San Diego State University, United States

A negative carbon isotope excursion (CIE) and widespread carbonate dissolution in deep-sea records indicate that the oceans were acidified by the rapid release of vast quantities of carbon during the Paleocene-Eocene thermal maximum (PETM, ca. 55 Ma). Three basic subdivisions of the CIE (pre-CIE, CIE, CIE-recovery) are used to decipher the history of carbonate sedimentation at a series of PETM sections from the Walvis Ridge depth transect in the southeastern Atlantic (ODP Sites 1263, 1266, 1262) and the Maud Rise in the Southern Ocean (ODP Sites 689, 690). Though carbonate content declined at all sites during the CIE onset, dissolution was far more intense in the southeastern Atlantic, where the CIE onset is marked by a layer of clay. Such spatial differences in the degree of carbonate dissolution likely reflect proximity to the carbon source and/or areas where thermohaline circulation transferred liberated carbon to the deep ocean. In contrast, grossly similar patterns of carbonate sedimentation are preserved within the CIE-recovery intervals of the Walvis Ridge and Maud Rise sites. Specifically, carbonate content gradually recovers, increasing into an extended maximum that is higher than pre-CIE levels. This carbonate-enrichment is initially paralleled by a brief increase in wt% coarse-fraction that is rapidly reversed with wt% coarse-fraction declining into an extended minimum that is lower than pre-CIE levels. We attribute this sedimentological shift to a dilution effect driven by increased coccolithophorid production/preservation during the CIE recovery. A notable exception to this CIE-recovery pattern is the deepest of the PETM sites (Site 1262), where both carbonate content and wt% coarse-fraction increase above pre-CIE levels. This discrepancy is owed to the deeper water depth (3.6 km) of Site 1262, and reflects improved foraminiferal-shell preservation as the lysocline/CCD descended to depths deeper than initial levels, an interpretation confirmed by shell fragmentation data. Though the Maud Rise sites do not capture the full range of lysocline movement, the coherent patterns of carbonate sedimentation typifying the CIE recovery period do conform to a predicted "over-deepening" of the lysocline/CCD likely fostered by an alkalinity overshoot driven by increased continental weathering/runoff.