Abstract:
Large forms of sorted patterned ground belong to the most prominent geomorphic features ofperiglacial and permafrost environments of the mid-latitudes and polar regions, but they were hitherto unknownin 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 High-lands. These geomorphic features are enigmatic since forms of patterned ground exceeding several metres arecommonly associated with distinct seasonal ground temperatures, oscillating around 0◦C. To systematicallyinvestigate present frost phenomena and relict periglacial landforms in the Bale Mountains, we conducted ex-tensive geomorphological mapping. The sorted stone stripes were studied in more detail by applying aerialphotogrammetry, ground-penetrating radar measurements, and36Cl surface exposure dating. In addition, weinstalled ground temperature data loggers between 3877 and 4377 m to analyse present frost occurrence andseasonal 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 occurrencewould require a mean annual ground temperature depression of about 11◦C, corresponding to an air tempera-ture decrease of about 6–8◦C (relative to today) as inferred from a simple statistical ground temperature modelexperiment. Our results suggest the formation of the large sorted stone stripes under past periglacial conditionsrelated to lateral and vertical frost sorting in the course of cyclic freezing and thawing of the ground. It is likelythat the stone stripes formed either in proximity to a former ice cap on the Sanetti Plateau over the last glacial pe-riod 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 thesegeomorphic features provides independent evidence besides glacial landforms for unprecedented palaeoclimaticand palaeoenvironmental changes in the tropical Bale Mountains during the (Late) Pleistocene.
