Quaternary megapaleolake system in Northwest Botswana: evidence of Lacustrine deposition and geographical extent using multiple geochemical proxies

Reconstruction of climate dynamics in southern Africa has been immensely

constrained by the absence of continuous proxies records. Sediments provide

potential sources of studying past environments because their makeup is a direct

response to variabilities in the environment and climate. In this study a multiple

geochemical proxy approach was used to investigate two 30 m deep sediments cores

from northern Botswana to construe their various geological processes and

environments of deposition. Based on these proxy studies two distinct but dissimilar

hydrological and climatic settings were identified on the two cores. For BH 11

lithological studies reveal a sediments sequence that has a bottom unit with several

repetitive and alternating layers of sand-silt-clay, and a dominantly silty/clayey unit at

the top. This arrangement of units likely indicates lake-related processes with input

from regional rivers. Magnetic susceptibility measurements indicate a dual source of

the sediments. Sediments in the lower unit (30 to 7 m depth) could have come from a

distal sub-tropical source with rocks that are richer in magnetic minerals. Sediments

in the upper section (from 7 m depth to surface) were strongly influenced by a more

felsic source that was likely of local surrounding landscapes. Carbon dynamics in the

middle portions of the lower unit of BH 11 suggests deposition under more humid

conditions than in the upper section, within a periodically drying out fluvial system.

Isotopically lighter δ

13Corg values (-26 to -28 ‰) for the lower section compared to the

upper section (~ -20‰) reflect difference in the vegetation cover, moisture conditions

and probably climate between the two sections. For core BH 10, sediments in the

lower unit were sourced locally from a less magnetic and humid landscape. The upper

unit signifies continually drying up condition to present day. The two sites may have

hosted major lakes at different times in the past. This study validates the effectiveness

of sediments in unravelling environmental and climate change in fluvial-lacustrine

depositional settings, and gives the possibilities of connecting the sediment record to

regional stratigraphic markers.