Ocean currents complicate reconstructions of past climate

Reconstruction of the past ocean water temperature, based on shells of micro-plankton from the ocean sediment, to study past climates, has to take into account the transportation of these shells by currents.

03/03/2015 | 4:39 PM

Paolo Scussolini, climatologist (Institute of Environmental Studies), and Frank Peeters, marine palaeontologist (Earth and Climate cluster), are part of an international team of researchers that reconstructed the displacement of floating microscopic animals, the planktic foraminifera, in the ocean. Using state-of-the-art computer models of ocean circulation they could simulate, for the first time, the provenance of fossil foraminifera: what they dubbed the “footprint” of a sediment core (see example in the figure). 

Core footprintTheir study in Nature Communications explaines how the team discovered that foraminifera are transported by currents along very long distances, depending on the location, up to thousands of kilometers. The temperature recorded in a sediment cores is therefore not from the ocean above, but from a footprint that is often larger than previously thought, and geographically skewed. This is both a stark warning for paleoceanographers, and a powerful tool to improve the interpretation of climate records.

Shells contain “proxies” of ocean properties
Studying of 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.

Simulating shells amidst currents in a model
”We know that that the ocean is a turmoil of powerful currents, and that the tiny foraminifera just get carried along with them”, explains Scussolini. “We ran the models after carefully evaluating the traits of these bugs (such as depth of habitat, sinking speed, life span), and we compared the results with analysis of real fossil shells. Our 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.”

“Ocean currents generate large footprints in marine palaeoclimate proxies”
Erik van Sebille 1,* , Paolo Scussolini 2,#, Jonathan V Durgadoo 3, Frank Peeters 2, Arne Biastoch 3, Wilbert Weijer 4, Chris Turney 1, Claire B. Paris 5, and Rainer Zahn 6
1. ARC Centre of Excellence for Climate System Science & Climate Change Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, Australia
2. Earth and Climate Group, Faculty of Earth and Life Sciences, VU University Amsterdam, the Netherlands
3. GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany
4. Los Alamos National Laboratory, Los Alamos, New Mexico
5. Rosenstiel School for Marine and Atmospheric Science, University of Miami, Florida
6. Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain and Universitat Autònoma de Barcelona, Institut de Ciència i Tecnologia Ambientals (ICTA), Departament de Física, 08193 Bellaterra (Cerdanyola), Spain

* Now at: Grantham Institute – Climate Change and the Environment, Imperial College London, United Kingdom
# Now at: Institute for Environmental Studies (IVM), VU University Amsterdam, the Netherlands