Microfossil fragmentation across the Paleocene-Eocene transition at ODP Site 1209 (North Pacific): Implication for reconstructing nannofossil fluxes.

The Paleocene-Eocene period was punctuated by several hyperthermal events likely triggered by massive injection of greenhouse gases, the best-known being the PETM (∼56 Ma). Environmental changes during hyperthermals severely impacted planktonic organisms, especially calcareous nannoplankton producing tiny calcite platelets. These calcite platelets (coccoliths) accumulate at the sea floor and constitute a major component of deep-sea sediments. As calcareous nannoplankton contribute to the biological carbon pump and carbonate counter pump, a better knowledge of their production dynamics in surface ocean and their dissolution throughout the water column is pivotal to produce realistic carbon budgets across hyperthermal events. In this work, we quantitatively assessed calcareous nannofossil abundances and nannofossil fragments from deep sea sediments of late Thanetian to early Ypresian age (55.63–57.42 Ma) from ODP Site 1209 (North Pacific). We developed a new method, based on the shape and number of fragments, to reconstruct an all-inclusive nannofossil absolute abundance (including entire specimens and fragments) in the sediments. The reconstructed values of absolute abundance estimated integrating fragments are on average 25% higher. We also analysed different dissolution proxies (planktonic foraminifera fragmentation, benthic/planktonic ratios, coarse fraction). The fragmentation of planktonic foraminifera does not covary with nannofossil fragmentation, suggesting distinct controlling factors. Foraminiferal proxies and nannofossil assemblages point to episodes of intense dissolution across the PETM as well as during an event pre-dating the PETM by 300–700 kyrs that we identify as the Late Paleocene Event (LPE).