Changes in pH in the eastern equatorial Pacific across stage 5–6 boundary based on boron isotopes in foraminifera

Abstract

Estimates of paleo‐pH for the eastern equatorial Pacific Ocean across the oxygen isotopic stage 5–6 boundary have been made based on the boron isotopic composition of planktonic (Orbulina universa) and benthic (mixed species) foraminifera from core V19‐28. The estimated deep ocean pH during the penultimate glacial period was about 0.3 ± 0.1 pH units higher compared to the modern deep ocean. This is consistent with previously estimated deep ocean pH changes across the stage l–2 boundary in the western equatorial Pacific and tropical Atlantic, thus arguing against the possibility that the benthic foraminifera analyzed to estimate deep ocean pH changes have been significantly affected by anomalous local environment and/or diagenesis. The estimated changes in the deep ocean carbonate chemistry require a decoupling (of several kilometers) between the saturation horizon and the lysocline during the glacial periods. Though such a decoupling could be achieved by enhanced respiration CO2 driven calcite dissolution in sediments during glacial periods, 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 surface ocean during the glacial‐interglacial transition. This is inconsistent with an expected higher surface ocean pH during the glacial period due to lower atmospheric pCO2 and is also in contrast with the previously 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 ocean pH between glacial‐interglacial periods could be attributed to less nutrient utilization efficiency and/or enhanced calcite production during glacial periods. Such a decrease in nutrient utilization efficiency and/or increase in calcite production would lead to a greater disequilibrium between the pCO2 of eastern equatorial Pacific surface ocean and that of the atmosphere, making this part of the ocean a greater source of CO2 to the atmosphere during glacial periods compared to today.