Patterns and rates of soil movement and shallow failures across several small watersheds on the Seward Peninsula, Alaska

Abstract

Abstract. Thawing permafrost can alter topography, ecosystems, and sediment and carbon fluxes, but predicting landscape evolution of permafrost-influenced watersheds in response to warming and/or hydrological changes remains an unsolved challenge. Sediment flux and slope instability in sloping saturated soils have been commonly predicted from topographic metrics (e.g., slope, drainage area). In addition to topographic factors, cohesion imparted by soil and vegetation and melting ground ice may also control spatial trends in slope stability, but the distribution of ground ice is poorly constrained and hard to predict. To address whether slope stability and surface displacements follow topographic-based predictions, we document recent drivers of permafrost sediment flux present on a landscape in western Alaska that include creep, solifluction, gullying, and catastrophic hillslope failures ranging in size from a few meters to tens of meters, and we find evidence of rapid and substantial landscape change on an annual timescale. We quantify the timing and rate of surface movements using a multi-pronged, multi-scalar dataset including aerial surveys, interannual GPS surveys, synthetic aperture radar interferometry (InSAR), and climate data. Despite clear visual evidence of downslope soil transport of solifluction lobes, we find that the interannual downslope surface displacement of these features does not outpace downslope displacement of soil in locations where lobes are absent (downslope movement means: 7 cm yr−1 for lobes over 2 years vs. 10 cm yr−1 in landscape positions without lobes over 1 year). Annual displacements do not appear related to slope, drainage area, or modeled total solar radiation but are likely related to soil thickness, and volumetric sediment fluxes are high compared to temperate landscapes of comparable bedrock lithology. Time series of InSAR displacements show accelerated movement in late summer, associated with intense rainfall and/or deep thaw. While mapped slope failures do cluster at slope–area thresholds, a simple slope stability model driven with hydraulic conductivities representative of throughflow in mineral and organic soil drastically overpredicts the occurrence of slope failures. This mismatch implies permafrost hillslopes have unaccounted-for cohesion and/or throughflow pathways, perhaps modulated by vegetation, which stabilize slopes against high rainfall. Our results highlight the breadth and complexity of soil transport processes in Arctic landscapes and demonstrate the utility of using a range of synergistic data collection methods to observe multiple scales of landscape change, which can aid in predicting periglacial landscape evolution.

Permafrost extent sets drainage density in the Arctic

“…The proposed mechanism is consistent with field ( 40 , 41 ) and remote-sensing observations ( 27 ) of surface displacement on hillslopes with water tracks. Increased temperatures and rainfall can also generate active-layer detachments in convergent zones on hillslopes ( 42 ), an advective process that can drive higher sediment discharges for decades by creating new incised, contiguous channels ( 43 ).…”

“…Based on circumpolar carbon stock estimates to 100 cm depth ( 55 ), 4.35 10 7 Gt C are contained within these per-degree areas vulnerable to channel expansion within the studied watersheds. While carbon may accumulate on landscapes for hundreds or thousands of years ( 5 ), permafrost channel heads have been observed to migrate tens of meters per year ( 41 , 44 ), but quantifying these processes’ long-term impact on carbon fluxes has been limited. Post-glacial warming at the beginning of the Holocene (11,650 calendar years before present) denuded Arctic landscapes of sediment ( 56 ) and carbon ( 57 ) on centennial timescales.…”

Estimating Permafrost Distribution Using Co‐Located Temperature and Electrical Resistivity Measurements

“…T47 consists of south, east, and north–east facing slopes, separated by two streams that merge toward the bottom of the watershed (Figure 1b). The site is mostly covered by tussock tundra, dwarf shrubs and grasses with patches of tall shrubs, particularly near the streams (Del Vecchio et al., 2022), and outcropping bedrock at the highest elevations. KG is the site that is farthest inland.…”

High‐Resolution Maps of Near‐Surface Permafrost for Three Watersheds on the Seward Peninsula, Alaska Derived From Machine Learning

“…The metrics slope and aspect were calculated with the Geospatial Data Abstraction Library (GDAL/OGR contributors, 2022) using the 3 m DEM. Topographic curvature, a measure of landscape convexity and concavity, was calculated as a polynomial fit over a 45 m window, which is the window size that smooths out the microtopographic roughness present at the sites due to numerous hillslope failures (Del Vecchio et al, 2022), yet still captures the site-scale trends in topographic curvature, such as hollows and ridgetops. Vegetation canopy height models (CHM) were derived from the lidar point clouds at each site using the grid_canopy function in the lidR package.…”