Abstract
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.
Highlights
Antarctic sea ice has been suggested to have played a key role in glacial–interglacial atmospheric CO2 variability (e.g., Stephens and Keeling, 2000; Ferrari et al, 2014; Kohfeld and Chase, 2017; Stein et al, 2020)
Open Ocean Zone (POOZ) diatoms made up the largest proportion of diatoms identified, representing between 72 %–91 % of the assemblage (Fig. 4), with higher values observed during warmer interstadial periods of Marine Isotope Stage (MIS) 1, 3, and 5
Sea ice appears to have been largely absent during MIS 3 (57 to 29 ka), sampling resolution is low, but increased rapidly to 48 % cover during MIS 2 where winter sea ice was consolidated over the core site
Summary
Antarctic sea ice has been suggested to have played a key role in glacial–interglacial atmospheric CO2 variability (e.g., Stephens and Keeling, 2000; Ferrari et al, 2014; Kohfeld and Chase, 2017; Stein et al, 2020). Sea ice has been dynamically linked to several processes that promote deep ocean carbon sequestration, namely by (1) reducing deep ocean outgassing by ice-induced “capping” and surface water stratification (Stephens and Keeling, 2000; Rutgers van der Loeff et al, 2014) and (2) influencing ocean circulation through. Numerical modelling studies have shown that sea-ice-induced capping, stratification, and reduced vertical mixing may be able to account for a significant portion of the total CO2 variability on glacial– interglacial timescales (between 40–80 ppm) (Stephens and Keeling, 2000; Galbraith and de Lavergne, 2018; Marzocchi and Jansen, 2019; Stein et al, 2020). Debate continues surrounding the timing and magnitude of sea ice impacts on glacial-scale carbon sequestration (e.g., Morales Maquede and Rahmstorf, 2002; Archer et al, 2003; Sun and Matsumoto, 2010; Kohfeld and Chase, 2017)
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