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The input of massive amounts of carbon to the atmosphere and ocean at the Paleocene-Eocene Thermal Maximum (PETM; ~55.53 Ma) resulted in pervasive carbonate dissolution at the seafloor. At many sites this dissolution also penetrated into the underlying sediment column. The magnitude of dissolution at and below the seafloor, a process known as chemical erosion, and its effect on the stratigraphy of the PETM, are notoriously difficult to constrain. Here, we illuminate the impact of dissolution by analyzing the complete spectrum of sedimentological grain sizes across the PETM at three deep-sea sites characterized by a range of bottom water dissolution intensity. We show that the grain size spectrum provides a measure of the sediment fraction lost during dissolution. We compare these data with dissolution and other proxy records, electron micrograph observations of samples and lithology. The complete data set indicates that the two sites with slower carbonate accumulation, and less active bioturbation, are characterized by significant chemical erosion. At the third site, higher carbonate accumulation rates, more active bioturbation, and possibly winnowing have limited the impacts of dissolution. However, grain size data suggest that bioturbation and winnowing were not sufficiently intense to diminish the fidelity of isotopic and microfossil assemblage records. [ABSTRACT FROM AUTHOR]

Reconstruction from oxygen-isotope data of global sea-surface temperatures (SSTs) that prevailed during periods when greenhouse-gas levels were higher than at present (that is, in the early Eocene and Cretaceous epochs) indicates that temperatures were significantly warmer than present in mid- to high latitudes, but similar to or slightly cooler in the tropics. However, Pearson and colleagues claim, on the basis of oxygen-isotope data from planktonic foraminifers recovered from clay beds in Tanzania and other localities, that the tropics were warm during the Eocene, attributing the discrepancy between these and previous tropical SST estimates to the effects of diagenetic overprinting of the latter. Here we note some potential flaws in their interpretation of the new oxygen-isotope data. [ABSTRACT FROM AUTHOR]

Lithified marine sediments are not equitably distributed through time, raising the possibility that lithification masks biological signals when data from unlithified and lithified sediments are compared or combined. Using mollusk-dominated assemblages from the early Cenozoic of the Gulf Coastal Plain, we find that lithification conceals small taxa, decreases taxonomic resolution, and exacerbates the undersampling of rare taxa. Lithified assemblages appear less diverse and have less even abundance distributions than coeval unlithified samples. These limitations cannot be overcome by standardization procedures, nor are they likely to be circumvented by collecting larger samples. The effects of this bias, however, can be mitigated by restricting analyses to a single lithification state or to specific size classes. Since lithification selectively obscures small taxa, the magnitude of this bias will be most severe when organisms are particularly small, such as in the aftermath of mass extinctions. In the study area, lithification artificially protracts the recovery period following the Cretaceous-Paleogene mass extinction by ~7 m.y. [ABSTRACT FROM AUTHOR]

Stratigraphic features of the carbon isotope excursion (CIE) marking the Paleocene-Eocene Thermal Maximum (PETM; ca. 55.8 Ma) are used to study ocean-climate change and carbon cycling during this ancient global warming event. Yet discrepancies in its timing and amplitude exist between bulk-carbonate and planktic-foraminifera d13C records. Here we examine these disparities through the lens of d13C compositions of size-segregated planktic shells across the pre-CIE to CIE transition in the iconic PETM section of Ocean Drilling Program Site 690 in the Weddell Sea. Our results show that the stratigraphic position of the CIE onset is dependent upon shell size, which we attribute to preferential mixing of smaller shells with pre-CIE d13C values up into the overlying CIE interval. Hence, the transitory loss of size-dependent d13C signatures in photosymbiotic planktic foraminifera is a taphonomic artifact, not a geochemical signal of symbiont "bleaching" during the PETM. Our results also indicate that many salient features of the Site 690 bulk-carbonate d13C record are aberrations caused by size-dependent sediment mixing, and as such, should not be viewed as primary signals of ocean-climate change during what is arguably one of the best ancient analogs for future ocean-climate change. [ABSTRACT FROM AUTHOR]