Earth sciences: Interactions between ice and fire may affect ocean oxygen

Volcanic eruptions triggered by retreating ice sheets may have caused low oxygen levels in the North Pacific during the last deglaciation period (approximately 17,000–10,000 years ago) reports a Nature paper. The findings highlight the complex coupling between different earth systems that can affect marine ecosystems.

Deoxygenation of the subsurface ocean is projected to increase with global warming trends and have strong impacts on marine ecosystems, which rely on oxygen to survive, especially in areas of the North and East Pacific that have low levels of oxygen. However, identifying the mechanisms that trigger and sustain past long-term deoxygenation has been challenging due to variability from year to year.

Jianghui Du and colleagues examined seafloor sediment from two sites in the Gulf of Alaska, Northeast Pacific, to generate high-resolution records of ocean oxygenation. They found that initial deoxygenation in the North Pacific immediately after the Cordilleran ice sheet retreat during the last deglaciation period was associated with increased volcanic ash in seafloor sediments. This finding suggests that ice sheet melting triggered volcanic eruptions, caused by changes to stresses within the crust as it rises up after being pushed down by the ice. The iron in volcanic ash fertilized the region of ocean, fuelling organism productivity and leading to sustained deoxygenation. The authors went on to identify older deoxygenation events linked to ice sheet retreat over the past 50,000 years.

In an accompanying News & Views piece, Weiqi Yao and Ulrich Wortmann point out that today, loss of oxygen from some areas of the ocean may affect the world's most-extensive and least-explored oceanic ecosystems, with unknown consequences for food security. They add that the work from Du and colleagues “points to the urgent need for improved understanding of how biogeochemical feedbacks affect the health of oceans across the globe.”