From plankton shells to past oceans: How biominerals record seawater physics | Stergios Zarkogiannis

Title: From plankton shells to past oceans: How biomineral accretion records seawater physics Speaker: Stergios Zarkogiannis, Leibniz Centre for Tropical Marine Research Date of seminar: 2026.06.15 Planktonic foraminifera are single-celled marine organisms that build calcite shells and have done so since the Jurassic (~200 Ma). Their shells preserve a rich archive of past ocean conditions through geochemical proxies (δ¹⁸O, Mg/Ca, δ¹¹B), yet their potential to record past ocean physics remains largely unexplored. I will show that foraminifera actively regulate their calcification according to ambient seawater density, as a depth-stabilising mechanism in response to physical changes in the water column driven by temperature and salinity. Using X-ray micro-tomography (μCT) to measure shell volumes combined with independent geochemical temperature and salinity reconstructions, I established a quantitative relationship between shell mass and ambient seawater density. This opens the door to reconstructing past ocean density and, by extension, surface ocean physics and current velocity directly from sediment cores. I will also discuss how this plankton shell mass–seawater density relationship may have regulated atmospheric CO2concentrations and driven the global carbon cycle on both glacial–interglacial and longer timescales. The talk concludes with an outlook on ongoing work using μCT and these biomineral proxies that could particularly benefit from synchrotron X-ray imaging, as well as prospects for neutron-based boron analysis enabling non-destructive paleo-ocean pH reconstructions from museum collections.