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Sea ice across Dansgaard-Oeschger events in Greenland

Periodic Reporting for period 1 - SEADOG (Sea ice across Dansgaard-Oeschger events in Greenland)

Reporting period: 2015-10-01 to 2017-09-30

In the Last Glacial period, the climate of the Northern Hemisphere was punctuated by abrupt millennial scale changes called Dansgaard-Oeschger (D-O) events that are clearly recorded by Greenland ice core temperature proxies. The underlying mechanism responsible for the recurring cycles of ~10°C rapid increase, followed by gradual cooling, remains uncertain. Many studies suggest that large changes in sea ice extent played a major role. Sea ice is closely linked to climate; changes in sea ice extent feedback positively on Arctic temperature—a phenomenon of great relevance to the future of Arctic sea ice given our warming climate. By combining Greenland ice core chemistry records with atmospheric chemistry transport modeling, SEADOG aimed to constrain Arctic sea ice variability across D-O events.

Sea salt sodium (Na) was the focus of this work because substantial evidence suggests that the surface of the sea ice is a significant source of sea salt to the polar regions. This raises the possibility that ice core Na may be interpreted as a tracer of past sea ice conditions. Model simulations of sea salt aerosol at various Arctic sites, produced by this project, confirm the importance of the sea ice source of sea salt aerosol. For the first time, sea salt concentrations deposited at Greenland ice core sites were calculated using a global process-based model. Simulated values compared well with monthly-resolved ice core chemistry records. Based on tests conducted, we conclude that the influence of sea ice extent on year-to-year changes in Greenland ice core sea salt concentrations is minimal under present-day Arctic conditions. The majority of ice core sea salt variation is due to meteorological factors. However, ice cores from the High Arctic, away from central Greenland, offer promise for sea ice reconstruction.

Adaption of atmospheric chemistry transport models for simulations of D-O events under Last Glacial climatic conditions is still ongoing.
Initial work focused on modifying the chemical transport model in order to simulate the sea salt aerosol budget at Arctic sampling locations. To achieve this, the sea ice source of sea salt was required in addition to the open ocean source. I also found that some sea salt emissions were required from the thick, multi-year sea ice, as well as the thinner sea ice that re-forms annually. The next step was to include capability to simulate concentrations of sea salt deposited on the ice sheet to allow direct comparison with the ice core record. Using the optimised chemical transport model, Greenland ice core sea salt concentrations can be simulated to within a factor of two and the seasonal cycle, with maximum sea salt in winter, is well-captured. Model simulations suggest that the sea ice source contributes relatively little sea salt to central Greenland ice cores under present-day conditions. In contrast, simulation suggest that more than 75% of the sea salt found in ice cores from the High Arctic, located closer to the sea ice pack, originates from the sea ice surface. Furthermore, the model simulations captured up to 60% of the year-to-year variability in sea salt concentrations in some Greenland ice cores.

To test what controls the year-to-year variability in sea salt recorded in Arctic ice cores, I performed simple sensitivity tests to compare the influence of sea ice changes versus changes in meteorology i.e. temperature, wind strength. Results suggests that while meteorology is the dominant control on Greenland ice core sea salt signals, ice core records from the High Arctic are more likely to be influenced by sea ice conditions.

A peer-reviewed article detailing the development of Greenland ice core sea salt modelling capability was published in the journal Atmospheric Chemistry and Physics. A second manuscript presenting examining controls on Arctic ice core sea salt variability is undergoing final edits prior to submission. I have presented results from this project at the international PAGES Open Science Meeting (2017) and through invited talks at the UK Paleoclimate Climate Society meeting (2017) and University of Copenhagen (2017). Co-authored studies, including work to simulate the sea ice and climate change across D-O events, have also been presented at international conferences.
SEADOG involves developing fundamental process-based understanding of how a proposed chemical tracer of sea ice conditions can be interpreted under different climate conditions, sea ice states and over different timescales. It represents the first time that Greenland ice core sea salt records have been simulated comprehensively, accounting for the processes of emission, transport and deposition. Findings will impact how ice core sea salt data are interpreted in the future. More work is needed to conclude on whether or not Greenland ice core sea salt records across D-O events provide information about sea ice change.
Simulated Greenland ice core Na concentrations compared to ice core data