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.

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<p>Secondary minerals such as iron and aluminum (oxyhydr)oxides are a well-known key factor determining the accumulation and persistence of organic carbon (OC) in soil. Manganese (Mn) oxides, although being less abundant in soil than other oxide minerals, may also bind and stabilize organic matter. In addition, they exhibit a high redox activity that may promote oxidation of refractory organic compounds into substrates easily available to microorganisms. However, little is known about the adsorption and oxidation of dissolved OC (DOC) by Mn oxides. Therefore, we investigated the adsorption of dissolved organic matter (DOM) to vernadite, acid birnessite, and cryptomelane, by varying DOM type (beech and pine-derived), pH (4 and 7), and background electrolyte composition (no salt addition, 0.01 M NaCl or CaCl<sub>2</sub>). Preliminary results show that the extent and kinetics of DOM adsorption as well as oxidative DOM transformation strongly differed with Mn oxides and sorption conditions. Overall, DOM adsorption was higher at pH 4 than at pH 7. Vernadite was most sorptive, retaining 68% to 85% of added DOC at pH 4. At pH 7, on average 30% less DOC was adsorbed by Mn oxides. After reaction, reduced specific ultraviolet absorbance at 280 nm of DOM indicates preferential adsorption of aromatic moieties. Contact of DOM with Mn oxides also resulted in high concentrations of dissolved low-molecular-weight (LMW) organic acids, consisting mainly of formic, acetic, oxalic, and citric acid. In addition, we will present results from liquid chromatography/Fourier transform ion cyclotron resonance mass spectrometry and X-ray diffraction on the molecular transformation of reacted DOM and reductive changes of reacted Mn oxides, respectively. Consequently, interactions of DOM and Mn oxides may promote selective sorptive stabilization of organic matter as well as support microbial growth due to oxidative production of easily available organic compounds.</p>