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キーワード:
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要旨:
The spacing of ridges and valleys reflects the competition between diffusive transport on hill slopes and advective transport in channels, although the underlying lithologic, tectonic, and climatic controls have not been untangled. Here, we measure geochemical and geomorphic properties of catchments in Kruger National Park, where granitic lithology and erosion rates are invariant, enabling us to evaluate how varying rainfall (MAP=470, 550, and 730mm) impacts ridge-valley spacing. Catchment-average erosion rates, based on 10Be concentrations in river sands, are low (3 to 6 m My-1) and vary minimally across the three sites. Our lidar-based, slope-area trends reveal that hill slopes in the dry site are gentle (3%) and short (area of hill-valley transition is 75m2). The terrain has low relief and is highly dissected by small channels. With increasing rainfall, hill slopes lengthen (transition area increases from 220 to 700m2) and increase in gradient (6 to 8%), resulting in fewer, higher relief catchments. The chemical depletion fraction of ridge-crest regoliths increases with rainfall from 0.3 to 0.7, indicating progressive increase in chemical relative to physical erosion. In the dry site, catenas are relatively undifferentiated, but intermediate and wet site catenas have leached sandy crests and upper side slopes complemented by clay accumulation zones further down slope. The texture of this granitic landscape appears to be set by climate-driven feedbacks among chemical weathering, regolith fabric differentiation and hydrological routing that enhances the vigor of diffusive transport relative to advective transport.
