Where fast weathering creates thin regolith and slow weathering creates thick regolith

Abstract

Weathering disaggregates rock into regolith – the fractured or granular earth material that sustains life on the continental land surface. Here, we investigate what controls the depth of regolith formed on ridges of two rock compositions with similar initial porosities in Virginia (USA). A priori, we predicted that the regolith on diabase would be thicker than on granite because the dominant mineral (feldspar) in the diabase weathers faster than its granitic counterpart. However, weathering advanced 20× deeper into the granite than the diabase. The 20 × ‐thicker regolith is attributed mainly to connected micron‐sized pores, microfractures formed around oxidizing biotite at 20 m depth, and the lower iron (Fe) content in the felsic rock. Such porosity allows pervasive advection and deep oxidation in the granite. These observations may explain why regolith worldwide is thicker on felsic compared to mafic rock under similar conditions. To understand regolith formation will require better understanding of such deep oxidation reactions and how they impact fluid flow during weathering. Copyright © 2012 John Wiley & Sons, Ltd.

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

“…Although steady-state is actually observed at Hakgala, we note that in settings where even deeper regolith might form the advance of the weathering front might be totally decoupled from erosion at the surface and regolith might accumulate . Interestingly, our weathering profile at Hakgala fits well with the observation made by Bazilevskaya et al (2013) that slow weathering profiles developed on granitoid lithology obtain similar thicknesses of mostly 10 -20 m.…”

“…Exploring the way these pathways are generated is important because their understanding is fundamental to quantitative models of the weathering front advance (Bazilevskaya et al, 2013(Bazilevskaya et al, , 2015Brantley et al, 2008;Goddéris et al, 2006;Lebedeva et al, 2007;Moore et al, 2012;Navarre-Sitchler et al, 2011). These models essentially characterize the transport of solutes and gases through regolith profiles and the participating weathering reactions.…”

Relating Weathering Fronts for Acid Neutralization and Oxidation to pCO2 and pO2

“…Regolith exposed to weathering for more than 10 My on diabase and granitoid ridgetops in the Piedmont of Virginia comprise completely developed profiles weathering profiles from the Piedmont, VA (United States) (data from Pavich et al (1989) and Bazilevskaya et al (2013) x ð Þ values were multiplied by ten to aid in visualization. Acid-generating capacity (stars) and CaCO 3 concentrations (gray rectangles) are shown in diabase (g) and granite (h) profiles.…”

Mineralogical Transformations and Soil Development in Shale across a Latitudinal Climosequence

“…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.…”