The enigma of relict large sorted stone stripes in the tropical Ethiopian Highlands

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.

A multiannual ground temperature dataset covering sixteen high elevation sites (3493–4377 m a.s.l.) in the Bale Mountains, Ethiopia

“…Similar to the thermal conditions on Mount Kenya (Grab et al, 2004), nocturnal superficial ground frost is common in the Bale Mountains during the dry season (Fig. 6) and favours the formation of typical small-scale periglacial landforms (Groos et al, 2021c). Because of the strong insolation in the tropics, the diurnal temperature amplitude at the ground surface can reach up to 50°C.…”

“…Although an in-depth evaluation of the presented dataset is beyond the scope of this contribution, the conducted statistical analysis provides fundamental insights in the ground thermal regime and in the spatio-temporal variations of ground temperature in the Ethiopian Highlands. Relict large sorted stone stripes and polygons on the Sanetti Plateau are a strong indicator for sporadic permafrost or at least seasonal ground frost in the Bale Mountains during the Late Pleistocene (Groos et al, 2021c). However, the current mean annual ground temperature in the order of 7 to 11°C at the highest sites (Fig.…”

“…While the southern margin of the plateau is bounded by the Harenna Escarpment, broad U-shaped valleys characterise the western, northern, and eastern declivity of the mountain range. An exceptional feature on the Sanetti Plateau that provides evidence for seasonal or permanent ground frost in the past are relict large sorted stone stripes with a width of up to 15 m and a length of up to 1000 m (Groos et al, 2021c).…”

“…To monitor ground temperature in the Bale Mountains and to establish a modern reference for the paleoclimatic interpretation of periglacial landforms such as the large relict stone stripes on the Sanetti Plateau (Groos et al, 2021c), we have installed two different types of data loggers (see Fig. 2): high-quality UTL-3 Scientific Dataloggers (hereafter abbreviated as GT) and lowcost tempmate.®-B2 ground temperature data loggers (hereafter abbreviated as TM).…”

“…All generated datasets are stored in an on-demand processing database (Wöllauer et al, 2020). Part of the data has already been presented in previous studies (Lemma et al, 2020;Bittner et al, 2021;Groos et al, 2021c), but neither the meteorological nor the ground temperature datasets are yet openly accessible. The aim of this contribution is therefore to describe the ground temperature monitoring network that has been established in the Bale Mountains between 3493 and 4377 m and to share a consistent and comprehensive multiannual ground temperature dataset from a remote afro-alpine study site.…”