Southern Ocean Physical and Biological Response to Millennial-scale Warming Events During Marine Isotope Stage 3

The generally cold conditions that prevailed in Antarctica between the previous interglacial and the current interglacial were punctuated by a series of warming events lasting several thousand years. These events during marine isotope stage 3 (MIS3), were associated with the cooling phase of abrupt temperature changes in Greenland, and with transient increases in atmospheric CO₂. The timescale of the climate variability during MIS3 makes these variations particularly relevant for studying the coupled earth system response to anthropogenic climate change. Growing evidence from models and climate archives points to a role for the ocean in driving CO₂ change associated with Antarctic warming. However, the paucity of high accumulation rate sediment records, and the difficulty of accurately dating records beyond the radiocarbon time frame, means the ocean's response to these events, and its role in the CO₂ change, remains uncertain. To explore the Southern Ocean's response during Antarctic warm events we have compiled existing high-resolution SST records to demonstrate a consistent warming across all sectors of the Southern Ocean, north of the polar front, during Antarctic warm events. Across all events, the magnitude of warming observed in the Southern Ocean is proportional to, but less than, the warming observed in Antarctica. We explore the biogeochemical response with new analyses from a Pacific Subantarctic sediment core from south of New Zealand (TAN1106-28; 2798 m water depth; 48.37S;165.65E). Here we see decreases in export production (Th-normalised xsBa flux) and lithogenic flux during Antarctic warm events, consistent with a role for iron fertilisation and consistent with findings from the Atlantic Subantarctic. In contrast to the Atlantic records, peaks in authigenic uranium, and thus lower oxygen levels, occur during maxima in lithogenic flux. Minima in xsBa flux coincide with maxima in CaCO₃ burial flux, which may reflect better preservation of CaCO₃ associated with flushing of carbon from the ocean. Together, these results provide further evidence that the Southern Ocean contributed to millennial-scale changes in atmospheric CO₂, both through changes in biological carbon export linked to iron fertilisation, and through changes in ocean ventilation.