Abstract. Silicate weathering, which is of great importance in regulating the global carbon cycle, has been found to be affected by complicated factors, including climate, tectonics and vegetation. However, the exact transfer function between these factors and the silicate weathering rate is still unclear, leading to large model–data discrepancies in the CO2 consumption associated with silicate weathering. Here we propose a simple parameterization for the influence of vegetation cover on erosion rate to improve the model–data comparison based on a state-of-the-art silicate weathering model. We found out that the current weathering model tends to overestimate the silicate weathering fluxes in the tropical region, which can hardly be explained by either the uncertainties in climate and geomorphological conditions or the optimization of model parameters. We show that such an overestimation of the tropical weathering rate can be rectified significantly by parameterizing the shielding effect of vegetation cover on soil erosion using the leaf area index (LAI), the high values of which are coincident with the distribution of leached soils. We propose that the heavy vegetation in the tropical region likely slows down the erosion rate, much more so than thought before, by reducing extreme streamflow in response to precipitation. The silicate weathering model thus revised gives a smaller global weathering flux which is arguably more consistent with the observed value and the recently reconstructed global outgassing, both of which are subject to uncertainties. The model is also easily applicable to the deep-time Earth to investigate the influence of land plants on the global biogeochemical cycle.
Understanding weathering processes in landslide-dominated catchments is critical for evaluating the role of landslides in chemical weathering and the global carbon cycle. Previous studies have focused on solute concentrations in landslide-impacted landscapes, but have paid less attention to developing isotopic tracers of landslide-induced weathering fluxes. Recent work found that the dissolved radiogenic uranium isotopes in river water are closely related to the denudation rates in catchments draining steep mountains where landslides are thought to be a major erosion mechanism, suggesting the potential of uranium isotopes to trace landslide-induced weathering fluxes. Here we compile the dissolved radiogenic uranium isotopes ( 234 U/ 238 U ratios) in the river water samples from a group of catchments with variable landslide activities in the Minjiang River Basin at the eastern margin of the Tibetan Plateau. We derive three metrics of landslide activity from the analyses of digital topography and an inventory map of the co-seismic landslides triggered by the 2008 Mw7.9 Wenchuan earthquake: the normalized volume of landslides, the mean catchment slope angle, and the fraction of slopes steeper than a threshold angle beyond which slopes are mechanically unstable. The riverine dissolved 234 U/ 238 U ratios correlate negatively with the metrics of landslide activity in each catchment, which likely reflect the influence of landslides on the dissolved 234 U/ 238 U ratios. Mechanistically, enhanced bedrock landsliding would accelerate the exposure of fresh rock, promoting bedrock weathering and congruent dissolution of 234 U and 238 U contained in minerals; reduced landslide activities and enhanced regolith weathering would lead to preferential accumulation of 234 U against 238 U in solutes through alpha-recoil ejection, thus increasing dissolved 234 U/ 238 U. Our findings provide field evidence of using the riverine dissolved 234 U/ 238 U ratio to trace weathering fluxes driven by landslides, shedding new light on chemical weathering processes in uplifting mountains.
Abstract. Silicate weathering, which is of great importance regulating global carbon cycle, has been found to be affected by complicate factors including climate, tectonics, vegetation, and etc. However, the exact transfer function between these factors and silicate weathering rate is still unclear, leading to large model-data discrepancies of the CO2 consumption associated with silicate weathering. Here we propose a simple parameterization for the influence of vegetation cover on erosion rate to improve the model-data comparison based on a state-of-the-art silicate weathering model. We found out that the current weathering model tends to overestimate the silicate weathering fluxes in the tropical region, which can hardly be explained by either the uncertainties in climate and geomorphological conditions or the optimization of model parameters. We show that such an overestimation of tropic weathering rate can be rectified significantly by considering the shielding effect of vegetation cover on the erosion rate of the leached soils considering that the geographic distribution of such soils is coincident with regions with the highest leaf area index (LAI). We propose that the heavy vegetation in the tropical region likely slows down the erosion rate, much more so than thought before, through reducing extreme stream flow in response to precipitation. The silicate weathering model thus revised gives a smaller global weathering flux which is arguably more consistent with the observed value and the recently reconstructed global outgassing, both of which are subject to uncertainties. The model is also easily applicable to the deep-time Earth to investigate the influence of land plant on global biogeochemical cycle.
Abstract Projecting future changes in effective precipitation under global warming, which requires constraints on both the precipitation and evapotranspiration, is still challenging in the semi‐arid region of East Asia. Here we present proxies, based on Sr/Ca and 87 Sr/ 86 Sr of soil carbonate, to reconstruct the evolution of both effective precipitation and temperature from a high‐resolution loess profile in the East Asian monsoon marginal area covering the last interglacial period as a geological analog for future conditions. We find that intervals with a stronger summer monsoon are characterized by lower effective precipitation during Marine Isotope Stage (MIS) 5 due to a larger increase in evapotranspiration than in monsoonal precipitation in response to higher temperature. The greater response of the effective precipitation to temperature than summer monsoon intensity during MIS 5 thus suggests the risk of further aridification in the semi‐arid region of East Asia under climate warming.
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