Marine carbon cycle evolution in the eastern equatorial Pacific over the last deglaciation

Resumo

Although the glacial-interglacial climate cycles of the late Pleistocene were very likely paced by changes in solar insolation, the full amplitude of these climate cycles was only achieved with the help of strong feedbacks within the Earth system, including variations in the atmospheric CO2 concentration. It is thought that these CO2 changes resulted primarily from perturbations of the marine carbon cycle. Thus, for example, an increase in the efficiency of the marine carbon pumps (more specifically the solubility pump and biological pumps) may have permitted the accumulation of larger amounts of CO2 in the ocean interior, away from the atmosphere. The goal of this thesis is to investigate the existence of such a theoretical stock of CO2 in the glacial ocean and to assess the mechanisms through which it may have arisen. To this end, I have analysed a combination of proxies in sediment core ODP1240 from the Eastern Equatorial Pacific (EEP) over the last deglaciation, all of which related to past changes in ocean interior carbon content. These proxies include: radiocarbon “ventilation ages” as a proxy for deep ocean turn-over time, benthic foraminiferal B/Ca ratios as a proxy for deep water Δ[CO3 2-]in situ-sat (and therefore [CO3 2-]), δ13C as a proxy for respired carbon content, and Δδ13C(epifaunal-infaunal) as an additional estimate of water oxygenation. The results demonstrate that the deep- and shallow-subsurface EEP were much more poorly ventilated during the Last Glacial Maximum (LGM) relative to the Holocene epoch. This would suggest an increased residence time for carbon in the deep Pacific and an increase in the efficiency of the marine biological carbon pump via a decrease in its “leakiness”, which could have been further enhanced by an increase in local export productivity during the last glacial period. The increase in deep ocean respired carbon levels that would be expected from these changes, is confirmed by deep-water [CO3 2-] reconstructions (derived here using the novel LA-ICPMS technique), as well as benthic δ13C and oxygenation estimates. All together indicate a more respired CO2 enriched and more oxygendepleted deep EEP during the last glacial period. This greater accumulation of respired CO2 in the glacial ocean would have been achieved at the expense of the surface ocean and atmosphere. However, the Δ[CO3 2-]LGM-Holocene reconstructed in this study location is relatively small compared to the changes estimated in radiocarbon ventilation for the same period, suggesting that a “counteracting” mechanism, such as carbonate dissolution at the seafloor, should have also played a role. This mechanism would have increased average ocean alkalinity, allowing even more atmospheric CO2 to be “sequestered” by the ocean during the glaciation. An initial comparison with results from the South Pacific and Southern Oceans suggests that a promising avenue for future work might be to extend the approach adopted in this study to a larger number of sites from around the global ocean, with the aim of moving closer to a quantification of the glacial marine respired carbon stock and the proposed increase in average ocean alkalin