Density and stable oxygen isotope profiles of four snow pits from Berkner Island, Filchner-Ronne Ice Shelf, Antarctica

The ice cap on Berkner Island is grounded on bedrock within the Filchner-Ronne Ice Shelf and is, therefore, expected to be a well-suited place to retrieve long-term ice-core records reflecting the environmental situation of the Weddell Sea region. Shallow firn cores were drilled to 11 m at the two main summits of Berkner Island and analysed in high depth resolution for electrical d.c. conductivity (ECM), stable isotopes, chloride, sulphate, nitrate and methane-sulphonate (MSA). From the annual layering of dD and non-sea-salt (nss) sulphate, a mean annual snow accumulation of 26.6 cm water at the north dome and 17.4 cm water at the south dome are obtained. As a result of ineffective wind scouring indicated by a relatively low near-surface snow density, regular annual cycles are found for all species at least in the upper 4-5 m. Post depositional changes are responsible for a substantial decrease of the seasonal dD and nitrate amplitude as well as for considerable migration of the MSA signal operating below a depth of 3-4 m. The mean chemical and isotopic firn properties at the south dome correspond to the situation on the Filchner-Ronne Ice shelf at a comparable distance to the coast, whereas the north dome is found to be more influenced by maritime air masses. Persistent high sea-salt levels in winter snow at Berkner Island heavily obscure the determination of nss sulphate probably due to sulphate fractionation in the Antartic sea-salt aerosols. Estimated time-scales predict ages at 400 m depth to be ca. 2000 years for the north and ca. 3000 years for the south dome. Pleistocene ice is expected in the bottom 200 and 300 m, respectively.

伯克纳岛（Berkner Island）上的冰帽直接坐落于菲尔希纳-龙尼冰架（Filchner-Ronne Ice Shelf）内部的基岩之上，因此被视为获取反映威德尔海（Weddell Sea）区域环境状况的长期冰芯记录的理想位点。研究团队在伯克纳岛的两个主要穹顶处钻取了深度达11米的浅粒雪芯，并以高深度分辨率对其开展分析，检测指标包括直流电导率（electrical d.c. conductivity, ECM）、稳定同位素、氯离子、硫酸根、硝酸根以及甲烷磺酸（methane-sulphonate, MSA）。通过氘同位素（dD）与非海盐硫酸盐（non-sea-salt, nss sulphate）的年层特征，测算得到北穹顶的年平均积雪积累量为26.6厘米水当量，南穹顶则为17.4厘米水当量。受近地表雪密度相对偏低的影响，该区域风蚀作用较弱，因此至少在上部4-5米深度内，所有检测指标均呈现规则的年周期特征。沉积后作用导致季节性氘同位素与硝酸根振幅显著降低，同时甲烷磺酸信号在3-4米以下深度发生明显迁移。南穹顶的平均化学与同位素粒雪特征，与菲尔希纳-龙尼冰架上距海岸距离相近区域的状况一致；而北穹顶则更易受海洋气团的影响。伯克纳岛冬季积雪中持续偏高的海盐水平极大地干扰了非海盐硫酸盐的测定，这一现象可能与南极海盐气溶胶中的硫酸盐分馏作用有关。基于估算的时间标尺，北穹顶400米深度处的年龄约为2000年，南穹顶则约为3000年；两地底部200米和300米深度分别有望发现更新世（Pleistocene）冰体。