Calcareous nannofossil evidence for Marine Isotope Stage 31 (1 Ma) in Core AND-1B, ANDRILL McMurdo Ice Shelf Project (Antarctica)

ANDRILL Core AND-1B, recovered in the Western Ross Sea of Antarctica, has been examined in search of calcareous nannofossils. Exhaustive and detailed analyses of the interval from 86.61 to 98.99 mbsf revealed for the first time at an extreme southern high latitude (77.88° S) the presence of lower Pleistocene calcareous nannofossils, together with Tertiary and Upper Cretaceous reworked species. Other calcareous microfossils in the assemblage include, spicules of calciosponges and small foraminifers. The short normal magnetozone between 84.97 and 91.13 mbsf is correlated with the Jaramillo Subchron (C1r.1n) (Wilson et al., 2007). The presence of nannofossils in the biogenic interglacial sediments is consistent with an episode of warm surface waters and open-marine conditions during the Jaramillo subchron, at ~1 Ma, which corresponds with Marine Isotope Stage (MIS)-31 (Scherer et al., 2007; Naish et al., 2007). Climate proxies such as oxygen isotope stratigraphy and calcareous nannofossils at ODP Site 1165 (Pospichal, 2003; Villa et al., 2008) and the diatom assemblage in a shelly carbonate sequence at Cape Roberts 1 (Bohaty et al., 1998) also support a warming event during this time and suggest it extended around the Antarctic Continent. This in turn implies a total or partial collapse of the McMurdo Ice Shelf and a concurrent shift or temporary dissipation of the Polar Front (Antarctic Convergence) and Antarctic Divergence that currently serve as barriers to the influx of calcareous nannofossils to the margins of Antarctica.