Silicate weathering of soil-mantled slopes in an active Alpine landscape
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Denudation
Soil production function
Cosmogenic nuclide
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Denudation
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Abstract. Cosmogenic radionuclides (CRNs) are the standard tool to derive centennial-to-millennial timescale denudation rates, however, it has been demonstrated that chemical weathering in some settings can bias CRNs as a proxy for landscape denudation. Currently, studies investigating CRN weathering biases have mostly focused on the largely insoluble target mineral quartz in felsic lithologies. Here, we examine the response of CRN build-up for both soluble and insoluble target minerals under different weathering scenarios. We assume a simple box model in which bedrock is converted to regolith at a constant rate, and denudation occurs by regolith erosion and weathering either in the regolith or along the regolith-bedrock interface, as is common in carbonate bedrock. We show that weathering along the regolith-bedrock interface increases CRN concentrations compared to a no-weathering case, and how independently derived weathering rates or degrees can be used to correct for this bias. If weathering is concentrated within the regolith, insoluble target minerals will have a longer regolith residence time and higher nuclide concentration than soluble target minerals. This bias can be identified and corrected using paired nuclide measurements coupled with knowledge of either the bedrock or regolith mineralogy to derive denudation and long-term weathering rates. Similarly, single nuclide denudation measurements can be corrected if a weathering rate and compositional data are available. Our model highlights that for soluble target minerals, the relationship between nuclide accumulation and denudation is not monotonic. We use this understanding to map the conditions of regolith mass, weathering, and denudation rates at which weathering corrections for cosmogenic nuclides become large and ambiguous as well as identify environments in which the bias is mostly negligible, and CRN concentrations reliably reflect landscape denudation. We highlight how measurements of CRNs from soluble target minerals, coupled with bedrock and regolith mineralogy, can help to expand the range of landscapes for which centennial-to-millennial timescale denudation and weathering rates can be obtained.
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This paper reviews the methodology and applications of terrestrial cosmogenic nuclides as a tool for quantifying rates of geomorphic processes. The review starts from systematics in the production of cosmogenic 10Be and 26Al in quartz, and 36Cl in calcite, and then describes the basic modeling of the accumulation of those nuclides under varying denudation rates. Procedures for sample preparation and nuclide measurement using accelerator mass spectrometry are also summarized. Recent research reveals denudation rates of bare rock surfaces for both silicates and carbonates, as well as soil production rates from saprolite beneath the soil layer on hillslopes. The empirical formulation of soil production rates as a function of soil thickness enables us to test hypothetical transport laws of soil particles through a combined analysis with topographic parameters of hillslopes. Chemical processes contributing to soil production and denudation have been quantified with a coupled approach using cosmogenic nuclide analysis and geochemical mass balance method. However, linkages across climate conditions, element leaching, and denudation rates are still debated because of timescale discrepancies between soil and saprolite formation. Climate seems to affect soil production indirectly by reducing the mechanical strength of saprolite resulting from chemical weathering of bedrock. A theoretical framework is presented for modeling saprolite weakening and denudation, which connects bedrock weathering, erodibility of uppermost saprolite, soil production and transport with steady-state topography of hill-noses.
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Abstract Terrestrial cosmogenic nuclides (TCN) are widely employed to infer denudation rates in mountainous landscapes. The calculation of an inferred denudation rate ( D inf ) from TCN concentrations is typically performed under the assumptions that denudation rates were steady during TCN accumulation and that soil chemical weathering negligibly impacted soil mineral abundances. In many landscapes, however, denudation rates were not steady and soil composition was significantly impacted by chemical weathering, which complicates interpretation of TCN concentrations. We present a landscape evolution model that computes transient changes in topography, soil thickness, soil mineralogy, and soil TCN concentrations. We used this model to investigate TCN responses in transient landscapes by imposing idealized perturbations in tectonically (rock uplift rate) and climatically sensitive parameters (soil production efficiency, hillslope transport efficiency, and mineral dissolution rate) on initially steady‐state landscapes. These experiments revealed key insights about TCN responses in transient landscapes. (a) Accounting for soil chemical erosion is necessary to accurately calculate D inf . (b) Responses of D inf to tectonic perturbations differ from those to climatic perturbations, suggesting that spatial and temporal patterns in D inf are signatures of perturbation type and magnitude. (c) If soil chemical erosion is accounted for, basin‐averaged D inf inferred from TCN in stream sediment closely tracks actual basin‐averaged denudation rate, showing that D inf is a reasonable proxy for actual denudation rate, even in many transient landscapes. (d) Response times of D inf to perturbations increase with hillslope length, implying that response times should be sensitive to the climatic, biological, and lithologic processes that control hillslope length.
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