Late Quaternary nearshore molluscan patterns from Patagonia: Windows to southern southwestern Atlantic-Southern Ocean palaeoclimate and biodiversity changes?

Varied approaches (palaeobiodiversity, palaeobiogeography, bioerosion, geochemistry) to unique Patagonian late Quaternary molluscan assemblages in the southwestern Atlantic, with ages especially from interglacial Marine Isotope Stage (MIS) 5e and MIS 1, provide large-scale and long-temporal palaeoenvironmental data for the southern SWA. Together with new patterns of δ18O and δ13C variations in modern, mid-Holocene, and Late to Middle Pleistocene shells of Protothaca antiqua (Bivalvia) and the coeval Pleistocene Tegula atra (Gastropoda), the overall sources of evidence illustrate possible responses to recent palaeoclimate and sea-ice changes around the southernmost SWA-western Antarctica, leading to modern conditions. For the mid-Holocene, the influence of the Hypsithermal is confirmed. In the northern Golfo San Matías, the highest δ18O and δ13C values support higher salinity and sea surface temperatures (SST), and a Golfo San Matías Front stronger than today. Lower δ18O values in the northern Golfo San Jorge (GSJ) compared to the Late to Middle Pleistocene suggest warmer mid-Holocene waters, independently supported by thermally anomalous molluscan taxa, geographical shifts of areas of endemism and absence of T. atra (cold water proxy); overall higher δ13C values compared to present suggest higher productivity. For the Late to Middle Pleistocene (particularly MIS 5e), highest δ13C values (relative to modern and mid-Holocene trends) match with the location of tidal fronts and areas of maximum chlorophyll-a concentrations today. Accordingly, these fronts may have been already active and significantly intensified due to the prevailing climate conditions that included colder waters and stronger upwelling from the southern GSJ southwards. This is independently supported by palaeobiogeographical and bioerosion trends and the dominance of the cold water species T. atra during the Pleistocene, which is dispersed from the SE Pacific into the SWA by rafting on kelps and whose occurrence is controlled by SST, light, winds, and nutrient concentration/productivity. Repeated, abrupt climate oscillations during the last glacial cycle with significant impact on SST, ice melting and surface-ocean stratification in the western Antarctica-Weddell Sea-Antarctic Circumpolar Current realm are so far the only available plausible explanations to account for the different mid-Holocene and modern patterns, and for the regional disappearance of T. atra after MIS 5e. Further palaeoceanographic research in this key area is needed to understand how all these mechanisms operated in the past, potentially influencing the Patagonian shelf waters and coastal fronts.