HR: 16:45h
AN: PP14A-04    [Abstracts]
TI: Revisiting ODP Site 690 to Assess the Responses of Marine Carbonate Chemistry to the Paleocene-Eocene Thermal Maximum
AU: * Kelly, D C
EM: ckelly@geology.wisc.edu
AF: University of Wisconsin - Madison, Dept. of Geology & Geophysics 1215 W. Dayton Street, Madison, WI 53706 United States
AU: Zachos, J C
EM: jzachos@es.ucsc.edu
AF: University of California - Santa Cruz, Earth Sciences Department 1156 High Street, Santa Cruz, CA 95064 United States
AU: Bralower, T J
EM: tjb26@psu.edu
AF: Pennsylvania State University, Geosciences Department 0503A Deike Bldg, University Park, PA 16802 United States
AU: Schellenberg, S A
EM: schellenberg@geology.sdsu.edu
AF: San Diego State University, Deparment of Geological Sciences 5500 Campanile Drive, San Diego, CA 92182 United States
AB: The close of the Paleocene epoch (ca. 55 Ma) is punctuated by a transient ($<$100 kyr) global warming event referred to as the Paleocene-Eocene Thermal Maximum (PETM). In the marine realm, hallmark signatures of the PETM are a negative carbon isotope excursion (CIE) on the order of 3 per mil, widespread carbonate dissolution, and a benthic foraminiferal mass extinction. The rapid onset and anomalous magnitude of the CIE has been attributed to a sudden release of some 2000 Gt of methane into earth's surficial carbon reservoir. Presumably, oxidation of this methane elevated pCO2 levels in the atmosphere/ocean system, fueling global greenhouse warmth and carbonate dissolution. Here we revisit what is arguably the most complete deep-sea record of the PETM recovered from ODP Site 690 to explore the dynamic coupling between atmospheric CO2 levels, marine carbonate chemistry, continental weathering and global climate. The abrupt onset of the CIE is accompanied by a sharp decline in wt.% carbonate, yet wt.% coarse-fraction ($>$63 microns, foraminiferal shells) values remain fairly constant. These sedimentological shifts collectively point toward the selective removal of fine-fraction ($<$63 microns) carbonate produced by calcareous nannoplankton during peak oceanic warmth. We believe this selective pattern of "dissolution" actually reflects, in part, reduced calcification among some calcareous nannofossil taxa. An important corollary of this interpretation is that rising pCO2 levels attained a critical threshold that inhibited nannoplankton calcification. Decreased surface-ocean carbonate production triggered a shoaling of the local lysocline and concomitantly enhanced the ocean's carbon-storage capacity providing an important sink for atmospheric CO2. The character of carbonate sedimentation is reversed during the later, recovery stages of the CIE. It is within this stratigraphically expanded portion of the CIE that wt.% carbonate values and the relative proportion of wt.% fine-fraction increase markedly. This secondary shift coincides with a 5 degrees C cooling of intermediate waters and a sharp influx of kaolinite. Concurrent cooling of sea-surface temperatures is also suggested by the disappearance of warm-water microplankton. We believe these changes to be interrelated. The kaolinite spike likely reflects intensified silicate weathering on Antarctica as well as increased continental runoff. Thus, enhanced silicate weathering reactions (CaSiCO3 + CO2 -$>$ SiO2 + CaCO3) may have served as an added sink for atmospheric CO2 and a source of oceanic Ca2+ and HCO3- that drove an alkalinity overshoot thereby fostering increased carbonate sedimentation within the Weddell Sea region. The remarkable sequence of oceanic changes preserved in the Site 690 PETM record is consistent with the hypothesis that both marine carbonate chemistry and continental weathering acted as negative feedbacks to curb PETM warmth.
DE: 9310 Antarctica
DE: 9604 Cenozoic
DE: 4805 Biogeochemical cycles (1615)
DE: 4806 Carbon cycling
DE: 3030 Micropaleontology
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting