MIOCENE FORAMINIFERA FROM IODP SITE 374-U1521: A CENTRAL ROSS SEA RECORD OF THE MCO AND MMCT

The Ross Sea record of the Miocene Climate Optimum (MCO, ~16.9-14.7 Ma) and the Middle Miocene Climate Transition (MMCT, ~14.7-13.8 Ma) can provide crucial insight into the Antarctic ocean-cryosphere system. The MCO and MMCT would have had a profound impact on the ice sheet dynamics, water mass behavior, and productivity in the Ross Sea. Here we investigated the central Ross Sea Miocene foraminiferal assemblages from IODP Site U1521 (McKay et al., 2019) to identify shifts in environmental conditions, trace the continental shelf evolution, and compare with other Ross Sea Neogene drill sites.

Based on a cluster analysis, the foraminiferal assemblages follow shifts in the lithologic units. Unit IV (MCO) contains the most diverse and relatively abundant assemblage including Uvigerina, and the endemic planktic Antarcticella antarctica. Unit III (MCO) generally has the lowest foraminiferal recovery. During much of Unit III deposition, we suspect dissolution of calcareous benthics by corrosive waters from open water diatom productivity and higher flux of organic matter to the seafloor. The lowest Unit III cluster, with abundant Globocassidulina and Nonionella, coincides with alternating diatom-rich and mud-rich lithologies and heterotrophic dinoflagellates. The short-lived middle cluster of Globobulimina coincides with an increase in brackish algae, suggesting stratification due to meltwater and low oxygen at depth. The uppermost cluster in Unit III, with Globocassidulina and Uvigerina, coincides with a dominance of autotrophic dinoflagellates. Both Units IV and III have rare occurrences of subpolar/temperate planktics during interglacials, suggesting episodic incursions of warm surface water into the Ross Sea during the MCO. Unit II (MMCT) contains increased neritic taxa, a decrease in Uvigerina, and absence of “warm” planktics. We suggest the decrease in Uvigerina (a shelf edge proxy) between Unit IV and II tracks the northward progradation of the Ross continental shelf. The absence of agglutinated benthic foraminifera suggests there was no High Salinity Shelf Water production during the middle Miocene, therefore no polynya and weak katabatic winds. Lastly, our initial comparison with AND-2A yields similar environmental interpretations supported by the foraminifera.