Hydrology of surface waters and thermohaline circulation during the last glacial period

Sedimentological studies on oceanic cores from the north Atlantic have revealed, over the last glacial period, abrupt climatic changes with a periodicity of several thousand years which contrasts strongly with the glacial-interglacial periodicity (several tens of thousand years). These periods of abrupt climate changes correspond to massive icebergs discharges into the north Atlantic. The aim of this work was to study the evolution of the thermohaline circulation in relation to these episodic iceberg discharges which punctuated the last 60 ka. To reconstruct the oceanic circulation in the past, we have analysed oxygen and carbon stable isotopes on benthic foraminifera from north Atlantic deep-sea cores. First of all, the higher temporal resolution of sedimentary records has enabled us to establish a precise chrono-stratigraphy for the different cores. Then, we have shown the close linkage between surface water hydrology and deep circulation, giving evidence of the sensibility of thermohaline circulation to melt water input in the north Atlantic ocean. Indeed, changes in deep circulation are synchronous from those identified in surface waters and are recorded on a period which lasted ∼ 1500 years. Deep circulation reconstructions, before and during a typical iceberg discharge reveal several modes of circulation linked to different convection sites at the high latitudes of the Atlantic basin. Moreover, the study of the last glacial period gives the opportunity to differentiate circulation changes due to the external forcing (variations of the orbital parameters) and those linked to a more local forcing (icebergs discharges). 105 refs., 50 figs., 14 tabs., 4 appends