Shaping post-orogenic landscapes by climate and chemical weathering

Abstract

The spacing of hills and valleys refl ects the competition between disturbance-driven (or diffusive) transport on hillslopes and concentrative (or advective) transport in valleys, 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, South Africa, where granitic lithology and erosion rates are invariant, enabling us to evaluate how varying mean annual precipitation (MAP = 470 mm, 550 mm, and 730 mm) impacts hill-valley spacing or landscape dissection. Catchment-averaged erosion rates, based on 10 Be concentrations in river sands, are low (3-6 m/m.y.) and vary minimally across the three sites. Our lidar-derived slope-area analyses reveal that hillslopes in the dry site are gentle (3%) and short, such that the terrain is low relief and appears highly dissected. With increasing rainfall, hillslopes lengthen and increase in gradient (6%-8%), resulting in less-dissected, higher-relief catchments. The chemical depletion fraction of hilltop regoliths increases with rainfall, from 0.3 to 0.7, refl ecting a climate-driven increase in chemical relative to physical erosion. Soil catenas also vary systematically with climate as we observe relatively uniform soil properties in the dry site that contrast with leached sandy crests and upper slopes coupled with downslope clay accumulation zones in the intermediate and wet sites. The geomorphic texture of this slow-eroding, granitic landscape appears to be set by climate-driven feedbacks among chemical weathering, regolith fabric differentiation, hydrological routing, and sediment transport that enhance the vigor of hillslope sediment transport relative to valley-forming processes for wetter climates.

Timescales of carbon turnover in soils with mixed crystalline mineralogies

“…Soil residence times, estimated from average regolith depth and erosion rates determined using cosmogenic isotopes are > 10 5 years (Chadwick et al, 2013), providing ample time for crystalline mineral differentiation, ripening, and depletion of metastable SRO minerals. In addition to strong geological differences across KNP, variation in clay mineralogy is imposed by a regional north-south gradient in rainfall that ranges from about 470 to 740 mm annually, and locally by differentiation of clay content along hillslopes.…”

“…Low rates of landscape erosion and exceptionally long soil residence times (Chadwick et al, 2013) ensure that nearly all soil minerals have evolved past the metastable SRO stage and that there are few free trivalent metal ions available for direct sorption by organic ligands (Khomo et al, , 2012. We evaluate radiocarbon ( 14 C) in bulk soil, and fractions separated by density into free particulate and mineral-associated components.…”

Quantification of colloidal and aqueous element transfer in soils: The dual-phase mass balance model

“…Briefly, the catena is in a semiarid, monsoon rainfall regime ($550 mm mean annual rainfall) and extends 150 m along a gentle (65%) slope underlain by Archaean granite and gneiss of the Nelspruit suite within Kruger National Park, South Africa (Barton et al, 1986). Dissolved and suspended material flow through the sandy, quartz-rich skeleton formed from the granitic parent material, and low denudation rates (catchment derived rate = 6.6 ± 1.0 m/m.y., Chadwick et al, 2013) have allowed unusually well-differentiated catenas to form throughout much of the landscape (Venter, 1986;Khomo et al, 2011Khomo et al, , 2013. The catena has three zones: a clay-poor upslope position with reddish soils, a clay-poor seepline position with whitish soils, and a clay-rich downslope position with graybrown soils.…”

The Influence of Lithology on the Forms of Hillslopes and Drainage Basins in Soil Mantled Landscapes

“…Although this view of soil-mantled landscape development appears sensible in many natural landscapes (Perron et al, 2009;Chadwick et al, 2013), past research on soils, and recent research on processes of bedrock disaggregation challenge the view that the bedrock underlying a landscape is always more resistant to incision than the soils formed from it. In some arid environments soils may be stabilized by silica or carbonate cements that have been precipitated during soil formation (e.g., Machette, 1985), which can entomb landscape forms as erosion is retarded (Alonso-Zarza et al,…”