Ocean acidification during the Cenozoic

The oceanic uptake of anthropogenic CO2 causes a progressive decrease of seawater-pH, a phenomenon that has been termed ‘ocean acidification’. Such are duction in pH and associated carbonate system changes is of concern because it may affect ocean biogeochemical processes such as biological calcification. The geological record provides a unique opportunity to study climate effects and the long-term response of marine organisms and ecosystems to variations in the Earth’s carbon cycle, and in particular the Paleocene/Eocene Thermal Maximum (PETM,~55 Ma) bears evidence for rapid C injection that may allow comparison with anthropogenic ocean acidification.

Boron isotopes (δ11B) in marine carbonates have been established as a proxy for paleo-acidity, and estimates of Plio-Pleistocene surface and deep ocean pH are consistent with ice core measurements of varying pCO2 and climate change, as inferred from the benthic O isotope record. In addition, the C fractionation into alkenones appears to reflect dissolved [CO2] in surface seawater, and B/Ca ratios in planktic foraminifera have recently been suggested as an additional proxy for seawater acidity. Several records using these proxies describe estimates of carbonate chemistry variations for the earlier Cenozoic (last ~66 Ma). These records will be reviewed, expanded on, and discussed, with a particular focus on Bbased proxies.

One of the challenges for absolute pH-estimates from δ11B in marine carbonates is the uncertainty of the B isotopic composition of seawater priot to the past ~3Ma.Approaches to solve this problem include the use of δ11B of surface-dwelling foraminifera, numerical modeling, and most recently, measuring the δ11B of fluid inclusions in ancient halites. All methods estimate lower than modern δ11B of seawater for most of the Cenozoic, but absolute values diverge significantly. This problem is approached by analyzing and cross-calibrating the δ11B of benthic foraminifera from Atlantic and Pacific deep-sea sediment cores over the past 50 Ma. Similar to other studies, consistent trends are found towards lower δ11B in the early Cenozoic, but these data are also evaluated with independent estimates of surface ocean[CO2], through modeling estimates of deep-ocean pH, and B concentrations in Paleocene planktonic foraminifera. The goal is to characterize Paleocene seawater B chemistry, so that surface and deep-water pH variations across the Paleocene/Eocene Thermal Maximum can be quantified.

Honisch B., Allen K., Hyams O., Penman D., Raitzsch M., Ruprecht J., Thomas E., & Zachos J., in press. Ocean acidification during the Cenozoic. Applied Geochemistry doi:10.1016/j.apgeochem.2011.03.083. Article (subscription required).

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