Mercury content and isotopic composition of frost flower, surface snow and brine samples near Barrow, Alaska@en

Frost flowers are ice crystals that grow on refreezing sea ice leads in Polar Regions by wicking brine from the sea ice surface and accumulating vapor phase condensate. These crystals contain high concentrations of mercury (Hg) and are believed to be a source of reactive halogens, but their role in Hg cycling and impact on the fate of Hg deposited during atmospheric mercury depletion events (AMDEs) are not well understood. We collected frost flowers growing on refreezing sea ice near Barrow, Alaska (U.S.A.) during an AMDE in March 2009 and measured Hg concentrations and Hg stable isotope ratios in these samples to determine the origin of Hg associated with the crystals. We observed decreasing Delta199Hg values in the crystals as they grew from new wet frost flowers (mean Delta199Hg = 0.77 ± 0.13 per mil, 1 s.d.) to older dry frost flowers (mean Delta199Hg = 0.10 ± 0.05 per mil, 1 s.d.). Over the same time period, mean Hg concentrations in these samples increased from 131 ± 6 ng/L (1 s.d.) to 180 ± 28 ng/L (1 s.d.). Coupled with a previous study of Hg isotopic fractionation during AMDEs, these results suggest that Hg initially deposited to the local snowpack was subsequently reemitted during photochemical reduction reactions and ultimately accumulated on the frost flowers. As a result of this process, frost flowers may lead to enhanced local retention of Hg deposited during AMDEs and may increase Hg loading to the Arctic Ocean.霜花（Frost flowers）是极地地区重新冻结海冰裂缝中，通过毛细作用从海冰表面汲取盐水并累积气相凝结物而生长形成的冰晶。这类冰晶含有高浓度的汞（Hg），被认为是活性卤素的来源之一，但其在汞循环中的作用，以及对大气汞损耗事件（Atmospheric Mercury Depletion Events, AMDEs）期间沉积汞归趋的影响尚不明确。我们于2009年3月的一次大气汞损耗事件期间，在美国阿拉斯加州巴罗附近的重新冻结海冰上采集了生长的霜花样本，并测定了样本中的汞浓度与汞稳定同位素比值，以明确与该晶体结合的汞的来源。我们观察到，随着霜花从新生湿润霜花（平均Δ¹⁹⁹Hg（Delta199Hg）= 0.77 ± 0.13‰，1倍标准差（1 s.d.））发育为老化干燥霜花（平均Δ¹⁹⁹Hg = 0.10 ± 0.05‰，1倍标准差（1 s.d.）），其晶体中的Δ¹⁹⁹Hg值逐渐降低。同期，样本中的平均汞浓度从131 ± 6 ng/L（1倍标准差（1 s.d.））升至180 ± 28 ng/L（1倍标准差（1 s.d.））。结合此前关于大气汞损耗事件期间汞同位素分馏的研究，本研究结果表明：最初沉积在当地积雪层中的汞，随后在光化学还原反应中被重新释放，并最终在霜花上累积。该过程使得霜花可能增强大气汞损耗事件中沉积汞的本地留存能力，并提升北冰洋（Arctic Ocean）的汞负载量。