# Colloidal organic carbon and trace elements in peat porewaters across a permafrost gradient in Western Siberia

Tatiana Raudina, Sergey Loiko, Daria Kuzmina, Liudmila Shirokova, Sergey Kulizhskiy, Evgeniya Golovatskaya, Oleg Pokrovsky

► To cite this version:

Tatiana Raudina, Sergey Loiko, Daria Kuzmina, Liudmila Shirokova, Sergey Kulizhskiy, et al., Colloidal organic carbon and trace elements in peat porewaters across a permafrost gradient in Western Siberia. Geoderma, 2021, 390, pp.114971. 10.1016/j.geoderma.2021.114971. hal-04843200

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# Colloidal organic carbon and trace elements in peat porewaters across a permafrost gradient in Western Siberia

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Tatiana V. Raudina¹,², Sergey V. Loiko², Daria M. Kuzmina², Liudmila S. Shirokova³,⁴,

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Sergey P. Kulizhsky², Evgenia A. Golovatskaya¹, Oleg S. Pokrovsky³*

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¹ Institute of Monitoring of Climatic and Ecological Systems, Siberian Branch of the Russian Academy of Sciences, Tomsk, Russia

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² BIO-GEO-CLIM Laboratory, Tomsk State University, Tomsk, Russia

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³ Geoscience and Environment Toulouse, UMR 5563 CNRS, Avenue Edouard Belin, 31400, Toulouse, FRANCE

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⁴ N. Laverov Federal Center for Integrated Arctic Research, UrB Russian Academy of Science, Arkhangelsk, Russia

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*Corresponding author. Email: oleg.pokrovsky@get.omp.eu

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28 **Abstract**

The majority of organic carbon (OC), nutrients, and dissolved trace elements in soil porewaters are present in the form of colloids which determine element transport, bioavailability, and overall impact on ecosystems. Climate warming and permafrost thaw in high latitudes will primarily affect the soil liquid phase thereby modifying delivery of colloids to the hydrological network and their role in C transport and emission. Here we studied colloids in peat porewaters across a natural gradient of sporadic, isolated, discontinuous and continuous permafrost zone in the Western Siberian Lowland (WSL), the largest peatland in the world. The depth of sampling and the microrelief (mounds and hollows) had a generally weak impact on the proportion of colloidal forms (3 kDa - 0.45 µm) of OC, major (Fe, Al, P, alkali and alkaline-earth metals) and trace elements (TE) including micronutrients (Zn, Mn, Ni, Co, Cu), toxicants (Sb, As, Cd, Pb) and geochemical tracers (trivalent and tetravalent cations). Considering all micro-landscapes together, there was no sizable change in the proportion of colloidal fraction of OC, Fe, Al, P, micronutrients and toxicants across the permafrost zones. The majority of colloidal forms of all elements of these groups were represented by a size fraction between 3 and 30 kDa and were essentially Fe-Al-organic compounds with an average Fe:Al:OC molar ratio of 1.9:1:308. Overall, the degree of impact from environmental factors on OC and metal distribution among various colloidal fractions can be classified as depth ≤ permafrost type < microlandscape. Applying a substitution “space for time scenario” for the climate warming and permafrost thaw in Western Siberia, we do not expect sizable changes in C and element colloidal status during active layer thickness increase and permafrost boundary shift northward. Future studies of colloids in peat ice (below the active layer) are needed to assess possible changes in delivery of C and metals from soil to rivers and onward into the Arctic Ocean under massive permafrost thawing in the WSL.

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# 1. Introduction

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Organic and organo-mineral colloids (solute entities having sizes between ~1 nm and ~1 µm or 0.45 µm) are the main vectors of organic carbon (OC) and trace element (TE) transport in soil porewaters (Aiken et al., 2011; Philippe and Schaumann, 2014). Numerous studies have focused on characterization of elemental colloidal status (i.e., proportion of colloids relatively to total dissolved element concentration and distribution of this element between different size fractions of colloids) of aqueous OC and major (Ca, Al, Fe, P, Si) and TE in soils (Riise et al., 2000; Stutter et al., 2003; Pokrovsky et al., 2005; Regelink et al., 2011; Yan et al., 2016; Bazilevskaya et al., 2018). Among TE in natural waters, one can distinguish micro-nutrients (B, Mn, Fe, Co, Ni, Cu, Zn, Rb, Mo, Ba), potential toxicants (As, Cd, Sb, Pb) and geochemical tracers (Li, Al, Ti, Cr, Ga, Sr, Y, REEs, Zr, Cs, Nb, Hf, Th, U) as outlined in recent studies of rivers (i.e., Cuss et al., 2020; Pokrovsky et al., 2020). The soils of arctic and subarctic regions subjected to climate warming and permafrost thaw are especially important with regard to the colloidal status of their solutes because these soils contain high concentrations of organic carbon originated from massive partially frozen peat deposits. Upon contact with water, the peat is capable of releasing a sizable amount of the dissolved organic matter (DOM) (Dutta et al. 2006; Jansson et al. 2008; Vonk et al., 2015; Shirokova et al., 2017a, b; Payandi-Rolland et al., 2020a). Due to the dominance of precipitation over evaporation at high latitudes, Histosols exhibit relatively low salt concentrations in their pore waters (Raudina et al., 2017, 2018). Altogether, these factors provide optimal conditions for colloid generation in peat soil and surface waters (Pokrovsky et al., 2016a). For these reasons, studying dissolved C, macro- and micronutrients and toxicants in permafrost peat soils is crucially important for understanding factors controlling the changes in element lateral export under permafrost thawing scenarios.
The majority of available studies on soil colloids deal with tropical (Chorover and Sposito, 1995; Chorover et al., 2004; Ranville et al., 2005; Thompson et al., 2006; Buettner et al., 2014), temperate (Gasser et al., 1994; Goody et al., 1995; Keller and Domergue, 1996; Huber and Denaix, 2000; Riise et al., 2000; Kögel-Knabner et al., 2008; Pédrot et al., 2008, 2009; Montalvo et al., 2015; Bazilevskaya et al., 2018) or boreal (Berggren et al., 1989; Van Hees et al., 2000; Pokrovsky et al., 2005; Gangloff et al., 2016) permafrost-free regions. Thus, the response of the aqueous phase in soil ecosystems to changing environmental conditions in Arctic and subarctic settings is not possible to assess currently. Such assessment is especially important for large territories of continental lowlands containing frozen and thawed peat capable of strongly enriching soil solutions in organic colloids.

In this work, we used a case study of the Western Siberian Lowland (WSL), an extensive peatland (> 1 million km² located within the permafrost zone) exhibiting a sizeable gradient of climate and permafrost across variable micro-landscapes. The WSL is relatively well studied from the view point of bulk (< 0.45 µm) chemistry of OC and TE in soil porewaters (Raudina et al., 2017), suprapermafrost waters (Raudina et al., 2018), rivers (Pokrovsky et al., 2015, 2016a, b, 2020; Krickov et al., 2020) and lakes (Shirokova et al., 2013; Polishchuk et al., 2017; Manasypov et al., 2015, 2020), but the speciation and size distribution of C and metals in the aqueous phase of peat soils remain completely unknown. The frozen peat in the WSL has an average temperature just slightly below 0°C and therefore is highly vulnerable to thaw (Romanovsky et al., 2010). For this reason, the response of peat porewaters in the WSL to climate warming may be quite fast and highly significant. This may provide massive export of C and associated micronutrients and metal pollutants from soils to rivers and further into the Arctic Ocean. Although colloids in the WSL surface waters (lakes and rivers) were occasionally characterized (Pokrovsky et al., 2011, 2016a; Shirokova et al., 2013; Loiko et al., 2017; Krickov et al., 2019), the chemical composition and stoichiometry of permafrost
peat porewater colloids in this and other permafrost regions of the world remain virtually
unknown.

Using a set of conventional size-separation techniques, here we characterize the
distribution of OC and major and trace elements among several colloidal and low molecular
weight (LMW) pools in peat porewaters of the WSL across a sizeable gradient of permafrost
and climate zones. Specific objectives were to i) provide average colloidal concentrations of C
and major and trace elements in the WSL, particularly across its various permafrost zones, ii)
reveal the group of major and trace elements depending on their degree of colloidal transport,
iii) quantify the impact of peat core depth, micro-landscapes (mounds, depressions) and
permafrost distribution on element concentration in various colloidal fractions, and iv)
characterize the change of colloidal status of OC, major and trace metals along the hydrological
continuum, from soil porewaters to large surface reservoirs such as rivers and lakes. Our
ultimate goal was to use the current colloidal status of OC and metal in southern (sporadic and
isolated) permafrost zones as a possible proxy for future changes in northern (discontinuous
and continuous) permafrost zones. For this, we followed a "space for time substitution"
approach (Blois et al., 2013), well constrained for western Siberia (Frey et al., 2007; Raudina et
al., 017; Porkovsky et al., 2020). Eventually, this should allow foresight into possible future
changes in organic and organo-mineral colloids in peat porewaters in arctic and subarctic
regions.

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## 2. Materials and methods

### 2.1. Geographical setting and local micro-landscapes

Peat soil porewaters were extracted from peat deposits located in the Western Siberian
Lowland. The WSL is represented by Pleistocene sands and clays which are covered by thick
(1 to 4 m) peat. The territory investigated in this work includes 4 main permafrost zones:

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isolated, sporadic, discontinuous and continuous. Details of lithology, climate, permafrost distribution, and active layer thickness are described in previous studies (Raudina et al., 2017, 2018). For sampling, we selected 4 main test sites (Fig. 1) across a latitudinal and permafrost gradient: Kogalym (1) which is located within the isolated/sporadic permafrost zone, Khanymey (2) which is located in the discontinuous/sporadic permafrost zone, Pangody (3) which is within the discontinuous to continuous permafrost zone, and Tazovsky (4) located in the continuous permafrost zone (Table 1).

A typical feature of all selected sites is presence of positive and negative forms of relief (i.e., micro-landscapes, see Fig. 1). The positive relief forms (raised areas) include ridges in the sporadic permafrost zone, mounds in discontinuous permafrost zones, and polygons in the subarctic tundra of continuous permafrost zones. The negative forms (depressions) are typified by hollows (across all zones), permafrost subsidences (in discontinuous and continuous permafrost zones), and frost cracks (in the polygonal tundra biome). Overall, we sampled 9 peat mounds, 2 polygons, 8 hollows, 2 permafrost subsidences and 2 frost cracks (Table 1). Discrete depth of pore water sampling ranged from 10 to 65 cm in all sites and corresponded to entire thickness of the active (unfrozen) peat layer. Seasonal (winter) freezing was always below 60 cm in each sampling site.

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## 2.2. Soil porewater sampling and size fractionation

Altogether, 23 soil pore waters in 4 main sampling sites were collected from the end of July through the beginning of August 2015 using the PTFE suction cup lysimeters (3 cm in diameter; 5 cm in length) as described in detail elsewhere (Raudina et al., 2016, 2017). Soil porewaters were sampled from individual depths (Table 1), with a typical vertical distance between suction cups from 15 to 40 cm. The first 3 portions of collected water (100-200 mL in total) were discarded. Within 2 hours of soil porewater collection, it was filtered through

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single-use Minisart filter units (Sartorius, acetate cellulose filter) with a diameter of 25 mm and a pore size of 0.45 µm. The size separation procedure was performed immediately after 0.45 µm-filtration in a specially designed laboratory space within the vicinity of each sampling site.
We used parallel (non-cascade) ultrafiltration through 3, 30 and 100 kDa using Amicon Ultracel® centrifugal filters made of regenerated cellulose. These disposable 50-mL polypropylene vials were fitted with membranes of various molecular poresize and were first treated by 1% HNO₃ and rinsed 3 times via passing between 50 and 100 mL of MilliQ water through the membrane. The DOC and metal blanks were a factor of 3 to 10 lower than the minimal concentrations of these components in experimental fluids. We routinely ran MilliQ blanks (every 10 samples, at least once per day) in the field laboratory at each site. The first portion of the sample supernatant that passed through membranes was rejected. Each fraction of filtrate was sub-devised into non-acidified and acidified portions for carbon/anions/UV absorbency/pH and trace element measurements, respectively. All samples were preserved in a refrigerator for 1-2 months prior to analysis.

In this work we define colloids as entities having the size between 0.45 µm and 3 kDa.
Further, we distinguish two group of high-molecular weight (HMW) (HMW<100 kDa - 0.45 µm and HMW<30 kDa - 100 kDa), medium molecular weight (MMW) colloids (MMW<3kDa - 30 kDa), and low molecular weight (LMW) or truly dissolved fraction (LMW<3kDa).

## 2.3. Analyses

In filtrates and ultrafiltrates, we measured pH (in the field, with an uncertainty ±0.02 pH units), all major anions, cations, Si, and about 40 trace elements. The DOC and dissolved inorganic carbon (DIC) were analyzed using a Carbon Total Analyzer (Shimadzu TOC VSCN) with an uncertainty better than 3%. The UV absorbance of the filtered samples was measured at 280 nm using a quartz 10-mm cuvette on a Cary-50 spectrophotometer. Major anion
concentrations ($Cl^{-}$, $SO_{4}^{2-}$) were measured by ion chromatography (HPLC, Dionex ICS 2000) with an uncertainty of 2%. Major cations (Ca, Mg, Na, K), Si, and ~40 trace elements (TE) were analyzed with an ICP-MS Agilent ce 7500 with In and Re as internal standards and 3 various external standards (1, 10 and 100 ppb), placed every 10 samples in a series of samples. The SLRS-5 (Riverine Water Reference Material for Trace Metals certified by the National Research Council of Canada) was used to check the accuracy of analyses (Yeghicheyan et al., 2013).

## 2.4. Statistical treatment

The concentration of dissolved carbon, major and trace elements in soil porous waters at each sampling site was tested for normality. The normality of data distribution was checked separately for each site and microlandscape and for the full set of the data using the criteria of Kolmogorov-Smirnov. The significance value was < 0.01 and therefore non-parametric criteria for data comparisons were used. Major and TE concentrations in soil porewaters of the 4 main sampling sites and the two main micro-relief landscapes (polygon or mound and depressions) were first processed using a non-parametric H-criterion Kruskal-Wallis test (p < 0.05). This test is suitable for evaluation of the difference of each component among several simultaneous samplings. In the event of significant differences, a comparison of DOC, major and TE concentrations between soil porewaters sampled in one main pair of micro-landscapes (mound/polygon-depression) of each of the 4 major sampling sites was conducted using a non-parametric pair Wilcoxon-Mann Whitney test. Furthermore, two way ANOVA test was used to reveal the effect of two factors (micro-landscape and permafrost) on elementary composition of peat porewater colloids at p < 0.05.

## 2.5. Element speciation modeling
Trace element speciation in the presence of major colloidal constituents (OC, Fe and Al) was assessed using Visual MINTEQ and the vMinteq database (Gustafsson, 2014). We used a NICA-Donnan humic ion binding model (Benedetti et al., 1995; Milne et al., 2003) in conjunction with a diffuse layer model for hydrous ferric oxide (Traina, 1996). Details of this procedure for surface waters in boreal and permafrost landscapes are described elsewhere (Ilina et al., 2016; Pokrovsky et al., 2016a; Loiko et al., 2017). The input parameters of the model were the *in-situ* temperature, pH, DIC, DOC, anions, cations, and filtered (0.45 µm and 3 kDa) TE concentrations. For this model, we assumed that the DOM<3kDa fraction consists of fulvic acid (FA) and the DOM0.45µm-3kDa fraction consists of humic acid (HA). The colloidal HFO concentrations were calculated as a difference between the Fe (< 0.45) and the Fe in the < 3kDa filtrate. The model yielded a theoretical degree of complexation for major and trace metals with DOM in two major fractions of soil porewater (0.45 µm and 3 kDa) and divalent metal and sulphate adsorption on ferric colloids (3 kDa - 0.45 µm). It also provided a degree of solution supersaturation with respect to possible autigenic minerals of the WSL soils.

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## 3. Results

### 3.1. Effect of sampling depth (within each micro-landscape and each permafrost zone)

The concentrations of colloidal forms of major and trace elements in peat porewaters of Western Siberia averaged across all sampling depth, micro-landscapes, and 4 permafrost zones are listed in **Table 2**. In this presentation, we distinguish major components of colloids (DOC, Al, Fe), macronutrients (P, Ca, K, Si), micronutrients (Mn, Zn, Co, Ni, Cu, Mo), two representative toxicants (Cd, Pb) and several geochemical tracers (Ti, La, Ce, Th, U). The other major and trace components of peat porewaters are provided in **Table S1** of the Supplement.

Sampling depth did not exert a measurable impact on the colloidal status of DOC and all major and trace elements (Fig. S1 of the Supplement). The depth-averaged distributions of
elements among the 4 main colloidal fractions in positive and negative forms of micro-landscapes are illustrated in **Fig. S2** of the Supplement. Overall, variations between samples from the same micro landscape and same permafrost zone greatly exceeded the variations between depths, as confirmed by the statistical treatment of all data (**Table S2** of the Supplement). For DOC and most major and trace elements, the differences in concentrations at the deepest (30-60 cm) and surface (0-30 cm) horizons were not significant (p > 0.05) with the exception of P, Y, and REEs in depressions and Mn and Cd in mounds. In order to test the effect of micro-landscape and permafrost zone (see section 3.2 below), we combining concentrations and percentages of colloidal fraction measured at several depths in each micro-landscape. A two way ANOVA test of the effect of two factors (micro-landscape and permafrost) on elementary composition of peat porewater colloids (**Table S2 C**) demonstrated that only a few elements (P, Mn and Cu) exhibited significant (p < 0.05) impact of two factors. This result is in full agreement with Wilcoxon-Mann Whitney tests (**Tables S2 A** and **B**).

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## 3.2. Effect of micro-landscapes and permafrost zones

The depth-averaged percentages of element distribution among colloidal and LMW<3kDa fractions in dominant positive (mound, polygon) and negative (hollow, subsidence, frost crack) micro-landscapes of the 4 studied permafrost zones together with the mean values for all permafrost-affected WSL territory are listed in **Table S3**. The effect of individual micro-landscapes (mound/polygon, hollow, subsidence, and frost crack) on the depth-averaged element concentrations, tested via a Mann-Whitney U-test, was not pronounced (p > 0.05) for most elements affected by the filtration and ultrafiltration procedure (**Table S2 B**). The major colloidal constituents (DOC, Al, Fe) are illustrated in **Fig. 2** whereas some macro- (P, Si), micro-nutrients (Mn, Zn, Co, Ni, Cu), toxicants (As, Cd, Pb) and geochemical trace elements

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251 (Y, La, Th, U) distribution among the 3 major colloidal pools and the LMW< 3kDa fraction are
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253 Statistical differences in depth-averaged element concentrations in different colloidal
254 fractions were also determined between different forms of micro-landscape by Mann-Whitney
255 U test for each fraction and component as listed in **Table S2 B** of the Supplement. Virtually
256 none of the major and trace elements affected by the size separation procedure demonstrated
257 significant differences in concentration between mound and hollow, mound and subsidence,
258 and hollow and subsidence. Exceptions are Mn (all size fractions), Cu (< 100 kDa and < 30
259 kDa), and REEs (< 3 kDa), for which the differences between positive and negative forms of
260 microrelief were significant (p < 0.05).

261 The results presented above (the negligible effect of depth of sampling and the position
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264 obtained in different micro-landscapes at different depths for each permafrost zone. Permafrost
265 type distribution and active layer thickness (ALT) exerted a generally weak impact on colloids
266 as latitudinal variations of the truly dissolved LMW< 3 kDa fraction were within ±10-20%. This
267 range did not exceed variations among different depths and micro-landscapes as illustrated for
268 OC, major components of colloids (Fe, Al, P, Ca), selected micronutrients (Cu, Ni), toxicants
269 (Cd, Pb) and geochemical tracers (La, Th, U) in **Fig. 3**. The DOC and all other major and trace
270 elements did not demonstrate any statistically significant systematic impact of permafrost type
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273 3.3. Stoichiometry of colloids and groups of elements depending on their colloidal status
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The total colloidal fraction (3 kDa - 0.45 µm) exhibited the following order of main constituents (in molar concentrations): OC > Fe ≥ Al. The depth- and micro-landscape-averaged OC:Al and Fe:Al ratios plotted as a function of latitude demonstrated ca. 2 to 3 times higher DOC:Al values in the sporadic/discontinuous compared to the discontinuous/continuous permafrost zones (Fig. 4 A, B). The average OC:Fe ratio in colloids did not evolve across various permafrost zones and ranged from 100 to 400 (Fig. 4 C). Depth-averaged ratios of Fe:Al and OC:Fe for positive and negative forms of microrelief in colloids (3 kDa - 0.45 µm) of peat porewaters across the 4 permafrost zones ranged from 2.3 to 3.2 and 178 to 330, being generally lower in depressions and at the northern sites Pangody and Tazovsky.

Almost half of the total dissolved (< 0.45 µm) P was present in the LMW<3kDa fraction. Its colloidal forms were dominated (20-30%) by the 3-30 kDa fraction. The HMW colloidal fractions (100 kDa-0.45 µm, 30-100 kDa) accounted for 12 and 17%, respectively, of P. The highest percentage of organic sulphur (Sorg) and sulfate (SO4²⁻) were essentially in the LMW<3kDa fraction (98% and 40-90%, respectively). The SUVA typically decreased from 4.3-2.1 in the 0.45µm fraction to 2.6-2.0 L mg⁻¹ m⁻¹ in the LMW<3 kDa fraction. There was no impact for soil depth, type of micro-landscape and permafrost coverage in the WSL on SUVA in all fractions of peat porewaters.

Depending on their affinity to colloids, and considering all micro-landscapes together in both permafrost-free and permafrost-bearing zones, we distinguished several groups of elements. Group 1 members are weakly affected by colloids and present mainly (≥ 80%) in the LMW<3kDa fraction: Li, B, Si, DIC, Cl, SO4 and Sorg (on mounds). For elements of this group, the contribution of three colloidal fractions (100 kDa-0.45 µm, 30-100 kDa, and 3-30 kDa) was approximately equal. Group 2 members are represented by OC and soluble (or usually mobile) elements including some alkalis and alkaline-earth elements (Na, Mg, Ca, K, Rb, Sr, Ba, Cs), divalent metals (Ni, Mn, Co, Zn, Cd), oxyanions (P, As, V, Cr, Mo, Sb), and Tl. Between 40
and 80% of group 2 elements were present in the LMW< 3 kDa form, and the majority of colloids had a molecular size between 3 and 30 kDa. Group 3 members are divalent metals tightly bound to OM (Cu, Pb) and low soluble trivalent cations (Al, Fe, Ga, Y, REEs), Hf, Nb, W, and U. They were dominated by colloids (60 to 80%). Lastly, group 4 members are elements present essentially (≥ 80%) in colloidal form and include tetravalent cations (Ti, Zr, Th), Nb, and Ag. It is worth noting that the majority of colloidal forms of the elements within these groups were represented by a size fraction between 3 and 30 kDa **(Table S3)**.

The concentration of main colloidal constituents - OC, Fe and Al - systematically decreased with size fraction (0.45 µm → 100 kDa → 30 kDa → 3 kDa) in all permafrost zones. Progressive decrease in TE concentrations from large to small pore size demonstrated two distinct patterns. These patterns were dependent on TE affinity to either OC or Fe and Al (**Fig. S3**). To distinguish between the main TE carriers, we quantified the coefficients of linear correlations between TE and Fe, Al or DOC during filtration and ultrafiltration (**Table S4**). For example, alkalis and alkaline-earth elements, V, Mo and U decreased their concentrations in parallel with OC and exhibited higher correlation coefficients with OC than with Fe. Siderophilic (Cr, Mn, Co, Ni) and chalcophilic elements (Cu, Cd, Zn, Pb) followed a decrease with Fe/Al rather than OC. Finally, trivalent and tetravalent cations were tightly associated with Fe and Al (indistinguishable between two).

### 3.4. Thermodynamic modeling of mineral saturation and element speciation in colloids

The vMinteq calculation of major (Al, Fe, Ca) and trace element speciations in the < 0.45 µm and in < 3 kDa fractions demonstrated that some decrease in the complexation degree of divalent metals (Mg, Ca, Sr, Ba, Ni, Zn, Mn, and Co) and $U^{VI}$ occurred in northern peat soil solutions compared to the southern (sporadic) permafrost zone. This was most visible in the < 3 kDa fraction which demonstrated a decrease in pH and DOC northward (**Fig. S4**). The
predicted complexation of Al, Fe, Cd, Cu, REE, Pb, Th and U VI with DOM in the < 0.45 µm fraction remained very high, specifically, between 98 and 100%.

The proportions of TEs that were complexed with DOM as calculated by the vMinteq model were generally consistent with experimental findings. The model predicted that between 95 and 100 % of all dissolved metals were associated with DOM in both the < 0.45 µm and < 3 kDa fractions. Even Na and K were ca. 5 to 20% complexed with DOM in the < 0.45 µm fraction. This is lower than the measured proportion of colloidal Na and K (40-60%) and suggests that even the LMW< 3kDa fraction may contain a sizable amount of organic complexes (3-25% according to the vMinteq model). At the same time, the vMinteq model could not account for TEs that were present in the form of HMW organo-ferric and organo-aluminium colloids. These colloids are formed by coprecipitation of Fe and Al hydroxide and stabilization by OM (Vasyukova et al., 2010). The dominance of Fe and Al hydroxides in the colloidal fraction is consistent with supersaturation of the < 0.45 µm fraction with respect to Al hydroxide, goethite and ferrihydrite (Table S5). In contrast, the LMW< 3 kDa fraction was undersaturated with respect to these compounds. Therefore, a sizeable proportion of trace metals, notably trivalent and tetravalent cations, are present inside HMW colloidal Al, Fe-oxy(hydr)oxide forms but are unaccounted in the vMinteq complexation approach.

**4. Discussion**

The first, and most important, result of this study is the rather low impact of peat depth, microlandscape and permafrost coverage on major and trace element concentrations in colloidal (3 kDa - 0.45 µm) fractions relative to the total dissolved (< 0.45 µm) phase of peat porewaters. A possible reason for this could be generally weak involvement of frozen mineral horizons in peat porewater transfer in the active (unfrozen) layer of the WSL (Raudina et al., 2017). The major colloidal chemical composition (OC, Fe, Al) and TE partitioning among
different size fractions are therefore weakly dependent on environmental conditions. This reflects the primary processes of peat constituent release into aqueous solution with minimal biotic transformation and photolysis. Note that colloids in peat porewaters are strongly dependent on pH (Andreasson et al., 1988) because the higher pH and salt concentration likely result in higher colloidal transport (McCarter et al., 2020). However, both pH and total dissolved (< 0.45 µm) solids of the WSL peat porewaters are weakly dependent on environmental and landscape context (Raudina et al., 2017, 2018) and this can explain relatively low variability in colloidal status of major and TE across the depths, micro-landscapes and permafrost zones. We believe that the “universal” colloidal features of peatland porewaters sampled in this study reflect an overall homogeneity of the upper 0-60 cm of the WSL peat in terms of its age, chemical and botanical composition and the identity of peat-forming species (Stepanova et al., 2015; Raudina and Loiko, 2019). These factors lead to remarkable similarity of organo-mineral colloids formed via leaching of DOM and trace metals from the peat by aqueous solutions.

In peat porewaters of the WSL, there were several major element groups regarding their affinity to colloidal (3 kDa - 0.45 µm) or LMW< 3kDa fraction (Fig. S5). This general classification of elements on 4 groups according to the degree of colloidal binding is consistent with the earlier assessment of colloids in lakes (Shirokova et al., 2013; Pokrovsky et al., 2016a) and rivers (Krickov et al., 2019) of this region. This classification reflects the capacity of a trace element to 1) complex with carboxylic groups of DOM (alkalis, alkaline-earths and divalent heavy metals), or 2) be associated, via adsorption or coprecipitation, with the main inorganic constituents of soil colloids, such as Fe and Al oxy(hydr)oxides (trivalent and tetravalent cations). The latter feature is responsible for exceptionally high colloidal mobility in peat porewaters of usually inert, low-soluble geochemical tracers (REEs, Ti, Zr, Th).
A typical hydrological continuum of surface and subsurface waters in permafrost peatlands (Fig. 5) includes water transfer from soil porewaters to small soil depressions (subsidences), lakes, streams and large rivers (Palmer et al., 2016; Shirokova et al., 2019; Payandi-Rolland et al., 2020b). Because of the permafrost context, the peat is disconnected from groundwater and underlaying mineral horizons via an impermeable (frozen) layer. As such, peat porewaters are influenced solely by peat - aqueous solution interactions (Raudina et al., 2017). Thaw ponds and thermokarst lakes are located within the peat substrate and often exhibit frozen bottom sediments composed of organic (peat) detritus (Manasypov et al., 2015; Audry et al., 2011). This is especially true for small depressions of the palsa peatbogs (Pokrovsky et al., 2016a). In contrast, rivers of the WSL are partially affected by underlying rock lithology (Pokrovsky et al., 2020). As a result, the rivers exhibit sizable heterogeneity in colloidal fraction and stoichiometry that is dependent on watershed landscape parameters and the permafrost coverage (Krickov et al., 2019). Furthermore, in contrast to soil waters, colloids of open surface waters are subjected to various autochthonous processes occurring in the water column such as photo- and bio-transformation, exometabolite production and biological uptake. These processes are combined with external input of ionic forms of labile elements (alkalis, alkaline-earths, oxyanions) from groundwater through unfrozen water paths (taliks) within the underlaying silicate rocks. Note that elevated Fe:Al ratio in colloids of sporadic/absent permafrost zone relative to the other permafrost zones (Fig. 4 B) may be due to some input of underground, Fe-rich waters to peat soil profile in the southern part of the WSL.

Results of the present study and previous work for the WSL discontinuous permafrost zone (Pokrovsky et al., 2016a; Payandi-Rolland et al., 2020b) suggest that sizable transformation of DOM and TE already occurs between soil porewaters and waters of surface depressions (Fig. 5). Despite approximately similar percentage of OC in colloids, there is a clear decrease in colloidal fraction of alkalis (Na, K, Rb, Cs) and micronutrients (V, Cr, Cu,
Zn, Cd and Mo) between porewaters and surface waters. This may be caused by a concomitant decrease (by a factor of 2 to 3) in the concentration of DOC. In contrast, an increase in the colloidal proportion of Al, Fe, Ni, Y, REEs and U may be caused by an increase in pH along the hydrological continuum (from 0.3 units in depressions to 3 units in rivers). Overall, a decrease in DOC and an increase in pH and total dissolved solid (ionic strength) between peat soil and surface waters of the WSL lead to: i) a decrease in electrostatic interaction between alkali and organic matter due to shrinkage of the electric double layer thereby leading to a decrease of colloidal alkalis; ii) a deprotonation of carboxylic groups of the DOM leading to a stronger binding of metals; iii) an increase in the degree of polymerization of Fe and Al hydroxides leading to an increase in HMW colloids capable of incorporating insoluble trivalent and tetravalent cations.

Given the quite low effect of micro-landscape type and permafrost zone on the colloidal status of OC and various elements, we do not expect a drastic change in colloidal and truly dissolved fractions upon gradual thawing of permafrost and northward boundary shifting. Moreover, because the sampling depth had a negligible impact on the colloidal fractions of OC, major and trace elements, any sizable changes in the colloidal status of solutes upon the increase in ALT of the WSL peatlands are unlikely. However, thawing of deep peat and mineral horizons may mobilize organic- and element-rich colloids from the dispersed ice (Lim et al., 2020) to the hydrological network. This may strongly modify the colloidal status of C, macro- and micronutrients and potential toxicants in soil and surface waters of permafrost peatlands.

420

421

## 5. Conclusions

422

423

Colloidal (3 kDa - 0.45 µm) forms of C and major and trace elements in peat porewaters of the Western Siberian Lowland were weakly dependent on sampling depth, type of
424 microlandscape and the permafrost coverage. The medium molecular weight colloids
425 (MMW<sub>3Da</sub> ~ 30 kDa) dominated the size distribution of DOC, major and most trace elements.

426 The change in elemental composition of colloids and dominant colloidal fractions
427 between peat porewater and surface waters (rivers and lakes) consisted of a decrease in
428 colloidal forms of alkalis and an increase in the degree of colloidal binding of some divalent
429 and trivalent metals. A likely reason for this is an increase in pH and a decrease in DOC within
430 a hydrological continuum "peat porewater → depression → lake → river". This change in
431 solution chemistry could occur due to progressive increase in the groundwater impact on
432 surface waters.

433 Under a climate warming scenario, the increase in ALT, change in dominant
434 microlandscapes and permafrost boundary shift northward are not expected to provide
435 significant modification in colloidal status of OC, major and trace elements unless the thawing
436 depth exceeds the peat thickness and reaches the mineral horizon. Further studies of colloids in
437 ice which is dispersed in deep peat layers and mineral horizons are needed to constrain the
438 effect of strong active layer deepening on DOC and trace metal export and speciation in soil
439 porewaters of permafrost peatlands.

440
441 **Data availability**
442 The full data set of measured DOC, major and trace element concentrations in the 4 main
443 fractions of peat porewaters (< 0.45 µm, < 100 kDa, < 30 kDa and < 3 kDa) in all sites, micro-
444 landscapes and different sampling depths is presented in the Mendeley Data Repository
445 (Raudina et al., 2020).

446
447 **Acknowledgements:**
448 The reported study was funded by RFBR (projects № 19-29-05209, № 19-35-60030 and № 18-
449 05-60264) for data acquisition, laboratory and field researches, and manuscript preparation.
450 Data analysis was carried out under support from the Russian Science Foundation (project №
451 18-77-10045). We thank the Editor David Laird and 3 anonymous reviewers for constructive
452 remarks.

453
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<table><caption>**Table 1.** Sampling site description, permafrost coverage and micro-landscapes and sampling depths. Each sample was processed for 0.45 µm filtration and 100, 30 and 3 kDa ultrafiltration. ALT is active (unfrozen) layer thickness.</caption><thead><tr><th>Site, latitude</th><th>Permafrost zone</th><th>Micro-landscape</th><th>Peat thickness, m</th><th>ALT, cm</th><th>Number of samples</th><th>Sampling depth, cm</th></tr></thead><tbody><tr><td rowspan="3">Tazovsky 67.2°N</td><td rowspan="3">continuous</td><td>polygon</td><td rowspan="3">2.0-4.0</td><td>45</td><td>2</td><td>10-15<br>38-42</td></tr><tr><td>frost crack</td><td>48</td><td>2</td><td>7-12<br>30-35</td></tr><tr><td>hollow</td><td>65</td><td>2</td><td>10-15<br>40-45</td></tr><tr><td rowspan="2">Pangody 65.5°N</td><td rowspan="2">discontinuous</td><td>peat mound</td><td>0.2-1.3</td><td>49</td><td>2</td><td>10-15<br>40-45</td></tr><tr><td>hollow</td><td>0.3-1.0</td><td>82</td><td>4</td><td>10-15<br>30-35<br>55-65</td></tr><tr><td rowspan="2">Khanymey 63.5°N</td><td rowspan="2">discontinuous/ sporadic</td><td>peat mound</td><td>0.1-1.4</td><td>90</td><td>5</td><td>8-13<br>25-35<br>35-45<br>55-65</td></tr><tr><td>permafrost subsidence</td><td>0.7-1.1</td><td>165</td><td>2</td><td>10-15<br>35-45</td></tr></tbody></table>
<table><tr><td rowspan="2">Kogalym<br>62.2°N</td><td rowspan="2">isolated/<br>sporadic</td><td>peat mound</td><td>1.7-2.3</td><td>> 200</td><td>2</td><td>10-15<br>35-45</td></tr><tr><td>hollow</td><td>1.0-1.5</td><td>> 200</td><td>2</td><td>10-15<br>35-45</td></tr></table>

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<table><caption>**Table 2.** Concentration (mean±SD) of elements in different size of colloids in peat porewaters of Western Siberia, averaged across all sampling depth, micro-landscapes, and 4 permafrost zones. DOC, Al, Si, P, K, Ca, Fe concentration is in mg L⁻¹, all others TE is in µg L⁻¹.</caption><thead><tr><th rowspan="2">Element</th><th colspan="4">Khanymey (63.78°N)</th><th colspan="4">Kogalym (62.25°N)</th><th colspan="4">Pangody (65.87°N)</th><th colspan="4">Tazovsky (67.36°N)</th><th colspan="4">WSL average</th></tr><tr><th>&lt; 0.45 µm</th><th>&lt; 100 kDa</th><th>&lt; 30 kDa</th><th>&lt; 3 kDa</th><th>&lt; 0.45 µm</th><th>&lt; 100 kDa</th><th>&lt; 30 kDa</th><th>&lt; 3 kDa</th><th>&lt; 0.45 µm</th><th>&lt; 100 kDa</th><th>&lt; 30 kDa</th><th>&lt; 3 kDa</th><th>&lt; 0.45 µm</th><th>&lt; 100 kDa</th><th>&lt; 30 kDa</th><th>&lt; 3 kDa</th><th>&lt; 0.45 µm</th><th>&lt; 100 kDa</th><th>&lt; 30 kDa</th><th>&lt; 3 kDa</th></tr></thead><tbody><tr><td>DOC</td><td>77.6±20</td><td>75±21</td><td>65.5±17</td><td>30.2±15</td><td>45±19</td><td>38.6±11</td><td>32.7±17</td><td>20.9±5</td><td>60.9±15</td><td>54.8±17</td><td>52.2±16</td><td>31.9±15</td><td>53.7±16</td><td>46.3±19</td><td>42.2±12</td><td>24.3±9</td><td>61.3±14</td><td>55.9±13</td><td>50.2±13</td><td>27.5±12</td></tr><tr><td>Al</td><td>0.31±0.1</td><td>0.29±0.1</td><td>0.25±0.1</td><td>0.1±0.03</td><td>0.16±0.03</td><td>0.15±0.03</td><td>0.12±0.03</td><td>0.09±0.02</td><td>0.77±0.06</td><td>0.57±0.5</td><td>0.52±0.5</td><td>0.34±0.4</td><td>0.4±0.2</td><td>0.39±0.1</td><td>0.35±0.1</td><td>0.15±0.02</td><td>0.43±0.4</td><td>0.36±0.3</td><td>0.32±0.3</td><td>0.17±0.2</td></tr><tr><td>Si</td><td>0.98±0.5</td><td>0.92±0.5</td><td>0.92±0.5</td><td>0.87±0.4</td><td>1.2±0.5</td><td>1.17±0.5</td><td>1.14±0.5</td><td>1.12±0.5</td><td>4.28±2.7</td><td>4.54±3.1</td><td>4.5±3.2</td><td>4.38±3</td><td>2.81±1.3</td><td>2.75±1.3</td><td>2.74±1.3</td><td>2.86±1.4</td><td>2.35±2</td><td>2.39±2.2</td><td>2.37±2.2</td><td>2.33±2.2</td></tr><tr><td>P</td><td>0.03±0.02</td><td>0.03±0.02</td><td>0.02±0.01</td><td>0.01±0.01</td><td>0.17±0.2</td><td>0.16±0.2</td><td>0.16±0.2</td><td>0.14±0.2</td><td>0.04±0.03</td><td>0.04±0.03</td><td>0.03±0.02</td><td>0.02±0.01</td><td>0.11±0.1</td><td>0.09±0.1</td><td>0.08±0.1</td><td>0.07±0.1</td><td>0.08±0.1</td><td>0.07±0.1</td><td>0.06±0.1</td><td>0.05±0.1</td></tr><tr><td>K</td><td>0.92±1.3</td><td>0.85±1.3</td><td>0.8±1.2</td><td>0.61±0.9</td><td>1.24±0.4</td><td>0.99±0.6</td><td>0.69±0.5</td><td>0.34±0.2</td><td>0.83±0.3</td><td>0.61±0.3</td><td>0.45±0.4</td><td>0.37±0.3</td><td>0.34±0.2</td><td>0.27±0.1</td><td>0.22±0.1</td><td>0.14±0.1</td><td>0.8±0.8</td><td>0.66±0.8</td><td>0.54±0.7</td><td>0.39±0.6</td></tr><tr><td>Ca</td><td>0.95±0.2</td><td>0.85±0.3</td><td>0.78±0.2</td><td>0.41±0.2</td><td>1.29±0.5</td><td>1.16±0.4</td><td>0.86±0.3</td><td>0.66±0.2</td><td>1.36±0.5</td><td>0.98±0.3</td><td>0.86±0.3</td><td>0.63±0.3</td><td>2.51±0.5</td><td>2.44±0.5</td><td>2.2±0.4</td><td>1.4±0.3</td><td>1.52±0.7</td><td>1.35±0.8</td><td>1.19±0.7</td><td>0.74±0.5</td></tr><tr><td>Ti</td><td>4.78±4.1</td><td>4.66±4</td><td>3.22±2.6</td><td>0.46±0.6</td><td>1.99±1.6</td><td>1.77±1.5</td><td>1.2±0.9</td><td>0.3±0.1</td><td>5.14±4.2</td><td>4.12±2.6</td><td>3.5±2.3</td><td>0.87±1</td><td>4.13±1.4</td><td>3.72±1.3</td><td>3.14±0.7</td><td>0.57±0.4</td><td>4.22±3.3</td><td>3.77±2.8</td><td>2.92±2</td><td>0.57±0.6</td></tr><tr><td>Mn</td><td>2.78±1.8</td><td>1.59±0.9</td><td>1.49±0.8</td><td>0.85±0.4</td><td>7.97±3.4</td><td>7.15±3.5</td><td>6.7±3.7</td><td>5.23±3.5</td><td>10±9.9</td><td>9.7±9.8</td><td>9.2±9.9</td><td>7.3±8.3</td><td>85.4±57</td><td>84.1±57</td><td>79.8±54</td><td>51.5±41</td><td>27.1±45</td><td>26.2±45</td><td>24.8±43</td><td>14.9±28</td></tr><tr><td>Fe</td><td>0.78±0.2</td><td>0.67±0.3</td><td>0.57±0.3</td><td>0.22±0.1</td><td>1.35±1</td><td>1.25±1</td><td>1.04±0.8</td><td>0.55±0.5</td><td>2.99±2.3</td><td>2.36±1.6</td><td>2.25±1.6</td><td>1.37±1.2</td><td>1.91±0.8</td><td>1.81±0.7</td><td>1.64±0.6</td><td>0.68±0.4</td><td>1.75±1.5</td><td>1.51±1.2</td><td>1.37±1.1</td><td>0.7±0.8</td></tr><tr><td>Zn</td><td>12±4.9</td><td>10.6±4.5</td><td>8.34±3.5</td><td>5.33±2.2</td><td>13.9±3.5</td><td>12.5±3.6</td><td>9.4±1.8</td><td>7.9±1.7</td><td>5.38±2.7</td><td>4.61±1.1</td><td>3.56±1.1</td><td>2.31±0.5</td><td>8.55±3.4</td><td>7.05±3.1</td><td>5.85±3.3</td><td>4.58±3</td><td>9.71±4.8</td><td>8.62±4.4</td><td>6.63±3.4</td><td>4.94±2.7</td></tr><tr><td>Co</td><td>0.29±0.1</td><td>0.27±0.1</td><td>0.26±0.1</td><td>0.14±0.1</td><td>0.18±0.1</td><td>0.17±0.1</td><td>0.16±0.1</td><td>0.11±0.1</td><td>1.67±1</td><td>1.6±1</td><td>1.57±1.1</td><td>1.21±0.9</td><td>1.4±0.5</td><td>1.36±0.5</td><td>1.26±0.4</td><td>0.73±0.3</td><td>0.92±0.9</td><td>0.89±0.8</td><td>0.84±0.8</td><td>0.56±0.7</td></tr><tr><td>Ni</td><td>1.57±0.8</td><td>1.5±0.6</td><td>1.42±0.5</td><td>1.05±0.5</td><td>1.5±0.5</td><td>1.22±0.6</td><td>0.89±0.5</td><td>0.78±0.4</td><td>3.97±2.4</td><td>3.99±2.2</td><td>3.66±2.4</td><td>2.81±2.3</td><td>3.43±1.2</td><td>3.31±1.1</td><td>2.99±0.8</td><td>1.82±0.4</td><td>2.67±1.7</td><td>2.57±1.7</td><td>2.39±1.7</td><td>1.66±1.4</td></tr><tr><td>Cu</td><td>5.79±6.2</td><td>5.36±5.8</td><td>4.72±5.5</td><td>2.4±3.2</td><td>6.6±6.6</td><td>5.1±4.5</td><td>4.03±3.4</td><td>2.93±2.7</td><td>8.9±10</td><td>7.52±9.1</td><td>7.51±9.8</td><td>3.83±4.3</td><td>5.26±7.6</td><td>4.44±7.1</td><td>4.06±7.1</td><td>2.02±3.1</td><td>6.61±7.7</td><td>5.64±6.6</td><td>5.15±6.7</td><td>2.8±3.3</td></tr><tr><td>Mo</td><td>0.12±0.1</td><td>0.11±0.1</td><td>0.1±0.1</td><td>0.05±0.03</td><td>0.07±0.05</td><td>0.04±0.01</td><td>0.03±0.01</td><td>0.02±0.002</td><td>0.06±0.04</td><td>0.05±0.04</td><td>0.05±0.03</td><td>0.03±0.03</td><td>0.09±0.1</td><td>0.06±0.03</td><td>0.06±0.03</td><td>0.06±0.03</td><td>0.09±0.06</td><td>0.07±0.05</td><td>0.07±0.01</td><td>0.04±0.01</td></tr><tr><td>Cd</td><td>0.96±0.8</td><td>0.62±0.5</td><td>0.56±0.5</td><td>0.33±0.3</td><td>0.3±0.16</td><td>0.1±0.07</td><td>0.1±0.05</td><td>0.1±0.04</td><td>0.19±0.1</td><td>0.17±0.1</td><td>0.15±0.1</td><td>0.12±0.1</td><td>0.12±0.1</td><td>0.11±0.1</td><td>0.1±0.05</td><td>0.08±0.1</td><td>0.27±0.3</td><td>0.19±0.2</td><td>0.17±0.2</td><td>0.12±0.1</td></tr><tr><td>La</td><td>0.3±0.2</td><td>0.28±0.1</td><td>0.21±0.1</td><td>0.1±0.03</td><td>0.25±0.2</td><td>0.21±0.2</td><td>0.13±0.1</td><td>0.07±0.1</td><td>0.8±1.1</td><td>0.64±0.8</td><td>0.59±0.8</td><td>0.33±0.6</td><td>0.32±0.2</td><td>0.3±0.2</td><td>0.25±0.1</td><td>0.1±0.03</td><td>0.43±0.6</td><td>0.37±0.4</td><td>0.31±0.4</td><td>0.14±0.3</td></tr><tr><td>Ce</td><td>0.63±0.3</td><td>0.6±0.3</td><td>0.46±0.2</td><td>0.15±0.1</td><td>0.51±0.5</td><td>0.48±0.5</td><td>0.33±0.3</td><td>0.16±0.1</td><td>2.01±2.9</td><td>1.64±2.3</td><td>1.56±2.3</td><td>0.92±1.7</td><td>0.77±0.5</td><td>0.71±0.4</td><td>0.61±0.3</td><td>0.23±0.1</td><td>1.01±1.6</td><td>0.88±1.2</td><td>0.77±1.2</td><td>0.38±0.9</td></tr><tr><td>Pb</td><td>0.01±0.00</td><td>0.008±0.003</td><td>0.007±0.003</td><td>0.05±0.003</td><td>1.38±0.6</td><td>1.18±0.5</td><td>0.84±0.3</td><td>0.78±0.2</td><td>0.93±0.5</td><td>0.65±0.3</td><td>0.49±0.2</td><td>0.24±0.1</td><td>0.82±0.1</td><td>0.78±0.1</td><td>0.68±0.2</td><td>0.27±0.1</td><td>1.03±0.4</td><td>0.89±0.4</td><td>0.69±0.2</td><td>0.38±0.2</td></tr><tr><td>Th</td><td>1.099±0.36</td><td>1.003±0.49</td><td>0.786±0.3</td><td>0.346±0.2</td><td>0.038±0.03</td><td>0.037±0.04</td><td>0.027±0.03</td><td>0.004±0.003</td><td>0.093±0.03</td><td>0.085±0.01</td><td>0.072±0.01</td><td>0.011±0.01</td><td>0.07±0.02</td><td>0.066±0.02</td><td>0.059±0.011</td><td>0.01±0.003</td><td>0.07±0.03</td><td>0.065±0.03</td><td>0.052±0.02</td><td>0.007±0.005</td></tr><tr><td>U</td><td>0.07±0.03</td><td>0.067±0.03</td><td>0.047±0.02</td><td>0.004±0.004</td><td>0.02±0.02</td><td>0.018±0.02</td><td>0.014±0.01</td><td>0.006±0.006</td><td>0.06±0.06</td><td>0.055±0.06</td><td>0.053±0.06</td><td>0.03±0.04</td><td>0.018±0.007</td><td>0.016±0.006</td><td>0.015±0.004</td><td>0.006±0.003</td><td>0.033±0.03</td><td>0.031±0.03</td><td>0.028±0.03</td><td>0.015±0.02</td></tr></tbody></table>
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Map of study sites with permafrost boundaries (Brown et al., 2001; http://portal.intermap.com, NSIDC) in the Western Siberian Lowland, photos of individual peat profiles, the technique of sampling and size separation procedure.</figcaption></figure>

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<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Examples of DOC (upper row), Al (medium row) and Fe (bottom row) percentage distribution among LMW and 3 colloidal pools in dominant microlandscapes of sporadic/absent (Kogalym), discontinuous/sporadic (Khanymey), discontinuous/continuous (Pangody) and continuous (Tazovsky) permafrost zone. See other elements in Fig S2.</figcaption></figure>
<figure><img src="image_4.png" /></figure>

**Fig. 3** Low molecular weight (< 3 kDa) fraction of OC, major (Fe, Al, P, Ca) and trace (Cu, Ni, Cd, Pb, La, Th, U) elements (R² > 0.4) in dominant micro-landscapes across the permafrost gradient of Western Siberia peatlands. The symbols represent the average of samples from several discrete horizons.
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Depth-averaged molar ratios of OC:Al (A), Fe:Al (B), and OC:Fe (C) in colloids (3 kDa - 0.45 µm) of peat porewaters across four permafrost zones of the WSL.</figcaption></figure>
<figure><img src="image_8.png" /></figure>

B

<figure><img src="image_9.png" /></figure>

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**Fig. 5.** **A:** Percentage of measured colloidal (3 kDa - 0.45 µm) forms in depressions, lakes, rivers (Pokrovsky et al., 2016a), and peat porewaters (this study) of the discontinuous permafrost zone (mean±SD). See pH and other major and trace elements in Table S6 of the Supplement.

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**B:** The evolution of colloidal forms along the hydrological continuum (porewater → depression → lake → river) in permafrost peatlands of western Siberia.
<figure><img src="image_10.png" /></figure>

Cond, pH, SUVA, DIC, DOC, Major and trace elements