Sea ice changes in the southwest Pacific sector of the Southern Ocean during the last 140 000 years

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 (similar to 21 ka) to Holocene, with few quantitative reconstructions extending to the onset of glacial Termination II (similar to 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 (similar to 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 degrees S throughout the region, which may have contributed to early reductions in atmospheric CO2 concentrations through its impact on air-sea gas exchange.

Automated summary

Limited data on past sea ice coverage has hindered our understanding of its impact on atmospheric CO2 concentration. This study provides new estimates of sea ice concentrations and sea surface temperatures using fossil diatom assemblages, and finds that sea ice expansion may have influenced intermediate ocean circulation changes.