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

Abstract

The Okavango Delta in North Botswana is one of the few active alluvial mega-fans developing in an endorheic setting (Assine & Soares, 2004;McCarthy, 2013) (Figure 1). This fan forms a vast swamp flooded annually by water from the highlands of Angola to the North and it supports an exceptional ecosystem in the middle of the semi-arid Kalahari Desert. Due to their isolation, endorheic ecosystems result from tight coupling between climatic, topographic and geologic settings. Changes in one or several of these controlling factors have dramatic effects on the fragile equilibrium of the ecosystems, especially in semi-arid or arid climate contexts (Yapiyev et al., 2017). In the context of rapid climate change and of a regionally increasing anthropogenic pressure, especially on water resources (Ministry of Mineral, Energy and Water Affairs, Republic of Botswana, 1997), improving our knowledge of the structure and functioning of the sensitive Okavango geo-ecosystem is a fundamental challenge. Among the major threats to the ecosystem of the Delta, a decrease in the amount and quality of the water is probably the most important. In order to evaluate the impact of global change on the ecosystem, it is thus necessary to fully understand the hydrology of the Delta. In that respect, describing the geometry, sedimentological structure and geochemical pattern of the various aquifers is essential to understand the geochemical exchanges between the surface and groundwater, the soil and the biosphere.Many studies have already addressed the sedimentation, the hydrology and the vegetation dynamics of the fan in terms of sediment input or water dynamics. A majority of these studies also deal with the effects of anthropogenic activity such as, water pumping, or those of tectonic activity along the faults bordering the rift (Ellery

Comment on “Highly Contrasted Geochemical Pattern in Sediments of the Okavango Delta, Botswana Driven by Dust Supply, Hydrological Heritage and Biogeochemical Reactions” by Jolivet et al.

“…Samples containing high concentrations of CaO are enriched in REEs (Figure 2a) and exhibit clear fractionation of the Gd/Yb ratio (Figure 2b), reflecting the varying ability of REEs to incorporate into the solid phase (log D Gd = 3.4 vs. log D Yb = 1.9). We suggest that this process is responsible for the distinctive concave REE concentration patterns Jolivet et al (2023) observed in the island subsurface samples they analyzed. Rather than indicating the presence of two non-connected aquifers, the differing REE patterns observed in sediment samples from Nxaraga Island reflects the preferential fractionation of REEs into calcite, which precipitates as a consequence of evapotranspiration.…”

“…The implication of this is that large regions of the floodplain may revert to dry land and remain so for decades (McCarthy et al, 2003), during which time termites are able to colonize the dry surfaces. Jolivet et al (2023) and Dauteuil et al (2021) proposed that the groundwater beneath floodplains and islands are hydrologically separate. However, several studies have demonstrated there is a complete evolutionary link between fresh groundwater beneath the floodplain and saline water beneath islands as mentioned above (e.g., McCarthy & Ellery, 1994 Figures 1c and 1d).…”

“…During the hot summer months, saline groundwater which has risen to surface evaporates, leaving efflorescent crusts on the soil surface. Jolivet et al (2023) are of the opinion that the near surface sediment which forms the islands is stratified, with discrete clay-and sand-rich layers. They suggest the clay-rich layer beneath the island extends laterally beneath the floodplain, although the auger holes they drilled in the floodplain did not intersect this presumed layer.…”

“…The partitioning of REEs between calcite and aqueous solution generally decreases systematically with atomic number (Voigt et al, 2017). In Figure 2, we plot data collected by Jolivet et al (2023) to demonstrate the influence of calcite (represented by CaO) on the concentration and fractionation of REEs in soils from Nxaraga Island. Samples containing high concentrations of CaO are enriched in REEs (Figure 2a) and exhibit clear fractionation of the Gd/Yb ratio (Figure 2b), reflecting the varying ability of REEs to incorporate into the solid phase (log D Gd = 3.4 vs. log D Yb = 1.9).…”

“…This model has been subject to rigorous testing by a number of researchers, which has included geophysical imaging and quantitative modeling of subsiding brine plumes (Bauer et al, 2006;Bauer-Gottwein et al, 2007), and has been widely accepted (e.g., Milzow et al, 2009;Ramberg & Wolski, 2008). Jolivet et al (2023) propose an alternative hydrological model for the Okavango. Their model is based on chemical and mineralogical analyses of samples from six auger holes drilled on Nxaraga Island and is augmented with trace element water chemistry data from the same holes by Dauteuil et al (2021).…”