HR: 13:55h
AN: PP32C-02    [PDF]
TI: The Paleocene-Eocene Thermal Maximum in the Southern Ocean: Middle Bathyal Constraints from ODP Sites 689 and 738
AU: * Schellenberg, S
EM: schellenberg@geology.sdsu.edu
AF: Geological Sciences, San Diego State University, San Diego, CA 92182 United States
AU: Zachos, J
EM: jzachos@es.ucsc.edu
AF: Earth Sciences, University of California, Santa Cruz, CA 95064 United States
AU: Kelly, D C
EM: ckelly@geology.wisc.edu
AF: Geology and Geophysics, University of Wisconsin, Madison, WI 53706 United States
AB: The Paleocene-Eocene Thermal Maximum (PETM; ~55 Ma) is marked by a 2-3$\permil$ negative carbon-isotope excursion (CIE), increased temperatures, reduced ocean-atmosphere circulation, and intensified oceanic carbonate dissolution. The most parsimonious mechanism to drive these global patterns was a massive methane hydrate destabilization that released methane to the ocean-atmosphere and elevated pCO$^{2}$. PETM reconstructions of the Southern Ocean are based largely on the lower-bathyal ODP Site 690 (South Atlantic Sector; ~1,900 m paleodepth). To improve our understanding of this climatically sensitive region, we determined cm-scale variations in bulk-carbonate stable-isotopes and wt% carbonate through the PETM at the lesser known middle-bathyal ODP Sites 689 (Atlantic Sector; ~1.1 km paleodepth) and 738 (Indian Sector; ~1.3 km paleodepth). Sites 689 and 738 both contain three stratigraphic intervals of relatively stable $\delta$$^{13}$C values through the CIE onset; we view these intervals as coeval to similar intervals at Site 690, which Bains et al. (1999) interpret as pauses between distinct methane releases. This congruence suggests that Sites 689 and 738 are as stratigraphically complete as Site 690 through the CIE onset, although much of the subsequent CIE recovery interval is lost in a coring gap at Site 689. Through the CIE recovery interval, Site 738 $\delta$$^{13}$C increases by ~0.8$\permil$ over an $\sim$5-cm interval compared to an equivalent increase over a greater than 150-cm interval at Site 690. Rather than a hiatus, we hypothesize that this Site 738 pattern records ocean-atmosphere $\delta$$^{13}$C changes at a relatively constant MAR, which is consistent with Farley and Eltgroth's (2003) Site 690 $^{3}$He data indicating increased MARs through the CIE recovery interval. These findings imply that (1) the MAR increase was not circum-polar, and (2) carbon cycling/removal from the ocean-atmosphere reservoir was more rapid than previously estimated. Carbonate saturation is a critical parameter in carbon-cycle dynamics; wt% carbonate may provide a first-order estimate of carbonate saturation changes by assuming constant carbonate export production. Sites 689 and 738 wt% carbonate values drop markedly at the CIE onset as predicted by the methane hydrate hypothesis and documented at deeper pelagic PETM sections. Unlike deeper records, Site 689 and 738 wt% carbonate values then transiently increase, stabilize, and drop again at the third and final $\delta$$^{13}$C step-decrease during the CIE. Following this second drop, wt% carbonate values at both middle-bathyal sites rapidly increase to pre-CIE values within the CIE minimum, while values at the lower-bathyal Site 690 gradually increase through the CIE recovery interval. We hypothesize that these patterns reflect carbonate saturation profile changes in response to pulsed pCO$^{2}$ increases from distinct methane releases. Specifically, given that wt% carbonate values remain suppressed at Site 690, we interpret the "double-dip" pattern at Sites 689 and 738 as two distinct lysocline shoalings to at least middle-bathyal depths. If true, current PETM models may underestimate the magnitude of carbonate undersaturation, and thereby the methane volume released, during the PETM.
DE: 1050 Marine geochemistry (4835, 4850)
DE: 3344 Paleoclimatology
DE: 4267 Paleoceanography
DE: 9310 Antarctica
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting