Sea-Ice-Driven Mercury Depletion over the Arctic Marginal Ice Zone Revealed by Direct Shipborne Observations

The Arctic is undergoing rapid environmental change, with sea-ice loss altering key exchanges among the cryosphere, atmosphere, and biosphere. During spring, atmospheric mercury and ozone depletion events (AMDEs and ODEs) drive rapid declines in gaseous elemental mercury (Hg 0 ) and ozone (O 3 ), promoting mercury deposition to snow, ice, and marine ecosystems. Despite three decades of research, AMDEs have been documented almost exclusively at coastal land-based stations, leaving their occurrence and mechanisms over oceanic sea-ice poorly constrained. To address this gap, we conducted a research cruise in the Arctic marginal ice zone to study the connections between sea-ice and atmospheric Hg chemistry. We measured atmospheric Hg 0, reactive gaseous Hg (RGM), and O 3 along the cruise track, identifying the simultaneous occurrences of AMDEs and ODEs. Air mass back-trajectory analysis combined with sea-ice images from satellite remote sensing revealed that these depletion events occurred exclusively when air masses originated from over sea-ice and were near its surface. Our findings confirm that Arctic sea-ice plays a pivotal role in initiating AMDEs observed in the marginal ice zone, driven by halogen release from the sea-ice surface. This study advances understanding of the interplay between sea-ice dynamics and atmospheric chemistry, with important implications for Hg deposition in polar ecosystems and the consequences for human and wildlife health, given the potential impacts of a rapidly changing Arctic.