Reconstructing rainfall in sandy drylands of southern Africa: exploring the potential of the chloride mass balance hydrostratigraphy approach in Kalahari sand dunes.

Sand dunes sediments are a commonly used archive for palaeoenvironmental reconstruction, with chronologies for their accumulation rates, and migration rates, used in a large number of palaeoenvironmental reconstructions across global sand-rich drylands. A less common use of sand dunes as an archive is to explore the chemical and isotopic composition of the pore moisture stored within the sand. Sand dunes, constitute the uppermost unit above dryland aquifers, and represent part of the unsaturated (or vadose) zone in terms of hydrogeology. Chloride is a chemical tracer within the vadose zone that is commonly used to understand, and quantify rates of, recharge to the groundwater table, using a chloride mass balance (CMB) approach (e.g. Scanlon et al., 2006). In doing so, any variations in the concentration of pore-moisture chloride between discrete depths in the sand profile can also be used to provide a novel archive for tracking changes in palaeomoisture availability and land-use change (see review in Stone and Edmunds, 2016). This approach is known as a ‘hydrostratigraphy’.

This presentation will explore the utility of the CMB hydrostatigraphy approach for Kalahari linear dunes above the Stampriet Basin, which is a transboundary aquifer within southern Africa. This is a region for which palaeoenvironmental proxies for rainfall are extremely scarce, owing to poor preservation of organic-rich material in this oxygen-rich environment. Three repeat field visits for sampling the dunes (2011, 2013 and 2016) were used in the research design in order to explore the repeatability of this CMB hydrostratigraphy approach at this location. In addition, a transect of dunes, including dunes close to a pan, were sampled. It was hypothesised that those dunes in close proximity to the pan would be unsuitable, owing to the possible presence of Cl-rich evaporites and capillary zone influences on the behaviour of moisture in the sand-rich sediment. The trends in these profiles will be presented, along with attempts to understand the moisture pathway behaviour in these dune sediments.  

 

References

Scanlon, B.R., Mukerhjee, A., Gates, J., Reedy, R., Sinha, A.K., 2010. Groundwater recharge in natural dune systems and agricultural ecosystems in the Thar Desert region, Rajasthan, India. Hydrogeology Journal 18, 959–972.

Stone, A. E. C., Edmunds, W. M. (2016) Unsaturated zone hydrostratigraphies: A novel archive of past climates in dryland continental regions. Earth-Science Reviews 157, 121-144.