Coral, climate memory

© Lauric Thiault

Analysis performed on coral samples collected during the Tara Pacific expedition, organized by the Tara Ocean Foundation in partnership with the Centre scientifique de Monaco (CSM) and the CNRS, provided first results on paleoclimatology. Recent discoveries based on samples of coral skeletons have enabled scientists to reconstruct changes in sea surface temperature over time.

During this mission, scientifically coordinated by the CSM and CNRS, extensive coral sampling was carried out as the schooner Tara crossed the Pacific Ocean from east to west and from south to north. 40% of the world’s coral reefs are concentrated in this region. After 2 and a half years of sailing, the 35,000 samples collected are now being carefully analyzed.

Analyses of 40 tropical coral fragments lead to new discoveries in paleoclimatology

These analyses, partly funded by Monaco Explorations, were presented on June 23 at the Goldschmidt Virtual 2020 (annual international conference on geochemistry). Marine Canesi, PhD student in geochemistry at the Laboratoire des sciences du climat et de l’environnement (LSCE) — a CNRS-CEA-UVSQ joint research unit on climate and environmental sciences — presented her research and the tools of paleoclimatology.

What is paleoclimatology ?

As coral grows, it records environmental changes, revealing past episodes of intense ocean warming that led to massive bleaching. Retracing past climate conditions on Earth provides valuable information to help scientists better understand and predict coral’s ability to adapt to climate change.
Several glacial and interglacial periods were studied through the analysis of continental deposits, marine sediments, and polar ice. Since 1950, scientists have developed high-performance tools to measure with great precision the elemental and isotopic chemical composition of samples from different sources, including tropical coral.

Did you say “coral core” ?

Reef-building coral (Scleractinia) produces a calcareous skeleton inhabited by colonies of polyps. Over time, coral grows. Some species, such as Porites or Diploastrea, can become very resistant to environmental changes. Growth rate varies from one species to another, from a few millimeters to a few centimeters per year. Some colonies are several million years old, thus enabling long-term reconstructions of climate.

By drilling core samples through the entire mass of the coral colony, from the most recent living part to the skeleton base, scientists can study the environmental conditions under which the coral developed since birth. This research is based on geochemical analysis of coral skeletons across the whole length of each core. Strontium/calcium and lithium/magnesium concentration ratios are reliable indicators (called “tracers” or “proxies”) to reconstruct changes in surface seawater temperature. More recently, the multiproxy approach has provided even more robust reconstructions.

20171121_Tete_carotte_Diploastrea_heliopora@NPansiotCore of a colony of Diploastrea (brain coral). © Noëlie Pansiot / Tara Ocean Foundation

What coral teaches us

The elemental composition of 40 fragments of tropical coral of genera Porites and Diploastrea, collected from two colonies in the Palau Archipelago during the Tara Pacific expedition, was analyzed by Marine Canesi at the LSCE, thanks to joint financing from Monaco Explorations. This work first led to new calibrations for geochemical tracers of surface seawater temperature,

that were then tested and validated, before being applied to past climate reconstructions. Second, the study focused on the geochemical response of Palau’s coral to thermal stress associated with El Niño Southern Oscillation (ENSO) that has been occurring  in this region since the late 19th century.

Anomalies de température de l’eau de mer de surface à Palaos depuis 1880 à nos joursSurface seawater temperature anomalies (SST) in Palau (NW Pacific Ocean) from 1880 to the present day. These data highlight periods of ocean surface warming or cooling. 

Purpose of this research

Over the last centuries, intense ocean warming has caused the health status of tropical coral to deteriorate, sometimes leading to death. Paleoclimatic reconstructions of such events help develop increasingly reliable models to better predict the impact of human activities on coral ecosystems. It is therefore crucial to produce reconstructions of sea surface temperature over decades and even hundreds of years.

Marine Canesi’s PhD thesis is co-supervised by Dr Éric Douville of the GEOTRAC team (GEOchronology, Tracers, ARChetometry) at LSCE and Dr Stéphanie Reynaud at the Centre scientifique de Monaco.

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