Developing a high-resolution record of nitrogen isotopes in the sub-polar and polar North Atlantic during two key interglacial periods

As Arctic warms about twice as fast as the rest of the planet, Greenland ice-sheet has been melting during the last century, contributing to about 30% of the observed sea-level rise. The potential effects of the major freshwater inputs from the demise of the Greenland ice sheet are still unresolved. We still do not fully understand how the intensity, timing and location of freshwater inputs could impact open-ocean convection. While we generally though freshwater input would reduce drastically or even completely shut down the convection in the Nordic Seas, recent studies contradicted this view. These studies concluded that a relatively thick, cold, and fresh, summer surface-mixed layer originating from ice sheet melting characterized the past interglacial period known as Marine isotope stage 11 (~400 000 years ago). This interglacial period with active ocean convection was thus, paradoxically, characterized by upper-ocean conditions, which are thought to prohibit convective mixing and deep-water formation. Thus, unravelling the effects of freshwater inputs from the demise of the Greenland ice sheet is crucial to better predict near-future climate. A change in the upper-ocean stratification in the polar North Atlantic could have major impacts on convective processes and North Atlantic circulation, which, in turn, could create major climate disturbances including rainfall and drought frequency, Arctic sea ice, and even hurricane activity in the Atlantic.

The proposed work will document the impact of high meltwater fluxes on the evolution of the upper-ocean stratification in the sub-polar and polar North Atlantic throughout the Holocene and Marine isotope stage 1 1 periods. To do this, samples from sediment cores along a North-South transect will be analysed to reconstruct potential latitudinal changes in surface stratification during these climatic periods. We will measure the nitrogen isotope composition of foraminifera-bound organic matter, which has proven to be a robust proxy for relative nitrate consumption and related upper-water stratification in this exact region. This proxy will then be used to compare the surface stratification to a suite of other oceanic proxies (temperature, salinity, etc) to describe the relationship between stratification and the intensity of deep-water formation, and to identify the controlling factors. This study will provide information about when freshwater input affected the upper-ocean stratification in this region, and if that had any influence on oceanic circulation. This will greatly improve our capacity to predict near-future consequences of the present climatic trends and the potential demise of the Greenland ice sheet.