Ancient DNA from a sea-ice microbe tells tales of past climate in the Arctic

Published 12-01-2024

A new research method utilises ancient DNA preserved in ocean sediments to provide reliable information of sea ice occurrence in the Arctic.

Icy arctic waters off the coast of Greenland. The coast of Greenland is seen in the background. A polar bear is resting on some sea ice in the waters off the coast. There are several other flakes of sea ice in the water.
The researchers spotted a polar bear taking a rest on sea ice during an expedition to the high Arctic, summer 2023. (Daniel Amirault, Amundsen Science).

The Arctic is warming around four times faster than the rest of the globe. According to the latest IPCC projections, there will be no summer sea ice in the Arctic region within decades.

Data on sea ice is of great interest to many climate researchers because the occurrence of sea ice both geographically speaking and over time affects conditions in the atmosphere and the ocean. However, we have a poor understanding of sea ice before the satellite era.

Especially accurate longer time series of past sea ice occurrence that gives us an understanding of sea ice in the Arctic in a geological timescale are in demand, as they can provide insights into climate change and be used to make future climate projections.

Filling the gap via proxy

We have satellite data from 1979 and onward but before that the best we can do is qualified guesses.

“Using this method, we are able to produce data that are the next best thing to an actual measurement– measurements of a proxy,” says Sara Harðardóttir, former postdoc at GEUS and now Researcher at the Marine and Freshwater Research Institute, Iceland, and first author on a research article published in the Nature portfolio journal Communications Earth and Environment this week, about the newly developed method.

The method can help us fill the gap in our knowledge about past sea ice occurrence in the arctic via the so-called proxies. A proxy is something other than the actual thing – in this case sea ice – that acts like a tattletale about the thing we want to know. It is like how you can gain insights into the biodiversity of an area you might not have access to by counting the number and variety of bugs on the windshields of cars passing through the area.

“Sea ice harbours a unique microscopic ecosystem. What we did here was to test a new proxy for sea ice by targeting DNA of a sea ice microbe that is very specific but also rare” says Sofia Ribeiro, senior author of the article and senior researcher, Geological Survey of Denmark and Greenland (GEUS).

The ancient DNA of the microbe in question, Polarella glacialis or just P. glacialis for short, can be preserved in marine sediments for thousands of years. It is like an archive of notes on sea ice just waiting to be read – and the researchers now figured out how to read them.

“We compared the gene copy numbers of P. glacialis in modern samples from around Greenland and they follow the distribution of sea ice we see today. That is our “calibration” for going back in time,” says Sara Harðardóttir.

The researchers quantified P. glacialis DNA in Arctic marine and fjord surface sediments and a sediment core from northern Baffin Bay spanning 12,000 years. Sea ice and sediment trap samples confirmed that cysts of P. glacialis are common in first-year sea ice and sinking particulate matter following sea-ice melt.

“The time-series we are now able to create using this method can be used to inform climate modelling on future climate and ecosystem adaption to climate change by feeding the models with even more accurate input,” says Sofia Ribeiro.

Climate modelling and paleoclimatology

When we create future climate models, we feed the models with knowledge of past climate, and how it has changed over time. The reliability of the results depends on the quality and accuracy of the input. Researchers include a vast variety of parameters in the models, some of which are based on very accurate measurements thanks to modern technology, and some which are the most qualified guesses we can come up with based on current knowledge. Some things leave little or no traces for us to estimate their occurrence by – like sea ice. However, indicators like the ancient microbe DNA mentioned in this news item can act like proxies and enable us to inform our modelling with the next best thing to direct measurements. Paleoclimatology, the science of studying past climate, relies on such proxy methods. It is based on those that we know how air and ocean temperatures, salinity, sea-level and CO2 concentrations evolved for millions of years of Earth’s history, way before there were any humans to tell the story.

A person wearing a safety helmet and protective laboraty clothes like gloves and a face mask is seen taking samples from a sediment core aboard a ship. An ocean is visible in the background.
Photo: Sara Harðardóttir, taking samples from a marine sediment core at sea. (Underground Channel, Frederik Wolff)

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Research article

Millennial-scale variations in Arctic sea ice are recorded in sedimentary ancient DNA of the microalga Polarella glacialis.
By Sara Harðardóttir, James S. Haile, Jessica Louise Ray, Audrey Limoges, Nicolas Van Nieuwenhove, Catherine Lalande, Pierre-Luc Grondin, Rebecca Jackson, Katrine Sandnes Skaar, Maija Heikkilä, Jørgen Berge, Nina Lundholm, Guillaume Massé, Søren Rysgaard, Marit-Solveig Seidenkrantz, Stijn De Schepper, Eline D. Lorenzen, Connie Lovejoy and Sofia Ribeiro.
Published in Communications Earth and Environment, January 9 2024.

Sofia Ribeiro
Senior Researcher
Glaciology and Climate
Malene David Jensen-Juul
Communications Officer
Press and Communication