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

Abstract

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 stagel-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 alecoupling (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 glacialinterglacial 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 amaosphere, making this pan of the ocean a greater source of CO2 to the atmosphere during glacial periods compared to today.

Empirical relationship between pH and the boron isotopic composition of Globigerinoides sacculifer: Implications for the boron isotope paleo‐pH proxy

“…Furthermore, it is not certain how applicable the calculated isotopic fractionation between the two aqueous species of boron [Kakihana et al, 1977] is to the natural system. In this regard, it is noteworthy that the parallel trend between theoretical trend and the empirical curves suggests that the paleo-pH estimations in previous studies [Sanyal et al, 1995[Sanyal et al, , 1997 …”

Geochemistry of Major Constituents, Boron and Boron Isotopes in Pore Waters from ODP Site 1202, Okinawa Trough

“…Other important B and δ 11 B applications are oceanic crust alteration Spivack et al 1990;Smith et al 1995), environmental pollution (Gabler and Bahr 1999), and pH reconstruction in past oceans (Spivack et al 1993;Sanyal et al 1997;Palmer and Pearson 2003). Recent studies also demonstrate that B and δ 11 B provide crucial information on the study of fluid origin and the degree of water/ sediment interaction at depth, as well as methane hydrates formation (Brumsack and Zuleger 1992;You et al 1993a;You et al 1995You et al 1996Kopf et al 2000;Deyhle and Kopf 2002).…”

Modelling δ13C in benthic foraminifera: Insights from model sensitivity experiments

“…The same is true for pH: Hönisch et al (2008) found that pH in the southeast Atlantic Ocean during the LGM was increased by up to 0.1 pH units above 3500 m water depth, but decreased below that depth (−0.07 pH units). The Pacific may have experienced increases of up to 0.5 pH units (Sanyal et al, 1997). A logical next step would be to apply our model to a combined carbon cycle/general ocean circulation model in order to obtain spatial patterns for δ 13 C foram .…”