Comment on cp-2021-107

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 (sSSTs) for a full glacial-interglacial cycle from the southwestern Pacific sector of the Southern Ocean using fossil diatom assemblages from deep-sea core TAN1302-96 (59.09° S, 157.05° E, water depth 3099 m). We find that winter sea ice was consolidated over the core site during the latter part of the penultimate glaciation, Marine Isotope Stage (MIS) 6 (from at least 140 to 134 ka), when sSSTs were between ~1 and 1.5 °C. The winter sea ice edge then retreated rapidly as sSSTs increased during the transition into the Last Interglacial Period (MIS 5e), reaching ~4.5 °C by 125 ka. As the Earth entered the early glacial stages, sSSTs began to decline around 112 ka, but winter sea ice largely remained absent until ~65 ka during MIS 4, when it was sporadically present but unconsolidated (< 40 % wSIC). WSIC and sSSTs reached their maximum concentration and coolest values by 24.5 ka, just prior to the Last Glacial Maximum. Winter sea ice remained absent throughout the Holocene, while SSSTs briefly exceeded modern values, reaching ~5 °C by 11.4 ka, before decreasing to ~4 °C and stabilizing. The absence of sea ice coverage over the core site during the early glacial period suggests that sea ice may not have been a major contributor to CO2 drawdown at this time. During MIS 5d, we observe a weakening of meridional SST gradients between 42° to 59° S throughout the region, which may have contributed to early reductions in atmospheric CO2 concentrations through its impact on air-sea gas exchange. Sea ice expansion during MIS 4, however, coincides with observed reductions in Antarctic Intermediate Water production and subduction, suggesting that sea ice may have influenced intermediate ocean circulation changes.

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