Reconstruction of paleoenvironments and paleoclimate during the last and present interglacials over the Patagonian coast (Argentina), with particular reference to the shell geochemistry

The present study aims a better understating of the paleoceanographic, paleoclimatic and paleoenvironmental conditions of the Patagonia Atlantic coast in the in the vicinity of San Jorge Gulf (Chubut and Santa Cruz Provinces, Argentina) through the study of the geochemistry (stable isotopes and trace elements) of molluscs collected in the late Quaternary marine deposits. On the basis of XRD powder analysis, petrographic and trace element screening tests (comparison between marine shells and paleosols), a large dataset of unaltered bivalves with aragonite (Ameghinomya antiqua species) or mixed aragonite-calcite (Mytilus edulis and Aualcomya atra species) composition of the shells was identified. Analytical data definitively demonstrate the suitability of Ameghinomya antiqua and Mytilidae family specimens as potential proxies for paleoclimatic studies. Oxygen isotopic composition of modern Mytilus edulis in the study area is progressively more positive moving southward. The isotopic composition on modern mussels along the Atlantc coast of Patagonia seems to reflect the changes in seawater temperature. Stable isotope (13C/12C and 18O/16O ratios) and trace elements (Ba/Ca, Sr/Ca, Mg/Ca, Mn/Ca, Fe/Ca) data indicate large variability of modern condition in the gulf, but in the past too systematic differences between old beach ridges sets existed and can be interpreted as substantial different paleoceanographic conditions, partly related also to local mixing with freshwater. Paleontological and geochemical data indicate that the Holocene and the previous interglacial appear consistently different from the present. In particular they seem to contain molluscs indicating a stronger influence of the warm and saltier Brazil current in the area and, then, fluctuations in the relative position between the two surface currents (Brazilian and Malvinas) over the time.