LAUSR

Abstract The Paleocene-Eocene transition is well represented in the bathyal Zumaia section (Basque-Cantabric Basin, Northern Spain), where numerous studies have documented the sedimentological, environmental and biotic effects of the Paleocene-Eocene Thermal Maximum (PETM). The extinction of more than half of the species of deep-sea benthic foraminifera has been previously reported, but the triggering causes and the exact sequence of events leading the to the extinction event still remain uncertain. The combined study of benthic foraminifera and geochemical proxies allowed us to assess the consequences of an extreme, global warming event, and to investigate its environmental precursors, with special attention to the analysis of the uppermost Paleocene. 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.