Mineralogical transformations set slow weathering rates in low-porosity metamorphic bedrock on mountain slopes in a tropical climate

Abstract

Originally published as:Behrens, R.; Bouchez, J.; Schuessler, J. A.; Dultz, S.; von Blanckenburg, F. (2015): Mineralogical transformations set slow weathering rates in low-porosity metamorphic bedrock on mountain slopes in a tropical climate. The spheroidal weathering produces rounded corestones and spalled rindlets at the rock-saprolite interface. We used detailed textural, mineralogical and chemical analyses to reconstruct the sequence of weathering reactions and their causes. The first mineral attacked by weathering was found to be pyroxene initiated by in situ Fe oxidation. Volumetric calculations suggest that this oxidation leads to the generation of porosity due to the formation of micro-fractures allowing for fluid transport and subsequent dissolution of biotite and plagioclase. The rapid ensuing plagioclase weathering leads to formation of high secondary porosity in the corestone over a distance of only a few cm and eventually to the final disaggregation of bedrock to saprolite. The first secondary phases are oxides or amorphous precipitates from which secondary minerals (mainly gibbsite, kaolinite and goethite) form. As oxidation is the first weathering reaction, the supply of O 2 is a rate-limiting factor for chemical weathering. Hence, the supply of O 2 and its consumption at depth connects processes at the weathering front with those at the Earth's surface in a feedback mechanism. The strength of the feedback depends on the relative weight of advective versus diffusive transport of O 2 through the weathering profile. The feedback will be stronger with dominating diffusive transport. The low weathering rate is explained by the nature of this feedback that is ultimately dependent on the transport of O 2 through the whole regolith, and on lithological factors such as low bedrock porosity and the amount of Fe-bearing primary minerals. Tectonic quiescence in this region and low pre-development erosion rate (attributed to a dense vegetation cover) minimize the rejuvenation of the thick and cohesive regolith column, finally leading to low denudation rates.

Toward a conceptual model relating chemical reaction fronts to water flow paths in hills

“…This in turn suggests that feedback mechanisms may couple erosion at the land surface to weathering and vice versa in some locations (e.g., Carson and Kirkby, 1972;Stallard, 1995;Fletcher et al, 2006;Lebedeva et al, 2007;Fletcher and Brantley, 2010;Behrens et al, 2015). Thus, regolith-mantled systems may not constantly maintain W = E, but they may commonly be moving toward such a steady state, driven by feedbacks.…”

“…Feedbacks could include the effects of porewater chemistry, soil gas chemistry, particle size or fracture spacing (Fletcher et al, 2006;Fletcher and Brantley, 2010;Behrens et al, 2015).…”

Mineralogical Transformations and Soil Development in Shale across a Latitudinal Climosequence

“…Some mineral weathering reactions, such as carbonate dissolution, pyrite oxidation or plagioclase feldspar dissolution, are described as profile initiating reactions that begin the process of disaggregating bedrock into regolith; weathering reactions such as clay dissolution, however, are likely more important in controlling the overall regolith thickness . Indeed, studies on various lithologies have documented the deepest weathering reactions as carbonate dissolution (White et al, 2005;Williams et al, 2007;Jin et al, 2010), biotite oxidation (Buss et al, 2008;Behrens et al, 2015;Bazilevskaya et al, 2015), or plagioclase feldspar dissolution (Brantley and White, 2009;Behrens et al, 2015) but these minerals sometimes constitute a small fraction of the overall parent mineralogical composition.…”

“…An increasing number of quantitative weathering models are advancing our ability to predict weathering advancement and landscape evolution (e.g., Godderis et al, 2006;Lebedeva et al, 2007Lebedeva et al, , 2010Brantley et al, 2008;Minasny et al, 2008;Maher, 2010;Brantley and Lebedeva, 2011;Bazilevskaya et al, 2013Bazilevskaya et al, , 2015 but field observations of soil thickness, geochemistry, and mineralogy are needed across a range of environmental gradients to validate these models (Behrens et al, 2015). In an effort to investigate soil formation as a function of climate, a climosequence of sites was established on residual shale parent material in the Northern Hemisphere.…”

Tank Cascade Systems as a Sustainable Measure of Watershed Management in South Asia

“…The weathering grade of the parent metamorphic bedrock is locally highly heterogeneous, resulting from the petrographic character and small scale changes in environmental conditions [39][40][41]. The transition between saprolite and superimposing allochtonous deposits is characterized in profiles NA01 and NA03 by a diffuse change from gravels in a matrix of sandy loam to alternating layers of sandy and loamy material at a depth of 170 cm b. s. (profile NA01) and 200 cm b. s. (profile NA03).…”