Variations to calcareous nannofossil CaCO3 content during the middle Eocene C21r-H6 hyperthermal event (~Â 47.4Â Ma) in the Gorrondatxe section (Bay of Biscay, western Pyrenees)

The carbonate content of calcareous nannofossils is dependent on seawater composition. One of the factors that affect seawater chemistry and consequently the degree of calcification in coccolithophores is temperature, as seen in present day warming oceans. The depth at which carbonates are dissolved (Calcite Compensation depth, CCD) can rise due to an increase in HCO3- and decrease in pH, leading to a major dissolution on the seabed and burndown. Similar processes have also been deduced for Eocene hyperthermal events, such as the PETM and ETM2. This study reports changes in coccolith carbonate mass from a hemipelagic setting (Gorrondatxe, at 1500 m paleodepth) during the core of a minor Eocene hyperthermal event, namely the C21r-H6 event (47.44-47.32 Ma). Image analysis techniques were used to determine differences in the carbonate mass of selected calcareous nannofossil taxa, revealing species-specific patterns. The CaCO3 mass of Chiasmolithus solitus decreased by 50% over the course of the C21r-H6 event, and many specimens also lost their crossed central bars, an additional indication of mass loss; Reticulofenestra sp. (3-5 μm) showed a similar trend, but the percentage of mass lost was lower; Toweius pertusus, interpreted as being reworked, mirrored the behaviour of Chiasmolithus solitus, suggesting that the CaCO3 mass loss may have occurred on the seabed, rather than in the water column. In general, it can be concluded that the lysocline rose to 1500 m paleobathymetry in the Bay of Biscay during the C21r-H6 event. Formation of corrosive bottom water in the North Atlantic Ocean is regarded as being responsible for the rise in the lysocline.