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

• Published in: Earth surface processes and landforms 2022-03, Vol.47 (3), p.854-871
• Main Authors: Tooth, Stephen, McCarthy, Terence S., Duller, Geoff A. T., Assine, Mario L., Wolski, Piotr, Coetzee, Grace
• Format: Article
• Language: English
• Subjects: Arid lands; Arid zones; Aridity; Beach ridges; Catchment area; Catchments; Channels; Geomorphology; Holocene; Holocene climate; Hydroclimate; Kalahari; Landforms; luminescence dating; Okavango Delta; palaeodischarge; Pollen; Rain; Rainfall; Ridges; scroll bar; Sediments; Tributaries; Tropical climate; Valleys
• ISSN: 0197-9337; 1096-9837
• DOI: 10.1002/esp.5289

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 m3 s−1, at least nine times those of the present‐day channels. These palaeochannels represent the last major phase of meander activity in the middle and lower Delta. The palaeodischarges imply significantly higher rainfall over the catchment in the mid Holocene, possibly resulting from a southwards shift of the African tropical rainbelt. Over the last few thousand years, diminished fluvial activity in the Xugana region has continued;