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This article explores the role of fossil organic carbon (FOC) weathering in global biogeochemical cycles. FOC is a unique type of organic carbon that has accumulated and fossilized in the Earth's lithosphere over time. Recent studies have shown that FOC is easily reacted, mobilized, and transformed under surface conditions, releasing CO2 and impacting global biogeochemical cycles. However, there are challenges in understanding FOC weathering, including limited data for calibrating its kinetics and the influence of factors like rock types, tectonic activities, geomorphic processes, and climate. The article suggests that future research should focus on studying FOC weathering in different environments and developing proxies to reconstruct past weathering. It also emphasizes the importance of understanding the connection between FOC weathering and the biogeochemical cycles of other nutrient elements. [Extracted from the article]

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 CO 2 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. [ABSTRACT FROM AUTHOR]

Radiogenic uranium isotope disequilibrium (234U/238U) has been used to trace a variety of Earth surface processes, and is usually attributed to direct recoil of 234Th and preferential dissolution of radioactively damaged lattices at the mineral surface. However, the relative contribution of these two mechanisms in the natural environment remains unresolved, making it hard to use the extent of disequilibrium to quantify processes such as weathering. This study tests the contribution of preferential dissolution using well-characterized weathered moraines and river sediments from the southeastern Tibetan Plateau. The observations show that weathering of recent moraines where the contribution from direct recoil is negligible and is not associated with depletion of 234U at the mineral surface. It suggests a limited role for preferential dissolution in this setting. We attribute this lack of preferential dissolution to a near-to-equilibrium dissolution at the weathering interfaces, with little development of etch pits associated with radioactively damaged energetic sites. [ABSTRACT FROM AUTHOR]

Earthquakes play a fundamental role in the evolution of Earth's topography through co-seismic uplift and subsidence, as well as erosion through widespread landslides induced by ground motion. Earthquake-induced landslides can result in exceptional increases in the transfer of mass from landscapes, supplying sediment to rivers where impacts can last for decades or longer. Landslides can also erode vegetation and soils, in addition to exposing freshly ground rock mass, which can both result in regionally significant carbon transfers. Recent work has sought to quantify the fluxes, rates, and temporal patterns of the impacts on earth surface processes generated by these tectonic events. In this review, we focus on the impacts of earthquake-induced landslides on erosion and sediment flux, river water chemistry, and carbon export through the lens of the well-studied 2008 Wenchuan earthquake. We then discuss these themes in the context of works from the 1999 Taiwan Chi-Chi earthquake, 2015 Nepalese Gorkha earthquake, and New Zealand historical earthquakes, altogether highlighting an important role of earthquake-induced landslides in erosion and weathering along tectonically active mountain ranges. We suggest that more monitoring, geochemical tracing, paleo-records, and/or modelling studies are needed to compare the processes and fluxes of catchment erosion and weathering before and after earthquake events, which will help to deepen our understanding not only of the impacts of earthquake-induced landslides on earth surface processes, but also the linkage among high magnitude events, continental erosion and weathering, and the long-term global carbon cycle. [ABSTRACT FROM AUTHOR]

We analyzed the Sr/Sr ratios of basaltic rocks and basin water in Xuyi, China, and found a big difference, which may challenge conventional wisdom on the contribution of basalt weathering in end-member analysis. Results of an in-house weathering experiment suggest that rainwater and dust are not responsible for the difference. By isolating the major minerals in basalt, we found that plagioclase has much higher Sr/Sr ratios than bulk basalt and basin water, which might explain the difference in Sr/Sr ratios of basalt and basin water. We infer that low-temperature hydrothermal alteration increased the Sr/Sr ratios of plagioclase. Future analyses of end-member contribution in a mixed-rock basin should take into account that basin water and plagioclase have higher Sr/Sr ratios than basaltic rocks. [ABSTRACT FROM AUTHOR]

Abstract: It is generally accepted that progressive cooling of global climate since the Late Cretaceous results from decreasing partial pressure of atmospheric CO2 (pCO2). However, details on how and why the carbon cycle evolved and how it would affect pCO2 have not been fully resolved. While the long-term decline of pCO2 might be caused by the decrease of volcanic degassing through the negative feedback between pCO2 and silicate weathering, seafloor spreading, the major control of CO2 degassing, seems to have remained relatively constant. Alternative explanation, known as ‘uplift driven climate change’ hypothesis, proposes that tectonic uplift may have enhanced the sink of atmospheric CO2 by silicate weathering, and thus produced the decline of pCO2. However, increasing weathering sink of CO2 could deplete atmosphere all of its CO2 within several million years while holding volcanic outgassing constant. In this work, major fluxes of long-term carbon cycle are calculated based on a reverse model constrained by marine C, Sr and Os isotopic records and the spreading rate of sea floor. Weathering of island basalt and continental silicate rocks are separated in the new model. The results indicate a long-term decline of island basalt weathering in consistent with the global cooling trend over the past 100 million years. Dramatic changes of the CO2 fluxes associated continental silicate weathering, reverse weathering, volcanic degassing and the growth of organic carbon reservoir have been observed. Disturbance of atmospheric CO2 cycle by these fluxes seems to be maintained by the concomitant adjustments of island basalt weathering that were sensitive to the pCO2 controlled environment factors such as temperature and runoff. The negative feedbacks between pCO2 and weathering of island basalt might have played a significant role in stabilizing the long-term carbon cycle. [Copyright &y& Elsevier]

Abstract: Eolian deposits of loess on the Chinese Loess Plateau are investigated to decipher the influence of sequentially changed monsoon climate on silicate weathering process. Detailed hydrochloric acid dissolution experiments are applied to establish a sensitive proxy for silicate weathering. Combined elemental and mineralogical studies show that the minerals which are susceptible to incipient chemical weathering can be totally dissolved in hot hydrochloric acid (80°C, 3mol/L) after 4h, while other stable minerals are nearly unaffected. Thus, the hydrochloric acid dissolvable fractions (ADF) of loess and paleosol are more sensitive than bulk samples in the weathering reaction. Since Mg is tend to be leached out from the ADF while Al is conserved during the incongruent weathering, Mg/Al ratio of the ADF (Mg/AlADF) could indicate the weathering intensities of the Mg-bearing minerals, mainly chlorite. The Mg/AlADF of the modern soils on the Chinese Loess Plateau is closely correlated to the local precipitation amounts, suggests that the intensity of summer monsoon is a key factor in the weathering of Mg-silicate. The silicate weathering intensity of the loess-paleosol deposits of past 130Kyrs shows strong procession cycle of ∼ 23Kyrs coupled by glacial-interglacial variation. The ∼ 23Krys cycles are coherent to the variations of the intensity of summer monsoon, while the ∼ 100Kyrs glacial-interglacial variation may relates to the changes in winter monsoon. Phased intensification of East Asian summer monsoon, and thus enhanced silicate weathering and atmosphere CO2 drawdown, in response to the uplift of Tibetan Plateau, may be another mechanism relating the late Cenozoic tectonics to global cooling. [Copyright &y& Elsevier]

Abstract The ratio between the activity of dissolved 234U and 238U in river water, (234U/238U) Riv , shows promise for tracing weathering processes. It has been suggested that preferential 234U release generates a high (234U/238U) Riv under low weathering intensity while congruent weathering, as expected for high weathering intensity, would release uranium with a secular equilibrium (234U/238U) Riv of 1. However, weathering intensity depends on the combination of exposure age of weathering interfaces and dissolution rate, complicating the use of (234U/238U) Riv as a tool in weathering studies. This work attempts to resolve the weathering dynamics reflected by (234U/238U) Riv using catchment scale denudation rate (D) as a first order approximation for the age of weathering interfaces. A global dataset (n = 653), including 132 new measurements from Chinese catchments, indicates changing responses of (234U/238U) Riv to D. Higher values (>1.5) and larger variability of (234U/238U) Riv are observed in catchments both with high (>3 mm/yr) and low D (<0.1 mm/yr). In contrast, the lowest values (approaching secular equilibrium) and the least variable (234U/238U) Riv are associated with catchments of medium to high D (0.3–2 mm/yr). The observed pattern can be explained by a model that involves competing uranium release from active- and inactive-weathering interfaces in response to a shifting weathering regime, from kinetically-limited to supply-limited. (234U/238U) Riv decreases with decreasing D under a kinetically-limited weathering regime due to the increasing contribution of congruent weathering relative to the preferential dissolution of 234U from radioactively damaged sites as D decreases. In contrast, (234U/238U) Riv increases with decreasing D under a supply-limited weathering regime because the minerals that remain under these conditions continue to release 234U to solution by recoil, while contributing little to the dissolved 238U because of their slow dissolution rate. A much deeper depth of the weathering interface is suggested for catchments with high D due to higher weathering contribution from deep fractures relative to the poorly developed surface regolith. A young age of ∼10 kyrs is constrained by the model for complete dissolution of exposed weathering-active minerals, which implies a rapid shifting from kinetically-limited to supply-limited weathering regime and limited influence of weathering on the calculation of comminution age based on (234U/238U) in sediments. A site-specific dissolution rate that is consistent with those estimated from weathering profiles is also constrained, which suggests that the discrepancy between field and laboratory weathering rate may largely be controlled by the difference in chemical affinity associated with stagnant micro-pores, low permeability, and high rock/fluid ratios rather than the passivation of weathering interfaces. Highlights • We observed changing responses of riverine dissolved (234U/238U) value to changes in catchment denudation rates. • The observation reflects shifting weathering limitation regimes. • We proposed a model that captures most variability of the riverine (234U/238U). • Site-specific dissolution rates ∼3 orders of magnitude lower than laboratorial experiments are constrained. [ABSTRACT FROM AUTHOR]