Paris, 28 March 2012
Reef-building corals are organisms that live exclusively in tropical waters. Since they are very sensitive to light and temperature, they grow near the water surface within a very narrow depth range, which makes them good indicators of sea level. By studying fossil corals that formed over the past few hundred thousand years it is possible, therefore, to reconstruct past variations in sea level and environmental change. Such records thus provide valuable information about the dynamics and behavior of ice sheets in the past. A better understanding of these dynamics should eventually help to improve modeling and prediction of future variations in sea level.
As part of an international drilling campaign carried out on modern reef slopes in Tahiti in 2005(2), researchers from the European Center for Research and Teaching in Environmental Geosciences (CEREGE) (Aix-Marseille Université/CNRS/IRD/Collège de France) took core samples from three sites located on coral reefs off the island of Tahiti. By dating these records, they were able to reconstruct variations in sea level over the past 16 000 years(3). The dating revealed an extremely rapid sea level rise during the last deglaciation, which took place between around 21 000 and 11 000 BP. In the course of this transition from the last glacial period to the warm climate that Earth currently enjoys, global sea level rose by around 120-130 meters over a period of nearly 15 000 years. It was already known that this elevation was not constant but punctuated by rapid phases of sea level rise connected to the massive collapse of ice sheets. The highest of these sea level rises, called Melt-Water Pulse 1A (MWP-1A), nonetheless remained puzzling in many ways.
The new research has confirmed the existence of this major climate event, while for the first time revealing its extent, sequence of events and duration. The beginning of MWP-1A was dated at 14 650 BP, which means that this event coincided with the beginning of the first warm phase that marked the end of the glaciation in the Northern hemisphere. This period, known as the Bølling(4) period, lasted just under two thousand years, during which the temperature of the Northern hemisphere rose by around 5 °C in just a few years. According to the CEREGE researchers, during MWP-1A, global sea level rose by nearly 14 meters in a mere 350 years. The rate of sea level rise was at least 40 mm/year, which should be compared with the average rate of 10 mm/year estimated for the last deglaciation, or with the 3 mm/year increase observed today by satellite. Based on simulations using geophysical models, the researchers established that the Antarctic ice sheet made a highly significant contribution, probably as much as fifty percent, to MWP-1A. The work illustrates the instability of ice sheets, especially in the Antarctic, during major climate disturbance, and sheds new light on the future contribution of the Antarctic ice sheet to sea level rise in the current context of global climate warming.
The work was supported in particular by the Comer Foundation (USA), the European Science Foundation (ESF-EuroMARC), the European Commission (Past4Future project), Collège de France, CNRS and IRD.
The 'DP Hunter', the drilling vessel used to take core samples from the coral reef off the island of Tahiti.
Members of the team examining coral samples.
1 - Centre européen de recherche et d'enseignement en géosciences de l'environnement
2 - The campaign was implemented thanks to the Integrated Ocean Drilling Program (IODP), an international program in which France plays a major part, together with seventeen other countries, via the ECORD European consortium.
3 - The precision of the dating obtained using the U/Th method at CEREGE is on the order of around thirty years for samples of this type.
4 - During this first warming phase, stone age peoples were able to settle in Northern Europe as far north as Scandinavia, before temperatures fell again 12 900 years ago.
Ice sheet collapse and sea-level rise at the Bølling warming 14,600 yr ago. Pierre Deschamps, Nicolas Durand, Edouard Bard, Bruno Hamelin, Gilbert Camoin, Alexander Thomas, Gideon Henderson, Jun'ichi Okuno, Yusuke Yokoyama, Nature, 29 March 2012.
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