As our planet has been warming over the past decades, Arctic sea ice has been retreating rapidly. Model projections show that we, and certainly the generation of our kids, will experience an Arctic without summer sea ice. This is already transforming the Arctic region drastically and new Arctic shipping routes have already been used. But it is very likely that Arctic sea ice will disappear even more rapidly than projected, because most models underestimate the trends we observe over the past decades. This shows that we do not fully understand sea ice in the climate system.
It is thus essential that we get a better understanding of Arctic sea ice and its role in climate change. This can be achieved by looking into the past, beyond satellite and historical records, where major climate shifts are documented in the geological record. To investigate the past sea ice cover, we rely on proxies or physical evidence of past sea ice stored in sediments. While the currently used sea ice proxies like microfossil assemblages or biomarkers provide crucial insights in sea ice evolution, they still have limitations. Here, the idea to exploit DNA stored in marine sediments as a new proxy can significantly advance the field of paleo sea ice research.
The idea to apply molecular ecology for sea ice reconstructions originated from reading a paper on sea ice biology one rare sunny afternoon in Bergen in 2014. The study showed that the DNA signature of sea ice is unique and that the DNA of individual sea ice organisms such as diatoms or dinoflagellates could be detected in sea ice. Being a paleoceanographer, I remember thinking that if these signatures could also be detected in sediments, there is a huge potential for developing environmental DNA in sediments as a sea ice proxy. So, by combining the expertise of paleoceanographers and molecular ecologists, we set up a pilot study. We were lucky to get fresh material from the Ice2Ice cruises in the Greenland Sea in 2015 and 2016. We selected one sediment core to test whether we could use environmental ancient DNA to detect sea ice changes.
We employed metabarcoding and quantitative PCR on marine sediments from a the Greenland Sea and found diverse DNA signatures and paleodiversity shifts back to circa 100,000 years ago. We identified several taxa that were never reported from the fossil record before and detected the sea ice dinoflagellate Polarella glacialis. Together, these are the building blocks for a new sea ice proxy. Our challenge for the coming years is to take this proof of concept to the next level and test the applicability of ancient DNA for sea ice reconstructions on a wider scale in the polar regions. Our first results show that techniques from molecular ecology can make a possibly game-changing difference in sea ice and paleoclimate research.