06 March 2015

Ocean currents complicate reconstructions of past climate


Reconstruction of the past ocean water temperature, based on shells of microplanktonfrom the ocean sediment, to study past climates, has to take into account the transportation of these shells by currents. A recent study, partially funded by DOE, reconstructed the displacement of floating microscopic animals, the planktic foraminifera, in the ocean. Using state-of-the-art computer models of ocean circulation, the investigators could simulate, for the first time, the provenance of fossil foraminifera; or the “footprint” of a sediment core. They discovered that foraminifera are transported by currents along very long distances, depending on the location, up to thousands of km. The temperature recorded in a sediment cores is therefore not from the ocean above, but from a footprint that is often pretty vast, and geographically skewed. This is both a stark warning for paleoceanographers, and a powerful tool to improve the interpretation of climate records. Studying the climate of the past is essential to understand the climate system, and to predict what the future holds for us. Planktic foraminifera live close to the ocean’s surface, where they “record” the water temperature, and as they die they settle to the ocean’s floor. Here is where paleoceanographers find them, much later, when they dig up cores of sediment, looking for the information still preserved in the shells. Since the 1950’s, the earth’s climate history has been reconstructed from the fossil shells of these organisms. Still, in decades of research, never were the trajectories of living and settling foraminifera rigorously investigated. For this study, the investigators carefully evaluated the traits of these bugs (such as depth of habitat, sinking speed, life span), and compared the results with analysis of real fossil shells. The study, a collaboration between European, Australian, and US institutes, is also a successful example of how two research communities, paleoclimatologists and ocean modelers, can join forces to solve a long-standing problem, and move the climate science forward.

Erik van Sebille
University of New South Wales
van Sebille, E, P Scussolini, JV Durgadoo, FJ Peeters, A Biastoch, W Weijer, C Turney, CB Paris, and R Zahn.  2015.  "Ocean Currents Generate Large Footprints in Marine Palaeoclimate Proxies."  Nature Communications 6(6521), doi:10.1038/ncomms7521.