Constraining soil formation and development through quantitative analyses of chemical weathering and physical erosion

At the Earth surface, the terrestrial life is supported by a thin soil mantle. Human activities can alter soil physical and chemical properties to the extent that humans are now identified as the sixth soil forming factor. Yet, quantitative data on natural soil development processes are hardly needed to distinguish anthropogenic from natural changes in the soil system. This dissertation aims to provide new insights on soil development on time scales of 103 to 105 years for two critical zone observatories: the Betic Cordillera (southeast Spain) and the Rio Grande Do Sul (southern Brazil). In the first part, the coupling between chemical weathering and physical erosion is analyzed based on chemical depletion and total denudation. In the semi-arid environment of the Betic Cordillera, soil weathering extents increase with soil thickness and decrease with increasing surface denudation rates, consistent with kinetically limited weathering. In the deeply weathered soils of southern Brazil, the soil development is strongly controlled by local topography, with increasing chemical depletion downslope. In the second part, the widely accepted concept of steady state soil thickness is challenged for thin soil profiles based on analytical data from in-situ produced cosmogenic nuclides and U-series disequilibria. In the third part, meteoric 10Be is used as a geochemical tracer for quantifying long-term soil denudation rates. Given the low retention of meteoric 10Be in highly weathered soil profiles, a new laboratory protocol involving sequential extraction of Be from the mineral reactive phases is developed and tested. The dissertation demonstrates the potential of employing multiple geochemical techniques to provide new insights in the rates of soil development, chemical weathering and surface denudation.(SC - Sciences) -- UCL, 201

Long-term soil erosion derived from in-situ 10Be and inventories of meteoric 10Be in deeply weathered soils in southern Brazil

Meteoric 10Be is commonly used as a geochemical tracer of soil erosion and regolith residence time over long time scales (> 103 yr). The acidic conditions of tropical soils are less favorable for meteoric 10Be retention, and make a translation of meteoric 10Be inventories into surface erosion rates complex. In this empirical study, we evaluate the potential loss of meteoric 10Be in the weathering zone. Along a toposequence in southern Brazil, three regolith profiles were sampled to explore the mobility of Be in the soil system. The sequential chemical extractions of four reactive fractions indicate that the amorphous oxy-hydroxide and crystalline oxide fractions are the main carriers of meteoric 10Be. Our data from the Brazilian toposequence reveal significantly higher inventories of meteoric 10Be in the slope concavities compared to the upslope positions. This demonstrates that chemical leaching and deep percolation of meteoric 10Be within the soil profile leads to vertical and lateral redistribution of meteoric 10Be across the hillslope. Our data show that more than 50% of the delivered meteoric 10Be is leached from the upslope regolith profiles. After correcting the inventories for incomplete retention of meteoric 10Be, surface erosion rates are derived for the slope convexities. Upslope erosion rates are below 5 mm/ kyr and agree with the in-situ 10Be-derived denudation rates. Our results suggest that soil and sediment fluxes can accurately be derived from meteoric 10Be inventories in well-developed soils, when the meteoric 10Be mobility is constrained using differential mass balance

Dynamic soil properties in response to anthropogenic disturbances

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Impact of low denudation rates on soil chemical weathering intensity: A multiproxy approach

Quantifying the influence of denudation, i.e. physical erosion and chemical weathering, on soil weathering intensity is an important component for a comprehensive understanding of element biogeochemical cycles. The relation between the weathering intensity and the denudation rate is not clear and requires further investigation in a variety of climatic and erosional settings. Here, in the Betic Cordillera (southern Spain), we assess the soil chemical weathering intensity with a multiproxy approach combining different indicators of chemical weathering of the soil: the Total Reserve in Bases (TRB), the content in Fe-oxides, the quartz and clay content, the soil cation exchange capacity (CEC), and the silicon (Si) isotope composition of the clay-sized fraction. Our multiproxy approach demonstrates that in this semi-arid environment at low denudation rates, an increasing denudation rate decreases the soil weathering intensity, whereas Si mobility remains limited. Our results converge with previous conclusions based on chemical mass balance methods in the same geological setting. Mass balance methods, and particularly Chemical Depletion Fractions (CDF), are based on the immobility of a refractory element (commonly zirconium, Zr) relative to major cations in soils. Interestingly, our study suggests that a weathering index such as the TRB may provide a useful complement to assess soil chemical weathering intensity in eroding landscapes where the application of chemical mass balances may be hampered by potential Zr mobility in the soil or by heterogeneity of Zr concentrations in the bedrock.SCOPUS: ar.jinfo:eu-repo/semantics/publishe