<jats:p>Abstract. Sea ice expansion in the Southern Ocean is believed to have contributed to
glacial–interglacial atmospheric CO2 variability by inhibiting air–sea gas exchange and influencing the ocean's meridional overturning circulation. However, limited data on past sea ice coverage over the last 140 ka (a complete glacial cycle) have hindered our ability to link sea ice expansion to oceanic processes that affect atmospheric CO2 concentration. Assessments of past sea ice coverage using diatom assemblages have primarily focused on the Last Glacial Maximum (∼21 ka) to Holocene, with few quantitative reconstructions extending to the onset of glacial Termination II (∼135 ka). Here we provide new estimates of winter sea ice concentrations (WSIC) and summer sea surface temperatures (SSST) for a full glacial–interglacial cycle from the southwestern Pacific sector of the Southern Ocean using the modern analog technique (MAT) on
fossil diatom assemblages from deep-sea core TAN1302-96. We examine how the
timing of changes in sea ice coverage relates to ocean circulation changes
and previously proposed mechanisms of early glacial CO2 drawdown. We
then place SSST estimates within the context of regional SSST records to
better understand how these surface temperature changes may be influencing
oceanic CO2 uptake. We find that winter sea ice was absent over the
core site during the early glacial period until MIS 4 (∼65 ka), suggesting that sea ice may not have been a major contributor to early
glacial CO2 drawdown. Sea ice expansion throughout the
glacial–interglacial cycle, however, appears to coincide with observed
regional reductions in Antarctic Intermediate Water production and
subduction, suggesting that sea ice may have influenced intermediate ocean
circulation changes. We observe an early glacial (MIS 5d) weakening of
meridional SST gradients between 42 and 59∘ S throughout
the region, which may have contributed to early reductions in atmospheric
CO2 concentrations through its impact on air–sea gas exchange.
                    </jats:p>