Delineation of groundwater provenance in Arctic environment using isotopic compositions of water and sulphate

Abstract

Little is known about water–rock interaction related to deeper groundwater flows along tectonic discontinuities in the Arctic glacial catchments. Therefore, we analyzed oxygen and sulfur isotope compositions (δ18O, δ34S) of dissolved sulfate (SO42−) to better understand this process in groundwater from mineral and thermal springs and subglacial outflows of southern Spitsbergen. Our results show that coupled δ18O-δ34S of SO42− is a useful measure of groundwater provenance and its relation to water–rock interaction along flow paths in different lithologies. In the studied area, groundwater-rock interaction with carbonate and crystalline rocks contributes two different sources of sulfide-derived SO42− with distinctive isotope compositions and leaves similar isotopic signature as in shallow surface environment (e.g., proglacial lakes and streams). The sulfide-derived SO42− from Paleozoic carbonates has lower δ34S (+5‰) and higher δ18O (−5‰) compared to higher δ34S (+10 to +15‰) and more negative δ18O (−10‰) in the Precambrian crystalline rocks. Minor contributions of SO42− are from sea spray. Conversely, direct inflows of seawater appear to be negligible, if any. Moreover, the more negative δ2H and δ18O values (−85 to −70‰ and −12 to −9‰, respectively) suggest that majority of the studied groundwater and surface water is from melting of snow/ice formed in fall/winter months compared to lower contributions from spring/summer precipitation with higher δ2H and δ18O (−65 to −55‰ and −8 to −6‰). Interestingly, the water-soluble SO42− from the Antarctic soils with high δ34S (+12 to +16‰) and negative δ18O (−15 to +5‰) is remarkably similar to the sulfide-derived SO42− from the Precambrian crystalline rocks analyzed by us in southern Spitsbergen. This implies that sulfide oxidation might be a widespread process in polar environments, even under extremely dry conditions as in Antarctica.