Changes to sea-surface characteristics during the middle Eocene (47.4 Ma) C21r-H6 event: evidence from calcareous nannofossil assemblages of the Gorrondatxe section (western Pyrenees)

The study of Eocene hyperthermal events is crucial to deciphering the potential responses of the environment to different global warming scenarios. To this end, the present study characterizes the calcareous nannofossil assemblages from the C21r-H6 (47.48 to 47.22 Ma) event of the Gorrondatxe section, thus providing new insights into the environmental impact of this carbon-cycle perturbation event on ocean surface waters. The proportion of reworked calcareous nannofossils was found to increase notably during the event, representing up to 40 % of the assemblage, whereas the number of autochthonous calcareous nannofossils decreased due to dilution with terrigenous material. Autochthonous taxa were found not to show significant changes, as only the abundance of warm and oligotrophic indicators decreased slightly. Hardy taxa that inhabited epicontinental environments and were able to adapt to drastic changes in shallow water characteristics, such as salinity, peaked within the core of the event. This scenario strongly suggests that temperature was not the main factor controlling the distribution of calcareous nannofossil assemblages in the Gorrondatxe area during the core of the C21r-H6 event. The combination of a coeval decrease in delta C-13 and an increase in clay minerals (especially kaolinite) suggests increased continental input to the ocean. Therefore, it can be assumed that terrestrial input increased as a consequence of intensified hydrologic cycle and weathering on land, also suggesting increased freshwater input into the oceans. This interpretation supports the hypothesis that increased silicate weathering leads to a reduction in atmospheric CO2 levels.