Geologists are today anticipating a rise in ocean levels of between 80 and 180 centimetres by 2100. But to fine-tune this prediction and upgrade the models underpinning it, we need to know about the recent past in greater detail – on a scale of a few thousand years instead of the millions of years that geologists usually work with. This tour de force was achieved by an international team of researchers that included the University of Geneva (UNIGE). The scientists succeeded in reconstructing the sea-level curve over the last 6,000 years in French Polynesia with unmatched accuracy: to within one centimetre. The research, which is based on analysing coral microatolls, is published in Nature Communications.
Geologists are today anticipating a rise in ocean levels of between 80 and 180 centimetres by 2100. But to fine-tune this prediction and upgrade the models underpinning it, we need to know about the recent past in greater detail – on a scale of a few thousand years instead of the millions of years that geologists usually work with. This tour de force was achieved by an international team of researchers that included the University of Geneva (UNIGE). The scientists succeeded in reconstructing the sea-level curve over the last 6,000 years in French Polynesia with unmatched accuracy: to within one centimetre. The research, which is based on analysing coral microatolls, is published in Nature Communications.
Studies based on coral reef growth have always been approximate up to now because corals evolve at depths varying from 0 to 20 metres. Although the existence of these reefs provides evidence about sea level, the information lacks precision. "By concentrating on microatolls – small islands formed by a specific variety of coral, Porites – we managed to improve the accuracy to one centimetre," says Elias Samankassou, a researcher in the Department of Earth Sciences in the UNIGE Faculty of Sciences. During four successive campaigns from 2012 to 2015, the international team studied microatolls ranging from ten centimetres to eight metres in diameter, all scattered across twelve South Pacific islands as far apart as the distance between Amsterdam and Sofia. It then required two years of hard work to process and analyse all the data that was collected.
Several factors explain the unparalleled quality of the measurements taken by the researchers. First, the laboratories at the Helmholtz Centre for Ocean Research in Kiel, Germany, were used to measure the uranium-thorium ratio, making it possible to date the samples collected from the microatolls within a few decades or even a few years. Uranium and thorium degrade at different rates: their age can be deduced by comparing the relative proportions found in the samples with their initial proportions at the time of the coral’s formation.
Read more at University of Geneva
Photo: Fieldwork on a microatoll, next to the Makemo Island. (Photo UNIGE/Elias Samankassou)
