Calcareous plankton evolution and the Paleocene/Eocene thermal maximum event: new evidence from Tanzania

The biotic response of calcareous microplankton at the Paleocene/Eocene thermal maximum (PETM) climate event principally comprised poleward migrations, the appearance of excursion taxa and elevated turnover in calcareous nannoplankton. Here we examine whether this relatively muted evolutionary signal may be due to the quality of the stratigraphic and fossil record by analysing exceptionally well preserved microfossils from an expanded, hemipelagic PETM section in southern Tanzania (TDP Site 14). Both nannofossils and planktonic foraminifera show major abundance declines at the onset of the PETM that are not explained by dilution or dissolution. The PETM onset is also marked by the extinction of eight nannofossil species, possibly as many as 12, and the intra-PETM assemblages are quite distinct from background, and dominated by Coccolithus including the excursion taxon C. bownii. This biotic response is not clearly biased in favour of particular ecological groups, although the nannofossil excursion taxa were all warm-water favouring, and there was a net diversity loss within cool-water-favouring eutrophic groups. A shift from Toweius- to Coccolithus-dominated assemblages is suggestive of increased oligotrophy, although the presence of abundant Gladiolithus indicates upper photic zone oligotrophy throughout the time interval represented by the TDP Site 14 section. The significant plankton assemblage shifts, diversity decline and possible reduction of calcareous plankton production and/or export suggest the rapid onset of highly disruptive environmental change at the PETM onset. This change must have included rapid temperature increase, and heat stress may have played a role in shaping the PETM assemblage compositions, causing a limited number of extinctions and favouring taxa from warmer areas of the groups' ecological range. However, the muted evolutionary response in these plankton groups indicates that they were highly resilient to this environmental change and through adaptation to the new conditions (e.g. the excursion taxa), but probably principally through range shifts (i.e. migration) or simply the ability to acclimatize, the vast majority of the biodiversity survived and thrived again once conditions returned to normal