SUSTAINED SEA-ICE FREE CONDITIONS IN THE ANTARCTIC NEARSHORE ZONE DURING MARINE ISOTOPE STAGE 31 (1.07 MA)

Antarctic drilling during the Cape Roberts Project (CRP), McMurdo Sound, Antarctica (77°S), demonstrated that Oligocene-Miocene glacial cycles recorded in nearshore Antarctic continental shelf sequences can be calibrated to Milankovitch-paced records of ice volume changes (Naish et al., 2001, Nature, 423, 719-723). Here we report the first robust correlation of an extreme mid-Pleistocene interglacial recorded on the Antarctic continental shelf with the global oxygen isotope stratigraphic record.

CRP-1 Lithostrat. Unit 3.1 is two meters thick (33.82 – 31.89 mbsf), and is composed of up to 80% unconsolidated biogenic carbonate, with abundant and diverse diatoms. The diatom floras are almost entirely pelagic, including many taxa that occur commonly only north of the Polar Front. The common occurrence of well-preserved pelagic diatoms and the scarcity of sea-ice associated taxa (generally <3%) argues for an extended period of significantly warmer than present surface water temperatures and reduced sea-ice extent and duration along the margin of the Antarctic continent.

Lithostratigraphy strongly suggests that the carbonate unit represents deposition during a single interglacial, despite the fact that it is punctuated by an ice-rafting event. New magnetostratigraphic analyses demonstrate that the lower part is reversed polarity, and the upper part normal polarity. Independent chronostratigraphy is constrained by diatom biostratigraphy, 87Sr/86Sr ratios in biogenic carbonates, and 40Ar/39Ar dating of a pumice clast, as between 1.11 and 0.83 Ma. The polarity record thus indicates, with a high degree of confidence, that CRP-1 Unit 3.1 includes the base of the Jaramillo (Chron C1r.1n) at 1.07 Ma. This unprecedented chronostratigraphic control allows virtually unequivocal correlation to MIS-31 of the deep-sea oxygen isotope record.

MIS-31 was the last significant warm interglacial of the obliquity-dominated world, and may represent a precursor to the high-amplitude eccentricity-dominated cycles that followed the mid-Pleistocene climate shift. Significant surface water changes coincident with C1r.1n are apparent in both northern and southern high latitude DSDP/ODP records. The warm surface water event reported here is especially significant given its proximal position to the Antarctic ice sheet.