TY - JOUR A1 - Steinhöfel, Grit A1 - Breuer, Jörn A1 - von Blanckenburg, Friedhelm A1 - Horn, Ingo A1 - Sommer, Michael T1 - The dynamics of Si cycling during weathering in two small catchments in the Black Forest (Germany) traced by Si isotopes JF - Chemical geology : official journal of the European Association for Geochemistry N2 - Silicon stable isotopes have emerged as a powerful proxy to investigate weathering because Si uptake from solution by secondary minerals or by the vegetation causes significant shifts in the isotope composition. In this study, we determined the Si isotope compositions of the principle Si pools within two small catchments located on sandstone and paragneiss, respectively, in the temperate Black Forest (Germany). At both settings, clay formation is dominated by mineral transformation preserving largely the signature of parental minerals with delta Si-30 values of around -0.7%. Bulk soils rich in primary minerals are similar to bulk parental material with delta Si-30 values close to -0.4%. Topsoils are partly different because organic matter degradation has promoted intense weathering leading to delta Si-30 values as low as -1.0%. Water samples expose highly dynamic weathering processes in the soil zone: 1) after spring snowmelt, increased release of DOC and high water fluxes trigger clay mineral dissolution which leads to delta Si-30 values down to -0.7% and 2) in course of the summer, Si uptake by the vegetation and secondary mineral formation drives dissolved Si to typical positive delta Si-30 values up to 1.1%. Groundwater with delta Si-30 values of around 0.4% records steady processes in bedrock reflecting plagioclase weathering together with kaolinite precipitation. An isotope mass balance approach reveals incongruent weathering conditions where denudation of Si is largely driven by physical erosion. Erosion of phytoliths contributes 3 to 21% to the total Si export flux, which is in the same order as the dissolved Si flux. These results elucidate the Si dynamics during weathering on catchments underlain of sedimentary origin, prevailing on the Earth surface and provide therefore valuable information to interpret the isotope signature of large river systems. KW - Weathering KW - Sedimentary rocks KW - Biogeochemical Si cycle KW - Silicon isotopes KW - UV femtosecond laser ablation Y1 - 2017 U6 - https://doi.org/10.1016/j.chemgeo.2017.06.026 SN - 0009-2541 SN - 1878-5999 VL - 466 SP - 389 EP - 402 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Oeser, Ralf Andreas A1 - Stroncik, Nicole A1 - Moskwa, Lisa-Marie A1 - Bernhard, Nadine A1 - Schaller, Mirjam A1 - Canessa, Rafaella A1 - van den Brink, Liesbeth A1 - Köster, Moritz A1 - Brucker, Emanuel A1 - Stock, Svenja A1 - Pablo Fuentes, Juan A1 - Godoy, Roberto A1 - Javier Matus, Francisco A1 - Oses Pedraza, Romulo A1 - Osses McIntyre, Pablo A1 - Paulino, Leandro A1 - Seguel, Oscar A1 - Bader, Maaike Y. A1 - Boy, Jens A1 - Dippold, Michaela A. A1 - Ehlers, Todd A1 - Kühn, Peter A1 - Kuzyakov, Yakov A1 - Leinweber, Peter A1 - Scholten, Thomas A1 - Spielvogel, Sandra A1 - Spohn, Marie A1 - Ubernickel, Kirstin A1 - Tielbörger, Katja A1 - Wagner, Dirk A1 - von Blanckenburg, Friedhelm T1 - Chemistry and microbiology of the Critical Zone along a steep climate and vegetation gradient in the Chilean Coastal Cordillera JF - Catena : an interdisciplinary journal of soil science, hydrology, geomorphology focusing on geoecology and landscape evolution N2 - From north to south, denudation rates from cosmogenic nuclides are similar to 10 t km(-2) yr(-1) at the arid Pan de Aziicar site, similar to 20 t km(2) yr(-1) at the semi-arid site of Santa Gracia, -60 t km(-2) yr(-1) at the Mediterranean climate site of La Campana, and similar to 30 t km(-2) yr(-1) at the humid site of Nahuelbuta. A and B horizons increase in thickness and elemental depletion or enrichment increases from north (similar to 26 degrees S) to south (similar to 38 degrees S) in these horizons. Differences in the degree of chemical weathering, quantified by the chemical depletion fraction (CDF), are significant only between the arid and sparsely vegetated site and the other three sites. Differences in the CDF between the sites, and elemental depletion within the sites are sometimes smaller than the variations induced by the bedrock heterogeneity. Microbial abundances (bacteria and archaea) in saprolite substantially increase from the arid to the semi-arid sites. With this study, we provide a comprehensive dataset characterizing the Critical Zone geochemistry in the Chilean Coastal Cordillera. This dataset confirms climatic controls on weathering and denudation rates and provides prerequisites to quantify the role of biota in future studies. KW - Weathering KW - Denudation KW - Microbial abundance KW - Climate KW - Chile Y1 - 2018 U6 - https://doi.org/10.1016/j.catena.2018.06.002 SN - 0341-8162 SN - 1872-6887 VL - 170 SP - 183 EP - 203 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Behrens, Ricarda A1 - Bouchez, Julien A1 - Schuessler, Jan A. A1 - Dultz, Stefan A1 - Hewawasam, Tilak A1 - von Blanckenburg, Friedhelm T1 - Mineralogical transformations set slow weathering rates in low-porosity metamorphic bedrock on mountain slopes in a tropical climate JF - Chemical geology : official journal of the European Association for Geochemistry N2 - In the Sri Lankan Highlands erosion and chemical weathering rates are among the lowest for global mountain denudation. In this tropical humid setting, highly weathered deep saprolite profiles have developed from high-grade metamorphic charnockite during spheroidal weathering of the bedrock. 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. (C) 2015 Elsevier B.V. All rights reserved. KW - Weathering KW - Critical zone KW - Corestone KW - Regolith KW - Sri Lanka Y1 - 2015 U6 - https://doi.org/10.1016/j.chemgeo.2015.07.008 SN - 0009-2541 SN - 1878-5999 VL - 411 SP - 283 EP - 298 PB - Elsevier CY - Amsterdam ER -