What Sea-Surface Temperatures, Sea Ice Extent, Carbon Isotopes, and Iron-Induced Marine Productivity Tell Us About Ocean Carbon Uptake During Marine Isotope Stages 4 and 2

Atmospheric carbon dioxide concentrations during Marine Isotope Stage (MIS) 4 (59-68 ka) were 77 ppm lower than during the Last Interglacial Period (MIS 5e, 118-127 ka), and only 10 ppm higher than during MIS2 (18-28 ka). Several processes affect ocean carbon uptake during cold stadial periods, including temperature-induced changes in CO₂ solubility, isolation of the deep ocean, reduced air-sea gas exchange due to sea ice expansion, and iron-induced changes in marine productivity. Here we analyze previous proxy compilations to compare ocean carbon uptake mechanisms between MIS2 and 4.

First, reconstructed whole ocean sea-surface temperatures (SSTs) are within 0.5ºC of each other for MIS2 and 4, suggesting very similar CO₂ solubility between these two periods. Regionally, SSTs between 50 and 60ºN were about 1ºC warmer during MIS4 than MIS2, with potential impacts on the Atlantic Meriodional Overturning Circulation. However, benthic foraminiferal ∂¹³C data imply that isolation of deep waters reconstructed for MIS2 was already in place during MIS4, and in some basins may have been more extreme.

Second, Antarctic ice core proxies suggest that sea ice cover near Antarctica was already 95% of full glacial levels during MIS4, suggesting that effects of sea ice on air-sea gas exchange would have been close to their full extent during MIS4. However, the few reconstructions of sea ice cover in the Antarctic Polar Front (APF) Zones are less coherent, suggesting that sea ice was anywhere between 50% and 100% of its full glacial extent, depending on location.

Finally, our compilation of opal fluxes suggests that the Southern Ocean had not yet shifted to a full glacial state during MIS4. Relative to MIS2, MIS4 opal fluxes were higher south of the modern-day APF, comparable in the APF, and lower in the Subantarctic zone. Similarly, dust fluxes to the Antarctic EPICA Dome C ice core during MIS4 were only 67% of their full glacial, MIS2 levels, suggesting that effects of Fe fertilization were not fully realized during MIS4. However, the majority of marine-based estimates of terrigenous fluxes to the Southern Ocean exhibit the same or higher terrigenous fluxes during MIS4 relative to MIS2, suggesting different behavior between marine core terrigenous and ice core dust fluxes and a more complicated link between terrigenous and opal fluxes.