A molluscan record of Late Cenozoic climate and palaeoseasonality from Antarctica and South America

Shallow marine late Neogene and Quaternary sedimentary deposits around coastal Antarctica and South America contain abundant fossil bivalves, but these have rarely been utilised for palaeoclimate work. Due to their incremental growth, bivalves contain a wealth of information relating to the environment they were living in, including temperature (using δ[superscript 18]O) and productivity (using δ[superscript 13]C). A repeatable method of assessing the preservational state of fossil bivalves using a combination of techniques has been developed to ensure only pristine material is analysed for stable isotopes. These include inspection of the lamellar structure under an optical microscope, observation of luminescence using cathodoluminescence and identification of internal crystal structure using scanning electron microscopy. A targeted study of modern pectinid, cardiid and hiatellid bivalves confirm their suitability for reconstructing sea surface temperatures (SSTs), although in the case of hiatellids an understanding of the local environment is essential for an accurate interpretation of oxygen isotope records. Stable isotope analysis of pristine late Neogene (ca. 6.5 to 2.5 Ma) pectinid bivalves from James Ross Island, Antarctic Peninsula and pectinid, cardiid and hiatellid bivalves from the Vestfold and Larsemann Hills in the East Antarctic, record two distinct environmental signals: 1) warmer than present summer SSTs of up to +4.5°C, consistent with a number of other shallow-shelf and deeper water marine fossil proxies, including bryozoans, silicoflagellates and cetaceans, and with General Circulation Model simulations (e.g. PlioMIP) and, 2) seasonal fluxes of freshwater from local ice sheets, artificially decreasing δ[superscript 18]O ratios. Stable isotope analysis of late Pliocene to Quaternary (ca. 3.5 to 0.2 Ma) Chlamys bivalves and Argopecten purpuratus from the Mejillones Peninsula and Coquimbo, northern Chile, identify similar SSTs (+13 to +18°C) compared to present and suggest El Niño-Southern Oscillation (ENSO) variation has been present in the south-eastern Pacific since the late Pliocene, a signal consistent with climate model predictions. This study emphasises the importance of developing new climate proxies that identify seasonal variation, and which can be used in conjunction with other environmental proxies to provide detailed palaeoclimate data for little studied Neogene successions of the coastal zones of Antarctica and the south-eastern Pacific.