Silicate weathering is the primary control of atmospheric CO2 concentrations on multiple timescales. However, tracing this process has proven difficult. Lithium isotopes are a promising tracer of ...silicate weathering. This study has reacted basalt sand with natural river water for ~9 months in closed experiments, in order to examine the behaviour of Li isotopes during weathering. Aqueous Li concentrations decrease by a factor of ~10 with time, and δ7Li increases by ~19‰, implying that Li is being taken up into secondary phases that prefer 6Li. Mass balance using various selective leaches of the exchangeable and secondary mineral fractions suggest that ~12–16% of Li is adsorbed, and the remainder is removed into neoformed secondary minerals. The exchangeable fractionation factors have a Δ7Liexch-soln = −11.6 to −11.9‰, while the secondary minerals impose Δ7Lisecmin-soln = −22.5 to −23.9‰. Overall the experiment can be modelled with a Rayleigh fractionation factor of α = 0.991, similar to that found for natural basaltic rivers. The mobility of Li relative to the carbon-cycle-critical cations of Ca and Mg changes with time, but rapidly evolves within one month to remarkably similar mobilities amongst these three elements. This evolution shows a linear relationship with δ7Li (largely due to a co-variation between aqueous Li and δ7Li), suggesting that Li isotopes have the potential to be used as a tracer of Ca and Mg mobility during basaltic weathering, and ultimately CO2 drawdown.
Chemical weathering plays an important role in sequestering atmospheric CO2, but its potential influence on global climate over geological timescales remains debated. To some extent, this uncertainty ...arises from the difficulty in separating the respective contribution of sedimentary and crystalline silicate rocks to past weathering rates in the geological record; two types of rocks having presumably different impact on the long-term carbon cycle. In this study, we investigate the use of rare earth element (REE) and neodymium isotopes (εNd) in leached iron oxide fractions of river sediments for tracing the origin of weathered rocks on continents. A new index, called ‘concavity index’ (CI), is defined for measuring the degree of mid-REE enrichment in geological samples, which enables the determination of the source of iron oxides in sediments, such as seawater-derived Fe-oxyhydroxide phases, ancient marine Fe oxides derived from the erosion of sedimentary rocks, and recent secondary oxides formed in soils via alteration of crystalline silicate rocks or pyrite oxidation. Using this index, we demonstrate that the εNd difference between paired Fe-oxide and detrital fractions in river sediments (defined here as ∆εNd Feox-Det) directly reflects the relative contribution of sedimentary versus crystalline silicate rocks during weathering. While rivers draining old cratons and volcanic provinces display near-zero ∆εNd Feox-Det values indicative of dominant silicate weathering (0.5 ± 1.1; n = 30), multi-lithological catchments hosting sedimentary formations yield systematically higher values (2.7 ± 1.2; n = 44), showing that sedimentary rock weathering can be traced by the occurrence of riverine Fe oxides having more radiogenic Nd isotope signatures compared to detrital fractions. This assumption is reinforced by the evidence that calculated ∆εNd Feox-Det values agree well with previous estimates for carbonate and silicate weathering rates in large river basins.
Examining the influence of climate and tectonics on measured Nd isotopic compositions, we find that ∆εNd Feox-Det is strongly dependent on temperature in lowlands, following an Arrhenius-like relationship that reflects enhanced alteration of silicate rocks and formation of secondary Fe oxides in warmer climates. In contrast, in high-elevation catchments, ∆εNd Feox-Det defines striking correlation with maximum basin elevation, which we also interpret as reflecting the intensification of silicate weathering and associated Fe oxide formation as elevation decreases, due to the combined effects of thicker soils and warmer temperature.
Overall, our new findings are consistent with previous assertions that the alteration of sedimentary rocks prevails in high-elevation environments, while silicate weathering dominates in floodplains. This novel approach combining REE and Nd isotopes opens new perspectives for disentangling the weathering signals of sedimentary and crystalline silicate rocks in the geologic record, which could be used in future studies to reassess the causal relationships between mountain uplift, erosion and climate throughout Earth's history.
•World survey of REE and Nd isotopes in riverine Fe-oxides and detrital sediments•A new REE index (concavity index) is defined to determine the types of iron oxides.•ΔεNd Feox-Det as a new proxy for sedimentary versus crystalline rock weathering•ΔεNd Feox-Det correlates with temperature and maximum elevation in watersheds.•Shift from silicate- to carbonate-weathering regime from lowlands to mountains
A 1.21-ha pilot study in SE Ireland investigated crushed returned concrete (CRC), applied at a rate of 7.5 tonnes/ha as a soil amendment for carbon dioxide removal (CDR) by enhanced weathering (EW) ...over a 10-month period. Most, but not all amended sites showed enhanced concentrations of calcium and bicarbonate in shallow soil waters compared with adjacent controls. Soil pH increased rapidly by 0.3 to >1 pH units (5.75 ± 0.25 (n = 8) in controls to 6.5 ± 0.7 (n = 24) at amended sites), with greatest increases in soils that had lowest initial pH (<6.0). Aside from one site where concentrations continued to increase, calcium (1.76–0.39 mM Ca) and bicarbonate (4.06–0.64 mM) values in soil waters declined gradually over the course of the study, consistent with rapid initial dissolution and progressive exhaustion of the fine-grained fraction, as indicated by a shrinking-core model. Most sites exhibited high bicarbonate to cation ratios (1.2–2.2) consistent with CRC dissolution by soil carbonic acid, although two sites adjacent to the field-boundary river that floods periodically showed evidence for strong acid (nitric) dissolution. Enhanced bicarbonate levels in soil waters at amended sites compared with controls, coupled with a downward-flush hydrological model driven by annual effective rainfall to calculate bicarbonate export and a simple stoichiometric correction for nitric acid weathering, indicate gross CDR rates in the range zero to 0.52 tonnes CO2/ha (69 kg CO2/tonne), limited by bicarbonate export rather than weathering rates. Further optimisation of the CDR efficiency of this material should be possible using strategies to avoid or pre-neutralise non-carbonic soil acids and by taking account of sub-aerial carbonation at the post-crushing, pre-amendment stage that arises when the material is crushed in preparation for EW applications. A simplified life-cycle analysis indicates that a discount of about 11% should be applied to account for CO2 emissions from crushing, transport and spreading operations.
•Crushed returned concrete (CRC) investigated as a soil amendment for carbon dioxide removal (CDR).•CDR rates of up to 0.55 tonnes CO2 per hectare at low nitrate sites, but zero at high nitrate sites.•CRC dissolution kinetics are fast; most material weathers within 12 months.•Detection and characterisation of spatial heterogeneities in soil-water dissolved bicarbonate are challenges for commercial scale CDR.
Weathering of organic carbon contained in sedimentary rocks (petrogenic OC, OCpetro) is an important control on the concentrations of carbon dioxide (CO2) and oxygen in the atmosphere. Of particular ...significance are steep mountainous catchments, where high rates of physical erosion introduce OCpetro to the surface, where oxygen in air and water can help drive oxidative weathering reactions, yet measurements of CO2 emissions from OCpetro oxidation are still scarce. Here, we use in situ gas accumulation chambers and show that CO2 fluxes, and their environmental controls, can be determined during a stand-alone, short-term (8 days) field campaign, applied to a remote setting. In the rapidly eroding Waiapu River catchment, New Zealand, dominated by mudstones, we measured high rates of CO2 release (222–1590 mgC m−2 d−1) in five accumulation chambers in the near-surface of naturally fractured and bedded rock outcrops. The corresponding CO2 concentrations are very high (pCO2 ~4700–27,100 ppmv), and such values could influence acid-hydrolysis reactions during chemical weathering of co-occurring silicate minerals. The CO2 is radiocarbon depleted (fraction modern, F14C = 0.0122–0.0547), confirming it is petrogenic in origin. Stable carbon isotopes suggest a source from OCpetro, but δ13C values of the CO2 are lower by ~3.5–3.7 ± 0.1 ‰ from those of OCpetro (−25.9 ± 0.1 ‰), consistent with isotope fractionation associated with microbial respiration of OCpetro. Over 6 days of measurement, we find that CO2 fluxes respond quickly to changes in temperature and humidity, indicating an environmental regulation that is captured by our short-term installation. The approaches applied here mean that future research can now seek to constrain the climatic, lithological and biological controls on OCpetro oxidation across regional to global scales.
•CO2 fluxes from oxidative weathering can be measured during short field campaigns.•Their size, regulated by temperature and humidity, reaches that of soil respiration.•Carbon isotopes reveal that measured CO2 is sourced from petrogenic organic matter.•Microbial respiration is potentially associated with carbon isotope fractionation.•Partial pressures of CO2 in rock can be high and induce silicate weathering.
This study has analysed Li isotopes ratios from well-studied soil and pore water profiles from Iceland that have the same parent material but have experienced different degrees of chemical ...weathering. Thus, from least to most weathered, we have analysed vitrosols (V), gleyic andosols (GA), brown andosols (BA), Histosols (H) and Histic Andosols (HA). Although the most weathered H and HA soils have the highest content in clay-sized material, they have the least fractionated δ7Lipore water values. In contrast, the least weathered GA and BA pore waters are most fractionated for Li isotopes. Given that Li isotope ratios are fractionated by clay mineral formation, this appears counter-intuitive. A single trend for all samples of δ7Li as a function of Li/Na ratios suggests that they are all controlled by a process with a single fractionation factor, in this case likely the formation of poorly-crystalline allophane, which dominates in the “least weathered” soils. This rapidly forming secondary mineral dominates Li isotope fractionation over more slowly-forming crystalline clays. The fractionation along a single path shows that the key process here in controlling the Li isotope ratio of surface waters is the degree of Li uptake by secondary minerals. This does not necessarily correspond to the amount of clay minerals present in the soil, but to the amount of clay minerals that are being newly formed in a single passage of the pore water through the soil, or are in equilibrium with soil solutions at the time of sampling.
Weathering is a foundational process in most Earth systems, but there has been a lack of data directly quantifying what influences mechanical weathering. Here we use multiple years of in situ field ...data, “listening” to acoustic emissions of naturally cracking rocks, to test a hypothesized link between climate and subcritical crack‐tip processes (i.e., the bond‐breaking mechanism thought to embody most mechanical weathering). Our results challenge the assumption of a singular dependence of mechanical weathering on stresses. We find that mechanical weathering rates exponentially increase as functions of atmospheric vapor pressure (VP), temperature, and relative humidity, even when controlling for stress‐loading. VP exerts the most pronounced influence on the observed mechanical weathering rates. Put in the context of global climate change, our results underscore the potential for climate‐dependent subcritical crack‐tip processes to influence all weathering‐allied problems including the long‐term stabilization of the climate by weathering‐carbon‐cycle feedbacks.
Plain Language Summary
Weathering refers to the mechanisms by which rocks physically and chemically break down into soil, sediment, and dissolved molecules. Chemical weathering rates are frequently inferred to be strongly coupled to climate, because chemical reactions depend on factors like moisture and temperature. In past studies, climate has been connected to physical weathering only through its influence on stress‐inducing processes like freezing or temperature cycling. In this study, we use field observations of the sounds that natural rock cracking makes and find that cracking accelerates in warmer, wetter conditions, even when controlling for stresses. Thus, this study provides field data to support a climatic influence on weathering via a pathway—molecular bond‐breaking at crack‐tips—that is additional to, and distinct from, how climate may influence stresses or chemical weathering. Accordingly, any system connected to weathering, such as Earth's carbon cycle, surface erosion, or biosphere, could be impacted by this additional process of accelerated rock breakdown during warmer and wetter climates, such as those predicted under modern global warming trends.
Key Points
After controlling for stress‐loading, mechanical weathering rates—measured in situ—still strongly correlate with moisture and temperature
Vapor pressure's influence on crack‐tip subcritical cracking processes appears to govern observed trends, independent of stress magnitude
Recognizing this novel facet of weathering's climate‐dependence will enhance understanding of all climate‐dependent systems like the carbon cycle and landscape evolution
Most of the ions in rivers come from the migration of weathering products of continental rocks. However, there is no clear understanding of the dynamic changes in ions derived from the chemical ...weathering of rocks and their feedback to climate change. This affects our comprehension of the evolution of life and the cycle of matter on Earth. In the present study, we established a dataset with a 0.25° × 0.25° spatial resolution for the major dissolved ions and their total fluxes in the riverine transported (ICWR) at the global scale from 1980 to 2020 using the Lechuga-Crespo model and the random forest algorithm. The results show that from 1980 to 2009, the total amount of the ICWR was 5.4·109 Mg yr−1, of which alkalinity accounted for 45%, Ca2+ accounted for 17%, SO42− accounted for 14%, Na+ accounted for 9%, Cl− accounted for 8%, Mg2+ accounted for 6%, and K+ accounted for 2%. In addition, the ICWR increased at a rate of 6.47·106 Mg yr−1. The most obvious trend was in the area with high ionic activity coefficients between 30° N and 30° S. The growth trend of the ICWR during 2010–2020 was seven times that during 1980–2009, resulting in a 30% increase in the ICWR over the past 40 years. This study highlights the critical role of climate change in the transport and evolution of the ICWR, and explores their impact on vegetation change. It has important reference value for responding to the eco-environmental problems caused by climate change.
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•Climate change has caused a 30% increase in the ICWR.•The rate of change in alkalinity was the fastest, and the rate of change in Mg2+ was the slowest.•Runoff, nitrogen deposition, and soil moisture were important factors affecting the changes in the ICWR.•A dataset was created for the riverine transported ICWR on a global scale, from 1980 to 2020.•The positive effect of ICWR on NPP was greater than the negative effect.
Apatite's ubiquity in crystalline rocks, variable trace element contents (particularly with regard to the REE, actinides and Sr), and amenability to various dating techniques based on the decay of ...the radioisotopes U and Th, permit specific provenance determinations. In this study, we first present a comprehensive description of the trace element behaviour of apatite in various kinds of bedrocks (igneous rocks from felsic through to ultramafic compositions, metamorphic rocks from low to high grades and of diverse protolith composition, and authigenic apatite) in which we explain why apatite is so highly diverse in terms of its trace element composition. Next, we present a synthesis of bedrock apatite trace-element compositional data from previous work, assembling a library of apatite compositions that includes the most abundant apatite-bearing lithologies in the Earth's crust, and many other less abundant rock types. Compositional statistics, classification, and a machine learning classifier are then applied to this dataset to generate biplots that can be used to determine the broad source lithology of detrital apatite, with misclassification averaging 15%. This methodology is tested in three case studies to demonstrate its utility. In these examples, detrital apatite can be convincingly linked to different lithology types, and combined apatite trace-element and UPb data can determine the terranes from which individual apatites were likely derived. The addition of apatite trace-element information therefore enables the determination of the source lithology, making the extraction of novel information and more specific provenance determinations possible, and opening up new avenues in source-to-sink modelling.
•Apatite trace element composition is diverse, and faithful to its source rock.•Apatite UPb peak de-convolution is possible using trace element composition.•Combined apatite UPb and trace element analysis permits hyper-specific provenance•Apatite trace element composition is a low-bias provenance indicator
•U-series isotopes directly determined weathering duration and rates in weathering clasts.•Clast weathering rates correlate strongly and positively with annual runoff values in watersheds.•∼1800 ...times constantly higher weathering rates observed in watersheds than in clasts.•Rate discrepancy attributed to the fractal nature of the roughness and surface area across scales.
To elucidate the factors that contribute to the orders of magnitude difference generally observed in apparent weathering rates across scales, we utilized a U-series isotopic technique to directly determine the duration and rates of chemical weathering recorded in weathering clasts. In this study, we systematically compiled 15 individual volcanic weathering clasts collected from eight andesitic watersheds along a gradient of mean annual precipitation ranging from ∼1600 mm to 6400 mm on the tropical volcanic Basse-Terre Island of French Guadeloupe. We measured U-series isotope compositions to quantify weathering advance rates along the core-rind transects in these weathering clasts. Total rind formation ages, the length of time it took for these weathering clasts to form from andesitic rock fragments or volcanic clasts in the soil zones, range from 60 kyr to 300 kyr. As expected for each individual core-rind weathering transect, the rind ages generally increase almost linearly with distance away from the core-rind boundary. The derived clast weathering rates range from 0.08 ± 0.04 to 0.34 ± 0.01 mm kyr−1, and the rates exhibit a strong positive correlation (R2=0.74) with the annual runoff values of these watersheds. This correlation documents the first direct evidence that weathering over geological timescales on the island is controlled by the amount of precipitation and corroborates the relationship that was previously reported for the riverine weathering fluxes at Basse-Terre Island in Guadeloupe, but measured with much shorter time scales. The clast-scale rates (inferred for the characteristic length scale of mm) are compared to the watershed-scale rates of Basse-Terre Island (length scale of km): constantly higher rates are observed at the watershed scales than the clast scale and the discrepancy equals ∼1800 ± 400 times, despite changes in watershed characteristics such as watershed size, relief, and runoff values. The discrepancy is attributed to the fractal nature of the roughness and surface area across scales. For example, the presence of undulations and fractures (e.g. roughness) at the bedrock-saprolite contact of the watershed, which is generally not assessed in the watershed surface area estimates, is most likely the main contributor to the constant rate difference of ∼1800 times.
The chemical weathering of rocks currently absorbs about 1.1 Gt CO2 a−1 being mainly stored as bicarbonate in the ocean. An enhancement of this slow natural process could remove substantial amounts ...of CO2 from the atmosphere, aiming to offset some unavoidable anthropogenic emissions in order to comply with the Paris Agreement, while at the same time it may decrease ocean acidification. We provide the first comprehensive assessment of economic costs, energy requirements, technical parameterization, and global and regional carbon removal potential. The crucial parameters defining this potential are the grain size and weathering rates. The main uncertainties about the potential relate to weathering rates and rock mass that can be integrated into the soil. The discussed results do not specifically address the enhancement of weathering through microbial processes, feedback of geogenic nutrient release, and bioturbation. We do not only assess dunite rock, predominantly bearing olivine (in the form of forsterite) as the mineral that has been previously proposed to be best suited for carbon removal, but focus also on basaltic rock to minimize potential negative side effects. Our results show that enhanced weathering is an option for carbon dioxide removal that could be competitive already at 60 US $ t−1 CO2 removed for dunite, but only at 200 US $ t−1 CO2 removed for basalt. The potential carbon removal on cropland areas could be as large as 95 Gt CO2 a−1 for dunite and 4.9 Gt CO2 a−1 for basalt. The best suited locations are warm and humid areas, particularly in India, Brazil, South-East Asia and China, where almost 75% of the global potential can be realized. This work presents a techno-economic assessment framework, which also allows for the incorporation of further processes.
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