Chemical Weathering Rates Research Articles

Soil Formation and Mass Redistribution during the Holocene Using Meteoric 10Be, Soil Chemistry and Mineralogy

Soil development and erosion are important and opposing processes in the evolution of high-mountainous landscapes, though their dynamics are not fully understood. We compared soil development between a calcareous and a siliceous chronosequence in the central Swiss Alps at high altitudes, which both cover soil formation over the Holocene. We calculated element mass balances, long-term erosion rates based on meteoric 10Be and we determined the rates of soil formation. We also analyzed the shifts in the mineralogical composition, weathering indices, the particle size distribution, carbon stocks and oxalate extractable Fe, Al, and Mn. The siliceous soils had high chemical weathering rates at the early stage of soil formation that strongly decreased after a few millennia. The development of calcareous soil was characterized by high carbonate losses and a shift to finer soil texture. Soil erosion hampered the upbuilding of soil horizons in the early stages of soil development, which led to a delay in soil and vegetation development. This study shows how soil formation drivers change over time. In the early stages of soil development, the parent material predominantly drives soil formation while at later stages the vegetation becomes more dominant as it influences surface stability, hydrological pathways, and chemical weathering that determine water drainage and retention.

Comparison of soil production, chemical weathering, and physical erosion rates along a climate and ecological gradient (Chile) to global observations

Abstract. Weathering of bedrock to produce regolith is essential for sustaining life on Earth and global biogeochemical cycles. The rate of this process is influenced not only by tectonics, but also by climate and biota. We present new data on soil production, chemical weathering, and physical erosion rates from the large climate and ecological gradient of the Chilean Coastal Cordillera (26 to 38∘ S). Four Chilean study areas are investigated and span (from north to south) arid (Pan de Azúcar), semi-arid (Santa Gracia), Mediterranean (La Campana), and temperate humid (Nahuelbuta) climate zones. Observed soil production rates in granitoid soil-mantled hillslopes range from ∼7 to 290 t km−2 yr−1 and are lowest in the sparsely vegetated and arid north and highest in the Mediterranean setting. Calculated chemical weathering rates range from zero in the arid north to a high of 211 t km−2 yr−1 in the Mediterranean zone. Chemical weathering rates are moderate in the semi-arid and temperate humid zones (∼20 to 50 t km−2 yr−1). Similarly, physical erosion rates are lowest in the arid zone (∼11 t km−2 yr−1) and highest in the Mediterranean climate zone (∼91 t km−2 yr−1). The contribution of chemical weathering to total denudation rates is lower in the arid north than further south. However, due to heterogeneities in lithologies and Zr concentrations, reported chemical weathering rates and chemical depletion fractions are affected by large uncertainties. Comparison of Chilean results to published global data collected from hillslope settings underlain by granitoid lithologies documents similar patterns in soil production, chemical weathering, and total denudation rates for varying mean annual precipitation and vegetation cover amounts. We discuss the Chilean and global data in the light of contending model frameworks in the literature and find that observed variations in soil production rates bear the closest resemblance to models explicitly accounting for variations in soil thickness and biomass.

Geochemical and palynological records of rapid climatic change from the early to late Aptian: evidence from the Jiaolai Basin, eastern China

The Laiyang Group in the Jiaolai Basin, eastern China, contains Lower Cretaceous continental river and lake deposits that provide a complete record of the evolution of the paleoclimate in the Laiyang area from the early to late Aptian. This study used novel chemical weathering indices and palynological data for Lower Cretaceous mudstone in the Laiyang Group coupled with existing geochemical data, clay mineralogy and plant fossil record to reconstruct the geochemical weathering conditions and paleoclimatic evolution of the Jiaolai Basin over this period. Four distinctive episodes of paleoclimatic change were identified, with one linked to an extremely dry–hot climatic event correlated with the Oceanic Anoxia Event (OAE)1a. During Stage 1, strong chemical weathering was correlated with a relatively warm–humid climate during the early Aptian but the intensity of chemical weathering tended to decrease with an increasingly dry and hot climate. In contrast, Stage 2 was characterised by stable conditions with weak chemical weathering and an extremely dry, hot climate during the early and middle Aptian. The presence of organic-rich black shale and the associated distribution of organic carbon isotope values verified that the extremely dry–hot climate during this stage was related to OAE 1a. Stage 3 was a transitional period extending through the middle and late Aptian during which the climate became relatively warm and humid, triggering increases in chemical weathering. Finally, Stage 4 during the late Aptian was characterised by a return to high chemical weathering rates and a warmer climate. Notably, the paleoclimatic changes in the Aptian period indicated by the geochemical record of the Jiaolai Basin are synchronous with those indicated by marine records. The results of this study provide a reference for further study of this climatic event in terrestrial systems within the context of the greenhouse climate in the Cretaceous. KEY POINTS Geochemical and palynological records of rapid climatic change are preserved in lower to upper Aptian terrestrial sediments in eastern China. Chemical weathering in inland east Asia during OAE 1a first increased, then decreased and then increased again. Terrestrial vegetation responded to global climate change during the early Aptian period in eastern China.

U-isotopes and weathering rates in watersheds of Araxá city, Minas Gerais State, Brazil

This work was conducted at Araxá city, Minas Gerais State, Brazil. The aim was to characterize by different approaches weathering rates at six watersheds occurring there. The study area is well-known in the country due to economic and touristic reasons. Beginning in the 1960's and 1970's, nowadays huge mining activities for niobium and phosphate fertilizer exploitation by different companies are taking place there along with the use of natural mineral waters for health treatment, following a tradition that started in the 19th century for tuberculosis treatment. The dataset utilized in this investigation comprised results obtained in the analysis of distinct geochemical compartments, i.e. rocks, soils, bottom sediments, rainwater and surface waters from small hydrographic basins. The waters of three catchments are extensively used by water-supply systems of Araxá city in order to meet the demand of this resource as a possible supply of drinking water for the local community. Hydrochemical (major and trace constituents) and radionuclides (238U, 234U, and 210Po) analyses of rainwater and water bodies samples allowed estimates of the fluxes in each drainage. These fluxes were subtracted from rainfall deposition, yielding positive net values only for bicarbonate and U-isotopes as natural tracers in all watersheds, which allowed to calculate chemical weathering rates of 2.6–38.9 ton/km2yr (bicarbonate) and 0.09–19.8 ton/km2yr (U-isotopes). Physical weathering rates were obtained from 210Pb data in bottom sediments and exceeded 200 to 1.3 × 105 times the chemical weathering rates evaluated by the U-isotopes approach, a finding compatible with others reported in literature but adopting a diverse conceptual framework. Thus, the development of all analytical protocols along this investigation permitted an integrated appraisal of distinct approaches applied to the same selected site, as well as a comparison of weathering rates with other values reported in the literature, improving the knowledge about this subject in Araxá city watersheds. The novel dataset reported in this paper constitutes an aid to the already existing number of weathering rates elsewhere, helping modellers engaged on predicting future landscape changes.

Mechanisms controlling the dissolved load, chemical weathering and CO2 consumption rates of Cauvery River, South India: role of secondary soil minerals

Hydrochemical assessment of the Cauvery River Basin (CRB), an east flowing Western Ghats (WG) river, is carried out to understand the dissolved load sources and controlling mechanisms with quantification of source-wise input to the dissolved load. Silicate weathering rates (SWR) and associated CO2 consumption rates (CCR) are evaluated on account of silicate basement of CRB comprising of granulites and supracrustal rocks. The source-wise solute load contributions estimated using the chemical mass balance model signify that 68% of total load is from chemical weathering followed by anthropogenic (18.5%) and atmospheric (13.5%) inputs respectively, indicating that chemical weathering is the major solute load controlling mechanism for CRB. The intensity of silicate chemical weathering occurring in the CRB is measured by degree of weathering index (Re). It is found to be higher (> 3), suggesting an incomplete weathering of drainage rocks (primary minerals) resulting in formation of soils comprising of secondary minerals including oxides, alumino-silicates and clay minerals (kaolinite, montmorillonite and smectite). Detailed understanding of the mechanism of chemical weathering is carried out using Ca/Na and Mg/Na elemental ratios of different end-members including primary minerals from rocks and secondary minerals from soils. The Na-normalized mixing diagram reveals that chemical weathering of secondary minerals is dominating and the solute load contribution to the total dissolved load is significantly higher (35.5%) from secondary minerals than primary minerals (23.5%). The SWR and associated CCR are estimated to be 13 t km−2 year−1 and 3.3 × 105 mol km−2 year−1, respectively, at outlet (Musiri) of the CRB. Results additionally signify that SWR of the east flowing WG river, Cauvery, are several times (~ 4) lower than the average SWR of west flowing WG rivers even though the associated CCR are comparable for both river systems.

Globally elevated chemical weathering rates beneath glaciers

Physical erosion and chemical weathering rates beneath glaciers are expected to increase in a warming climate with enhanced melting but are poorly constrained. We present a global dataset of cations in meltwaters of 77 glaciers, including new data from 19 Asian glaciers. Our study shows that contemporary cation denudation rates (CDRs) beneath glaciers (2174 ± 977 Σ*meq+ m−2 year−1) are ~3 times higher than two decades ago, up to 10 times higher than ice sheet catchments (~150-2000 Σ*meq+ m−2 year−1), up to 50 times higher than whole ice sheet means (~30-45 Σ*meq+ m−2 year−1) and ~4 times higher than major non-glacial riverine means (~500 Σ*meq+ m−2 year−1). Glacial CDRs are positively correlated with air temperature, suggesting glacial chemical weathering yields are likely to increase in future. Our findings highlight that chemical weathering beneath glaciers is more intense than many other terrestrial systems and may become increasingly important for regional biogeochemical cycles.

Source to sink analysis of weathering fluxes in Lake Baikal and its watershed based on riverine fluxes, elemental lake budgets, REE patterns, and radiogenic (Nd, Sr) and 10Be/9Be isotopes

We present a detailed analysis of weathering fluxes at Lake Baikal, the largest lake in the world, using the major element, trace element and isotope geochemistry of major inflowing rivers, the lake itself, and its sediments. Our objective is to assess how lake records could be used to understand river-catchment-scale denudation and weathering processes.Total denudation rates at Lake Baikal, as obtained from meteoric 10Be/9Be, are an order of magnitude lower than the global average, at 16–35 t km−2 yr−1. Chemical weathering rates obtained from the riverine dissolved load and discharge are, on the other hand, in the same range as global values, at 6–29 t km−2 yr−1. Chemical weathering rates are higher in the north of the catchment than in the south, consistent with higher runoff in the north. In contrast, 10Be/9Be-derived denudation rates are higher in the south. We hypothesize that this pattern of variation may be due to the stabilizing effect of permafrost soils preventing erosion in the north. An inverse model shows that the Selenga River, Lake Baikal’s major tributary, has a silicate weathering contribution to riverine dissolved cation fluxes of 42 mol%; this and other characteristics are representative of large rivers globally. Many trace elements have much lower concentrations in the lake than in inflowing rivers (Be (5%), Mn (3%), Fe (0.4%) and REE (1–2%)). We suggest, based on REE patterns and Mn, Fe-depth profiles in the lake, that this removal is the result of pH induced changes in dissolved-adsorbed partitioning at the river-lake interface, and the incorporation of trace elements into authigenic Fe-Mn (oxyhydr)oxide phases forming within the lake. Strontium is isotopically uniform within the lake, demonstrating that the whole lake mixes on a timescale shorter than its residence time (<330 years). Neodymium and Be, in contrast, show isotopic variability between the basins. While the Sr isotope budget of the lake is largely consistent with observed riverine Sr fluxes, an unradiogenic Nd source is needed to explain lake Nd isotope compositions, especially in the Northern Basin. This source appears to derive from old crustal rocks in this part of the catchment and could be hydrothermal, or small rivers that were not sampled here. Similarly, elevated 10Be/9Be ratios in the lake basins relative to the river input imply variable but significant atmospheric inputs of 10Be into the lake.Overall, this study demonstrates that records of the paleo-chemistry of lakes, and Lake Baikal in particular, hold promise for understanding denudation and weathering on the continents, in ways that are more directly relatable to the environmental conditions of the catchment than is possible with marine records.

Elemental geochemical evidence for controlling factors and mechanisms of transitional organic matter accumulation: The upper Permian Longtan Formation black shale in the Lower Yangtze region, South China

The marine-continental transitional shale in the upper Permian Longtan Formation is not only regarded as an excellent source rock, but also one of the crucial layers of shale gas exploration in the Lower Yangtze region, South China. However, controlling factors and mechanisms of transitional organic matter (OM) accumulation in such a setting are not well understood. In this study, the characteristics of total organic carbon (TOC) and elemental geochemistry of 22 rock samples collected from one core and two different types of outcrops are systematically investigated to characterize the paleoenvironmental conditions and OM accumulation mechanisms. Results show that shales developed in a tidal flat-lagoon environment display high TOC contents ranging from 6.16% to 10.10% (average 8.50%), whereas these formed in a deltaic environment exhibit medium-high TOC contents varying from 0.93% to 6.70% (average 3.26%). These data also elucidate that the target shales in the Longtan Formation were deposited in a complex paleoenvironment with strong water-mass restriction that was mainly characterized by warm and humid paleoclimate, high primary productivity, oxic-to-dysoxic conditions, and a high sedimentary rate. Besides, the transitional OM accumulation is not determined by a single factor, but is the result of the mutual configuration and coupling of multiple factors such as paleoclimate, paleoproductivity, paleoredox, and sedimentary rate. All these factors will directly or indirectly affect OM supply or preservation. Finally, the “integrated model” for transitional OM accumulation is proposed. The model stresses two aspects: on one hand, the warm and humid paleoclimate not only facilitates the growth of higher plants, but also accelerates the chemical weathering rate of the parent rocks and increases the input of nutrients to the water column, which is conducive to the blooms of lower aquatic organisms. Both higher plant debris and lower aquatic organisms together provide abundant OM sources. On the other hand, although the oxidized water environment is usually unfavorable for OM preservation, a higher sedimentary rate can greatly shorten OM exposure time in the decomposition region of aerobic bacteria, and a number of organic matters cannot be oxidized or degraded through the rapid burial. These findings also add to our knowledge that despite the oxygenated water environment during transitional shale deposition, TOC contents are not necessarily lower.

Anomalous weathering trends indicate accelerated erosion of tropical basaltic landscapes during the Permo-Triassic warming

Weathering of basalt in tropical environments is sensitive to climate change and considered to play a significant role in regulating climate evolution. Though extensively studied on the modern Earth and hypothesized for the geological past, basalt weathering is difficult to directly track in geological archives. We here focus on the basaltic sediments derived from the Emeishan large igneous province (LIP) in South China and determined their organic carbon content and isotope, mineral and geochemical compositions. Based on the newly obtained high-precision zircon U-Pb age of 251.79 ± 0.12/0.13/0.36 Ma, we establish temporal variations of weathering intensity for the Emeishan LIP basaltic landscapes in the tropical latitudes through the Permo-Triassic rapid climate warming periods. Analysis of weathering trends indicates a significant decrease in weathering intensity across the Permo-Triassic transition. This decrease in intensity is anomalous considering its coincidence with the climate warming at the Permo-Triassic transition, which tends to elevate chemical weathering rate of basalts and produce sediments with high chemical weathering intensity. Combining these weathering trends with present-day basalt weathering-temperature relationship and reported paleo-temperatures, total erosion rate is quantitatively estimated to increase from 282 ± 43 t/km2/yr in the latest Permian to > 1618 ± 187 t/km2/yr in the Early Triassic. This massive erosion in the Early Triassic is plausibly related to amplified extreme precipitation associated with intensification of the hydrological cycle in the Early Triassic hothouse climate. This study underlines the weathering responses of tropical landscapes to rapid climate warming.

Quantifying Non‐Thermal Silicate Weathering Using Ge/Si and Si Isotopes in Rivers Draining the Yellowstone Plateau Volcanic Field, USA

AbstractIn active volcanic regions, high‐temperature chemical reactions in the hydrothermal system consume CO2 sourced from magma or from the deep crust, whereas reactions with silicates at shallow depths mainly consume atmospheric CO2. Numerous studies have quantified the load of dissolved solids in rivers that drain volcanic regions to determine chemical weathering rates and atmospheric CO2 consumption rates. However, the balance between thermal and non‐thermal components to riverine fluxes in these areas remains poorly constrained, hindering accurate estimates of atmospheric CO2 consumption rates. Here we use the Ge/Si ratio and the stable silicon isotopes (δ30Si) as tracers for quantifying non‐thermal silicon contributions in rivers draining the Yellowstone Plateau Volcanic Field, USA. The Ge/Si ratio (µmol.mol−1) was determined for seven thermal water samples (183 ± 22), eight rivers (35 ± 23) and six creeks flowing into Yellowstone Lake (5 ± 3) during base flow and during peak water discharge following snowmelt. The δ30Si value (‰) was determined for thermal waters (−0.09 ± 0.04), Yellowstone River at Yellowstone Lake outlet (1.91 ± 0.23) and creek samples (0.82 ± 0.29). The calculated atmospheric CO2 consumption associated with non‐thermal waters flowing through Yellowstone's rivers during peak discharge is ∼3.03 ton.km−2.yr−1, which is ∼2% of the annual mean atmospheric CO2 consumption in other volcanic regions. This study highlights the significance of quantifying seasonal variations in chemical weathering rates for improving estimates of atmospheric CO2 consumption rates in active volcanic regions.

Source Region Geochemistry From Unmixing Downstream Sedimentary Elemental Compositions

AbstractThe geochemistry of river sediments is routinely used to obtain information about geologic and environmental processes occurring upstream. For example, downstream samples are used to constrain chemical weathering and physical erosion rates upstream, as well as the locations of mineral deposits or contaminant sources. Previous work has shown that, by assuming conservative mixing, the geochemistry of downstream samples can be reliably predicted given a known source region geochemistry. In this study, we tackle the inverse problem and “unmix” the composition of downstream river sediments to produce geochemical maps of drainage basins (i.e., source regions). The scheme is tested in a case study of rivers draining the Cairngorms, UK. The elemental geochemistry of the μm fraction of 67 samples gathered from the beds of channels in this region is used to invert for concentrations of major and trace elements upstream. A smoothed inverse problem is solved using the Nelder‐Mead optimization algorithm. Predictions of source region geochemistry are assessed by comparing the spatial distribution of 22 elements of different affinities (e.g., Be, Li, Mg, Ca, Rb, U, V) using independent geochemical survey data. The inverse approach makes reliable predictions of the major and trace element concentration in first order river sediments. We suggest this scheme could be a novel means to generate geochemical baselines across drainage basins and within river channels.

Impacts of chemical weathering and human perturbations on dissolved loads of the Wei River, the Yellow River catchment

As the largest tributary of the Yellow River, the Wei River drains a basin mainly comprising two different geological settings, i.e., the Chinese Loess Plateau (CLP) in the north covered by Quaternary loess and the Qinling Mountains area in the south. Exploring the influence of natural weathering processes and human activities on river chemistry has high significance in understanding chemical transport from the Wei River to the Yellow River. The geochemical characteristics of the Wei River mainstream and its tributaries as well as chemical weathering were determined using a geochemistry and isotope dataset of high spatial resolution. The mainstream clearly exhibited a mixture of major elements and stable isotopes from tributaries on both the north and south banks, with values within the range of those of the tributaries. North-bank tributaries draining thick loess deposits showed higher contents of major elements and the enrichment of stable isotopes, reflecting the influence of geological setting on river chemistry. The dominant processes accounting for the water chemistry of the mainstream are evaporite (39–75%) and carbonate weathering (21–54%). Within the watershed, evaporite dissolution and silicate weathering are higher in the northern CLP area, whereas carbonate weathering is higher in the southern Qinling Mountain area. The CO2 consumption by the Wei River is 19.9 × 109 mol/yr, accounting for 32.5% of the total CO2 consumption by the middle reaches of the Yellow River. During the high-flow period, the chemical weathering flux of the Wei River was found to be within the range of northern rivers in China, while the chemical weathering rate was relatively high in the semi-arid region. This suggests that chemical denudation is a significant factor for the river chemistry of the CLP at least in the high-flow period.

Elucidating Pathfinding Elements from the Kubi Gold Mine in Ghana

X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) are applied to investigate the properties of fine-grained concentrates on artisanal, small-scale gold mining samples from the Kubi Gold Project of the Asante Gold Corporation near Dunwka-on-Offin in the Central Region of Ghana. Both techniques show that the Au-containing residual sediments are dominated by the host elements Fe, Ag, Al, N, O, Si, Hg, and Ti that either form alloys with gold or with inherent elements in the sediments. For comparison, a bulk nugget sample mainly consisting of Au forms an electrum, i.e., a solid solution with Ag. Untreated (impure) sediments, fine-grained Au concentrate, coarse-grained Au concentrate, and processed ore (Au bulk/nugget) samples were found to contain clusters of O, C, N, and Ag, with Au concentrations significantly lower than that of the related elements. This finding can be attributed to primary geochemical dispersion, which evolved from the crystallization of magma and hydrothermal liquids as well as the migration of metasomatic elements and the rapid rate of chemical weathering of lateralization in secondary processes. The results indicate that Si and Ag are strongly concomitant with Au because of their eutectic characteristics, while N, C, and O follow alongside because of their affinity to Si. These non-noble elements thus act as pathfinders for Au ores in the exploration area. This paper further discusses relationships between gold and sediments of auriferous lodes as key to determining indicator minerals of gold in mining sites.

Study of Chemical Weathering and CO2 Consumption Rate of Granite in a Small Subtropical Watershed

Study of Chemical Weathering and CO2 Consumption Rate of Granite in a Small Subtropical Watershed

Deciphering the signatures of weathering and erosion processes and the effects of river management on Li isotopes in the subtropical Pearl River basin

Lithium (Li) isotopes signatures in sedimentary archives are promising proxies to reconstruct paleo-weathering rates and paleo-seawater composition. Nonetheless, the interpretation of the seawater Li isotope composition evolution over the Cenozoic is still debated. In this aim, the study of large rivers can provide constraints for continental weathering flux to the ocean. However, in the recent past, human activities like flow regulation via reservoir construction, have significantly modified the hydrodynamic of riverine systems, impacting the transfer of matter from the continent to the ocean. Hence, these effects need to be considered and investigated when unravelling the signature of natural processes from river borne materials.We report Li concentrations and Li isotope compositions (δ7Li) for the dissolved and sediment loads of subtropical river waters from the Xijiang basin, the largest tributary of the Pearl River (also known as the Zhujiang), Southern China. These rivers display low dissolved Li concentrations. Although they are potentially impacted by carbonate dissolution in the upstream karst terrains, the Li budget of these rivers is dominated by silicate weathering. The dissolved Li isotope compositions of rivers vary over a range from +16‰ to +27‰. These values are high compared to the estimated composition of the bedrock (~0‰), and are explained by the preferential incorporation of the light 6Li in solid weathering products formed during the incongruent dissolution of silicate rocks. We show that the variations of the dissolved δ7Li values across the basin reflect changes in silicate weathering intensity which controls the formation of secondary phases. The high δ7Li values in the headwaters reflect greater degree of secondary phase formations associated with the higher erosion rates in the mountainous part of the basin, whereas the lower δ7Li values in the lowlands characterized by low soil erosion rates and more abundant rainfall reflect a more intensive weathering. This interpretation is supported by the positive relationship between soil erosion rates and the Li dissolved fluxes. The isotope compositions of the suspended particulate material (SPM) transported by rivers in the Xijiang Basin (from −3.8‰ to −0.7‰) are systematically lower than the composition of the bedrock. We observe that the δ7Li of most SPM samples collected during the wet season are lower than those of samples collected during the dry season. We interpret this seasonality as a consequence of seasonal changes in the sediment properties due to water regulation. In spite of the influence induced by river managements on the river sediments, our data also highlight that the imprint of weathering and erosion processes is still preserved in the sedimentary load of the Xijiang as suggested by the relationship between the SPM δ7Li values and the weathering intensity defined as the ratio between the chemical weathering rates and the total denudation rates. Altogether, our study confirms the robustness of Li isotopes as a proxy of weathering and erosion processes, even in this heavily-managed river system.

Potential influence of rapid climate change on elemental geochemistry distributions in lacustrine sediments—A case study at a high Arctic site in Ny-Ålesund, Svalbard

Metal contamination has become an increasingly severe environmental issue due to intense anthropogenic activities in recent decades. Many studies have reported a rapidly increasing trend of heavy metal contents in sedimentary records. In this study, two lacustrine sediment cores (LDL and YL) far away from scientific research stations were collected in Ny-Ålesund and analyzed for the vertical distributions of 17 elemental concentrations (Cu, Zn, Pb, Co, Ni, Cr, Sr, Ba, Mn, P, Ti, K2O, Na2O, CaO, MgO, Fe2O3, Al2O3), CIA and TOC contents. The results indicated that only the proxies Pb, P, CaO, TOC, and CIA showed an increasing trend in the upper 7 cm section of the sediment cores, while most of the elements' concentrations decreased towards the surface. The rapid increase of TOC contents is likely related to the climate warming over the past 200 years, which promotes the prosperity of vegetation and thus leads to more input of organic matter into the lakes. Moreover, a large number of seabirds live around the sampling position and the seabird guano contains high concentrations of P, which could be regarded as an important nutrient source for vegetation. Additionally, the rapid climate warming could accelerate the chemical weathering rates, and thus lead to increased CaO contents in the sediment profiles according to its geological background. Therefore, the concentrations of other elements are very likely diluted by the high contents of organic matter and CaO in the upper part of the sediment cores. It is noteworthy that the rapidly increasing trend of Pb contents are related to the gas-oil powered generators in Ny-Ålesund and long-range atmospheric transport from Europe. This study highlighted the nonnegligible influence of climate warming on the inorganic elemental geochemistry distributions in remote lakes.

Denudation rates of granitic regolith along climatic gradient in Eastern China

In order to assess the impact of the monsoonal climate on denudation, physical erosion, and chemical weathering processes, we measured cosmogenic nuclides 10Be, 26Al and immobile element Zr in five granitic regolith profiles across Northeast to South China. The 10Be and 26Al concentrations are 2.98 × 104–25.20 × 104 atoms g−1 and 2.18 × 105–16.62 × 105 atoms g−1, respectively. Compiling previous depth profile data from Leymon and Beacon Heights, we find that the 26Al/10Be ratios systematically increase or decrease with the depth. Our numerical model shows that muons contributions can produce higher 26Al/10Be ratios (greater than 6.75) in both steady-state and non-steady state scenarios, which can result in underestimated burial age (initial 26Al/10Be ratio) and exaggerated denudation rates (due to non-negligible production from muons) for the application of dual-nuclide in Earth Science. The denudation rates, chemical weathering rates and physical erosion rates derived from cosmogenic nuclides and Zr are 28–45, 9–31 and 6–22 m Ma−1, respectively. Chemical weathering rates, excluding the JLN profile, comprise a large proportion of the denudation rates (greater than 50%). Despite the wide range of mean annual precipitation (MAP) and mean annual temperature (MAT), the denudation rates vary without an apparent gradient from northeast to south. However, the physical erosion and chemical weathering results show good relationships with climate, suggesting the limitation type changed from kinetic-limited in the north, to supply-limited in the south. Chemical weathering indices (chemical index of alteration, CIA and weathering intensity factor, WIF) also indicate that the extent of chemical weathering strengthens from northeast to south and increases with chemical weathering rates, but decreases with physical erosion rates.

Evaluating Freshwater Mussel Shell δ13C Values as a Proxy for Dissolved Inorganic Carbon δ13C Values in a Temperate River

AbstractThe stable isotope ratio of dissolved inorganic carbon (δ13CDIC) in rivers reflects the dominant vegetation type in the surrounding watershed, rates of chemical weathering, atmospheric CO2 fluxes, and the relative rates of photosynthesis and respiration. Reconstructing past δ13CDIC values may reveal changes in these characteristics before watersheds experience land‐use change and/or climate change. This study uses freshwater mussels, Elliptio complanata, and coeval environmental data to assess how high‐resolution changes in the oxygen and carbon isotope ratios of shell carbonate (δ18Oshell and δ13Cshell, respectively) can be used as proxies of paleotemperature and paleo‐DIC in rivers. To test our hypotheses, we analyzed time‐series δ18Oshell and δ13Cshell values, δ13C values in bivalve tissues, and environmental data collected fortnightly from the Neuse River, North Carolina. Shell microsamples milled every 150 μm along the maximum growth axis represent an average of 12 ± 5 days (n = 524; excluding periods of growth cessation of &gt;30 days), which is similar to the environmental data resolution (∼14 days). Serially sampled δ18Oshell and δ13Cshell values did not capture the full range of environmental conditions due to growth cessation during winter shutdown and extreme weather events. Low water temperature and elevated turbidity appear to be significant drivers of growth cessation. Spatial and temporal differences in the amount of metabolic carbon incorporated in the shell (∼0%–44%) likely occur due to variable mussel metabolic rates within and among study sites. Though high‐resolution δ13Cshell values did not reflect variations in δ13CDIC values, average δ13Cshell values were indicative of average δ13CDIC values in the Neuse River.

Chemical weathering and carbon dioxide consumption in a small tropical river catchment, southwestern India

Studies done on small tropical west-flowing river catchments located in the Western Ghats in southwestern India have suggested very intense chemical weathering rates and associated CO2 consumption. Very less studies are reported from these catchments notwithstanding their importance as potential sinks of atmospheric CO2 at the global scale. A total of 156 samples were collected from a small river catchment in the southwestern India, the Payaswini–Chandragiri river Basin, during pre-monsoon, monsoon and post-monsoon seasons in 2016 and 2017, respectively. This river system comprises two small rivers originating at an elevation of 1350 m in the Western Ghats in peninsular India. The catchment area is dominated by biotite sillimanite gneiss. Sodium is the dominant cation, contributing ~ 50% of the total cations, whereas HCO3− contributes ~ 75% of total anions. The average anion concentration in the samples varied in the range HCO3− > Cl− > SO42− > NO3− > F−, whereas major cation concentration varied in the range Na+ > Ca2+ > Mg2+ > K+. The average silicate weathering rate (SWR) was 42 t km−2 y−1 in the year 2016 and 36 t km−2 y−1 in 2017. The average annual carbon dioxide consumption rate (CCR) due to silicate rock weathering was 9.6 × 105 mol km−2y−1 and 8.3 × 105 mol km−2 y−1 for 2016 and 2017, respectively. The CCR in the study area is higher than other large tropical river catchments like Amazon, Congo-Zaire, Orinoco, Parana and Indus because of its unique topography, hot and humid climate and intense rainfall.

Small-catchment perspective on chemical weathering and its controlling factors in the Nam Co basin, central Tibetan Plateau

Very little data is available on chemical weathering and its controlling factors in small catchments of the central Tibetan Plateau (TP) despite being the main part of the plateau. Hydrochemistry of river water in 24 small catchments in the Nam Co basin draining different strata and recharge forms was investigated monthly during the 2018 monsoon season. The results show that carbonate dissolution is the most important geochemical process controlling the evolution of hydrochemistry of most river water in the Nam Co basin (the average contribution to riverine solutes reaches 46%) even though its contribution shows significant spatial distribution: low values occur in some granite regions (e.g. 27% in H10) but is high in catchments where carbonate rocks are prevalent (e.g. 68% in H23). The contribution of silicate weathering is low, with an average of 11% in catchments located on the east and south banks draining sandstone/mudstone, while this value increases to 32% in catchments on the south bank which pass through granite, and on the west bank. The contribution of sulfide oxidation in most rivers on the south bank is unexpectedly high (40%) but is much lower in other rivers (13%). This spatial difference is attributed to the influence of topography, particularly slope. The carbonate weathering rate (CWR) in small catchments of the Nam Co basin shows spatial heterogeneity and is significantly and positively correlated with the lithologic coefficient which proposed to indicate the degree of rock weatherability and negatively correlated with mean slope and mean altitude. The spatial distribution of the silicate weathering rate (SWR) is not as significant as the CWR and is mainly governed by runoff depth. Chemical weathering rates in the glacierised catchments are 1.3 times higher than those in the non-glacierised regions which share similar lithology. Thus, glaciation enhances chemical weathering in the study area. The area-weighted average value of CWR and SWR in the study area is 23.2 t/km2/y and 5.2 t/km2/y, respectively. Chemical weathering rates in the Nam Co basin, central TP are relatively low when compared with those in catchments located on the edge of the TP, indicating the carbon sequestration effect caused by this supergene geological process in this area may be insignificant.