Planktonic foraminifera organic carbon isotopes as archives of upper ocean carbon cycling

The carbon cycle is a key regulator of Earth’s climate. On geological time-scales, our understanding of particulate organic matter (POM), an important upper ocean carbon pool that fuels ecosystems and an integrated part of the carbon cycle, is limited. Here we investigate the relationship of planktonic foraminifera-bound organic carbon isotopes (δ(13)C(org-pforam)) with δ(13)C(org) of POM (δ(13)C(org-POM)). We compare δ(13)C(org-pforam) of several planktonic foraminifera species from plankton nets and recent sediment cores with δ(13)C(org-POM) on a N-S Atlantic Ocean transect. Our results indicate that δ(13)C(org-pforam) of planktonic foraminifera are remarkably similar to δ(13)C(org-POM). Application of our method on a glacial sample furthermore provided a δ(13)C(org-pforam) value similar to glacial δ(13)C(org-POM) predictions. We thus show that δ(13)C(org-pforam) is a promising proxy to reconstruct environmental conditions in the upper ocean, providing a route to isolate past variations in δ(13)C(org-POM) and better understanding of the evolution of the carbon cycle over geological time-scales.