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.

Models that inform how magma moves and volcanic eruptions unfold may need an update, according to a new study. It reports that gas bubbles in magmas can form through the mechanical forces of shear as magmas flow and deform–  a new physical mechanism for magma bubble nucleation that challenges conventional degassing models. The formation of gas bubbles within magma – also known as nucleation – is a fundamental process that shapes how volcanic eruptions unfold. The timing and rate at which these bubbles appear and expand influences key magma features, including its buoyancy, viscosity, and explosive potential. Understanding nucleation is therefore vital for building accurate models of magma movement and predicting eruptive styles. Traditionally, bubble nucleation has been attributed mainly to depressurization as magma rises to the surface, which causes dissolved gases like water vapor and carbon dioxide (CO₂) to separate from the mix. Although these bubbles can form spontaneously, it’s thought that they usually form more easily on tiny mineral crystals within the magma, which act as microscopic catalysts for nucleation within magmatic reservoirs and conduits.

Here, Olivier Roche and colleagues investigated a new way that gas bubbles might form in magma. Instead of focusing on decompression as the trigger, Roche et al. propose that the mechanical energy from shear forces, created as magmas move, can also drive bubble nucleation. In a series of laboratory experiments, the authors observed bubble formation in a pressurized molten polymer infused with dissolved CO₂ – an analog for magma – as different shear rates were applied to the liquid. They found that, while some bubbles formed naturally, most bubbles formed after shear was applied, especially in regions where shear stress was the strongest, and that the required shear stress needed to trigger nucleation decreased as dissolved CO₂ content within the liquid increased. Overall, the findings demonstrate that viscous shear – a mechanical force common in flowing magmas – can supply the energy needed to trigger bubble formation without a reduction in pressure. Moreover, sudden mechanical shocks to the liquid resulted in rapid, widespread nucleation, further illustrating that deformation and motion within magma can actively drive bubble formation. Roche et al. use theoretical and computational models to show that shear-induced nucleation can occur naturally in volcanic conduits, especially in high-viscosity magmas, and that it may be enhanced by magma decompression during ascent. According to the authors, shear could promote efficient degassing and explain why highly-viscous, gas-rich magmas can sometimes erupt quietly and effusively, and without explosive fragmentation.