Coral, an archive of ocean acidification

© Noëlie Pansiot / Tara Expeditions Foundation

CEA research engineer Eric Douville recently spent 10 days aboard Tara, working in the laboratory and collecting samples at sea. Back at home base — the Laboratory of Climate and Environmental Sciences (LSCE) on the Paris-Saclay campus — Eric is studying the core samples of coral collected in the Pacific Ocean since the beginning of the expedition. Interview with a geochemist.

 

Can you tell us more about your mission with Tara Pacific?

In collaboration with the Scientific Center of Monaco (MSC), I proposed to set up precise pH measurements of surface seawater from the schooner by installing on board a UV-Vis spectrophotometer with fiber optics. The pH is the key indicator of ocean acidification resulting from the ongoing absorption into the sea of carbon dioxide massively released into the atmosphere by human activities. We study this parameter to characterize and quantify the current acidification of water masses and better assess its impact on the life of corals.

 

You have received all the coral cores collected during Tara’s expeditions.

I lead a research team at the Laboratory of Climate and Environmental Sciences (LSCE). We carry out work in both geochronology and geochemistry. In geochronology, we develop tools to precisely date climate records or archaeological remains. For example, certain techniques were used during an earlier expedition to date the deep fossil corals collected in the North Atlantic. Currently, we’re using these same tools to confirm the growth pattern of surface coral we sampled in the Pacific Ocean. In a second step, we will develop our work in geochemistry to establish and quantify the evolution over time (especially over the last 100 years) of the temperature and pH of the Pacific Ocean.

 

©erottinger-1105Coral reefs studied by the scientists on board Tara © Eric Rottinger / Tara Expeditions Foundation

 

What are the established protocols to study these growth models?

The first step, before studying the geochemistry of Tara’s coral samples, is to establish an age model, to understand how the drilled and studied colonies developed over time. There are 2 methods to establish the growth rate of coral colonies. The first is simply to track and count the annual density bands after core analysis by medical scanner or x-ray. Each year, the coral forms a particular density band linked to the succession of seasons. If necessary, to confirm the counting of tropical coral growth bands, we use dating methods based on the radioactive decay of certain isotopes.

Have you started working on the cores taken by the Tara Pacific teams?

Of the 20 or so cores recently received since the beginning of the expedition, we’ve begun the analysis of about 10 cores of the species Porites. We were able to establish the growth patterns of the concerned colonies while beginning the first geochemical analyses. The first cores taken in the eastern Pacific were a bit short and only covered the last 50 years, but in agreement with the Tara Pacific consortium, we decided to drill a new species of massive coral of the genus Diploastrea heliopora. Since then, we’ve received new cores taken from different sites in the western Pacific, enabling us today to cover the last 200 years or even beyond.

 

20171121_Carotte_Diploastrea_heliopora@NPansiotGroup of polyps in a colony of Diploastrea helioporae © Noëlie Pansiot / Tara Expeditions Foundation

 

What happens after the dating of the samples?

After establishing the colony growth model, we analyze their geochemistry, in other words the chemical composition of the coral skeleton, which allows us to reconstruct the evolution of the ocean temperature and pH in the past.

 

What information is provided by these Tara coral samples?

It’s too early to tell! But there’s one important thing to do: we have to compare the evolution of the ocean’s characteristics here in relation with the parameters of coral growth and physiology, in other words their capacity to develop and grow over time. This is to study the impact of ongoing global changes on the development of coral reefs in the Pacific. What does stand out from this research is that the major changes in pH and temperature since the beginning of the industrial era have already changed the rate of coral growth.
Today, all our recordings systematically show a drop in the pH of the oceans over the last 150 years, which corresponds to the industrial era. We note that since the beginning of the industrial era, ocean acidification has accelerated at a rate never seen since 300 million years. And its impact on marine life, which will be more and more pressing in future years, remains unknown.

Noëlie Pansiot

 

* Spectrophotometer: optical instrument which can measure water pH with the help of a colour indicator.

Related articles