka y 292 ± 17 ka) indican que la terraza y los yacimientos que contienen se formaron durante los estadios isotópicos MIS 9 y MIS 8, siendo correlacionables con la Unidad Arganda II en el valle del Jarama, aunque Pinedo podría representar parcialmente a la Unidad inferior de Arganda I (MIS 11 – MIS 9). La datación AAR obtenida para la Unidad superior de Pinedo indica una edad de 226 ± 37 ka. Aunque esta edad ha de tomarse con precauciones debido a la escasa contextualización estratigráfica y los bajos niveles de dentina de las muestras analizadas, podría indicar que el depósito de la terraza estudiada se continuó hasta bien entrado el MIS 7, como también parece suceder en el Jarama (Valdocarros). En el tramo del valle analizado los procesos de sobre-sedimentación ligados a los fenómenos de subsidencia evaporítica (aguas arriba) y la existencia del nivel de base interno impuesto por el meandro encajado de Toledo (aguas abajo), favorecieron los procesos de agradación fluvial sobre los de encajamiento. El método de datación utilizado (post IR–IRSL) ha permitido obtener la edad más antigua hasta la fecha sobre sedimentos fluviales arenosos en la Cuenca del Tajo.

Palabras clave: Geocronología, OSL-IRSL, AAR, terrazas fluviales, Achelense, Valle del Tajo, España Central.


ABSTRACT
This study presents the first geochronological data from research projects focused in the chronostratigraphy of the Tagus fluvial terraces in the environs of Toledo (Central Spain). The research is focused on Middle to Late Pleistocene fluvial terraces with palaeontological and acheulean sites of similar palaeolithic technology than those showed by the lithic assemblage of the Pinedo site (Máximo Martín Aguado Collection). This study includes first ISRL and AAR dating for the fluvial levels at +25-30 m containing the acheulean sites of Pinedo (Toledo) and Cien Fanegas (Aranjuez).  The obtained ISRL dates (>280 ky and 292 ± 17 ky) indicate that this fluvial level and their inset palaeolithic sites formed during the marine isotopic stages MIS 9 y MIS 8, being possible to correlate them with the Arganda II Unit of the adjacent Jarama valley. However, data from Pinedo suggest that this acheulean site will also correspond to the lower Arganda I Unit of the Jarama valley (MIS 11 – MIS 9).  In spite of the obtained AAR date (226 ± 37 ky) has to be carefully considered, due to the scarce stratigraphic data and low dentine levels of the Equus sp. analyzed samples, it will indicates that sedimentation of the studied terrace level reached the MIS 7, as may also occur in the Jarama valley (Valdocarros). The analyzed sector of the Tagus valley, continuous fluvial sedimentation was favored by the occurrence of karstic evaporitic subsidence (upstream) and the presence of an outstanding internal base-level represented by the Toledo Rocky Meander (downstream). The dating method used in this study (post-IR IRSL) let to obtain the oldest date at present from fluvial sandy deposits in the Tagus fluvial basin.

Keywords: Geochronology, OSL-IRSL, AAR, fluvial terraces, Acheulean, Tagus Valley, Central Spain.