Figure 7. Variations in (a) dFe concentration, (b) DOC concentration, and (c) EC with an increase in the coverage of wetland (Mari ). Figure panels on the right-hand side shows variations in dFe concentration with increases in (d) EC and (e) DOC concentration. As written in the subsection 2.6, the approximation line or curve on the each graph is the most suitable regression equation among the calculated liner function and non-liner functions (power, exponential, and logarithmic).
4 Discussion
4.1 Permafrost distribution in the Tyrma region and comparison of dFe and DOC concentrations in other similar permafrost-regime regions.
Since the produced landcover map in this study (Figure 6) was based on NDXI and slope in the ground truth areas, the areas determined to be wetland (Mari ) most likely have the same vegetation as wetland (Mari ). Moreover, according to the authors’ previous work and field experience, permafrost existence underneath the wetland (Mari ) was confirmed in many areas of the Tyrma region (Tashiro et al., 2020). These facts allow us to expect that permafrost is distributed with a fairly high possibility under the wetland-covered areas shown in the landcover map (Figure 6). Given this perspective and the calculated wetland coverage in the large river basins (Table 1), the permafrost coverage percentage in the Tyrma region can be estimated at around 15-35%. This coverage corresponds with the estimated permafrost regime (sporadic permafrost distribution: 10–50%) by Obu et al. (2019) using a hemispheric-scale model.
In the Ob River, flows north and west across the western Siberian lowland, there have been some studies about river water chemistry in the sporadic permafrost areas (around N60°–N62°). Pokrovsky et al. (2016) showed that riverine dFe concentrations (<0.45 µm like this study) in these regions in summer were in the range of 0.5–3.5 mg L–1, and Vorobyev et al. (2017) showed that they were 0.2 to 1.8 mg L–1, respectively. Compared with these data in the Ob River Basin, riverine dFe concentrations (<0.02–0.540 mg L–1) in the Tyrma regions were relatively low. However, riverine DOC concentrations in the Tyrma region and those in sporadic permafrost regions in the Ob River Basin are almost in the same range: 7.4–29.5 mg L–1 in the Tyrma region and 6–33 mg L–1 in the Ob region (Pokrovsky et al., 2015; Vorobyev et al., 2017) . One of the reasons for the difference in dFe concentration among the Tyrma region and the Ob region (N60°–N62°) despite the same permafrost regime is likely soil profile in the active layer. Since soil thawing in summer allows water to interact with mineral soil horizon under the peat soil layer, this can increase dFe discharge into rivers as known in Siberian watersheds with sporadic or less permafrost distribution (Bagard et al., 2011; Pokrovsky et al., 2016; Vorobyev et al., 2017). In the Tyrma region, thick peat soil layers from the surface to near the permafrost table are formed in the wetland (Tashiro et al., 2020); therefore, water interaction with mineral soils may be restricted in the flow path from the permafrost wetlands (Mari ) to rivers in summer.
4.2 Role of permafrost wetland (Mari ) in supplying dFe and DOC to rivers
Our fine-scale landcover map enabled us to understand the distribution of permafrost wetland (Mari ) in the Tyrma region and detect a relationship between its coverage and water chemistry. As shown in Figure 7, river water chemistry was greatly influenced by the coverage of wetland (Mari ): dFe and DOC concentrations increased, but EC decreased with an increase in the wetland coverage. These results agree well with the findings in northern Sweden and northwestern Canada where permafrost is present under peatlands (Olefeldt et al., 2013, 2014). It is widely accepted that whether water predominantly passes through the peat soil layer or the mineral soil layer is important in determing the river water chemistry. For example, DOC discharge generally reduces during transport through the mineral soil layer because of adsorption on clay minerals (Kothawala et al., 2012; Smedberg et al., 2006), which may also reduce the discharge of Fe-organic complexes to rivers. According to the soil profiles in the Tyrma region reported by Tashiro et al. (2020), peat soil accumulates more than 40 cm from the surface to the permafrost table in the wetlands (Mari ), whereas peat soil occupies only 7–12 cm of the surface in the forests. Based on this fact and our findings, it follows that river water chemistry in the Tyrma region is greatly regulated by the combination of two water resources: the peat soil layer in the wetlands (Mari ) and the mineral soil layer in the forests. Moreover, this study found a strong positive correlation between riverine dFe and DOC concentrations in the Tyrma region (Figure 7d), suggesting that wetland(Mari )-derived dFe comprises mainly Fe-organic complexes which are stable in river waters with neutral pH and can be transported to the ocean (Tipping, 2002). This is consistent with Levshina (2012) which investigated the ratio of Fe bounded with humic acids to riverine dFe concentration in the Amur-Mid Basin, and also with other previous studies in the boreal regions (Björkvald et al., 2008; Ingri et al., 2006).
The observed dFe concentration in the large rivers in the Tyrma region was 0.12–0.38 mg L–1 (Table 1), which is as high as the dFe concentration observed in July in the Bureya River and the Zeya River which are the representative large rivers in the Amur-Mid Basin (Nagao et al., 2007). Given the considerable water discharge of such large rivers, there will be no doubt that great quantities of dFe are supplied from Amur-Mid Basin to the Amur Main River, although riverine dFe concentration varies seasonally (Tashiro et al., 2020). According to the previous study and the existent wetland map in the Amur River Basin, wetlands are widely distributed in the Bureya River Basin and Zeya River Basin (Egidarev & Simonov, 2007; Egidarev et al., 2016). There is unfortunately little information about wetland types and permafrost distribution in these regions, it is thus difficult to understand how important the wetland (Mari ) is as a dFe source for the whole of Amur-Mid Basin. From our findings, however, it should be emphasized that wetland (Mari ) is major dFe source for rivers in the Tyrma region and can be important dFe source for rivers in other regions of the Amur-Mid Basin where permafrost is sporadically distributed. In addition, this study warns the possibility of change in riverine dFe concentration in the Amur-Mid Basin due to permafrost degradation under a warming climate. Recent studies pointed out that permafrost degradation will influence on biogeochemical cycle of iron because of change in redox conditions and water flow path induced by increase in active layer thickness (Patzner et al., 2022; Pokrovsky et al., 2016). In the Amur River Basin, the evidence of permafrost degradation was recently found by Winterfeld et al. (2018) using Δ14C, and it may be more serious near the permafrost boundary like the Tyrma region. Considering the great contribution of wetland-derived dFe to the marine ecosystem in the Sea of Okhotsk (Nishioka et al., 2014; Shiraiwa, 2012; Suzuki et al., 2014), we will need to pay careful attention to the influence of permafrost degradation on riverine dFe concentration in the Amur-Mid Basin.
5 Conclusions
To assess the importance of permafrost wetlands, called Mari , as a dFe source for rivers, we made a landcover map with fine resolution (30 m) using Landsat-8 data and a machine learning technique (decision tree analysis). As a result, this study clearly demonstrated that river water chemistry in the Tyrma region was greatly influenced by the coverage of wetland (Mari ): dFe and DOC concentrations increased, but EC decreased with an increase in the wetland coverage in the watershed. To our knowledge, this study is the first to discuss the role of permafrost wetlands (Mari ) in river water chemistry and to show the direct evidence of the importance of permafrost wetlands (Mari ) for riverine dFe and DOC concentrations in the Amur-Mid Basin. Given that permafrost degradation is predicted to occur due to ongoing climate change, further research will be required to assess the influence of permafrost degradation on the watershed hydrological cycle, iron dynamics, and riverine dFe cocentration in the Amur-Mid Basin because this issue may have the potential to change the amount of dFe discharged to the Sea of Okhotsk.