Tracing recent environmental changes and

Geomorphology 268 (2016) 312–321

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Geomorphology journal homepage: www.elsevier.com/locate/geomorph

Tracing recent environmental changes and pedogenesis using geochemistry and micromorphology of alluvial soils, Sabie-Sand River Basin, South Africa Peter N. Eze a,⁎, Jasper Knight b, Mary Evans c a b c

Department of Earth & Environmental Science, Botswana International University of Science & Technology, Private Bag 16, Palapye, Botswana School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Wits 2050, Johannesburg, South Africa School of Geosciences, University of the Witwatersrand, Wits 2050, Johannesburg, South Africa

a r t i c l e

i n f o

Article history: Received 19 November 2015 Received in revised form 14 June 2016 Accepted 15 June 2016 Available online 17 June 2016 Keywords: Soil development Flooding Weathering indices Kruger National Park South Africa

a b s t r a c t Three pedons on the alluvial terraces of the Sabie-Sand River Basin within Kruger National Park, South Africa, were studied to improve our understanding of recent environmental changes, and assess degree of chemical weathering and pedogenesis in the area using geochemical and micromorphology proxies. Particle-size distributions were obtained using Malvern Mastersizer; soil geochemistry was determined by XRF and thin sections by routine laboratory procedures. The soils are predominantly sandy (N 94% sand in all samples). The mean phi-values of the soils had little variation suggesting that reworking of sediments upwards in individual profiles produced a more uniform pedogenesis rather than coming from different physical sources. Calcification is the dominant pedogenic process in these alluvial soils. The Chemical Index of Alteration (CIA) proved a more suitable index than Chemical Index of Weathering (CIW) for evaluating weathering in the terraces. The micromass and b-fabrics are mostly granostriated and partly brown mosaic speckled. MISECA values for the degree of soil development range from 4 to 9, which mean weakly to moderately-developed soils. Coarse secondary calcite nodules and coatings are responsible for cementation as observed in pedon 2, which suggests calcium carbonate precipitation from periodical flooding and evaporating groundwater events. The features and diagnostic properties of the soils on the alluvial terraces along the Sabie-Sand River provide evidence for land surface impacts of recent environmental changes in this internationally important conservation area. Precise dating of calcium carbonate precipitates is, however, needed to put the observed evidence into a wider geochronological perspective. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Sabie River is the most biologically diverse river in southern Africa and is particularly important because it is one of the most significant rivers draining through Kruger National Park (KNP), a global tourist destination and known for its biodiversity. The status of KNP as an internationally important conservation area has made it necessary to better understand the evolution and dynamics of the geomorphic, hydrologic and biotic landscape elements and, where necessary, restore or simulate natural function, structure and composition in the face of global environmental and climate change (DWAF, 1997). Chemical weathering and pedogenesis of soils on the alluvial terraces of the Sabie River, as index of past environmental change, are still largely understudied. Most of what we know about the soils is limited to studies done principally for ecological and tourism purposes at KNP (Water Research Commission, WRC, 2001; Mostert, 2007). Quaternary-age ⁎ Corresponding author. E-mail address: [email protected] (P.N. Eze).

http://dx.doi.org/10.1016/j.geomorph.2016.06.023 0169-555X/© 2016 Elsevier B.V. All rights reserved.

soils of varying depths and morphologies are important features on the alluvial terraces from the upper to lower Sabie River. Chemical weathering is a precursor of pedogenesis, and it exerts huge influence on many physical and chemical properties of soils and geomorphology. Environmental conditions including climate, geomorphic settings and parent materials impact significantly on soil development and properties, which are of relevance within KNP because of their relationships to ecosystem health. Geochemical proxies have been used successfully in the evaluation of chemical weathering and pedogenesis in sediments, soils and palaeosols (Yang et al., 2004; Adams et al., 2011), determination of mean annual precipitation (Adams et al., 2011) and evaluation of soil fertility (Delvaux et al., 1989). The Chemical Index of Alteration (CIA) and Chemical Index of Weathering (CIW) in addition to elemental ratios such as K/Na and K/Ca are some of the quantitative methods used in assessing the degree of chemical weathering with respect to their mobility in soil profiles during weathering (e.g. Nesbitt and Young, 1982, 1984; Sawyer, 1986; Harnois, 1988). Consequently, high K/Na ratios generally suggest enhanced silicate dissolution, while high K/Ca ratios

P.N. Eze et al. / Geomorphology 268 (2016) 312–321

reflect strong carbonate dissolution in a weathering profile. Also known as indices of alteration, chemical weathering indices resolve major element oxide chemistry to a single value per unit sample of soil. Ideally, they are applied by plotting specific index values against depth in the weathering profile or each sedimentary layer for soils formed from transported parent materials, thereby providing a pictorial representation of changes in bulk chemistry with presumed increasing or decreasing weathering of the parent materials (Price and Velbel, 2003). Geochemical mass balance, on the other hand, shows the degree and pattern of major element mobility/translocation down a soil profile with respect to the relatively immobile element such as zirconium (Zr). Micromorphology can also be used as a marker of pedogenesis and for soil classification (Stoops et al., 2010). Imprints of dominant pedogenic processes under various climatic and/or environmental settings are identifiable from soil micromorphology. Hence the application of micromorphology is effective in discriminating inherited from pedogenic properties, evaluating the development and degradation of structures, the re-arrangement of soil solids by pedofauna and other physical agents, explaining the nature of clay enriched subsoil, as well as the behaviour of gypsum and carbonates (Gerasimova and Lebedeva, 2008). Micromorphology has also been used successfully to establish changes related to climate oscillations in South African soils, such as clay coating formation resulting from clay illuviation (Eze and Meadows, 2014). Khormali et al. (2003) proposed a soil micromorphological index of soil development (MISECA) to evaluate the degree of argillic horizon development in calcareous soils of arid and semi-arid environments such as is found in the KNP. The criteria used in the computation of MISECA include microstructure, b-fabric, area and size of clay coating, area of decalcified zone, area of Fe/Mn oxide and alteration degree according to Bullock et al. (1985). MISECA classifies the degree of development from weakly developed to strongly developed soils, with increasing numerical values of 0–24. A good correlation has been reported between MISECA and soil properties (Ghergherechi et al., 2009; Khormali and Kehl, 2011). Most alluvial soil formation occurs on terrace surfaces, and soil development in alluvial landscapes is strongly controlled by climatically driven aggradation-degradation episodes (Bull, 1990; Aslan and Autin, 1998). Similar to most alluvial soils found elsewhere, soils on the alluvial terraces of the Sabie River are prone to episodic flooding, with the last and most remarkable being in 2001 (Heritage et al., 2001). Seasonal

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floods also result in sediment reworking within the channel and overbank sedimentation on floodplains where present. Given this context, we may infer that soils on the alluvial landscapes of Sabie River would be affected by the aggradation-degradation action of episodic floods, and that specific biopedological and geochemical processes act at both microscopic and landscape scales in the channel, terraces and floodplain geomorphic settings. The main objective of this study, therefore, is to provide a pedological characterisation, description and interpretation of the Quaternary soils on the alluvial terraces of Sabie-Sand River Basin complex. The focus is on the application of total element geochemistry and micromorphology to improve our understanding of climate and environmental change in this area. 2. Physical setting The study area is located on three alluvial positions on the Sabie River which is located in the Mpumalanga Province of South Africa and the lower region of Mozambique (Fig. 1). Soils in this region correspond to long-term climate-controlled weathering patterns on mainly granitic bedrock (Khomo et al., 2013). There is no precise geochronology for these soils, but it is most likely that soils developed in alluvial floodplains and river terraces are of similar age to the landforms themselves, which is dominantly late Pleistocene to early Holocene (Venter, 1986). The Sabie River, a major tributary of the wider Sabie-Sand River Basin, originates in the Drakensberg Mountains at an altitude of about 2200 m and flows eastwards for about 210 km to the Incomati River in Mozambique (van Coller et al., 1997). The alluvial terrains selected for this study are situated on the downstream lowveld (wide open flat area covered in grass or low scrub) geomorphic zone (at around 100–300 m asl), where the river has started to meander and deposit alluvium on terraces, by avulsion outside of perennial river channels, and on in-channel bars. Along the Sabie River gradient, the three studied pedons (P) fall within the high alluvial terrace (P1), mid-alluvial plain (P2), and the low alluvial floodplain (P3). P1 and P2 are in-channel geomorphic settings, while P3 is on a floodplain. The heights of the terraces above present river channel level are approximately 2.5 m for P1; 2.2 m for P2, and 1.5 m for P3. The pedons are located on elevations of 258 m (P1), 167 m (P2) and 119 m asl (P3), respectively. The sampled locations cover around 50 km distance of the lower Sabie River between Skukuza and Malelane within KNP (Fig. 1). The

Fig. 1. Geographical location of the study area.

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climate of the area is temperate to subtropical, characterised by warm dry winters and hot rainy summers. This region has a mean annual temperature of 22.5 °C and mean annual precipitation of 553 mm. River flow is perennial with overbank flooding strongly associated with sporadic episodes of intense rainfall events. The soils on the terraces are developed on transported coarse-textured alluvial parent material. Freshwater mollusc shells are also common in the overbank deposits. Outcrops of resistant rocks including dolerite dykes, diabase and gabbro sills (Chesire, 1994) are common in the mid to lower Sabie River. These rocks control channel geomorphology of the Sabie River where they crop out. The dominant vegetation within the floodplain zone is dominantly reed grass (Phragmites mauritianus), potato bush (Phyllanthus reticulatus), Rhodesian redwood (Breonadia salicina), and river bushwillow (Combretum erythrophyllum) (van Coller et al., 1997). 3. Methodology

in the equation: CIA ¼ 100 ½Al2 O3 =ðAl2 O3 þ CaO þ Na2 O þ K2 OÞ

ð1Þ

CIW ¼ 100 ½Al2 O3 =ðAl2 O3 þ CaO þ Na2 OÞ:

ð2Þ

A geochemical mass balance calculation was used to quantify the net result of pedogenic weathering. Eq. (3) assumes that an immobile element behaves conservatively and corrects mobile element concentrations for volumetric strain during weathering and pedogenesis (Chadwick et al., 1990), τi; j ¼

Ci; p Cj; w −1 Ci; w Cj; p

ð3Þ

Three soil profiles were dug at varying elevations along the Sabie River alluvial terraces: higher terrace (P1), mid-alluvial (P2) and low alluvial floodplain positions of the slope (P3). Undisturbed soil monoliths (500 g) were taken from horizons of the three profiles with peds and taken to the laboratory for impregnation with resin. Additional representative bulk samples were collected from the same soil horizons and bagged for particle-size and geochemical analysis. The macromorphological properties were described in accordance with the guidelines for soil profile description (FAO, 2006).

where C is the concentration (weight percentage or molar mass), i the immobile element, j the element in question, w the weathered material and p for parent material. If τi, j = −1, then the element i is completely depleted during chemical weathering. In most cases, this equation provides a tool for estimating elemental loss or gain within a profile, although the mass-balance equation has critical assumption that since the calculations are highly dependent on immobile elemental concentrations, the reference element should be conservative. Zirconium is the chosen immobile element for this study because zircon is highly resistant to chemical weathering and inter-sample variations in Zr are less than for Ti (Chadwick et al., 1990).

3.2. Laboratory methods

4. Results

Pre-treatment of air-dried samples included gentle grinding to break up peds and subsequently passing it through a 2 mm sieve to separate gravels and roots/rhizomes from the b2 mm soil fraction. Moist colours were determined using a Munsell colour chart. Granulometric analysis to determine the particle-size distribution of the samples was done using Malvern Mastersizer 3000. Mean phi-values were determined to compare the center of gravity for each particle-size curve (Folk, 1968). Standard pellets of 5 g were made out of homogenised samples for total elemental oxide compositions by X-ray fluorescence spectrophotometry (XRF) at the School of Geosciences, University of the Witwatersrand, using the Axios mAX® apparatus from PANalytical. Major elements were obtained by fusion method with tetra-borate flux using a SuperQ® program, while trace elements were done by pressed pellet method, with organic binder (MOWIOL) using the ProTrace® program. Thin sections of about 5 cm by 3 cm were prepared from air-dried, undisturbed peds using standard techniques (Murphy, 1986). Photomicrographs of the thin sections were acquired from slides viewed with a light polarizing petrographic microscope (Nikon) and images captured with an Olympus ALTRA 20 camera. Micromorphological descriptions were made according to Bullock et al. (1985) and Stoops (2003), while interpretation of micromorphological features followed the criteria of Stoops et al. (2010).

4.1. Lithostratigraphy

3.1. Field sampling

3.3. Weathering indices Potassium and sodium are profoundly abundant in silicate minerals such as feldspar and mica, whereas calcium resides in carbonate minerals in soils and sediments. We therefore used K/Ca and K/Na ratios to evaluate carbonate dissolution and silicate dissolution respectively. Salinization was evaluated by (K + Na)/Al ratio. The justification for this as a measure of salinity is that alkali metals accumulate as soluble salts not removed in the soil (Sheldon and Tabor, 2009). The Chemical Index of Alteration (CIA) as proposed by Nesbitt and Young (1984), and Chemical Index of Weathering (CIW or CIA-K), which is a modified version of CIA (Harnois, 1988), addresses post-burial addition of K by metasomatism and/or illitization of clay mineral soils by removing K

There is a slight contrast in the horizons of the pedons. The profile on the upper terrace (P1) has A1-A2-Ab; lower terrace pedon (P2) has A1-A2-AC-C1-C2; while pedon P3 showed A1-A2-C-Ab horizon designations (Fig. 2). The results of the macromorphological properties (Table 1) show that the colour varies from brown to dark reddish brown, with P2 having lighter hues than P1 and P3. The redder horizons are pedon 2, A1 (5YR 3/3) and C1 (5YR 5/6) and pedon 3, A2 (5YR 3/4) – an indication of higher iron oxide contents in the soils (Barron and Torrent, 1984). Soils on P1 also have the weakest structure which ranges from granular to weak subangular blocky in the lowest buried horizon. Soils in P2 show the clearest structure which is medium to strong subangular blocky structure. The two upper horizons of P3 have better developed structure than the two underlying horizons. P1 and P3 had a gravelly sedimentary layer indicating lithological discontinuity at certain depths (Table 1; Fig. 2). In accordance with the principles of soil horizon designation, Roman numeral prefixes were not used here since the soils are developing on an alluvial setting with similar parent material and no outstanding pedogenic horizon formation. The moist consistence of P2 and P3 soils is firm while that of P1 soils is loose to friable. Texture, as determined in the field by hand feel, shows that all the soils are dominantly sandy down the profile. Laboratory analysis confirmed that all the soils have higher contents of finer materials at the two uppermost horizons. Weak cementation was evident only in P2. Precipitated secondary calcites from intermittent surface water flooding are responsible for the cementation as observed in the P2 profile. Even though similar shells were found in P1and P3 profiles, they are fewer than that in P2 and the levels of decomposition/dissolution were not sufficient to provide a cementing effect on the soils. This fact is further supported by the occasional to moderate reaction of P1 and P3 soils to dilute hydrochloric acid. As would be expected of alluvial soils, the three profiles harboured varying quantities of other features including excreta pellets, freshwater mollusc shells, crystals and nodules of pedogenic carbonates. There is clear evidence for the impacts of deposition


315

Fig. 2. Lithostratigraphy of the three pedons.

by flooding events on the alluvial terraces, as seen from lithological discontinuities of P1 and P3 profiles. A truncation of buried (Ab) ochric horizons suggests deposition of transported material on an existing geomorphically stable landscape, most likely during flood events. The three soil profiles lack well-developed pedogenic horizons. Although P2 sits at a lower elevation than P1, it is able to have a relatively more stable structure and a more distinct horizon development and structure because it is farther away from the Sabie River channel and therefore would not be as readily affected by intermittent floods. The soils are predominantly sandy (N94% sand in all samples). Coarse and medium sand particles account for over 50% of the soil particles. The silt distribution ranged from 0.2 to 3.5% while clay ranged from virtually non-existent (0%) to 0.2% in the three soil profiles (Table 2). The mean phi-values of the soils had little variation within and across the profiles; it ranges from 1.8 to 3.0 (Table 2).

considerably and do not follow any particular trend down the profiles. However, NiO followed by Cr2O3 and P2O5 was least abundant in all three pedons. Loss on Ignition (LOI) is fairly constant across the profiles, with the exception of the two uppermost horizons of the P3 soils where dark colouration of organic matter contents was more visible (Fig. 2). Minor element distributions (Table 4) across the profiles followed a Ba N Zr N Sr N Cr N Rb N Zn N Ni N Cu trend. Uranium levels were below detection limits in almost all the horizons. The geochemical mass-balance results (Fig. 3a–c) show the patterns of distribution of the elements down the respective pedons: MgO, CaO and Al2O3, followed a strikingly similar pattern down the P1 profile increasing by over 100% from A1 to A2, but decreased in the ABb horizon (Fig. 3a). Oxides P2O5 and Fe2O3 were fairly uniform down the soil profile while MnO depleted with depth. There is neither gain nor loss of Na2O and K2O down-profile (Fig. 3a). There was no significant addition or loss of SiO2 across P1. In the soils of P2, MgO, CaO and Al2O3 were almost uniformly distributed on the four uppermost horizons but were lost in the C2 horizon (Fig. 3b). Similar to P1, P2O5 and Fe2O3 remained evenly distributed. On the other hand, MnO was lost with depth. Despite the high mobility of Na2O and K2O in soils, they neither accumulated nor

4.2. Geochemistry Silicon dioxide (SiO2), the dominant mineral, ranged from 64 to 83.21% (Table 3). The quantities of other element oxides vary

Table 1 Macromorphological properties of alluvial soils in Sabie-Sand River Basin. Horizon

Depth (cm)

Pedon 1 A1 A2 Ab

0–27 27–85 85–103+

Pedon 2 A1 A2 AC C1 C2

0–28 28–49 49–76 76–123 123–200+

Pedon 3 A1 A2 C Ab

0–21 21–44 44–57 57–90+

a b c

Colour (moist)

Structurea

Roots

Boundaryb

Consistence (moist)

Fieldc texture

Cementation

React HCl

Other features

10YR 2/2 (very dark brown) 10YR 3/4 (dark yellowish brown) 10YR 4/3 (dark brown)

1gr 1sbk 2sbk

Many Common Few

gs cw –

Loose Friable Friable

Sand Sand Sand

None None None

Occasional Occasional Occasional

Shells Shells Gravels

5YR 3/3 (dark reddish brown) 7.5YR 5/6 (strong brown) 7.5YR 5/6 (strong brown) 5YR 5/6 (yellowish red) 10YR 5/8 (yellowish brown)

2gr 3sbk 3sbk 3sbk 3m

Many Common Few Occasional None

cs cw gw gw –

Firm Firm Firm Very firm Very firm

Lo.Sa Lo.Sa Sa.Lo Sa.Lo Sand

None None Strong Strong Indurated

Moderate Moderate Strong Strong Strong

Pellets Pellets Crystals Crystals Nodules

10YR 3/4 (dark yellowish brown) 5YR 3/4 (dark reddish brown) 10YR 3/3 (dark brown) 7.5YR 4/4 (brown)

2sbk 2sbk 1sbk 1sbk

Many Common None None

gs cs as –

Firm Firm Friable Loose

Lo.Sa Sand Sand Sand

None None None None

Occasional Occasional Moderate Moderate

Pellets Pellets Gravels Shells

1 – weak; 2 – medium; 3 – strong; gr – granular; sbk – subangular blocky; m – massive. a – abrupt; c – clear; s – smooth; g – gradual; w – wavy. Lo.Sa – loamy sand; Sa-Lo – sandy loam.

316


Table 2 Particle size distribution of the alluvial soils. Horizon

Origin

Coarse sand 500–2000 μm

Medium sand 250–500 μm

Fine sand 150–250 μm

Very fine sand 63–150 μm

Silt 63–2 μm

Clay b2 μm

Meana phi-value

(g kg−1) Pedon 1 A1 A2 Ab

Fluvial Fluvial Fluvial

369 292 139

255 297 259

197 245 377

147 143 213

31 22 11

1 1 1

2.4 2.2 2.1


Fluvial Fluvial Fluvial Fluvial Fluvial

247 285 298 388 531

271 239 232 246 264

275 203 210 197 101

193 247 240 166 99

13 25 19 4 5

1 1 1 0 0

2.3 2.5 2.5 2.3 2.1

Pedon 3 A1 A2 C Ab

Fluvial Fluvial Fluvial Fluvial

300 290 492 74

125 152 301 264

100 124 141 451

438 404 58 190

35 28 9 2

2 2 0 1

3.0 2.7 2.1 1.8

a

Mean phi determined from: Σ25th + 50th + 75th percentiles.

were lost across P2; TiO2 in this profile however was slightly depleted with depth. SiO2 gained with depth in P2, especially in the lowest horizon. Similar to P1, the mass balance of major elements in P3 followed similar patterns in the uppermost horizons, with addition or removal or elements sharply striking a difference in the buried horizon (Ab), which confirms lithological discontinuity brought about by sedimentary deposition (Fig. 3c). There was addition of MgO, CaO and Al2O3 in the Ab horizon of P3 with the upper three horizons, exhibiting a loss from A1 to C. There were no additions or losses in the Na2O and K2O contents of the P3 soils. Similar to P1 and P2, SiO2 in P3 varied very slightly (b 0.6%). The two upper horizons (which are organic matter-rich) and the last two mineral-rich horizons had a similar pattern of P2O5, Fe2O3 and MnO distributions within them. Results of the pedogenic ratios used to evaluate the degree of carbonate and silicate dissolution in the profiles (Table 4) show carbonate dissolution to be more dominant than silicate dissolution in the three soil profiles. This is to be expected considering the alluvial nature of the parent materials which have the potential of containing considerable carbonate-enriched freshwater mollusc shells. The low ratios of Al/ Si in the soils, especially in the C horizons (Table 4) reflect the low clay

values of the soils; they are dominantly sandy, displaying poor to weak soil development and chemical weathering. Salinization is moderate in the soils as seen from the moderate (K + Na)/Al ratios, with the exception of the A1 and A2 horizons of P3. The values for CIA are lower than that of CIW. Both indices have optimum weathered values of 100 and optimum fresh values of ≤ 50. Therefore, CIA results indicate that the soils have undergone less chemical weathering than the estimation of weathering intensity shown by the CIW data. 4.3. Micromorphology The main micromorphological properties of the soils are summarized in Table 5. The overall microstructure of the soils ranges from granular to weakly developed subangular blocky with voids ranging from simple to complex packing voids. The coarse-fine (c/f) related distribution is chitonic with the exception of the A1 and A2 horizons of P3 which are gefuric (Fig. 4a–c). The c/f ratios in the b 20 μm soil fraction vary from 2/8 to 8/1. Quartz, feldspar and fragments of carbonate grains are the main coarse constituents of the soil matrix (Fig. 4a–n). The dominant b-fabric varies from crystallitic to partly speckled and mosaic speckled

Table 3 Major oxides and inorganic carbonate composition of the alluvial soils. SiO2

TiO2

Al2O3

Fe2O3

MnO

MgO

CaO

Na2O

K2O

P2O5

Cr2O3

NiO

LOI

CO3

Profile 1 A1 74.62 A2 70.42 Ab 73.44

0.67 0.88 0.89

13.39 16.07 14.37

0.37 0.41 0.34

0.08 0.05 0.04

0.75 1.57 0.99

1.30 1.98 1.23

2.32 2.15 2.42

2.88 2.83 3.20

0.12 0.06 0.05

0.03 0.05 0.02

0.004 0.013 0.005

5.89 6.78 3.78

1.74 1.69 0.79

Profile 2 A1 A2 AC C1 C2

71.21 71.86 72.12 74.18 81.44

0.99 0.92 0.78 0.69 0.32

15.27 14.91 14.95 14.44 10.29

0.40 0.41 0.39 0.35 0.17

0.07 0.07 0.04 0.04 0.01

0.85 1.00 1.03 0.92 0.39

2.16 2.04 2.12 1.47 0.91

2.71 2.65 2.61 2.49 2.35

2.81 2.45 2.42 2.44 2.30

0.06 0.05 0.04 0.04 0.03

0.02 0.03 0.02 0.02 0.01

0.007 0.006 0.006 0.004 0.007

6.38 5.93 6.17 5.51 2.50

1.06 1.16 1.30 0.96 0.44

Profile 3 A1 A2 C Ab

65.41 64.00 83.21 76.09

1.02 1.06 0.59 0.71

18.70 19.63 8.52 12.51

0.72 0.83 0.19 0.23

0.16 0.19 0.04 0.04

1.25 1.24 0.39 0.49

1.59 1.41 0.91 1.17

2.00 1.66 1.90 2.58

2.64 2.39 2.42 3.38

0.11 0.12 0.04 0.05

0.04 0.04 0.01 0.01

0.015 0.012 0.002 0.002

11.26 11.63 1.66 2.15

1.69 1.99 0.24 0.74

(%)

LOI – Loss on Ignition in a furnace at 950 °C (= chemically bound H2O and CO2). Fe2O3 is expressed as total Fe.


317

Table 4 Minor elemental composition of the alluvial soils. Nb

Zr

Y

Sr

Rb

Ua

Profile 1 A1 0.55 A2 1.22 Ab 0.20

10.52 14.94 14.67

406.56 648.51 518.06

16.98 21.01 12.45

242.83 270.82 261.57

80.27 86.70 87.68

d.l d.l 0.27

Profile 2 A1 1.05 A2 0.19 AC 0.15 C1 0.97 C2 0.25

12.74 10.64 11.18 11.67 5.64

463.79 348.97 299.72 261.36 126.78

18.22 15.78 16.24 14.82 7.50

284.44 309.47 299.24 273.14 231.70

77.37 71.67 66.81 67.31 54.91

d.l d.l d.l d.l d.l

Profile 3 A1 0.76 A2 1.72 C 1.31 Ab 0.94

18.33 17.93 11.37 13.59

427.19 414.78 264.96 471.69

33.47 36.87 10.36 11.46

223.85 190.77 205.88 276.34

87.36 79.90 57.68 84.71

d.l 0.71 d.l d.l

Mo

Th

Pb

Zn

Cu

Ni

Co

Cr

V

4.27 9.07 5.96

15.93 15.56 18.03

44.79 41.58 34.34

31.96 20.02 15.41

39.13 38.04 25.80

16.55 10.31 10.59

182.53 154.36 128.59

63.95 70.33 75.65

8.23 3.64 5.34 9.62 4.39

17.62 15.80 14.64 13.55 13.30

38.09 41.43 38.00 35.17 16.52

31.54 31.80 29.89 19.08 9.18

36.04 39.62 42.86 37.71 13.47

16.30 15.10 14.82 10.00 6.44

144.28 167.05 174.40 150.29 81.88

10.84 12.26 5.49 8.38

23.01 23.75 12.82 15.85

77.17 87.60 22.14 24.96

65.26 79.40 18.28 11.83

68.03 70.08 16.51 14.67

21.28 29.52 6.21 9.97

226.67 211.27 72.39 75.99

Sc

Ba

Ga

8.51 8.16 7.71

560.33 502.32 560.69

16.28 19.83 18.00

79.45 82.04 82.58 70.50 39.77

10.95 10.54 10.57 10.65 4.90

506.45 441.69 439.38 436.99 428.95

17.56 17.94 16.91 16.23 10.74

94.01 96.78 45.77 43.41

11.16 14.25 6.57 3.89

569.55 538.63 504.12 652.77

23.75 23.66 11.76 14.85

(ppm)

a

d.l – below detection limit.

in the C horizons. The alteration degree ranges from 0 to 1 as defined by Bullock et al. (1985). The MISECA values, as calculated from a combination of microstructure, b-fabric, clay coating area and size, decalcified zone area, Fe/Mn oxide and alteration (after Khormali et al., 2003), range from 4 to 8. This implies that the overall soil development in the area is weak. Hypo-coatings are iron oxide and various forms of calcitic pedofeatures, which are mostly microcrystalline impregnative and pure calcitic nodules (Fig. 4d, l, m) where they are present in the soils. The moderately oriented coarser silt-sized materials occurred in the A1 and A2 horizons of the P2 soils (Fig. 4e–g). The presence of organic excreta and plant roots at various stages of decomposition was observed mainly in the upper unit (A1 and A2) horizons of P3 (Fig. 4j, k). The peds (measure of the degree to which adjacent faces are moulds of each other) of the moderately developed horizons are partially accommodated and unaccommodated, as defined by Brewer (1964). 5. Discussion Inferences from soil geochemical and micromorphological properties have demonstrated in this study that the soils on the alluvial terraces of Sabie River are weakly developed (Table 6). Ideally, strongly developed soils should have well-defined horizonation. The little variation in the mean grain size of the soils across and within the soil profiles would suggests that reworking of sediments upwards in individual profiles produced a more uniform pedogenesis rather than coming from different physical sources. Overbank deposition during episodic flood events can cause a change in surface sediment properties (mineralogy, grain size) as well as causing erosion and/or deposition, and these can change soil properties through geochemical processes (Lorz and Phillips, 2006). This phenomenon explains why there is always a sharp change in the distribution of some elements including MgO and CaO between different soil horizons (e.g., Fig. 3a–c), which may reflect the episodic nature of flood impacts. Biochemical secondary carbonates, most likely from freshwater molluscs, play an important role in the soils because their dissolution provides a binding/cementation property which helps in the development of soil structure. These carbonates may also accumulate as calcrete either in situ or as secondary fragments transported by floods (Blumel and Eitel, 1994). Soils in P2 reacted more vigorously with dilute acid and they appear to have a relatively more stable structure. Higher iron oxide contents in a well-drained soil environment could perhaps explain why P2 has a redder value than the other soil samples (e.g., Schwertmann, 1993). LOI values in the top soils of P3 suggest that they most likely have more organic matter than the lower horizons and soils of P1 and P2. The non-decomposed organic matter

enriched horizons most likely stabilized the top of P3, and this could have come from deposition by receding flood waters which are known to transport large amounts of organic debris during floods in this region (Pettit and Naiman, 2005). The soils were developed on allochthonous alluvial parent materials. This is supported by the high quartz, iron and aluminium oxide contents of the soils. Titanium oxide and P2O5 contents were relatively uniform across the soil profiles. In soils, P2O5 and Ti2O are not used in calculating chemical erosion as they contribute a very small fraction of the total long-term average, and should be of minor importance with respect to present weathering rates (Kholopova, 1980). Both CIW and CIA are fundamentally a measure of the conversion of feldspar to clay, which occurs primarily through chemical weathering. However, CIA gave a better representation of what is demonstrated by the soil profiles when compared to CIW (Table 4) in terms of the degree of chemical weathering. However, when evaluating chemical weathering in heterogeneous weathering residua, it is always better to apply weathering indices which take into consideration all the mostly mobile elements during weathering (i.e., alkali and alkaline earth metals) (Price and Velbel, 2003). Aluminium is leached out in soils preferentially with respect to silicon; the low Al/Si ratios of the soils also confirm the low clayeyness of the soils (Table 4). The low values of these indices in the soils suggest low levels of chemical weathering. Salinization is, however, quite significant in the soils as seen from the (K + Na)/Al ratios (Table 4). Increased sodium content is known to hinder aggregation of soil particles (García-Orenes et al., 2005) and this could explain why soils on the upper horizons of P3 have more stable structure because they contain lower Na. Micromorphology has further demonstrated that pedogenesis is not advanced in these soils, as seen from the weak pedality controlled by the high (c/f) ratios of the groundmass. Weakly developed micromass with crystallitic b-fabrics in the soils may not signify advanced weathering, but rather the nature of the parent material. Clay coatings indicate illuviation and are known to help in soil structure formation. The absence of clay coatings on the soils suggests that they possibly developed under dry conditions which could not have promoted translocation of colloidal materials down the profile. The classification of degree of soil development, according to MISECA values, suggests that all the soil horizons are weakly developed. Soil development requires that the rate of material deposition and removal be in equilibrium. This, however, is not the case here on the alluvial terraces as they are prone to episodic events including flooding. This could have gone a long way in distorting the morphology and overall bulk soil geochemistry of the area. There is strong evidence of the effects of aggradation-degradation cycles related to climate and flood events in the area, as seen from the

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Fig. 3. Geochemical mass-balance plots of the pedons: a) P1; b) P2; and c) P3.

P.N. Eze et al. / Geomorphology 268 (2016) 312–321 Table 5 Pedogenic ratios and chemical weathering indices for the alluvial soils. Horizon

K/Caa

K/Nab

Al/Sic

(K + Na)/Ald

CIAe

CIWf

Pedon1 A1 A2 Ab

1.32 0.85 1.55

0.82 0.87 0.87

0.11 0.13 0.12

0.52 0.41 0.52

67.32 69.78 67.72

78.72 79.55 79.74


0.77 0.71 0.68 0.99 1.50

0.68 0.61 0.61 0.64 0.64

0.13 0.12 0.12 0.11 0.07

0.49 0.47 0.46 0.47 0.62

66.54 67.62 67.65 69.29 64.92

75.82 76.07 75.97 78.48 75.94

Pedon 3 A1 A2 C Ab

0.99 1.01 1.58 1.72

0.87 0.95 0.84 0.86

0.17 0.18 0.06 0.10

0.33 0.27 0.67 0.63

75.01 78.24 61.96 63.70

83.89 86.48 75.20 76.94

a b c d e f

Carbonate dissolution. Silicate dissolution. Clayeyness. Salinization. Chemical Index of Alteration (CIA) = 100 × [Al2O3/(Al2O3 + CaO + Na2O + K2O). Chemical Index of Weathering (CIW) = 100 × [Al2O3/(Al2O3 + CaO + Na2O).

horizon designations. The soil profiles of P1 and P3 formed on the marginal parts of the alluvial terraces have signs of overlapping of two different pedogenic events. The buried (Ab) horizons show that there were erosion events which eroded the A horizons of P1 and P3 and then deposited transported materials on top. The upper units of the profiles are enriched with freshwater shells embedded in coarse alluvium which superimpose on lower units. We interpret this as a result of extension of alluvial plain deposition caused by episodic floods, which impact on the areas previously occupied by upland grassland ecosystems and soils. We also explain the partial cementation of P2 groundmass (originally carbonate-free) by crystals of secondary calcite to the same landscape dynamics, which led to the periodic saturation of soils with surface water, from which calcium carbonate could precipitate. This is similar to a model for silcrete formation proposed for the (climaticallysimilar) Okavango delta region, Botswana (Shaw and Nash, 1998). The time of these environmental changes along the Sabie River needs to be precisely established. Relative dating using archaeological evidence is one such approach (e.g., Kuman et al., 2005). There is no independent archaeological or instrumental geochronology for the soil units, but it is likely that the catastrophic river flood of 2001 (Heritage et al., 2001) may be preserved as a laterally-extensive marker horizon. The driving force of Sabie River geomorphic dynamics also is uncertain, and could be a combination of climate change, changing groundwater table position, ecosystem changes, KNP management practices, or a combination of these. 6. Conclusions Using geochemical and micromorphological proxies, this study has provided evidence for recent environmental changes and soil development patterns in the Sabie-Sand River Basin complex. The soils on the alluvial terraces of the Sabie River bear imprints of recent extreme flood events, as seen from soil weak horizonation. The upper horizons are dynamic formations. In this area, deposition and sedimentation processes which are driven by episodic river flooding cycles account for the transient, weak pedality, weakly stable structure of the soils, and hence

Fig. 4. Photomicrographs of the soils: a) Pedon 1, A1 horizon (PPL); b) a under XPL; c) Pedon 1, A2 horizon; d) higher magnification of c; e) Pedon 2, A1 horizon; f) higher magnification of “e”; g) Pedon 2, A2 horizon; h) Pedon 2, Bk1 horizon; i) Pedon 2 Ck horizon (PPL); j) XPL of i; k) Pedon 3, A1 horizon; l) Pedon 3 A2 horizon; m) Pedon 3, ABb horizon (XPL); and n) “m” under PPL.

319

320


Table 6 Micromorphological properties of the selected horizons. Horizon Microstructure and voids

c/f ratio (20 μm)

Micromass, b-fabric colour and limpidity

Weakly developed crumbs with simple packing voids; chitonic

8/1

Granostriated (mostly) and 0 partly speckled (very few) brown

6, weakly developed

Weakly developed subangular blocky (sbk), simple packing voids

8/2

As above

0

5, weakly developed

7/3

As above

1

7/3

As above

1

2/8

1

Ck

Granules

9/1

Crystallitic (mostly) and partly brown mosaic speckled Granostriated

7, weakly developed 8, weakly developed 8, weakly developed 4, weakly developed

Hypo-coatings of iron oxide; and few dark reddish mottles; OM As above

AC

Moderately developed sbk with complex packing voids; chitonic Moderately developed sbk with complex packing voids; chitonic As above

Weakly developed sbk with complex packing voids; gefuric As above Chitonic; weakly developed sbk with channels and complex packing voids

2/8

Crystallitic reddish brown speckled As above Granostriated and partly mosaic speckled

0

6, weakly developed As above As above

Very few quasi coating of coarse clay; organic matter excrement As above, with infillings of calcite Loose continuous infillings and coating of iron oxide; calcite nodule

Pedon 1 A1

A2

Pedon 2 A1 A2

Pedon 3 A1 A2 Ab

2/8 3/7

the considerable heterogeneity shown in the microfabrics of the soils. These properties clearly demonstrate the impact of climate-driven variations in river flooding as a soil-forming factor in the Sabie-Sand River Basin. To correlate the observed environmental changes within this catchment to climatic events on the scale of southern Africa, more accurate dating of the soil units is needed. For alluvial soils formed from coarse sandy parent materials, the Chemical Index of Alteration (CIA) provides a more consistent proxy to assess the degree of chemical weathering. MISECA values, weak horizonation and pedality, and absence of significant clay coatings in the micromass of the soils show that the present regional warm, dry climate conditions coupled with intermittent deposition and sedimentation of materials after flood events could not have promoted pedogenesis; hence the soils are generally weakly developed. MISECA as a micromorphological index for soil development is suitable for soils in this and other areas with similar climatic and geomorphic conditions. The methods and results of this study could thus be applied to studies of environmental change in alluvial basins elsewhere in the world. Acknowledgements This project was funded by the National Research Foundation (South Africa), grant 91344 (to JK). Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version, at doi: http://dx.doi.org/10.1016/j.scitotenv.2016.06. 120. These data include the Google map of the most important areas described in this article. References Adams, J.S., Kraus, M.J., Wing, S.L., 2011. Evaluating the use of weathering indices for determining mean annual precipitation in the ancient stratigraphic record. Palaeogeogr. Palaeoclimatol. Palaeoecol. 309, 358–366. Aslan, A., Autin, W.J., 1998. Holocene flood-plain soil formation in the southern lower Mississippi Valley: implications for interpreting alluvial paleosols. Geol. Soc. Am. Bull. 110, 433–449. Barron, V., Torrent, J., 1984. Influence of aluminum substitution on the color of synthetic hematites. Clay Clay Miner. 32, 157–158. Blumel, W.D., Eitel, B., 1994. Tertiary calcic sediment covers and calcretes in Namibia origin and geomorphic significance. Z. Geomorphol. 38, 385–403. Brewer, R., 1964. Fabric and Mineral Analysis of Soils. Wiley, New York, N.Y. (470 pp).

Alteration MISECA (soil degree development)

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0 0

Pedofeatures

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As above, but with hypocalcite coatings None observed

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