Isotopic compositions of ground ice in near-surface permafrost in relation to vegetation and microtopography at the Taiga–Tundra boundary in the Indigirka River lowlands, northeastern Siberia

Abstract

The warming trend in the Arctic region is expected to cause drastic changes including permafrost degradation and vegetation shifts. We investigated the spatial distribution of ice content and stable isotopic compositions of water in near-surface permafrost down to a depth of 1 m in the Indigirka River lowlands of northeastern Siberia to examine how the permafrost conditions control vegetation and microtopography in the Taiga–Tundra boundary ecosystem. The gravimetric water content (GWC) in the frozen soil layer was significantly higher at microtopographically high elevations with growing larch trees (i.e., tree mounds) than at low elevations with wetland vegetation (i.e., wet areas). The observed ground ice (ice-rich layer) with a high GWC in the tree mounds suggests that the relatively elevated microtopography of the land surface, which was formed by frost heave, strongly affects the survival of larch trees. The isotopic composition of the ground ice indicated that equilibrium isotopic fractionation occurred during ice segregation at the tree mounds, which implies that the ice formed with sufficient time for the migration of unfrozen soil water to the freezing front. In contrast, the isotopic data for the wet areas indicated that rapid freezing occurred under relatively non-equilibrium conditions, implying that there was insufficient time for ice segregation to occur. The freezing rate of the tree mounds was slower than that of the wet areas due to the difference of such as soil moisture and snow cover depends on vegetation and microtopography. These results indicate that future changes in snow cover, soil moisture, and organic layer, which control underground thermal conductivity, will have significant impacts on the freezing environment of the ground ice at the Taiga–Tundra boundary in northeastern Siberia. Such changes in the freezing environment will then affect vegetation due to changes in the microtopography of the ground surface.

Stable Water Isotope Assessment of Tundra Wetland Hydrology as a Potential Source of Arctic Riverine Dissolved Organic Carbon in the Indigirka River Lowland, Northeastern Siberia

“…The DOC concentration in wetlands was also higher than that of the six largest Arctic rivers (Ob', Yenisey, Lena, Kolyma, Yukon and Mackenzie), which ranged from 4.39 to 10.14 mg C L −1 in summer (from 4.35 to 11.37 mg C L −1 annual average) (Cooper et al, 2008;Amon et al, 2012;Holmes et al, 2012). Furthermore, the organic layer of the surface soil is known to store large amounts of organic carbon in the Arctic region (Hugelius et al, 2014), and in this study area, the surface organic layer is relatively thick, with a depth of approximately 30 cm (Takano et al, 2019). Therefore, the high DOC concentration in the wetlands is considered to be the result of sufficient dissolution from the soil organic carbon stored in the organic layer.…”

“…The growing season is from late June to early August, with a thaw depth of approximately 20-30 cm (Shingubara et al, 2019;Takano et al, 2019). The maximum thaw depth (more than 50 cm in depth) typically occurs in the first half of September, whereas the depth of the surface organic layer is approximately 30 cm (Takano et al, 2019). Active layer freezing usually starts in the second half of September to October and freezes completely from November to December.…”

“…The water samples for isotope analysis were stored in 3-or 6-ml glass vials. For this study, the isotopic compositions of the Indigirka mainstream and tributaries in 2011 and 2012, summer precipitation in 2012-2014, and snow cover were obtained from Takano et al (2019).…”

“…The limitation of drainage due to high river levels not only disturbs DOC release but also maintains high soil moisture during the growing season. This may have several effects such as an increase in methane emissions (Shingubara et al, 2019;Murase et al, 2020), the withering of trees at the taiga-tundra boundary (Liang et al, 2014), and thickening of the seasonally thawed active layer (Takano et al, 2019). Such biogeochemical changes can affect the overall carbon cycle in the Arctic region (McGuire et al, 2010a).…”