Implications of present ground temperatures and relict stone stripes in the Ethiopian Highlands for the palaeoclimate of the tropics

Large sorted patterned grounds are the most prominent features of periglacial and permafrost environments of the mid and high latitudes, but have not yet been verified for the tropics. Here, we report on relict large sorted polygons (up to 8 m in diameter) and large sorted stone stripes (up to 1000 m long, 15 m wide, and 2 m deep) on the ~ 4000 m high Sanetti Plateau in the Bale Mountains, southern Ethiopian Highlands. For a systematic investigation of past and present frost-related processes and landforms in the Bale Mountains, we conducted geomorphological mapping both in the field and on satellite images. 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 29 ground temperature data loggers between 3493 and 4377 m to analyse present frost occurrence and seasonal temperature variations from 2017 to 2020. Finally, we ran a simple experiment and combined recent ground temperature measurements with meteorological data in a statistical model to assess the air temperature depression needed for the past formation of deep seasonal frost and cyclic freezing and thawing on the plateau. Our results show that relict and modern periglacial landforms are common in the Bale Mountains. Nocturnal superficial ground frost on the plateau occurs at 35–90 days per year, but the mean annual ground temperature (~ 11 °C) is far off from seasonal or permanent frost conditions. The modelling experiment suggests a minimum air temperature depression on the plateau of 7.6 ± 1.3 °C for the emergence of several decimetre deep seasonal frost. The stone stripes probably formed under periglacial conditions in proximity of a palaeo ice cap on the plateau during the coldest period(s) of the last glacial cycle. We hypothesise that the slightly inclined and unglaciated areas of the plateau, the coexistence of regolith and large blocks, the occurrence of deep seasonal frost, as well as relatively dry conditions beyond the ice cap provided ideal conditions for frost heave and sorting and the formation of large sorted patterned grounds. The presence of these landforms and the associated air temperature depression provide further evidence for an amplified cooling of high tropical mountains during the last glacial period that is yet not well captured in global climate models.