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.

学界普遍认为，工业时代格陵兰冰芯记录的甲磺酸（methanesulfonic acid）浓度下降，预示着北大西洋海洋浮游植物种群因大西洋经向翻转环流（Atlantic meridional overturning circulation）减弱而出现崩溃。与之相悖的是，海洋生源硫生成量保持稳定，这与前述解读相矛盾，并提示大气氧化过程的变化可能是甲磺酸浓度下降的潜在诱因。然而，目前尚未量化大气氧化过程对甲磺酸生成的影响，该假说也未经过严格检验。
本研究基于阿拉斯加迪纳利冰芯，构建了一段跨越数个世纪的甲磺酸记录，该记录显示甲磺酸浓度下降幅度与格陵兰冰芯记录相近，但较格陵兰记录滞后93年。采用更新后的二甲基硫（dimethyl sulfide）氧化路径开展的箱式模型（box model）模拟结果表明：由污染驱动的硝酸根自由基（nitrate radicals）氧化作用抑制了大气中甲磺酸的生成，这一机制可以解释迪纳利与格陵兰冰芯中绝大多数（即便不是全部）甲磺酸浓度下降现象，而无需假定浮游植物种群数量发生变化。
相较于北大西洋区域，北太平洋甲磺酸浓度下降的时间滞后，这一现象反映了上风区域工业化进程的独特历史，且与迪纳利和格陵兰冰芯的硝酸根记录相一致。本研究结果证实：工业时代北极冰芯中观测到的甲磺酸多年代际变化趋势，反映的是人为污染加剧而非海洋初级生产水平下降。