Environmental instability during the latest Paleocene at Zumaia (Basque-Cantabric Basin): The bellwether of the Paleocene-Eocene Thermal Maximum

Geochemical and micropaleontological data indicate that environmental instability started ~42.6 kyr before the onset of the carbon isotope excursion (CIE) that officially marks the Paleocene/Eocene boundary. Perturbations during the late Paleocene include fluctuations in trophic and redox conditions, as well as in detrital input. Previous studies on calcareous nannofossils suggest that early warming started during the last ~18.8-12 kyr of the Paleocene. The gradual but rapid disappearance of benthic foraminifera reveals significant environmental stress during the last ~12 kyr of the Paleocene, and the main phase of extinction of benthic foraminifera coincided with the onset of the negative CIE. The gradual onset of the extinction is recorded across an interval with high % CaCO3 content, suggesting that carbonate corrosiveness was not the main trigger of the extinctions. Geochemical and micropaleontological data point to oligotrophy and increased temperatures in surface waters prior to the onset of the CIE. The increasing food demand of benthic foraminifera due to accelerated metabolic rates at higher temperatures was exacerbated by more oligotrophic conditions during the last ~12 kyr of the Paleocene, possibly triggering the gradual but rapid extinction, which culminated during the onset of the CIE, when carbonate dissolution was pervasive at Zumaia. Dilution of carbonate-rich material by terrigenous matter, combined with effects of dissolution, resulted in a very low % CaCO3 content in the lowermost 350 cm of the Eocene. The environmental changes observed during the late Paleocene are the bellwether of the PETM, and are in line with the early warming recorded prior to the PETM at other locations.