Reefs and climate

A month in the Eparses Islands to reveal the secrets of the reefs and coral

Contact : Stéphan Jorry 

Located in the south-west Indian Ocean, the Eparses Islands are extremely effective as sentinels both from environmental/climatic and biological/geological/ oceanographic/atmospheric and anthropological perspectives.  The islands are unspoilt and practically exempt of human presence offering a rare and precious pallet for multidisciplinary studies.

In order to settle on a logical research setting to meet with the expectancies of the state in terms of knowledge, notably in the field of biodiversity, the French Southern and Antarctic Territories, administrator of the Eparses Islands, requested the support of the CNRS in 2009.

 This reflection led to launching a call for projects further to a call for interest through inter-organism reflection coordinated by the CNRS for a consortium of partners including AAMP, FRB, IRD, TAAF.

Of the 36 projects submitted, 19 were selected for multi-annual support from 2011 to 2013 which included the availability of the Marion Dufresne, travel expenses and small equipment.

The Marion Dufresne, a research and supply vessel belonging to the Terres Australes et Antarctiques Françaises (TAAF) (the French Southern and Antarctic Territories) started its rotation on 1st April 2011 around the Eparses Islands in the Indian Ocean. The TAAF organized this rotation further to a call for projects duly called "The Eparses Islands" coordinated by the CNRS aiming to develop a corpus of data and knowledge on the islands.

The Eparses Islands, a French territory in the Indian Ocean

These small islands are located in the south-west Indian Ocean around Madagascar. The five entities are Juan de Nova, Europa, Bassas da India, Tromelin and the Glorieuses Islands (composed of Grande Glorieuse and Ile du Lys).

The Eparses Islands constitute one of the five districts of the TAAF, a French overseas authority with no permanent population. The other districts are Crozet, Kerguelen, Saint-Paul and Amsterdam in the sub Antarctic zone and Terre Adélie in the Antarctic.

Due to their remoteness and the relative distance between them, the EEZ of the Eparses Islands is extensive totalling 640 400 km². It is the second largest French EEZ (including Crozet, Kerguelen, Saint-Paul and Amsterdam) (2.39 million km²) after Polynesia.

The Eparses Islands are all of coral origin as reflected in their morphology in the form of an atoll for Bassas da India, the Glorieuses, Juan-de-Nova and Europa and in the form of a raised atoll for Tromelin. Accumulating a surface area of 44 km², the highest point does not exceed a few metres of altitude. Located at altitudes of between 22° south for Europa and 11° south for the Ile of Lys of the Glorieuses, the Eparses Islands have a tropical climate and are located in the path of hurricanes from the north-east.

A particularly interesting zone for scientific research 

The Indian Ocean extends from the Tropics to the sub Antarctic, presents a unique latitudinal gradient and is of exceptional biological abundance in extremely diversified environmental conditions. Through its overseas territories in the Indian Ocean, France is the only country able to conduct scientific research along such a diverse climatic latitudinal gradient.

The sub Antarctic islands have been greatly coveted for research studies for several decades thanks to the combined effort of the TAAF and IPEV. In contrast, the inherent features of the Eparses Islands and their environments remain little studied.

The major scientific challenges

Climatic change and sea level rise are major social issues. Over the last century, sea level has risen by between 1 to 3 mm per year, mainly due to thermal expansion, and over the last decade, retreating sea ice.

The melting polar ice caps represent a significant risk for the future due to their great volume (64 m equivalent sea level).  Projections for the next century tend to show that the sea level rise due to thermal expansion may double and that the contribution of melting polar ice caps may increase 18 to 20 fold.

However, there are a number of uncertainties concerning the dynamics of polar caps, specifically those in Greenland and the Antarctic and the evolution of the climate system. 

To improve these projections we need to study recordings of past sea levels associated with episodes of global warming on scales comparable to those of current climatic changes in order to complete measurements made through instruments over the last century. The last deglaciation (between 25000 and 6000 years ago) represents the most recent event of this type and consequently represents an exemplary study period.


Coral reefs are excellent climate recorders, at different timescales (104-105 years for reefs, 102-104 years for coral colonies). For the last 10 years they have held a central position in the reconstitution of past sea level variations and environmental parameters during the Quaternary (specifically paleo temperatures). The most precise fluctuations in sea level, as much in terms of amplitude as in terms of chronology, are determined by datings with uranium/thorium and radiocarbon methods of shallow water fossil corals. 

Works conducted during the 2011 rotation of the Marion Dufresne

Based on the study of fossilized coral reefs and their associated sedimentary environments, the objective of the REEFCORES project (REEFs and CORals from the EparseS) is to reconstitute sea levels and environmental and climatic changes from the Quaternary to date and to understand the impact on the coral systems of the Eparses Islands.

During the first rotation of the Marion Dufresne from the 1-26 April 2011, the efforts of the REEFCORES team concentrated on the study of fossilized coral reefs on the islands and their associated sedimentary environments, situated in the lagoons in the reef atoll.

Many sediment samples were made (with the grab sampler at sea and the hand corer on the coastal zone) and we also sampled coral outcrops and fossilized beaches. Generally, all the islands show great sedimentological diversity which in part appears to be influenced by the geomorphology of the reefs and the significant tide amplitude. The first recordings of coral reef fossils tend to confirm that the Eparses Islands are strategic for studying the high sea levels of the Holocene (a few thousand years ago) and those of the last inter-glacial age (around 125 000 years ago).

This expedition was a great opportunity for scientists of diverse disciplines (biologists, geologists, archaeologists, etc.) to compare and share their results and approaches of the study of these still preserved natural environments.

The pooling of equipment and means provided optimum cohesion between the different research teams, favoured new collaboration and proved to be an extremely enriching experience for the people involved in the REEFCORES project.

The Marion Dufresne expedition around the Eparses Islands marked the beginning of the REEFCORES programme. During this first mission we discovered the vast expanses which were to prove extremely interesting for this study.

The Eparses Islands are indeed small in size (0.5 to 35 km²) but the reefs surrounding them delineate lagoons sometimes exceeding 250 km².

The few days spent around these islands were sufficient to discover the different sediment and reef environments on land and at sea.

The first results will determine the work to be accomplished over the coming years, the final objective being to apprehend the recent evolution of sea levels over the last several thousands of years and to understand the capacity of adaptation of coral reef environments to global climate change.

An international and multidisciplinary team

The REEFCORES team is made up of staff from French Institutes - CEREGE, CNRS/Collège de France/IRD/Universities Aix-Marseille 1 & 3, Ifremer, IUEM and three foreign laboratories from the IFM-GEOMAR (Kiel, Germany), the University of Amsterdam, Rice University (Houston, USA) who are associated within this project which includes geologists, geochemists and geophysicists.

The Eparses Islands, a key site in the reconstitution of sea levels and climate change over the last deglaciation.

Precise reconstitution of variations in sea level implies obtaining recordings in diverse tectonic contexts (volcanic islands, microcontinents) and in sites located relatively far from previously “far-field” areas for deconvolution of tectonic and isostatic signals.

 Based on a great number of simulations, taking into account the uncertainties associated with the input parameters of the Glacial Isostatic Adjustment models, it was shown that the zone located around the Seychelles is ideal to precisely reconstitute sea level variations during the last deglaciation.

 The sea level in this area is practically equivalent to eustatic level (in other words, the isostatic component is very low) and, moreover, the sensitivity of the GIA models to the imprecision of input parameters (ice and rheology model of the Earth mantle) is very limited.

 The Glorieuses Islands, and in particular, the most northern isles located in the Mozambique Channel, benefit from a particularly ideal situation and the presence of well-developed coral reefs constituting a key site for the reconstitution of sea level variations since the last ice age.

The Mozambique Channel has a tropical climate and offers study sites located across a wide latitudinal belt (11 to 22°S) and therefore favourable for paleoclimatic reconstitution for half of the intertropical belt. Local fluctuations in marine currents and thermal exchange with the atmosphere in this transect of the Indian Ocean towards the South Atlantic makes it an extremely important area for global ocean circulation and represents a key zone for heat exchange between tropical and subtropical zones. Surface water temperatures in the Mozambique Channel are closely linked to precipitations and temperature variations in East African regions. Coral reefs located on the East African coast show different responses to the ENSO (El Niño Southern Oscillation) and the variability in sea surface temperatures of the Indian Ocean due to their specific oceanographic location. To date, there are no seasonal resolution recordings in this part of the Indian Ocean and this type of recording is absolutely necessary to evaluate long-term variability in African climates in relation to the Indian Ocean and ENSO. The sites of Europa and Glorieuses correspond to oceanic sites with no coastal influence.