Ice core methanesulfonic acid from the Begguya (Mount Hunter) plateau, Denali National Park, Alaska, 2013

An industrial-era drop in Greenland ice core methanesulfonic acid is thought to herald a collapse in North Atlantic marine phytoplankton stocks related to a weakening of the Atlantic meridional overturning circulation. In contrast, stable levels of marine biogenic sulfur production contradict this interpretation, and point to changes in atmospheric oxidation as a potential cause of the methanesulfonic acid decline. However, the impact of oxidation on methanesulfonic acid production has not been quantified, nor has this hypothesis been rigorously tested. Here we present a multi-century methanesulfonic acid record from the Denali, Alaska, ice core, which shows a methanesulfonic acid decline similar in magnitude but delayed by 93 years relative to the Greenland record. Box model results using updated dimethyl sulfide oxidation pathways indicate that oxidation by pollution-driven nitrate radicals has suppressed atmospheric methanesulfonic acid production, explaining most, if not all, of Denali’s and Greenland’s methanesulfonic acid declines without requiring a change in phytoplankton production. The delayed timing of the North Pacific methanesulfonic acid decline, relative to the North Atlantic, reflects the distinct history of industrialization in upwind regions and is consistent with the Denali and Greenland ice core nitrate records. These results demonstrate that multidecadal trends in industrial-era Arctic ice core methanesulfonic acid reflect rising anthropogenic pollution rather than declining marine primary production.