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).