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

Cryospheric weathering processes in permafrost and glaciated environments play an essential role in carbon cycling within the Earth system. Chemical weathering of silicate, carbonate and sulfidebearing rocks releases cations and anions that can consume (or release) atmospheric carbon dioxide (CO2), as well as biologically important nutrients such as phosphorous, iron and silicon, which can impact downstream ecosystems (Figure 1). How these cryospheric weathering processes will respond to future climate-driven changes in permafrost thaw and glacial melt is difficult to predict due to the role of complex forcing mechanisms and feedbacks. Isotope geochemistry utilizes changes in the relative abundance of different isotopes due to physical, chemical and biological reactions, allowing some of the complexities of cryospheric weathering processes to be unpicked. In recent years, there has been an explosion in the range of stable and radiogenic isotope systems used for the study of high-latitude environments, including isotopes of major elements such as carbon, oxygen, and silicon (e.g., Opfergelt et al., 2013; Kutscher et al., 2017), and trace metal isotopes such as strontium (Hindshaw et al., 2014), lithium (Murphy et al., 2019), iron (Zhang et al., 2015), uranium-series (e.g., Arendt et al., 2018) and rare earth elements (e.g., Clinger et al., 2016). This research topic explores some of the developments in high-latitude field 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 (δ34S) and oxygen (δ18O) isotopes show that iron and sulfate reduction processes dominate in water saturated, high organic carbon environments, whereas sulfide oxidation dominates in drier areas with less organic carbon. Zolkos and Tank use an experimental approach in combination with stable carbon isotopes (δ13CO2) to show that carbonate weathering coupled with sulfide oxidation in recently or previously unthawed Canadian permafrost sediments is a net source of inorganic CO2 to the atmosphere, albeit partly counterbalanced by carbonate buffering

Strontium isotopes trace the dissolution and precipitation of mineral organic carbon interactions in thawing permafrost

Interactions between minerals and organic carbon (OC) in soils are key to stabilize OC and mitigate greenhouse gas emissions upon permafrost thaw. However, changes in soil water pathways upon permafrost thaw are likely to affect the stability of mineral OC interactions by inducing their dissolution and precipitation. This study aims to assess and quantify how mineral OC interactions are affected by dissolution and precipitation in thawed relative to unthawed layers. We hypothesize that a change in the radiogenic strontium (Sr) isotopic ratio (87Sr/86Sr) involved in mineral OC interactions upon changing water saturation conditions implies a destabilization of the mineral OC interaction. We quantified mineral OC interactions using selective extractions in soils facing gradual thaw (Eight Mile Lake, AK, USA) and in sediments with a thawing history of abrupt thaw (Duvanny Yar, Russia), and we measured the 87Sr/86Sr ratio of the selective extracts targeting the Sr associated to mineral OC interactions. Firstly, for water saturated layers with a higher proportion of mineral OC interactions, we found a difference in the 87Sr/86Sr ratio relative to the surrounding layers, and this supports the preservation of a Sr “stable” pool in these mineral OC interactions. We estimated that a portion of these mineral OC interactions have remained undissociated since their formation (between 4% and 64% by Sr isotope mass balance). Secondly, we found no difference in 87Sr/86Sr ratio between layers accumulating Fe oxides at redox interfaces regularly affected by water table changes (or upon thermokarst processes) relative to surrounding layers. This supports the dominance of a Sr “labile” pool inherited from processes of dissolution and precipitation of the mineral OC interactions. Thirdly, our estimations based on a Sr isotope mass balance support that, as a consequence of permafrost thaw, a larger proportion of Sr from primary mineral weathering (>80%) controls the Sr in mineral OC interactions in the saturated zone of deeply thawed soils relative to poorly thawed soils (∼50%). In conclusion, we found that the radiogenic Sr isotope method, applied for the first time in this context, is promising to trace dissolution-precipitation processes of mineral OC interaction in thawing permafrost

Iron and silicon isotope behaviour accompanying weathering in Icelandic soils, and the implications for iron export from peatlands

peer reviewe

Distribution of mineral constituents in Yedoma permafrost: implications for Yedoma formation

Ice-rich permafrost deposits such as Yedoma are highly sensitive to thaw and given that they contain up to one third of the organic carbon content of the Northern circumpolar permafrost region, their degradation is considered to be a potential climate tipping point on Earth. Accurately predicting the impact of climate warming on the fate of organic carbon in Yedoma requires better constraints on the mineral element reserve in these deposits. This study provides evidence for the homogeneity of chemical composition and mineralogy of Yedoma deposits with depth. This suggests that upon deep thaw through thermokarst or thermo-erosion a high reserve in mineral nutrients is likely to be exposed also from deeper deposits

Pulses of sub-ice microbial activity during winter: evidence from nitrate concentrations and silicon isotopes in the Lena River

Large Arctic rivers are key locations for nitrogen processing, which controls the supply of this limiting nutrient to the Arctic Ocean. In a warming Arctic, longer ice-free periods increase riverine productivity and modulate nitrogen consumption and delivery to the ocean. In this study, the annual variability of nitrate concentrations at the Lena River outlet (Samoylov station) was investigated. Significantly higher nitrate concentrations in water were observed sub-ice (winter) than in the open water (summer), and the higher nitrate concentrations follow phases of colder air temperature at the Lena catchment scale (ERA5 reanalysis data). We hypothesize that colder phases result in thicker river ice leading to darker under-ice conditions preferred by nitrifying microbial communities, thereby inducing increasing sub-ice nitrification. We tested this hypothesis using silicon isotopes known to fractionate upon freezing. The high nitrate concentrations in the winter are associated with heavier silicon isotope compositions in river water. This can be explained by the supersaturation and precipitation of amorphous silica preferentially incorporating the lighter silicon isotopes, leaving the water isotopically heavier. Supersaturation of amorphous silica can result from thicker ice formation upon colder air temperature at catchment scale. The silicon isotope data support phases of thicker ice formation, and indirectly support darker sub-ice conditions at the river base creating pulses of increasing nitrification. Our hypothesis is also supported by a change in the value of an index for dissolved organic carbon aromaticity (SUVA) during the colder phases: this suggests that conditions favour the decomposition of dissolved organic matter during periods of thicker river ice. Air temperature, nitrate concentration, silicon isotopes and SUVA are supporting evidence for pulses of sub-ice microbial activity in the river during winter. It follows that decreasing ice cover duration throughout the catchment is likely to decrease winter nitrate fluxes from the Lena River to the Arctic Ocean

Silicon isotopes reveal a non-glacial source of silicon to Crescent Stream, McMurdo Dry Valleys, Antarctica

In high latitude environments, silicon is supplied to river waters by both glacial and nonglacial chemical weathering. The signal of these two end-members is often obscured by biological uptake and/or groundwater input in the river catchment. McMurdo Dry Valleys streams in Antarctica have no deep groundwater input, no connectivity between streams and no surface vegetation cover, and thus provide a simplified system for us to constrain the supply of dissolved silicon (DSi) to rivers from chemical weathering in a glacial environment. Here we report dissolved Si concentrations, germanium/silicon ratios (Ge/Si) and silicon isotope compositions (d30SiDSi) in Crescent Stream, McMurdo Dry Valleys for samples collected between December and February in the 20142015, 20152016, and 20162017 austral seasons. The d30SiDSi compositions and DSi concentrations are higher than values reported in wet-based glacial meltwaters, and form a narrow cluster within the range of values reported for permafrost dominated Arctic Rivers. High d30SiDSi compositions, ranging from C0.90h to C1.39h, are attributed to (i) the precipitation of amorphous silica during freezing of waters in isolated pockets of the hyporheic zone in the winter and the release of Si from unfrozen pockets during meltwater-hyporheic zone exchange in the austral summer, and (ii) additional Si isotope fractionation via long-term Si uptake in clay minerals and seasonal Si uptake into diatoms superimposed on this winter-derived isotope signal. There is no relationship between d30SiDSi compositions and DSi concentrations with seasonal and daily discharge, showing that stream waters contain DSi that is in equilibrium with the formation of secondary Si minerals in the hyporheic zone. We show that d30SiDSi compositions can be used as tracers of silicate weathering in the hyporheic zone and possible tracers of freeze-thaw conditions in the hyporheic zone. This is important in the context of the ongoing warming in McMurdo Dry Valleys and the supply of more meltwaters to the hyporheic zone of McMurdo Dry Valley streams

Much more than carbon: Element stocks in ice-rich permafrost of the Yedoma domain

Soils of the permafrost zone store globally relevant reservoirs of frozen matter, such as organic matter, mineral elements as well as other biogeochemical relevant compounds like contaminants. Besides the well-studied organic carbon (OC), other compounds can become available in active biological and hydrological element cycling as global climate change is warming northern permafrost regions nearly four times faster than the global average. Current heating in Siberia is unprecedented during the past seven millennia, triggering widespread permafrost degradation and collapse. This is especially relevant for our study region, the Yedoma domain. In this region, a large amount of belowground ice is present and the ground can become unstable with warming, allowing the mobilisation of previously frozen sediments with their geochemical element contents. With this presentation, we synthesise recent studies, which have improved the understanding of various frozen stocks. Here, we estimated that the Yedoma domain contains 41.2 Gt of nitrogen (N), which increases the previous estimate for the circumpolar permafrost zone by ~46 %. The highest element stock within the Yedoma domain is estimated for Si (2739 Gt), followed by Al, Fe, K, Ca, Ti, Mn, Zr, Sr, and Zn. The stocks of Al and Fe (598 and 288 Gt, respectively) are in the same order of magnitude as OC (327-466 Gt). Concerning contaminants, we focused on mercury. Using the ratio of mercury to OC (R(HgC), value based on own measurements: 2.57 μg Hg g C−1) and the OC levels from various studies for a first rough estimation of the Hg reservoir, we estimate the Yedoma mercury pool to be ~542,000 tons. In conclusion, we find that deep thaw of the Yedoma permafrost domain and its degradation will bear the potential to change the availability of various elements in active biogeochemical and hydrological cycles in northern regions, which will have the potential to change crucial ecosystem variables and services

More than one third of the organic carbon exposed by the world’s largest thaw slump (Batagay, Siberia) is not directly available for mineralization but geochemically stabilized

Mineral-organic carbon (OC) interactions account for 30 – 80 % of the total permafrost OC pool. Quantifying the nature and controls of mineral-OC interactions is necessary to better assess permafrost-carbon-climate feedbacks. This is particularly true for ice-rich environments that are impacted by rapid thaw and the development of thermokarst landforms. Retrogressive thaw slumps are amongst the most dynamic forms of slope thermokarst and they expand through the years due to the ablation of an ice-rich headwall. These phenomena are important to consider in the permafrost carbon budget since they expose deep OC sometimes tens of thousands of years old that would not have re-entered the modern carbon cycle if these disturbances had not occurred. Here, we analyzed sediment samples collected from the headwall of the Batagay megaslump, East Siberia, locally reaching 55 m high. The series of discontinuous deposits comprises also older sediment up to ~650 ka old. We present total element concentrations, mineralogy, and mineral-OC interactions in the different stratigraphic units. The mineralogy in the deposits is very similar across the sedimentary series. Our data show that up to 34 ± 8 % of the total OC pool is stabilized by mineral-OC interactions. For most of the analyzed samples, associations to poorly crystalline iron oxides do not have a significant role in OC stabilization. Hypothesizing a retreat rate of 26000 m²/yr and constant thickness of stratigraphic units within the headwall, we provide a first order estimate of ~2 × 10^7 kg of OC is exported annually downslope of the headwall, with ~ 38 % being geochemically stabilized by complexation with metals or associations to poorly crystalline iron oxides. These data support that more than one third of the organic carbon exposed by this massive thaw slump is not directly available for mineralization, but rather stabilized geochemically

More than carbon: Frozen element inventories in ice-rich Yedoma permafrost

Soils of the permafrost zone store globally relevant reservoirs of frozen matter, such as organic matter, mineral elements as well as other biogeochemical relevant compounds like contaminants. Besides well-studied organic carbon (OC), other compounds can become available in active biological and hydrological element cycling as global climate change is warming northern permafrost regions nearly four times faster than the global average. Current heating in Siberia is unprecedented during the past seven millennia, triggering widespread permafrost degradation and collapse. This is especially relevant for our study region, the Yedoma domain. In this region, a large amount of belowground ice is present and the ground can become unstable with warming, allowing the mobilisation of previously frozen sediments with their geochemical element contents. With this presentation, we want to synthesise recent studies, which have improved the understanding of various frozen stocks. Here, we estimated that the Yedoma domain contains 41.2 Gt of nitrogen, which increases the previous estimate for the circumpolar permafrost zone by ~46%. The highest element stock within the Yedoma domain is estimated for r Si (2739 Gt), followed by Al, Fe, K, Ca, Ti, Mn, Zr, Sr, and Zn. The stocks of Al and Fe (598 and 288 Gt) are in the same order of magnitude as OC (327–466 Gt). Concerning contaminants, we focused on mercury. Using the ratio of mercury to OC (RHgC, our found value: 2.57 μg Hg g C−1) and the OC levels from various studies for a first rough estimation of the Hg reservoir, we estimate the Yedoma mercury pool to be ~542000 tons. In conclusion, we find that deep thaw of the Yedoma permafrost domain and its degradation will bear the potential to change the availability of various elements in active biogeochemical and hydrological cycles, which will have the potential to change crucial ecosystem variables and services