Ocean acidification during the Paleocene-Eocene Thermal Maximum: constraints from multiple proxies

The massive release of carbon and subsequent acidification of the ocean during the Paleocene – Eocene Thermal Maximum (PETM) is often cited as a potential paleo analog for the future. Model experiments, however, demonstrate that the rate of carbon emission (>4500 Pg C in 5 to 10 k.y.) relative to the mixing time of the ocean (<1 k.y.) was sufficiently slow to limit the decline in surface ocean pH and carbonate saturation to within tolerable limits for surface-dwelling calcifiers, consistent with the lack of a long-term impact on these biota. The deep sea, however, should have, and did in fact, become highly undersaturated as is evident by the widespread dissolution of seafloor carbonate sediment, and possibly extinction of a number of deep-sea benthic calcifiers. Our current understanding of the changes in carbonate saturation state of seawater during the PETM is largely based on semi-quantitative or qualitative measures, for example, the carbonate content of sediments and microfossil fragmentation indices. While very useful for constraining changes in conditions on the sea floor, for example, the position of the CCD, additional information is desired to assess changes in saturation over the entire water column. To address this, efforts are currently underway to develop quantitative constraints on ocean carbonate chemistry, including pH and [CO3] for surface and bottom water, respectively. These studies include the application of proxies based on the sensitivity of the partitioning of B/Ca and B isotopes in seawater as recorded in planktic and benthic foraminifers. Application of these proxies to sediment cores from depth transects spanning the PETM are yielding new insight into the scale and pattern of change in ocean carbonate chemistry. This presentation will review the findings of these studies, and discuss implications.

Zachos, J. C., Anderson, L. D., Brown, R. E., Hoenisch, B., Kelly, D., Penman, D. E., Thomas, E. & Zeebe, R. E., 2010. Ocean acidification during the Paleocene-Eocene Thermal Maximum: constraints from multiple proxies. American Geophysical Union, Fall Meeting 2010, abstract #PP13E-06. Abstract.

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