RECONSTRUCTING SURFACE WATER [CO32-] CHANGES IN THE EASTERN EQUATORIAL PACIFIC ACROSS TWO GLACIAL-INTERGLACIAL CLIMATE 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 perturbations in the climate system can cause large shifts in the uptake or outgassing of CO₂ in this region. Here, I focus on two transitional periods, the last deglaciation (25 kyr to present) and the last glaciation (the Marine Isotope Stage (MIS) 5a-4 transition, 96 to 70 kyr), to investigate how the carbon system in the EEP responds to major climate changes. In preliminary studies, I measured B/Ca ratios in the non-symbiotic planktic foraminifera Globigerina bulloides from core MV1014-17JC (00º10.83’S, 85º52.00’W; 2846 m water depth) as a proxy for changes in seawater carbonate ion concentration ([CO₃^2-]) in the EEP over the last 25 kyr. As isotopic fractionation of boron species in foraminifera is controlled by calcification rate (which is controlled by [CO₃^2-]), [CO₃^2-] can be calculated from B/Ca ratios. In addition, I measured δ¹³C in the symbiotic planktic foraminifera Globigerinoides ruber and Trilobatus sacculifer to calculate downcore Δ[CO₃^2-], based on the finding that surface water [CO₃^2-] affects the δ¹³C value of each species differently. As surface water [CO₃^2-] is directly linked to surface water CO₂ concentrations, which in turn is linked to atmospheric CO₂, I reconstructed [CO₃^2-] to determine when the EEP was either a source or sink for atmospheric CO₂ across the past 25 kyr. Initial results showed general agreement between the two methods, with lower than modern carbonate ion values across most of the last 25 kyr. The records do, however, diverge during the Last Glacial Maximum when the δ¹³C derived Δ[CO₃^2-] record rises above modern values while the B/Ca derived record remains lower and in the mid-Holocene (~7 kyr) with the opposite trend occurring. Now, using the same multi-proxy approach from core MV1014-17JC, I am reconstructing surface water [CO₃^2-] changes across the MIS 5a-4 transition, when global climate transitioned from the previous interglacial into the last ice age. I will compare changes in carbon cycling in the EEP across both a glacial-interglacial and interglacial-glacial transition and evaluate what role (if any) the EEP played in regulating global atmospheric CO₂ concentrations.