CHANGES IN PH IN THE EASTERN EQUATORIAL PACIFIC ACROSS STAGE 5-6 BOUNDARY BASED ON BORON ISOTOPES IN FORAMINIFERA

Estimates of paleo-pH for the eastern equatorial Pacific Ocean across the oxygen isotopic stage 5-6 boundary have been made based on the bor on isotopic composition of planktonic (Orbulina universa) and benthic (mixed species) foraminifera from core V19-28. The estimated deep ocea n pH during the penultimate glacial period was about 0.3+/-0.1 pH unit s higher compared to the modern deep ocean. This is consistent with pr eviously estimated deep ocean pH changes across the stage1-2 boundary in the western equatorial Pacific and tropical Atlantic, thus arguing against the possibility that the benthic foraminifera analyzed to esti mate deep ocean pH changes have been significantly affected by anomalo us local environment and/or diagenesis. The estimated changes in the d eep ocean carbonate chemistry require a decoupling (of several kilomet ers) between the saturation horizon and the lysocline during the glaci al periods. Though such a decoupling could be achieved by enhanced res piration CO2 given calcite dissolution in sediments during glacial per iods, it lacks support from the calcite sedimentary records. The boron isotopic compositions of planktonic foraminifera, on the other hand, indicate no significant pH change in the eastern equatorial Pacific su rface ocean during the glacial-interglacial transition. This is incons istent with an expected higher surface ocean pH during the glacial per iod due to lower atmospheric pCO(2) and is also in contrast with the p reviously estimated boron isotope based glacial-interglacial pH change of 0.2+/-0.1 pH units in the western equatorial Pacific and tropical Atlantic. The lack of change in eastern equatorial Pacific surface oce an pH between glacial-interglacial periods could be attributed to less nutrient utilization efficiency and/or enhanced calcite production du ring glacial periods. Such a decrease in nutrient utilization efficien cy and/or increase in calcite production would lead to a greater diseq uilibrium between the pCO(2) of eastern equatorial Pacific surface oce an and that of the atmosphere, making this part of the ocean a greater source of CO2 to the atmosphere during glacial periods compared to to day.