Osmium concentration and isotope composition at ultra-low levels in polar ice and snow

The abundances of Platinum Group Elements (PGEs: Ruthenium, Rhodium, Palladium, Osmium, Iridium, and Platinum) are high in meteorites and extremely low in terrestrial rocks and water and accumulations of mainly platinum and iridium in ancient polar archives have been argued to trace terrestrial (continental/volcanic dust) and extra-terrestrial sources. The PGE concentration data, however, lack specificity. For example, the extent to which terrestrial dust compared to cosmic dust has contributed to the PGE inventory of polar ice cannot be readily evaluated from the PGE concentration data alone. Since the osmium isotopic compositions (R(187Os/188Os) ratio) of terrestrial (= 1.40 ± 0.30) and extraterrestrial/volcanic sources (= 0.13) are distinctly different from each other, osmium isotopic composition has the potential to elucidate relative contributions from these sources in ancient polar ice. However, the determination of osmium isotopes in polar ice core archives is challenging due to extremely low concentrations (∼10E-15 g g−1), and due to the availability of small sample sizes (tens of grams). The main objective of this study is to develop a highly sensitive procedure that allows accurate and precise determination of osmium concentration and isotope composition using ~50 g of melted Greenland ice or snow. By substantially improving previously established clean lab chemistry and high sensitivity mass spectrometry we analyzed snow collected from Summit, Greenland during 2009, 2014, and 2017. We find that the average osmium concentration of the snow is 0.459 ± 0.018 (95% C.I.) fg g−1 corresponding to an osmium flux of 0.0579 ± 0.0023 (95% C.I.) fmol cm−2 yr−1. The average R(187Os/188Os) ratio of the Summit snow is 0.264 ± 0.026 (95% C.I.). Assuming that the volcanic source is negligible, the average ratio indicates that about 0.0518 ± 0.0040 (95% C.I.) fmol cm−2 yr−1 of osmium is of cosmic derivation, corresponding to an accretion rate of extra-terrestrial osmium to the Earth of 264 ± 21 mol yr−1. This assessment is similar to the present-day accretion rate of extra-terrestrial osmium to the Earth determined by previous studies. Because of its sensitivity our procedure can be extended to study changes in the accretion of extra-terrestrial osmium over the last several hundred thousand years using samples of ice core. The data contains osmium concentration and isotope composition of (1) reagents and procedural blanks of two Osmium purification methods (Table 1), (2) Antarctic sea snow, sea water, and NEEM firn for method comparison (Table 2), and finally (3) modern snow from Summit, Greenland for sample application (Table 3).铂族元素（Platinum Group Elements, PGEs：钌、铑、钯、锇、铱、铂）在陨石中丰度较高，而在陆相岩石与水体中丰度极低。古极地档案中以铂和铱为主的物质富集被认为可用于示踪陆源（大陆/火山尘埃）与地外来源。然而，铂族元素浓度数据缺乏特异性。例如，仅依靠铂族元素浓度数据，无法直接评估陆源尘埃与宇宙尘埃对极地冰中铂族元素储量的贡献比例。由于陆源（1.40 ± 0.30）与地外/火山源（0.13）的锇同位素组成（187Os/188Os比值）差异显著，锇同位素组成可用于阐明古极地冰中上述两种来源的相对贡献。但极地冰芯档案中的锇同位素测定极具挑战：一是样品中锇浓度极低（约10^-15 g·g^-1），二是样品量仅为数克级别。本研究的核心目标是开发一套高灵敏度分析流程，可利用约50 g融化的格陵兰冰/雪样品，精准测定锇的浓度与同位素组成。通过大幅优化现有洁净实验室化学前处理方法与高灵敏度质谱技术，我们对2009、2014及2017年采集自格陵兰冰盖顶峰（Summit）的雪样开展了分析。分析结果显示，雪样的平均锇浓度为0.459 ± 0.018（95%置信区间）fg·g^-1，对应锇通量为0.0579 ± 0.0023（95%置信区间）fmol·cm^-2·yr^-1。格陵兰冰盖顶峰雪样的平均187Os/188Os比值为0.264 ± 0.026（95%置信区间）。假设火山源贡献可忽略不计，该平均比值表明约0.0518 ± 0.0040（95%置信区间）fmol·cm^-2·yr^-1的锇来自宇宙，对应地球接收的地外锇沉积速率为264 ± 21 mol·yr^-1。该估算结果与前人研究测定的当代地球地外锇沉积速率一致。得益于该流程的高灵敏度，我们可将其推广至利用冰芯样品研究过去数十万年间地外锇沉积速率的变化。本数据集包含以下三类样品的锇浓度与同位素组成数据：（1）两种锇纯化方法的试剂与流程空白（表1）；（2）用于方法比对的南极海雪、海水及东北格陵兰冰芯钻探（NEEM）粒雪样品（表2）；（3）用于样品应用验证的格陵兰冰盖顶峰现代雪样（表3）。