Environmental ancient DNA as proxy for sea ice reconstructions

Arctic sea ice extent has been declining rapidly over the past decades. Projections indicate that the Arctic will be sea ice free during summer well before the end of the century. This will bring about a major change in the Arctic with consequences for local communities as well as global climate. To understand the implication of changes in Arctic sea ice cover, it is essential to document the variability and rates of such changes. Satellite and historical accounts fail to grasp these sufficiently, therefore we have to document sea ice extent on longer, geological timescales. To reconstruct past sea ice extent, we use signals of sea ice stored in ocean sediments (proxies). There are few proxies available, and each have strengths but also limitations. In aDNAPROX, a cross-disciplinary project that brings together paleoceanographers and molecular ecologists, we developed and assessed sedimentary ancient DNA as a novel proxy for reconstructing Arctic sea ice evolution. The project aimed to be the starting point for developing an innovative technique that will allow us to get better insight into the variability of sea ice extent in the past. Eventually, this will increase the knowledge about the changing Arctic cryosphere and its local and global impact. In this project, we have generated sea ice biomarker data, palynological records and molecular data from surface sediments and sediment cores in the Arctic Ocean, Greenland Sea and Labrador Sea. In a sediment core from the Greenland Sea dating back to almost 100,000 year ago, we recorded sedimentary ancient DNA which showed changes in the molecular assemblage related to changes in sea ice cover. We also successfully tracked the genetic sequence of a single, sea ice organism in this sediment core and in seafloor sediments from different locations in the Arctic. We analysed the sedimentary DNA in recent seafloor sediments from areas with different sea ice regimes, which reveal strong differences in biological diversity. Bringing all our new insights together, we demonstrated a strong potential of sedimentary ancient DNA as a sea ice proxy. This pioneering work will be continued in a new ERC funded research project (2019?2024), where the results from aDNAPROX will be integrated with new data to fully explore the value of using sedimentary ancient DNA for sea ice reconstructions and even broader, for paleoceanography.

The project has demonstrated that techniques from microbial ecology can be applied to address paleoceanographic questions, hereby opening a new cross-disciplinary research field. The pioneering work within the project was crucial to secure further funding from the Europen Research Council, which ensures the continuation of ancient DNA work by the aDNAPROX research team. This team has learned the skills and competences to drive this emerging field forwards and operate a newly established cross-disciplinary ancient DNA research lab in Norway.

Sea ice cover is a 'canary in the coal mine' for the Arctic and global climate state. Sea ice extent is rapidly decreasing and Arctic summers are predicted to be sea-ice free by 2050. Because sea ice is crucial for sustaining local Arctic communities, international ecosystem services and biological diversity, there is an urgent need to understand variability in sea ice cover over longer time scales. Our best tool to predict future sea ice coverage is to understand past trends. Research activities investigating past natural variability in sea ice extent rely on microfossils and/or organic biomarkers produced by organisms that are associated with sea ice. Both methods have inherent assumptions and limitations, which make sea ice reconstructions non-trivial. To improve our understanding of sea ice history in the recent geological past, with special attention to rapid climate change events (e.g. Dansgaard/Oeschger cycles, Heinrich events), we propose to develop a new proxy for determining past sea ice cover using environmental ancient DNA (aDNA) from Arctic sediments. We have obtained preliminary results indicating that aDNA can be a valuable proxy to reconstruct past sea ice cover. Here we propose to further develop and assess the aDNA proxy and compare with traditional microfossil and organic biomarker proxies in order to provide a reference framework for understanding present-day and future accelerated sea ice loss as a consequence of anthropogenic climate change. More reliable proxies will aid policy makers and end-users of ecosystem services to make well-informed decisions on climate change mitigation in the Arctic.