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.

Abstract

Abstract The Okavango Delta in southern Africa has been the subject of geomorphological and hydrological investigations since the 1970s. In the early 1990s, McCarthy and co‐workers developed a geomorphological and hydrological model which describes the processes that give rise to the gently undulating topography and the lack of saline surface water in this semi‐arid environment. This model is based on extensive investigations of soil and water chemistry conducted across the Okavango Delta, and has been subject to rigorous testing by a number of independent researchers. In their recent paper, Jolivet et al. (2023, https://doi.org/10.1029/2023GC010978 ) proposed an alternative hydrological model based on chemical and mineralogical analyses of samples from six auger holes drilled at a site on the lower fringe of the permanent swamp. We provide a critique of their model and explain how this model cannot describe the geohydrological functioning of the Okavango Delta. Jolivet et al. suggest there are two shallow aquifers, one hosted in sand and the other in clay‐rich material. Previous studies of subsurface soils on 18 islands have failed to reveal a deeper clay‐rich layer. Rather, the shallow groundwater is laterally fully connected and salinity varies in response to evapotranspiration. We attribute REE fractionation in subsurface soils observed by Jolivet et al. to be caused by the precipitation of calcite from groundwater which strongly accumulates REE. We are of the opinion that Jolivet et al. have insufficient data to suggest revisions to the current understanding of the functioning of the Okavango hydrological and sedimentological system.