CONICET | Buscador de Institutos y Recursos Humanos

Abstract The lower Miocene Patagonian transgression is widely recognized all along the foot hills of the Andes in Patagonia. At the southern tip of Patagonia, it comprises a continuous 200 meters thick succession of shallow marine to estuarine sediments, with abundant fossiliferous levels. One of the most representative fossil group in this unit are oysters, which appear across the entire section. In situ or slightly remobilized oyters were sampled in order to check if they preserved in their shells the isotopic signature of the Miocene sea. As oysters are capable of living in normal to low salinity environments they isotopic signature was correlated with the sedimentary facies in two well exposed stratigraphic columns. The thick shells of the oysters allowed detailed sampling for isotope analysis. Samples were obtained with a microdrill from polished surfaces cut perpendicular to the shell surface in a dorsal to ventral (radial) sense. Growth increments could be identified in hand specimens, and petrographic analysis showed a complex arrange of three types of microstructures: prismatic layers, composed of coarse, prismatic calcite crystals, elongated perpendicular to shell surface; foliated layers, composed of long calcite crystals arranged oblique to parallel to shell surface; and chalky layers with a general opaque appearance, composed of fine grained carbonate, in which crystals cant be individualized. Modern living oysters show these three types of layers in their shells, pointing to a biogeinc origin of the microstructure. Sampled oysters came from many different stratigraphic levels, representing distinctive sedimentary paleoenvironments. Oxygen, carbon, and strontium isotope ratios were measured in these samples aiming to verify whether they present isotopic variations. Additionally, few shells were selected to carry out detailed intra-shell isotopic analysis, such as layer by layer sampling across an entire shell. Prismatic and foliated layers gave values close to 0 for both δ13C and δ18O ratios. Chalky layers from any part of the stratigraphic column give very negative values of both δ13C (xxx) and δ18O (-5 to -15). This large difference is also present within individual layers of the same shell. On the other hand, Sr isotopes on these contrasting layers, gave very similar values. This probably means that the isotopic signature of the biogenic carbonate of chalky layers was originally similar as in the foliated and prismatic layers (this is true for modern oysters), and it was subsequently altered by pore or meteoric waters. This alteration was very selective, invading chalky layers due to its high porosity with waters with a very negative δ13C and δ18O signature, probably meteoric waters. We disregarded an environmental control (i.e. to the shells achieved such negative isotope values because of a great input of freshwater into the sea or estuary), because a rich, normal-marine, fossil community was found together with the oysters. In addition, this hypothesis is also contrasted by the lack of intermediate isotope values, which should reflect mixing of marine and fresh water. Taking into account that only foliated and prismatic layers preserve the original δ13C and δ18O of the biogenic carbonate, a base-to- top decrease of both δ13C and δ18O values is detected in the stratigraphic column. This could be interpreted as an increasing input of freshwater to the system, which is consistent with the paleoenvironmental reconstructions based of facies analysis. The layer by layer sampling of a thick shell of the lower part of the column gave subtle variations in isotopic values around zero, pointing to seasonal variations in sea water conditions (i.e. temperature, or isotopic composition). Few chalky layers in this shell gave very negative anomalous values. 87Sr/86Sr ratios show little variations across the stratigraphic column. No relationship between layer microstructure and Sr isotope values were found. Only a subtle variation to higher values to the top of the stratigraphic column was found, and is interpreted to be a response of long-term fluctuation in the isotopic composition of the ocean, consistent with the global Cenozoic 87Sr/86Sr variations.

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