(Supplementary Table 1) Mercury isotope values for common and thick-billed murre (Uria aalge, Uria lomvia) eggs from Alaska

Atmospheric deposition of mercury to remote areas has increased threefold since pre-industrial times. Mercury deposition is particularly pronounced in the Arctic. Following deposition to surface oceans and sea ice, mercury can be converted into methylmercury, a biologically accessible form of the toxin, which biomagnifies along the marine food chain. Mass-independent fractionation of mercury isotopes accompanies the photochemical breakdown of methylmercury to less bioavailable forms in surface waters. Here we examine the isotopic composition of mercury in seabird eggs collected from colonies in the North Pacific Ocean, the Bering Sea and the western Arctic Ocean, to determine geographical variations in methylmercury breakdown at northern latitudes. We find evidence for mass-independent fractionation of mercury isotopes. The degree of mass-independent fractionation declines with latitude. Foraging behaviour and geographic variations in mercury sources and solar radiation fluxes were unable to explain the latitudinal gradient. However, mass-independent fractionation was negatively correlated with sea-ice cover. We conclude that sea-ice cover impedes the photochemical breakdown of methylmercury in surface waters, and suggest that further loss of Arctic sea ice this century will accelerate sunlight-induced breakdown of methylmercury in northern surface waters.

自工业化前时代以来，偏远地区的大气汞沉降量已增长至原来的三倍，其中北极区域的汞沉降现象尤为显著。汞沉降至表层海洋与海冰后，可转化为甲基汞（methylmercury）——一种具备生物可利用性的毒素形态，并沿海洋食物链发生生物放大效应。在表层水体中，甲基汞经光化学分解为生物可利用性更低的形态时，会伴随汞同位素的非质量分馏（mass-independent fractionation）过程。本研究对采集自北太平洋、白令海及北冰洋西部海鸟繁殖群落的海鸟卵中的汞同位素组成展开分析，以此探究北纬地区甲基汞分解过程的地理差异。研究证实了汞同位素非质量分馏现象的存在，且该分馏程度随纬度升高而逐渐减弱。觅食行为、汞来源与太阳辐射通量的地理差异均无法解释这一纬度梯度特征，但非质量分馏程度与海冰覆盖面积呈负相关关系。据此我们认为，海冰覆盖会阻碍表层水体中甲基汞的光化学分解，并提出：本世纪北极海冰的进一步消融，将加速北纬表层水体中阳光诱导的甲基汞分解进程。