Calcareous nannofossil paleofluxes as proxies of coccolithophore production and calcification: Insights into the Albian-Cenomanian Tethys Ocean

Among the strategies proposed to mitigate accelerated climate change, Ocean Alkalinity Enhancement aims to replicate natural processes to increase the CO2 storage capacity of the ocean, but the potential efficiency and impact on marine phytoplankton, including coccolithophores, is largely unknown. Therefore, natural analogs in the geological past may have the potential to clarify longer-term responses. We focused on the quantification of nannofossil calcite paleofluxes to understand the response of coccolithophores to different marine carbonate chemistry that influenced the absorption of atmospheric CO2 into the oceans. The investigation was carried out for the Upper Albian-Lower Cenomanian interval of the Piobbico Core-Monte Petrano composite section (Umbria-Marche Basin, Italy), consisting of pelagic marlstones and marly limestones (Marne a Fucoidi) overlain by limestones (Scaglia Bianca). High-resolution integrated biostratigraphy, chemostratigraphy and cyclostratigraphy allowed accurate estimates of sedimentation rates. Calcareous nannofossils were quantified in ultrathin sections by counting all specimens observed in 1 mm2. Total nannofossil calcite paleofluxes were gathered based on absolute abundances and volume/mass of individual taxa in a unit area (1 mm2) and unit time (1 year). Out of 36, only 10 genera constitute 95% of nannofossil assemblages. The amount of biogenic calcite produced by individual nannofossil taxa is highly variable and strictly dependent on the ultrastructure and dimensions. Therefore, some genera were separated into morphometric groups to calculate paleofluxes with highest accuracy. Watznaueria barnesiae dominates the nannofloral assemblages in the investigated interval, with abundances fluctuating from 44% to 70%. Nannoconus abundance is very variable, reaching relatively high paleoflux values in the lower portion of the Scaglia Bianca. Lithostratigraphically, the boundary between the Marne a Fucoidi and the Scaglia Bianca does not correspond to a shift in nannofossil paleofluxes: an increase has been observed in the transitional interval where the terrigenous input starts to decrease. The results show that nannofossil absolute abundances, calcite paleofluxes and CaCO3 content are not directly correlated and that a sedimentation rate normalization is crucial to derive total assemblages and single taxa behavior. Paleoenvironmental changes in temperature, fertility, chemistry impacted calcareous nannoflora with species-specific responses. Overall, the data show that a relatively high terrigenous input (Marne a Fucoidi) limited coccolithophore biocalcification. At times without terrigenous input (Scaglia Bianca), fluctuations in nannofossil calcite paleofluxes suggest a potential controlling role of alkalinity, which may have differentially affected single species/genera. Thus, it is possible that individual taxa had maximal calcification at specific (optimal) alkalinity values.