Periglacial puzzles: Unravelling environmental controls on alpine solifluction lobe dimensions and shape

Freeze-thaw processes cause periglacial creep called solifluction that forms solifluction lobes: lobate features characterized by a steep riser (front) and a smooth tread (body). These lobes are a widespread phenomenon, yet there is little known about how they develop and what controls their dimensions and shape. Research into solifluction lobe morphometry is crucial for understanding their role in landscape evolution, hydrology, sediment budgets, and providing microhabitats in a warming climate. While their dimensions vary greatly between (and within) different regions, previous research has mainly focussed on lobes in Arctic regions, so our focus is on lobes in an alpine environment. We mapped geomorphic (e.g. length, width, riser height) and vegetation properties (e.g. vegetation cover) of 40 solifluction lobes in the Turtmann Valley, Swiss Alps and equipped all lobes with a TOMST TMS4 logger recording soil moisture and temperature at a 10-minute interval for nearly two years. To determine environmental controls on lobe dimensions, we combined these data with derivatives of a high-resolution digital elevation model (e.g. slope, aspect, flow accumulation) and conducted a Spearman ranked correlation test. Our results revealed that: (i) studied lobes are on average 18 m wide and 28 m long, with a length-width ratio of 2.0, a riser height of 1.8 m and a slope of 31 degrees. Thus, these lobes are longer and more tongue-shaped, with higher risers and on steeper slopes, than many Arctic lobes previously studied. (ii) Largest lobes occur in areas with high flow accumulation (𝝆 = 0.69-0.75), which, together with high fine material contents promotes high risers (𝝆 = 0.40-0.41). (iii) Wider, lobate lobes dominate at higher elevations (𝝆 = 0.49) on north-exposed slopes (𝝆 = 0.53) with long snow (𝝆 = 0.38-0.51) and sparse vegetation cover, while narrow, tongue-shaped lobes occur at lower elevations with high vegetation cover (𝝆 = 0.38-0.55), suggesting a strong topoclimatic control. Overall, our results suggest that alpine solifluction lobe dimensions and shape are controlled by different set of environmental factors than arctic lobes, possibly due to effects of stronger topoclimatic gradients, steeper slopes and higher flow accumulation on mountain slopes.