A new record of Arctic sea-ice coverage – informed by the slow and steady sedimentation of cosmic dust on the sea floor – reveals that ancient ice waxed and waned with atmospheric warming, not ocean heat, over the last 300,000 years. The findings provide rare insights into how modern melting in the region could reshape the Arctic’s nutrient balance and biological productivity. The Arctic is warming more rapidly than any other region on Earth, driving a precipitous decline in sea ice coverage. This loss not only affects the region’s marine ecosystems and coastal communities, but it also has far-reaching implications on global climate and economics. However, predicting when the Arctic Ocean will become perennially ice-free remains uncertain due in large part to the general lack of long-term sea ice records and the fact that the processes controlling ice loss are not fully understood.

To address this gap and measure the abundance of sea ice over the past 300,000 years, Frank Pavia and colleagues developed a new geochemical technique using two naturally occurring isotopes – extraterrestrial helium-3 (³He_ET) and thorium-230 (²³⁰Th_xs,0) – preserved in Arctic Ocean sediments. Under ice-free conditions, both isotopes settle onto the seafloor at steady, predictable rates, but they originate from very different sources. Helium-3 arrives from space, delivered uniformly to Earth’s surface by a constant influx of cosmic dust particles. In contrast, thorium-231 is produced consistently within the ocean as dissolved uranium decays. During open water conditions, both isotopes accumulate together. However, during periods of sea-ice cover, the deposition of helium-3 is blocked, altering the ratio of the two isotopes accumulating on the sea floor. Pavia et al. use the ratio of these two isotopes in Arctic sediment cores to measure when and where ocean surface was covered by ice in the past. The record shows that during the last ice age, the central Arctic Ocean remained covered by sea ice year-round. As the Arctic began to warm ~15,000 years ago, ice started to retreat, leading to mostly seasonal sea ice during the warm early Holocene. Later, as the global climate cooled again, sea-ice cover expanded once more. According to the authors, these changes were driven mostly by atmospheric warming, rather than ocean temperatures, challenging assumptions that oceanic inflows of warm water dominated past Arctic sea-ice extent. What’s more, Pavia et al. found that sea ice variation was closely coupled with biological nutrient use, suggesting that as sea ice retreats, surface productivity increases. These findings indicate that future reductions in Arctic sea ice are likely to enhance biological nutrient consumption, with implications for long-term marine productivity in a warming Arctic Ocean.

Each year, vast mats of Sargassum spread across the tropical Atlantic, fouling Caribbean coastlines. Analyses of coral drill cores help explain the mechanism that drives these brown algal blooms.

Phosphorus-rich deep water, driven to the surface by winds, promotes nitrogen-fixing cyanobacteria that live in symbiosis with Sargassum algae and supply them with essential nutrients in this nitrogen-poor region.

Understanding how the blooms are caused can improve predictions of Sargassum stranding events