Makgadikgadi Research Articles

Late Pleistocene hydrological settings at world heritage Tsodilo Hills (NW Kalahari, Botswana), a site of ancient human occupation

Based on genetic studies, a Pleistocene Kalahari “palaeo-wetland”, which spanned the region of the Okavango Delta and the Makgadikgadi Basin, was recently considered the geographic origin of evolutionary modern humans. It was proposed that subsequent out-of-homeland migration was induced by climate shifts. The Tsodilo Hills, which are in relative proximity to the Okavango Delta, represent a site of ancient human occupation since at least 100 ka. Local hydrological dynamics were predominately controlled by climate variability and are archived in the sediments of Palaeolake Tsodilo. This study seeks to better understand the Late Pleistocene environments of the ancient Tsodilo people with a focus on palaeo-hydrological settings, which played a major role for their livelihoods.Our multidisciplinary approach included different remote sensing and geophysical methods, comprehensive application of differential GPS, and sedimentological analyses concentrating on the lake beds. Four palaeo-shorelines could be identified, three of which indicate highstands during which the Tamacha palaeo-river drained Palaeolake Tsodilo towards the Okavango Panhandle. Two highstands during MIS 3b and LGM are related to periods of largely increased fish consumption by humans as has been documented by archaeologists. The palaeolake was likely most extended about 100 ka ago or earlier, when it covered ca. 70 km2 and was 16 m deep. A single (neo-)tectonic fault could be detected.We assume that the Tamacha palaeo-river was a gateway for ancient humans to reach the Tsodilo Hills from these palaeo-wetlands. The people took advantage of the Tsodilo Hills as shelter from weather hazards and as a natural fortress against predators and elephants. Geologically, the Tsodilo Hills were comparatively calm. They represented a relatively safe haven where the social behaviour of early modern humans could evolve to a higher complexity, which relates to the fundamental question when and where modern human behaviour began.

Migratory Okavango zebra navigate to water holes

![Figure][1] A zebra in the Okavango Delta wearing a GPS collar. Photo credit: Tatjana Hubel. When the north Nxai Pan fence, which had divided the Okavango Delta from the lush grasslands of the Makgadikgadi saltpans in Botswana for almost 40 years, was removed in 2004, no one knew whether

A closer look at mineral aerosol emissions from the Makgadikgadi Pans, Botswana, using automated SEM-EDS (QEMSCAN®)

ABSTRACT This paper demonstrates the use of Quantitative Evaluation of Minerals by SCANning electron microscopy (QEMSCAN®), an automated scanning microscopy technique, which combines scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), providing ultra-fast analysis of particle grains at a micron-scale resolution. We evaluate its application in aerosol studies by comparing surface and airborne samples from the Makgadikgadi Pan in Botswana. The playa is a major global dust emitter and its aerosols have a widespread effect on atmospheric, biological and terrestrial processes. Sampling was conducted at a carefully selected surface location and associated BSNE dust trap stack at 0.25, 0.5, 0.85 1.65 meters. The dominant minerals identified here are quartz, halite, thernadite, mica, calcite and feldspar. Surface sample results from QEMSCAN are in line with other forms of elemental and mineralogical analyses. When comparing surface samples with elevated trap samples, we noted a fining and fractionation during grain entrainment, resulting in a compositional shift with height. We also observed some ultra-fine fraction losses from the BSNE traps. Overall, the single location here establishes the link between fluvial playa basin inputs, sediment storage, evaporation products and aeolian losses and outputs from a dry lake surface, not unlike semi-arid evaporative dust sources elsewhere.

Importance of old bulls: leaders and followers in collective movements of all-male groups in African savannah elephants (Loxodonta africana)

In long-lived social species, older individuals can provide fitness benefits to their groupmates through the imparting of ecological knowledge. Research in this area has largely focused on females in matrilineal societies where, for example, older female African savannah elephants (Loxodonta africana) are most effective at making decisions crucial to herd survival, and old post-reproductive female resident killer whales (Orcinus orca) lead collective movements in hunting grounds. In contrast, little is known about the role of older males as leaders in long-lived social species. By analysing leadership patterns of all-male African savannah elephant traveling groups along elephant pathways in Makgadikgadi Pans National Park, Botswana, we found that the oldest males were more likely to lead collective movements. Our results challenge the assumption that older male elephants are redundant in the population and raise concerns over the biased removal of old bulls that currently occurs in both legal trophy hunting and illegal poaching. Selective harvesting of older males could have detrimental effects on the wider elephant society through loss of leaders crucial to younger male navigation in unknown, risky environments.

Layered mound, inverted channels and polygonal fractures from the Makgadikgadi pan (Botswana): Possible analogues for Martian aqueous morphologies

Layered mounds and inverted channels with polygonal fractures from the Ntwetwe Pan in the Makgadikgadi Basin (central Botswana) have been herein investigated. These morphologies are from an evaporitic basin (the Makgadikgadi Basin) that is the remnant of an ancient Pleistocene lake and is currently part of the world’s largest evaporitic system.The mounds in the Ntwetwe Pan are characterized by a layered structure and low relief (max. 5 m above the pan floor) and can be in excess of 2 ​km wide. The mounds consist mainly of loose (non-lithified) sand and silt with high moisture contents, even during the dry season. Geophysical investigations have shown that groundwater processes, particularly those related to the capillary fringe that rises and conveys moisture through the mounds, are factors that make mound sediments resistant to wind erosion.The inverted channels, identified in the southern part of the Ntwetwe Pan, are characterized by gentle reliefs and depressions, which depend upon the distribution of calcretes and indurated sediments. Large scale (up to 100 ​m wide) polygonal fractures localized at the front of the channels, disappear at the transition with the present-day pan floor.We consider that these particular mounds, within the Ntwetwe Pan, are remnants of the strandline of the paleo-Makgadikgadi Lake, and that the inverted channels represent distributary channels of a relict fan delta, formed by an ephemeral river, most likely the paleo-Boteti River, during a Lake Paleo-Makgadikgadi highstand stage. We consider that large scale (up to 100 ​m wide) polygonal fractures, located on the channel-mouth lobes, represent large-scale desiccation cracks formed by rapid water evaporation from delta deposits.The results of this investigation highlight the importance of the paleo-drainage system and its interactions with the water table and wind-deflation as main geomorphological factors within salt pan environments. The mounds in the Makgadikgadi pans also show strong geomorphic similarities to spring mounds on the surface of Mars, localized in equatorial layered deposits (ELDs). These ELDs mounds are considered to result from cyclical groundwater upwelling, evaporation and wind deflation. The geological processes that resulted in the formation of mounds within the Makgadikgadi pans may, therefore, help to explain how similar layered deposits formed on Mars and confirm existing theories.

Dust provenance and its role as a potential fertilizing agent for the Okavango Delta, Botswana

AbstractDust plays a globally important role in supplying biologically essential elements to landscapes underlain by nutrient‐poor substrates. Here we show that dust may play a significant role in sustaining productivity in the vast wetlands of the Okavango Delta in southern Africa, one of the world's richest biodiversity hotspots. Dust accumulates preferentially on tree‐covered islands in the seasonal swamps of the Delta, creating pockets of fine‐grained, nutrient‐rich material within the semi‐arid landscape of the Kalahari Desert. Strontium and neodymium isotopes reveal that this dust likely originates predominantly from the Makgadikgadi salt pans, located 300 km away, and contributes 10–80% of the fine‐grained material present in Okavango island soils. Surface material sourced from the Makgadikgadi Pans contains relatively high amounts of bioavailable phosphorus and iron, potentially influencing Okavango Delta biological productivity. We propose that long‐term ecosystem productivity and nutrient availability in the Okavango may be strongly mediated by regional dust inputs. Understanding the influence of dust deposition on nutrient loads and biogeochemical cycling is thus critical for predicting the response of the Okavango Delta to future changes in climate. We suggest that dust inputs may play a significant role in the supply of nutrients to other large, global wetland systems located in dryland environments. © 2020 John Wiley & Sons, Ltd

Elephants for Africa: male Savannah elephant Loxodonta africana sociality, the Makgadikgadi and resource competition

The resurgence of the Boteti River, at the western border of the Makgadikgadi Pans National Park (MPNP), has had a large influence on the importance of this area in the framework of ecosystem conservation in Botswana and, in particular, for the movement of elephants into historical rangelands. The River represents a hard border to the MPNP and community lands to the east. Cultural knowledge about how to deal with elephants is deficient because of the lack of recent exposure to this species, aggravating the issues in the region. Working towards human–elephant coexistence, Elephants for Africa's research focuses on the social and ecological requirements of male Savannah elephants Loxodonta africana, alongside those of the human population, and applies this in the context of human–wildlife resource competition in the region. Strong partnerships with the communities bordering the MPNP and engaging a local non‐governmental organization board of experts to guide our work are the foundation of evolving working solutions to decrease the socio‐economic impacts of living alongside elephants. The Boteti River and the MPNP constitute a key area for the long‐term sustainability of populations of large mammalian species, including elephants.

Fluctuating food resources and home ranges in brown hyaenas living in a semi‐arid environment

AbstractWhilst food availability influences space use by animals, how fluctuating resources influence the location of home ranges and space use patterns is little understood. In this study, we first determined the home range of brown hyaenas (Parahyaena brunnea) living inside, and adjacent to, the Makgadikgadi Pans National Park, Botswana. We then examined home range use and how home range fidelity varied depending on the spatio‐temporal variability of available resources that were important for brown hyaenas. Finally, we determined if there was any variation in space use between male and female brown hyaenas. We found that brown hyaenas lived in well‐defined home ranges and showed strong spatial associations with other group members. Brown hyaenas shifted their seasonal home ranges to match food distribution. Hyaenas living in variable food environments demonstrated greater fluctuations in seasonal home range sizes and lower levels of home range retention than individuals from clans in more stable food environments. While home range edges remained more stable for clans located in areas with more food, they still showed a high degree of spatial plasticity in seasonal and annual movements in response to food availability and distribution over time. Clans living inside the Makgadikgadi Pans National Park did not defend an area sufficiently large to provide adequate food resources all year and individuals foraged into adjacent clan ranges when food was scarce. Hyaenas in clans outside Makgadikgadi Pans National Park lived in an area that contained adequate food resources all year. Gender did not influence space use by brown hyaenas.

Hydrogeological conceptual model of large and complex sedimentary aquifer systems – Central Kalahari Basin

Successful groundwater resources evaluation and management is nowadays typically undertaken using distributed numerical groundwater flow models. Such models largely rely on hydrogeological conceptual models. The conceptual models summarize hydrogeological knowledge of an area to be modelled and thereby providing a framework for numerical model design. In this study, an efficient data integration method for developing hydrogeological conceptual model of the large and hydrogeologically-complex, Central Kalahari Basin (CKB) aquifer system, was undertaken. In that process, suitability of 3-D geological modelling with RockWorks code in iterative combination with standard GIS (ArcGIS) was tested. As a result, six hydrostratigraphic units were identified, their heads and related flow system interdependencies evaluated and hydraulic properties attached. A characteristic feature of the CKB is a thick unsaturated Kalahari Sand Unit (KSU), that restricts the erratic recharge input to <1 mm yr−1 in the centre to about 5–10 mm yr−1 in the eastern fringe. The analysis of the spatial distribution of topological surfaces of the hydrostratigraphic units and hydraulic heads of the aquifers, allowed to identify three flow systems of the three aquifers, Lebung, Ecca and Ghanzi, all three having similar radially-concentric regional groundwater flow patterns directed towards discharge area of Makgadikgadi Pans. That pattern similarity is likely due to various hydraulic interconnections, direct or through aquitard leakages, and also due to the presence of the overlying unconfined, surficial KSU, hydraulically connected with all the three aquifers, redistributing recharge into them. The proposed 3-D geological modelling with RockWorks, turned to be vital and efficient in developing hydrogeological conceptual model of a large and complex multi-layered aquifer systems. Its strength is in simplicity of operation, in conjunctive, iterative use with other software such as standard GIS and in flexibility to interface with numerical groundwater model. As a result of conceptual modelling, fully 3-d, 6 layer numerical model, with shallow, variably-saturated, unconfined layer is finally recommended as a transition from conceptual into numerical model of the CKB.

Measurements of windblown dust characteristics and ocean fertilization potential: The ephemeral river valleys of Namibia

Abstract Delivery of nutrients to the ocean by mineral aerosol deposition involves complex biogeochemical interactions that include atmospheric processing, dissolution and biotic uptake of available nutrients in the surface waters. Research into the fertilization potential of aeolian dust is currently constrained by a lack of understanding of the nutrient composition and bioavailability in dust source areas. Further, research into hot-spots of dust emission has largely focused on paleo-lacustrine sources and pans, to the detriment of other potential sources such as ephemeral river valleys in desert regions. Here, we investigate the sediment characteristics and nutrient content of windblown and surface sediments of a largely overlooked southern African dust source, Namibia’s ephemeral river valleys. We deployed monitoring equipment in three river valleys to capture deflated sediments and monitor airborne dust concentration and meteorological conditions throughout an annual dust season. Our results show that windblown dust within the river valleys is easily transportable offshore from Namibia over the Benguela Upwelling System, an intensely productive region of the South Atlantic Ocean. We demonstrate that the windblown dust contains iron, phosphorus and nitrogen nutrients, each of which may positively impact primary production rates when deposited in the complex upwelling system. The river valley dust has a significantly higher content of nutrients than either of southern Africa’s major dry lake bed dust sources, Etosha and Makgadikgadi Pans. This aeolian work builds on previous source sediment findings proposing the ephemeral river valleys of Namibia as regionally important sources of dust with enhanced ocean fertilisation potential.

Paleolimnological features of a mega-lake phase in the Makgadikgadi Basin (Kalahari, Botswana) during Marine Isotope Stage 5 inferred from diatoms

The Makgadikgadi–Okavango–Zambezi basin (MOZB) is a structural depression in the south-western branch of the East African Rift System of the northern and middle Kalahari, central southern Africa. In the present day, the mainly dry subbasins of the MOZB are part of a long-lived lacustrine system that has likely existed since Early Pleistocene and from which an extant freshwater fish radiation emerged seeding all major river systems of southern Africa. During hydrologically favourable periods the subbasins were connected as a single mega-lake termed Lake Palaeo-Makgadikgadi. Previous geomorphological studies and OSL dates have provided evidence for repeated mega-lake periods since approximately 300 ka. The environmental and climatic implications of such large scale late Quaternary lake-level fluctuations are controversial, with the duration of mega-lake phases poorly constrained. Here, we present the first evidence for a Marine Isotope Stage (MIS) 5 mega-lake period (about 935–940 m a.s.l.) reconstructed from a diatom-rich, 30-cm-thick lacustrine sediment section, exposed close to a palaeo-shoreline of the Makgadikgadi Basin. Based upon the environmental setting and in comparison with sedimentation rates of other similar lake environments, we tentatively estimated that the highstand lasted approximately 1 ka during MIS 5d–b. The 30-cm section was sampled in 0.5-cm steps. Diatom species diversity ranges from 19 to 30 through the section. The dominant species are Pseudostaurosira brevistriata, Rhopalodia gibberula, Cyclotella meneghiniana and Epithemia sorex. The total of 60 sediment samples provide us with a record at decadal to bi-decadal resolution. Based on diatom assemblages and their oxygen isotope composition (δ18O) we infer an alkaline and mostly oligohaline lake with shallow water conditions prevailing in MIS 5, and is potentially analogous to a Heinrich event. The climate over southern Africa during MIS 5 has been considered very arid but the hydromorphological context of our sediment section indicates that we captured a mega-lake period providing evidence that short-term excursions to significantly higher humidity existed. A hydrologically more favourable environment during MIS 5 than formerly presumed is in line with the early human occupation of the Kalahari.

Iron and nutrient content of wind-erodible sediment in the ephemeral river valleys of Namibia

Research concerning the global distribution of aeolian dust sources has principally focussed on salt/clay pan and desiccated lacustrine emission areas. In southern Africa such sources are identified as Etosha Pan in northern Namibia and Makgadikgadi Pans in northern Botswana. Dust emitting from ephemeral river valleys, however, has been largely overlooked. Rivers are known nutrient transport pathways and the flooding regimes of ephemeral river valleys frequently replenish stores of fine sediment which, on drying, can become susceptible to aeolian erosion. Such airborne sediment may be nutrient rich and thus be significant for the fertilisation of marine waters once deposited. This study investigates the dust source sediments from three ephemeral river valleys in Namibia in terms of their particle size distribution and their concentrations of bioavailable N, P and Fe. We compare the nutrient content of these sediments from the ephemeral river valleys to those collected from Etosha and Makgadikgadi Pans and consider their relative ocean fertilising potential. Our results show that the ephemeral river valleys contain fine grained sediment similar in physical character to Etosha and Makgadikgadi Pans yet they have up to 43 times greater concentrations of bioavailable iron and enriched N and P macronutrients that are each important for ocean fertilisation. The known dust-emitting river valleys of Namibia may therefore be contributing a greater fertilisation role in the adjacent marine system than previously considered, and not-yet investigated. Given this finding a re-assessment of the potential role of ephemeral river valleys in providing nutrient-rich sediment into the aeolian and marine systems in other dryland areas is necessary.

Enhancing weak transient signals in SEVIRI false color imagery: Application to dust source detection in southern Africa

A method is described to significantly enhance the signature of dust events using observations from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI). The approach involves the derivation of a composite clear-sky signal for selected channels on an individual time step and pixel basis. These composite signals are subtracted from each observation in the relevant channels to enhance weak transient signals associated with either (a) low levels of dust emission or (b) dust emissions with high salt or low quartz content. Different channel combinations, of the differenced data from the steps above, are then rendered in false color imagery for the purpose of improved identification of dust source locations and activity. We have applied this clear-sky difference (CSD) algorithm over three (globally significant) source regions in southern Africa: the Makgadikgadi Basin, Etosha Pan, and the Namibian and western South African coast. Case study analyses indicate three notable advantages associated with the CSD approach over established image rendering methods: (i) an improved ability to detect dust plumes, (ii) the observation of source activation earlier in the diurnal cycle, and (iii) an improved ability to resolve and pinpoint dust plume source locations.

Mapping the surface geomorphology of the Makgadikgadi Rift Zone (MRZ)

The Makgadikgadi basin and wider Middle Kalahari region of Botswana and beyond host landforms which have been attributed to Quaternary environmental change, including palaeolake level fluctuations and aeolian activity. Tectonic processes and landforms on the other hand, have mostly been linked to the Okavango graben and associated rift zone (ORZ) to the west of the Makgadikgadi. In this paper we establish the extent of tectonic surface expression associated with the Makgadikgadi Rift Zone (MRZ). We identify a series of parallel, NNE-SSW, normal faults and scarps, expressing horst and graben structures linked to seven major blocks in the northern Makgadikgadi basin, using both Shuttle Radar Topographic Mission (SRTM) and geomagnetic data. Subtle expression of rifting has controlled endorheic drainage topology, replicated regional dune-field patterns and displaced the 945 m palaoelake contour since lake desiccation. These observations underscore the role of neotectonic “piano key” block movement in shaping surface landforms across a large expanse of the Kalahari region. This paper provides the first detailed map and introduction to the Makgadikgadi Rift Zone (MRZ) and its geomorphology.

Mixed methods evaluation of targeted selective anthelmintic treatment by resource-poor smallholder goat farmers in Botswana

Due to the threat of anthelmintic resistance, livestock farmers worldwide are encouraged to selectively apply treatments against gastrointestinal nematodes (GINs). Targeted selective treatment (TST) of individual animals would be especially useful for smallholder farmers in low-income economies, where cost-effective and sustainable intervention strategies will improve livestock productivity and food security. Supporting research has focused mainly on refining technical indicators for treatment, and much less on factors influencing uptake and effectiveness. We used a mixed method approach, whereby qualitative and quantitative approaches are combined, to develop, implement and validate a TST system for GINs in small ruminants, most commonly goats, among smallholder farmers in the Makgadikgadi Pans region of Botswana, and to seek better understanding of system performance within a cultural context. After the first six months of the study, 42 out of 47 enrolled farmers were followed up; 52% had monitored their animals using the taught inspection criteria and 26% applied TST during this phase. Uptake level showed little correlation with farmer characteristics, such as literacy and size of farm. Herd health significantly improved in those herds where anthelmintic treatment was applied: anaemia, as assessed using the five-point FAMACHA© scale, was 0.44–0.69 points better (95% confidence interval) and body condition score was 0.18–0.36 points better (95% C.I., five-point scale) in treated compared with untreated herds. Only targeting individuals in greatest need led to similar health improvements compared to treating the entire herd, leading to dose savings ranging from 36% to 97%. This study demonstrates that TST against nematodes can be implemented effectively by resource-poor farmers using a community-led approach. The use of mixed methods provides a promising system to integrate technical and social aspects of TST programmes for maximum uptake and effect.

The glass beads of Kaitshàa and early Indian Ocean trade into the far interior of southern Africa

Abstract

Break on Through to the Other Side: The Effectiveness of Game Fencing to Mitigate Human—Wildlife Conflict

Game fences are widely used to mitigate human—wildlife conflict and, where fencing is deemed the right choice, appropriate design, alignment and maintenance are the keys to effectiveness. We studied the effectiveness of the Makgadikgadi Pans National Park game fence along the Boteti River in central Botswana, one of the highest human—wildlife conflict areas in the country, using a spoor survey method and counting holes made by transgressing animals. We investigated the fence's effectiveness in separating humans and wildlife, as evidenced by the fence transgression frequencies of different hole-digging and conflict species. Since the fence prevents access to long stretches of river along the national park boundary, fence transgressions for lions (Panthera leo), elephants (Loxodonta africana) and all the digging species were higher during the dry season when there was no surface water available in the park. Conversely, cattle (Bos taurus and Bos indicus) crossed the fence and moved into the park in high numbers during the rainy season, probably to access grazing. Wildlife damage to the fence was highest in the dry season and without the implementation of prompt and constant maintenance and repair, game fences such as the Boteti fence cannot be effective in alleviating human—wildlife conflict.

Airborne and ground‐based transient electromagnetic mapping of groundwater salinity in the Machile–Zambezi Basin, southwestern Zambia

ABSTRACTThe geological and morphological evolution of the Kalahari Basin of Southern Africa has given rise to a complex hydrogeological regime that is affected by water quality issues. Among these concerns is the occurrence of saline groundwater. Airborne and ground‐based electromagnetic surveying is an efficient tool for mapping groundwater quality variations and has been used extensively to explore the Kalahari sediments, e.g., in Botswana and Namibia. Recently, airborne and ground‐based mapping of groundwater salinity was conducted in the Machile–Zambezi Basin, southwestern Zambia, using the versatile time‐domain electromagnetic system and WalkTEM system, respectively, incorporating earlier ground‐based ProTEM 47D measurements. The data were inverted using the laterally constrained inversion technique followed by a separate spatially constrained inversion scheme. WalkTEM data were inverted as ordinary single‐site one‐dimensional inversions. The regional electrical resistivity signature of the Machile–Zambezi Basin was found to be characterized by high elevation (1000 m–1050 m above mean sea level), high electrical resistivity (above 100 Ωm) areas that form the western and eastern boundaries of a low‐resistivity (below 13 Ωm) valley that extends southwestwards into the Makgadikgadi salt pans. The electrical resistivity distribution is indicative of a full graben related to the Okavango–Linyati Fault system as a result of propagation of the East African Rift Valley System into Southern Africa. The saline lacustrine sediments infilling the Machile Graben are responsible for the low formation resistivity (below 13 Ωm) and high salinity (above 7000 μS/cm) observed in the groundwater and are probably related to the complex evolutionary history of Palaeo‐Lake Makgadikgadi.

Integrated interpretation of helicopter and ground-based geophysical data recorded within the Okavango Delta, Botswana

Integration of information from the following sources has been used to produce a much better constrained and more complete four-unit geological/hydrological model of the Okavango Delta than previously available: (i) a 3D resistivity model determined from helicopter time-domain electromagnetic (HTEM) data recorded across most of the delta, (ii) 2D models and images derived from ground-based electrical resistance tomographic, transient electromagnetic, and high resolution seismic reflection/refraction tomographic data acquired at four selected sites in western and north-central regions of the delta, and (iii) geological details extracted from boreholes in northeastern and southeastern parts of the delta. The upper heterogeneous unit is the modern delta, which comprises extensive dry and freshwater-saturated sand and lesser amounts of clay and salt. It is characterized by moderate to high electrical resistivities and very low to low P-wave velocities. Except for images of several buried abandoned river channels, it is non-reflective. The laterally extensive underlying unit of low resistivities, low P-wave velocity, and subhorizontal reflectors very likely contains saline-water-saturated sands and clays deposited in the huge Paleo Lake Makgadikgadi (PLM), which once covered a 90,000km2 area that encompassed the delta, Lake Ngami, the Mababe Depression, and the Makgadikgadi Basin. Examples of PLM sediments are intersected in many boreholes. Low permeability clay within the PLM unit seems to be a barrier to the downward flow of the saline water. Below the PLM unit, freshwater-saturated sand of the Paleo Okavango Megafan (POM) unit is distinguished by moderate to high resistivities, low P-wave velocity, and numerous subhorizontal reflectors. The POM unit is interpreted to be the remnants of a megafan based on the arcuate nature of its front and the semi-conical shape of its upper surface in the HTEM resistivity model. Moderate to high resistivity subhorizontal layers are consistent with this interpretation. The deepest unit is the basement with very high resistivity, high P-wave velocity, and low or complex reflectivity. The interface between the POM unit and basement is a prominent seismic reflector.

The dynamism of salt crust patterns on playas

Playas are common in arid environments and can be major sources of mineral dust that can influence global climate. These landforms typically form crusts that limit evaporation and dust emission, modify surface erosivity and erodibility, and can lead to over prediction or underprediction of (1) dust-emission potential and (2) water and heat fluxes in energybalance modeling. Through terrestrial laser scanning measurements of part of the Makgadikgadi Pans of Botswana (a Southern Hemisphere playa that emits significant amounts of dust), we show that over weeks, months, and a year, the shapes of these surfaces change considerably (ridge thrusting of >30 mm/week) and can switch among continuous, ridged, and degraded patterns. Ridged pattern development changes the measured aerodynamic roughness of the surface (as much as 3 mm/week). The dynamic nature of these crusted surfaces must be accounted for in dust entrainment and moisture balance formulae to improve regional and global climate models.