Reconstructing Surface Water Carbonate Ion Concentration Changes in the Eastern Equatorial Pacific Across Glacial Transitions

Today, the eastern equatorial Pacific (EEP) plays a critical role in the global CO2 budget as a major source of CO2 to the atmosphere, but recent studies suggest the region may shift to a sink for atmospheric CO2 under different climate states. Here, I focus on two transitional periods, the last deglaciation (25 kyr to present) and last glaciation (the Marine Isotope Stage (MIS) 5a-4 transition, 96 to 60 kyr), to investigate how the carbon system in the EEP responds to major climate changes. I measured B/Ca ratios in the planktic foraminifera Globigerina bulloides from core MV1014-17JC (00º10.83’S, 85º52.00’W; 2846 m water depth) as a proxy for changes in surface water carbonate ion concentration ([CO32-]) in the EEP across both climate transitions. Because calcification rate (controlled by [CO32-]) drives the uptake of boron in foraminiferal tests, [CO32-] can be calculated from B/Ca ratios. In addition to the B/Ca proxy, the relationship between δ13C values in the planktic foraminifera Globigerinoides ruber and Trilobatus sacculifer to surface water [CO32-] differ in response to changes in seawater [CO32-]. Therefore, I also measured δ13C in these two species as another proxy for surface water Δ[CO32-] change. Because surface water [CO32-] is linked to surface water CO2 concentrations and thus atmospheric pCO2, I use reconstructed [CO32-] to indicate if the EEP was more or less of a source of CO2 to the atmosphere in the past. Results indicate that across both the deglaciation and glaciation, the EEP remained as much or more of a source of CO2 than today. Enhanced upwelling across these glacial transitions coupled with an expansion of the oxygen minimum zone (OMZ) likely delivered CO2- and nutrient-rich water to the surface. However, this increase in nutrient concentrations coupled with dust fertilization across cold Heinrich events failed to stimulate biological productivity to the point where the region switched to being a sink for atmospheric CO2. Sustained lower-than-modern surface water [CO32-] across both climate transitions indicate that the EEP may remain a source of CO2 to the atmosphere across anthropogenic climate changes in the future