Changes in <i>p</i>H in the eastern equatorial Pacific across stage 5–6 boundary based on boron isotopes in foraminifera

Abstract

Estimates of paleo‐ p H 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 p H during the penultimate glacial period was about 0.3 ± 0.1 p H units higher compared to the modern deep ocean. This is consistent with previously estimated deep ocean p H 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 p H 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 CO 2 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 p H change in the eastern equatorial Pacific surface ocean during the glacial‐interglacial transition. This is inconsistent with an expected higher surface ocean p H during the glacial period due to lower atmospheric p CO 2 and is also in contrast with the previously estimated boron isotope based glacial‐interglacial p H change of 0.2 ± 0.1 p H units in the western equatorial Pacific and tropical Atlantic. The lack of change in eastern equatorial Pacific surface ocean p H 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 p CO 2 of eastern equatorial Pacific surface ocean and that of the atmosphere, making this part of the ocean a greater source of CO 2 to the atmosphere during glacial periods compared to today.