Recognition and palaeoclimatic implications of late Quaternary niche glaciation in eastern Lesotho

Abstract

Geomorphic evidence of former glaciation in the high Drakensberg of southern Africa has proven controversial, with conflicting glacial and non-glacial interpretations suggested for many landforms. This paper presents new geomorphological, sedimentological and micromorphological data, and glacier mass-balance modelling for a site in the Leqooa Valley, eastern Lesotho, preserving what are considered to be moraines of a former niche glacier that existed during the Last Glacial Maximum (LGM). The geomorphology and macro-sedimentology of the deposits display characteristics of both active and passive transport by glacial processes. However, micromorphological analyses indicate a more complex history of glacial deposition and subsequent reworking by mass movement processes. The application of a glacier reconstruction technique to determine whether this site could have supported a glacier indicates a reconstructed glacier equilibrium line altitude (ELA) of 3136 m a.s.l. and palaeoglacier mass balance characteristics comparable with modern analogues, reflecting viable, if marginal glaciation. Radiocarbon dates obtained from organic sediment within the moraines indicate that these are of LGM age. The reconstructed palaeoclimatic conditions during the LGM suggest that snow accumulation in the Drakensberg was significantly higher than considered by other studies, and has substantial relevance for tuning regional climate models for southern Africa during the last glacial cycle.

multi-disciplinary review of late Quaternary palaeoclimates and environments for Lesotho

“…However, more recent quantitative approaches, including detailed mapping, macro-and micro-sedimentology, and obtaining ages for small moraines (Figure 2a), have provided more substantive evidence for Quaternary glaciation in eastern Lesotho. Such glaciation is dated to ~19 500-17 350 calibrated years before present (cal yr BP) 6,12,13 , which coincides with the terminal stage of the LGM in southern Africa 2 . While the accuracy of the term 'niche glaciation' has been queried 29 , evidence based on these moraines for small-scale, spatially restricted glaciation during the LGM is undisputed to date 18 .…”

“…3,4 Existing literature for Lesotho is based almost entirely on archaeological and geomorphological evidence that spans discrete periods of human occupation or glacial and periglacial activity ( Figure 1). Such past research has offered numerous inferences on possible palaeoclimatic changes throughout the late Quaternary in Lesotho 5,6 , but the chronological continuity and quantification of past climates lack both detail and objective confirmation [7][8][9] .…”

“…A variety of Quaternary periglacial and glacial studies on Lesotho's eastern high mountain region have emphasised colder conditions during the late Pleistocene, including but not limited to the Last Glacial Maximum (LGM) 6,[10][11][12][13] and Holocene neoglacial episodes. The latter events are presumed to have occurred at ~4500 years before present (yr BP), 4000 yr BP, and 3000-2000 yr BP respectively.…”

“…However, palaeoenvironmental interpretations of associated precipitation levels during these cold periods are conflicting. 6,8,15,31 Boelhouwers and Meiklejohn 8 suggested that some periglacial features, including large patterned ground, solifluction mantles and blockstreams ( Figure 2c) were indicative of dry, cold conditions at their time of formation, whereas typically erosional features, including nivation hollows, cirques and cryoplanation terraces are likely indicative of wetter conditions during cold periods 36 . Other scholars 11,26 have argued that similar periglacial features, including large sorted patterned ground near Mafadi Summit ( Figure 2b) and at Thabana Ntlenyana, and pronival ramparts at Thabana Ntlenyana, imply relatively high moisture availability during their formation.…”

A review of topographic controls on moraine distribution

“…The distribution and dimensions of cirque glaciers are strongly controlled by topography (Mills et al, 2009;Bendle and Glasser, 2012). For the purpose of this paper, a distinction is made between topoclimatic factors (such as aspect, which regulates the receipt of solar radiation or allows snow to accumulate in the lee of prevailing winds), which directly impact local climatic conditions, and topographic factors (such as ice and snow avalanching from surrounding peaks), which regulate glacier size without a direct impact on local climate.…”