Boron isotopes (δ11B) in marine carbonates have been evaluated as a proxy for paleoacidity and Plio/Pleistocene estimates of surface and deep ocean pH are consistent with ice core measurements of varying pCO2 and deep ocean carbonate preservation. We have applied this proxy to quantify the extent of deep ocean acidification associated with the Paleocene-Eocene Thermal Maximum (PETM), an extreme climate event at ~55 Ma, which was characterized by a sudden increase in Earth’s surface temperature of 5-9°C. A ~3.5-4‰ negative carbon isotope excursion, as well as dissolution of carbonate sediments throughout the deep sea provide evidence that this warming was caused by a rapid, massive (>4,500 Pg) release of isotopically depleted carbon, which also acidified the ocean. Our boron isotope estimates are based on the benthic foraminifer species Nuttallides truempyi and Cibicidoides praemundulus, selected from Walvis Ridge ODP Sites 1263 and 1262 in the eastern south Atlantic. Our data display an ~4‰ negative shift across the event, although the low abundance and small size of foraminiferal tests within the interval of extreme carbonate dissolution did not allow us to capture the entire pH excursion. Translation of early Cenozoic boron isotope records into pH estimates requires an estimate of the δ11B of seawater at that time, but an unambiguous archive for this parameter does not exist and model estimates are largely contradictory. We have approached this problem by analyzing the δ11B of benthic foraminifers from Atlantic and Pacific deep sea cores over the past 50 million years and find consistent trends towards lower δ11B in the early Cenozoic. Although we cannot independently estimate how much of this trend is due to overall more acidic bottom waters in the past, we have used these data to estimate an early Cenozoic δ11B of seawater. With this value we can quantify that PETM deep ocean acidification at this location amounted to ~0.5-1 pH unit, a shift that is on the extreme side of current model estimates, and the absolute pH values are higher than predicted by modeling. The boundary constraints on these estimates will be discussed.
Hoenisch, B., Hyams, O., Raitzsch, M. S., Thomas, E., Zachos, J. C. & Zeebe, R. E., 2010. Reconstructing deep-sea acidification during the early Cenozoic. American Geophysical Union, Fall Meeting 2010, abstract #PP13E-07. Abstract.
