Abstract
Pedogenic carbonates provide valuable archives of past environmental conditions through their elemental and isotopic compositions. This study integrates stable carbon (δ13C) and oxygen (δ18O) isotopes, obtained from carbonate features alongside manganese (Mn) and iron (Fe) concentrations in carbonate features and soil horizons across four pedosedimentary sections (SC01–SC04), to reconstruct vegetation dynamics, climatic stability, and local hydrological variability. Elemental concentrations of Mn and Fe are reported in molar percent (mol%). Soil profiles exhibit low Mn content (0.04%–0.11%), in contrast to higher Fe concentrations (2.68%–7.47%). Carbonate features are systematically depleted in Fe relative to their host matrices (0.42%–3.13%), while Mn is selectively enriched in several nodules, reaching up to 0.22% (Nd07, SC02). Stable isotope data reveal tightly clustered values within each section. δ13C values range from −3.17‰ to −0.24‰ for nodules, which is consistent with a C4-dominated savannah ecosystem. However, deep-seated pseudomycelia exhibit a distinct δ13C signature of −6.85‰, interpreted as a biogenic signal from C3 shrub roots in a closed, riparian sub-surface environment rather than a vegetation shift. δ18O values display a narrow range of −5.52‰ to −3.39‰ across all sections, indicating limited variability in meteoric water composition and evaporative conditions. High Mn/Fe ratios correlate with depleted δ18O values (≤−5‰), marking periods of increased freshwater input and seasonal waterlogging. Crucially, the decoupling of redox signals in some nodules, characterised by a combined high Mn/Fe ratio with δ18O enrichment, highlights the occurrence of episodic fluvial pulses and overbank flooding superimposed on a generally semi-arid background. These findings demonstrate that while the regional macro-climate remained stable, local hydrology was highly dynamic, driven by high-frequency fluctuations in river-water influence.
1 Introduction
Pedogenic carbonate serves as a vital natural pathway for carbon sequestration in arid and semi-arid environments, while also acting as a proxy for past climatic and environmental change (; ). These carbonates typically form either through the dissolution and recrystallisation of precursor lithogenic or biogenic minerals such as dolomite or calcite or through the weathering of silicate minerals (). Both processes play a critical role in the global carbon cycle by facilitating carbon dioxide (CO2) sequestration through fluvial and marine systems, as well as terrestrial carbon storage within the soil profiles of dryland ecosystems (). While these carbonates are recognised as active pathways for CO2 sequestration that can be enhanced or degraded by current land use (; ; ; ), they are also frequently identified as the result of long-term accumulation over geological timescales, particularly within palaeosols (). Consequently, they serve as accurate proxies for palaeoenvironmental and palaeoclimatic reconstruction (; ; ; ).
The established methodological framework for using pedogenic carbonate as a palaeoenvironmental proxy relies on the origin of the carbon trapped within the mineral lattice, which is primarily derived from root respiration and organic matter decomposition (). Consequently, analysing the stable carbon isotope ratios (δ13C) within these carbonates makes it possible to infer the dominant vegetation type (C4 vs. C3 plants) prevailing during their formation (; ). However, the δ13C signal may also be influenced by the dissolved inorganic carbon (DIC) from which carbonates precipitate, including potential contributions from lithogenic carbonate dissolution (; ). In addition, kinetic fractionation during rapid precipitation and post-depositional diagenetic processes may lead to deviations from equilibrium conditions, thereby modifying the primary δ13C signature (). As a result, while broad ecological interpretations (e.g., C3 vs. C4 dominance) remain robust, more detailed quantitative reconstructions should be treated with caution ().
Palaeoclimatic reconstruction is supported by the relationship established between the meteoric water, from which the oxygen present in pedogenic carbonate was derived and the resulting isotopic signature (). Thus, the stable isotopic ratio of oxygen in pedogenic carbonate is mainly controlled by the climatic conditions prevailing during mineral precipitation. This is because the fractionation of oxygen isotopes (18O/16O) during carbonate precipitation is temperature-dependent, effectively recording the climatic conditions at the time of their formation (). Although seasonal variations and the multiple dissolution-reprecipitation cycles inherent in the development of pedogenic carbonates are often cited as limitations to palaeoenvironmental and palaeoclimatic reconstructions based on stable isotopes (; ; ), the approach remains demonstrably effective. These processes, while complex, do not negate the capacity of the carbonates to record significant environmental signals, particularly when interpreted within their specific pedosedimentary context (; ; ).
In pedosedimentary environments where active pedogenesis is interrupted by allochthonous deposits, it is often difficult to determine whether these interruptions signify a sustained transition to a humid climate or whether they simply represent isolated large-scale flooding events. However, as established by , pedogenic carbonates in hydromorphic or fluctuating environments are dynamic archives rather than static minerals. They exist in complex “paragenetic” relationships with other redox-sensitive phases. Mn and Fe in pedogenic carbonates often occur as secondary phases (e.g., oxyhydroxides), and therefore reflect redox dynamics within the soil system rather than strictly syn-depositional conditions. Manganese is reduced and mobilised more rapidly than iron under conditions of increasing anoxia (). Consequently, the relative abundance of these elements in the pedogenic carbonate reflects the specific duration and intensity of the wetting event, allowing transient pulses to be distinguished from prolonged waterlogging.
Pedogenic carbonates play a central role in shaping the pedological landscape of Botswana (; ; ). Recently, pedosedimentary sections characterised by significant accumulations of carbonate features have been described along an ephemeral watercourse in the Central District (, under review). The result demonstrated a pedogenic origin of the carbonate features, in theses sections. Furthermore, mineralogical characterisation identified calcite as the predominant phase. The calcium trapped in these pedogenic carbonates comes mainly from the basaltic bedrock (). However, the high-resolution environmental conditions, particularly the specific vegetative cover and the transient redox dynamics that governed carbonate precipitation, remain poorly constrained. This study aims to reconstruct these palaeoenvironmental conditions, including palaeohydrological fluctuations and redox potentials, by combining stable isotope (δ13C and δ18O) analyses with the Mn/Fe ratio as a novel redox proxy.
2 Materials and methods
2.1 Study site and sampling strategy
As mentioned above, the study site has already been the subject of other research, particularly with the aim of understanding the formation of the landscape and tracing the origin and processes of calcium ion mobilisation by carbonates (). It is in the Central District of Botswana, between 22.34° and 22.42° south latitude and between 26.67° and 26.76° east longitude.
The general environmental setting highlights a relict landscape composed of gently sloping plains interspersed with isolated ravines subject to active erosion, characteristic of the Eastern Hardveld geomorphological unit of Botswana (). The geological substrate is mainly dominated by the Late Carboniferous to Early Jurassic Karoo Supergroup formations, covered in places by Cenozoic Kalahari Group sands (). According to the lithostratigraphic description of , the Lower Karoo Supergroup (comprising the Ecca and Beaufort groups) is characterised by a basal sequence of conglomerates and breccias that fines upward into coal-rich mudrocks with organic shales, followed by poorly lithified, grey-to-purple claystones (Beaufort Group). Field observations at the study site reveal a complex stratigraphy where three primary lithologies belonging to the Upper Karoo Supergroup are interbedded. These include alternating mudstones and sandstones from the Lebung Group and basaltic deposits from the Stormberg Lava Group. The Kalahari Group consists of loose aeolian sand containing buried palaeosols rich in carbonate nodules ().
The local climate is hot and semi-arid (corresponding to BSh type, Köppen–Geiger classification), characterised by long dry periods and a short rainy season from November to March. Rainfall data analysed by over a 60-year period indicate a mean annual precipitation of approximately 431 mm. However, rainfall is marked by strong interannual variability, with frequent droughts and episodic flooding (; ). The winter season (May to September) is dry and sunny. Daytime temperatures remain warm, whereas nights and early mornings can be cold, with temperatures occasionally reaching 0 °C. Maximum temperatures occur during the summer months and may exceed 45 °C ().
Vegetation corresponds to a bush savannah ecosystem, dominated by woody species such as Acacia, Terminalia, and Boscia, with a diverse herbaceous layer of perennial grasses (). This vegetation assemblage reflects a mixed C3–C4 system (), in which woody plants are predominantly C3 photosynthetic, whereas the grass layer is largely composed of C4 species (). Such a vegetation structure is characteristic of semi-arid regions of Botswana (; ).
Four pedosedimentary sections were selected along an ephemeral river locally known as Metsimasweu (Figure 1). These sections were selected for their representativity of the local landscape marked by prominent carbonates features in soil. Full description of the sections is given by . In general, all these sections reflect an alternation between periods of stability, favouring pedogenesis, and periods of instability characterised by fluvial sedimentary deposits resulting from river flooding during wet periods. Thus, the stratigraphy reveals ancient soils (palaeosol) mostly palaeo-Calcisols buried under fluvial sediments, which are themselves covered by poorly developed modern soils, mainly Arenosols or Cambisols. Chemical weathering is generally low, characterised by a low clayeyness and robust chemical weathering index (RW). However, the grain size distribution revealed finer particles towards the downstream sections (SC03 and SC04), reflecting local translocation. The sedimentary layers also show a dominance of angular to sub-angular quartz grains and the presence of opaque fragments, characteristic of accessory minerals in basaltic rocks. This reflects the dominance of local contribution also supported by the presence of basalt and sandstone fragments in soil horizons. The main carbonate features observed are nodules and diffuse powders. Nevertheless, some horizons at the bottom of the sections, generally in contact with or adjacent to the watercourse, often show accumulations in the form of pseudomycelia.
FIGURE 1
Section (SC01) stands out as the most significant profile in the study. Nodules are present throughout this section, occurring in both the modern soil (Nd01 in the Bk2 horizon; 6.35 wt.% CaCO3) and the buried palaeosols. In the latter, CaCO3 equivalents increase significantly, ranging from 9.39 wt.% in the AB1b horizon (Nd02) to a peak of 24.61 wt.% in the Bkkb horizons (Nd03, Nd04). Additionally, the base of SC01 is characterised by pseudomycelia (Ps) with a lower carbonate content of 2.18 wt.%. In contrast, carbonate features in the modern soils of SC02, SC03 and SC04 are poorly developed, appearing only in powder form with CaCO3 contents generally below 5 wt.%. Identifiable features are much more prominent in the buried palaeosols of these sections. Specifically, the BCkkb and BCkb horizons across SC02, SC03, and SC04 show a consistent abundance of nodules. The BCkkb horizons consistently exhibit higher concentrations, ranging from 14.24 wt.% in SC02 to 21.09 wt.% in SC04, while the BCkb horizons maintain lower equivalents, fluctuating between 4.00 wt.% and 6.19 wt.%. Full morphological characteristics and physicochemical properties of the four sections are given in Supplementary Table S1.
These carbonate features result primarily from pedogenic processes, with calcite serving as the dominant mineralogical phase. The transition from powdery forms in modern horizons to well-defined nodules in buried layers suggests varying stages of carbonate development and secondary mineral precipitation over time.
demonstrated that local basaltic rocks are the dominant source of calcium ions for pedogenic carbonates observed in these sections. This supply involves two distinct pathways: first, the mechanical transport of basaltic fragments which are reworked into sediments and subsequently incorporated into the soil, where their weathering releases calcium ions to the soil solution that are mobilised by capillary rise and evaporation during the dry season to form secondary carbonates. Second, the in situ chemical weathering of the parent material releases calcium ions directly into solution; these ions are then transported by the river and move from the fluvial system into the soil profiles during dry periods, where they are eventually fixed as pedogenic carbonates.
Stratigraphy and locations of carbonate features is presented in Figure 2. Carbonate features (nodules and pseudomycelia) were selectively sampled from horizons displaying significant carbonate accumulation for elemental geochemistry, as well as stable carbon and oxygen isotope analyses. In Section 1 (SC01), five features were collected: nodules Nd01 (Bk2), Nd02 (AB1b), and Nd03–Nd04 (Bkkb), alongside pseudomycelia (Ps) from AB3b. Section 2 (SC02) yielded three nodules, comprising Nd05 (BCkkb) and Nd06–Nd07 (BCkb). Sampling in Section 3 (SC03) included five nodules: Nd08–Nd11 (BCkkb) and Nd12 (BCkb). Finally, six nodules were recovered from Section 4 (SC04), consisting of Nd13–Nd15 (BCkkb) and Nd16–Nd18 (BCkb).
FIGURE 2
Soil samples were also collected in each horizon of the four soil profiles for elemental geochemical analyses.
2.2 Sample preparation
To ensure the analysis targeted the pedogenic carbonate phase specifically, the samples were cleaned of adhering silicate particles and organic matter. Soil and carbonates samples were then ground into a fine homogeneous powder, using an agate mortar.
2.3 Iron and manganese elemental analysis
Iron and Mn concentrations, originally reported as oxide weight percentages, were determined for all samples (soil and carbonate) using a Bruker AXS S4 Explorer wavelength dispersive X-ray fluorescence spectrometer (WDX), housed at the Laboratory for the Environmental and Raw Materials Analysis (LERA), Institute of Applied Geosciences, Karlsruhe Institute of Technology (KIT), Germany. Analyses were conducted on fused beads. Prior to element quantification, the loss on ignition (LOI) of the pulverised samples was determined by heating them to 950 °C for 3 h and recording the weight difference before and after heating. Fused beads were prepared by mixing the ignited sample powder with a 66.5:33.5 ratio of Li-tetraborate and Li-metaborate (Spectroflux 110, Alfa Aesar) before melting it at 1,000 °C–1,100 °C using a fusion instrument. An Rh X-ray tube (50 keV, 1 kW) was used as the radiation source. Signals were detected using a proportional flow counter (Ar-CH4 gas) and a scintillation counter. XS-55, LIF2000 and PET were used as analyser crystals. Four certified reference materials, including AGV-1, GXR-2, GXR-5 and Soil 7, were used to verify the accuracy of the produced data. All the uncertainty obtained were below the detection limit.
Mn/Fe ratios in pedogenic carbonates were used as an indirect proxy for redox fluctuations within the soil system, as demonstrated that trace Mn and Fe, primarily occurring as secondary oxyhydroxides, are sensitive to hydrological dynamics and redox cycling. To facilitate stoichiometric comparison, bulk geochemical data from soil samples, originally reported as oxide weight percentages (wt.%), were recalculated into elemental molar proportions. For each oxide, the reported weight percentage was divided by its molecular weight to obtain the corresponding oxide molar proportion. This value was then multiplied by the cation stoichiometric coefficient of the oxide (e.g., 2 for Al2O3; 1 for CaO) to derive the unnormalised cation molar proportion. Final elemental molar percentages (mol%) were obtained by normalising these values to the total molar sum of all analysed elements (). This approach ensures that the geochemical signal represents the actual chemical activity and redox-driven availability of these metals in the soil solution at the time of mineral precipitation (; ).
2.4 Stable isotope analyses
The ratios of stable isotopes of carbon (13C/12C) and oxygen (18O/16O) in carbonate features were determined at LERA. The samples were analysed for the stable isotope composition of oxygen (δ18O) and carbon (δ13C) by isotope-ratio mass spectrometry (IR-MS) using a Delta V Advantage mass spectrometer (Thermo Fisher Scientific) coupled to an on-line, automated carbonate preparation system (GasBench II; Thermo Fisher Scientific). In this system, the sample vials sealed by rubber septa are flushed with He so that the atmosphere inside the vials is free of CO2 and O2, and 103% orthophosphoric acid is added to release CO2 from the complete dissolution of the ∼1 mg carbonate samples over 90 min at 72 °C. Using He as carrier gas, the sample CO2 then passes a gas-phase chromatograph to separate it from other gases and is injected into the mass spectrometer for analysis (e.g., ), in this case in ten pulses of 100 μL each. The results obtained from these ten injections are then averaged to yield the final results.
Calibration was conducted using the NBS19 standard material. Precision is determined by the absolute standard deviation (ASD) of repeated measurements of the Carrara Marble standard material, with an average accuracy of 0.03‰ for δ13C and of 0.09‰ for δ18O, respectively (n = 150) All results are reported in the δ notation [‰] relative to the V-PDB standard (Supplementary Table S2).
3 Results
3.1 Manganese and iron concentration in the sections
As detailed in Section 2.3, elemental concentrations are reported as molar percentages (mol%). Figure 3 illustrates the vertical variations of Mn and Fe for each study section (numerical values are presented in Supplementary Table S3). Throughout the profiles, Mn occurs primarily as a minor or trace element, ranging from 0.04% to 0.11%, whereas Fe constitutes a major component, with concentrations ranging from 2.68% to 7.47%.
FIGURE 3
In SC01, the distribution of Mn reveals relative enrichment in horizons Bk1, Bk2, AB1b and Bkkb, where values generally exceed 0.09%. Regarding Fe, a clear stratigraphic distinction emerges: the lower horizons (CB1b to AB3b) exhibit notably higher concentrations (5.86%–7.47%) than the upper sequence (A1 to Bkkb; 2.74%–4.97%).
Sections SC02, SC03 and SC04 display more variable patterns. In SC02, both elements exhibit largely uniform profiles, typically remaining below 0.05% for Mn and 4% for Fe. The BCkb horizon is a notable exception, with higher values of 0.10% Mn and 4.70% Fe. SC03 exhibits a coupled distribution, with peak values for both Mn and Fe occurring in the AC, C2, and ABb horizons, while other horizons remain below 0.05% and 5%, respectively. Similarly, in SC04, both elements follow a twin-peak pattern, with the highest concentrations in the ABb (5.88% for Fe and 0.09% for Mn) and Bt1b (5.23% for Fe and 0.09% for Mn) horizons, while the rest of the profile shows significant depletion.
3.2 Manganese and iron concentration in carbonate features
Table 1 shows the concentrations of Mn and Fe in the carbonate features. The distribution of these elements within the carbonate features contrasts with the patterns observed in the bulk soil profiles. While Fe concentrations are consistently lower than those in the surrounding matrix, Mn concentrations exhibit selective enrichment in several features.
TABLE 1
| Section | Horizon | Depth (cm) | Sample | Mn | Fe |
|---|---|---|---|---|---|
| (mol%) | |||||
| SC01 | Bk2 | 120–180 | Nd01 | 0.06 | 2.49 |
| AB1b | 240–300 | Nd02 | 0.11 | 1.16 | |
| Bkkb | 300–360 | Nd03 | 0.09 | 1.22 | |
| 360–420 | Nd04 | 0.08 | 1.53 | ||
| AB3b | 740–840+ | Ps | 0.03 | 3.13 | |
| SC02 | BCkkb | 240–360 | Nd05 | 0.01 | 0.62 |
| BCkb | 500–540 | Nd06 | 0.16 | 1.01 | |
| 540–580 | Nd07 | 0.22 | 0.91 | ||
| SC03 | BCkkb | 245–285 | Nd08 | 0.10 | 0.59 |
| 285–305 | Nd09 | 0.05 | 0.61 | ||
| 305–345 | Nd10 | 0.04 | 0.69 | ||
| 345–375 | Nd11 | 0.05 | 0.42 | ||
| BCkb | 375–400 | Nd12 | 0.15 | 0.69 | |
| SC04 | BCkkb | 160–190 | Nd13 | 0.06 | 0.86 |
| 190–220 | Nd14 | 0.03 | 0.62 | ||
| 220–260 | Nd15 | 0.07 | 1.04 | ||
| BCkb | 260–300 | Nd16 | 0.03 | 0.78 | |
| 300–330 | Nd17 | 0.05 | 0.61 | ||
| 330–400+ | Nd18 | 0.02 | 0.90 | ||
Manganese and iron contents (mol%) in the selected carbonate features.
In SC01, most features (Nd01, Nd03, Nd04 and Ps) are depleted in Mn relative to their respective host horizons (Bk2, Bkkb and AB3b). However, Nd02 (0.11%) is an exception, with higher Mn levels than its host, AB1b (0.09%). More pronounced enrichment is observed in SC02, where Nd06 (0.16%) and Nd07 (0.22%) significantly exceed the Mn content of their host BCkb (0.09%), though Nd05 (0.01%) remains depleted compared to BCkb (0.04%).
In SC03, Nd08 (0.10%) and Nd12 (0.15%) are similarly enriched relative to their hosts BCkkb (0.06%) and BCkb (0.06%). Nd09 (0.05%), Nd10 (0.04%) and Nd11 (0.05%) in BCkkb follow a contrasting depletion pattern. In SC04, Nd13 (0.06%) and Nd15 (0.07%) have higher Mn content than their host BCkkb (0.04%), while Nd14 (0.03%), Nd16 (0.03%), Nd17 (0.05%) and Nd18 (0.02%) have lower concentrations than their surrounding matrices (BCkb; 0.06%).
The different trends of the ratio Mn/Fe of the selected carbonate features are presented in Figure 4. Overall, features from sections SC01 (0.01–0.09) and SC04 (0.02–0.08) exhibit lower ratios compared to those from SC02 (0.02–0.24) and SC03 (0.06–0.22). Within SC01, the highest ratio is observed in nodule Nd02 (0.09), followed by Nd03 (0.07), whereas the pseudomycelia (Ps) yield the lowest values (0.01). In SC02, the maximum ratio occurs in Nd07 (0.24), with Nd05 (0.02) representing the minimum for that section. A similar trend is observed in SC03, where Nd12 (0.22) and Nd08 (0.16) display the highest ratios, while Nd10 (0.06) shows the most significant depletion. Finally, in SC04, Nd17 (0.08) reaches the highest ratio, contrasting with Nd18 (0.02) at the base of the section, which records the lowest value.
FIGURE 4
3.3 Stable isotope of carbon and oxygen in carbonate features
Figure 5 shows the δ13C and δ18O values of the carbonate features from the four sections (SC01–SC04), plotted against depth (numerical value are presented in Supplementary Table S2). Across the four sections, δ13C and δ18O values define well-structured datasets characterised by relatively narrow ranges and clear clustering within each section. For both isotopic systems, most measurements fall within restricted intervals, with limited dispersion among samples from the same section. Differences between sections are mainly expressed as shifts in the central value and overall range, rather than as large internal variability. Where present, contrasts between carbonate features are expressed as offsets in isotopic values relative to the dominant clusters.
FIGURE 5
In SC01, δ13C values of nodules are tightly grouped between −3.17‰ and −2.19‰. Though within the same range, a slight shift is observed when moving from Nd01 (−3.17‰) in modern soil to Nd02 (−2.63‰) in buried palaeosols. Pseudomycelia at the base of the section is displaying a distinctly lower δ13C value (−6.85‰), clearly separated from the nodule cluster. The δ18O values of the nodules in SC01 form a consistent group ranging from −4.49‰ to −3.39‰. Although pseudomycelia have a slightly lower value (−4.94‰), this is not far from the range observed in the nodules.
In SC02, the δ13C values of the nodules fall within a relatively narrow range between −2.40‰ and −0.24‰. However, there is a slight discrepancy between Nd05 (in BCkkb), exhibiting a δ13C value of −0.24‰, and the nodules in the horizon below (BCkb), Nd06 and Nd07, with δ13C values of −2.00 and −2.40‰, respectively. No significant differences were observed in the δ18O values. They are clustered between −5.17‰ and −4.67‰. In SC03, nodules display compact fields for both stable isotopes. δ13C values are concentrated between approximately −2.39‰ and −1.08‰. Even though a slight shift toward more negative values is observed with depth. The δ18O values form a more tightly clustered group, ranging from −5.52‰ to −5.39‰, indicating minimal dispersion within the section.
In SC04, except for a more negative δ13C observed in Nd18 (−3.00‰), the other nodules have δ13C values that form a compact group between −1.22‰ and −1.70‰. The δ18O is characterised by a cluster of values ranging from −5.01‰ to −4.86‰.
4 Discussion
4.1 Vegetation dynamics and stable carbon isotopes in pedogenic carbonates
The relationship between the carbon source of pedogenic carbonates and soil carbon dioxide dynamics has been well established by numerous studies, which show that the carbon incorporated into pedogenic carbonate features is predominantly derived from soil CO2 produced by plant respiration (; ; ; ). Consequently, the stable carbon isotope composition (δ13C) of these carbonates is closely linked to the dominant vegetation type (specifically the ratio of C3 to C4 plants) present at the time of precipitation (). In this study, nodules (Nd), which represent the dominant carbonate features, are identified in the upper horizons of section SC01 and throughout the other sections (SC02–SC04). They have δ13C values ranging from −3.17‰ to −0.24‰. Considering the theoretical enrichment of 14‰–16‰ between soil CO2 and pedogenic carbonate (), the observed values correspond to a soil CO2 source ranging from −14 to −17‰. Although these values are slightly more negative than a pure C4 biomass signal (average −12‰), they confirm strong C4 dominance. The minor offset suggest that carbonate precipitation occurred in equilibrium with a soil CO2 reservoir influenced by atmospheric exchange, a condition typical of open-canopy environments with low soil respiration rates (). Moreover, seasonal bias should be considered as pedogenic carbonate accumulation primary occurs during the dry season, reflecting vegetation setting and soil dynamics at that specific time (). The similarity between the δ13C value of Nd01, the nodules observed in modern soil, and the δ13C values of the numerous nodules in the buried palaeosol in SC01 reveals the dominance of the same type of vegetation over a prolonged period of time. The type of vegetation then remained the same, dominated by C4 plants during the various pedosedimentary cycles.
However, the pseudomycelia (Ps) identified at the bottom of section SC01 (horizon AB3b) show a significantly lower δ13C value (−7‰). At first glance, this isotopic jump could suggest a transition from vegetation, i.e., a mixed C4-C3 plants to more C3-dominated vegetation (). Nevertheless, this interpretation, which would support vegetation different from that currently predominant, is probably erroneous. A distinction in formation dynamics must be made between the different carbonate morphologies identified. While nodules represent cumulative features resulting from long-term pedogenic processes, often averaging environmental signals over millennia (; ; ), pseudomycelia typically reflect an incipient stage of carbonate accumulation. In riparian contexts, these features are frequently linked to high-frequency seasonal fluctuations of the water table (; ). Consequently, the pseudomycelia likely capture a more “instantaneous” or modern signal compared to the time-averaged archive preserved within the more recalcitrant nodules. The appearance of pseudomycelia in SC01 is consistent with the immediate proximity of to watercourse. Thus, rather than reflecting a change in vegetation type, this signal seems to reflect current root respiration within this horizon, dominated by shrub roots, which are the only ones to reach this depth. Field observations confirm accumulation along shrub roots (Figure 2). The significant depth of the unit, combined with the hydric influence of the river, would isolate the system from atmospheric CO2 infiltration, thus allowing the capture of a pure biogenic signal from roots ().
There is, nonetheless, an apparent contradiction between the dominance of herbaceous species, as indicated by the δ13C values of the nodules, and the bush savannah characterised by the dominance of shrubs (woody species such as Acacia, Terminalia, and Boscia), as observed in the field and reported in the literature (). This discrepancy is consistent with the previously mentioned seasonal bias observed in δ13C values inherent to carbonate formation (; ). The enriched δ13C values recorded in the nodules are likely influenced by the timing of carbonate precipitation. As noted by , pedogenic carbonates in semi-arid environments predominantly form during the dry season via evapotranspiration. During these periods, low soil respiration rates allow for greater atmospheric CO2 exchange, which further shifts the isotopic composition toward more positive values (). On the other hand, the value of −7‰ for deep pseudomycelia reflects the specific signature of C3 shrub roots in a closed environment, thus confirming the coexistence of these two biological forms within a stable savannah structure.
4.2 Climate dynamics and stable oxygen isotopes in pedogenic carbonates
The source of oxygen in pedogenic carbonate is meanly related to soil water and therefore, meteoric water (). According to , the δ18O in pedogenic carbonates records climate because it faithfully tracks the isotopic composition of precipitation, which is itself controlled by temperature and rainfall regime at the time of carbonate formation. In tropical and subtropical regions, the “amount effect” dictates that high precipitation rates lead to a significant decrease in the 18O content of meteoric water (; ). This leads to lower δ18O values in soil carbonates for a given temperature, while dry climates produce enriched 18O values due to low rainfall and intense evaporation (; ). This enrichment occurs because evaporation preferentially removes the lighter 16O isotope to the atmosphere, leaving residual soil water and the resulting pedogenic carbonates enriched in 18O (). As suggested by our results, the stable oxygen isotopes (δ18O) of the carbonate features in these pedosedimentary sections are clustered around a very narrow range of values from −5.52‰ to −3.39‰. This apparent isotopic homogeneity under comparable hydrological conditions across multiple pedosedimentary sections suggests a period of significant climatic stability. Specifically, it indicates that the isotopic composition of the meteoric water source and the subsequent evaporative enrichment in the soil profile remained relatively constant throughout the period of carbonate precipitation (; ). This is further highlighted by the cross-plot of δ13C and δ18O (Figure 6), which shows a highly concentrated cluster of data points rather than a clear linear trend or several distinct groups. In stable isotope geochemistry, a cluster reflects the absence of significant changes in vegetation type (C4 dominance) or regional water balance (precipitation vs. evaporation). If there had been significant transitions from wet to dry conditions, the samples would likely have been distributed along a “covariation line”; on the contrary, the overlap between modern surface nodules and deeper features reinforces the conclusion that environmental parameters, particularly seasonal aridity and open-canopy savannah structure, remained fundamentally unchanged during the various deposition cycles.
FIGURE 6
Nevertheless, vertical trends in SC01 highlighted some slight variations in the δ18O signatures. Nd02 and Nd03 are characterised by relatively high δ18O values, respectively −3.64‰ and −3.39‰, while Nd01 and Nd04 showed relatively low values, respectively −4.50‰ and −4.48‰. These internal fluctuations likely reflect vertical gradients in soil water evaporation during the time of carbonate precipitation. The enriched (higher) values in Nd02 and Nd03 suggest formation during periods of peak evaporative stress or at shallower depths more exposed to solar radiation. Conversely, the more depleted (lower) values in Nd01 and Nd04 may represent formation during slightly wetter seasons or at depths where soil moisture was better insulated from atmospheric evaporation (; ) The stratigraphic framework established by strongly supports this “depth effect” interpretation. According to this stratigraphy, the AB1b horizon containing Nd02 constituted the upper part of the buried palaeosol, likely representing an exposed or near-surface landform prior to the deposition of C1 sediments. The enriched δ18O signal of Nd03, located in the upper part of the Bkkb horizon, follows this same logic of surface-proximal evaporation. In contrast, the more depleted values in Nd01 (Bk1) and Nd04 (lower Bkkb) reflect carbonate precipitation in deeper accumulation horizons, where the isotopic composition of soil water remains more closely aligned with that of local meteoric water and is less influenced by secondary evaporative enrichment ().
This model of depth-controlled buffering also extends to the pseudomycelia, though their signature is further modified by the specific hydrological dynamics of the riparian zone. Given their proximity to the river channel, these features likely precipitated from groundwater associated with the local watercourse. Their isotopic value represents a balance between the original depleted signature of the river water and subsequent evaporative enrichment at the capillary fringe during periods of water-table recession (; ). This distinguishes the pseudomycelia from the nodules; while the former are controlled by fluctuating groundwater levels near the channel, the latter are primarily influenced by the downward infiltration and evaporation of meteoric waters in the upper soil profile.
4.3 Hydrological conditions and carbonate precipitation
Despite a stable regional climate, highlighted localised fluvial changes that interrupted soil development. This dynamic demonstrates the need to analyse variations in local hydrological conditions using Mn/Fe ratios to better understand the specific environment of carbonate formation. Chemical comparisons between pedogenic carbonate features and their host horizons reveal a systematic depletion of iron in the nodules compared to the pedogenic matrix. Conversely, some features (e.g., Nd02, Nd06, Nd07) show significant Mn enrichment. It is important to note that Mn and Fe in pedogenic carbonates are commonly present as secondary oxyhydroxide phases rather than being structurally incorporated into calcite. As a result, Mn/Fe ratios likely integrate both syn- and post-pedogenic redox processes, reflecting fluid circulation, transient saturation, and redox cycling within the soil system (). This chemical signature aligns with the stratigraphic position of the AB1b horizon (hosting Nd02) in SC01; the overlying C1 deposit indicates a period of fluvial overbank flooding, which would have induced the temporary saturation and reducing conditions necessary to drive this Mn-Fe partitioning.
Figure 7 shows a scatter plot of Mn/Fe ratios versus δ18O values for carbonate samples. Overall, higher Mn/Fe ratios tend to be associated with more depleted δ18O values (≤−5‰), whereas samples with less negative δ18O values cluster at lower Mn/Fe ratios. This likely reflects reducing conditions affecting the soil system, possibly associated with periods of increased moisture and redox fluctuations. Such conditions are consistent with more reducing soil or pore-water environments, in which Mn4+ is preferentially reduced and mobilised compared to Fe3+. In contrast, samples characterised by low Mn/Fe ratios likely reflect more oxidising and/or better-drained settings, where Fe remains relatively immobile and Mn availability is limited (; ).
FIGURE 7
The dispersion of Mn/Fe ratios at similar δ18O values indicates that, in addition to the isotopic composition of soil water, local redox heterogeneity and hydrological dynamics (such as fluctuating water tables or episodic saturation) played a major role in controlling trace element signatures. Within this framework, samples Nd02 and Nd03 are distinctive. They combine relatively high Mn/Fe ratios with less negative δ18O values (around −3.64 to −3.39‰), setting them apart from the main cluster. The δ18O enrichment suggests precipitation from more evaporated soil waters as mentioned in Section 4.2, and commonly associated with drier conditions. However, their elevated Mn/Fe ratios imply that carbonate recorded episodes of locally reduced conditions, which promoted Mn mobilisation despite an overall evaporative hydrological context (). Nd02 and Nd03 therefore record a partial decoupling between hydrological and redox signals and are best interpreted as reflecting short-lived reducing events superimposed on a generally dry background climate. These samples likely capture episodic hydrological pulses (e.g., transient saturation following rainfall or perched water tables) rather than sustained wet conditions, highlighting the importance of micro-environmental variability during pedogenic carbonate formation ().
The group formed by samples Nd06, Nd07, Nd08, Nd11 and Nd12 exhibits high Mn/Fe ratios that contrast with the stability of their δ18O signatures. This geochemical trend probably reflects a change related to lithological heritage rather than climatic changes. The local geology consists of alternating basaltic rocks (Mn/Fe ≈ 0.01–0.02), mudstones (Mn/Fe ≈ 0.02) and sandstones (Mn/Fe ≈ 0.03) (). Furthermore, demonstrated that the weathering of these parent materials, individually or in combination, directly influences the development of these pedosedimentary sections. In addition, the absence of oxygen isotopic deviation confirms that these chemical variations occurred under a constant regional climate regime.
In summary, the combination of stable isotopes (δ13C, δ18O) and elemental ratios (Mn/Fe) allows for a reliable reconstruction of a resilient shrub savannah ecosystem. The main strength of this multifaceted approach lies in its ability to dissociate regional climate from local hydrology. While the close clustering of isotopic data indicates long-term regional climate stability and a stable plant structure dominated by C4 plants, Mn/Fe ratios reveal the ‘hidden’ complexity of the site’s fluvial history. Specifically, the ability to correlate Mn enrichment in nodules with stratigraphic evidence of off-channel flooding (e.g., deposit C1) validates the use of the ratio as a high-resolution indicator of transient redox events that isotopic averages might otherwise overlook. However, certain limitations must be acknowledged. The intrinsic “temporal averaging” nature of larger carbonate nodules means that they likely smooth out decades or even centuries of environmental variability, potentially masking short-term ecological changes. Furthermore, the “seasonal bias” of carbonate precipitation, which favours the dry season, means that δ13C and δ18O values may disproportionately reflect arid phase conditions rather than the absolute annual mean. While pseudomycelia offer a more “instantaneous” hydrological snapshot, their localised formation along modern root paths and near the riverbed complicates their use as a direct analogue for older, deeper pedosedimentary cycles. Finally, the absence of independent chronological constraints means that a robust temporal framework cannot be established, so interpretations are necessarily process-based rather than time-resolved. Despite these constraints, converging evidence suggests that the study area has maintained a consistent ecological character, punctuated periodically by river pulses, throughout the recorded sections.
5 Conclusion
Overall, the combined analysis of stable isotopes (δ13C and δ18O) and elemental ratios (Mn/Fe) in pedogenic carbonate features enables the reconstruction of long-term environmental stability and short-lived hydrological disturbances within the study area. The δ13C values of the carbonate nodules (−3.17‰ to −0.24‰) consistently indicate a resilient, C4-dominated savannah ecosystem. The relatively narrow range of δ18O values (−5.52‰ to −3.39‰) suggests limited variability in the isotopic composition of soil water; however, this signal should not be interpreted as a direct indicator of regional climatic stability. Instead, it likely reflects a combination of local controls, including evaporation, soil water residence time, and seasonal bias in carbonate precipitation. Minor vertical variations, particularly in SC01, are therefore more plausibly linked to depth-dependent evaporative processes than to climatic shifts. Meanwhile, Mn/Fe ratios provide valuable insights into redox dynamics and hydrological variability within the soil system, although their interpretation must consider the potential contribution of secondary processes and post-depositional redistribution. Although the time-averaging nature of carbonate precipitation and its seasonal bias may smooth out short-term variability, the convergence of isotopic and elemental evidence suggests that there is a stable ecological framework which is periodically interrupted by river-driven hydrological pulses. Applying this multi-proxy approach to further pedosedimentary sections in different geomorphic settings would enable the evaluation of the regional representativeness of these signals and help to distinguish between local and climate-driven influences. Furthermore, improved chronological constraints would refine the timing and significance of hydrological events.
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FE: Conceptualization, Formal Analysis, Investigation, Methodology, Validation, Visualization, Writing – original draft. TB: Investigation, Methodology, Resources, Supervision, Validation, Writing – review and editing. EE: Data curation, Methodology, Software, Validation, Writing – review and editing. SN: Methodology, Resources, Supervision, Writing – review and editing. PE: Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Writing – review and editing.
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Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenvs.2026.1821521/full#supplementary-material
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Summary
Keywords
fluvial influence, palaeosols, redox processes, semi-arid environments, soil geochemistry, Southern Africa
Citation
Dina Ebouel FJ, Bineli Betsi T, Eiche E, Norra S and Eze PN (2026) Decoupling climate and local hydrology in Central Botswana: new insights from carbonate δ13C, δ18O isotopes, and elemental geochemistry. Front. Environ. Sci. 14:1821521. doi: 10.3389/fenvs.2026.1821521
Received
02 March 2026
Revised
03 April 2026
Accepted
15 April 2026
Published
08 May 2026
Volume
14 - 2026
Edited by
Cláudia Pascoal, University of Minho, Portugal
Reviewed by
K. Prasanna, Birbal Sahni Institute of Palaeosciences (BSIP), India
Xolane Mhlanga, University of Mpumalanga, South Africa
Updates
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© 2026 Dina Ebouel, Bineli Betsi, Eiche, Norra and Eze.
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*Correspondence: Peter N. Eze, peter.eze@uni-potsdam.de
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All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.