A team of researchers, amongst them the ULg’s Anne Mouchet, has unveiled part of the mystery which has up until recently surrounded the hypothesis of a formation of deep water in the North Pacific. Research which has been published in the journal Science (1).

Today the whole of the ocean depths are fed by two convection areas, two ‘motors’ where the waters plummet to participate in the deep currents. These waters then climb back to the surface as they warm up (read the article: Twenty thousand years under the sea). These sites of deep water formation are situated in the North Atlantic and the offshore Antarctic. The waters of other regions of the globe are not dense enough (cold and rich in salt) to dive to such depths. If this configuration is today experiencing a certain stability, paleo-oceanographyhas already shown us that it has been subject to a series of reorganisations in the past. Sometimes minor, sometimes more significant. But the research in which has participated Anne Mouchet, a researcher at the ULg’s Laboratory of Atmospheric and Planetary Physics (LPAP), has permitted a reproduction of upheavals which are unusual due to their significance. In other words, a series of circulation shutdowns in the Atlantic, coupled with the creation of a zone of deepwater production in the North Pacific at the end of the last interglacial period. This research has also enabled its causes and consequences to be understood.

If it was difficult to vouch for such a reorganisation, it was in part because the sediments, witnesses which recount quite like no other the history of our Earth, resist time less well in the North Pacific. The deep waters of the Pacific are more corrosive than the others to limestone shells. It is thus difficult to retrace the different evolutions of ocean circulation in this region. ‘But a certain number of sediments seemed to reveal this formation of deepwater in the North Pacific,’ explains Anne Mouchet. ‘Along with researchers from Hawaii and Japan, we thus had the idea of compiling a maximum of data with new sediment samples, which enabled us to verify these variations in currents.’ An observation made possible, for example, by studying two types of zooplankton: plankton foraminifera and benthic foraminifera. One lives on the surface, the other in the ocean’s depths. These foraminifera, as they become fossilized, create limestone shells in assimilating carbon, amongst other things, and thus also one of its isotopes, radiocarbon (or carbon 14, whose radioactivity allows elements which contain it to be dated). Studying the difference in age between the two foraminifera belonging to the same sedimentary layer thanks to carbon 14 allows us to have an idea of the evolution of the ventilation of deep water over the years.