THE BENTHIC FORAMINIFERAL RECORD OF RAPID CLIMATE CHANGE IN THE PALEOGENE

The Paleogene began with the fifth largest mass extinction event in the Phanerozoic, caused by the impact of an asteroid at the Cretaceous/Paleogene boundary (K/Pg, 66 Ma). The early-middle Paleogene was characterized by a global warming trend, punctuated by short-lived, extreme warming events called hyperthermals, with the Paleocene-Eocene Thermal Maximum (PETM, 55.8 Ma) the most extreme. Hyperthermals were associated with global negative carbon isotope excursions, carbonate dissolution, increased continental weathering, and biotic perturbations. Benthic foraminifera (unicellular eukaryotes) are common and widely distributed on the seafloor, and their fossil record allows detailed analysis of mass extinctions and climate change.

This group did not suffer significant extinction at the K/Pg boundary, in contrast to marine pelagic calcifiers which may have been severely affected by rapid acidification of surface waters; benthic foraminifera abruptly decreased in diversity and were affected by geographic heterogeneity of marine export productivity and waters oversaturated in CaCO₃ due to extinction of calcareous plankton. During the PETM, benthic foraminifera suffered severe extinction, but marine pelagic and terrestrial groups diversified. Benthic turnover across other hyperthermal events resembles that recorded across the PETM in decreased diversity, but without extinctions. The cause of the extinction is debated, but analysis of widely distributed drill sites suggests that trophic conditions (strength of bentho-pelagic coupling, supply of refractory organic matter, processes controlling food availability on seamounts, and increased food demands triggered by higher metabolic rates at higher temperatures) and carbonate undersaturation, may have been synergistic causes.

Decreased diversity and assemblage turnover may be used to infer sensitivity of benthic foraminifera to environmental turnover, including food availability, warming and ocean acidification.