Coccoliths as Recorders of Paleoceanography and Paleoclimate over the Past 66 Million Years

Coccolithophores are a major group of oceanic calcifying phytoplankton, and their calcite skeletal remains, termed calcareous nannofossils, are a major component of deep-sea sediments accumulating since the Jurassic. Coccolithophores play a role in both the biological pump and the carbonate pump, exporting organic and inorganic carbon, respectively, out of the surface ocean. This means that they are key responders to and recorders of ocean carbon cycle and climate changes over geological and shorter timescales, and studying these responses can help elucidate the uncertain fate of calcifying phytoplankton under projected climate change scenarios. Here, we review established and emerging approaches for reconstructing (a) mixed-layer ocean temperature, (b) marine productivity, and (c) aspects of the ocean carbon cycle, using calcareous nannofossils from deep-sea sediments. For each parameter, we discuss the different proxies that have been proposed, based on abundance or species composition, inorganic geochemistry, and/or coccolith morphology, and explore their applications and limitations in Cenozoic paleoceanography. ▪ Calcareous nannofossils can be used to reconstruct upper ocean conditions and changes over centennial to million-year timescales. ▪ Key coccolith-based proxies for temperature, productivity, and the carbon cycle are reviewed. ▪ Approaches based on assemblages, geochemistry, and morphology provide novel insights into the evolution and adaptation of coccolithophores and past climate.