Southern Ocean upwelling, Earth’s obliquity, and glacial-interglacial atmospheric CO ₂ change

Abstract

Previous studies have suggested that during the late Pleistocene ice ages, surface-deep exchange was somehow weakened in the Southern Ocean’s Antarctic Zone, which reduced the leakage of deeply sequestered carbon dioxide and thus contributed to the lower atmospheric carbon dioxide levels of the ice ages. Here, high-resolution diatom-bound nitrogen isotope measurements from the Indian sector of the Antarctic Zone reveal three modes of change in Southern Westerly Wind–driven upwelling, each affecting atmospheric carbon dioxide. Two modes, related to global climate and the bipolar seesaw, have been proposed previously. The third mode—which arises from the meridional temperature gradient as affected by Earth’s obliquity (axial tilt)—can explain the lag of atmospheric carbon dioxide behind climate during glacial inception and deglaciation. This obliquity-induced lag, in turn, makes carbon dioxide a delayed climate amplifier in the late Pleistocene glacial cycles.

Bottom water oxygenation changes in the Southwestern Indian Ocean as an indicator for enhanced respired carbon storage since the last glacial inception

“…Sea-ice formation and seasonally induced melt water would lead to stronger surface water stratification and affect air-sea gas exchange and impede lower waters to rise to the productive surface (Watson and Naveira Garabato, 2006;Wolff et al, 2010). This barrier mechanism is consistent with the observed first increase in nitrate consumption associated with enhanced stratification of the surface ocean (Ai et al, 2020;Studer et al, 2015). Only at the second CO 2 drop at the 355 MIS 5/4 transition would deep ocean circulation slow, as is indicated by increased aU and decreased Mn/Ti and in good agreement with other proxy data (Jimenez-Espejo et al, 2020;Kohfeld and Chase, 2017;Oliver et al, 2010;Wilson et al, 2015).…”

“…This antiphasing at TERM I with invigorated circulation and thus rejuvenation of the deep ocean is linked to enhanced supply of nutrient-rich waters to the surface, which fueled biological production. However, this nutrient-fueled phytoplankton growth was not efficient to quantitatively fix dissolved carbon, thus allowing CO 2 to escape to the atmosphere (Ai et al, 2020;Sigman et al, 2010;Studer et al, 2015). The increase in opal 345 fluxes in connection with more ventilated bottom waters at the end of the glacial periods fits well with higher atmospheric https://doi.org/10.5194/cp-2021-29 Preprint.…”

“…Upwelling and thus nutrient supply to the surface water south of the PF were substantially reduced during glacial times (Anderson et al, 2009;Francois et al, 1997). Biological productivity was reduced, but at the same time nitrate consumption was more complete (Ai et al, 2020;Studer et al, 2015), stemming CO 2 outgassing from the ocean interior. Part of these surface waters are transported towards the north and when they reach 275 the location of DCR-1PC, they are largely depleted in Si(OH) 4 .…”

“…Relaxed Fe-limitation as a result of enhanced bioavailable Fe-input with dust would have lowered the Si:N uptake ratio in diatoms (Brzezinski et al, 2002). Nitrogen isotope studies show that nitrate utilization was more complete in glacial periods when compared to interglacials (Ai et al, 2020;Horn et al, 2011;Martínez-García et al, 2014;Studer et al, 2015), whereas 280 silicon isotopes show an opposite pattern (Brzezinski et al, 2002;Dumont et al, 2020). Despite preferred nitrate uptake and https://doi.org/10.5194/cp-2021-29 Preprint.…”

Ice Age‐Holocene Similarity of Foraminifera‐Bound Nitrogen Isotope Ratios in the Eastern Equatorial Pacific

“…The combined effects of Subantarctic iron fertilization (which reduced the nutrient content of Subantarctic surface water and Subantarctic Mode Water; Martinez‐Garcia et al., 2014; Robinson et al., 2005) and the shift from NADW to GNAIW (Lynch‐Stieglitz et al., 2007) cause a reduction in regenerated nutrient burden and O 2 utilization in the Pacific that translates to an additional and more substantial rise in mid‐depth O 2 . However, further strengthening of the ocean's biological pump through an increase in the degree of nutrient consumption in Antarctic surface waters as well as a decline in Antarctic overturning (Ai et al., 2020; François et al., 1997; Sigman et al., 2021; Studer et al., 2015; Wang et al., 2017) leads to a buildup of regenerated nutrients and a strong decline in deep ocean O 2 , consistent with available O 2 reconstructions (see above) and indicative of a more efficient global biological pump (Hain et al., 2014). The circulation communicates some of this O 2 decline to the mid‐depth Pacific boxes as well.…”