Boron isotopic composition of foraminifera as a paleo-pH tool and its implications for the glacial ocean chemistry

Paleo-pH estimations based on boron isotopic composition of foraminifera suggest that during the last glacial maximum, both in western equatorial Pacific and equatorial Atlantic, the deep ocean and surface ocean pH were respectively 0.3 and 0.2 pH units higher than today. Such a deep ocean pH increase indicates an increase in deep ocean carbonate ion concentration of 100 {dollar}mu{dollar}mol/kg. This could be attributed to higher respiration-CO{dollar}sb2{dollar} driven calcite dissolution within the sediments during the glacial periods. The estimated carbonate chemistry of the glacial surface ocean is consistent with an 80 {dollar}mu{dollar}atm decrease in atmospheric pCO{dollar}sb2{dollar} as recorded in ice cores. However the expected calcite preservation event accompanying a 100 {dollar}mu{dollar}mol/kg increase in deep ocean carbonate ion concentration is not prominent in sedimentary records.; To establish the validity of the boron isotope paleo-pH tool, a single species of foraminifera (Orbulina universa) was cultured in seawater with pH values adjusted to 7.70, 8.15, 8.60 and 9.00. The results convincingly show a relationship between pH and boron isotopic composition of foraminifera. O. universa cultured under ambient conditions as well as those obtained from coretops were isotopically lighter (by {dollar}{lcub}sim{rcub}3.3perthous){dollar} than coretop Globigerinoides sacculifer suggesting that a vital effect is active for one or both these species.; Boron isotope based pH changes were also estimated across stage 5-6 boundary, in samples from an eastern equatorial Pacific core, to determine if the estimated pH changes are similar to the previously estimated changes across stage 1-2 boundary in the western equatorial Pacific and equatorial Atlantic. While the deep ocean pH changes were similar across both stage 5-6 and stage 1-2 boundaries, there was no significant glacial-interglacial pH change in the eastern equatorial Pacific surface ocean. This could be explained by higher calcite production and/or lower nutrient utilization in the eastern equatorial Pacific during glacial periods suggesting also that this part of the ocean was a larger source of CO{dollar}sb2{dollar} to the atmosphere compared to today.