Severe flood modulates the sources and age of dissolved organic carbon in the Yangtze River Estuary.

Floods in global large rivers modulate the transport of dissolved organic carbon (DOC) and estuarine hydrological characteristics significantly. This study investigated the impact of a severe flood on the sources and age of DOC in the Yangtze River Estuary (YRE) in 2020. Comparing the flood period in 2020 to the non-flood period in 2017, we found that the flood enhanced the transport of young DOC to the East China Sea (ECS), resulting in significantly enriched Δ14C-DOC values. During the flood period, the proportion of modern terrestrial organic carbon (OC) was significantly higher compared to the non-flood period. Conversely, the proportion of pre-aged sediment OC was significantly lower during the flood period. The high turbidity associated with the flood facilitated rapid transformation and mineralization of sedimentary and fresh terrestrial OC, modifying the sources of DOC. The flux of modern terrestrial OC transported to the ECS during the flood period was 1.58 times higher than that of the non-flood period. These findings suggest that floods can modulate the sources and decrease the age of DOC, potentially leading to increased greenhouse gas emissions. Further research is needed to understand the long-term impacts of floods on DOC dynamics in global estuaries.

Escalating Carbon Export from High-Elevation Rivers in a Warming Climate.

High-elevation mountains have experienced disproportionately rapid warming, yet the effect of warming on the lateral export of terrestrial carbon to rivers remains poorly explored and understood in these regions. Here, we present a long-term data set of dissolved inorganic carbon (DIC) and a more detailed, short-term data set of DIC, δ13CDIC, and organic carbon from two major rivers of the Qinghai-Tibetan Plateau, the Jinsha River (JSR) and the Yalong River (YLR). In the higher-elevation JSR with ∼51% continuous permafrost coverage, warming (>3 °C) and increasing precipitation coincided with substantially increased DIC concentrations by 35% and fluxes by 110%. In the lower-elevation YLR with ∼14% continuous permafrost, such increases did not occur despite a comparable extent of warming. Riverine concentrations of dissolved and particulate organic carbon increased with discharge (mobilization) in both rivers. In the JSR, DIC concentrations transitioned from dilution (decreasing concentration with discharge) in earlier, colder years to chemostasis (relatively constant concentration) in later, warmer years. This changing pattern, together with lighter δ13CDIC under high discharge, suggests that permafrost thawing boosts DIC production and export via enhancing soil respiration and weathering. These findings reveal the predominant role of warming in altering carbon lateral export by escalating concentrations and fluxes and modifying export patterns.

Elucidation of the dominant factors influencing N2O emission in water-level fluctuation zones in a karst canyon reservoir, southwest China.

The water-level fluctuations zones (WLFZs) are crucial transitional interfaces within river-reservoir systems, serving as hotspots for N2O emission. However, the comprehension of response patterns and mechanisms governing N2O emission under hydrological fluctuation remains limited, especially in karstic canyon reservoirs, which introduces significant uncertainty to N2O flux assessments. Soil samples were collected from the WLFZs of the Hongjiadu (HJD) Reservoir along the water flow direction from transition zone (T1 and T2) to lacustrine zone (T3, T4 and T5) at three elevations for each site. These soil columns were used to conduct simulation experiments under various water-filled pore space gradients (WFPSs) to investigate the potential N2O flux pattern and elucidate the underlying mechanism. Our results showed that nutrient distribution and N2O flux pattern differed significantly between two zones, with the highest N2O fluxes in the transition zone sites and lacustrine zone sites were found at 75 % and 95 % WFPS, respectively. Soil nutrient loss in lower elevation areas is influenced by prolonged impoundment durations. The higher N2O fluxes in the lacustrine zone can be attributed to increased nutrient levels resulting from anthropogenic activities. Furthermore, correlation analysis revealed that soil bulk density significantly impacted N2O fluxes across all sites, while NO3-and SOC facilitated N2O emissions in T1-T2 and T4-T5, respectively. It was evident that N2O production primarily contributed to nitrification in the transition zone and was constrained by the mineralization process, whereas denitrification dominated in the lacustrine zone. Notably, the annual N2O efflux from WLFZs accounted for 27 % of that from the water-air interface in HJD Reservoir, indicating a considerably lower contribution than anticipated. Nevertheless, this study highlights the significance of WLFZs as a vital potential source of N2O emission, particularly under the influence of anthropogenic activities and high WFPS gradient.

Microbial contribution estimated by clumped isotopologues (13CH3D and 12CH2D2) characteristics in a CO2 enhanced coal bed methane reservoir.

Carbon capture and storage (CCS) of CO2 is a key technology for substantially mitigating global greenhouse gas emissions. Determining the biogeochemical processes in host rocks after CO2 injection informs the viability of carbon storage as a long-term sink for CO2, the complexity of reservoir CH4 cycling, as well as the direct and indirect environmental impacts of this strategy. The doubly substituted ('clumped') isotopologues of methane (13CH3D and 12CH2D2) provide novel insights into methane origins and post-generation processing. Here, we report the chemical compositions of hydrocarbons (C1/C2+ molecular ratios), and methane bulk and clumped isotopes (δ13C, δD, Δ13CH3D and Δ12CH2D2) of a CO2 enhanced coal bed methane recovery (CO2-ECBM) area in Qinshui basin, China and is an analogue for carbon capture and storage. The clumped isotopologue compositions observed in the study area are generally consistent with a range of temperatures spanning 73 to 193 °C. The range in apparent temperature and correlations among clumped and bulk isotopic indices are best explained by mixing between a high maturity thermogenic methane (high in δ13C and δD, with a clumped isotope composition equilibrated near ∼249 °C) and biogenic methane formed or processed in the reservoir (low in δ13C and δD, with a clumped isotope composition equilibrated near 16-27 °C). We hypothesize that the biogenic endmember may result from slow methanogenesis and/or anaerobic oxidation of methane (AOM). This study demonstrates that the potential of methane clumped isotope approach to identify in situ microbial metabolic processes and their association with carbon cycling in CO2-ECBM area, improving our understanding of biogeochemical mechanisms in analogous geological reservoirs.

Quantifying the contribution of methane diffusion and ebullition from agricultural ditches.

Agricultural ditches are significant methane (CH4) sources since substantial nutrient inputs stimulate CH4 production and emission. However, few studies have quantified the role of diffusion and ebullition pathways in total CH4 emission from agricultural ditches. This study measured the spatiotemporal variations of diffusive and ebullitive CH4 fluxes from a multi-level ditch system in a typical temperate agriculture area, and assessed their contributions to the total CH4 emission. Results illustrated that the mean annual CH4 flux in the ditch system reached 1475.1 mg m-2 d-1, among which 1376.7 mg m-2 d-1 was emitted via diffusion and 98.5 mg m-2 d-1 via ebullition. Both diffusive and ebullitive fluxes varied significantly across different types of ditches and seasons, with diffusion dominating CH4 emission in middle-size ditches and ebullition dominating in large-size ditches. Diffusion was primarily driven by large nutrient inputs from adjacent farmlands, while hydrological factors like water temperature and depth controlled ebullition. Overall, CH4 emission accounted for 86 % of the global warming potential across the ditch system, with 81 % attributed to diffusion and 5 % to ebullition. This study highlights the importance of agricultural ditches as hotspots for CH4 emissions, particularly the dominant role of the diffusion pathway.

Seasonal variation and dissolved organic matter influence on the distribution, transformation, and environmental risk of pharmaceuticals and personal care products in coastal zone: A case study of Tianjin, China.

Pharmaceuticals and personal care products (PPCPs) are emerging contaminants that have raised urgent environmental issues. The dissolved organic matter (DOM) plays a pivotal role on PPCPs' migration and transformation. To obtain a comprehensive understanding of the occurrence and distribution of PPCPs, a seasonal sampling focused on the riverine system in coastal zone, Tianjin, Bohai Rim was conducted. The distribution and transformation of thirty-three PPCPs and their interaction with DOM were investigated, and their sources and ecological risks were further evaluated. The total concentration of PPCPs ranges from 0.01 to 197.20 µg/L, and such value is affected by regional temperature, DOM and land use types. PPCPs migration at soil-water interface is controlled by temperature, sunlight, water flow and DOM. PPCPs have a high affinity to the protein-like DOM, while the humus-like DOM plays a negative influence and facilitates PPCPs' degradation. It is also found that protein-like DOM can represent point source pollution, while humus-like substances indicate non-point source (NPS) emission. Specific PPCPs can be used as markers to trace the source of domestic discharge. Additionally, daily use PPCPs such as ketoprofen, caffeine and iopromide are estimated to be the main risk substances, and their ecological risk varies on space, season and river hydraulic condition.

High-frequency data significantly enhances the prediction ability of point and interval estimation.

Accurate prediction of dissolved oxygen (DO) dynamics is crucial for understanding the influence of environmental factors on the stability of aquatic ecosystem. However, limited research has been conducted to determine the optimal frequency of water quality monitoring that ensures continuous assessment of water health while minimizing costs. To address these challenges, the present study developed a hybrid stochastic hydrological model (i.e., ARIMA-GARCH hybrid model) and machine learning (ML) models. The objective of this study is to identify the best-performing model and establish the optimal monitoring frequency. Results revealed that high-frequency DO monitoring data exhibit greater variability compared to low-frequency data. Moreover, the ARIMA-GARCH model demonstrates promising potential in predicting DO concentrations for low-frequency monitoring data, surpassing ML models in performance. Furthermore, increasing the monitoring frequency significantly improves the prediction accuracy of models, regardless of whether point (with lower R2 values of 0.64 and 0.51 for daily detection than these of every 15 min (0.96 and 0.99) at CHQ and LHT, respectively) or interval predictions (with RIW higher values of 2.00 and 1.55 for daily detection higher than these of 0.02 and 0.16 in every 15 min at CHQ and LHT, respectively) are considered. Additionally, a 4 hourly monitoring frequency was found to be optimal for water quality assessment using each model. These findings identify the superior performing of the ARIMA-GARCH model and highlight the crucial role of monitoring frequency in enhancing DO prediction and improving model performance.

Influences of hydrodynamics on dissolved inorganic carbon in deep subtropical reservoir: Insights from hydrodynamic model and carbon isotope analysis.

Dam construction significantly impacts river hydrodynamics, subsequently influencing carbon biogeochemical processes. However, the influence of hydrodynamic conditions on the migration and transformation of Dissolved Inorganic Carbon (DIC) remains uncertain. To bridge this knowledge gap, we integrated hydrochemistry, isotopic composition (δ13CDIC), and a hydrodynamic model (CE-QUAL-W2) to examine the distinctions, control mechanisms, and environmental effects of DIC biogeochemical processes in a typical large and deep reservoir (Hongjiadu Reservoir) under different hydrodynamic conditions. We evaluated hydrodynamic alterations through the Schmidt stability index and relative water column stability. The analysis disclosed that during weak hydrodynamics periods, the energy necessary for complete mixing the surface and deep water was 34 times higher (3615.32 J/m2 vs.106.86 J/m2), and stability was 13 times greater (312.96 vs. 24.69) compared to periods of strong hydrodynamics. Additionally, the spatiotemporal heterogeneity of DIC concentrations (1.4 % to -9.1 %) and δ13CDIC (-1.7 % to -19.5 %) from the dry to wet seasons reflected disparities in DIC control mechanisms under varied hydrodynamic conditions. Based on model simulations, our calculations indicate that during weak hydrodynamics periods, the enhancement of the biological carbon pump effect resulted in substantial sequestration of DIC, reaching up to 379.6 t-DIC·d-1 in the water. Conversely, during strong hydrodynamics periods, DIC retention capacity decreased by 69.2 t·d-1, resulting in reservoir CO2 emissions of 22.7 × 104 t, which were more than 7 times higher than during weak hydrodynamics periods (3.2 × 104 t). Our findings emphasize the discernible impact of hydrodynamic conditions on reservoir biogeochemical processes related to DIC. Considering the increasing construction of reservoirs globally, understanding and controlling hydrodynamic conditions are crucial for mitigating CO2 emissions and optimizing reservoir management.

Molecular and optical signatures of photochemical transformation of dissolved organic matter: Nonnegligible role of suspended particulate matter in urban river.

Natural dissolved organic matter (DOM) is one of the Earth's dynamic carbon pools and a key intermediate in the global carbon cycle. Photochemical processes potentially affect DOM composition and activity in surface water. Suspended particulate matter (SPM) is the integral component of slow-moving rivers, and holds the potential for photochemical reactivity. To further investigate the influence of SPM on DOM photochemical transformation, this study conducted experiments comparing samples with and without SPM irradiated under simulated sunlight. Surface water samples from slow-moving urban rivers were collected. DOM optical characteristics and molecular features obtained by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were investigated. Photolabile DOM was enriched in unsaturated and highly aromatic terrestrial substances. Photoproduced DOM had low aromaticity and was dominated by saturated aliphatics, protein-like substances, and carbohydrates. Study results indicated that the presence of SPM had a nonnegligible impact on the molecular traits of DOM, such as composition, molecular diversity, photolability, and bioavailability during photochemical reactions. In the environment affected by SPM, molecules containing heteroatoms exhibit higher photosensitivity. SPM promotes the photochemical transformation of a wider range of chemical types of photolabile DOM, particularly nitrogen-containing compounds. This study provides an essential insight into the more precise simulation of photochemical reactions of DOM influenced by SPM occurring in natural rivers, contributing to our understanding of the global carbon cycle from new theoretical perspectives.

Dynamic N transport and N2O emission during rainfall events in the coastal river.

The coastal zone exhibited a high population density with highly impacted by anthropogenic activities, such as river impoundment to prevent saline intrusion, which resulted in weak hydrological conditions. Rainfall events can result in dramatic changes in hydrological and nutrient transportation conditions, especially in rivers with weak hydrological conditions. However, how the nitrogen transport and N2O emissions or biogeochemistry responds to the different types of rainfall events in the weak hydrodynamics rivers is poorly understood. In this study, the hydrological, nitrogenous characteristic, as well as N2O dynamics, were studied by high-frequency water sampling during two distinct rainfall events, high-intensity with short duration (E1) and low-intensity with long duration (E2). The results displayed that the hydrologic condition in E1 with a wider range of d-excess values (from -9.50 to 32.1 ‰), were more dynamic than those observed in E2. The N2O concentrations (0.01-3.33 µmol/L) were higher during E1 compared to E2 (0.03-1.11 µmol/L), which indicated that high-intensity rainfall has a greater potential for N2O emission. On the contrary, the concentrations of nitrogen (e.g., TN and NO3--N) were lower during E1 compared to E2. Additionally, hysteresis was observed in both water and nitrogen components, resulting in a prolonged recovery time for pre-rainfall levels during the long-duration event. Moreover, the results showed that the higher average N2O flux (78.3 µmol/m2/h) in the rainfall event period was much larger than that in the non-rainfall period (1.63 µmol/m2/h). The frequency dam regulation resulted in the water level fluctuation, which could enhance wet-dry alternation and simulated N2O emissions. This study highlighted the characteristic of N dynamic and hydrological responses to diverse rainfall events occurrences in the coastal river. Rainfall could increase the N2O emission, especially during high-intensity rainfall events, which cannot be ignored in the context of annual N2O release.

Constraining the sources and cycling of dissolved inorganic carbon in an alpine river, eastern Qinghai-Tibet Plateau.

It is generally acknowledged that riverine dissolved inorganic carbon (DIC) behaviors play a critical role in global carbon cycling and hence have an impact on climate change. However, little is known about the intricate DIC dynamics under various meteorological conditions in the alpine areas. Here, we investigated DIC biogeochemical processes in the Bailong River catchment, eastern Qinghai-Tibet Plateau (QTP), by combining measurements of major ions, stable and radioactive isotopic compositions of DIC (δ13CDIC and Δ14CDIC), and physiographic parameters in the Bailong River catchment. Statistics and stoichiometry analyses suggest that multiple biogeochemical processes could affect carbon cycling in the Bailong River catchment. The "old" DIC with low Δ14C values (-472.4 ± 127.8 ‰, n = 3) and stoichiometry analysis of dissolved ions showed clear evidence that carbonate weathering is primarily responsible for water chemistry in the upstream (elevation >2000 m). However, upstream samples showed that δ13CDIC increased between 5 ‰ and 11 ‰ from the theoretical mixing line, concomitant with increasing pH and decreasing pCO2, suggesting that isotopic fractionation of DIC due to CO2 outgassing may be the primary cause of the increased δ13CDIC values. Additionally, the higher Δ14C values (-285.4 ± 123.5 ‰, n = 12) in the downstream region below 2000 m suggest that allochthonous modern carbon had a great impact on DIC variations. The presence of younger DIC may have important implications for the interpretation of inorganic carbon age in downstream rivers. Our study demonstrates that physiographic conditions can regulate DIC behaviors, which can improve estimations of carbon yield and comprehension of global carbon cycle.

Agricultural ditches are hotspots of greenhouse gas emissions controlled by nutrient input.

Agricultural ditches are pervasive in agricultural areas and are potential greenhouse gas (GHG) hotspots, since they directly receive abundant nutrients from neighboring farmlands. However, few studies measure GHG concentrations or fluxes in this particular water course, likely resulting in underestimations of GHG emissions from agricultural regions. Here we conducted a one-year field study to investigate the GHG concentrations and fluxes from typical agricultural ditch systems, which included four different types of ditches in an irrigation district located in the North China Plain. The results showed that almost all the ditches were large GHG sources. The mean fluxes were 333 µmol m-2 h-1 for CH4, 7.1 mmol m-2 h-1 for CO2, and 2.4 µmol m-2 h-1 for N2O, which were approximately 12, 5, and 2 times higher, respectively, than that in the river connecting to the ditch systems. Nutrient input was the primary driver stimulating GHG production and emissions, resulting in GHG concentrations and fluxes increasing from the river to ditches adjacent to farmlands, which potentially received more nutrients. Nevertheless, the ditches directly connected to farmlands showed lower GHG concentrations and fluxes compared to the ditches adjacent to farmlands, possibly due to seasonal dryness and occasional drainage. All the ditches covered approximately 3.3% of the 312 km2 farmland area in the study district, and the total GHG emission from the ditches in this area was estimated to be 26.6 Gg CO2-eq yr-1, with 17.5 Gg CO2, 0.27 Gg CH4, and 0.006 Gg N2O emitted annually. Overall, this study demonstrated that agricultural ditches were hotspots of GHG emissions, and future GHG estimations should incorporate this ubiquitous but underrepresented water course.

Machine Learning Predicts the Methane Clumped Isotopologue (12CH2D2) Distributions Constrain Biogeochemical Processes and Estimates the Potential Budget.

Methane (CH4) is a matter of environmental concern; however, global methane isotopologue data remain inadequate. This is due to the challenges posed by high-resolution testing technology and the need for larger sample volumes. Here, worldwide methane clumped isotope databases (n = 465) were compiled. We compared machine-learning (ML) models and used random forest (RF) to predict new Δ12CH2D2 distributions, which cover valuable and hard-to-replicate methane clumped isotope experimental data. Our RF model yields a reliable and continuous database including ruminants, acetoclastic methane, multiple pyrolysis, and controlled experiments. We showed the effectiveness of utilizing a new data set to quantify isotopologue fractionations in biogeochemical methane processes, as well as predicting the steady-state atmospheric methane clumped isotope composition (Δ13CH3D of +2.26 ± 0.71‰ and Δ12CH2D2 of +62.06 ± 4.42‰) with notable biological contributions. Our measured summer and winter water emitted gases (n = 6) demonstrated temperature-driven seasonal microbial community evolution determined by atmospheric clumped isotope temporal variations (Δ 13CH3D ∼ -0.91 ± 0.25 ‰ and Δ12CH2D2 ∼ +3.86 ± 0.84 ‰), which in turn is relevant for future models quantifying the contribution of methane sources and sinks. Predicting clumped isotopologues translates our methane geochemical understanding into quantifiable variables for modeling that can continue to improve predictions and potentially inform global greenhouse gas emissions and mitigation policy.

Anthropogenic Disturbance Stimulates the Export of Dissolved Organic Carbon to Rivers on the Tibetan Plateau.

The impacts of human activities on the riverine carbon (C) cycle have only recently been recognized, and even fewer studies have been reported on anthropogenic impacts on C cycling in rivers draining the vulnerable alpine areas. Here, we examined carbon isotopes (δ13CDOC and Δ14CDOC), fluorescence, and molecular compositions of riverine dissolved organic matters (DOM) in the Bailong River catchment, the eastern edge of the Tibetan Plateau to identify anthropogenic impacts on the C cycle. Human activities show limited impact on dissolved organic carbon (DOC) concentration, but significantly increased the age of DOC (from modern to ∼1600 yr B.P.) and changed the molecular compositions through agriculture and urbanization despite in the catchment with low population density. Agricultural activities indirectly increased the leaching of N-containing aged organic matter from deep soil to rivers. Urbanization released S-containing aged C from fossil products into rivers directly through wastewater. The aged DOC from agricultural activity and wastewater discharge was partly biolabile and/or photolabile. This study highlights that riverine C is sensitive to anthropogenic disturbance. Additionally, the study also emphasizes that human activities reintroduce aged DOC into the modern C cycle, which would accelerate the geological C cycle.

[Mechanisms of moxibustion preconditioning underlying improving learning-memory ability by regulating polarization of microglia via TLR4/NF-κB signaling pathway in AD rats].

OBJECTIVE: To observe the effect of moxibustion preconditioning on learning-memory ability, Toll like receptor 4(TLR4)/nuclear factor-kappa B(NF-κB) signal pathway related proteins and microglia in rats with Alzheimer's disease (AD), so as to explore its possible mechanisms underlying improvement of AD. METHODS: Male SD rats were randomly divided into normal, sham operation, AD model and pre-moxibustion groups, with 9 rats in each group. Moxibustion was applied to "Baihui"(GV20), "Shenshu"(BL23) and "Zusanli"(ST36) for 15 min, once daily, 6 days as a course of treatment for 3 courses. At the end of moxibustion, the AD model was established by injection of Aß25-35 aggregation solution into the bilateral hippocampus. The sham operation group was only injected with the same amount of 0.9% Nacl solution. The spatial learning-memory ability of rats was detected by Morris water maze test, the ultrastructure of hippocampal neurons was observed by transmission electron microscope (TEM). The histopathological changes of hippocampus tissue were observed by HE staining, and the protein expression levels of TLR4 and NF-κB p65 in the hippocampus detected by Western blot, and the positive expressions of Iba-1, CD80 and CD206 in the hippocampal CA1 region were detected by immunofluorescence labeling. The contents of inflammatory factors IL-1ß, TNF-α and IL-10 in the hippocampus were measured by ELISA. RESULTS: Compared with the sham operation group, the escape latency was significantly increased (P<0.01), and the number of platform quadrant crossing times was decreased (P<0.01) in the model group. In comparison with the model group, the increased escape latency and the decreased platform quadrant crossing times were reversed in the pre-moxibustion group (P<0.01). TEM and light microscope observation showed loose arrangement of cells, enlarged cell space, degeneration, swelling and deformation of hippocampal neurons, rupture of membranes of a large number of cells, reduction of mitochondria, dilation of endoplasmic reticulum, and matrix vacuoles, uneven distribution of organelles and cytoplasm, and being difficult in distinguishing the nuclear cytoplasm in the model group, which was relatively milder in the pre-moxibustion group. The expression levels of hippocampal NF-κB p65 and TLR4, the mean immunofluorescence density of Iba-1 and CD80, as well as the contents of IL-1ß and TNF-α in hippocampal CA1 region were significantly increased in the model group than those in the sham operation group (P<0.01), and obviously decreased in the pre-moxibustion group than those in the model group (P<0.05, P<0.01). Whereas the expression of CD206 and the content of IL-10 were evidently decreased in the model group than those in the sham operation group (P<0.01), and strikingly increased in the pre-moxibustion group than those in the model group (P<0.01). No significant differences were found between the sham operation group and the normal group in all the indexes mention above (P>0.05). CONCLUSION: Pre-moxibustion at GV20, BL23 and ST36 can improve learning-memory ability in AD rats, which may be associated with its functions in promoting the polarization of microglia from M1 to M2 and reducing the neuroinflammatory response by way of TLR4/NF-κB signaling pathway.

High-temporal-resolution of lithium isotopes in Yangtze River headwater: Hydrological control on weathering in high-relief catchments.

How climate change regulates silicate weathering in tectonically active areas remains clear. To evaluate the roles of temperature and hydrology in continental-scale silicate weathering in high-relief catchments, we applied a high temporal resolution of lithium isotopes in the Yalong River, which drains the high-relief borders of the eastern Tibetan Plateau. The dissolved δ7Li values range from +12.2‰ to +13.7‰ in the non-monsoon season and are higher and significantly vary from +13.5‰ to +19.4‰ in the monsoon season. The negative correlation between dissolved δ7Li and the Li/Na ratio is attributed to the formation of various proportions of δ7Li-low secondary minerals during weathering. From non-monsoon to monsoon season, the weathering intensity decreases with increasing secondary minerals formation and the weathering transforms from a supply limited to a kinetically limited weathering regime, indicated by a negative correlation between dissolved δ7Li value and SWR/D ratio (SWR = silicate weathering rate and D = total denudation rate). No correlations between temperature and dissolved δ7Li values were observed, and SWR suggested that temperature is not the direct control factor of silicate weathering in high-relief areas. The dissolved δ7Li values display positive correlations with discharge, physical erosion rates (PERs), and SWR. This positive correlations was attributed to an increase in the PER which caused the formation of more secondary minerals with increasing discharge. These results indicate the rapid temporal variability of riverine Li isotopes and chemical weathering process in response to changes in hydrology rather than temperature. Combined with the compiled PER, SWR, and Li isotopes at various altitudes, we further suggest that weathering in high-altitude catchments is more sensitive to hydrological changes than weathering in low-altitude catchments. These results highlight the key role of the hydrologic cycle (runoff and discharge) and the geomorphic regime in controlling global silicate weathering.

Isolation of dissolved organic matter from aqueous solution by precipitation with FeCl3: mechanisms and significance in environmental perspectives.

Ferric ions can bind strongly with dissolved organic matter (DOM), including humic acids (HA), fulvic acids (FA), and protein-like substances, whereas isolation of Fe-DOM precipitates (Fe-DOMP) and their biochemical characteristics remain unclear. In this work FeCl3 was used to isolate DOM components from various sources, including river, lake, soil, cow dung, and standard tryptophan and tyrosine, through precipitation at pH 7.5-8.5. The Fe-DOMP contribute to total DOM by approximately 38.6-93.8% of FA, 76.2% of HA and 25.0-30.4% of tryptophan and tyrosine, whilst fluorescence spectra allowed to monitor/discriminate the various DOM fractions in the samples. The relative intensity of the main infrared peaks such as 3406‒3383 cm-1 (aromatic OH), 1689‒1635 cm-1 (‒COOH), 1523-1504 cm-1 (amide) and 1176-1033 cm-1 (‒S=O) show either to decline or disappear in Fe‒DOMP. These results suggest the occurrence of Fe bonds with various functional groups of DOM, indicating the formation of π-d electron bonding systems of different strengths in Fe‒DOMP. The novel method used for isolation of Fe-DOMP shows promising in opening a new frontier both at laboratory and industrial purposes. Furthermore, results obtained may provide a better understanding of metal-organic complexes involved in the regulation of the long-term stabilization/sequestration of DOM in soils and waters.

Calcium isotopic geochemistry of geothermal systems in the tectonically active southeastern Tibetan Plateau.

The global Calcium (Ca) cycle is closely coupled to the carbon cycle, and Ca isotopes have potential in tracing it. Even though groundwater is one of the main reservoirs of Ca at the Earth's surface, few data are available for groundwater, and the behavior of Ca and its isotopes in geothermal systems remains unknown. Here we analysed the stable Ca and radiogenic Sr isotope compositions of thermal waters distributed along the Jinsha and Yalong river valleys in the southeastern Tibetan Plateau. The Ca isotopic composition of the thermal water ranges from 0.45 to 2.16 ‰ (δ44/40Ca values relative to SRM 915a). The thermal waters collected from carbonate aquifers have higher δ44/40Ca values than bedrocks, which was attributed to secondary carbonate precipitation accompanied by CO2 degassing. In contrast, δ44/40Ca values in thermal waters collected from clastic and igneous rocks are similar to bedrock. Despite some thermal waters undergoing secondary silicates formation and CaNa ion exchange, such processes maybe not play a significant role in governing the Ca isotopic composition of these thermal waters. This suggests that Ca isotopes can be used to trace secondary carbonate precipitation driven by CO2 degassing (e.g. travertine) in geothermal systems located in tectonically active areas.

Source availability and hydrological connectivity determined nitrate-discharge relationships during rainfall events in karst catchment as revealed by high-frequency nitrate sensing.

Karst terrain seasonal monsoonal rainfall is often associated with high concentrations of nitrate-N in streams draining agricultural land. Such high concentrations can pose problems for environmental and human health. However, the relationship between rainfall events that mobilize nitrate and resulting nitrate export remains poorly understood in karst terrain. To better understand the processes that drive nitrate dynamics during rainfall events, the characteristics of individual rainfall events were analyzed using sensor technology. Thirty-eight rainfall events were separated from the high-frequency dataset spanning 19 months at a karst spring site. The results revealed that nitrate-discharge (N-Q) hysteresis in 79% of rainfall events showed anticlockwise hysteresis loop patterns, indicating nitrate export from long distances within short event periods. Karstic hydrological connectivity and source availability were considered two major determining factors of N-Q hysteresis. Gradual increase in hydrological connectivity during intensive rainfall period accelerated nitrate transportation by karst aquifer systems. Four principal components (PCs, including antecedent conditions PC1&3 and rainfall characteristics PC2&4 explained 82% of the cumulative variance contribution to the rainfall events. Multiple linear regression of four PCs explained more than 50% of the variation of nitrate loading and amplitude during rainfall events, but poorly described nitrate concentrations and hydro-chemistry parameters, which may be influenced by other factors, e.g., nitrate transformation, fertilization time and water-rock interaction. Although variation of N concentration during event flow is evident, accounting for antecedent conditions and rainfall factors can help to predict rainfall event N loading during rainfall events. Pollution of the karstic catchment occurred by a flush of nitrate input following rainfall events; antecedent and rainfall conditions are therefore important factors to consider for the water quality management. Reducing source availability during the wet season may facilitate to reduction of nitrogen loading in similar karst areas.