During the fellowship, the experienced researcher Christopher V. Henri and his supervisors Anker Lajer Højberg and Jens Christian Refsgaard of the Geological Survey of Denmark and Greenland (GEUS) will lead an effort improving the prediction of nitrate levels and of the production of nitrous oxide in soils by developing advanced and accurate modeling techniques. The widespread use of agrochemicals has led to the contamination of many surface and groundwater bodies around the world. Nitrate is a highly problematic contaminant due to its adverse effect on human health and on ecosystems. Understanding the fate of nitrate in the subsurface is, however complex. Indeed, the pollutant undergoes in many cases a sequential biochemically induced degradation, which will reduce levels but also produce nitrous oxide, a potent greenhouse gas. Today, numerical models are essential in the management of such groundwater contamination. Yet, virtually all available numerical solutions for reactive transport in soils use Eulerian methods that present serious numerical issues, which significantly reduce their applicability. The proposed project will represent a breakthrough in our representation of reactive transport in unsaturated soils by developing a stable and reliable Lagrangian method able to simulate reactive systems as the Nitrate biodegradation. The method will also allow to identify key processes triggering reactions in soils, which is primordial to improve our management of groundwater contaminations and to better understand the implication that the subsurface production of nitrous oxide can potentially have on climate. The project will also allow the high-potential applicant to secure a position in Europe and the world-class host institution to maintain excellence through a series transfer of knowledge, training and communication strategy.

The Arctic is undergoing a profound transformation. The striking increase in observed Greenland Ice Sheet melt and sea-ice retreat are two symptoms of the multiple impacts of climate change at high latitudes. In the sea-ice covered oceans surrounding northern Greenland however, large areas of year-round open water, termed polynyas, have been present since historical times. Located at two major Arctic-Atlantic gateways, these biological productivity hotspots support not only large mammals and birds but are a vital natural and cultural resource for indigenous Arctic communities. Polynyas also act as a potentially significant carbon sink. Three large polynyas recurrently form around northern Greenland. Paleoceanographic reconstructions of the largest, the North Water Polynya, indicate that the polynya and its associated ecosystem are extremely sensitive to rapid climatic fluctuations during the Holocene. Taking advantage of new, readily available marine sediment cores, I propose to reconstruct, for the first time, ocean circulation and sea-ice dynamics at the two main polynyas in the Greenland Sea (North East Water and Sirius Water). This action will target key climatic intervals in the early (Holocene Climatic Optimum) and late (Little Ice Age, Medieval Warm Period) Holocene and provide direct input to a fully-coupled earth system model to evaluate past and, more vitally, predict future dynamics under different greenhouse gas concentration pathways defined by the IPCC. This action provides a unique opportunity for me to learn a new proxy for Arctic paleoclimate reconstructions, skills in the applicability of paleo-data to modelling and a number of essential transferable skills such as scientific writing and mentorship, coupled with an extensive new network of expert collaborators. Through this action, I believe I can contribute to bringing this field of research forward, while gaining invaluable assets for my career development as an independent Arctic researcher.

Through this MSC action, I propose to design, test, and apply new genetic proxies for reconstruction of past sea ice extent. Satellite observations show an abrupt decrease in the extent of seasonal sea ice and state-of-the-art projections point to a seasonally ice-free Arctic Ocean by mid-century. Predicting the full range of climate change impacts on the Earth system requires an understanding of past variability. Past sea ice information is, however, limited by the fact that sea ice leaves no direct fingerprint in the geological record. The identification of new reliable proxies for sea ice is one of the most important challenges facing paleoclimatology today. DNA from sea ice microalgae offers an exciting possibility, as the sea ice environment harbours an enormously diverse and highly specialized community of protists, including diatoms and dinoflagellates. Because only a small fraction of sea ice microalgae leave a microfossil imprint in marine sediments and many species are only identifiable by advanced microscopy techniques, one limitation is that molecular references are lacking for many potential DNA gene markers. To identify the potential proxies I will apply classical taxonomy and DNA metabarcoding to study the vertical export of microalgae in the spring, after sea ice melt. The potential of the new sea ice proxies will be tested and selected markers will be applied by exploring sedimentary ancient DNA preserved in marine sediments records from the Arctic.