Southern Ocean upwelling, Earth’s obliquity and glacial-interglacial atmospheric CO2 change

Abstract:

The interaction of Southern Ocean circulation and biogeochemistry can affect the concentration of atmospheric CO₂ and thus global climate. Previous studies suggest that, during recent ice ages, surface/deep exchange was weaker in the Southern Ocean’s Antarctic Zone (AZ), which reduced the leakage of deeply sequestered CO₂ and contributed to the lower atmospheric CO₂ levels of ice ages. Here we report diatom-bound nitrogen isotope ratios in two sediment cores from the Indian sector of the AZ. The data, when compiled with data from the Pacific AZ, indicate that a vast area of the AZ experienced reduced surface-deep exchange and increased nitrate consumption during the ice ages. The new, high-resolution records are consistent with the strength and/or latitude of the Southern Westerly Winds as the driver of AZ changes. We identify three modes of change, each of which appears to be reflected in atmospheric CO₂. Two modes, related to global climate and the “bipolar seesaw” mechanism, have been proposed previously. We propose a third mode, arising from the changes in meridional temperature gradient driven by obliquity (Earth’s rotational tilt angle), which can explain the lag of atmospheric CO₂ behind climate during glacial inception and deglaciation. It makes CO₂ a delayed climate amplifier in the late Pleistocene glacial cycles and may help to draw out recent glacial cycles to their 100kyr “saw-tooth” pattern.