Silicate Weathering Research Articles

Hydrochemical Appraisal and Driving Forces of Groundwater Quality and Potential Health Risks of Nitrate in Typical Agricultural Area of Southwestern China

Elucidating the hydrogeochemical processes and quality assessment of groundwater holds significant importance for its sustainable development. In this paper, 53 groundwater samples were collected from a typical agricultural area in the northeastern Chongqing municipality in SW China. The integration of multivariate statistical analysis, ion ratio analysis, geomodelling analysis, the entropy water quality index, health risks assessment, and sensitivity analysis was carried out to explore the hydrochemical processes and quality assessment of groundwater in this study. The statistical results reveal that the cationic concentrations followed the order of Ca2+ > Mg2+ > Na+ > K+, while the anionic components were in the order of HCO3− > SO42− > NO3− > Cl−. Based on the Piper trilinear diagram, the hydrochemical types were shown as Ca-HCO3 and Ca-Mg-HCO3 types. Hierarchical cluster analysis indicated that the groundwater samples could be categorized into three groups. The hydrochemical compositions were primarily influenced by water–rock interactions (e.g., carbonate dissolution and silicate weathering). In terms of irrigation suitability, the sodium adsorption ratios (SARs) ranged from 0.05 to 1.82, and the electrical conductivity (EC) varied from 116 to 1094 μs/cm, indicating that most groundwater samples were suitable for irrigation. The entropy-weighted water quality index ranged from 15 to 94, suggesting that the groundwater samples were suitable for drinking purposes. Non-carcinogenic human health risks followed the order of children > adult females > adult males, within the average values of 0.30, 0.21, and 0.18, respectively. Sensitivity analysis showed that the parameters had the weight order of NO3 > body weight (BW) > ingestion rate (IR) > exposure frequency (EF). Hence, we recommend prioritizing the management of areas with high salinity levels, while avoiding the excessive use of nitrogen fertilizers, raising awareness among local residents about safe groundwater, and providing robust support for the sustainable development of groundwater in typical agricultural areas.

Erosional modulation of the balance between alkalinity and acid generation from rock weathering

The balance between alkalinity generation by carbonate and silicate weathering and sulfuric acid generation by sulfide weathering controls the effect of weathering on atmospheric pCO2 over geologic timescales. How this balance varies across environmental gradients remains poorly constrained. Here, we analyze this balance across an erosional gradient of two orders of magnitude in the Three Rivers (Yangtze, Mekong, and Salween Rivers) Headwater Region (TRHR), eastern Qinghai-Tibet Plateau (QTP). By employing major element chemistry and multiple isotopes (δ34SSO4, δ18OSO4, and δ18OH2O) coupled with forward and inverse approaches, we unmix contributions of silicates, carbonates, evaporites, and sulfides to the total weathering budget. Across the TRHR, riverine SO42– is derived mainly from a mixture of an evaporite source with uniform values of δ34SSO4 and δ18OSO4, and a sulfide source that contributes highly variable values of δ34S (−12.2 ‰ to +4.1 ‰) and δ18O (−17.7 ‰ to −1.6 ‰). Contributions of sulfide oxidation to riverine SO42– vary from 16 % to 94 %, and sulfuric acid consumes 6 % to 63 % of the alkalinity produced by weathering. The fractions of weathering alkalinity derived from carbonate weathering range from 36 % to 98 % relative to silicate weathering. The combination of silicate, carbonate, and sulfide weathering suggests that the instantaneous weathering fluxes of most sampled catchments in the TRHR act as a sink of atmospheric CO2 over timescales shorter than marine carbonate burial (∼104 years), but as a carbon source over timescales longer than carbonate burial and shorter than sulfide burial (∼107 years). The spatial variability of the balance between alkalinity and acid generation, and, thus, the relationship between chemical weathering and atmospheric pCO2, are largely dependent on lithology. However, within comparable lithologic settings, sulfide and carbonate weathering rates rise with increasing erosion, whereas silicate weathering rates remain constant. Consequently, plateau weathering shifts from a sink to a source of atmospheric CO2 with increasing erosion. These findings suggest that sulfide weathering is more sensitive to erosion than carbonate and silicate weathering, and that it could play an important role in the long-term carbon cycle during mountain building.

A Comprehensive Study on the Hydrogeochemical and Isotope Characteristics and Genetic Mechanism of Geothermal Water in the Northern Jinan Region

Geothermal water (GW) resources are highly valued as clean, renewable energy sources. In this study, a comprehensive analysis of water chemistry and isotope data from 25 GW samples was conducted to gain insights into the hydrochemical characteristics and formation mechanisms of the GW in the northern Jinan region (NJR). Statistical analysis and hydrochemical methods were employed for relevant analysis. The findings reveal that the GW in the NJR exhibits high salinity, with an average total dissolved solids (TDS) concentration of 9009.00 mg/L. The major ions identified are Na+ and Cl−, with mean concentrations of 2829.73 mg/L and 4425.77 mg/L, respectively, resulting in a hydrochemical type of Cl−Na. The analysis of δ2H and δ18O isotopes indicates that the GW originates from atmospheric precipitation that undergoes deep cycling and interaction with older groundwater. The composition of 3H suggests that the GW in the NJR is a mixture of waters, while radiocarbon dating (14C) suggests that the recharge of the GW may have occurred in the late Pleistocene era. The GW in the NJR is classified as partially equilibrated waters. The temperature range of geothermal reservoirs is 57.13 to 99.74 °C. The hydrochemical components primarily result from water–rock interactions, including silicate weathering, cation exchange, as well as carbonate weathering and the dissolution of halite and gypsum. Moreover, taking into account the hydrogeological conditions, hydrochemistry, and isotope analysis, a conceptual model of the geothermal reservoir in the NJR was developed. The research findings serve as a valuable reference and foundation for the development and utilization of geothermal resources in the Jinan region. These originate from the Taiyi mountains in the south or the Taihang mountains in the west, and experience deep circulation and long runoff times. This study provides a reference for the sustainable development and utilization of regional geothermal resources.

Mg isotopes of siliciclastic sediments on continental marginal sea: Insights for the potential to trace silicate weathering

In the last decade, much attention has been devoted to Mg isotopic behavior during various weathering processes. Nonetheless, whether Mg isotopes in terrigenous siliciclastic sediments in modern oceans can reflect continental weathering regimes remains unresolved. Understanding the major controls of Mg isotope fractionation in siliciclastic sediments is thus a prerequisite for decoding past Mg isotope records for tracing continental weathering history. In this study, we present mineralogical, elemental, and Mg isotopic compositions of terrigenous siliciclastic sediments in the Changjiang (Yangtze River) estuary and the adjacent continental shelf of the East China Sea. δ26Mg values of the clay-sized sediments vary from −0.15‰ to 0.00‰ and lack of the correlation with the proportion of different clay minerals. Our results suggest that the incipient to intermediate weathering processes are characterized by the progressive leaching of isotopically light Mg from silicates, leading to the enrichment of 26Mg in secondary clay minerals. Therefore, Mg isotopic variability of clay-sized sediments is primarily determined by catchment weathering, rather than by provenance lithology or mixing of clay minerals. An isotopic mass balance calculation indicates that the δ26Mg value of silicate weathering flux on a continental scale is ca. -0.60‰ to −0.40‰. The new constraint on the δ26Mg value of silicate weathering flux provides a new opportunity to better understand the Mg cycle. The Mg flux from silicate weathering in the Changjiang catchment is estimated as ∼15–17 × 1010 mol/yr, accounting for ca. 50% to 60% of the total Mg influx of Changjiang River into the ocean. In addition, numerical modeling with the up-to-date flux and isotope fractionation data have indicated that a slight enhancement of silicate weathering can potentially drive the rapid rise of Mg/Ca in seawater since late Cenozoic, but maintain the constancy of seawater δ26Mg as reported by literature. Our study demonstrates that Mg isotopes of clay-sized sediments can be used to constrain the relevant δ26Mg value of silicate weathering flux on a continental scale, which is a key parameter to trace the Mg cycle on Earth's surface.

Redox trapping of arsenic in hyporheic zones modified by silicate weathering beneath floodplains

Groundwater containing high concentrations of dissolved arsenic (As) and iron (Fe(II)) discharges to rivers across the Ganges-Brahmaputra-Meghna delta. Observed Fe(III)-oxyhydroxide (FeOOH)–As deposits lining the riverbanks of the Meghna River may have been created by bidirectional mixing in the hyporheic zone (HZ) from ocean tides. This process has been named the Natural Reactive Barrier (NRB). Sedimentary organic carbon (SOC) is deposited annually on floodplains. Floodwaters that infiltrate through this layer may chemically transform the groundwater prior to discharging through the HZ in ways that influence the capture and retention of As in the NRB. The goal of this study is to understand how the interaction of these two scales of river-groundwater mixing influence the fate of As trapped within an NRB. Monitoring wells were installed to 1–17 m depth, up to 100 m distance from the river's edge during the dry season on the East (Site 1) and West (Site 2) sides of the river. They were sampled during the dry season (January) under gaining river conditions. The physical properties and elemental composition of the sediment was described by hand observation and hand-held X-Ray Fluorescence (XRF), respectively. Mixing with river water was quantified using the sum of charge of major cations (TC). Site 1 has a sloping bank that is only partially inundated during the wet season. The aquifer is composed of homogeneous sand. Site 2 is flat and therefore fully inundated in the wet season. The aquifer is composed of sand with thin (1–20 cm thick) clay layers. Both sites generate the dissolved products of FeOOH-reduction coupled to organic carbon oxidation, and silicate weathering beneath the floodplain. These products are dissolved Fe, As, silica, bicarbonate, calcium and phosphate. This chemistry is conducive to the formation of crystalline iron oxide minerals such as goethite which may co-precipitate with As, trapping it long-term.

Hydrochemical characteristics and salt intrusion in groundwater of the lower Chao Phraya river basin: Insights from stable isotopes and hydrochemical analysis

Saltwater intrusion is a significant hydrogeological challenge in coastal regions. This study investigates salt intrusion and hydrogeological processes in the Lower Chao Phraya River basin using stable isotopes and hydrochemical analysis. One Hundred groundwater wells sampled from different aquifers were analyzed for major ions spanning from 2010 to 2021, as well as stable isotopes specifically for the samples in the year 2021. The results reveal four distinct groundwater types: Ca(HCO3)2, NaHCO3, NaCl, and CaSO4, based on their predominant cations and anions. Shallow groundwater, which represents the discharge zone affected by salt intrusion, halite dissolution, and cation exchange reactions, exhibits a significant concentration of NaCl. Water-rock interactions play a crucial role in shaping groundwater chemistry, with carbonate dissolution and precipitation dominating the Ca(HCO3)2 type and silicate weathering influencing the CaSO4 type. The NaHCO3 type is associated with ion exchange and seawater intrusion. Silicate and carbonate rock weathering are the dominant processes in the region. Stable isotope analysis indicates that groundwater recharge primarily comes from rainfall and surface water, with limited contributions from deep sources. Overall, this study provides valuable insights into the hydrochemical characteristics and complex water-rock interactions within the groundwater system of the Chao Phraya River basin. As a result, it holds significant potential for informing effective groundwater management practices and facilitating resource planning in the region. Furthermore, the findings and methodologies presented here offer valuable lessons for addressing saltwater intrusion and hydrogeological challenges in other coastal regions worldwide.

Global Cooling‐Driven Summer Monsoon Weakening in South China Across the Eocene‐Oligocene Transition

AbstractRecent research has suggested that East Asia has experienced a prevailing monsoon climate since the Eocene. However, there is little knowledge about the development of the East Asian monsoon system before the Miocene, particularly in southern China, due to a lack of well‐dated continuous sediment records. Here, we present new magnetic proxy records from International Ocean Discovery Program Site U1501 in the northern South China Sea. We conducted rock magnetic experiments, made scanning electron microscopy observations and performed diffuse reflectance spectrum analysis on the late Eocene‐early Oligocene core sediments. The magnetic signal of the sediments is dominated by detrital magnetite and titanomagnetite formed during the silicate weathering and erosion processes, which were used to infer the evolution of summer monsoon precipitation in southern China. Our results along with geochemical and clay mineral data from Site U1501 strongly indicate that the East Asian summer monsoon generally weakened across the Eocene‐Oligocene transition. This change was linked to coeval global cooling rather than tectonic processes. Prior to the Eocene‐Oligocene transition, the summer monsoon intensified, likely due to the latest Eocene warming event caused by the enhancement of the Atlantic Meridional Overturning Circulation, which may have preconditioned the Earth system for the greenhouse‐to‐icehouse transition.

Passive carbon sequestration associated with wollastonite mining, Adirondack Mountains, New York

Abstract Crushed ore in Adirondack wollastonite mines (New York) shows textural evidence for wollastonite dissolution and cementation by calcite and opal. The reaction CaSiO3 + CO2 = CaCO3 + SiO2 is a model reaction for silicate weathering and carbonation that has not been characterized in the field until now (outside of controlled experiments). Cemented samples from the Lewis and Fox Knoll mines contain up to 3% and 6% calcite, respectively, and contain modern 14C. Carbon isotope ratios have an organic signature at both mines but more strongly at Lewis (δ13C from –9‰ to –29‰ VPDB), which, along with observed filamentous biofilms, supports a microbial role in mineralization. Differences are seen between wollastonite weathering in these mines vs. wollastonite weathering in lab experiments and field studies of carbonate formation in other rock types. Grains surrounded by reaction products reach complete dissolution here, indicating that passivation by jacketing is not important at the field sites. Also, dissolved ions do not all form in situ reaction products, suggesting that solutes are leaving the system. A key finding of this study is the strong organic δ13C signature of calcite cements at the Lewis mine, which also show higher calcite content per years of exposure compared to cements at the Fox Knoll mine. Although microbial fractionation complicates isotopic assessment of atmospheric CO2 sequestration, our findings suggest sequestration rates are enhanced by geomicrobiological activity.

Assessment of some bottled natural mineral waters and spring waters in Algeria using multivariate statistical analysis, hydrogeochemical approaches and water quality index (WQI)

ABSTRACT In Algeria, the consumption of mineral waters has grown enormously, indicating that these waters have safe quality for drinking use. The objectives of the present study are to evaluate the physicochemical characteristics of 43 bottled mineral waters and to identify the main hydrogeochemical processes governing water chemistry using a combination of geochemical techniques and multivariate statistical approaches. The results revealed that the abundance order of cations and anions (mg/L) are Ca2+> Na+> Mg2+> K+, HCO3 −> SO4 2−> Cl−> NO3 −. In addition, the comparison between the obtained results and World Health Organization (WHO) standards showed that the analysed physicochemical parameters (pH, Na+, Cl−, SO4 2−, NO3 −) are within the acceptable limits of the World Health Organization (WHO) for drinking except TDS, Ca 2+, Mg2+, K+, HCO3 −, which exceed the WHO permissible limits in few water samples. Hydrogeochemical types in the study area were Ca2+-HCO3 −, Ca2+-Mg2+-HCO3 −, Ca-Mg2+-SO4 2—Cl-, Na+-HCO3 − and Na+-Cl−types, with a low mineral and intermediate mineral contents based on Van der Aa’s classification. The Gibbs plot specifies that almost all samples (93%) fall in water–rock interaction dominance. The major hydrogeochemical processes acting are dissolution of carbonate minerals, ion-exchange mechanisms and silicate weathering. Principal component analysis (PCA) was applied to hydrochemical data of mineral water samples. The results suggest that hydrochemistry in the region is affected not only by geogenic processes but also is impacted by anthropogenic activities. The comparative evaluation of the different water quality indices (WQI) shows that there is a difference between indices values for the same water sample, which may be attributed to the weighting methods, aggregation methods, and the limits of classes used in the calculation of each index. The results from this study can provide the basis for local decision-makers to develop suitable water management strategies and protection against any pollution risk.

Hydrochemical Characterization and Quality Assessment of Groundwater in the Southern Plain of Hebei Province, China

The purpose of this research was to understand the hydrogeochemical characteristics and assess the quality of phreatic and confined groundwater in southern Hebei Province. A total of 107 groundwater samples were collected, representing different aquifer conditions over the study area. Multivariate statistical analysis, hydrochemical maps, ionic ratio coefficients, geographic information system (GIS) and geochemical simulation were comprehensively and systematically used to reveal the hydrochemical characteristics of groundwater and its controlling mechanism. The results revealed that both phreatic (pH = 7.02–9.08) and confined groundwater (pH = 7.00–10.60) were slightly alkaline. The hydrochemical types were mainly present as the HCO3-Ca-Mg type in the western premontane area and mixed Ca-Mg-SO4-Cl and Na-Cl-SO4 types in the eastern plains. The hydrochemical composition was dominated by water–rock interactions of natural processes, including silicate weathering, dissolution of sulfate minerals (gypsum, anhydrite), and cation-exchange adsorption. Anthropogenic activities were the main factor causing NO3− content in some groundwater samples to exceed the geochemical baseline. The hydrogeochemistry of groundwater in different aquifers was significantly varied. The average contents of TH, TDS, Na+, Ca2+, Mg2+, Cl− and SO42− in phreatic aquifers were significantly higher than those in confined aquifers. The Entropy Weighted Water Quality Index (EWQI) results revealed that 17.78% of phreatic and 50% of confined water samples were meeting the purpose of drinking water. The groundwater samples with EWQI values exceeding 100 were mainly situated in the Handan urban area and the eastern region of Xingtai City, which should be avoided for direct utilization and needs to be improved through protection and management measures, to enhance the quality of groundwater. Correlation analysis showed that groundwater quality was significantly dominated by TH, TDS, Na+, Mg2+, Cl− and SO42− concentrations.

The influence of secondary weathering processes on dissolved nickel isotope compositions under cold climatic conditions – Observations from the Mackenzie Basin

Nickel (Ni) and its stable isotope signature (δ60Ni) have recently gained momentum as a tracer of nutrient cycling in the modern and past oceans. A robust understanding of Ni isotope cycling in the ocean rests on an accurate understanding of the Ni sources and sinks to and from the oceans. In particular, rivers, which are the dominant Ni source to the oceans, show significant variation in Ni isotopes compared to rocks, thought to reflect variable extents of formation of secondary phases that scavenge light Ni isotopes. The current estimate of the global isotope composition of riverine Ni is based on a few large rivers in warm climates, thus preventing the assessment of a potential climatic control on the Ni isotope flux to the ocean. In this contribution, we investigate the Ni elemental and isotope signatures of river catchments in cold climates, namely the Mackenzie Basin tributaries (Canada) and two rivers, the Nass and Skeena, draining the Western Cordillera.The river solid load in the Mackenzie Basin shows almost no resolvable variation in terms of Ni abundances and Ni isotopes, which are similar to silicate rocks. Furthermore, dissolved Ni and Ni isotopes do not show any relationship with source tracers. This suggests a minimal source control on variations in riverine Ni. This result is particularly intriguing as riverine chemistry in the Mackenzie Basin is often dominated by lithological controls, including rocks other than silicate, such as carbonate and black shale. Instead, the variations in dissolved Ni isotopes are related to the removal of dissolved Ni, reflecting the dominant control by secondary weathering processes. The Ni isotope fingerprint of these secondary weathering processes reflects Ni scavenging into metal oxides, consistent with the literature. The data presented here and literature data show that dissolved Ni and lithium isotopes (a tracer of clay formation) exhibit contrasting patterns between the Mackenzie and the Amazon basins. This suggests that climatic conditions might couple or decouple the formation of metal oxide and clay. In addition, the Ni isotope signatures of the river dissolved load vary with the specific weathering intensity of Ni (the fraction of riverine Ni exported as dissolved species). At high specific weathering intensity of Ni, the Ni isotope signature reflects that of silicate, consistent with the congruent release of Ni from the weathering of silicate. Lower specific weathering intensities of Ni are associated with heavier Ni isotope signatures, suggesting greater scavenging of Ni onto secondary metal oxide. Finally, at the lowest Ni weathering intensity, the dissolved Ni isotope composition is lower again, potentially implying limited metal oxide formation under such conditions. Therefore, and though more data are required to confirm the hypothesis from this initial data, while climate might play a role in the relative rate of Li- and Ni- scavenging phase formation, the specific weathering intensity of Ni may be more significant in controlling dissolved Ni isotope composition.

No evidence for a volcanic trigger for late Cambrian carbon-cycle perturbations

Abstract The early Paleozoic was marked by several carbon-cycle perturbations and associated carbon-isotope excursions (CIEs). Whether these CIEs are connected to significant (external) triggers, as is commonly considered to be the case for CIEs in the Mesozoic and Cenozoic, or result from small carbon-cycle imbalances that became amplified through lack of efficient silicate weathering or other feedbacks remains unclear. We present concentration and isotope data for sedimentary mercury (Hg) and osmium (Os) to assess the impact of subaerial and submarine volcanism and weathering during the late Cambrian and early Ordovician. Data from the Alum Shale Formation (Sweden) cover the Steptoean positive carbon-isotope excursion (SPICE; ca. 497–494 Ma), a period marked by marine anoxia and biotic overturning, and several smaller CIEs extending into the early Ordovician. Our Hg and Os data offer no strong evidence that the CIEs present in our record were driven by (globally) elevated volcanism or continental weathering. Organic-carbon and Hg concentrations covary cyclically, providing further evidence of an unperturbed Hg cycle. Mesozoic and Cenozoic CIEs are commonly linked to enhanced volcanic activity and weathering, but similar late Cambrian–early Ordovician events cannot easily be connected to such external triggers. Our results are more consistent with reduced early Paleozoic carbon-cycle resilience that allowed small imbalances to develop into large CIEs.

Rare Hydrated Magnesium Carbonate Minerals Nesquehonite and Dypingite of the Obnazhennaya Kimberlite Pipe, in the Yakutian Kimberlite Province

The result of a complex mineralogical study of the first discovery of the rare hydrated magnesium carbonate minerals of Nesquehonite and Dypingite in the Obnazhennaya kimberlite pipe (of the Yakutian kimberlite province) is presented. The methods of X-ray phase analysis, electronic microscopy, and Raman spectroscopy have established that the main minerals of the samples found in the form of white crust on a small area of rock outcrop of kimberlite breccia are hydrated carbonates: Nesquehonite is MgCO3•3H2O, Dypingite is Mg5(CO3)4(OH)2•5H2O. The formation of Dypingite over Nesquehonite was shown using Raman imaging for the first time. Nesquehonite is represented as aggregates consisting of chaotically oriented prismatic crystals or kidney-shaped formations. Dypingite in the examined samples appears less frequently as rose-shaped aggregates formed from lamellar crystals. It is assumed that the formation of rare carbonates of the Obnazhennaya kimberlite pipe is mainly the result of the weathering of silicates, formation of mineralized solutions, and their subsequent crystallization, including the capture of CO2 from the air.

Enrichment mechanism of arsenic in deep confined aquifers of Guide Basin under the influence of geothermal activities

High arsenic (As) concentrations in groundwater affected by geothermal activity have been reported worldwide, but the genesis mechanism is not adequately understood. To address this issue, 22 groundwater samples and 69 sediment samples were collected from the Guide basin in northwest China for hydrochemical and geochemical analyses and laboratory experiments. Results indicated that Fe/Mn oxide minerals were the main carriers of As, which was proved by a positive correlation between As and Fe or Mn in sediments. Analysis of hydrologic characteristics showed that direct contribution of As from geothermal water to confined groundwater was probably not significant in the Guide basin. Results of laboratory experiments indicated that As release from aquifer sediments was accelerated by increased temperature under different pH and ORP conditions. Raising temperature may stimulate the release of As by promoting dissolutions of As-bearing Fe (hydr)oxides, weathering of As-bearing silicates and As desorption from sediments into groundwater. The research results can not only increase the understanding of arsenic enrichment mechanism in groundwater under geothermal geological background, but also provide a scientific basis for other high-As groundwater with similar geological background in the world.

Widespread clay authigenesis and highly congruent silicate weathering in the Marinoan aftermath

The termination of the Cryogenian period marks a transition from an extreme ice- to a greenhouse climate, likely initiated by increased atmospheric CO2, however uncertainties persist on continental weathering regarding this climate transition. Here we present paired lithium isotope data from carbonate (δ7Licarb) and carbonate-hosted silicate (δ7Lisili) components of the Marinoan cap carbonates to constrain C and Si cycles in the deglacial period. We find that the silicate component of cap carbonates is dominated by marine authigenic clays with minor contributions from terrestrially detrital materials and oxides. Built from δ7Lisili data, large variations of seawater δ7Li suggest a heterogeneous marine Li reservoir due to a stratified ocean in the deglacial period. Overall, widespread clay authigenesis and low seawater δ7Li values indicate active carbon-silicon cycling and congruent silicate weathering due to warm climate and exposure of fresh rock surface in the Marinoan aftermath, which promotes the deposition of cap carbonates and stabilization of Earth's surface environment.

Contrasting riverine K and Li isotope signatures during silicate weathering in the southeastern Tibetan Plateau

Mountain uplift produces steep slopes and rapid erosion rates, and hence exposes more silicate rocks to weathering and accelerates the drawdown of atmospheric CO2, driving global cooling over the Cenozoic. Riverine potassium (K) and lithium (Li) isotopes are two promising proxies for silicate weathering because the isotopes fractionate during secondary mineral formation associated with weathering and these two elements are highly enriched in silicate rocks. This study explores similarity and difference between K and Li isotope behaviors during weathering across the Lancang River in the southeastern Tibetan Plateau, in order to evaluate the role of these two isotopes in tracing silicate weathering in tectonically active mountains. Dissolved K and Li in the Lancang River mainstream are mainly derived from the dissolution of silicates. River waters are enriched in the heavy isotope of K (−0.35‰ to +0.05‰) and Li (+7.1‰ to +20.4‰) relative to suspended sediments (−0.56‰ to −0.42‰ and −2.3‰ to 0‰, respectively), and the element partitioning between dissolved and suspended loads controls dissolved δ7Li and δ41K values. Interesting, dissolved δ7Li and δ41K show the opposite evolution trends toward downstream. As the weathering intensity increases downstream (higher Si/(Na + K)⁎ ratios), the dissolved δ7Li values increase from 7.4‰ to 13.6‰, whereas dissolved δ41K values decrease from −0.04‰ to −0.26‰. Compared to its headwater with limited weathering, more secondary minerals (e.g., clays) are formed in the lower reaches (e.g., floodplains) with low physical erosion rates and high annual precipitation, driving more 6Li incorporated into clays, thus high dissolved δ7Li values downstream. While dissolved δ41K values and the fraction of 39K incorporated into secondary minerals are low in floodplains, this is consistent with observations that illite is a main K-bearing phase among clay minerals, and its content in the clay fraction in river sediment decreases as weathering intensity increases. Our study further supported that the increase in Cenozoic seawater δ7Li is related to weathering in floodplains rather than weathering of mountains, whereas K isotope fractionation mainly occurs in mountain regions. If Cenozoic seawater δ41K rise, as with δ7Li change, weathering at higher elevation may exerts more effect on high riverine and seawater δ41K values. This study improves our knowledge of K-Li isotope behaviors during present-day silicate weathering, and the response of K-Li isotopes in sedimentary archives to silicate weathering over geological time.

The enhanced weathering potential of a range of silicate and carbonate additions in a UK agricultural soil

Enhanced weathering (EW) is a carbon dioxide removal (CDR) technology which aims to accelerate silicate and/or carbonate weathering in agricultural land. At present, the rate and magnitude of CDR from EW remains uncertain. In this study, soil cores extracted from a typical UK agricultural site in Oxfordshire were used to geochemically assess the efficacy of EW while simulating field conditions. Six material “treatments” were applied to soil cores at a rate equivalent to 50 t ha−1: agricultural lime (aglime), basalt, cement kiln dust (CKD), olivine, steel slag, and volcanic ash. A range of chemical measurements were used to constrain the rate of dissolution, fate of dissolution products, and the CDR potential and environmental impact of treatment.After a single application, the CDR rates were, in decreasing order: steel slag (20 ± 4 kgCO2 ha−1 yr−1) > CKD (16 ± 4 kgCO2 ha−1 yr−1) > basalt (5 ± 3 kgCO2 ha−1 yr−1) > volcanic ash (3 ± 3 kgCO2 ha−1 yr−1) > aglime (2 ± 1 kgCO2 ha−1 yr−1) > olivine (0 ± 2 kgCO2 ha−1 yr−1). Despite its drawdown potential, steel slag addition is not advised because application raised the dissolved concentration of heavy metals in the soil. CKD application will be limited due to availability of this material. Liming of agricultural soil is normally considered a source of CO2, but this study indicates liming could cause CDR in some UK soil conditions.Extrapolating from this site to a wider scale supports the conclusions of recently published research which suggests 10 years of basalt application over UK cropland could remove 1.8 ± 0.9 MtCO2 yr−1. Given the low rates of CDR observed in this study, EW is only likely to be worthwhile, at least in soil and climate conditions common to SE England, where there are co-benefits beyond carbon uptake.

Lithium isotope systematics and water/rock interactions in a shallow-water hydrothermal system at Milos Island, Greece

The active venting fluids of Milos Island, located within the southern Aegean Sea, belong to a shallow-water hydrothermal system (< 200 m depth) that shows chemical compositions and evolution processes comparable to those of mid-ocean ridges (MOR). In this study, we analyze Li and δ7Li in 69 vent water samples, grouped into two types based on their salt content. The low-Cl end-member (EM) Cave fluids show a relatively high Li content (0.39–0.54 mM) with MORB-like δ7Li (∼4.5 ‰, MORB = 3.7 ‰) compared to that of seawater, and the high-Cl brine fluids contain remarkably high Li (6.14–10.6 mM) and variable δ7Li (1.4–8.7 ‰). The latter fluids may have derived from metamorphic basement modified by seawater interactions at ∼300 °C. A scenario using a steady-state dissolution/precipitation model can generate consistent Li and δ7Li patterns, where linear correlations of Cl and Li suggest phase separation occurred after water/rock interaction at depth. On the contrary, no significant δ7Li variation in most Milos fluids suggests limited isotopic fractionation occurred during phase separation. More importantly, the detected Li enrichment in the high-Cl fluids implies a large Li flux, ∼3.4 × 107 mol/yr, to the ocean from the Milos system. Assuming that 10% of the world's shallow-water systems discovered to date have similar Li outputs to those of Milos, this Li flux would represent ∼1.8% of MOR hydrothermal fluxes which is on the order of ∼13 × 109 mol/yr. These results emphasize the importance of Li flux derived from shallow-water hydrothermal systems, which should not be excluded from the calculation of the marine Li budget and its impact on the global silicate weathering cycles.

Microbial Catalysis for CO2 Sequestration: A Geobiological Approach.

One of the greatest threats facing the planet is the continued increase in excess greenhouse gasses, with CO2 being the primary driver due to its rapid increase in only a century. Excess CO2 is exacerbating known climate tipping points that will have cascading local and global effects including loss of biodiversity, global warming, and climate migration. However, global reduction of CO2 emissions is not enough. Carbon dioxide removal (CDR) will also be needed to avoid the catastrophic effects of global warming. Although the drawdown and storage of CO2 occur naturally via the coupling of the silicate and carbonate cycles, they operate over geological timescales (thousands of years). Here, we suggest that microbes can be used to accelerate this process, perhaps by orders of magnitude, while simultaneously producing potentially valuable by-products. This could provide both a sustainable pathway for global drawdown of CO2 and an environmentally benign biosynthesis of materials. We discuss several different approaches, all of which involve enhancing the rate of silicate weathering. We use the silicate mineral olivine as a case study because of its favorable weathering properties, global abundance, and growing interest in CDR applications. Extensive research is needed to determine both the upper limit of the rate of silicate dissolution and its potential to economically scale to draw down significant amounts (Mt/Gt) of CO2 Other industrial processes have successfully cultivated microbial consortia to provide valuable services at scale (e.g., wastewater treatment, anaerobic digestion, fermentation), and we argue that similar economies of scale could be achieved from this research.

The rise of New Guinea and the fall of Neogene global temperatures

The ~2,000-km-long Central Range of New Guinea is a hotspot of modern carbon sequestration due to the chemical weathering of igneous rocks with steep topography in the warm wet tropics. These high mountains formed in a collision between the Australian plate and ophiolite-bearing volcanic arc terranes, but poor resolution of the uplift and exhumation history has precluded assessments of the impact on global climate change. Here, we develop a palinspastic reconstruction of the Central Range orogen with existing surface geological constraints and seismic data to generate time-temperature paths and estimate volumes of eroded material. New (U-Th)/He thermochronology data reveal rapid uplift and regional denudation between 10 and 6 Mya. Erosion fluxes from the palinspastic reconstruction, calibrated for time with the thermochronological data, were used as input to a coupled global climate and weathering model. This model estimates 0.6 to 1.2 °C of cooling associated with the Late Miocene rise of New Guinea due to increased silicate weathering alone, and this CO2 sink continues to the present. Our data and modeling experiments support the hypothesis that tropical arc-continent collision and the rise of New Guinea contributed to Neogene cooling due to increased silicate weathering.