Abstract
Abstract. Large forms of sorted patterned ground belong to the most prominent geomorphic features of periglacial and permafrost environments of the mid-latitudes and polar regions, but they were hitherto unknown in the tropics. Here, we report on relict large sorted stone stripes (up to 1000 m long, 15 m wide, and 2 m deep) on the ca. 4000 m high central Sanetti Plateau of the tropical Bale Mountains in the southern Ethiopian Highlands. These geomorphic features are enigmatic since forms of patterned ground exceeding several metres are commonly associated with distinct seasonal ground temperatures, oscillating around 0 ∘C. To systematically investigate present frost phenomena and relict periglacial landforms in the Bale Mountains, we conducted extensive geomorphological mapping. The sorted stone stripes were studied in more detail by applying aerial photogrammetry, ground-penetrating radar measurements, and 36Cl surface exposure dating. In addition, we installed ground temperature data loggers between 3877 and 4377 m to analyse present frost occurrence and seasonal ground temperature variations. Superficial nocturnal ground frost was measured at 35–90 d per year, but the ground beneath the upper few centimetres remains unfrozen the entire year. Seasonal frost occurrence would require a mean annual ground temperature depression of about 11 ∘C, corresponding to an air temperature decrease of about 6–8 ∘C (relative to today) as inferred from a simple statistical ground temperature model experiment. Our results suggest the formation of the large sorted stone stripes under past periglacial conditions related to lateral and vertical frost sorting in the course of cyclic freezing and thawing of the ground. It is likely that the stone stripes formed either in proximity to a former ice cap on the Sanetti Plateau over the last glacial period due to seasonal frost heave and sorting or they developed over multiple cold phases during the Pleistocene. Although certain aspects of the genesis of the large sorted stone stripes remain unresolved, the presence of these geomorphic features provides independent evidence besides glacial landforms for unprecedented palaeoclimatic and palaeoenvironmental changes in the tropical Bale Mountains during the (Late) Pleistocene.
Highlights
Sorted patterned ground in the form of stone polygons, circles, or stripes is one of the most striking features of periglacial and permafrost environments
Groos et al.: The enigma of relict large sorted stone stripes in the tropical Ethiopian Highlands ground on the order of centimetres to decimetres are common on many mid-latitude and high tropical mountains as superficial nocturnal frost is sufficient for their formation (e.g. Francou et al, 2001; Matsuoka, 2005)
The coarse stone stripes on the slightly inclined Sanetti Plateau, which alternate with fine regolith stripes, are an exceptional geomorphic feature as they consist of very large clasts and are up to 2 m deep, 15 m wide, and 1000 m long
Summary
Sorted patterned ground in the form of stone polygons, circles, or stripes is one of the most striking features of periglacial and permafrost environments. Sorted stone polygons are found in flat areas while stripes typically occur on slightly inclined slopes Both forms are the product of a self-organising process related to the cyclic freezing and thawing of the ground (Kessler and Werner, 2003). Groos et al.: The enigma of relict large sorted stone stripes in the tropical Ethiopian Highlands ground on the order of centimetres to decimetres are common on many mid-latitude and high tropical mountains as superficial nocturnal frost is sufficient for their formation (e.g. Francou et al, 2001; Matsuoka, 2005). Active large sorted stone circles, polygons, and stripes are well-documented for the High Arctic (e.g. Washburn, 1980; Kessler and Werner, 2003; Hallet, 2013) and in relict form for some mid-latitude mountains (e.g. Ball and Goodier, 1968; Vopata et al, 2006; André et al, 2008; Krížek et al, 2019), but they have not yet been reported for any site in the tropics
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