Periglacial puzzles: unravelling environmental controls on solifluction lobes

Freeze-thaw processes cause periglacial creep called solifluction that forms solifluction lobes, which are lobate features characterized by a steep riser (front) and a relatively smooth tread (body). These lobes are a widespread phenomenon, yet there is still little known about how they develop and what controls their dimensions. Research into their morphometry is important, as the dimensions of solifluction lobes vary greatly between different regions and slopes. While previous research has focussed on lobes in Arctic regions, where elevation and fine material content have been identified as important factors, they also occur in alpine regions, where they are subject to entirely different topographic and ecological conditions.

 

In this study, we mapped geomorphic (e.g. length, width, riser height) and vegetation properties (e.g. vegetation cover and species occurrence) of 44 solifluction lobes in the Turtmann valley, Swiss Alps. In addition, each lobe was equipped with a TOMST logger recording soil moisture and temperature at a 10-minute interval for nearly two years. This data, in combination with derivatives of a high-resolution DEM (e.g. slope, aspect, flow accumulation), was used in a Spearman ranked correlation test to determine the topographic, topoclimatic and vegetation factors that control lobe morphometry and dimensions within our study area.

 

Our statistical analysis reveals that:

• The lobes in our study area are on average 19.7 m wide and 39 m long, with an average length-width ratio of 2.1 and an average riser height of 1.8 m. They have an average slope of 31 degrees. Thus, our lobes are approximately four times as long, have a twice as big length-width ratio, twice as high risers and three times steeper slopes than many arctic lobes previously studied.
• Lobe morphometry is significantly (p< 0.05) correlated with elevation (width, L/W-ratio), temperature (width, L/W-ratio), snow cover duration (width), ripening date (width, size), melt-out date (width), flow accumulation (width, length, size, riser height) and vegetation cover (width, size, L/W-ratio). This leads to a general trend of larger, wider lobes at higher elevations (𝝆 = -0.32), and longer, narrower lobes at lower elevations, influenced by changing temperature, vegetation, flow accumulation and snow characteristics over the elevational gradient. This contrasts with arctic studies where lobe size and width tend to increase downslope.
• None of the lobes found in this study are influenced by permafrost, and most do not freeze every winter due to snow cover insulation.

 

Our research reveals that (i) alpine solifluction lobes differ in size, shape and steepness from those found in arctic areas and therefore need to be studied separately; (ii) the morphometry of alpine solifluction lobes is influenced by elevation through vegetation, temperature, snow and flow accumulation.