CHEMOSTRATIGRAPHY OF THE PALEOCENE-EOCENE THERMAL MAXIMUM AT ODP SITE 738C (KERGUELEN PLATEAU, SOUTHERN INDIAN OCEAN): IMPLICATIONS FOR THE CHRONOLOGY AND DYNAMICS OF A MAJOR GLOBAL CARBON CYCLE PERTURBATION

The pronounced global carbon-cycle perturbation and warming associated with the Paleocene-Eocene Thermal Maximum (PETM; ~55.5 Ma) is most parsimoniously explained by the massive sublimation and release of sedimentary methane hydrates to the ocean-atmosphere reservoir. Accurate reconstruction of the PETM requires that the chronology and completeness of stratigraphic sections be evaluated and compiled into a synoptic record of geographic and bathymetric variation. To this end, we evaluated a poorly known PETM section from ODP Site 738 (~1.3 km paleodepth, Kergeulen Plateau, Southern Indian Ocean) using cm-scale variations in bulk stable-isotopes, weight percent carbonate, and XRF-determined major elements.

Stratigraphic d¹³C and d¹⁸O variations at Site 738 are broadly congruent with those at the South Atlantic Site 690 "reference section", with the following exceptions: (1) an additional short interval of intermediate d¹³C values during the initial carbon isotope excursion (CIE), suggesting a more compete earliest PETM section, (2) a progressive relative decrease in sedimentation rates following the negative extrema of the CIE, and (3) a stepped ~0.7 ‰ increase in d¹³C in the latest PETM, indicating a hiatus or rapid increase in ocean-atmosphere d¹³C values through accerated carbon cycling and/or significant organic carbon burial.

Weight percent carbonate values drop precipitously from >90% to ~70% at the onset of CIE and then gradually rise to ~85%, drop precipitously to ~75%, and finally sharply increase to >90% ­ all prior to the CIE negative extrema. These decreases in weight percent carbonate are less extreme and shorter lived than those in deeper PETM sections and, if interpreted as a carbonate dissolution proxy, imply two distinct episodes of increased seawater corrosivity at ~1.3 km paleodepth, possibly through shoaling of the lysocline.

Coincident with the pronounced decrease in weight percent carbonate at the onset of the CIE, XRF-scanning reveals transient increases in Fe and Mn content (~600% and ~400% above background, respectively). These bulk element "spikes" may reflect increased concentrations of non-carbonate phases due to dissolution and/or changes in redox conditions associated with decreased oxygenation at the onset of the PETM.