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Introduction

Permafrost (perennially frozen soil layer) contains a large stock of organic carbon (OC; ∼1600 Gt in the northern circumpolar permafrost region; Strauss et al., 2025

Strauss, J., Fuchs, M., Hugelius, G., Miesner, F., Nitze, I., Opfergelt, S., Schuur, E., Treat, C., Turetsky, M., Yang, Y., Grosse, G. (2025) Organic matter storage and vulnerability in the permafrost domain. In: Elias, S. (Ed.) Encyclopedia of Quaternary Science. Third Edition, Elsevier, Amsterdam, 5, 399–410. https://doi.org/10.1016/B978-0-323-99931-1.00164-1

). Upon Arctic warming leading to permafrost thaw, a portion of this carbon is released into the atmosphere as CO₂ and CH₄, making permafrost a net source of greenhouse gases to the atmosphere. More specifically, projections suggest that, by 2100, arctic permafrost could release 55–232 Gt C (Strauss et al., 2025

Strauss, J., Fuchs, M., Hugelius, G., Miesner, F., Nitze, I., Opfergelt, S., Schuur, E., Treat, C., Turetsky, M., Yang, Y., Grosse, G. (2025) Organic matter storage and vulnerability in the permafrost domain. In: Elias, S. (Ed.) Encyclopedia of Quaternary Science. Third Edition, Elsevier, Amsterdam, 5, 399–410. https://doi.org/10.1016/B978-0-323-99931-1.00164-1

). This permafrost carbon feedback is well studied in summer, but the most severe climate amplification occurs in autumn and winter, seasons understudied in permafrost regions (Natali et al., 2019

Natali, S.M., Watts, J.D., Rogers, B.M., Potter, S., Ludwig, S.M., et al. (2019) Large loss of CO₂ in winter observed across the northern permafrost region. Nature Climate Change 9, 852–857. https://doi.org/10.1038/s41558-019-0592-8

; Shogren et al., 2020

Shogren, A.J., Zarnetske, J.P., Abbott, B.W., Iannucci, F., Bowden, W.B. (2020) We cannot shrug off the shoulder seasons: Addressing knowledge and data gaps in an Arctic headwater. Environmental Research Letters 15, 104027. https://doi.org/10.1088/1748-9326/ab9d3c

).

In permafrost regions, the growing season (summer) is surrounded by shoulder seasons, i.e. the periods of active layer thawing before summer and freeze-up after summer (Olsson et al., 2003

Olsson, P.Q., Sturm, M., Racine, C.H., Romanovsky, V., Liston, G.E. (2003) Five Stages of the Alaskan Arctic Cold Season with Ecosystem Implications. Arctic, Antarctic, and Alpine Research 35, 74–81. https://doi.org/10.1657/1523-0430(2003)035[0074:FSOTAA]2.0.CO;2

). More specifically, the late shoulder season integrates late autumn (air temperature 0–10 °C and positive soil temperature) and early winter (negative air temperature and soil temperature at near freezing; Swedish Meteorological and Hydrological Institute-SMHI). The lengthening of the late shoulder season, caused by the delay in freezing of the active layer, is considered to increase greenhouse gas emissions (Rößger et al., 2022

Rößger, N., Sachs, T., Wille, C., Boike, J., Kutzbach, L. (2022) Seasonal increase of methane emissions linked to warming in Siberian tundra. Nature Climate Change 12, 1031–1036. https://doi.org/10.1038/s41558-022-01512-4

). However, the extent to which water–soil interactions and export from soils will change with a warmer and lenghtened late shoulder season remains unclear (Shogren et al., 2020

Shogren, A.J., Zarnetske, J.P., Abbott, B.W., Iannucci, F., Bowden, W.B. (2020) We cannot shrug off the shoulder seasons: Addressing knowledge and data gaps in an Arctic headwater. Environmental Research Letters 15, 104027. https://doi.org/10.1088/1748-9326/ab9d3c

).

Evidence from non-winter studies in permafrost regions shows that an increase in water–soil contact time changes redox conditions, leading to changes in OC stabilisation by minerals (Herndon et al., 2017

Herndon, E., AlBashaireh, A., Singer, D., Roy Chowdhury, T., Gu, B., Graham, D. (2017) Influence of iron redox cycling on organo-mineral associations in Arctic tundra soil. Geochimica et Cosmochimica Acta 207, 210–231. https://doi.org/10.1016/j.gca.2017.02.034

; Patzner et al., 2020

Patzner, M.S., Mueller, C.W., Malusova, M., Baur, M., Nikeleit, V., Scholten, T., Hoeschen, C., Byrne, J.M., Borch, T., Kappler, A., Bryce, C. (2020) Iron mineral dissolution releases iron and associated organic carbon during permafrost thaw. Nature Communications 11, 6329. https://doi.org/10.1038/s41467-020-20102-6

, 2022

Patzner, M.S., Logan, M., McKenna, A.M., Young, R.B., Zhou, Z., Joss, H., Mueller, C.W., Hoeschen, C., Scholten, T., Straub, D., Kleindienst, S., Borch, T., Kappler, A., Bryce, C. (2022) Microbial iron cycling during palsa hillslope collapse promotes greenhouse gas emissions before complete permafrost thaw. Communications Earth & Environment 3, 76. https://doi.org/10.1038/s43247-022-00407-8

; Barczok et al., 2024

Barczok, M., Smith, C., Kinsman-Costello, L., Patzner, M., Bryce, C., Kappler, A., Singer, D., Herndon, E. (2024) Iron transformation mediates phosphate retention across a permafrost thaw gradient. Communications Earth & Environment 5, 635. https://doi.org/10.1038/s43247-024-01810-z

; Rooney et al., 2024

Rooney, E.C., VanderJeugdt, E., Avasarala, S., Miah, I., Berens, M.J., Kinsman-Costello, L., Weintraub, M.N., Herndon, E.M. (2024) Decoupling of redox processes from soil saturation in Arctic tundra. Communications Earth & Environment 5, 746. https://doi.org/10.1038/s43247-024-01927-1

). Up to 80 % of permafrost soil OC interacts with minerals contributing to OC stabilisation (e.g., Mueller et al., 2015

Mueller, C.W., Rethemeyer, J., Kao-Kniffin, J., Löppmann, S., Hinkel, K.M., Bockheim, J.G. (2015) Large amounts of labile organic carbon in permafrost soils of northern Alaska. Global Change Biology 21, 2804–2817. https://doi.org/10.1111/gcb.12876

) with a key role of iron (Kappler et al., 2021

Kappler, A., Bryce, C., Mansor, M., Lueder, U., Byrne, J.M., Swanner, E.D. (2021) An evolving view on biogeochemical cycling of iron. Nature Reviews Microbiology 19, 360–374. https://doi.org/10.1038/s41579-020-00502-7

); this includes aggregation, sorption onto Fe-oxides or complexation with metal ions, such as Fe. We hypothesise that, along a permafrost degradation gradient, the extent of freezing at the late shoulder season drives changes in water pathways, and thereby affects biogeochemical connectivity between soils and from soils to rivers. The objective of this study is to determine to what extent these changes influence Fe and OC release in soil solution and their lateral export to rivers. Silicon isotopes (δ³⁰Si) and Ge/Si ratio can be used (i) to identify conditions for Fe-oxides dissolution or precipitation, since Fe-oxides preferentially adsorb the light Si isotopes (Delstanche et al., 2009

Delstanche, S., Opfergelt, S., Cardinal, D., Elsass, F., André, L., Delvaux, B. (2009) Silicon isotopic fractionation during adsorption of aqueous monosilicic acid onto iron oxide. Geochimica et Cosmochimica Acta 73, 923–934. https://doi.org/10.1016/j.gca.2008.11.014

) and Ge over Si (Anders et al., 2003

Anders, A.M., Sletten, R.S., Derry, L.A., Hallet, B. (2003) Germanium/silicon ratios in the Copper River Basin, Alaska: Weathering and partitioning in periglacial versus glacial environments. Journal of Geophysical Research: Earth Surface 108, 6005. https://doi.org/10.1029/2003JF000026

), and (ii) to trace freezing-driven changes in biogeochemical connectivity (Hirst et al., 2023

Hirst, C., Monhonval, A., Mauclet, E., Thomas, M., Villani, M., Ledman, J., Schuur, E.A.G., Opfergelt, S. (2023) Evidence for late winter biogeochemical connectivity in permafrost soils. Communications Earth & Environment 4, 85. https://doi.org/10.1038/s43247-023-00740-6

; Opfergelt et al., 2024

Opfergelt, S., Gaspard, F., Hirst, C., Monin, L., Juhls, B., Morgenstern, A., Angelopoulos, M., Overduin, P.P. (2024) Frazil ice changes winter biogeochemical processes in the Lena River. Communications Earth & Environment 5, 738. https://doi.org/10.1038/s43247-024-01884-9

), since amorphous silica precipitates under freezing conditions (Dietzel, 2005

Dietzel, M. (2005) Impact of cyclic freezing on precipitation of silica in Me–SiO₂–H₂O systems and geochemical implications for cryosoils and -sediments. Chemical Geology 216, 79–88. https://doi.org/10.1016/j.chemgeo.2004.11.003

), preferentially incorporating the light Si isotopes (Oelze et al., 2015

Oelze, M., von Blanckenburg, F., Bouchez, J., Hoellen, D., Dietzel, M. (2015) The effect of Al on Si isotope fractionation investigated by silica precipitation experiments. Chemical Geology 397, 94–105. https://doi.org/10.1016/j.chemgeo.2015.01.002

) and Si over Ge (Fernandez et al., 2021

Fernandez, N.M., Perez-Fodich, A., Derry, L.A., Druhan, J.L. (2021) A first look at Ge/Si partitioning during amorphous silica precipitation: Implications for Ge/Si as a tracer of fluid-silicate interactions. Geochimica et Cosmochimica Acta 297, 158–178. https://doi.org/10.1016/j.gca.2021.01.007

).

The study site of Stordalen, Sweden (Fig. 1a–e), was selected as a natural gradient of permafrost degradation to collect soil porewater and river water samples during September–November 2021 (see Supplementary Information).

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Detecting the Transition Between Late Autumn and Early Winter

Winter starts when the average daily temperature is below 0 °C for five consecutive days (SMHI). As a result, 18 October 2021 marks the start of early winter (Fig. 1f), with the first snowfall and near freezing soil temperatures (0.3–0.5 ± 0.2 °C; Fig. 1g). In early winter, air temperature ranges between −18 °C and +5.2 °C, leading to increases and decreases in snow cover thickness (maximum of 0.37 m on 27 October 2021; Fig. 1f).

The season transition is also reflected in soil porewater (Fig. 2). The temporal variability of the conductivity during one season is limited (e.g., for one replicate at the palsa site at 30 cm in late autumn: 56.3 ± 5.30 μS cm⁻¹, n = 9) relative to the spatial variability (three replicates separated by less than one meter) for one date (e.g., 19 September for the three replicates of the palsa site at 30 cm: 60.5 ± 27.2 μS cm⁻¹, n = 3). However, for the same site (e.g., palsa at 30 cm), the conductivity is significantly different between the two seasons (p-value < 0.001). This is why, in this study, we investigate the temporal evolution by grouping data by season: late autumn (17 September to 18 October 2021) and early winter (18 October to 11 November 2021).

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Influence of Early Winter Snowmelt on Soil Porewater Along the Permafrost Degradation Gradient

At the palsa site along the profile, there are significant differences between late autumn and early winter in pH (p-value from <0.001 to <0.05—an increase of more than one unit; Figs. 2, 3a), electrical conductivity (a decrease of up to 54 %) and DOC concentrations (a decrease of up to 61 %). At the intermediate and degraded palsa sites, variations in pH, electrical conductivity, DOC and Fe concentrations between late autumn and early winter are depth-specific (Fig. 3b, c). Along the gradient, the DOC concentrations are slightly lower than those reported in summer (2019, 30 cm: palsa = 102 ± 91.9 mg L⁻¹, intermediate = 249 ± 37.8 mg L⁻¹; Patzner et al., 2022

Patzner, M.S., Logan, M., McKenna, A.M., Young, R.B., Zhou, Z., Joss, H., Mueller, C.W., Hoeschen, C., Scholten, T., Straub, D., Kleindienst, S., Borch, T., Kappler, A., Bryce, C. (2022) Microbial iron cycling during palsa hillslope collapse promotes greenhouse gas emissions before complete permafrost thaw. Communications Earth & Environment 3, 76. https://doi.org/10.1038/s43247-022-00407-8

), except for the intermediate site at 30 cm in winter (Fig. 3).

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The significant difference between seasons at the palsa site can be explained by water inputs from early winter snowmelt (Fig. 1f). Snowfall began on 21 October 2021, resulting in a snow cover approximately 40 cm thick. This early winter snow cover melted on 28 October to 8 November 2021 due to an increase in air temperature reaching +5.2 °C, representing 44 mm of snow precipitation between 18 October (first snow) and 27 October 2021 (melting). Snowmelt water infiltration in near freezing soil can dilute the soil porewater, especially in dry well-drained soils (Zhao and Gray, 1999

Zhao, L., Gray, D.M. (1999) Estimating snowmelt infiltration into frozen soils. Hydrological Processes 13, 1827–1842. https://doi.org/10.1002/(SICI)1099-1085(199909)13:12/13<1827::AID-HYP896>3.0.CO;2-D

). The intermediate and degraded palsa sites being up to 30 % wetter than the palsa site (Fig. S-1; Olefeldt and Roulet, 2012

Olefeldt, D., Roulet, N.T. (2012) Effects of permafrost and hydrology on the composition and transport of dissolved organic carbon in a subarctic peatland complex. Journal of Geophysical Research: Biogeosciences 117, G01005. https://doi.org/10.1029/2011JG001819

) can explain the lower influence of water infiltration from snowmelt on the dilution of the soil porewater at these sites. Overall, the snowmelt event in early winter dilutes, more extensively, the soil porewater in dry well-drained soils (palsa).

More specifically, at the intermediate site, a decrease in redox potential (by 30 %) was observed at 30 cm in early winter (−192 ± 6.58 mV) compared to late autumn (−148 ± 30.3 mV; Fig. S-1). At 30 cm, close to the average water table level at this site (Holmes et al., 2022

Holmes, M.E., Crill, P.M., Burnett, W.C., McCalley, C.K., Wilson, R.M., Frolking, S., Chang, K.-Y., Riley, W.J., Varner, R.K., Hodgkins, S.B., IsoGenie Project Coordinators, IsoGenie Field Team, McNichol, A.P., Saleska, S.R., Rich, V.I., Chanton, J.P. (2022) Carbon Accumulation, Flux, and Fate in Stordalen Mire, a Permafrost Peatland in Transition. Global Biogeochemical Cycles 36, e2021GB007113. https://doi.org/10.1029/2021GB007113

), snowmelt is leading to water table rise driving more reducing conditions (Fig. 4a). These redox conditions, combined with a pH decrease of more than one unit in early winter compared to late autumn (Fig. 3b), favour the dissolution of Fe-oxides present in Stordalen soils (Herndon et al., 2020

Herndon, E., Kinsman-Costello, L., Godsey, S. (2020) Biogeochemical Cycling of Redox-Sensitive Elements in Permafrost-Affected Ecosystems. In: Dontsova, K., Balogh-Brunstad, Z., Le Roux, G. (Eds.) Biogeochemical Cycles: Ecological Drivers and Environmental Impact. American Geophysical Union, Washington, D.C., and John Wiley & Sons, Hoboken, NJ, 245–265. https://doi.org/10.1002/9781119413332.ch12

; Patzner et al., 2020

Patzner, M.S., Mueller, C.W., Malusova, M., Baur, M., Nikeleit, V., Scholten, T., Hoeschen, C., Byrne, J.M., Borch, T., Kappler, A., Bryce, C. (2020) Iron mineral dissolution releases iron and associated organic carbon during permafrost thaw. Nature Communications 11, 6329. https://doi.org/10.1038/s41467-020-20102-6

; Barczok et al., 2024

Barczok, M., Smith, C., Kinsman-Costello, L., Patzner, M., Bryce, C., Kappler, A., Singer, D., Herndon, E. (2024) Iron transformation mediates phosphate retention across a permafrost thaw gradient. Communications Earth & Environment 5, 635. https://doi.org/10.1038/s43247-024-01810-z

). This is supported by the 4 times increase in DOC and Fe concentrations in early winter relative to late autumn. The simultaneous decrease in δ³⁰Si and increase in Ge/Si ratio in early winter soil porewater (−0.29 ± 0.03 ‰ for δ³⁰Si and 2.84 μmol mol⁻¹ for Ge/Si) compared to late autumn (0.07 ± 0.15 ‰ for δ³⁰Si and 0.74 ± 0.43 μmol mol⁻¹ for Ge/Si) is an additional support for the dissolution of Fe-oxides releasing the light silicon isotopes and the Ge previously adsorbed onto their surface (Anders et al., 2003

Anders, A.M., Sletten, R.S., Derry, L.A., Hallet, B. (2003) Germanium/silicon ratios in the Copper River Basin, Alaska: Weathering and partitioning in periglacial versus glacial environments. Journal of Geophysical Research: Earth Surface 108, 6005. https://doi.org/10.1029/2003JF000026

; Delstanche et al., 2009

Delstanche, S., Opfergelt, S., Cardinal, D., Elsass, F., André, L., Delvaux, B. (2009) Silicon isotopic fractionation during adsorption of aqueous monosilicic acid onto iron oxide. Geochimica et Cosmochimica Acta 73, 923–934. https://doi.org/10.1016/j.gca.2008.11.014

). Therefore, in early winter, water from snowmelt infiltration, leading to water table rising and more reducing conditions at poorly drained intermediate soils, drives Fe-oxides dissolution and the release of Fe and DOC in soil porewater (Fig. 4a).

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Once in solution, geochemical modelling (Visual Minteq) predicts that Fe exists only as dissolved Fe, with 82 % present as dissolved inorganic Fe and 18 % as dissolved Fe bound to DOC, and that the Fe-DOC associations in solution remain constant between late autumn and early winter (as also observed in palsa and degraded palsa; Fig. S-2). This supports that the increasing Fe and DOC concentrations do not modify the nature of Fe-DOC associations in solution.

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Impact of Early Winter Snowmelt on the Biogeochemical Connectivity From Palsa Soil to River

Evidence for lateral biogeochemical connectivity from the palsa to the degraded palsa. With the lateral water transfer from the palsa site to the degraded palsa site (lower topography) via the intermediate site (Fig. 1), the degraded palsa site receives water from the palsa and intermediate sites and is connected to the rivers (Olefeldt and Roulet, 2014

Olefeldt, D., Roulet, N.T. (2014) Permafrost conditions in peatlands regulate magnitude, timing, and chemical composition of catchment dissolved organic carbon export. Global Change Biology 20, 3122–3136. https://doi.org/10.1111/gcb.12607

; Fouché et al., 2025

Fouché, J., Hirst, C., Bonneville, S., Opfergelt, S., Haghipour, N., Eglinton, T.I., Vonk, J.E., Bröder, L. (2025) Rainfall Impacts Dissolved Organic Matter and Cation Export From Permafrost Catchments and a Glacial River During Late Summer in Northeast Greenland. Permafrost and Periglacial Processes 36, 3–21. https://doi.org/10.1002/ppp.2250

), which can lead to the export of nutrients, such as C and Fe, from soil porewater (Fig. 3) to rivers (Tananaev et al., 2021

Tananaev, N., Isaev, V., Sergeev, D., Kotov, P., Komarov, O. (2021) Hydrological Connectivity in a Permafrost Tundra Landscape near Vorkuta, North-European Arctic Russia. Hydrology 8, 106. https://doi.org/10.3390/hydrology8030106

). The lateral biogeochemical connectivity between these sites can explain the transfer of colloidal Si (from ∼1 nm to ∼0.15 μm) from the palsa to the degraded palsa via the intermediate site in line with the functioning of this site, highlighted by the significantly higher proportion of colloidal Si in soil porewater at 10 cm at the degraded palsa compared to the palsa and intermediate sites (p-value < 0.001; Fig. 3).

Colloidal Si can form in winter (Fig. 4b) and be remobilised in soil porewater following thaw, rain or snowmelt events (Fig. 4c). The formation of colloidal Si in soil porewater at the site is controlled by the freezing-driven amorphous silica precipitation (Dietzel, 2005

Dietzel, M. (2005) Impact of cyclic freezing on precipitation of silica in Me–SiO₂–H₂O systems and geochemical implications for cryosoils and -sediments. Chemical Geology 216, 79–88. https://doi.org/10.1016/j.chemgeo.2004.11.003

), which is associated with a Si isotope fractionation (Oelze et al., 2015

Oelze, M., von Blanckenburg, F., Bouchez, J., Hoellen, D., Dietzel, M. (2015) The effect of Al on Si isotope fractionation investigated by silica precipitation experiments. Chemical Geology 397, 94–105. https://doi.org/10.1016/j.chemgeo.2015.01.002

). The low variability of the δ³⁰Si values in soil porewater with depth at the palsa site relative to the other sites (Fig. 3) suggests that freezing-driven amorphous silica precipitation is the dominant process controlling the Si isotope fractionation at the palsa site, where the depth of summer thaw is equal to the depth of winter freeze (thin active layer 47 ± 2.6 cm), leading to the complete refreezing of the active layer in winter (Lader et al., 2017

Lader, R., Walsh, J.E., Bhatt, U.S., Bieniek, P.A. (2017) Projections of Twenty-First-Century Climate Extremes for Alaska via Dynamical Downscaling and Quantile Mapping. Journal of Applied Meteorology and Climatology 56, 2393–2409. https://doi.org/10.1175/JAMC-D-16-0415.1

). In contrast, when the depth of summer thaw exceeds the depth of winter freeze (deeper active layer of 91 ± 2.3 cm for intermediate site or deeper than 145 cm for degraded palsa site), freezing conditions for amorphous silica precipitation can only be reached in portions of the ground enriched in dissolved Si (a mineral layer) more susceptible to freeze closer to the permafrost. For example, at 60 cm at the intermediate site, the increase in colloidal Si, in Ge/Si ratio and in δ³⁰Si in soil porewater in early winter relative to late autumn result from the amorphous silica precipitation favouring Si over Ge (Fernandez et al., 2021

Fernandez, N.M., Perez-Fodich, A., Derry, L.A., Druhan, J.L. (2021) A first look at Ge/Si partitioning during amorphous silica precipitation: Implications for Ge/Si as a tracer of fluid-silicate interactions. Geochimica et Cosmochimica Acta 297, 158–178. https://doi.org/10.1016/j.gca.2021.01.007

) and favour the light Si isotopes (Oelze et al., 2015

Oelze, M., von Blanckenburg, F., Bouchez, J., Hoellen, D., Dietzel, M. (2015) The effect of Al on Si isotope fractionation investigated by silica precipitation experiments. Chemical Geology 397, 94–105. https://doi.org/10.1016/j.chemgeo.2015.01.002

), leaving the residual soil porewater enriched in Ge and with a heavier δ³⁰Si value (Fig. 3). This is why processes such as mineral weathering in mineral horizons, biological amorphous Si from Si-accumulating plants, Fe oxides dissolution and precipitation (highlighted in the previous section) and groundwater contribution (degraded palsa, see Supplementary Information) can contribute to the larger variability of the δ³⁰Si and the Ge/Si ratio in soil porewater with depth at the intermediate and degraded palsa sites relative to the palsa site (Fig. 3). Overall, the δ³⁰Si, the Ge/Si and the proportion of colloidal Si in soil porewater support the lateral connectivity from the palsa to the degraded palsa sites along the late shoulder season.

Evidence for early winter lateral export from the degraded palsa site to the river. Water infiltration following early winter snowmelt may enhance lateral export of water and dissolved elements from the degraded palsa site to the river (Fouché et al., 2025

Fouché, J., Hirst, C., Bonneville, S., Opfergelt, S., Haghipour, N., Eglinton, T.I., Vonk, J.E., Bröder, L. (2025) Rainfall Impacts Dissolved Organic Matter and Cation Export From Permafrost Catchments and a Glacial River During Late Summer in Northeast Greenland. Permafrost and Periglacial Processes 36, 3–21. https://doi.org/10.1002/ppp.2250

), an hypothesis that we can test by comparing riverine water chemistry between early winter and late autumn (Fig. 3d). The DOC and Fe concentrations in river water significantly increase up to 3 times in early winter, pH significantly decreases in early winter (6.9 ± 0.08) compared to late autumn (7.2 ± 0.20) and electrical conductivity remains constant over the period (86.5 ± 51.1 μS cm⁻¹). The changes in Fe and DOC concentrations and in pH in river water can result from lateral export of soil porewater from the soil to river at the season transition. This is in line with the decrease in Fe and DOC concentrations in soil porewater from the degraded palsa site early winter (60.7 ± 42.8 mg L⁻¹ for DOC; 6.34 ± 5.31 mg L⁻¹ for Fe) relative to late autumn (71.8 ± 41.4 mg L⁻¹ for DOC; 9.15 ± 8.52 mg L⁻¹; Fig. 3c). The higher specific ultraviolet absorbance (SUVA) values in soil porewaters compared to river also support the early winter lateral export from soil to river resulting in higher SUVA values in river water early winter (1.44 ± 0.06 L m⁻¹ mg⁻¹) compared to late autumn (1.09 ± 0.28 L m⁻¹ mg⁻¹; Fig. S-3).

The increase in Fe concentration in rivers early winter, combined with a higher pH in river water relative to soil porewater at the degraded palsa site, and to the more oxic conditions expected in rivers than in soil porewater (e.g., Street et al., 2016

Street, L.E., Dean, J.F., Billett, M.F., Baxter, R., Dinsmore, K.J., Lessels, J.S., Subke, J.-A., Tetzlaff, D., Wookey, P.A. (2016) Redox dynamics in the active layer of an Arctic headwater catchment; examining the potential for transfer of dissolved methane from soils to stream water. Journal of Geophysical Research: Biogeosciences 121, 2776–2792. https://doi.org/10.1002/2016JG003387

; Rooney et al., 2024

Rooney, E.C., VanderJeugdt, E., Avasarala, S., Miah, I., Berens, M.J., Kinsman-Costello, L., Weintraub, M.N., Herndon, E.M. (2024) Decoupling of redox processes from soil saturation in Arctic tundra. Communications Earth & Environment 5, 746. https://doi.org/10.1038/s43247-024-01927-1

), favour the precipitation of Fe-oxides as colloids in river waters. This is supported by the increase in the proportion of colloidal Fe in river water early winter (90 ± 7 %) compared to late autumn (34 ± 39 %; Fig. 3d). This is also supported by the lower Ge/Si ratio in river water early winter (0.28 ± 0.03 μmol mol⁻¹) compared to late autumn (0.36 ± 0.04 μmol mol⁻¹) reflecting the formation of Fe-oxides (colloidal-sized and beyond) and the preferential adsorption of Ge over Si onto their surface (Anders et al., 2003

Anders, A.M., Sletten, R.S., Derry, L.A., Hallet, B. (2003) Germanium/silicon ratios in the Copper River Basin, Alaska: Weathering and partitioning in periglacial versus glacial environments. Journal of Geophysical Research: Earth Surface 108, 6005. https://doi.org/10.1029/2003JF000026

), leaving the residual water depleted in Ge relative to Si. A contribution from the isotopically light colloidal Si exported from the degraded palsa site to the river can contribute to explain the slight decrease in δ³⁰Si in the river water early winter compared to late autumn. Overall, the increase in lateral export from soil to river in early winter leads to release of Fe and DOC into the river, with potential implications on DOC decomposition.

Outlook: consequences of increasing DOC export to Arctic rivers. Here, we show an increase of 32 % of DOC concentrations in the river water in early winter compared to late autumn, a portion of carbon exposed to decomposition. Importantly, a snowmelt event early winter is suggested to contribute to the lateral DOC export between soils and from soils to river. The occurrence of warmer winter and subsequent snowmelt events is expected to increase in the future (Gulev et al., 2021

Gulev, S.K., Thorne, P.W., Ahn, J., Dentener, F.J., Domingues, C.M., Gerland, S., Gong, D., Kaufman, D.S., Nnamchi, H.C., Quaas, J., Rivera, J.A., Sathyendranath, S., Smith, S.L., Trewin, B., von Schuckmann, K., Vose, R.S. (2021) Changing State of the Climate System. In: Masson-Delmotte, V., Zhai, P., Pirani, A., Connors, S.L., Péan, C., Berger, S., Caud, N., Chen, Y., Goldfarb, L., Gomis, M.I., Huang, M., Leitzell, K., Lonnoy, E., Matthews, J.B.R., Maycock, T.K., Waterfield, T., Yelekçi, O., Yu, R., Zhou, B. (Eds.) Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK and New York, NY, USA, 287–422. https://doi.org/10.1017/9781009157896.004

), highlighting the need to consider their consequences for the biogeochemical connectivity of the landscape. The fate of an early winter snowmelt driven pool of DOC in the river depends on subsequent microbial and photodegradation processes when daylight is short (<6 h). The simultaneous Fe release can play a role in driving photodegradation (Bowen et al., 2020

Bowen, J.C., Ward, C.P., Kling, G.W., Cory, R.M. (2020) Arctic Amplification of Global Warming Strengthened by Sunlight Oxidation of Permafrost Carbon to CO₂. Geophysical Research Letters 47, e2020GL087085. https://doi.org/10.1029/2020GL087085

) if Fe-DOC complexes are present. However, Fe is likely transported as Fe (oxy)hydroxides under the pH of the river waters (pH = 6.8–7.5; Fig. 3d), so Fe may either contribute to stabilise DOC co-precipitates in this sub-Arctic river (Hirst et al., 2017

Hirst, C., Andersson, P.S., Shaw, S., Burke, I.T., Kutscher, L., Murphy, M.J., Maximov, T., Pokrovsky, O.S., Mörth, C.-M., Porcelli, D. (2017) Characterisation of Fe-bearing particles and colloids in the Lena River basin, NE Russia. Geochimica et Cosmochimica Acta 213, 553–573. https://doi.org/10.1016/j.gca.2017.07.012

) or contribute to microbial and photodegradation of DOC (Bowen et al., 2020

Bowen, J.C., Ward, C.P., Kling, G.W., Cory, R.M. (2020) Arctic Amplification of Global Warming Strengthened by Sunlight Oxidation of Permafrost Carbon to CO₂. Geophysical Research Letters 47, e2020GL087085. https://doi.org/10.1029/2020GL087085

).

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Conclusions

Our results support that early winter is a key period, triggering changes in soil–water interactions along a natural gradient of permafrost degradation. More specifically, we show that:

Our study highlights the lengthening of biogeochemical processes during early winter, supporting the need to account for the early winter snowmelt driven pool of DOC in river to reduce the uncertainty of the estimates of permafrost carbon export fluxes.

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Acknowledgements

We would like to thank the Swedish Polar Research Secretariat and SITES for the support of the work done at the Abisko Scientific Research Station. SITES is supported by the Swedish Research Council. We are greatful to 'Länsstyrelsen Norrbotten’ for the access to the Stordalens naturreservat. We thank the analytical platform MOCA at UCLouvain, A. Monhonval and E. Mauclet for their contribution to field sampling, and the valuable input from the LandSense team K. Van Oost, V. Vanacker, F. Jonard and S. Lambot. We acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (ERC Starting Grant, WeThaw, n°714617) to S.O, and from the Fonds National de la Recherche Scientifique to S.O., M.V. and E. d.B.A. (FNRS: FC69480, FC49507 and FC54613).

Editor: Andreas Kappler

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Data Availability Statement

The data related to this article is available online at: https://doi.org/10.14428/DVN/WXTVXJ and https://doi.org/10.14428/DVN/I3SOD5.

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References

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Supplementary Information

The Supplementary Information includes:

• Materials and Methods
• Supplementary Figures
• Supplementary Information References

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