Abstract:
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.
