Bioerosion on Late Pleistocene Marine Mollusks: A Paleoclimatological and Paleoecological Comparison of MIS 7 and MIS 5e with Modern Beaches (Río Negro, Argentina)

The Late Pleistocene littoral ridges of southern South America, especially those of the north of the Argentinean Patagonia, contain remains of mollusc shells with bioerosion traces. Eleven sites (2 from MIS 7, 4 from MIS 5e and 5 sites from modern beaches) with 40 taxa (17 bivalves and 23 gastropods) were analyzed, in the area between west of Baliza San Matías and Las Grutas (41°S, Río Negro). Three ichnogenera were identified in deposits of MIS 7: Entobia, Gastrochaenolites (Domichnia) and Oichnus (Praedichnia) with one ichnospecies (G. torpedo). Six ichnogenera were identified in deposits of MIS 5e: Entobia, Meandropolydora, Pinaceocladichnus, Iramena, Caulostrepsis and Oichnus with one ichnospecies (O. simplex). And seven ichnogenera were indentified in modern beaches: Entobia, Meandropolydora, Iramena, Caulostrepsis, Pinaceocladichnus, Gastrochaenolites (Domichnia) and Oichnus (Praedicnia), with two ichnospecies (O. simplex and O. paraboloides). On this basis, it is inferred that Pleistocene benthic communities were constituted by cheilostome bryzoans and polychaete annelids, together with drilling bivalves, predator gastropods and warm water species of bivalves and gastropods. Among them, Chama iudicai was found on Plicatula gibbosa from MIS 7 and MIS 5e. The record of warm species suggests a higher SST (sea surface temperature) than the present one and/or the influence of the warm southward Brazilian Current affecting the waters of Golfo San Matías. As for the modern beaches, there is an increase in polychaete annelids, cheilostome bryzoans, temperate‐cold species and drilling bivalves, as well as sandy substrate species, which together with an increase of Gastrochaenolites and Caulostrepsis, suggest a colder climate due to the greater influence of the Malvinas Current during the Holocene.