Editorial: Novel Isotope Systems and Biogeochemical Cycling During Cryospheric Weathering in Polar Environments

Clinger et al., 2016). This research topic explores some of the developments in high-latitude ﬁ eld and experimental studies that utilize such geochemical tools to trace the degree and nature of weathering reactions that play a critical role in carbon cycling. The nine contributions to the research topic involve the analysis of traditional (C, N, S, O) and non-traditional (Mg, Li, Si, Ge) isotopes from different samples types such as river waters, lake waters, rocks, sediments, or mineral separates from locations both in the Northern (Greenland, Iceland, Canada, Svalbard) and Southern Hemisphere (Patagonia, Antarctica). Two papers use isotope geochemistry to explore organic and inorganic carbon cycling within permafrost and active layer soils. Jones et al. show that biogeochemical processes and decomposition pathways of organic carbon in ice-wedge polygons in Svalbard are dependent upon water and organic carbon content. Sulfur ( δ 34 S) and oxygen ( δ 18 O) isotopes show that iron and sulfate reduction processes dominate in water saturated, high organic carbon environments, whereas sul ﬁ de oxidation dominates in drier areas with less organic carbon. Zolkos and Tank use an experimental approach in combination with stable carbon isotopes ( δ 13 CO 2 ) to show that carbonate weathering coupled with sul ﬁ de oxidation in recently or previously unthawed Canadian permafrost sediments is a net source of inorganic CO 2 to the atmosphere, albeit partly counterbalanced by carbonate buffering.

Two papers add to the growing body of work combining different isotope systems to understand subglacial weathering processes under the Greenland Ice Sheet. New measurements of magnesium (δ 26 Mg) and lithium (δ 7 Li) isotopes by Hindshaw et al. (2014) from the Leverett Glacier catchment, together with a number of radiogenic isotope systems, highlight the importance of the preferential dissolution of silicate and carbonate phases on chemical weathering and carbon cycling in meltwaters. Baronas et al. present new germanium isotope (Δ 74 Ge) measurements of glacial meltwaters from the Watson River catchment, whichtogether with batch dissolution experiments -reveal that meltwaters are isotopically light and contrasting germanium fractionation mechanisms occur during subglacial dissolutionprecipitation weathering reactions when compared to its "geochemical twin," silicon.
Previous studies have shown that stable silicon isotopes (δ 30 Si) are isotopically lighter in glacial rivers compared to non-glacial counterparts as a result of subglacial physical crushing and/or precipitation-dissolution reactions involving amorphous silica phases (Hatton et al., 2019). To date, these studies have only focused on Arctic glacial and river systems. Pryer et al. detail the first δ 30 Si measurements across a climate gradient from non-glaciated to glaciated catchments from Chilean Patagonia. Using careful filtration methods, a colloidalsize silica phase comprised predominantly of feldspars was identified in isotopically light, glacial-fed rivers. Not only does this add another mechanism for explaining variation in silica export and isotope mass balance in Arctic glaciated environments, but likely has consequences for other isotope systems.
Glacial meltwaters in the McMurdo Dry Valleys (MDVs) in Antarctica originate from cold-based glaciers in one of the driest places on Earth. Combined δ 30 Si and Ge/Si measurements from Crescent Stream in MDVs (Hirst et al.) reflect the formation of amorphous silica phases and secondary clays in an extensive hyporheic zone during freeze-thaw cycles. Interestingly, these data highlight that rivers draining cold-based glaciers have higher δ 30 Si compositions than rivers draining polythermalbased glaciers and similar δ 30 Si compositions to rivers draining permafrost in the Arctic. Hatton et al. present δ 30 Si measurements from Commonwealth Stream in MDVs that show the opposite relationship with dissolved silicon concentrations to that observed in rivers draining polythermal glaciers in the Arctic or Antarctic. Such contrasting behavior highlights the different origins of meltwater, with cold-based glaciers characterized by a higher proportion of supraglacial relative to subglacial melt, and dominance of in-stream weathering. The MDVs are also explored by Diaz et al., who use stable isotopes of carbon, nitrogen and sulfur to investigate the transport of salts from the Central Transantarctic Mountains to the highly saline soils in the MDVs. Their findings show that regional variations in stable isotopes reflect both water availability and different atmospheric inputs of sulfates and nitrates, and evaporation-precipitation of carbonates.
Polar lakes are key sites of further chemical alteration of cryospheric weathering products, biological productivity and nutrient uptake. Pogge von Strandmann et al. present δ 26 Mg records from a groundwater-fed Icelandic lake, fed by both cold and hot springs. Magnesium isotopes are fractionated by changes in lake temperature, as well as a strong seasonal signal due to biogenic calcium carbonate production in summer.
This research topic highlights the important role that isotope geochemistry plays in understanding the mechanistic links between cryospheric processes and climate-weathering feedbacks. Looking to the future, combining multiple isotopic systems with inter-disciplinary approaches will provide insight on the different aspects of the weathering processes occurring under and around ice sheets, and within permafrost. Furthermore, encouraging wider collaborations and standardized field sampling and experimental methodologies (e.g., Bouchard et al., 2020), will allow the geochemical community to better use the large number of existing and complementary datasets from the high-latitude regions to understand cryospheric weathering in polar environments.