A soil stratigraphic unit provides evidence for Late Quaternary climate change in the Chobe Enclave, northern Botswana

A quarry in the Chobe Enclave, northern Botswana, exposes a prominent pedostratigraphic unit of approximately 8 metres consisting of two pedostratigraphic levels (PL), older PL1 and younger PL2. In a region where proxies for palaeoenvironmental reconstruction are terribly lacking, this study assessed the potentials of paleosols (fossil soils) to answer fundamental questions about weathering, pedogenic intensity and environmental change using macromorphology, geochemistry and clay mineralogy. SiO 2 is the dominant major oxide (40.6- 98.9 wt. %) followed by CaO (0.02-29.6 wt. %), Fe 2 O 3 (0.48- 2.64 wt. %), MgO (0.14 – 1.81 wt. %) and Al 2 O 3 (0.29 – 0.93 wt. %). The clay-sized minerals present in the paleosols are sepiolite, quartz, calcite and kaolinite. The carbonates had strong positive correlation with Sr (R 2 = 0.935), while Fe 2 O 3 had weak positive correlation with TiO 2 (R 2 = 0.0187). PL1 developed on materials of uvial origin and is genetically immature with A/C horizons. In comparison with modern soil, PL1 would qualify as Entisols (Fluvent or Fluvaquent in Soil Taxonomy). PL2 is comparable to modern Calsisol and formed from protracted period of post burial dissolution, downward translocation and recrystallization of the palustrine carbonates to form pedogenic carbonates. Calcication, incipient ferralization and leaching of soluble salts are the dominant pedogenic processes in the pedostratigraphic section. Weathering and pedogenesis were generally incipient in the unit. Late Quaternary hydrological dynamics in the Chobe Enclave was the major driver for the formation of the soil stratigraphic unit. This study demonstrates the applicability of paleosols for reconstructing and discerning palaeoenvironmental and climate dynamics and gives the possibility of connecting pedosedimentary processes in the area to regional palaeoenvironmental This study focused on geochemical indicators of weathering and pedogenesis in a prominent pedostratigraphic unit from Chobe Enclave, NW Botswana. At the depth of about 8 meters, a prominent soil stratigraphic unit shows two distinct pedostratigraphic levels consisting of a weakly developed soil-sediment beds and an overlying paleosol carbonate (PL1 and PL2). In comparison with modern soils, PL1 would qualify as Entisols (Fluvent or Fluvaquent in USDA Soil Taxonomy). This implies that going deep in time, the intermittent changes in the hydrological regimes of Chobe Enclave has been a strong driver of soil formation and landscape evolution. From the geochemical indicators, the study area has experienced only incipient chemical weathering and pedogenesis. Calcication, leaching of soluble cations and downward migration of iron oxides are the operating pedogenic processes established from this study. It also implies that that prior to the dry period (MIS 6 to MIS1) under which carbonate paleosols (carbonate island) formed, Chobe Enclave witnessed period of geomorphic stability and wet conditions. Post burial dissolution, translocation and recrystallization of palustrine carbonates commonly found in the Chobe Enclave has culminated in the formation of pedogenic carbonates as found in the unit. From the viewpoint of palaeopedology, this study provides evidence of late Quaternary environmental and climate change in the Chobe Enclave. To improve this study, a high resolution dating of the genetic horizons of PL1 is recommended as the available geochronology of the area only accounted for PL2.


Introduction
Evidence of global and regional climate oscillation during the late Pleistocene-Holocene epochs is well documented (Konecky et al., 2011;Timmermann and Friedrich, 2016). In the semi-arid areas of southern Africa, it is marked by increased frequency of extreme climatic events and hydrological responses (Kusangaya et al., 2014). The Kalahari Basin covers a large expanse of lowland area in Botswana and extends into Namibia, South Africa, Angola, Zambia, and Zimbabwe (Fig. 1). Being a huge basin of internal drainage into which aeolian, uvial and palustrine sediments have accumulated since the late Cretaceous (Grove, 1969), the middle Kalahari has been a hotspot for paleoenvironmental reconstructions mostly through geological, geomorphological, and lately ecological archives (e.g. Burrough et al., 2007;Cordova et al., 2017). Multiple proxy evidence has con rmed a varying Quaternary history for the middle Kalahari, but climate dynamics in this area is still poorly understood even after ve decades of paleoenvironmental research (Burrough et al., 2009).
Chobe Enclave in the middle Kalahari Basin of northern Botswana has been in uenced by the Pleistocene-Holocene climate variability (Burrough and Thomas, 2009). The present day Chobe Enclave is under the in uence of: (i) the Congo Air Boundary (CAB) and Intertropical Convergence Zone (ITCZ), which are major tropical air mass convergence systems; (ii) three main river systems of the Okavango, Kwando-Linyanti, and the Zambezi-Chobe which bring waters from the equatorial Angolan highlands, and (iii) tectonics, given that the local fault system is an extension of the East African Rift Zone (Moore et al., 2012). Therefore, Chobe Enclave owes it geomorphic properties to the complex Quaternary palaeohydrological dynamics in uenced by both climate and/or tectonic changes. Most paleoenvironmental reconstruction studies of the middle Kalahari have been focused on the Mababe sub-Basin (Fig. 1). This study is the rst to focus on the potential of alluvial paleosols (fossil soils) from Chobe Enclave to contribute to our knowledge of the paleoenvironments of the middle Kalahari.
Climate parameters, the most active of which are precipitation and temperature, exert strong in uence on the nature and properties of soils (Jenny, 1994). Sitting at the interface between the atmosphere, lithosphere, hydrosphere and biosphere, soils and paleosols have the potential to archive the prevalent environmental and climatic conditions under which they formed (Beverly et al., 2018). Globally, paleosols have been successfully used for regional and site-speci c palaeoenvironmental reconstruction. Proxies that include particle sizes distribution, macromorphology, geochemistry and clay mineralogy of paleosols and sediments are strong indicators of a combination of pedogenesis, provenance, weathering intensity, hydraulic sorting, abrasion, redox condition and diagenesis ( It has been shown that Chobe Enclave has more soil diversity than earlier reported (Romanens et al., 2019). A rare exposure of a deep sand quarry in Chobe Enclave reveals a soil stratigraphic (pedostratigraphic) unit consisting of two levels: a thick unit of paleosol carbonate overlying a genetically immature soil pro le developed on alluvial sediments. In a geochronological study, Diaz et al. (2019) reported different phases of active deposition and landscape stability spanning MIS6 to MIS1 during which carbonate islands were formed in the Chobe Enclave. Using geochemical indicators of weathering and pedogenesis, we seek to understand the environmental and climate dynamics, and the pedosedimentary processes at a deep pedostratigraphic section in the Chobe Enclave. In consequence, the objectives of this study are: (i) to characterise the paleosol carbonate of the pedostratigraphic unit using their macromorphological, physico-chemical, geochemical, and clay mineralogical properties; and (ii) to assess weathering and pedogenesis in the unit; and (iii) to reconstruct the environmental conditions of the area. The ndings will help us to establish the relationship between pedogenic and geomorphic processes during periods of stability and instability in a semi-arid climate. Trace element compositions are particularly useful as these elements generally remain immobile during the processes of weathering, transport and deposition.  (Fig. 1). The basin is characterised by an extensive body of sand of approximately 2.5 × 10 6 km 2 and this is the parent material on which soils in the area have developed (Jones, 1980). The wet season in area lasts from November to March and is characterised by heavy thunderstorms brought by uctuations of the Intertropical Convergence Zone (ITCZ). Mean annual rainfall is about 650 mm, making it one of the wettest areas in Botswana (Jones, 2002). The dry season is from April to October and has high temperatures ranging from 30 to 40 o C in the day. The foremost vegetation in the area includes Colophospermum mopane, Acacia sp., Terminalia sericea or Philenoptera nelsii, with riparian forests along the Linyanti River and isolated dry oodplains and wetlands in the northeast (Romanens et al., 2019).

Field investigation
Reconnaissance eld surveys resulted in the identi cation of a pedostratigraphic unit exposed at a sand Quarry site in Chobe Enclave. Macromorphological properties of the soils and paleosols, as well as features related to erosion and sedimentation processes such as soil horizon thickness, type and topography of boundaries and coatings; and all pro le designations were done in accordance with the FAO/WRB Guideline for soil pro le description (FAO, 2006). Samples were collected from each genetic horizon of the two pedostratigraphic levels (PLs), ve samples from PL1 and four from PL2 (Fig. 2).

Laboratory analysis
Pre-treatment of samples involved air drying at room temperature, removal of large roots and gravel and passing through 2 mm sieve. Particle size distributions of the samples (after the removal of carbonates with dilute HCl) were performed on laser granulometry using Malvern Mastersizer 3000 (Worcestershire, UK) connected to a Hydro MV, wet dispersion unit (Malvern, Worcestershire, UK) and the data were analysed using the Mastersizer 3000, v1.0.1 software. Soil reaction (pH) and electrical conductivity (EC) were measured in a 1:2 soil to solution ratio and the values read off using pH meter and EC glass electrodes respectively. The carbonate content was determined gasometrically by measuring the volume of carbon dioxide evolved during reaction with hydrochloric acid using an Eijkelkamp calcimeter.
The analysis of the total elemental composition of the samples was carried out in the Minerals-Geochem laboratory of SGS, Randfontein, South Africa. The suite of major rock-forming oxides (reported in %) was determined by the Borate Fusion Method. A sample weighing 0.7 g was mixed with lithium tetraborate ux and fused to form a homogenous glass bead. The borate fusion was followed by analysis of the glass bead on the WDXRF Spectrometer. The loss on ignition (LOI) at 1000 o C was determined separately by gravimetry. The minor and trace elements were determined by sodium peroxide fusion method.
Sample (0.2-2.0 g) was weighed into a crucible containing NaO 2 and NaOH. The mixture was then fused.
The sample was leached, acidi ed and made up to volume. The sample was then analysed by ICP-OES.
To assess elemental redistribution through weathering and pedogenesis, a fundamental approach which entails normalization of all chemical elements of interest by an immobile element (in this study, Ti) as proposed by Brimhall and Dietrich (1987) was applied. Titanium and Zr are good indicators of detrital minerals (Kebonye and Eze, 2019). Under post-depositional palustrine conditions, Ti and Zr are chemically immobile (Winchester and Floyd 1977) and have the potential to provide a measure of abundance of some heavy minerals such as Ti oxides before weathering. Changes in provenance are detectable from Zr/Ti ratios (Reynolds et al. 2001;Reynolds et al., 2004). The rationale behind the use of Zr/Ti ratio is that the ratio varies across rock types, e.g., from ma c (e.g., basalt) to more silicic rocks

Pedostratigraphy
In accordance with the FAO/WRB guideline of soil pro le description, the section quali ed as "Bkkm1: Bkkm2: Bkkm3: Bkkm4: ACb: Cb1: Cb2: Cb3: Cb4". The two bottom horizons (Cb3 and Cb4) were wet as a result of underground water capillarity. There are variations in the thickness and morphology of the pedostratigraphic levels at Chobe Enclave: an older genetically immature buried alluvial paleosol (PL1) overlain by a strongly cemented paleosol carbonate (PL2) (Fig. 2). A total of nine distinct genetic horizons were identi ed after careful observation and delineation of the approximately 8 m deep pedostratigraphic unit: ve horizons in PL1 and four in PL2 (Fig. 2). The horizon boundary between PL1 and PL2 is abrupt.

Macromorphology and physico-chemical descriptions
Some striking variations in morphological, physical and chemical properties typical of depositional environments were observed PL1 and PL2 (Fig. 2). Similar to the horizon depths, the colours of the soils varied remarkably across the PLs. The colours are brown on the surface horizon and faded to light brownish gray in the deepest horizons (Fig. 2). While PL1 had predominantly weak granular to single grained structure, the paleosol carbonates (PL2) were structureless (massive). Cementation by carbonates was principally responsible for the development of massive structure of PL2. There were few ne roots in the uppermost horizons of both PL1 (ACb and Ab2) and PL2 (Bkkm1 and Bkkm2) which gradually vanished as one moved down the respective PLs. All horizons of PL2 reacted vigorously to dilute hydrochloric acid indicating the presence of carbonates.
Particle size distribution could not be determined for PL2 due to its cemented massive nature making disaggregation of particle impossible. Fine sand and silt dominate the particle size of PL1 (Table 1). The pH of the soils and sediments (C horizons) ranged from slight to moderately alkaline (7.8-9.7). The electrical conductivity values of PL1 were relatively on the low side ranging from 0 to 80 µS/cm. (Table   1). The calcium carbonate contents of the paleosol carbonates (PL2) were high (23.1-43 g kg −1 ) and nonexistent in PL1 except in ACb possibly due to leaching. The presence of clasts (see arrows in Fig. 3) is indicative of pedogenesis resulting in pedogenic palaeosol carbonate in PL2.

Geochemistry
Major elemental oxide composition results presented in Table 2 show SiO 2 to be the dominant major oxide (40.6-98.9 wt. %) followed by CaO (0.02-29.6 wt. %). Other major oxides like  striking spike in Sr, Ni and S were recorded in PL2 while the rest of the trace elements showed no clear trend in their variations. A couple of geochemical ratios used to evaluate weathering (WIP, DI and CIA) and provenance (Zr/Ti) showed more variations across PL1 than PL2 which had all the indices relatively more uniformly distributed (Fig. 4). Calcium had a remarkably strong positive correlation with Sr (Fig. 5a), while Fe 2 O 3 had a weak positive correlation with TiO 2 (Fig. 5b).

Clay mineralogy
The mineralogy of the clay-sized particles of the paleosols is shown in Table 4. The minerals present include sepiolite, quartz, calcite and kaolinite. While PL1 had a predominance of quartz and calcite only in Ab1, PL2 had abundance of calcite, quartz and sepiolite.

Sedimentation and pedogenesis in the Chobe Enclave from the macromorphological perspective
The studied soil stratigraphic unit in the Chobe Enclave quali es as a truncated compound paleosol in an alluvial depositional environment. Although the surface mineral horizon (A) developed on Kalahari sands has been removed by erosion leaving an exposure of hardly cemented B horizon outcrop on the surface (Fig. 2) at the studied unit, the surrounding areas still have their A horizon intact. Carbonate accumulation in the soils of arid and semiarid environments form the basis for their classi cation as Calsisols in modern soil (e.g. Romanens et al., 2019). However, for cemented horizons of carbonates that are beyond 5 meters in depth should not be seen as part of a modern solum. As obtains in PL2, such massive carbonate horizons rather is an indication of pedogenesis that spanned over hundreds of thousands of years and should be described as relict paleosols (Soil Survey Staff, 2017). There is evidence of pedogenesis in the obliterated structure of the initial palustrine rock from which the palaeosol carbonate developed including illuvial accumulation of carbonates, clasts immersed in the matrix (Fig. 3), brittleness, and very powdery stains of carbonates on the ngers. PL1 on the other hand is a weakly developed buried pedosediments of alluvial sediments origin. According to Romanens et al. (2019), soil formation in the Chobe enclave is strongly driven by complex interactions of vectors including alluvial depositional processes, paleoenvironmental factors such as old alluvial deposits, ancient wind-blown sand deposits, and hydrological dynamics. ).
The horizon boundary between PL1 and PL2 is abrupt. The abrupt boundary between PL1 and PL2 could be explained by divergence in soil development pathways. A critical observation of the soil stratigraphic unit shows that events principally connected with hydrological changes in the region triggered the formation of two morphologically distinct pedostratigraphic levels (PL1 and PL2) as shown from the distinct boundary. Environmental and climate conditions which ensured the geomorphic stability of the landscape at the time PL1 formed was followed by a prolonged period of carbonate-rich sediment deposition, dissolution and translocation of carbonates and hiatus of dry period that led to the hardening of the paleosol carbonate (PL2). Palustrine carbonates deposited between MIS6 to MIS1 dots Chobe Enclave as carbonate islands (Diaz et. al., 2019). The horizonation of PL2 together with the abundant presence of clasts (Fig. 3) provides evidence to support pedogenesis through secondary recrystallization of carbonate out of solution which if often triggered by loss water or dryness.
Granulometric results of PL1 (Table 2) con rms that the parent materials are of alluvial origin and were deposited under low energy given the dominantly ne nature of the particle sizes, well sorted, and leptokurtic to very leptokurtic kurtosis (statistically means that more particle sizes in the distribution tails and more particle sizes close to the mean (i.e. particle size distribution of the samples sharply peaked with heavy tails) with weak textural maturity. These attributes are typical of low energy uvial or tidal sediment deposits (Blott and Pye, 2001).

Geochemical indication of provenance, weathering and pedogenesis
The parent material (alluvial sediments) in the Chobe Enclave consist of quartz, rock fragments (conglomerates and sandstones) and ferromagnesian minerals from the network of rivers south of the Okavango Alluvial fan (Diaz et  A major pedogenic process in the Chobe Enclave includes chemical weathering of the detrital minerals in the alluvial sediments and mobilisation, loss or precipitation of relatively soluble minerals. Geochemical indicators of carbonate precipitation, Ca/Ti, and iron oxide redistribution, Fe/Ti show that calci cation and oxidation of iron oxide were the dominant pedogenic processes that shaped the diagnostic horizons of PL2 and PL1 respectively. As expected, the dominantly single grained structure of PL1 enhanced higher rate of soluble salt (e.g. Na) leaching than PL2. The strong Sr and carbonates correlation (R 2 = 0.935) at Chobe Enclave (Fig. 5a) implies absence of Sr dissolution and mobility. As obtains elsewhere, this is due to the high pH and CO 2 concentration in PL2. On the other hand, the weak positive correlation of Fe 2 O 3 versus TiO plot points to weak dissolution and mobility of Fe in the PLs (Reynolds et al., 2004) and of Sr tend to precipitate as authigenic carbonate under high soil pH (Kabata-Pendias 2001).
Calci cation, a pedogenic processes observed in PL2 are moderate-term pedogenic features and is known to happen in 10 3 years (Targulian and Krasilnikov, 2007).
The stagnic nature of Cb3 and Cb4 horizons points to frequent groundwater table rising through capillarity in the ne grained sediments (Cb3 and Cb4). Since the C horizons of the stratigraphic unit did not react with dilute HCl, it is most likely that their high pH could be attributed to alkaline underground water..

Palaeoenvironmental inferences
The available contain non-pedogenic (palustrine carbonate) as reported by (Diaz et al., 2019). Our study shows that pedogenic carbonates are present in PL2. According to Soil Survey Staff (2017), carbonates that precipitated in place from soil solution and translocated within the soil pro le is considered pedogenic. Since PL2 developed on alluvial quartz rich clastic materials which are non-calcareous parent material, the rate of carbonate accumulation would depend on the rate of at which the CaCO 3 originally present in palustrine carbonate can be dissolved and translocated via leaching waters in a soil pro le, and inputs from dust and/or rainfall (Gocke et al., 2012).
Based on the law of superimposition, PL1 is clearly older than PL2 and is comparable in attributes to present day Entisols. They correlate as uvents in USDA Soil Taxonomy. PL1 has buried weakly developed surface horizons and formed on palustrine deposits. The horizons of PL1 are genetically immature which is also supported by the DI, WIP and CIA results (Fig. 4). Brownish colouration by downward translocation iron oxide formed the basis for A/C horizon differentiation. Imprints of hydrological activities are evident in the stagnic Cb3 and Cb4 horizons at the depth of about 8 meters. Entisols occur over a wide range of climates across the globe. Entisols were among the modern soils delineated in Chobe Enclave by Romanens et al (2019).
The mobility and redistribution of major and trace elements within the soil environments are often in uenced by the prevailing environmental and climate conditions affecting weathering .The higher Sr values observed in PL2 implies dryness and precipitation of calcium and magnesium carbonates under arid climate (Küҫükuysal and Kapur, 2014), whereas higher values signify more exposure of PL1 to moisture. In a regional context, the morphological and pedostratigraphy of the PL2 depict long term patterns in climate and/or basin settling prompted by strong regional palaeohydrology that gave rise to the Mababe Depression, Makgakgadi Pan and Lake Ngami of the middle Kalahari Basin (Burrough et al., 2009). The differences in geochemical weathering and pedogenic ratios between the paleosol resulted from alternating wet and dry climate cycle in the Chobe Enclave. The development and maturity of PL1 was disturbed by a period of geomorphic instability that produced PL2 (Fig. 2). Periods of geomorphic instability has been reported to precede the development of duplex soils (Eze and Meadows, 2014a). PL1 of the Chobe Enclave are therefore exposed to more humid environments than today. The uppermost horizons of PL2 have been lost and this indicates strong erosion in the Enclave.
The presence of sepiolite in PL2 indicates alkaline soil conditions and high aluminium activity in solution in arid to semi-arid paleoclimate. The dry palaeoenvironmental condition of PL2 is further supported by the calcite, a carbonate mineral which forms from direct precipitation of calcium-rich solution and grows bigger as the waters dry out (Eze and Meadows, 2014b; Tabor and Myers, 2015). The clay-sized minerals are possibly of allogenic (detrital) origin (with higher chances of the little amount of clays present being transported into Chobe Enclave from surrounding catchments.

Conclusions
This study focused on geochemical indicators of weathering and pedogenesis in a prominent pedostratigraphic unit from Chobe Enclave, NW Botswana. At the depth of about 8 meters, a prominent soil stratigraphic unit shows two distinct pedostratigraphic levels consisting of a weakly developed soilsediment beds and an overlying paleosol carbonate (PL1 and PL2). In comparison with modern soils, PL1 would qualify as Entisols (Fluvent or Fluvaquent in USDA Soil Taxonomy). This implies that going deep in time, the intermittent changes in the hydrological regimes of Chobe Enclave has been a strong driver of soil formation and landscape evolution. From the geochemical indicators, the study area has experienced only incipient chemical weathering and pedogenesis. Calci cation, leaching of soluble cations and downward migration of iron oxides are the operating pedogenic processes established from this study. It also implies that that prior to the dry period (MIS 6 to MIS1) under which carbonate paleosols (carbonate island) formed, Chobe Enclave witnessed period of geomorphic stability and wet conditions. Post burial dissolution, translocation and recrystallization of palustrine carbonates commonly found in the Chobe Enclave has culminated in the formation of pedogenic carbonates as found in the unit. From the viewpoint of palaeopedology, this study provides evidence of late Quaternary environmental and climate change in the Chobe Enclave. To improve this study, a high resolution dating of the genetic horizons of PL1 is recommended as the available geochronology of the area only accounted for PL2.

Declarations
Con ict of interest: 37. Winchester JA, Floyd PA (1976) Geochemical magma type discrimination: application to altered and metamorphosed basic igneous rocks. Earth and Planetary Science Letters, 28 (3): 459-469. Figure 1 Location map of the study area showing the sampled soil stratigraphic unit.

Figure 2
Schematic representation of the pedostratigraphy unit in the Chobe Enclave.