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 (δC) with δC of POM (δC). We compare δC of several planktonic foraminifera species from plankton nets and recent sediment cores with δC on a N-S Atlantic Ocean transect. Our results indicate that δC of planktonic foraminifera are remarkably similar to δC. Application of our method on a glacial sample furthermore provided a δC value similar to glacial δC predictions. We thus show that δC is a promising proxy to reconstruct environmental conditions in the upper ocean, providing a route to isolate past variations in δC and better understanding of the evolution of the carbon cycle over geological time-scales.