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.

格陵兰冰芯中记录的工业时代甲基磺酸（methanesulfonic acid, MSA）浓度下降，被学界认为预示着北大西洋海洋浮游植物生物量的崩溃，该现象与大西洋经向翻转环流（Atlantic meridional overturning circulation, AMOC）的减弱存在关联。与之相反，海洋生源硫生成量的稳定水平与该解释相悖，表明大气氧化过程的变化可能是甲基磺酸浓度下降的潜在诱因。然而，目前尚未量化氧化作用对甲基磺酸生成的影响，该假说也未经过严格的科学验证。本研究基于阿拉斯加德纳里峰冰芯，构建了一段长达数百年的甲基磺酸浓度序列，其记录的甲基磺酸浓度下降幅度与格陵兰冰芯记录相近，但相较格陵兰记录延迟了93年。采用更新后的二甲基硫（dimethyl sulfide, DMS）氧化路径开展的箱式模型模拟结果显示，受人为污染驱动生成的硝酸根自由基的氧化作用，抑制了大气中甲基磺酸的生成。该机制无需改变浮游植物种群规模，即可解释德纳里峰与格陵兰冰芯中绝大部分（若并非全部）的甲基磺酸浓度下降现象。北太平洋甲基磺酸浓度下降相较于北大西洋出现延迟，这一特征反映了上风区域截然不同的工业化历程，同时与德纳里峰和格陵兰冰芯的硝酸根浓度记录相吻合。本研究结果表明，工业时代北极冰芯中甲基磺酸浓度的年代际变化趋势，反映的是人为污染的加剧，而非海洋初级生产的衰退。