Borealisation of an Arctic marine ecosystem during the Last Interglacial

The Arctic is not just a frozen wasteland; it is a living, breathing ocean that plays a critical role in the Earth's climate system. At its surface, microscopic plants called phytoplankton act as the foundation of the marine food web. These tiny organisms are also powerful engines for the planet's health, absorbing vast amounts of carbon dioxide from the atmosphere and sinking it to the deep ocean, effectively locking it away. For decades, scientists have watched this region warm at an alarming rate, nearly four times faster than the rest of the globe. As the ice melts and warm water from the Atlantic Ocean pushes further north, temperate species are beginning to invade these cold waters, a shift researchers call "borealisation." The big question is how this rapid change will reshape the ecosystem and whether the Arctic will continue to act as a carbon sink or lose that ability. To answer this, scientists look to the past, specifically to a warm period known as the Last Interglacial, which occurred between 128 and 116 thousand years ago. This era serves as a natural laboratory, offering a glimpse into what a warmer Arctic might look like, helping us understand how marine life and carbon cycles responded to heat in a time before human industry.

A team of researchers has now peered into this ancient past using a technique that bypasses the limitations of traditional fossils. They collected sediment cores from the floor of the Fram Strait, a deep channel between Greenland and Svalbard where warm Atlantic water flows north. Instead of looking for the hard shells of ancient organisms that often fail to preserve in these specific sediments, they extracted ancient DNA directly from the mud. This genetic material, preserved for over 100,000 years, provided a detailed snapshot of the entire ecosystem, from the tiniest microbes to the largest whales. The results reveal a dramatic transformation of the Arctic Ocean during that warm period. The ecosystem did not simply warm up; it fundamentally changed its character, shifting from a cold, ice-dominated system to one dominated by temperate species, a process that had profound consequences for how the ocean handled carbon.

The story begins with the microscopic plants that drive the ocean's productivity. In the cold, glacial periods leading up to the warm interval, the waters were dominated by diatoms. These are single-celled algae with glass-like shells that are heavy and sink quickly, carrying carbon efficiently to the deep sea. However, as the climate warmed and the Last Interglacial began, the diatoms were largely replaced by a different group of phytoplankton called coccolithophores. These are also single-celled algae, but they are smaller and build their shells out of calcium carbonate rather than silica. The genetic record shows a massive surge in a specific type of coccolithophore, Gephyrocapsa, which is common in temperate waters but rare in the cold Arctic. This shift suggests that the warm Atlantic water, carrying these temperate species, flooded into the Arctic, pushing out the cold-water specialists. The change was so distinct that the researchers could trace the exact moment the ecosystem flipped, marking a clear transition from a glacial world to an interglacial one.

Before this major shift settled in, the researchers detected a brief but chaotic period of ecological turbulence. Just as the ice sheets began to melt and the climate started to warm, the ocean became a mix of fresh meltwater and salty Atlantic water. This created a layered, unstable environment where the usual rules of the ocean were suspended. During this time, the DNA record shows a burst of diversity among smaller, more flexible organisms. Many of these were tiny, single-celled eukaryotes that could move on their own or eat other organisms for food, traits that helped them survive in the changing, nutrient-poor waters. The researchers also found evidence of algae that are known to form harmful blooms today, suggesting that the stress of rapid environmental change can trigger the proliferation of these potentially toxic species. This transitory phase highlights how fragile the Arctic ecosystem can be when the balance between ice, fresh water, and warm ocean currents is disrupted.

As the warm period peaked, the ecosystem stabilized into a new normal dominated by the temperate coccolithophores. But the changes went far beyond the microscopic level. The genetic record also captured the presence of large marine animals, offering the earliest molecular evidence of their existence in the Arctic during this time. The DNA of whales and fish appeared in the sediments, signaling that the borealisation of the ocean extended all the way up the food chain. The researchers found genetic traces of species like the Atlantic cod and the minke whale, animals that today are associated with warmer, temperate waters but were venturing far north during this warm spell. Even the seals, which usually cling to the ice, showed up in the record, though their presence was linked to specific, colder moments within the warm period when sea ice briefly returned. This indicates that the entire marine community, from the smallest plankton to the largest predators, was reshaped by the influx of warm water and the species it carried.

The implications of this ancient shift are significant for our understanding of the future. The move from heavy, fast-sinking diatoms to smaller, slower-sinking coccolithophores suggests that the ocean's ability to pull carbon from the atmosphere and store it in the deep sea may have weakened during that warm period. While the coccolithophores still performed photosynthesis, their shells are made of calcium carbonate, a process that can actually release carbon dioxide back into the water, partially offsetting the benefits of their growth. Furthermore, the smaller size of these organisms means they sink more slowly, giving them more time to be eaten or broken down in the upper ocean, which keeps the carbon circulating near the surface rather than sequestering it deep down. This suggests that as the modern Arctic warms and undergoes a similar transformation, its capacity to act as a global carbon sink might diminish, potentially accelerating climate change in a feedback loop.

The study provides a rare and detailed look at how a complex marine ecosystem responds to rapid warming, using a time when the Earth was naturally warmer than it is today. It confirms that the Arctic is not just a passive victim of climate change but an active participant that can undergo rapid, system-wide reorganization. The invasion of temperate species, the disruption of the food web, and the potential reduction in carbon storage efficiency all point to a future where the Arctic Ocean looks and functions very differently than it does now. By reading the genetic history locked in the mud, scientists have found a powerful analogue for what might happen in the coming decades. The Arctic is changing, and the lessons from the past suggest that these changes will ripple through the entire global climate system, altering the balance of life and the chemistry of the air we breathe.