Australian National University, Australia

The Paleocene-Eocene Thermal Maximum ( approximately 55 Ma) is recognized as a rapid (<10 ka) input of a large mass ( approximately 4500-6000 GtC) of (super 12) C-enriched carbon into the ocean-atmosphere system. Patterns of CaCO (sub 3) dissolution at the suggest that this was accompanied by a rapid decrease in ocean pH, followed by a gradual recovery phase. A further result of such modeling studies is the suggestion of an 'overshoot' or supersaturated ocean phase after the recovery, when the carbonate lysocline deepened to below its Paleocene depth and surface water carbonate saturation states rose to above pre-excursion levels. In an effort to quantify changes in the carbonate chemistry of surface waters and infer potential effects on calcifying organisms and Mg/Ca and delta (super 18) O-based temperature estimates, we have measured B/Ca and Mg/Ca in mixed-layer planktic foraminifers from IODP site 1209 in the Pacific and sites 1262 and 1263 in the Atlantic. Previous work at these sites has documented large increases ( approximately 50%) in Mg/Ca ratios in the mixed-layer planktic foraminifer species M. velascoensis and A. soldadoensis consistent with 5 to 6 degrees C of sea surface warming. Our B/Ca measurements in both species suggest a large drop in surface water pH and [CO (sub 3) (super =) ] coincident with the rise in temperature at the onset of the carbon isotope excursion, followed by a gradual recovery to pre-excursion levels. The latter feature, coupled with the rise in total alkalinity caused by the dissolution of CaCO (sub 3) is taken as evidence of an overshoot phase starting approximately 100 ka after the onset of the event. We are currently measuring boron isotopes in the same taxa in order to quantify the pH changes suggested by the B/Ca data. Additionally, we plan to measure B/Ca and delta (super 11) B in thermocline-dwelling planktic species in order to examine the depth-dependence of the pH changes. Estimating the magnitude of the pH drop at the onset of the event will facilitate calculations of the mass and rate of carbon input that triggered the PETM, as well as the magnitude of change in atmospheric pCO (sub 2) levels and Paleogene climate sensitivity.