Abstract
Abstract. The canonical question of which physical, chemical or biological mechanisms were responsible for oceanic uptake of atmospheric CO2 during the last glacial is yet unanswered. Insight from paleo-proxies has led to a multitude of hypotheses but none so far have been convincingly supported in three dimensional numerical modelling experiments. The processes that influence the CO2 uptake and export production are inter-related and too complex to solve conceptually while complex numerical models are time consuming and expensive to run which severely limits the combinations of mechanisms that can be explored. Instead, an intermediate inverse box model approach of the soft tissue pump is used here in which the whole parameter space is explored. The glacial circulation and biological production states are derived from these using proxies of glacial export production and the need to draw down CO2 into the ocean. We find that circulation patterns which explain glacial observations include reduced Antarctic Bottom Water formation and high latitude upwelling and mixing of deep water and to a lesser extent reduced equatorial upwelling. The proposed mechanism of CO2 uptake by an increase of eddies in the Southern Ocean, leading to a reduced residual circulation, is not supported. Regarding biological mechanisms, an increase in the nutrient utilization in either the equatorial regions or the northern polar latitudes can reduce atmospheric CO2 and satisfy proxies of glacial export production. Consistent with previous studies, CO2 is drawn down more easily through increased productivity in the Antarctic region than the sub-Antarctic, but that violates observations of lower export production there. The glacial states are more sensitive to changes in the circulation and less sensitive to changes in nutrient utilization rates than the interglacial states.
Highlights
During the last 800 000 years atmospheric pCO2 has varied in concert with Antarctic surface air temperature (EPICA Community Members, 2004; Luthi et al, 2008; Petit et al, 1999; Siegenthaler et al, 2005)
Our results suggest that their reduction in atmospheric CO2 for weaker winds was a consequence of reduced Antarctic Bottom Water (AABW) formation in their model rather than a weaker residual circulation
In our model we explore the effect of the residual circulation beyond that of conceptual deductions and unlike what is possible in GCMs, we isolate the various processes that play a role in the region
Summary
During the last 800 000 years atmospheric pCO2 has varied in concert with Antarctic surface air temperature (EPICA Community Members, 2004; Luthi et al, 2008; Petit et al, 1999; Siegenthaler et al, 2005). Kohfeld and Ridgwel (2009) provide an excellent review of the processes that are known or proposed to play an important role in lowering glacial atmospheric pCO2 They conclude from previous work that lower sea surface temperatures can explain approximately 26 ppmv (21–30 ppmv) of the interglacial to glacial pCO2 decrease and that this decrease was countered by a reduction in the terrestrial biosphere and reduced ice volume which resulted in an increase of about 22 ppmv (12–36 ppmv) and 13 ppmv (11–17 ppmv) respectively. The observed pCO2-temperature correlation in the glacial record is much stronger in the Antarctic (AA) than in the Northern hemisphere Greenland records This is partly because of the more complex and variable atmospheric circulation patterns that prevail over Greenland. Most of the lesser understood hypotheses of glacial oceanic carbon uptake relates foremost to the efficiency of the soft tissue biological pump where the efficiency is defined as the percentage of all nutrients exported from the surface layer that was exported as organic matter and is affected by both the biology and the circulation of the ocean
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