Investigating paleoflooding in the Okavango Delta, northwest Botswana from sedimentary records.

摘要

Sediment samples were collected from the Shorobe Quarry located at the distal end of the Okavango Delta. The Okavango Delta is located in the middle Kalahari in northwest Botswana which currently has a semiarid environment but has hosted mega-lakes in the past. Sediment samples were used to investigate the presence of these mega-paleolakes, in particular Paleolake Makgadikgadi. Previous studies have only investigated mega-paleolakes within the present day lake basins, although the mega-lakes extended beyond these lake basins. The Shorobe Quarry sediments are located beyond the present day lake basins but within the spatial boundaries of the mega-lake in the lower Okavango Delta. Sediment samples were used to determine the presence and evolution of Paleolake Makgadikgadi at highstand by determining grain size variations, magnetic susceptibility (MS), total inorganic carbon (TIC), total organic carbon (TOC, and stable carbon isotopes of organic carbon (delta13C). The sediment record from the edge of Paleolake Makgadikgadi consists of an unconformity between a lower consolidated pre-lake sediment section that consists of calcretized alluvium upon which unconsolidated lacustrine sediments are deposited. The lacustrine sediments represent Paleolake Makgadikgadi at highstand. The filling of the lake is shown by high silt and clay content, increasing MS values, high TIC concentration and increasing TOC concentration. Decreasing hydrologic input as Paleolake Makgadikgadi nears highstand is shown by decreasing silt and clay content, decreasing TIC concentration and decreasing TOC concentration. The transition to drier climates and Paleolake Makgadikgadi reaching highstand is shown by increased sand content, decreasing MS values, negligible TIC concentration and slightly decreasing TOC concentration. A mostly C3 organic matter dominance in the sediment record is shown by delta13C values of organic carbon remaining around -21 /.