HIGHER TEMPERATURES AND LOWER OCEANIC pCO(2): A CLIMATE ENIGMA AT THE END OF THE PALEOCENE EPOCH

One of the largest and most abrupt climatic warming events documented in the geologic record occurred at the end of the Paleocene epoch. Oceanic deep waters warmed to 10 degrees C, and high-latitude surface waters warmed from similar to 10 degrees C to similar to 20 degrees C within several thousand years. T his coincided with weakened atmospheric circulation and the extinction of similar to 50% of deep-sea benthic foraminiferal species. It has been suggested that this warm excursion was forced by higher atmospheric pCO(2) and greenhouse effects caused by a pulse of hydrothermal activity and/or volcanism. Stable isotopic evidence is presented from two widely separated locations that suggest this warming was associated with a drop in oceanic pCO(2) rather than an increase. Oceanic pCO(2) change across this event was estimated using a model of C-13 fractionation in photosynthate organic carbon versus [CO2aq] with solubility constants for CO2 and stable isotopic paleotemperamer estimates. To derive a well-preserved record for surface ocean delta C-13 change the organic carbon bound within the calcite lattice of well-preserved planktonic for aminiferase extracted for isotopic analysis. With allowance for uncertainity in the isotopic differences between phytoplankton and foraminiferal organic matter, the initial results indicate a drop in surface ocean pCO(2) at high and low latitudes from 600-700 parts per million (ppm) to - 200 ppm. Lower pCO(2) persisted for at least 10,000 years. The duration of the pCO(2) excursion was long enough for the ocean and atmosphere to have reach a new steady state condition. There is no evidence of increased organic carbon burial in the deep sea during this period. Two alternative explanations are presented to account for such a rapid drop in oceanic pCO(2). One involves reduced upwelling induced by diminished wind stress as atmospheric circulation weakened in response to climate warming. This would have reduced the rate of metabolic CO2 recycling into the surface ocean. It will be necessary to obtain data from regions outside potential upwelling zones in order to evaluate this. The second involves a read just men of carbonate equilibria in the ocean to higher [CO3-] in the surface ocean, particularly at high latitudesw heres urfacew aters warmedt o approximately 20 degrees C. Such a shift in carbonate e quilibria would have lowered the ocean's capacity to take in CO2. If the initial results presented here do accurately reflect a change in the global ocean [CO2aq], the Paleocene/Eocene boundary event may provide clues a about the ocean's physical and biologicl respons to rapid, large-scale perturbations atmospheric pCO(2) and to global warming.