Weathering and soil properties on old granitic catenas along climo-topographic gradients in Kruger National Park
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Date
2009-09-08T07:37:05Z
Authors
Khomo, Lesego
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Abstract
Catenas, the geomorphologically and hydrologically linked soils on hillslopes between crests and toeslopes, are arguably the best heuristic tool to underpin pedological work for understanding soil landscape evolution. Under this pretext, I investigated the expression of soil forming factors in shaping weathering and soil development in Kruger National Park’s diverse granitic landscapes. The quantitative investigations in four data chapters are preceded by narrative on how the soil landscape can most compellingly be viewed in pedological research. This synthetic perspective provides an interdisciplinary framework derived from the fusing long-standing pedological, geological and geomorphological thinking. Briefly, the framework articulates (1) the origin and fate of rock derived soil constituents; (2) how those constituents, primarily clay minerals and soil’s dissolved load, interact and react to the soil forming factors viz. climate, rock type and topography and (3) the distribution of soil properties along gradients of soil forming factors.
My field sites were located in Kruger National Park, on the eastern seaboard of South Africa, nestled between the Great Escarpment to the west and the Lebombo Mountains to the east along the Mozambican border. Kruger affords an opportune test-case to study the interaction of climate, topography and geology on soil landscape patterns in a model ecosystem that acts as a natural laboratory. Kruger functions as a model ecosystem for earth scientists because physiographic gradients are well demarcated and can be studied iteratively by focusing on the variation in one gradient while others are kept constant. For example, the north-south axis of the park separates granite from basalt; and rainfall increases from north to south. Therefore the park has four well-bounded blocks within which the interaction of rainfall and geology can be investigated. Also, the influence of rainfall on soil properties can be isolated by fixing geology and varying climate, as illustrated in the first data chapter of the thesis.
There, I used new and established tools to set boundaries on granitic weathering and soil development. In addition to constraining geology along the 400 to 700 mm mean annual rainfall gradient, the topographic dial on weathering was also set to zero by restricting sampling to crest soils. In this way, the sole impact of increasing rainfall on weathering and soil development was examined in exclusion of added complications imposed by geology and topography. Results suggested by geochemical mass balance, cosmogenic 10Be concentrations as an erosion proxy, soil exchange properties and clay amount and mineralogy all demonstrated indiscriminant loss of rock-derived constituents as rainfall increases from north to south. This means that granitic crests in Kruger are losing material under all climatic regimes, from dry in the north to wet in the south. The next question then was: what is the destiny of the material lost from crests? Does the material remain on the catenas or is it lost to the rivers for transport towards the ocean? How are the patterns different on catenas in distinct topographic contexts? These questions were the subjects of subsequent chapters in the thesis.
Therein, analyses for crests were extended to capture patterns between those crests and toeslopes across three relief classes in each of three climate zones for a nested matrix of nine catenas. Material removal from crest soils was evident across all nine catenas, but was nuanced by depletion and accumulation zones whose extents were context-dependent within the topo-vii
climatic matrix. Depletion zones invariably occurred at or near crests whereas accumulation zones were near toeslopes in all catenas across climate and relief. Depletions were most expressed by base cation and silicon losses while zones of accumulation were of heightened concentrations of aluminium and titanium. In terms of concrete soil properties, the depleted soils were clay-poor with low exchangeable base cation concentrations whereas soils at distal ends of catenas were high in both clay and exchangeable base cations. These general patterns were modified by the amount of rainfall and hillslope relief. Clay-rich zones were narrow in both high and low rainfall areas but widest at intermediate rainfall. Similarly with relief, it appeared that intermediate steepness facilitated the retention of material on catenas. Hence, coupled rainfall-topography interactions provide a thrust on material transfers on catenas resulting in a spectrum of catena types across gradients in soil forming factors. Hence, the catena is far from the static entity it is commonly imagined to be, but is dynamic if quantified across gradients of variation in soil forming factors.