Nannoplankton Extinction and Origination Across the Paleocene-Eocene Thermal Maximum

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Even this massive geological carbon-cycle perturbation is dwarfed by anthropogenic rates of carbon emissions [∼0.2 gigatons (Gt) per year for PETM and 8 Gt per year for modern (30)].

The CCD is the depth at which the rate of calcite input from surface waters equals the rate of dissolution and, in practice, is mapped on the sea floor by the transition from carbonate-bearing (above the CCD) to carbonate-free (below the CCD) sediments.

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Because the two methods generate similar patterns of turnover, we refer only to proportional rates, which tend to be more intuitive, in the text (but both proportional and per-capita rates are shown in the tables and figures).

Our calculated rates of evolutionary turnover are considered to be conservative for a number of reasons. First, the practice of integrating turnover rates over intervals of geological time (binning) necessarily yields rates of evolutionary change that are time-averaged. At BR, WL, and site 1209, the PETM is binned into two intervals: from the onset to the peak CIE (70 ky) and the recovery (150 ky) (20). At site 690, the record is resolved into time intervals of about 10 ky (table S3). This higher resolution is possible at site 690 because the cyclostratigraphic age model that we have used (6) was developed at this site. Second, there is likely to have been differential postmortem dissolution resulting in the selective removal of some delicate species, both in surface waters (21) and in the sediment. This in part accounts for the minor geographic variations in rates, resulting from higher species numbers in shelf and lower-latitude areas as a function of better preservation in shelf areas, and a real increase in species diversity (20).

We have used the cyclostratigraphic age model of Röhl et al. (2000) (6) rather than an alternative age model based on extraterrestrial He (³He_ET) incorporation in sediments (13, 20). The choice of age model does not substantially alter our findings because the onset-to-peak interval is nearly identical in both age models (table S1 and fig. S2). Values differ for the recovery interval, but this discrepancy is not substantial to our findings because the high-resolution record from site 690 demonstrates that extinction and origination rates returned to near-background levels before carbon isotope values increased from their PETM minimum [marking the start of recovery (20)], irrespective of age model (fig. S2).

Despite large CCD changes at the PETM, we are confident that the patterns we observed are associated with evolutionary turnover and not dissolution. We have assessed nannofossil preservation through the sections, and where substantial dissolution is present, it is confined to short intervals during the event onset to peak. All samples that exhibited substantial dissolution were excluded (20).

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We thank A. Z. Krug and M. Patzkowsky for technical assistance; S. Wing, U. Röhl, T. Tyrell, and L. Lourens for discussion; and M. Foote and others for reviews. This work was supported by NSF grant EAR-0120727 to S.J.G. and T.J.B. and a National Natural Research Council research fellowship to S.J.G. This research used samples and data provided by the ODP and the U.S. Geological Survey.