Significantly enhanced mid Holocene fluvial activity in a globally important, arid‐zone wetland: The Okavango Delta, Botswana

Abstract

Information on past dryland environments is commonly derived from geomorphological landforms and sediments ('geoproxies'). The Okavango Delta in the middle Kalahari, Botswana, has been subject to a long history of arid-humid transitions but its potentially rich archive of fluvial geoproxies is largely untapped. Previous palaeoenvironmental studies in the Delta region have focused mainly on aeolian dunes, lacustrine beach ridges, and rare pollen sequences in surrounding locations, and the Delta's channel dynamics have remained poorly constrained, both chronologically and in quantitative palaeohydrological terms. Focusing on the Delta's Xugana region, we present the first optically stimulated luminescence ages for palaeochannel fills and scroll bars, revealing significantly enhanced fluvial activity in the $7-4 ka interval. Along 120-150 m-wide, sand-bed, sinuous palaeochannels, lateral migration and bend cutoffs were driven by palaeodischarges up to $350-450 m 3 s À1 , at least nine

Highly Contrasted Geochemical Pattern in Sediments of the Okavango Delta, Botswana Driven by Dust Supply, Hydrological Heritage and Biogeochemical Reactions

“…Sedimentation in the Okavango alluvial fan initiated during the Late Pliocene—Early Pleistocene during the retreat of Lake Deception (McCarthy, 2013; Moore et al., 2012; Podgorski et al., 2013; Ringrose, 2022). The hydrodynamics of the Delta has been strongly influenced by Quaternary climate fluctuations as indicated by a major switch from large (120–150 m wide) meandering channels prevailing between about 7 and 4 ka ago to the modern narrow (<20 m wide) channels system (Tooth et al., 2021). The 7 to 4 ka humid period was following a noticeably dry period that affected southern Africa from about 15 to 9 ka (Telfer & Thomas, 2007).…”

Reply to Comment by Terence McCarthy and Marc Humphries on “Highly Contrasted Geochemical Pattern in Sediments of the Okavango Delta, Botswana Driven by Dust Supply, Hydrological Heritage and Biogeochemical Reactions”

“…During the last 40 ka, the hydrodynamics of the fan varied several times, following regional climate changes. Tooth et al (2021) highlighted a period of high discharge in the 7 to 4 ka interval, leading to the formation of >100 m wide meandering channels. Remnants of some of these channels are still preserved in the topography.…”

“…The origin of the thousands of islands that form the Okavango alluvial fan is thus very variable. Some, especially north of Maun town derive from paleo-dunes that have been dissected by the invading channels, others are remnants of meander bars accumulated by large paleo-channels (possibly those described by Tooth et al (2021) at 7 to 4 ka), some are large portions of land emerging today from the floodplain but supporting evidence of paleo-channels, some may be either paleo sand-bars or inverted channels, and finally a large proportion of them have no clear origin, formed through multiple dissection episodes of surface morphologies. Given their highly variable origin, each island must have a specific sedimentary structure with varying grain size and sediment facies.…”

Professor Kenneth John Gregory (1938–2020): The career, contributions and legacy of an eminent geographer and fluvial geomorphologist

“…Tooth et al (2022) investigate large, sinuous palaeochannels and scroll bars preserved in the Okavango Delta, located in the arid Kalahari region of Botswana. By using luminescence dating to establish the timing of fluvial deposition, and extrapolating hydraulic geometry relationships established for modern channels, the authors demonstrate significantly enhanced inflows during the mid Holocene (~7–4 ka), which probably resulted from a sustained southward shift of the average austral summer position of the African rainfall belt.…”