Article

Island forming processes in the Okavango Delta, Botswana

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Abstract

The Okavango Delta in Botswana is a large (40,000 km2) alluvial fan that is characterised by the presence of numerous tree-covered islands. Thought to originate from the mound-building activities of termites or through fluvial processes associated with development of scroll bars and inverted channels, islands have been shown to play an important role in the structure and functioning of the Delta through the creation of habitat diversity, focusing of nutrients, and disposal of toxic salts. This study investigates the processes responsible for the maintenance and growth of two such islands in the seasonal swamps. Transpiration by vegetation is shown to result in substantial increases in groundwater salinity beneath the islands, contributing to their growth through chemical precipitation. Detailed chemical analyses revealed that the precipitation of magnesian calcite and silica within the island soils contributes 30–40% of the total island volume. Isotopic analyses of carbonate samples show that vegetation plays an important role in providing carbon for carbonate precipitation. Variations in δ13C carbonate values appear to reflect the relative proportion of C3 to C4 plants on the island, with C4 species becoming more dominant toward island centres in response to increases in groundwater salinity. The study suggests that continued island growth is also related to the deposition of aerosols and the accumulation of dust preferentially on islands and possibly to ongoing termite activity. Tall trees that characterise the island margins trap dust carried from the floodplains, resulting particularly in the lateral growth of islands. Islands in the Okavango are considered to be the product of long-term aggradation processes, with the two islands studied estimated to be in the order of tens of thousands of years old.

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... Pene-contemporaneous silica and carbonate formation is taking place in the contemporary Okavango delta (McCarthy et al., 2012) where transpiration from plant growth results in the progressive accumulation of dissolved salts under islands. As the solute-enriched groundwater becomes increasingly saline, silica and then magnesium calcite are the first to pre- cipitate ( McCarthy et al., 1993). ...
... Thus the plant contributions to the calcareous sediment appear to have been almost entirely produced from C4 plants (cf. McCarthy et al., 2012). The δ 18 O value of the carbonate depends on the δ 18 O value of the porewater from which the calcite precipitated and the tem- perature of formation. ...
... If correct, this habitat preference could help explain the overall increase of CaO (Figure 8) along the proximal to saline pan gradient whereas the general increase in disseminated CaCO 3 with depth is further considered below. Calcium and bicarbonate would over the years be introduced via freshwater inflow as high concentrations of Ca 2+ and HCO - 3 ions are prevalent in the Okavango region surface and groundwater (Mackay et al., 2011;McCarthy et al., 2012). Other possible sources include the direct Figure 11. ...
Article
Trace elements together with some O and C isotope analysis were undertaken on duricrust strandline deposits in the palaeo-Makgadikgadi sub-basin (PMSB) to provide insight into palaeo-climatic conditions through the interpretation of calcrete, silcrete–calcrete intergrade and silcrete deposits. Trace element content and relative abundance suggest that the duricrust origins are associated with the long-term weathering of the Karoo Large Igneous Province which underlies the PMSB. This work shows that duricrust origins are related to Ca2+ and Si (and associated trace elements) being transported mainly through the groundwater and then subsequently precipitated at different strandline elevations over time. Local groundwater feeding in towards the pan margin and accumulating in near-neutral pan-marginal pools, appears to facilitate Si concentration and permeation of pre-existing calcretes. The silica precipitates as the pH drops when renewed freshwater enters the pools. Hence the inferred palaeo-climatic regime for silcretisation may be similar to that occurring in Botswana at present being dry semi-arid with low seasonal rainfall. In contrast the extensive calcrete precipitation in the strandlines results from abundant Ca2+ in adjacent waters which appear to be derived from both local and regional sources. The arrival of Ca2+ from regional sources (shown by trace element evidence) infers heavy rainfall in the upper catchment suggesting a major humid event followed by regional drying. Palaeo-climatic inferences suggest the juxtaposition of major humid events interspersed with more normal semi-arid palaeo-climates with an exception obtained from isotope data, of drier and cooler conditions than usual for the region around 80–90000 years ago. Whereas trace element data can greatly assist in the interpretation of complex deposits such as duricrusts, care should be taken over the use of particular ratios (such as Yb/Gd ratio) which may produce spurious results.
... Briefly, transpiration from dense vegetation on island fringes lowers the local water-table and creates large hydraulic gradients, drawing water in bulk from the floodplains to the island subsurfaces. Here, the waters do become highly enriched and resulting precipitation of authigenic minerals permanently removes elements from solution (Ramberg and Wolski, 2008;McCarthy et al., 2012), while the more soluble components (e.g. Na + , Cl À ) are conveyed to deeper groundwater by a density fingering process (see Bauer et al., 2006. ...
... The absence of salinisation is therefore due to a partitioning of evapotranspiration skewed towards island surfaces -indeed, floodplain vegetation may even play a role in reducing open water losses through shading (Kihwele et al., 2012). A key point about the authigenic mineral precipitation in the island subsurfaces is that the water flow is a bulk, mass transfer, with no potential for discrimination or for influencing the geochemistry of the remaining surface water (Ramberg and Wolski, 2008;McCarthy et al., 2012). ...
... Besides soils and nearshore marine sediments (Michalopoulos and Aller, 1995;Righi and Meunier, 1995), clay mineral neoformation has been identified in many fluvial settings, notably in solute rich, saline and/or alkaline environments Furquim et al., 2010a,b;Souza-Jú nior et al., 2010;Bristow and Milliken, 2011;McCarthy et al., 2012). In freshwater environments, potentially authigenic smectites are observed in a floodplain of the Mkuze, South Africa (Humphries et al., 2010(Humphries et al., , 2011. ...
Article
Chemical weathering of silicate minerals releases elements into solution whereas the neoformation of secondary minerals works in the opposite direction, potentially confounding estimates of silicate weathering rates. Silicon isotopes (δ30Si) may be a useful tool to investigate these processes. Here, we present 82 δ30Si measurements from surface waters, pore waters, biogenic silica (BSi), clays, sand and vegetation from the Okavango Delta, Botswana, a freshwater sub-tropical, flood-pulse wetland. Hydrologically, the Okavango is dominated by evapotranspiration water losses to the atmosphere. It receives an annual pulse of water that inundates seasonal floodplains, while river baseflow is sufficient to maintain a permanent floodplain. δ30Si in dissolved silica (DSi) in surface waters along a 300 km transect at near-peak flood show a limited range (0.36–1.19‰), implying the Delta is well buffered by a balance of processes adding and removing DSi from the surface water. A key control on DSi concentrations is the uptake, production of BSi and recycling of Si by aquatic vegetation, although the net isotopic effect is necessarily small since all BSi re-dissolves on short timescales. In the sediments, BSi δ30Si (n = 30) ranges from −1.49‰ to +0.31‰ and during dissolution, residual BSi tends towards higher δ30Si. The data permit a field-based estimate of the fractionation associated with BSi dissolution, ε30BSi-DSi = −0.26‰, though it is unclear if this is an artefact of the process of dissolution. Clay δ30Si ranges from −0.97‰ to +0.10‰, (n = 15, mean = −0.31‰) and include the highest values yet published, which we speculate may be due to an equilibrium isotope effect during diagenetic transformation of BSi. Two key trends in surface water DSi δ30Si merit further examination: declining δ30Si in an area roughly corresponding to the permanent floodplains despite net DSi removal, and increasing δ30Si in the area corresponding to the seasonal floodplains. We infer that evaporative enrichment of surface waters creates two contrasting regimes. Chemical weathering of low δ30Si phases releases low δ30Si DSi in the relatively dilute waters of the permanent floodplains, whereas silicon removal via clay formation or vegetation uptake is the dominant process in the more enriched, seasonal floodplains.
... While relatively few islands occur in the permanent swamps, they form an important component of the seasonal swamp where they vary in size from a few square metres in area to several thousand square metres (Gumbricht et al. 2004). Islands on the Okavango fan have been studied in detail and are considered to either represent abandoned channels or originate through termite activity (Ellery et al. 1993;McCarthy et al. , 2012. These raised patches remain elevated above the surrounding swamp water level and support vegetation intolerant of flooding. ...
... Over time, islands slowly enlarge and eventually start to coalesce, leading to a diverse range of island shapes and sizes (Gumbricht et al. 2004). Mineral volume calculations suggest that precipitated silica and magnesian calcite together account for * 30-40% of the volume of typical islands in the seasonal swamps (McCarthy et al. 2012), although this contribution is likely to vary depending on the size of the island and its stage of development. The implications of island formation and growth for ecosystem structure have been well documented (Ellery et al. 1993;Dangerfield et al. 1998;Ellery et al. 2000). ...
... Even when there is clear evidence of chemical sedimentation, determining the rate at which solutes accumulate in wetland soils is difficult. Calculated age estimates for two islands in the seasonal swamps of the Okavango Delta suggest that these features are the products of long-term aggradation processes (on the order of 10,000-100,000 years), with CaCO 3 accumulation rates beneath individual islands ranging between 100 and 2500 kg yr -1 (McCarthy et al. 2012). However, accumulation rates are likely to be strongly influenced by solute inputs and internal water fluxes, and are thus expected to vary substantially both within wetlands and between wetlands. ...
Article
Full-text available
Freshwater wetlands located in dryland environments are characterised by high evapotranspiration rates and frequent periods of desiccation, which strongly influence the water chemistry and solute budgets of these systems. The transpiration of groundwater, especially by trees, is an important mechanism through which dryland wetlands can lose water. This process can lead to groundwater salinization and the precipitation of substantial quantities of minerals within the soil, the accumulation of which can have profound consequences for wetland structure and function. This paper aims to bring together current knowledge on the processes that result in solute accumulation and chemical sedimentation which assist in maintaining freshwater conditions in many seasonal dryland wetlands. Examples from central and southern Africa, Australia and South America are presented to illustrate the geomorphically diverse settings under which chemical sedimentation can occur, and the importance of these processes for the resilience and longevity of dryland wetlands. We show that the localised development of saline groundwater and subsurface precipitation of minerals within soils can play a key role in creating and maintaining the habitat diversity of dryland wetlands. Wetland vegetation localises the accumulation of deleterious constituents, thereby preventing widespread salinization and playa-lake formation, and thus ensuring that the bulk of the surface water remains fresh. Although such processes remain widely understudied, we suggest that chemical sedimentation could be a common phenomenon in many dryland wetlands and have important implications for the future management of these ecosystems.
... This raises the question "Is the palustrine origin of these limestones likely?". Fine carbonate deposits have been observed in the Okavango Delta (McCarthy et al. 2012) in island-mounds forming within the Delta. However, these tree-covered islands do not accumulate significant amounts of calcium carbonate, only traces as subsurface crystals. ...
... However, these tree-covered islands do not accumulate significant amounts of calcium carbonate, only traces as subsurface crystals. Their formation is related to the chemistry at the interfaces between fresh groundwater flow and trapped saline groundwater through tree uptake and transpiration (McCarthy et al. 2012), with dust contributing substantially to the material found on the surface of islands (Humphries et al. 2014). If such a process cannot generate large calcium carbonate accumulations, it is for the simple reason that the Okavango waters are presently extremely poor in Ca 2+ , i.e., 3-7 mg/L on average, with a conductivity of 33-40 lS cm −1 (Dr. ...
... but with the presence of a sodic horizon below 30 cm (Solonetz or other soil types, like Calcisols, with a salic horizons). These high sodium concentrations suggest that these areas have an origin comparable to the present islands in the Okavango Delta (Ramberg and Wolski 2008;McCarthy et al. 2012). Besides mopane, the typical species are the grasses Schmidtia pappophoroides, Aristida adscensionis, and Digitaria eriantha; the sedge Kyllinga buchananii; and the forbs Ipomoea coptica and Chamaecrista absus. ...
... Outward from the tree ring, the floodplain is characterized by dense high grass, and reeds cover the banks of the Boro River channel. The sediment in both the floodplain and the island is composed of fine to medium grained, rounded to sub-rounded quartzrich sand of mixed fluvial and aeolian origin Huntsman-Mapila, Ringrose, et al., 2006;McCarthy et al., 2012). A series of piezometers set on a line crossing the island from the channel to the middle of the barren zone were used to collect water samples and measure the water table depth ...
... Furthermore, the dynamics of the groundwater table inside the island is decorrelated to the flood cycle and rainfall supply: this strongly suggests the presence of two different, poorly connected to unconnected aquifers: a superficial one in the channel-floodplain system and a deeper one, possibly captive within the island. At least, the geochemical analysis of sediments reveals similar chemical composition both in the flood plain and in the barren zone, not consistent with the presence of evaporitic deposits in the barren zone, as previously described in other islands with percent in Ca0 higher than 5% (McCarthy, 2006(McCarthy, , 2013McCarthy et al., 2012). ...
... The geometry and spatial extend of the subsurface saline aquifer remain poorly defined mainly due to the lack of deep logging in the floodplain. Two hypotheses can be put forward: (a) a restricted aquifer localized in the middle part of the island, as previously suggested by other researchers (Bauer-Gottwein et al., 2007;McCarthy, 2006McCarthy, , 2013McCarthy et al., 2012;Milzow et al., 2009;Ramberg & Wolski, 2008), or (b) a wide aquifer spreading outside the island and roofing 2-3 m from the surface, slightly below the bottom of the Boro River channel. However, this first hypothesis is contradicted by the here above presented geochemical data and by the hydrological dynamics. ...
... The origin of islands on the Okavango alluvial fan (Fig. 1) has been the focus of detailed study for a number of years McCarthy, 2006;McCarthy et al., 2012). Morphological examination has revealed that some islands originate through fluvial activity on the fan surface and represent scroll bars or abandoned channels. ...
... We recently calculated that~30-40% of the total volume of islands could be attributed to chemical precipitation (McCarthy et al., 2012). The study suggested that continued island growth was thus a combination of processes possibly including the deposition of windborne aerosols and local accumulation of dust, as well as ongoing termite activity. ...
... Many islands in the Delta are believed to have originated as termite mounds (McCarthy et al., , 2012, and islands may therefore continue to grow as a result of ongoing termite activity. Chemical analyses of surface material collected from termite mounds show higher Fe 2 O 3 / Al 2 O 3 ratios, suggesting the local addition of Fe to these soils. ...
Article
The Okavango Delta, situated in the Kalahari Desert (Botswana), is host to extensive areas of seasonal swamp that are characterised by the presence of numerous tree-covered islands. Islands have been shown to play an important role in the landscape through the creation of habitat diversity, focusing of nutrients, and sequestration of salts. Islands are thought to grow through the subsurface precipitation of carbonate and silica, although recent work has suggested that other factors may be involved, notably airborne dust accumulation. In this study, we investigate the role of airborne dust in the maintenance and growth of tree islands in the seasonal swamps of the Okavango Delta. Chemical and grain size analyses indicate that whilst the channels and floodplains in the Okavango are dominated by well-sorted Kalahari sand that covers the entire region, material on the surface of islands is distinctly different. Island soils are enriched in Al2O3 and are characterised by higher proportions of poorly sorted, fine-grained material that we attribute to the addition of airborne dust. Large quantities of material that circulate in anticyclonic systems over southern Africa represent a potentially significant source of particulate sediment to the Okavango, whilst peat fires and the desiccation of the surrounding floodplains during the dry season are also considered to be important sources of local dust. The data suggest that varying proportions of Kalahari sand, dust, and chemical precipitate give rise to the range of compositions found on and within the islands of the Delta. Dust typically accounts for between 20 and 60% of material found on the surface of islands, whilst dust and chemical precipitate dominate the subsurface material. Islands thus appear to grow through a combination of ongoing surface and subsurface processes that result in considerable heterogeneity in soil composition. In both instances vegetation, especially trees, is the main driving force behind island development, not only causing the subsurface accumulation of CaCO3 and SiO2 in island soils, but also trapping airborne dust on the surface of islands. Our study suggests that dust fallout is an equally or possibly even more important contributor to the local topographic irregularities and thus habitat diversity in the Okavango Delta. Despite the potential importance of airborne material to the biogeochemistry and development of tree islands in wetland systems, our knowledge regarding these processes remains poor.
... The Okavango Delta (Botswana) is one of the world's largest tropical wetland systems, and hence an interesting study area in this regard. It consists of a mosaic of permanent and temporary floodplains, intersected with tree-covered islands (McCarthy et al. 2012). The Okavango Delta's water balance is dominated by evapotranspiration: 98 % of all water is lost to the atmosphere before reaching the outlets. ...
... Islands in the Delta initiate around a termite mound, gradually building from this basis (McCarthy et al. 1998). Large trees-which are especially dense on the island fringes (McCarthy et al. 2012) -play an essential role in maintaining the hydraulic pressure gradient through transpiration. However, while the role of islands in sequestering Si is known, we have little understanding of the role of vegetation in depositing and sequestering Si in the floodplains, though they actually cover a much greater surface area. ...
... The importance of amorphous Si precipitates in the build-up of islands has been pointed to in earlier research (e.g. Ramberg and Wolski 2008;McCarthy et al. 2012). ...
Article
Full-text available
We assessed the role of vegetation and hydrology in the Si cycle in the Okavango Delta. Our results show a large storage of biogenic Si (BSi) in vegetation and the sediments. The biological storage is among the highest observed so far for any ecosystem worldwide. Floodplain vegetation accumulates similar amounts of BSi in both the temporary floodplains and the permanent floodplains, with most values observed between 20 and 100 g Si m(-2). This vegetation Si, after litterfall, contributes to a large biogenic Si storage in the sediments. In temporary floodplains, sediments contain less BSi (375-1950 g Si m(-2) in the top 5 cm) than in the permanent floodplains (1950-3600 g Si m(-2) in the top 5 cm). BSi concentrations in the floodplain sediments decline exponentially indicating rapid dissolution. In the occasional and seasonal floodplains, unidirectional solute transfer from floodplains to the islands will remove Si from the riverine systems. Our work clearly emphasizes the crucial role of floodplains and wetlands in Si transport through tropical rivers, and the potential interference of hydrology with this role.
... Termites have been described as ecosystem engineers due to their ability to induce changes in whole landscapes via multiple feedback loops, emphasising plant-soil feedbacks on mounds (Jones et al. 1997;Jouquet et al. 2006;McCarthy et al. 2012). Plant communities on mounds have often been reported to be different from the surrounding savannas and to show higher diversities on mounds (e.g. ...
... A widely cited paper by Dangerfield et al. (1998), the keystone to a series of studies from the Okavango delta McCarthy et al. 1993McCarthy et al. , 1998McCarthy et al. , 2012, showed that the construction of termite mounds leads to the formation of vegetated islands: the combined effects of particular changes to soil composition and vegetation lead to further soil changes due to physico-chemical soil precipitation processes. This, in turn, contributes to enlarging the island and allows further plants to colonise, which further stabilise the emerging microhabitat with their roots, and cause further precipitation processes due to evapotranspiration. ...
... Vegetation can also benefit from higher water availability in mound soil, as foraging tunnels around the mounds lead to higher infiltration and percolation towards the mound, and clay enrichment leads to higher field capacity of mound soil (Turner 2006). As mentioned in the case of soil precipitation processes, vegetation can cause further feedback loops, for example by acting as collectors of aerosols and enhancing soil structural integrity with their roots (McCarthy et al. 2012). Further feedback may happen due to remineralisation of plant biomass contributing to mound soil fertility, which has been reported to be already elevated as compared to the surrounding savanna due to termite activity (Jouquet et al. 2006(Jouquet et al. , 2011Sileshi et al. 2010). ...
Article
Full-text available
Particularly in savannas, termites are ecosystem engineers and a keystone group in ecology. For the understanding of the savanna vegetation, mound building termites are of particular interest. Due to their special soil chemistry and physical structure, termite mounds often host other plants than the surrounding savanna. As our knowledge of the specific contribution of mound-building termites to overall savanna diversity and ecosystem dynamics doubtlessly is not complete, this paper summarises the state of the art in order to stimulate further research. According to the research interest of the authors , focus is laid on the West African savanna and on the genus Macrotermes. Résumé français: Les termites sont des ingénieurs écosystémiques et un groupe " clé de voûte " en écologie, notamment dans les sa-vanes. Pour comprendre la végétation des savanes, les termites à termitière présentent un intérêt particulier. En effet, de par la spécificité de leurs caractéristiques de sol et de leur structure physique, les termitières hébergent fréquemment vegetation differente de la savane environnante. Cependant, le rôle des termites à termitière dans la diversité [végétale] des savanes et la dynamique de ces écosystèmes n'est que partiellement connu. Cet article propose donc un état des lieux des connaissances et vise à stimuler la recherche sur cette thématique. L'accent est mis sur la savane d'Afrique de l'Ouest et sur le genre Macro-termes, qui sont au coeur des travaux de recherche des auteurs.
... The δ 18 O and δD values of shallow groundwater fall within the range of isotopic values of evaporated surface water (−7.2 to 17.4‰ for δ 18 O and −47 to 73‰ for δD) ( Table 1, Fig. 4), consistent with evaporated river water recharge of shallow groundwater. The δ 18 O and δD of island groundwater samples are enriched (−0.8 to 4.5‰ for δ 18 O and −22 to 14‰ for δD), lie along the ODEL and are within the δ 18 O and δD range of evaporated surface water (Table 1, Fig. 4), consistent with recharge by evaporated river water and/or evaporation in the shallow subsurface or in pools in the centre of the islands (McCarthy and Ellery 1995, Huntsman-Mapila et al. 2006, Bauer-Gottwein et al. 2007, McCarthy et al. 2012, Mladenov et al. 2013. River water and island groundwater, which are potential sources of recharge of groundwater, are indistinguishable as their δ 18 O and δD compositions overlap on the ODEL. ...
... The increases in the alkalinity concentrations with increase in pH have been observed along the longitudinal profile of the Okavango River (Hutton and Dincer 1976, Summerfield 1983, Sawula and Martins 1991, Akoko et al. 2013, Atekwana et al. 2016, Gondwe and Masamba 2016. The increasing pH and alkalinity concentrations during evapoconcentration to carbonate saturation have been observed in ponded surface water on the islands and in island groundwater (McCarthy et al. 1991(McCarthy et al. , 2012. ...
... As noted above, sedimentation within channels in the Delta plays a role in channel avulsion, for this promotes loss of water from the channel to the surrounding swamps, facilitating a decrease in flow velocity, vegetation encroachment and ultimately channel failure (Government of Denmark and Republic of Botswana, 2006). Avulsion leads to water, sediment and nutrient dispersal, rejuvenating parts of the wetlands and promoting biodiversity (Kgathi et al., 2006;McCarthy & Ellery, 1998;McCarthy, Humphries, Mahomed, Le Roux, & Verhagen, 2012). Other abandoned channels may become filled with fine sediment to create topographically elevated ridges that often give rise to tree-covered islands, and these also play a fundamental role in the functioning of the Okavango through their influence on water salinity, nutrient concentrations and habitat diversity (McCarthy et al., 2012). ...
... Avulsion leads to water, sediment and nutrient dispersal, rejuvenating parts of the wetlands and promoting biodiversity (Kgathi et al., 2006;McCarthy & Ellery, 1998;McCarthy, Humphries, Mahomed, Le Roux, & Verhagen, 2012). Other abandoned channels may become filled with fine sediment to create topographically elevated ridges that often give rise to tree-covered islands, and these also play a fundamental role in the functioning of the Okavango through their influence on water salinity, nutrient concentrations and habitat diversity (McCarthy et al., 2012). Overall, a significant change in sediment flux from the upstream catchment may translate to changes in sedimentation rates linked to avulsion processes in the Delta. ...
Article
A key skill that geomorphologists possess is the ability to use multi‐scale perspectives in their interpretations of landscapes. One way to gain these perspectives is with the use of nested hierarchical frameworks. In fluvial geomorphology, such frameworks help with assessment of large‐scale controls (e.g., tectonic activity, climate change) on the pattern and dynamics of smaller‐scale physical features (e.g., channels, floodplains, bars), and conversely illustrate how these smaller‐scale features provide the building blocks from which to make interpretations of fluvial processes and dynamics over larger spatial and temporal scales. Given the rapid pace of technological developments, the range of relatively inexpensive tools available for visualising and mapping landscapes at different spatial scales is expanding exponentially. In this paper, which focuses on the World Heritage‐listed Okavango Delta in Botswana, we demonstrate how various visualisations generated by different technologies at different spatial scales (catchment, landscape unit, reach, site and geomorphic unit) are providing critical baseline information to enhance interpretation and communication of fluvial geomorphology, with potential application in water resources management. In particular, our nested hierarchical approach could be used as an interactive communication tool for non‐specialists and embedded within existing and future management plans for the Delta. The construction of nested hierarchies that synthesise information and analyses can be a valuable addition to the environmental manager's toolkit.
... These conditions locally induced the deposition of lacustrine carbonate and diatomite layers on top of the original sand layers (Diaz et al. 2019), and sand dunes were stabilised on the shores of palaeolakes (Burrough & Thomas 2008). Islands probably emerged by accumulation of salts because of plant evapotranspiration -a process comparable to the present-day Okavango Delta system (McCarthy et al. 2012). The southwestern part of the system became disconnected from the Linyanti River and subsequently dried up following the uplift of the Linyanti geological fault. ...
... Some areas are characterised by tall specimens, whereas in other places C. mopane is limited to short specimens with a tree cover of < 1%. Most of plots were located at slightly higher elevations, corresponding to fossil islands surrounded by sandy, alluvial deposits (McCarthy et al. 2012). The soil data indicate that C. mopane size corresponded to soil conditions. ...
Article
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Precise vegetation descriptions and maps are essential tools for the management of natural areas, as well as for understanding animal habitat use. The Chobe Enclave (CE), adjacent to the Chobe National Park and the Chobe Forest Reserve, forms a critical dry season range for many large herbivores. As a tool for future management and studies about wildlife habitat use and migration, this study proposed to describe the plant communities in the CE and to study their relationships with microtopography and soils. Plant species were inventoried in 82 sampling plots (40 x 20 m), covering the vegetation diversity recognised by an unsupervised classification (Landsat images, 30-m resolution). A hierarchical clustering classified the inventories in eight plant communities, mapped with a supervised classification. This study was conducted in parallel with a soil study. Soil variations and degree of flooding largely determine community composition. Floodplains along the Linyanti River and dambos (concentrating local run off from rainfall) provide reliable green forage for wildlife during the dry season. Adjacent to floodplains, riverine forests also maintain green browse and grazing well into the dry season. In drylands, vegetation is largely determined by soil texture. Forests dominated by Baikiaea plurijuga occupy the acidic, red sands in the east, while sandveld vegetation grows on deep sands in paleo-river channels. These habitats support dominant grasses, which provide important forage for grazers during the wet season. Finally, woodlands dominated by Colophospermum mopane, characterised by sodium-rich and alkaline soils, enable herbivores to meet their mineral requirements during reproduction. Conservation implications: Our soil and vegetation studies provide important insights into factors determining plant communities. Their diversity and close vicinity play a critical role in enabling herbivores to adapt to seasonal variations in forage quantity and quality. Results will enable researchers to gain insights into animal habitat seasonal use in the Chobe Enclave. Keywords: plant community ecology; vegetation map; soil; water availability; microtopography.
... Indeed, the Chobe Enclave is thought to have been part of a large delta and a lake during the Late Pleistocene (Burrough et al., 2009). The formation of some of the islands can probably be compared to those of the present-day Okavango delta system (McCarthy et al., 2012), in which siliceous, low carbonate, and salt deposits occur due to evapotranspiration processes and aeolian dust deposition behind tree curtains (Humphries and McCarthy, 2014). In addition to this microtopography, the region shows a small difference in elevation between extremities (~10 m, with a E-W average slope of 0.025%); large areas of the north-east of the Enclave are regularly flooded by the Linyanti river, whereas the south-west is slightly higher without any flooding, except for small depressions accumulating water after heavy rains (Kinabo et al., 2007). ...
... The presently dry conditions of these areas do not necessary lead to an accumulation of Na + (a sufficient source of Na + is lacking). These deposits were probably formed during wetter conditions than today by the same evapotranspiration mechanism as described by McCarthy and Humphries in the islands of the Okavango Delta (McCarthy et al., 2012;Humphries and McCarthy, 2014). Therefore, the high concentrations of Na + in these soils are probably derived from ancient salt deposits, the parent material of Solonetz and Solonchaks being interpreted as sodic paleosols. ...
Article
Full-text available
The study area of the Chobe Enclave (northern Botswana) is defined as mostly covered by Arenosols in available maps. However, recent explorations of the area showed that soils are more diverse than expected. This is because of complex interactions between current alluvial deposition processes, paleo-environmental effects (ancient alluvial deposition, ancient wind-blown sand deposits) and ongoing hydrological effects and colluvial effects on topographic gradients. An in-depth exploration of both soils and vegetation in the area was conducted with the aim (i) to survey the soil diversity at the Chobe Enclave, (ii) to study soil dynamics and identify the key factors of this diversity, and (iii) to create a soil map based on the analysis of the soil-vegetation relationship. For this purpose, thirty-six soil profiles were extensively described according to the World Reference Base for soil resources. In order to better classify these soils, physicochemical parameters, such as pHH2O, exchangeable cations, and particle size distributions, were measured for a specific set of soils (n = 16), representative of their diversity. To assess Soil Organic Matter (SOM) dynamics, samples were studied using Rock Eval pyrolysis. Results show a high soil diversity and heterogeneity with the presence of (i) Arenosols, as expected, but also of (ii) organic-rich soils, such as Chernozems, Phaeozems, and Kastanozems, (iii) salty/sodic soils, such as Solonchaks and Solonetz, and finally (iv) calcium-rich soils, such as Calcisols. Analyses of the different actors driving the soil diversity emphasized the importance of the surficial geology, composed of different sand deposits (red sands/white sands), carbonate and diatomite beds, as well as ancient salt deposits, in which high proportions of exchangeable Na⁺ were found, associated with high pHH2O (up to 11.3). In addition, as a parameter, the topography creates a complex hydrological system in the Chobe Enclave and therefore, induces a notable soil moisture gradient. Moreover, this study stressed the key role of termites: not only do they modify physicochemical patterns of soils, but they also decay and incorporate large quantities of fresh plant materials into soils. Finally, the analysis of Organic Matter (OM) showed that the Soil Organic Carbon (SOC) is composed essentially by recalcitrant Organic Carbon (OC) substances, such as charcoal, a common carbon type of tropical soils.
... These conditions locally induced the deposition of lacustrine carbonate and diatomite layers on top of the original sand layers (Diaz et al. 2019), and sand dunes were stabilised on the shores of palaeolakes (Burrough & Thomas 2008). Islands probably emerged by accumulation of salts because of plant evapotranspiration -a process comparable to the present-day Okavango Delta system (McCarthy et al. 2012). The southwestern part of the system became disconnected from the Linyanti River and subsequently dried up following the uplift of the Linyanti geological fault. ...
... Some areas are characterised by tall specimens, whereas in other places C. mopane is limited to short specimens with a tree cover of < 1%. Most of plots were located at slightly higher elevations, corresponding to fossil islands surrounded by sandy, alluvial deposits (McCarthy et al. 2012). The soil data indicate that C. mopane size corresponded to soil conditions. ...
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Precise vegetation descriptions and maps are essential tools for the management of natural areas, as well as for understanding animal habitat use. The Chobe Enclave (CE), adjacent to the Chobe National Park and the Chobe Forest Reserve, forms a critical dry season range for many large herbivores. As a tool for future management and studies about wildlife habitat use and migration, this study proposed to describe the plant communities in the CE and to study their relationships with microtopography and soils. Plant species were inventoried in 82 sampling plots (40 x 20 m), covering the vegetation diversity recognised by an unsupervised classification (Landsat images, 30-m resolution). A hierarchical clustering classified the inventories in eight plant communities, mapped with a supervised classification. This study was conducted in parallel with a soil study. Soil variations and degree of flooding largely determine community composition. Floodplains along the Linyanti River and dambos (concentrating local run off from rainfall) provide reliable green forage for wildlife during the dry season. Adjacent to floodplains, riverine forests also maintain green browse and grazing well into the dry season. In drylands, vegetation is largely determined by soil texture. Forests dominated by Baikiaea plurijuga occupy the acidic, red sands in the east, while sandveld vegetation grows on deep sands in paleo-river channels. These habitats support dominant grasses, which provide important forage for grazers during the wet season. Finally, woodlands dominated by Colophospermum mopane, characterised by sodium-rich and alkaline soils, enable herbivores to meet their mineral requirements during reproduction. CONSERVATION IMPLICATIONS: Our soil and vegetation studies provide important insights into factors determining plant communities. Their diversity and close vicinity play a critical role in enabling herbivores to adapt to seasonal variations in forage quantity and quality. Results will enable researchers to gain insights into animal habitat seasonal use in the Chobe Enclave.
... Pene-contemporaneous silica and carbonate formation is taking place in the contemporary Okavango delta (McCarthy et al., 2012) where transpiration from plant growth results in the progressive accumulation of dissolved salts under islands. As the solute-enriched groundwater becomes increasingly saline, silica and then magnesium calcite are the first to precipitate (McCarthy et al., 1993). ...
... If correct, this habitat preference could help explain the overall increase of CaO ( Figure 8) along the proximal to saline pan gradient whereas the general increase in disseminated CaCO 3 with depth is further considered below. Calcium and bicarbonate would over the years be introduced via freshwater inflow as high concentrations of Ca 2+ and HCO -3 ions are prevalent in the Okavango region surface and groundwater (Mackay et al., 2011;McCarthy et al., 2012). Other possible sources include the direct weathering release of Ca 2+ from silicate minerals and via carbonate rich wind-blown dust (Eckardt et al., 2008). ...
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The Boteti palaeo-estuary in northern Botswana is located where the endoreic Boteti river, an overflow from the regional Okavango river system, enters the Makgadikgadi pans. The present work considers diagenetic silica and calcium carbonate dominated transformations. The aims are to help identify precursor conditions for the origin of microcrystalline silcrete-calcrete intergrade deposits while developing insight into pene-contemporaneous silica and calcite matrix formation. General precursor conditions require the presence of cyclical endoreic freshwater inflow into a saline pan. The pan should be deep enough to sustain a permanent watertable under climatic conditions sufficient to cause carbonate fractionation within the groundwater. Freshwater inflow into a saline pan drives the geochemistry of the system (from freshwater to saline, from neutral to high pH). The geochemistry is controlled by the periodicity of inflow relative to salinity levels of phreatic groundwater in the receptor saline pan. The source of most silica and localized CaCO3 is derived from the dissolution and precipitation of microfossils, while more general CaCO3 enrichment stems from saline pan based carbonate fractionation. Diagenetic change leads to colloidal then more consolidated bSiO2/CaO aggregate formation (amorphous silica) followed by transformations into opaline silica over time. Irregular zones of siliceous sediment forming in otherwise calcareous deposits may relate to the irregular occurrence of biogenic silica in the source sediments, inferring a source for local silica mobilization in intergrade deposits. The distribution of calcareous micro-fossils may have a similar converse effect. Diagenetic evidence from an intergrade deposit with a low SiO2/CaO ratio suggests that transformation occurred more into the pan, while an intergrade deposit with a high SiO2/CaO ratio more likely formed closer to a land margin and was frequently inundated by freshwater. Pene-contemporaneous silcrete-calcrete intergrade formation under the above conditions may take place where dissolved silica crystallizes out in the vicinity of calcite crystals due to local decreases in pH. The continuing consolidation of bSiO2/CaO aggregates may be facilitated by the presence of increasing amounts of calcite. It appears that CaCO3 may act as a catalyst leading to pene-contemporaneous bSiO2/CaO aggregate formation. However the processes involved require further work. This article is protected by copyright. All rights reserved.
... Pene-contemporaneous silica and carbonate formation is taking place in the contemporary Okavango delta (McCarthy et al., 2012) where transpiration from plant growth results in the progressive accumulation of dissolved salts under islands. As the solute-enriched groundwater becomes increasingly saline, silica and then magnesium calcite are the first to precipitate (McCarthy et al., 1993). ...
... If correct, this habitat preference could help explain the overall increase of CaO ( Figure 8) along the proximal to saline pan gradient whereas the general increase in disseminated CaCO 3 with depth is further considered below. Calcium and bicarbonate would over the years be introduced via freshwater inflow as high concentrations of Ca 2+ and HCO -3 ions are prevalent in the Okavango region surface and groundwater (Mackay et al., 2011;McCarthy et al., 2012). Other possible sources include the direct weathering release of Ca 2+ from silicate minerals and via carbonate rich wind-blown dust (Eckardt et al., 2008). ...
... In this study, the term "cone" was used to refer to both active mounds without pediments and the conical parts of active mounds with pediments. Dangerfield et al. (1998) and McCarthy et al. (1998McCarthy et al. ( , 2012 suggested that islands on the floodplain of the Okavango Delta originated as mounds built by Macrotermes michaelseni (Sjöstedt), with subsequent expansion of the original structures through accumulation of minerals in groundwater (as a result of transpiration in plants growing on the mounds). They also suggested that islands in the Okavango Delta developed through repeated colonisation of Macrotermes mounds. ...
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Mound-building termites function as ecological engineers in tropical savanna landscapes. Large termite mounds support a greater diversity of plants than off-mound areas, but little is known about the process by which diverse mound vegetation develops. This study examined the dispersal mode of woody plants on termite mounds and assessed the process of plant assemblage formation on mounds. This study focused on mound status (termites active vs. inactive) and mound microtopography (mound cones vs. pediments) to develop a chronology of vegetation development. The frequencies of occurrence of woody plants were lower on active mounds (67 %), especially on cones (46 %) compared to inactive mounds (95 %). Species richness and the abundance of woody plants increased in the order of cones, pediments of active mounds and inactive mounds. The proportion of bird-dispersed plant species was much higher on mounds (>40 %) than in off-mound areas (3 %). Salvadora persica was the main bird-dispersed plant and occurred preferentially on active mounds. A schematic plant succession model incorporating the process of endozoochory was developed to explain vegetation development on active mounds.
... Inactive mounds were not classified in this manner because the boundaries between conical parts and pediments were indistinct. Dangerfield et al. (1998) and McCarthy et al. (1998McCarthy et al. ( , 2012 suggested that islands in the Okavango Delta originated as mounds built by M. michaelseni, with subsequent expansion of the original structures through accumulation of minerals in ground water (as a result of transpiration in plants growing on the mounds). After classification, large, inactive mounds were therefore analysed as structures that had developed sequentially from conical, active mounds. ...
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In savanna, termite mounds support more diverse vegetation than off-mound areas, but little is known of the patterns in plant assemblages on mounds. To explain vegetation differentiation between (1) component structures of termite mounds (conical centre vs. pediment), (2) active and inactive mounds (termites present vs. termites absent), and (3) sites on and off mounds (on mounds vs. surrounding savanna), species composition, richness and abundances of woody plants were recorded on 70 mounds and in 13 savanna plots (each 20 × 20 m) in north-eastern Namibia, focusing on soil hardness, mound status (active or not) and mound micro-topography as explanatory factors. Woody plants were absent from 33% of active mounds (54% of active cones) but were absent from only 5% of inactive mounds. Species richness and abundance per mound (mean ± SD) were lower on active mounds with (2.0 ± 1.8 and 4.6 ± 6.6, respectively) and without pediments (0.6 ± 0.6 and 0.9 ± 1.1, respectively) than on inactive mounds (4.4 ± 2.7, 19.4 ± 18.8, respectively). Despite the lower woody plant cover, some characteristic species, such as Salvadora persica, occurred preferentially on active mounds; this species occurred on 42% of active mounds. Mean soil hardness (± SD) was higher on conical parts of active mounds (4300 ± 2620 kPa) than on adjacent pediments (583 ± 328 kPa) and inactive mounds (725 ± 619 kPa). This study suggested that mound status, mound micro-topography, and soil hardness promote variability in the vegetation on mounds.
... The hydrology of the Okavango Delta is characterized by an annual flood pulse which travels down the delta in 4-6 months. High water conditions occur in the dry season (April to October) when the delta is flooded by discharge from subtropical watersheds in Angola, while low water conditions occur during the rainy season (November to March) when local rainfall estimated annually at 450 mm (e.g., McCarthy et al., 2012;Milzow et al., 2009) river to the groundwater table, the river represents a major source of groundwater recharge in the region (McCarthy et al., 1998;DWA, 2003;McCarthy, 2006). ...
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Study region: Okavango Delta, Middle Kalahari, NW Botswana. Study focus: We investigated the effect of evapotranspiration on the evolution of dissolved inorganic carbon (DIC) and stable carbon isotopes of DIC (13 C DIC) in the Okavango River. We measured the DIC concentrations and the 13 C DIC for samples collected over a 400 km reach of the river in the Okavango Delta during flood conditions and non-flood conditions. In addition, we incubated river samples collected from the proximal portion (Mohembo) and the distal portion (Maun) of the Delta and subsequently evaporated the samples by ∼90% under ambient conditions. New hydrological insights: We found a 379% and 500% increase in the DIC concentrations and a 13 C DIC increase of 3.9‰ and 6.1‰ in the river during the flood non-flood conditions, respectively. The DIC concentrations of evaporated river samples increased by 535% for the Mohembo and by 850% for the Maun samples. The increase in the 13 C DIC of the evaporated river samples resulted from CO 2(g) loss during chemical equilibrium with atmospheric CO 2(g) followed by carbon exchange between DIC and atmospheric CO 2(g). Although the 13 C DIC increased spatially for the Okavango River, it never reached the value of ∼0‰ expected for equilibration of river DIC with atmospheric CO 2(g). The results of the evaporated river samples suggest that isotopic enrichment from equilibration in Okavango River was balanced by respiration and photo-oxidation of carbon-depleted dissolved organic matter.
... ). In the Okavango Delta, Botswana, the elevated transpiration rate of trees growing on islands establishes a concentric groundwater-flow pattern from the surrounding swamps towards the center of the islands, where groundwater salinity steadily increases inducing the chemical precipitation of calcite and silica(McCarthy et al., 2012). Additionally, the salinity increase in the groundwater beneath the clayey soils of those islands, triggers a process of density-driven groundwater flow transporting solutes downward into the deeper aquifer layers, where saline groundwater accumulates within sand layers contributing to the salt balance of the freshwater-wetland ecosystem(Bauer-Gottwein et al., 2007;Ramberg & Wolski, 2008).Seasonal cycles of precipitation and dissolution of carbonate minerals within soils and sediments are also regulated by the position of the water table. ...
Thesis
The decline of tree islands in the freshwater-Everglades wetland because of hydrologic manipulation, has compromised valuable ecosystem services. Although the role of tree islands in maintaining freshwater quality stems largely from evapotranspiration processes, fundamental questions remain about the effects of different geologic materials on their hydrogeochemical functioning. To reduce this uncertainty, the lithological composition of a set of man-made tree islands was investigated coupled with long-term hydrologic and hydrochemical data. Key results indicate that limestone substrates and peat substrates with elevated proportions of sand, facilitated surface water-groundwater interactions and mineral dissolution. However, limestone-based islands were more effective in lowering the water table and concentrating solutes in response to evapotranspiration during low surface water stages. Additionally, the peat substrate of an island with low sand content favored the thermodynamic conditions for calcite accumulation in the phreatic zone, while phosphorus concentrations in the groundwater were associated with the breakdown of organic matter.
... Briefly, evapotranspiration from island and channel vegetation induces a local lowering of the water table. Surface water follows the hydraulic gradient, flowing into the island subsurfaces where enrichment and authigenic mineral precipitation effectively trap solutes within the system (McCarthy et al. 2012;Ramberg et al. 2006;Struyf et al. 2015). ...
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Wetlands fed by rivers can be a sink for elements depending on elemental concentrations, wetland hydrology, geochemistry, vegetation and climate. In the case of the Okavango Delta, northern Botswana, the outflow discharge is a small fraction (2–5%) of the inflow. This has strong potential consequences for the Delta, as it strongly affects element cycling and storage within the Delta. We estimated the inputs, behaviour and distribution of multiple elements along a longitudinal transect within the Okavango Delta, to show potential effects of retention mechanisms of different elements. High annual element input is rather attributed to discharge than to the concentration within the water, which is generally extremely low. We observed minimal enrichment of the elements within the water pathway along the transect from inflow to outlets, implying that element output is negligible. For most elements, we observed a high correlation between storage and sediment organic matter content. The organic matter content within the sediments was higher in the vegetated sediments than in non-vegetated sediments (factor ∼ 10), and a similar trend was found for most elements. In conclusion, organic matter dominated in sediments from vegetated plots and thus plays an important role in retaining the elements within the sediments of the Delta. This finding has major implications for e.g. planning constructed wetlands for water purification or element retention especially in areas with high evapotranspiration.
... The Okavango Delta in Botswana, one of the world's largest tropical wetland systems, is an interesting system in which to study submerged macrophytes as ecosystem engineers (Fig. 1). It comprises a mosaic of permanent and seasonal floodplains and tree-covered islands (McCarthy et al. 2012). With about 98% of total annual inflow being lost to the atmosphere before reaching the outlets, the Delta could be expected to become a saline system. ...
Article
The role of in-stream aquatic vegetation as ecosystem engineers in the distribution of organic matter was investigated in the Okavango Delta, one of the world’s largest oligotrophic wetlands. The Okavango channel beds are covered up to 50% with submerged macrophyte patches. By accumulating and concentrating organic matter in the sediments below the patches, macrophytes are likely able to locally forestall a deficiency of nutrients. Up to 21 times more N, 18 times more C, 13 times more P and 6 times more Si can be found in vegetated sediments compared to non-vegetated sediments. Nutrient specific accumulation relates to its relative scarcity in the overlaying water. There is a depletion of dissolved N relative to P, whereas Si is relatively abundant. The Okavango Delta water can generally be characterised as oligotrophic based on plant species composition (e.g. presence of carnivorous plants and absence of floating plants), low plant N:P ratios, and low nutrient- and element-concentrations. Local mineralization and intensified nutrient cycling in the sediments is hypothesized to be crucial for the macrophytes’ survival because it provides a key source of the essential nutrients which plants otherwise cannot obtain in sufficient quantities from the nutrient poor water. By engineering the ecosystem as such, channel vegetation also retards the loss of elements and nutrients to island groundwater flow, contributing to one of the key processes driving the high productivity of the Okavango Delta, making it unique among its kind.
... The influence of tree vegetation on soil mineralization processes and groundwater flow patterns have been documented in wetland environments. For instance, in the Mkuze wetland in South Africa and the Okavango Delta in Botswana, chemical precipitation of calcite, silica, and other minerals was found to occur in soils of tree islands in response to elevated evapotranspiration (ET) rates (Humphries et al., 2011;McCarthy et al., 2012). Elevated tree-island ET rates in the Okavango Delta also results in a density-driven accumulation of saline water into deep sand layers beneath clayey soils across the wetland, contributing to the salt balance of the ecosystem (Bauer-Gottwein et al., 2007;Ramberg and Wolski, 2008). ...
Article
Tree islands have been hypothesized to induce calcite precipitation by concentrating ions in the groundwater through evapotranspiration processes. This research investigates how lithology regulates the hydrologic and geochemical conditions within two types of constructed Everglades tree islands: peat-based and limestone-core surrounded by peat. Three years of hydrochemical and hydrologic data (2013–2015) obtained from the constructed tree islands suggest that under current climatic and managed hydrologic conditions, calcite dissolution is prevalent within the top meter of soil in both types of tree islands. Mass-balance calculations along groundwater flow paths in one peat-based tree island that contained clay at depth, indicated that calcite precipitation is likely 1 m below the soil surface. The lithological characteristics of that peat-based island supported a persistently depressed groundwater table, while tree islands with elevated ratios of peat-to-sand content did not. Limestone-core tree islands also supported a depressed water table, particularly during the dry season. This study determined that the lithology of a set of man-made Everglades tree islands played a primary role in regulating the seasonal fluctuation of the water table and hydrogeochemical processes. Understanding the mechanisms of tree-island formation and maintenance is important for preserving the overall ecosystem function of the freshwater-Everglades.
... This conceptual model considers the role of the river in the accumulation of solutes in solute stores beyond the river channel and in groundwater and in the transfer of solutes from the solute stores to the river. The solute stores (Fig. 5a) beyond the Okavango River channel consist of (1) salt precipitates on floodplains and on island fringes and island centers that result from ET Gumbricht et al., 2004;Ramberg and Wolski, 2008), (2) post flood recession isolated wetland pools and wetland water subjected to ET (Dincer et al., 1979;McCarthy et al., 1991 and (3) saline groundwater (Ramberg and Wolski, 2008;McCarthy et al., 2012). River access to the solute stores and the transfer of solutes to the river is established by temporal activation of hydrologic flow pathways connecting the river to the solute stores. ...
Article
We measured the concentrations of the total dissolved ions (TDI), dissolved silica, major cations and the δ D and δ ¹⁸O at sub-weekly intervals for one year in the Okavango River at the outlet of the Okavango Delta (Delta). Our objectives were to (1) document the temporal variations in the concentrations of solutes in the Okavango River, (2) determine the processes controlling the transfer of solutes to the river and (3) assess the temporal solute load and outflux from the Delta. We found that the TDI and major cation concentrations in the river were anomalously high during the rainy season and before the arrival of the annual flood pulse. The anomalous increases in the solute concentrations are due to dissolution and mobilization of precipitated salts stored in the floodplains and on hundreds of thousands of islands scattered across the Delta wetlands, as well as from ‘flushing’ of remnant evaporated flood water of higher salinity trapped in isolated wetland pools. Overland flow generated by local rains and flooding connect the river to the solute stores in the watershed. The temporally activated hydrologic flow pathways transfer solutes to the river that flushes them out of the Delta. The solute load in the river was higher during the rainy season and during pulse flooding, and mimicked the discharge hydrograph. We estimate that 17,838 Mg/y of dissolved solutes was flushed out of the Delta, with 67% removed during pulse flooding (6 months) and 30% during the rainy season (4 months). The transfer of solutes from the watershed to the river during pulse flooding and the rainy season, and solute export from the Delta is an important mechanism that keeps the Delta’s surface water resources fresh, which is critical for supporting a freshwater wetland ecosystem. Our results highlight the importance of intermittent activation of hydrologic flow pathways in controlling solute cycling in this and other arid watersheds.
... The flood-wave takes until August to filter through the anastomosing channels and swamps of the Okavango, the largest wetland in southern Africa and the largest inland delta on Earth (McCarthy and Ellery, 1998). Suspended sediments supplied annually to the Okavango Delta amount to only ~19% of total load (McCarthy et al., 2012), suggesting that weathering intensity is presently quite low even in the northern Kalahari Basin. Sedimentation of fine sand dominates the upper delta, whereas chemical sedimentation in the form of calcrete and silcrete prevails in the lower delta (McCarthy and Metcalfe, 1990). ...
Article
We here review what is known about the dunefields and fluvial systems of the Kalahari Basin in terms of geological setting and Quaternary dynamics and set out what has been hypothesized about the provenance of Kalahari sand so far. Previous work has tackled this problem by applying a limited number of techniques (mostly sediment textures and heavy minerals) to parts of the large dryland. The generally highly quartzose mineralogy of aeolian dunes and their compositional variability have been only broadly evaluated and several sedimentological issues have thus remained controversial, including the relative role played by fluvial processes versus aeolian reworking of older sediments and weathering controls. This reveals a need for a systematic provenance study that considers the entire basin. For this reason, here we combine original petrographic, heavy-mineral, and detrital-zircon geochronology data with previously published clay-mineral, geochemical, and geochronological information to present the first comprehensive provenance study of the vast Kalahari sand sea. Our multi-proxy dataset comprises 100 samples, collected across the Kalahari Basin from 11°S (NW Zambia) to 28°S (NW South Africa) and from 15°E (Angola) to 28°30′W (Zimbabwe). Kalahari aeolian-dune sand mostly consists of monocrystalline quartz associated with durable heavy minerals and thus drastically differs from coastal dunefields of Namibia and Angola, which are notably richer in feldspar, lithic grains, and chemically labile clinopyroxene. The western Kalahari dunefield of southeastern Namibia is distinguished by its quartz-rich feldspatho-quartzose sand, indicating partly first-cycle provenance from the Damara Belt and Mesoproterozoic terranes. Within the basin, supply from Proterozoic outcrops is documented locally. Composition varies notably at the western and eastern edges of the sand sea, reflecting partly first-cycle fluvial supply from crystalline basements of Cambrian to Archean age in central Namibia and western Zimbabwe. Basaltic detritus from Jurassic Karoo lavas is dominant in aeolian dunes near Victoria Falls. Bulk-sediment petrography and geochemistry of northern and central Kalahari pure quartzose sand, together with heavy-mineral and clay-mineral assemblages, indicate extensive recycling via aeolian and ephemeral-fluvial processes in arid climate of sediment strongly weathered during previous humid climatic stages in subequatorial Africa. Distilled homogenized composition of aeolian-dune sand thus reverberates the echo of paleo-weathering passed on to the present landscape through multiple episodes of fluvial and aeolian recycling. Intracratonic sag basins such as the Kalahari contain vast amounts of quartz-rich polycyclic sand that may be tapped by rivers eroding backwards during rejuvenation stages associated with rift propagation. Such an event may considerably increase the sediment flux to the ocean, fostering the progradation of river-fed continental-embankments, as documented by augmented accumulation rates coupled with upward increasing mineralogical durability in the post-Tortonian subsurface succession of the Zambezi Delta. The Central Kalahari is not a true desert. It has none of the naked, shifting sand dunes that typify the Sahara and other great deserts of the world. In some years the rains may exceed twenty — once even forty — inches, awakening a magic green paradise.” Mark Owens, Cry of the Kalahari.
... On the plateau, rivers with low channel steepness sluggishly flow for large tracts through wetlands, where sediment is sequestered rather than produced, as in the Barotse floodplain and Chobe marshes on the main stem or the Machili and Kafue Flats traversed by the Kasaya, Ngwezi, and Kafue tributaries. Because data on Kwando sediment load are, to the best of our knowledge, unavailable, information from the Okavango River, similarly draining entirely within the Kalahari Basin in Angola (Shaw and Thomas 1992;McCarthy et al. 2012), allows us to broadly constrain the annual sediment yield and erosion rate in the Kalahari Basin as 2 5 2 tons/km 2 and 0:001 5 0:001 mm, respectively. This is notably lower than estimates based on cosmogenic nuclides for the Uppermost Zambezi catchment, including Precambrian terranes in the north (16 5 2 tons/km 2 and 0:006 5 0:001 mm; Wittmann et al. 2020). ...
Article
Elemental geochemistry, Nd isotopes, clay minerals, and U-Pb zircon ages integrated by petrographic and heavy-mineral data offer a multi-proxy panorama of mud and sand composition across the Zambezi sediment-routing system. Detrital-zircon geochronology highlights the four major episodes of crustal growth in southern Africa: Irumide ages predominate over Pan-African, Eburnean, and Neoarchean ages. Smectite, dominant in mud generated from Karoo basalts or in the equatorial/winter-dry climate of Mozambican lowlands, prevails over illite and kaolinite. Elemental geochemistry reflects quartz addition by recycling (Uppermost Zambezi), supply from Karoo basalts (Upper Zambezi), and first-cycle provenance from Precambrian basements (Lower Zambezi). Mildly negative for sediments derived from mafic granulites, gabbros, and basalts, εNd values are most negative for sand derived from cratonic gneisses. Intrasample variability among cohesive mud, very coarse silt, and sand is principally caused by the concentration of Nd-rich monazite in the fine tail of the size distribution. The settling-equivalence effect also explains deviations from the theoretical relationship between εNd and TNd,DM model ages, suggesting that monazite carries a more negative εNd signal than less dense and less durable heavy minerals. Elemental geochemistry and Nd isotopes reveal that the Mazowe-Luenha river system contributes most of the sediment reaching the Zambezi Delta today, with minor supply by the Shire River. Sediment yields and erosion rates are lower by an order of magnitude on the low-relief Kalahari Plateau than in rugged Precambrian Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation Open Rubric terranes. On the Plateau, mineralogical and geochemical indices testify to extensive breakdown of feldspars and garnet unjustified by the presently dry climate. Detrital kaolinite is recycled by incision of Cretaceous-Cenozoic paleosols even in the wetter lower catchment, where inefficient hydrolysis is testified by abundant fresh feldspars and undepleted Ca and Na. Mud geochemistry and surficial corrosion of ferromagnesian minerals indicate that, at present, weathering increases only slightly downstream the Zambezi River.
Article
The origin and the chemical and isotopic evolution of dissolved inorganic carbon (DIC) in groundwater of the Okavango Delta in semi-arid Botswana were investigated using DIC and major ion concentrations and stable oxygen, hydrogen and carbon isotopes (δ¹³CDIC, δD and δ¹⁸O). The δD and δ¹⁸O indicated that groundwater was recharged by evaporated river water and unevaporated rain. The river water and shallow (<10 m) groundwater are Ca-Na-HCO3 type and deep groundwater (≥10 m) is Na-K-HCO3 to HCO3-Cl-SO4 to Cl-SO4-HCO3. The mean DIC concentrations were two times higher in shallow groundwater, seven times higher in deep groundwater and twenty-four times higher in island groundwater. The δ¹³CDIC indicate that DIC production in groundwater is from organic matter oxidation and in island groundwater from organic matter oxidation and dissolution of sodium carbonate salts. The ionic and isotopic evolution of the groundwater relative to evaporated river water indicates two independent pools of DIC.
Article
Dust 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 predominately from the Makgadikgadi salt pans, located 300 km away, and contributes 10 to 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.
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Freshwater wetlands located in dryland environments are characterised by high evapotranspiration rates and frequent periods of desiccation, which strongly influence the water chemistry and solute budgets of these systems. The transpiration of groundwater, especially by trees, is an important mechanism through which dryland wetlands can lose water. This process can lead to groundwater salinization and the precipitation of substantial quantities of mineral phases within the soil, the accumulation of which can have profound consequences for wetland structure and function. This paper aims to bring together current knowledge on the processes that result in solute accumulation and chemical sedimentation which assist in maintaining freshwater conditions in many seasonal dryland wetlands. Examples from central and southern Africa, Australia and South America are presented to illustrate the geomorphically diverse settings under which chemical sedimentation can occur, and the importance of these processes for the resilience and longevity of dryland wetlands. We show that the localised development of saline groundwater and subsurface precipitation of minerals within soils can play a key role in creating and maintaining the habitat diversity of dryland wetlands. Wetland vegetation focuses the accumulation of deleterious constituents, thereby preventing widespread salinization and playa-lake formation, and thus ensuring that the bulk of the surface water remains fresh. Although such processes remain widely understudied, we suggest that chemical sedimentation could be a common phenomenon in many dryland wetlands and have important implications for the future management of these ecosystems.
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ZoogeomorphologyZoogeomorphology is a relatively new discipline and is set for changes this century in BotswanaBotswana that have not been seen for millennia. The co-evolution of landscapes and wildlife amidst a changing climate means that the precise role of each is difficult to determine. As southern Africa becomes hotter and drier due to anthropogenic climate change this century, past adaptation strategies such as wildlife movements along (Balinsky’s 1962) ‘drought corridor’ will no longer be possible due to land use/land cover change and agriculture and human-related expansion. As the Sixth Great Extinction unfolds, it offers a unique opportunity to study just how significant different biota are in determining geomorphology, albeit as our climate changes. FaunaFauna that has remained intact since the Miocene will largely disappear from African shores and the palaeo-duneDune fields of the Kalahari may well become reactivated. The real significance of micro-organisms and invertebrates will then be realised as the bioturbators remain to shape the landscapes around them without the distraction posed by humans and the unique mega-fauna that surrounded them.
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The role of water and airborne sediments and nutrients is examined in the nearly closed system of the Okavango Delta. The total annual sediment load carried by the Okavango River into the delta is estimated at 620 000 tonnes. Annual deposition on the delta from the atmosphere under anticyclonic conditions only is estimated at 250 000 tonnes. Nutrient supply to the delta system from these two sources is estmiated for the papyrus-dominated habitats of the permanent swamps. Maximum and minimum water-borne and aerosol contributions are determined for potassium, phosphorus and nitrogen species. On an annual basis, atmospheric inputs range from 6-60% of the total. The existence of large reservoirs of nutrients in the peat cannot account for the observed variability in papyrus productivity on the channel fringes and in the backswamp areas, due to low rates of mineralisation (8.1 and 0.12 kg/ha/yr for nitrogen and phosphorus, respectively). Enhanced productivity exists on channel fringes due to flowing water; more even distrution of nitrogen and phosphorus over the delta comes from the atmosphere. Aerosol inputs of 3.9 and 0.13 kg/ha/yr for nitrogen and phosphorus, respectively, are compared with corresponding aquatic supplies of 108 and 2.25 kg/ha/yr. The effect of these contributions on papyrus productivity is discussed. The results obtained for the Okavango Delta suggest that atmospheric supply of nutrients may be equally important in more complex southern African ecosystems.
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Soil and groundwater salinization is a major problem in aquatic systems throughout the semi-arid and arid regions. An interesting example of a natural terminal evaporative system is the Okavango Delta, a large inland Delta in Botswana. The system accumulates about 300,000 tons of dissolved solids per year. The majority of the accumulated solutes are deposited on small islands in the wetland. In the centre of the islands, the shallow groundwater is therefore highly saline and displays total dissolved solid (TDS) concentrations around 30 g/l. Fresh groundwater underlying the superficial brines gives rise to a hydrodynamically unstable situation with dense brine perched on less dense fresh water. Instabilities (density fingers) can potentially occur and effectively transport the superficial brines into deeper aquifer units. Electrical resistivity tomography (ERT) was used in this study to directly map the salinity distribution in the subsurface below two different islands in the Okavango Delta. Surface arrays as well as borehole-to-surface layouts were used. Due to differences in the regional hydrologic setting, a density finger could be observed on one island, whereas on the other, the high salinity anomaly was confined to the surface layer. To our knowledge, this is the first time that fingering instabilities were observed under field conditions in natural aquifers.
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The modeled best fit to observed isotopic profiles suggests that the profile differences in part result from differing soil-respiration rates at each site. Soil CO2 and soil-respiration rates increase systematically with elevation. Suggests that evaporative isotopic enrichment of soil waters may have occurred at all elevations prior to precipitation of carbonate, or that seasonal differences in the isotopic composition of meteoric waters coupled with differential infiltration may be taking place. -from Authors
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The four models postulated for the formation of calcrete layers all involve the three-stage process: provision of a carbonate-rich solution, the movement of that solution and the precipitation of solid carbonate. The last step originates from either or both of two main alternatives: loss of CO2 out of the solution increasing the pH and consequently the CO concentration, or evaporation of the water effecting an increase in the ionic concentration. This paper presents theoretical calculations for the isotopic composition of the carbonate resulting from these two alternative processes. The isotopic composition of calcrete deposits from France, Spain, Italy, Libya, Cyprus, Kenya, Morocco, South Africa and India are compared with the theoretical models. The calcrete samples from Spain, Cyprus and India are probably the result of evaporation processes.
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A distinctive feature of the Nhecolândia, a sub-region of the Pantanal wetland in Brazil, is the presence of both saline and freshwater lakes. Saline lakes used to be attributed to a past arid phase during the Pleistocene. However, recent studies have shown that saline and fresh water lakes are linked by a continuous water table, indicating that saline water could come from a contemporary concentration process. This concentration process could also be responsible for the large chemical variability of the waters observed in the area. A regional water sampling has been conducted in surface and sub-surface water and the water table, and the results of the geochemical and statistical analysis are presented. Based on sodium contents, the concentration shows a 1: 4443 ratio. All the samples belong to the same chemical family and evolve in a sodic alkaline manner. Calcite or magnesian calcite precipitates very early in the process of concentration, probably followed by the precipitation of magnesian silicates. The most concentrated solutions remain under-saturated with respect to the sodium carbonate salt, even if this equilibrium is likely reached around the saline lakes. Apparently, significant amounts of sulfate and chloride are lost simultaneously from the solutions, and this cannot be explained solely by evaporative concentration. This could be attributed to the sorption on reduced minerals in a green sub-surface horizon in the “cordilhieria” areas. In the saline lakes, low potassium, phosphate, magnesium, and sulfate are attributed to algal blooms. Under the influence of evaporation, the concentration of solutions and associated chemical precipitations are identified as the main factors responsible for the geochemical variability in this environment (about 92% of the variance). Therefore, the saline lakes of Nhecolândia have to be managed as landscape units in equilibrium with the present water flows and not inherited from a past arid phase. In order to elaborate hydrochemical tracers for a quantitative estimation of water flows, three points have to be investigated more precisely: (1) the quantification of magnesium involved in the Mg-calcite precipitation; (2) the identification of the precise stoichiometry of the Mg-silicate; and (3) the verification of the loss of chloride and sulfate by sorption onto labile iron minerals.
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The relationship between δ18O and δ13C in a closed-basin lake is a function of hydrological change, vapor exchange, lake productivity, and total CO2 (or carbonate alkalinity) concentration. Covariance of δ18O and δ13C usually occurs in lake sediments under the condition of hydrological closure for time periods of the order of 5000 yr or longer. On shorter time scales, however, certain subtleties in the use of the δ13C–δ18O covariance as a hydrology history indicator must be recognized. The covariant trend may not be found in hyper-alkaline lakes because of the insensitivity of δ13C to lake volume changes. For less alkaline closed-basin lakes, covariance of δ13C–δ18O will result from relatively rapid increase or decrease of lake volume, with the rapid-decrease condition giving rise to heavy and narrow-ranged isotopic values for both δ13C and δ18O due to the coupled evaporation-productivity effect. When the lake volume remains stable (as is the case for an over-flowing open lake), poor δ13C–δ18O covariance will ensue, due to the effect of vapor exchange with the atmosphere.We have used the measured δ13C–δ18O covariance in carbonate sediments of Mono Lake, California, to deduce paleo-hydrological and lake-water alkalinity (and salinity) variations and found the results in consonance with Holocene lake-level history derived from the δ18O and other information.
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Many organisms create ot alter resource flows that affect the composition and spatial arrangement of current and future organismal diversity. The phenomenon called eosystem engineering is considered with a case study of the mound-building termite Macrotermes michaelseni. It is argued that this species acts as an ecosystem engineer across a range of spatial scales, from alteration of local infiltration rates to creation of landscape mosaics, and that its impacts accrue because of initiation of biophysical processes that often include feedback mechanisms. These changes to resource flows are likely to persist for long periods and constrain the biological structure of the habitat. The value of ecosystem engineering is discussed as a holistic way of understanding the complexity of tropical ecology.
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Evapotranspiration from a phreatic groundwater table is an important input parameter for hydrological models. This parameter, however, is difficult to determine in the field, since plants take their water from various depths and the penetration depth of direct evaporation is unknown. We present a method to estimate evapotranspiration rates based on diurnal groundwater level fluctuations observed in the Okavango Delta, Botswana. To this end, the regional setting as well as the local processes in the unsaturated zone have to be analysed. Resulting estimates for the evapotranspiration rate range from 0.06 to 4.3 mm/day, for varying local vegetation cover and soil characteristics.
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The IntCal04 and Marine04 radiocarbon calibration curves have been updated from 12 cal kBP (cal kBP is here defined as thousands of calibrated years before AD 1950), and extended to 50 cal kBP, utilizing newly available data sets that meet the IntCal Working Group criteria for pristine corals and other carbonates and for quantification of uncertainty in both the 14C and calendar timescales as established in 2002. No change was made to the curves from 0–12 cal kBP. The curves were constructed using a Markov chain Monte Carlo (MCMC) implementation of the random walk model used for IntCal04 and Marine04. The new curves were ratified at the 20th International Radiocarbon Conference in June 2009 and are available in the Supplemental Material at www.radiocarbon.org.
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PREFACE Within the Florida Everglades, tree islands, which cover only a small percentofthis ecosystem, historically have provided essential habitat for a wide variety ofterrestrial and amphibious plants, birds, and animals. These tree islands, however, have been one ofits least studied features. Because of their less flood tolerant vegetation, tree islands are one ofthe most sensitive components ofthe Everglades to changes in hydrology, and many tree islands have been lost during periods when water levels have been abnormally high or low. Their sensitivity to water level changes makes tree islands potentially one ofthe best and surest measures ofthe overall hydrologic health of the Everglades. Consequently, the maintenance of healthy, functioning tree islands and the restoration ofthose that have been lost will be an important performance measures that will be used tojudge the success ofthe Comprehensive Everglades Restoration Plan (CERP). A symposium, Tree Islands ofthe Everglades, was held on July 14 and 15, 1998 at Florida Atlantic University, Boca Raton, Florida. It was sponsored by Florida Center for Environmental Studies and the South Florida WaterManagement District. This was the first scientific meeting ever devoted to tree islands. The organizers of this symposium were Drs. Arnold van der Valk, Florida Center for Environmental Studies and Iowa State University, Fred Sklar, South Florida Water Management District, and Wiley Kitchens, United States Geological Survey.
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Carbon isotope techniques were used to determine the photosynthetic pathway of a number of wetland plant species. The relative cover abundances of C3 and C4 plants were compared along a successional sequence of the Maunachira River system of the Okavango Delta. Plants with a C3 photosynthetic pathway were dominant in early successional stages and C4 plants were dominant in late successional stages. It is proposed that nutrient availability, particularly nitrogen limitation, could be a determinant of the change from C3 to C4 plant dominance during succession.
Article
The pronounced chemical fractionation that takes place in hydro logically closed basins between dilute inflow and concentrated brines can be accounted for by a variety of mechanisms. These include mineral precipitation, selective dissolution of efflorescent crusts and sediment coatings, sorption on active surfaces, degassing, and reactions. Major solutes are differentially affected by these mechanisms, and their response may differ from basin to basin. Using data from Lake Magadi, Kenya; Lake Abert, Oreg; Devils Lake, N. Dak; Deep Springs Lake, Calif.; Basque Lake, British Columbia; and Great Salt Lake, Utah, some of these differences can be delineated. -from Authors
Article
A topographic map of the Okavango Delta and environs has been constructed using a combination of elevation data including trigonometric beacons and spot heights from the government of Botswana, surveys of the navigable channels, U. S. Department of Defense data and measurements made during a geophysical survey of the region. The topography provides insight into the local tectonic and sedimentary history. Local tectonics are dominated by uplift and horst formation associated with the Ghanzi Ridge, and an arch to the north of the Panhandle, which appear to represent the tips of incipient rifts which are propagating from the northeast. The Delta has formed in the resulting depression between these arches. The Panhandle has developed along a fault, and may be largely an erosional feature incised into the northern uplift zone. The Delta itself is an alluvial fan of remarkably uniform gradient. There is no evidence of regional tilting of the fan surface. Local highs and lows are developed on the fan, but channel location is relatively insensitive to this local topography. Moreover, marked elevation differences exist between adjacent channels, creating hydrologically unstable conditions. These unusual features of the local hydrology arise because of the confining effect of channel-flanking vegetation. Sedimentation in the Delta appears to be causing crustal sagging of the central Delta, which has: tilted the major palaeo-shoreline of the Mababe Depression to the west; formed a local depression within the Ghanzi Ridge facing the Delta; and detached a sliver of the ridge along the Thamalakane fault. It is suggested that local seismicity also results mainly from sediment loading. The Selinda spillway occupies a marked local depression, which is a graben between the Gumare fault and an extension of the Linyanti fault. It is probable that southwesterly propagation of the uplift zone associated with the incipient rift will ultimately deflect the Okavango River into the Chobe-Zambezi river system via this graben.
Article
Scientists’ understanding of the role of tree islands in the Everglades has evolved from a plant community of minor biogeochemical importance to a plant community recognized as the driving force for localized phosphorus accumulation within the landscape. Results from this review suggest that tree transpiration, nutrient infiltration from the soil surface, and groundwater flow create a soil zone of confluence where nutrients and salts accumulate under the head of a tree island during dry periods. Results also suggest accumulated salts and nutrients are flushed downstream by regional water flows during wet periods. That trees modulate their environment to create biogeochemical hot spots and strong nutrient gradients is a significant ecological paradigm shift in the understanding of the biogeochemical processes in the Everglades. In terms of island sustainability, this new paradigm suggests the need for distinct dry-wet cycles as well as a hydrologic regime that supports tree survival. Restoration of historic tree islands needs further investigation but the creation of functional tree islands is promising.
Article
Primary carbonates are a common feature of many modern and ancient lacustrine deposits. Carbonates from hydrologically open lakes show little or no correlation between δ13C and δ18O. In short-residence-time open lakes, carbonate oxygen isotopic composition is relatively invariant and typically is closely related to the bulk isotopic composition of inflow waters to the lake. Suites of carbonates which display covarying 13C and 18O compositions precipitate from waterbodies having relatively long residence times. Where the correlation between carbon and oxygen isotopic variations is high (r⪖ 0.7 ), the carbonates have normally precipitated from a closed lake. In addition, because of large changes in water balance, the δ18O of closed-lake carbonates usually varies over a range of several %o. Therefore, the combination of degree of covariance and spread of δ18O-values can be used to discriminate between carbonates produced in hydrologically open and closed basins. Within individual basins, covariant trends may have remarkable long-term persistence despite major environmental changes, indicating considerable stability in basin hydrology. Each closed lake has a unique isotopic identity defined by its covariant trend, which is a function of the basin's geographical and climatic setting, its hydrology, and the history of the waterbody. Any major interruption or realignment of this trend reflects a fundamental change in basin hydrology. Isotopic trends based upon the carbon and oxygen isotopic composition of primary lacustrine carbonates have several applications in palaeolimnology. The oxygen isotopic composition of open-lake carbonates may, with caution, be used as a proxy indicator of the composition of regional rainfall. Covariant trends can be used to trace the hydrological history of a basin, the evolution of individual water masses, and to correlate carbonate-bearing sediments from different parts of a basin.
Article
In many salt lakes around the world, the relative abundance of preserved authigenic minerals is different from that predicted from solute mass balance calculations. Conventional mass balance models assume that chloride behaves conservatively over long periods of time and fail to take into account the role of diffusion, deflation and fractional crystallisation/dissolution of salts. An alternative approach is to use oxygen isotopes as these reflect directly water molecule rather than solute concentrations and have the added advantage of providing a palaeohydrological record in lacustrine carbonates. We present a steady-state, stable-isotope model, in conjunction with stable isotopic measurements of sub-surface brines, regional groundwaters and carbonate deposits from the Lake Malata–Lake Greenly playa complex in South Australia, to estimate the apparent leakage and palaeoleakage from these superficially closed playa lakes. The steady-state model calculations, using the present δ18O and δ2H compositions of the lake brines and inflowing groundwater, suggest that the apparent present-day leakage for the complex is 75 to 90% of inflow (∼35 times that calculated from Cl− and Br−). Under such conditions, only low magnesian calcite precipitates and the lake water experiences reduced effects of evaporation, gas and vapour exchange and, consequently, reduced isotopic and chemical enrichment. Further, our model shows that calcite becomes increasingly Mg-rich until leakage is reduced to ∼55 to 70% of inflow — a condition favourable for dolomitisation. δ18O and δ13C of the lacustrine carbonates show excursions on the order of 5‰ over the length of a 2.3 m core, indicating that the lake complex has varied from being throughflow dominated (presence of low Mg-calcite relatively depleted in δ18O and δ13C) to evaporation dominated (high Mg-calcite/dolomite relatively enriched in δ18O and δ13C) throughout late Quaternary. Our estimates of leakage fractions are consistent with the observed mineralogical suite, but there remains a discrepancy between apparent closure indicated by the presence of the highly saline brine reservoir (
Article
Radiocarbon dates from pedogenic carbonate in the southern Strzelecki Desert of South Australia are several thousand years too young, a conclusion contrary to previous work which suggested dates on such material were reasonably reliable within the semi-arid zone, or several thousand years too old in the arid zone. Multiple dates from the same horizon, and from carbonates in vertical sequence in the same exposure, were used to test the reliability of those dates. These revealed that there were reversals and inconsistencies along single horizons, and it was found that carbonate fragments in cores of nodules gave older dates; also some recrystallization of carbonate had taken place and more than one generation of micrite was present. Dates from soil carbonates in the arid zone are not reliable estimates of the age of pedogenesis, but can be useful if their limitations are recognized.
Article
δ18 O values, δ13C values and 14C ages were determined in ten CaCO3, nodule populations collected from soil and paleosols in the Israeli coastal plain. The selected soils were carbonate-free when formed and the nodules in them represent either reprecipitation of carbonate illuviated from overlying horizons (descending mode) or precipitated from a raised brackish water table (ascending mode). The 14C ages represent the times of migration (illuviation) or eustatic movement (both climate-related) and the stable isotopic conditions reflect the environmental conditions at such times.The δ 13C values of most populations were found to have a wide range and cannot be used to reconstruct former climates. This wide range is attributed to microvariations of pCO2 in the soil. Because the δ 18O ranges in the same samples are usually comparable to analytical uncertainties and because the δ 18O mean values follow an age-correlated pattern, they permit us to place the soils in one of three categories.In two cases where nodules were soft both turned out to be young (∼ 1000 years) and to have wide ranges in δ 18O values; the latter are apparently due to continual re-equilibration with new soil solutions. In three cases of the descending mode of nodule formation, 12,000–14,000 year-old hard nodules had very narrow δ 18O whose means reflect the rainfall value at that time. The period between 12,000–14,000 years B.P. is independently known to be one of higher precipitation/evaporation ratio and of more intensive soil formation. These ascending-mode populations of hard nodules had narrow ranges in δ 18O (reflecting a mixture of sea water and rainfall) and all turned out to be 3,000–4,000 years old. At this period the sea reached its highest elevation, resulting in raising the nearcoast water table.
Article
The Okavango Delta is a large, low gradient (ca. 1:3400), alluvial fan situated in a graben structure which is an extension of the East African Rift system. The catchment is situated in sub-tropical central Angola (rainfall ca. 1000 mm/a), and is underlain by Kalahari sand. Okavango River water therefore has a very low dissolved solid content (ca. 40 ppm), and a very low suspended load. The main clastic sediment load consists of eroded Kalahari sand, which is transported as bedload. Base flow in the river sustains about 4000 km of permanent swamps in the Panhandle and upper fan. The latter are flanked by seasonal swamps, which become inundated during advance of the seasonal flood wave. This flood wave takes four months to traverse the 250 km length of the fan. The extent of seasonal flooding is variable and can exceed 12 000 km. The fan is situated in a semiarid region (rainfall 513 mm/a), in which evapotranspiration exceeds rainfall by a factor of three. Only 15% of inflow plus rainfall leaves the fan as surface flow, the rest being lost to the atmosphere. All of the clastic sediment load and most of the solute load is deposited on the fan. In the Panhandle region, bedload deposition occurs on point bars in a meandering river system, and produces alluvial ridges. Periodic avulsions give rise to anastomosis of the channel. Sedimentation patterns on the fan at the lower end of the Panhandle are closely tied to ecosystem function. Channel margins in the permanent swamps consist of peat, stabilised by papyrus, and are permeable. Water leaks from the channels, causing channel bed aggradation. Loss of water and aggradation in primary channels leads to channel failure, and the destruction of the accumulated peat by fire. Leaked water flows along hippo trails in the backswamps, giving rise to secondary distributary channel systems. Constant change in primary channel location creates a mosaic of wetland habitats in different successional stages, from open water to near climax aquatic grassland habitats. Little clastic sediment reaches the seasonal swamps, and the main sedimentary process is by precipitation of solutes, primarily as a consequence of transpiration. Aquatic plants induce saturation in silica in interstitial water in the root zone. Trees, which grow exclusively on islands, are responsible for intense transpirational water loss which induces saturation in calcite, the precipitation of which results in vertical expansion and hence island growth. Groundwater beneath these islands eventually becomes saline, resulting in species zonation. Islands are initiated by termites, or result from topographic inversion associated with channel failure. The resulting undulating topography creates a variety of habitats which vary in their duration of flooding, and hence in their species composition.
Article
The Nylsvlei wetland is located on the 240 km floodplain of the lower Nyl River in the semi-arid Northern Province. The wetland is supplied seasonally with water and sediment by the Nyl and other small tributaries but has been little studied geomorphologically. The Nyl floodplain has formed within a basinal to synclinal structure on the downthrown side of a fault and the variable stratigraphy influences both ground and surface water flows, and the resultant fluvial landforms and ecology. Owing to downstream decreases in discharge and channel-bed gradient, the Nyl and several other tributaries presently decrease in size downstream and disappear at the margins of the floodplain, such that summer flooding occurs primarily as sheetflow. Thin clay layers deposited by these sheetflows effectively seal the floodplain surface, prolonging inundation and limiting groundwater recharge. Some groundwater recharge, however, occurs by floodwater infiltration through the channel beds of gravelly sand at the floodplain margins. Groundwater supply may contribute to growth of the numerous elevated, circular, floodplain islands which are characterised by woody fringes but sparsely-vegetated interiors. At the downstream end of the floodplain, bedrock outcrop induces convergence of the rare floodwaters and, together with a steepening of gradient, results in a well-defined channel again forming (Mogalakwena River). Correct identification of the factors giving rise to such wetlands, and the hydrogeomorphological processes governing their development, provides an essential context for long-term ecological studies as well as information for the design of effective management guidelines for these fragile habitats.
Article
A group of islands of varying size on the floodplain of the Okavango alluvial fan, were studied to establish the processes which lead to the initiation and growth of islands. It was found that islands are initiated by the mound-building activities of the termite Macrotermes michaelseni. These termites import fine grained materials to use as a mortar for the construction of epigeal mounds. Their activities create a topographic feature, raised above the level of seasonal flooding, and also change the physical properties and nutrient status of the mound soil. Shrubs and trees are able to colonize these mounds, which results in increased transpiration. As a result, precipitation of calcite and silica from the shallow ground water occurs preferentially beneath the mounds, resulting in vertical and especially lateral growth, causing island expansion. © 1998 John Wiley & Sons, Ltd.
Article
Channel blockage and abandonment in the Okavango Delta has been considered to be caused by either a combination of encroachment of Cyperus papyrus from the channel banks into the channels, or the development of papyrus debris blockages in the lower reaches of major distributary channels. This has been investigated in the present study by measuring rates of encroachment of papyrus from the banks into the channels, rates of debris production from the channel fringes, quantities of debris flowing along different channel sections, as well as the dimensions, colonization, decomposition and overall spatial dynamics of debris blockages along the lower reaches of the major distributary channel of the Okavango Delta. Results do not support suggestions that papyrus growth and/or debris production cause channel blockage and abandonment. First, encroachment is inversely related to current velocity within the channel fringe, and current velocities in the fringes of those distributary channels that are considered to be prone to blockage and abandonment are amongst the highest in the study area. Secondly, channel cross-sectional area is maintained by erosion of the channel bed beneath floating debris blockages. Thirdly, there is no evidence for the gradual upstream development of debris blockages in the lower reaches of major distributary channels. Data on hydrological aspects on the lower reach of a major distributary channel system suggest that sedimentation processes, leading to aggradation of the channel bed and to a decline in current velocity may be the cause of channel decline. This appears to be accompanied by encroachment of the channel from the margin by papyrus and by the development of more permanent blockages than were observed in the present study. This sequence of events (encroachment and blockage) is therefore considered to be a symptom and not the cause of channel decline and abandonment. On a considéré que l'obturation et l'abandon du canal dans le Delta de l'Okavango avaient été causés par soit l'envahissement des canaux à partir des berges par Cyperus papyrus, soit par de développement de barrages faits de débris de papyrus dans les parties basses des principaux canaux défluents. La présente étude a analysé cela en mesurant le taux d'envahissement des papyrus à partir des berges, le taux de production de débris des rives des canaux, les quantités de débris qui flottent le long de différentes sections de canal, ainsi que les dimensions, la colonisation, la décomposition et la dynamique spatiale générale des barrages de débris le long des parties basses du principal canal défluent du Delta de l'okavango. Les résultats ne corroborent pas les suggestions selon lesquelles la croissance et/ou la production de débris de papyrus causent l'obturation et l'abandon du canal. D'abord, l'envahissement est inversément proportionnel à la vitesse du courant au bord du canal, et la vitesse actuelle du courant au bord de ces canaux défluents que l'on considère être les plus enclins à s'obstruer et àêtre abandonnés est parmi les plus élevées dans toute la région de cette étude. Ensuite, la zone du canal qui traverse se maintient grâce à l'érosion du lit du canal sous le niveau des barrages de débris flottants. Troisièmement, il n'y a aucune preuve d'un développement progressif vers l'amont des barrages de débris dans les parties basses des principaux canaux défluents. Des données sur des aspects hydrologiques de la partie basse d'un des principaux canaux défluents suggèrent que le processus de sédimentation, conduisant à un rehaussement du lit du canal et à une diminution de la vitesse du courant, pourrait être à l'origine du déclin du canal. Ceci semble s'accompagner de l'envahissement du canal à partir du bord par les papyrus et par le développement de barrages plus permanents que la présente étude a constatés. Cette séquence des événements (envahissement puis blocage) est dès lors considérié comme un symptôme et non une cause du déclin et de l'abandon du canal.
Article
ABSTRACTA study of the avulsion of a major distributory channel on the alluvial fan (22 000 km2 in area) of the Okavango River in northern Botswana has revealed that channels serve as arterial systems distributing water which sustains large areas of permanent swamp. The channels are vegetatively confined. A primary channel, defined here as a channel which receives water and sediment directly from the fan apex, aggrades vertically as a result of bedload deposition. The rate of aggradation increases downchannel and may exceed 5 cm yr−1 in the distal reaches. Rapid aggradation is associated with a decline in flow velocity. This initiates a series of feedback mechanisms involving invasion of the channel by aquatic plants which trap floating plant debris, further reducing flow rate and causing the channel water surface to become elevated, thereby increasing rate of water loss from the channel, accelerating blockage and aggradation. The channel ultimately fails. Enhanced water loss from the channel promotes the growth of flanking swamp vegetation, which confines the failing channel. Increased flow through the swamp erodes pre-existing hippopotamus trails, producing a secondary channel system which overlaps but does not connect directly to the failing reach of the primary channel. The region of failure of the primary channel migrates upstream, accompanied by headward propagation of the secondary channel system. The swamp distal to the failed primary channel dessicates and is destroyed by peat fires. Secondary channels are stable and not prone to blockage. Comparison with avulsions described in other river systems indicates that the influence of plants in the Okavango River system is exceptionally strong.
Article
Vegetation on islands in the perennial swamps of the Okavango Delta exhibits a marked zonation pattern. Island fringes are generally characterized by a broadleaf evergreen riparian community of Syzigium cordatum, Ficus verruculosa, F. natalensis, F. sycamorus, Phoenix reclinata, Garcinia livingstonei and Diospyros mespiliformis. This gives way towards island interiors to a community dominated by Acacia nigrescens, Croton megalobotrys and Hyphaene ventricosa. The central regions are characterized either by short, sparse grassland dominated by Sporobolus spicatus or are completely devoid of vegetation with sodium carbonate (trona)-encrusted soil surrounding a central pan of extremely high conductivity. An indirect gradient analysis indicated the important determinants of vegetation distribution as being not only elevation, depth to groundwater and distance from the water source, but also as groundwater chemistry (conductivity and pH) and soil chemistry (sodium concentration). These studies were supported by a direct gradient analysis approach with species tolerances to depth to groundwater, groundwater conductivity and soil-surface sodium concentrations having been established. Feedback interactions of vegetation and groundwater elevation and chemistry, as well as soil chemistry, are evident and appear to be responsible for the vegetation dynamics in the study area.
Article
This study investigated the local-scale generation and movement of dust in the seasonal swamps of the Okavango Delta, Botswana, with a view to examining possible transfer of material between flood plains and islands. It was found that most of the dust load was carried in the lowest 3 m of the air column, and consisted mainly of amorphous silica, indicating that dust was generated largely on the flood plains. Dust loads were found to be highest above the flood plains and lowest over the interiors of islands, probably due to the baffling effect of the island trees on wind velocity. The contrast in dust loads between islands and flood plains suggests that there is a net transfer of dust from flood plains to islands, but it was not possible to quantify this transfer. It is evident, however, that flood plains experience net erosion and islands net aggradation. A strong seasonality in dust loads was observed, with the maximum dust loads coinciding with maximum wind velocity in October. This also coincides with peak seasonal flooding in the delta, and only non-inundated flood plains are capable of generating dust. Years of low flood therefore appear to be more dusty. There may also be transfer of material from higher-lying to lower-lying flood plains, which may reduce the topographic contrast on the flood plains. Copyright © 2004 John Wiley & Sons, Ltd.
Article
In the Okavango Delta 98–99% of the water from inflow and rainfall is lost to the atmosphere through evapotranspiration. As a consequence 94% of inflowing solutes are retained within the Delta landscape. This process might be expected to result in an entirely saline environment, but that is not the case: the surface waters have very low salinity, supporting a typical freshwater biota. It has been deduced that the numerous islands in the Delta (about 150,000 within an area of 13,500km2) have been formed through evapotransporative concentration in the groundwater, of infiltrating solutes, followed by precipitation and volume increase. Evidence of this is the large amount of calcrete in island soils. These islands of 3–10m thickness with clayey soils are underlain by fine Kalahari sand to a depth of 200–300m, which also indicates that they are formed through surface processes. The infiltration rate of surface water from floodplains and streams into islands is very high, and is predominantly a lateral process that is unidirectional. Evapotranspiration in the riparian woodland zone cause the ground-waters in the central area of islands—with halophyte grasslands—to have very high salinities. By use of chloride as a conservative element the concentration factor between central island groundwater and surface water is calculated to be 500–1,000. This groundwater is depleted of calcium and magnesium supporting the early deductions that these elements have precipitated as calcrete. There is also a large depletion of silicate and potassium that probably have precipitated as well forming the clayey soils typical of the islands. The central island groundwater is dominated by sodium, bicarbonate and dissolved organic matter. The gradual increase of salinity here causes a periodic let off of this water through a density-driven process to deeper layers. This process together with island growth through precipitation of solutes are the two major sink processes of inflowing solutes and explains why the Okavango Delta is at present a freshwater system. The whole island complex is calculated to be 100,000–400,000years old while some intensely studied islands may be younger: 80,000–240,000years. The discrepancy is explained by a biassed selection of islands currently in flooded areas with better growth conditions. The uniqueness of the Okavango Delta and ideas for future research are discussed.
Article
The Okavango Delta of semi-arid northern Botswana is a large alluvial fan (22,000 km2) covered by permanent and seasonal swamps from which 96% of the annual discharge is lost by evapotranspiration. Many small islands (∼1ha) within the permanent swamps are the sites of accumulation of sodium carbonate salts and many contain saline pans. The associated alkaline soils are toxic to vegetation. An understanding of the processes involved in alkalinization could be of potential benefit to long-term conservation planning in this unique ecosystem. The relation between soil chemistry and mineralogy, and swamp and groundwater chemistry were investigated on an island in the swamps. The study revealed that the water table beneath the island is depressed and swamp water enters the groundwater regime of the island from the margins and below, and flows toward the centre. The water becomes progressively more saline, initially owing to transpiration by trees and ultimately by evaporation in the central parts of the island. As a result of increasing salinity, amorphous SiO2 and magnesium calcite precipitate in the soils beneath the marginal zone of the island, raising the land surface, while the more soluble alkali carbonates are concentrated in the centre of the island as surface crusts and brine ponds. Leaching of these salts into the soil during the rainy season and gravity-driven flow of saline brines in the dry season causes the downward movement of Al and Fe in the central zone of the island. K-feldspar and possibly amorphous allophane develop in the deeper soils under the central zone of the island.
Article
The Okavango Delta of northern Botswana is a large (40,000 km2) alluvial fan located at the terminus of the Okavango River. The river discharges about 10 km3 of water onto the fan each year, augmented by about 6 km3 of rainfall, which sustains about 2500 km2 of permanent wetland and up to 8000 km2 of seasonal wetland. Interaction between this surface water and the groundwater strongly influences the structure and function of the wetland ecosystem. The climate is semi-arid, and only 2% of the water leaves as surface flow and probably very little as groundwater flow. The bulk of the water is lost to the atmosphere. The Okavango River also delivers about 170,000 tonnes of bedload sediment and about 360,000 tonnes of solutes to the Delta each year, most of which are deposited on the fan. Bedload is deposited in the proximal, permanent wetland, whilst much of the solute load is deposited in the seasonal wetland. Notwithstanding the high evapotranspirational loss, saline surface water is rare. Between 80 and 90% of the seasonal flood water infiltrates the ground, recharging the groundwater beneath the flood plains and the many islands on the flood plains. The remainder is lost by evaporation. This groundwater reservoir is transpired into the atmosphere by both aquatic vegetation on the flood plains and terrestrial vegetation on the islands, and the water table is steadily lowered following passage of the seasonal flood. Trees, which are almost exclusively confined to islands, are particularly important, as they lower the water table beneath islands relative to the surrounding wetlands. There is therefore a net flow of groundwater towards islands. Accumulation of dissolved salts in this groundwater leads to precipitation of solutes (mainly of silica and calcite) in the soils beneath island fringes and the islands grow by vertical expansion. Islands are thus an expression of the chemical sedimentation taking place on the fan. Sodium bicarbonate accumulates in the groundwater beneath island centres, and this impacts on the vegetation, leading ultimately to barren island interiors. Dense saline brine thus produced subsides under density-driven flow. This cycling of seasonal flood water through the groundwater reservoir thus plays a key role in creating and maintaining the biological and habitat diversity of the wetland, and inhibits the formation of saline surface water.
Article
Shallow saline groundwater has a patchy occurrence in superficial postglacial marine sediments beneath coastal meadows and salt marshes on the southern shores of the Island of Læsø, Denmark. The postglacial sequence consists of 1–2 m of marine sands and gravels resting on a slightly undulating platform of practically impermeable interglacial marine clay, which makes up a confining layer or aquiclude. The saline groundwater primarily occurs beneath the vegetated fringe and associated minor unvegetated sand pans of coastal meadows and salt marshes. The salinity varies from 2% to approximately 17% and analyses of major ions indicate a prevailing marine composition. The saline groundwater is supersaturated with respect to carbonates in the entire salinity range and slightly supersaturated with respect to gypsum with salinities above 10%. However, aragonite is the only precipitate recorded so far. Stable isotope analyses (2H and 18O) plot fairly well along the local groundwater–seawater mixing line indicating mixing of seawater with fresh meteoric groundwater in the proportion 0–50%. In contrast, the stable isotopic compositions show no correlation to the wide range of salinities recorded. It is proposed that the thin cover of superficial deposits resting on a clay platform results in a relatively closed hydrochemical system in which increasing concentration of dissolved solids in the interstitial water is caused by transpiration which is known to abstract water from groundwater without leading to isotopic fractionation. Periodical precipitation/dissolution of salt in topsoils of sand pans and from salt secreting halophytes may also be contributory causes of increasing salinity in the groundwater. The spatial distribution of the saline groundwater most likely depends on the microtopography of the slightly undulating clay platform. The dense, saline shallow groundwater, emerging from the vegetated fringe and sand pans and flowing toward the depressions in the clay platform, percolates downward displacing lower salinity seawater and groundwater.
Article
Interest in palustrine carbonates and calcretes has increased over the last 20 years since they contain significant environmental information. Much of the work performed in this area has focused on either of two types of terrestrial carbonate—palustrine carbonates or calcretes (pedogenic and groundwater)—yet their simultaneous study shows there may be a gradual transition from one form to the other, revealing the interplay between pedogenic, sedimentary, and diagenetic processes. Three main factors control the formation of these carbonates: the position of the water table, the host rock, and the period of sub-aerial exposure. In pedogenic calcretes, precipitation of carbonate takes places mostly in the vadose zone above the water table, and within a previous host rock or sediment. In groundwater calcretes, the precipitation of carbonate also occurs within a previous host rock and around the groundwater table. In palustrine carbonates, however, the precipitation of lime mud occurs in a lacustrine water body. Palustrine carbonates necessarily form on previous lacustrine mud, whereas both types of calcretes may form on any type of sediment or soil. The sub-aerial exposure time needed to form palustrine carbonates may by relatively short (even a season), whereas pedogenic calcretes need more time (several years to millions of years). Groundwater calcretes do not form on the topographic surfaces, so there is no need of sub-aerial exposure. However, stable surfaces favour the development of thick groundwater calcretes. Small fluctuations in the water table cause gradual transitions of these three types of terrestrial carbonates and the subsequent mixture of their characteristic features, causing difficulties in the interpretation of these carbonates.
Article
This paper studies the interactions of density driven flow and geochemical reactions under evapo-concentration. A numerical model that couples the relevant flow, transport and chemical processes was used to analyze density-driven flow on islands in the Okavango Delta, Botswana. Evapo-concentration on the islands leads to steadily increasing solute concentrations until the onset of density-driven flow against the evaporation-induced upward flow. The modelling results suggest that lag times to the onset of density-driven flow are strongly influenced by geochemical reactions. Mineral precipitation and carbon dioxide de-gassing increase the lag time. Carbon dioxide de-gassing is enhanced if humic substances are present at elevated concentrations. Simulation results were shown to be most sensitive to the longitudinal dispersivity. Modelling results are compared to field observations from three islands in the Okavango Delta. A semi-quantitative correspondence between modelled and observed concentration patterns is established for the major chemical constituents.
Article
Islands constitute an important geomorphological component of the permanent swamps of the Okavango alluvial fan. Studies of the topography, soil chemistry, groundwater chemistry and vegetation cover across several islands indicate that many islands form as a result of the subsurface precipitation of calcite and probably amorphous silica, which produces vertical expansion, creating topographic relief. The main agent responsible for this precipitation is transpiration, especially by large, deep-rooted trees, which increases the salinity of the groundwater, leading to saturation in calcite and silica. The process ultimately leads to the development of highly saline groundwater and results in marked zonation of vegetation across the islands. Numerical modelling of the process suggests that salinization of the groundwater beneath an island takes between 100 and 200 yr. High calcite contents in island soils appear to be the product of several cycles of salinization, which are interspersed with periods of swamp abandonment during which the more soluble alkali salts are leached from the islands. Island formation represents an important process in the overall aggradation of the alluvial fan.
Article
The oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water. The carbon isotopic cycle for CO2 in soils can be described in terms of the proportion of biomass using the C4 photosynthetic pathway and the CO2 respiration rate of the soil; at low soil respiration rates significant atmospheric CO2 mixing can occur. In general, the carbon isotopic composition of soil carbonate is related to the proportion of C4 biomass present in soil, but soils that freeze to the depth of carbonate formation often have a significant atmospheric component. This suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation. Because of these relationships, the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred.
Article
Evapotranspiration exceeds rainfall by a factor of three in the Okavango swamps of northern Botswana, yet saline surface water is rare. Brines develop in the groundwater beneath islands, however, and very strong lateral concentration gradients develop. These arise as a result of transpiration by trees which grow around the fringes of islands as well as by capillary evaporation of groundwater from the interior of islands. Precipitation of calcite and amorphous silica from the groundwater occurs beneath island fringes. Long-term monitoring of the water table and groundwater chemistry beneath an island in the seasonal swamps has revealed that groundwater rises and falls with the seasonal flood and that the saline groundwater remains centred beneath the island. The study shows that islands act as sinks for dissolved solids during intense evapotranspirational loss from the swamps.
Article
For accurate X-ray spectrographic analysis of geological samples of widely varying composition, a fusion with lithium borate containing lanthanum oxide is used to make a suitable glass disc. Mg, Al, Si, P, K, Ca, Ti, Mn and Fe are determined on this disc, using similar discs made with pure chemicals for calibration. “Nominal percentage” composition of the sample is obtained as a direct reading from the scaler, counting time adjusted to give 104 or 103 counts for 1 per cent. For normal silicates this value will be within ±3 per cent (relative) of the correct value. For more variable samples and for greater accuracy with silicates, matrix corrections are made using the “nominal percentage” in a first order equation. The matrix correction coefficients have been determined experimentally and are shown to agree well with theoretical values. They are presented in a form suitable for use in other laboratories. For Na, a separate pressed powder sample is prepared and matrix corrections are not necessary.The precision of the method has been studied and is largely controlled by counting errors. An estimate of the accuracy has been made by comparison of X-ray results with two sets of published data on five “standard” rock samples; observed differences could all be explained as due to precision errors in the X-ray and chemical results. The coefficient of variation between the X-ray and chemical results was less than 1 per cent where the concentration of an element, expressed as an oxide, was greater than 1 per cent.Results are given to show that when matrix corrections are applied, samples with extreme compositional variability, e.g. rocks, minerals or ores, may be analysed satisfactorily.With the borate discs, background determinations are not made on each sample so that determinations involve reading the Kα line only. Rapid analyses are therefore possible; one operator can prepare and analyse more than 12 samples per day for 10 elements.
Article
A topographic map of the Okavango Delta and environs has been constructed using a combination of elevation data including trigonometric beacons and spot heights from the government of Botswana, surveys of the navigable channels, U. S. Department of Defense data and measurements made during a geophysical survey of the region. The topography provides insight into the local tectonic and sedimentary history. Local tectonics are dominated by uplift and horst formation associated with the Ghanzi Ridge, and an arch to the north of the Panhandle, which appear to represent the tips of incipient rifts which are propagating from the northeast. The Delta has formed in the resulting depression between these arches. The Panhandle has developed along a fault, and may be largely an erosional feature incised into the northern uplift zone. The Delta itself is an alluvial fan of remarkably uniform gradient. There is no evidence of regional tilting of the fan surface. Local highs and lows are developed on the fan, but channel location is relatively insensitive to this local topography. Moreover, marked elevation differences exist between adjacent channels, creating hydrologically unstable conditions. These unusual features of the local hydrology arise because of the confining effect of channel-flanking vegetation. Sedimentation in the Delta appears to be causing crustal sagging of the central Delta, which has: tilted the major palaeo-shoreline of the Mababe Depression to the west; formed a local depression within the Ghanzi Ridge facing the Delta; and detached a sliver of the ridge along the Thamalakane fault. It is suggested that local seismicity also results mainly from sediment loading. The Selinda spillway occupies a marked local depression, which is a graben between the Gumare fault and an extension of the Linyanti fault. It is probable that southwesterly propagation of the uplift zone associated with the incipient rift will ultimately deflect the Okavango River into the Chobe-Zambezi river system via this graben.
Article
The Okavango wetland in northern Botswana is one of the world's largest inland deltas. The delta is a dynamic environment with shifting channel routes, causing growth and decay of flanking wetlands, and giving birth to islands. Primary island nuclei are formed by fluvial processes and bioengineering, and subsequently grow into secondary larger islands of irregular shape by clastic and chemical sedimentation, and later by coalescence. This article presents classifications and quantitative estimations of channels, wetlands and islands of the Okavango Delta. Islands were classified dependent on composition, pattern of composition, shape and juxtaposition. 90 per cent of all islands in the entire wetland were identified, with a classification accuracy of 60 to 85 per cent. Smaller islands of the nucleus types dominate the upper parts of the delta, whereas larger secondary islands are more common in the distal part, a reflection of the age of the islands. Islands in the entry valley of the delta, the Panhandle, are larger in the top end - the primary region of recent elastic sedimentation. The overall size distribution of islands in the delta, however, shows no clumps, indicating that island growth is a uniform process over time and space. The total area flooded at least every decade is approximately 14 000 km(2), of which 9000 km(2) is classified as actual wetland. Channel meandering decreases from the Panhandle to the distal part of the delta, with the abandoned Thaoge channel as an exception. Occurrence of fluvially formed islands in the distal delta indicates that the water flow and area of inundation must once have been much larger. Copyright (C) 2004 John Wiley Sons, Ltd.