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.

Weathering is both an acid-base and a redox reaction in which rocks are titrated by meteoric carbon dioxide (CO 2 ) and oxygen (O 2 ). In general, the depths of these weathering reactions are unknown. To determine such depths, cuttings of Rose Hill shale were investigated from one borehole from the ridge and four boreholes from the valley at the Susquehanna Shale Hills Observatory (SSHO). Pyrite concentrations are insignificant to depths of 23 m under the ridge and 8-9 m under the valley. Likewise, carbonate concentrations are insignificant to 22 and 2 m, respectively. In addition, a 5-6 m-thick fractured layer directly beneath the land surface shows evidence for loss of illite, chlorite, and feldspar. Under the valley, secondary carbonates may have precipited.The limited number of boreholes and the tight folding make it impossible to prove that depth variations result from weathering instead of chemical heterogeneity within the parent shale. However, carbonate depletion coincides with the winter water table observed at~20 m (ridge) and~2 m depth (valley). It would be fortuitous if carbonate-containing strata are found under ridge and valley only beneath the water table. Furthermore, pyrite and carbonate react quickly and many deep reaction fronts for these minerals are described in the literature. We propose that deep transport of O 2 initiates weathering at SSHO and many other localities because pyrite commonly oxidizes autocatalytically to acidify porewaters and open porosity. According to this hypothesis, the mineral distributions at SSHO are nested reaction fronts that overprint protolith stratigraphy. The fronts are hypothesized to lie subparallel to the land surface because O 2 diffuses to the water table and causes oxidative dissolution of pyrite. Pyrite-derived sulfuric acid (H 2 SO 4 ) plus CO 2 also dissolve carbonates above the water table. To understand how reaction fronts record long-term coupling between erosion and weathering will require intensive mapping of the subsurface.

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The standard sieving, pipette, and hydrometer methods for soil particle size analysis (PSA) have three main drawbacks: (i) the procedures are tedious, (ii) the procedures are time consuming, (iii) and the results are protocol dependent. Laser diffraction PSA delivers rapid results using standardized procedures, but so far it has been difficult to reconcile results with those from standard sedimentation methods. The objective of this study was to develop a protocol that would permit direct usage of laser diffraction PSA and render results compatible with current methods. The protocol was developed using 54 standard soil samples from different textural classes. Regression of the laser diffraction PSA against the hydrometer/pipette method yielded R2 values of 0.92/0.9, 0.92/0.94, and 0.99/0.99 and RMSE values of 0.04/0.05, 0.07/0.06 and 0.05/0.03 for clay, silt, and sand, respectively. These statistics are comparable to those obtained by regressing results of the hydrometer against the sieve and pipette methods. A key factor in securing accurate and precise results was limiting the particle size range of the samples by wet sieving the sand fraction. This created representative samples and stable soil dispersed suspensions, allowing accurate estimations of particle size distribution for clay and silt fractions without empirical transformations. Results obtained with the proposed protocol matched those of standard sedimentation analyses for a wide range of soils, encouraging further adoption of laser diffraction for soil PSA. Core Ideas Laser diffraction particle size analysis can produce results compatible with standard pipette and hydrometer methods. A key step is to wet‐sieve the sand fraction after suspending the soil sample in the dispersant solution. The proposed protocol is faster, uses smaller samples, and provides more detail than standard sedimentation methods.