Abstract: This study combines historical records of 14C and 3H in the atmosphere and soil with renewal rate and groundwater lumped parameter models to predict the abundance of 14C and 3H in groundwater over time. 624 groundwater samples from numerous studies, over four decades (1978–2019), in South Africa, Namibia, Botswana and Mozambique were collated to compare with predicted groundwater activities of 14C and 3H within the South African Development Community (SADC) region. Spatial datasets of carbonate bearing lithology, C3/C4 vegetation, summer/winter rainfall and coastal proximity were used to apply corrections to 14C and 3H data. Corrected values of 14C and 3H were compared with the theoretical abundance of these tracers, derived from the lumped parameter models, to estimate the general mean residence times and presence of groundwater mixing between modern recharge and older groundwaters. This study found that corrected values produced varying mean residence times derived from 14C ages (∼500–28500 years) and a wide range of potentially mixed waters within each aquifer system (0–100 % of tested wells) across the study area. The largest proportions of mixed groundwater, as well as the youngest mean residence times, were found in alluvial and primary fractured rock aquifers (e.g., western coast of South Africa and southern Mozambique). The smallest proportions of mixed groundwater were predicted in deep confined clay-rich aquifers as well as layered coal bearing carbonate sequences (e.g., Orapa, Malwewe and Serowe, Botswana). Insights into the proportions of mixed groundwater and mean residence times can help assess hydrological resilience on a regional scale. Such information is pertinent in promoting socio-economic development and increased water/food security in the SADC region. By understanding the resilience of groundwater resources, robust and informed strategies for water equality and GDP growth in the SADC region can be envisioned and implemented.

Abstract: Paleohydrological data from the African tropics and subtropics, including lake, groundwater and speleothem records, are reviewed to show how environments and climates from both hemispheres are inter-related. Although orbitally induced changes in the monsoon strength account for a large part of long-term climatic changes in tropical Africa, the Late Pleistocene–Holocene hydrological fluctuations rather appear to have been a series of abrupt events that reflect complex interactions between orbital forcing, atmosphere, ocean and land surface conditions. During the Last Glacial Maximum (23–18ka BP), most records indicate that generally dry conditions have prevailed in both hemispheres, associated with lower tropical land- and sea-surface temperatures. This agrees with simulations using coupled ocean–atmosphere models, which predict cooling and reduced summer precipitation in tropical Africa; the global hydrological cycle was weaker than today when the extent of large polar ice-sheets and sea-ice was a prominent forcing factor of the Earth’s climate. Glacial-interglacial climatic changes started early: a first wetting/warming phase at ca. 17–16ka BP took place during a period of rapid temperature increase in Antarctica. Next, two drastic arid-humid transitions in equatorial and northern Africa occurred around 15–14.5ka BP and 11.5–11ka BP. Both are thought to match the major Greenland warming events, in concert with the switching of the oceanic thermohaline circulation to modern mode. However, part of the climatic signal after 15 ka BP also seems related to the Antarctica climate. During the Holocene, Africa has also experienced rapid hydrological fluctuations of dramatic magnitude compared to the climatic changes at high latitudes. In particular, major dry spells occurred around 8.4–8ka and 4.2–4ka BP in the northern monsoon domain. Comparison with other parts of the world indicates that these events have a worldwide distribution but different regional expressions. In the absence of large polar ice sheets, changes in the continental hydrological cycles in the tropics may have a significant impact on the global climate system. Climate information gathered here allows to identify geographical and methodological gaps, and raise some scientific questions that remain to be solved to better understand how the tropics respond to changes in major climate-forcing factors, and how they influence climate globally.

Abstract: The application of optically stimulated luminescence (OSL) dating in the Namib Desert is casting new light on late Quaternary environments. OSL has been applied to: (i) complex linear dunes, alongside ground penetrating radar stratigraphy in order to establish dune migration rates, (ii) fluvial lithofacies associations that distinguish between flood deposits and river end points, in order to constrain the timing of periods of higher discharge and conditions relatively drier than present and (iii) aeolian sand interbedded with carbonate deposits in order to provide chronologies for water-lain interdune sediments. We present and review the contribution of these data to enhancing reconstructions of the palaeoenvironments and palaeohydrology of the west coast of Namibia, particularly the increased confidence in interpretations provided by lithofacies analysis of the river deposits. This includes major silt deposits, which have had a contested palaeohydrological interpretation, such as the Kuiseb River Homeb Silts. We conclude that OSL should remain a key chronological technique to further elucidate the palaeoenvironmental history of southern Africa.