Monomethylmercury (MMHg) is a potent toxin that bioaccumulates and magnifies in marine food webs. Recent studies show abundant methylated Hg in deep oceans (>1000 m), yet its origin remains ...uncertain. Here we measured Hg isotope compositions in fauna and surface sediments from the Mariana Trench. The trench fauna at 7000-11000 m depth all have substantially positive mass-independent fractionation of odd Hg isotopes (odd-MIF), which can be generated only in the photic zone via MMHg photo-degradation. Given the identical odd-MIF in trench fauna and North Pacific upper ocean (<1000 m) biota MMHg, we suggest that the accumulated Hg in trench fauna originates exclusively from MMHg produced in upper oceans, which penetrates to depth by sorption to sinking particles. Our findings reveal little in-situ MMHg production in deep oceans and imply that anthropogenic Hg released at the Earth's surface is much more pervasive across deep oceans than was previously thought.
A large number of gases are releasing from the medium-high temperature geothermal fields distributed along the large-scale strike-slip fault zones in the southeastern margin of the Tibetan Plateau. ...In this study, 11 hot spring water and the associated bubbling gas samples were collected along the Xianshuihe-Anninghe fault zones (XSH-ANHFZ) and analyzed for chemical and isotopic compositions. The
δ
18
O
H
2
O
and
δ
D
H
2
O
values indicate that hot spring waters are predominantly meteoric origin recharged from different altitudes. Most water samples are significantly enriched in Na
+
and HCO
3
−
due to the dissolution of regional evaporites, carbonates and Na-silicates.
3
He/
4
He ratios of the gas samples are 0.025–2.73 times the atmospheric value. The
3
He/
4
He ratios are high in the Kangding region where the dense faults are distributed, and gradually decrease with increasing distance from Kangding towards both sides along the Xianshuihe fault zones (XSHFZ). Hydrothermal fluids have dissolved inorganic carbon (DIC) concentrations from 2 to 42 mmol L
−1
,
δ
13
C
DIC
from −6.9‰ to 1.3‰,
δ
13
C
CO
2
from −7.2‰ to −3.6‰ and Δ
14
C from −997‰ to −909‰. Combining regional geochemical and geological information, the CO
2
sources can be attributed to deep-sourced CO
2
from mantle and metamorphism of marine carbonate, and shallow-sourced CO
2
from the dissolution of marine carbonate and biogenic CO
2
. The mass balance model shows that 11±6% of the DIC is sourced from the dissolution of shallow carbonate minerals, 9±8% formed by pyrolysis of sedimentary organic matter, 80±9% derived from deep metamorphic origin and mantle-derived CO
2
. Among them, the deep-sourced CO
2
in Anninghe fault zones (ANHFZ) is merely metamorphic carbon, whereas ca. 12% and ca. 88% of the deep-sourced CO
2
in the XSHFZ are derived from the mantle and metamorphic carbon, respectively. The average deep-sourced CO
2
flux in the Kangding geothermal field is estimated to be 160 t a
−1
. If all the hot springs in various fault zones in the southeastern margin of the Tibetan Plateau are taken into account, the regional deep-sourced CO
2
flux would reach ca. 10
5
t a
−1
. These results show that the deep-sourced CO
2
released from non-volcanic areas might account for a considerable proportion of the total amount of global deep-sourced carbon degassing, which should be paid more attention to.
The late Carboniferous-early Permian coal seams of the Qinshui Basin in Shanxi Province are the most prolific producer of coalbed methane (CBM) in China. Methane formed in the late Triassic during ...deep burial and reheating in late Jurassic-early Cretaceous driven by magmatic underplating. Basin inversion brought the coal seams to 400–700 m from the surface in the mid-late Cenozoic. Here we present results of a study aimed at understanding the origin of the methane, and how it was affected by Cenozoic exhumation of the basin. Methane from a 12 km traverse perpendicular to the basin margin in the southeast part of the basin have stable isotope compositions (δ13C = −30.2 to −35.2‰, and δD = −155 to −194‰) indicating a thermogenic origin with limited biogenic input. They are, however, lighter than expected based on coal maturity, and C1/(C2 + C3) (>1000) are significantly higher than typical thermogenic methane (<50). This is due to diffusive fractionation during commercial gas extraction. He-Ne-Ar isotopes are a mixture of crustal-radiogenic gas with air-derived noble gases. 4He concentrations (0.52 to 33.25 ppmv) and 4He/40Ar⁎ ratios (0.06–1.74) are unusually low. He-Ne-Ar concentrations are consistent with the open system Rayleigh fractionation of noble gases derived from air-saturated water with 4He/40Ar⁎ = 1 during gas extraction. The low 4He/40Ar⁎, compared with average crust (5) or local production (13) values, implies that >90% of the radiogenic 4He produced in the coals has been lost prior to equilibrium between gas and water phase in the reservoir. This likely occurred in response to gas loss process during rapid exhumation in Cenozoic, showing that the He and Ar content of natural gases is a sensitive indicator of gas loss event caused by recent basin inversion. The event may have led to the loss of up to 44% of the methane from the coal seams. This study demonstrates the importance of basin inversion on gas preservation in shallow CBM, and shows that, in contrast to δ13CCH4, the light noble gases are essential for tracing such a process.
•Gas compositions and stable isotopes indicate diffusive fractionation during commercial gas extraction.•There is no indication of mantle volatiles in gas samples, no evidence of mantle heating during the Yanshanian Orogeny.•The low radiogenic He and He/Ar ratio indicate loss of free gas during basin exhumation.
Identifying the mechanism for circulation of nutrient elements in the hydrological cycle is crucial for protection of water resource quality. In this study, 72 surface water and 21 groundwater ...samples were collected in three seasons for hydrochemical and multiple isotopic (δD-H2O, δ18O–H2O, δ15N–NO3−, and δ18O–NO3−) analysis in the Zhangxi watershed, a small valley plain in the southeastern Yangtze River Delta. The hydrochemical type in surface water and shallow groundwater was Ca–HCO3, and the δD and δ18O isotope values were similar. Thus, the hydraulic connection and its material interaction were similar. Quantitative evaluation of the mass balance model showed that shallow groundwater accounted for more than 90% of surface water in the dry season but only 33.0% in the wet season. On the average, nitrate in shallow groundwater was mainly derived from organic fertilizer, whereas nitrate in surface water was mainly sourced from soil nitrogen and chemical fertilizer carried by surface runoff. At the time scale, the nitrogen sources of surface water showed seasonal variation owing to the interaction between surface water and groundwater. The contribution of organic fertilizer in the wet season was approximately 16.2%, but in the dry season increased to 32.3% and exceeded that of chemical fertilizer (24.6%). Shallow groundwater contributed more to surface water in the dry season, and transferred more nitrogen from organic fertilizer to surface water, so that they had similar nitrate sources. At the space scale, nitrate sources of surface water differed significantly with the land use types. The NO3− and δ15N–NO3− concentrations of the surface water in the mid-valley plain and downstream were significantly higher than in the natural forest area upstream, indicating the increased contribution of organic fertilizer. Agricultural activities and the interaction between surface water and groundwater in the valley plain area have a significant impact on nitrogen pollution. Reduction in nitrogen fertilizer use and establishment of a riparian protection zone are crucial to protect the water resource quality.
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•Surface water was mainly recharged by shallow groundwater in the dry season.•Nitrate sources of surface water showed seasonal variation due to hydrological cycle.•Organic fertilizer contributed more nitrate to surface water in the dry season.•Spatially, nitrate sources of surface water were related to land use types.
Soil heterotrophic respiration (HR) is an important source of soil-to-atmosphere CO
flux, but its response to changes in soil water content (θ) is poorly understood. Earth system models commonly use ...empirical moisture functions to describe the HR-θ relationship, introducing significant uncertainty in predicting CO
flux from soils. Generalized, mechanistic models that address this uncertainty are thus urgently needed. Here we derive, test, and calibrate a novel moisture function, f
, that encapsulates primary physicochemical and biological processes controlling soil HR. We validated f
using simulation results and published experimental data, and established the quantitative relationships between parameters of f
and measurable soil properties, which enables f
to predict the HR-θ relationships for different soils across spatial scales. The f
function predicted comparable HR-θ relationships with laboratory and field measurements, and may reduce the uncertainty in predicting the response of soil organic carbon stocks to climate change compared with the empirical moisture functions currently used in Earth system models.
Red mud (bauxite residue) is the main waste product of the alkaline extraction of alumina from bauxite with high amounts of metals. In this study, bioleaching of heavy metals from red mud by using ...the fungus Aspergillus niger was investigated. Bioleaching experiments were examined in batch cultures with the red mud at various pulp densities (1–5%, w/v) under various bioleaching conditions (one-step, two-step and spent medium bioleaching). It was shown that the main lixiviant excreted by A. niger was citric acid. The highest leaching ratios of most various heavy metals were achieved under spent medium leaching at 1% pulp density. The increase in red mud pulp densities resulted in a general decrease in leaching ratios under all bioleaching conditions. However, in the case of the spent medium leaching the decrease in leaching ratios was lowest. The Toxicity Characteristic Leaching Procedure (TCLP) tests showed that the leaching toxicity of the bioleaching residue was far below the levels of relevant regulations. The micromorphology of the red mud particles were changed by the fungal activity during bioleaching process.
•We examined the biological leaching efficiency of heavy metals from red mud.•Three different bioleaching methods were compared in our research.•Spent medium process is proposed to be the best option for bioleaching of red mud.•The leaching toxicity of red mud decreased obviously after bioleaching.•The micromorphology of red mud particles was changed by fungus activity.
Rare earth elements (REE) are sensitive tracers of rock-water interactions and soil formation processes. In this study, we investigated the REE concentration and distribution patterns for both bulk ...samples and acid leachable phase for a series of regolith and soil profiles developed on a deeply weathered granite hillslope in a subtropical climate. The exogenous input had little impact on the REE distributions on the studied hillslope. The REE distribution in the profiles was controlled by changes in the physical and chemical environmental parameters and mineral transformation during weathering. However, the specific mechanisms differed among the different weathering stages and profiles located at different positions along the slope. Among the environmental parameters, the profile redox state and acidity were the dominant factors controlling the REE behavior, with organic materials as the main regulators. A strong positive relationship between chemical index of alteration (CIA) and La/Yb was observed for the regolith samples, but the La/Yb ratio decreased upward from a depth of 120 cm (the bottom depth of the soil horizon) as the CIA value increased, suggesting a preference for light REE (LREE) fixation in kaolinite formed during the dissolution of feldspar; however, a conversion occurred during the subsequent pedogenesis period. The hilltop soil profile was characterized by the strongest LREE depletion and the highest positive Ce anomaly (~7.31–14.28), compared with other profiles down the hillslope (~0.84–2.78). The strongest LREE depletion on the hilltop profile JLN-S4 suggested a preference for LREE release resulting from the transformation of kaolinite to gibbsite, and the highest Ce anomaly indicates that Ce was fixed in a more oxidized environment within a well-drained profile. The hill foot profile JLN-S1 had the highest LREE leaching rate, but also the lowest heavy REE (HREE) leaching rate, which was ascribed to its lowest erosion rate, lowest pH value, and reductive conditions in a more hydromorphic zone down the hill. The releasing rates of REE calculated by a mass balance model at different positions along the catena ranged from 1395 to 14,379 μg m−2y−1. All the observed REE fractionation regimes and weathering rate variations along the hillslope illustrated that topography-regulated hydrologic condition played a critical role in the release and migration of REE during granite weathering.
•Incongruent granite weathering at different weathering stages and topography dominated light and heavy REE fractionation.•Topography exerted strong effects on REE release, migration, and fractionation through hydromorphic conditions.•Organic materials regulated REE behaviors by affecting the profile pH and redox condition.•The pH value control of REE behavior was realized by regulating the absorption-desorption process.
Increasing lines of evidence question the homogenous response of Asian Summer Monsoon (ASM) precipitation patterns, requiring rethinking of the forcing mechanisms. Here we show a ~15,000‐year ...quantitative precipitation history based on well‐dated lake levels at Lake Chenghai, subtropical China. Lake levels and the inferred precipitation were high during the Bølling‐Allerød, early and late Holocene, but low during the middle Holocene. The orbital scale precipitation trend is out of phase with boreal summer insolation, the later has been widely suggested as the driver of ASM precipitation. Lake Chenghai long‐term lake levels are synchronous with trends in tropical Pacific sea surface temperatures, the related zonal sea surface temperature gradients, and interhemispheric temperature gradients. We propose that changes in either the interhemispheric or zonal Pacific temperature gradients modulate the intensity and location of the western Pacific subtropical high, which is juxtaposed on the ASM, leading to heterogeneous hydroclimatic conditions over subtropical East Asia.
Plain Language Summary
We present a 15,000‐year record of southern China precipitation based on well‐dated (n=130 radiometric ages) lake level variations of a closed basin lake. The reconstructed long‐term southern China precipitation trend is out of phase with boreal summer insolation, which has been shown to drive ASM precipitation, but broadly follows trends in tropical Pacific sea surface temperatures (SSTs), the related zonal SST gradients, and interhemispheric temperature gradients. We propose that changes in either the interhemispheric or zonal Pacific temperature gradients modulate the intensity and location of the western Pacific subtropical high and are juxtaposed on the ASM, leading to heterogeneous hydroclimatic conditions over subtropical East Asia, including an unexpected southern China mid‐Holocene drought. The results of this study are unique in that (1) precipitation was robustly reconstructed from beach evidence, which is a first‐order measure of the balance between rainfall amount and evaporation. (2) A new mechanism driving long‐term precipitation changes over subtropical East Asia is proposed. Providing the mechanism stands, increasing SST gradients under the expected global warming scenario could enhance the role of western Pacific subtropical high and lead to less precipitation over subtropical East Asia.
Key Points
A 15,000‐year precipitation history over subtropical China was reconstructed based on well‐dated (n=130 radiometric ages) lake levels
Subtropical China precipitation trend does not resemble Asian summer monsoon trend but is synchronized with trends in tropical Pacific SST
Juxtaposition of western Pacific subtropical high on Asian summer monsoon could have shaped the subtropical East Asian precipitation
The terrestrial biogeochemical silicon (Si) cycle may significantly influence the stabilization of soil organic carbon (SOC), and thus plays an important role in regulating the global carbon (C) ...balance and climate change. Processes involved in Si-enhanced SOC stability at a decadal or centennial scale include protection of SOC through amorphous Si and interactions of Si-iron and Si-aluminum. Strategic manipulation of the Si cycle in terrestrial ecosystems offers a new opportunity to enhance soil C sequestration. Rock powder amendment, establishment of Si-rich plant buffer strips and innovative management practices that return Si-rich biomass materials to soil can be implemented as strategies to enhance soil C sequestration through regulating the terrestrial Si cycle. However, quantifying (i) the contribution of different processes to the enhancement of soil dissolved Si and secondary Si minerals, (ii) the relative importance of different SOC stabilization mechanisms, and (iii) the potential and cost of different measures has not been attempted.
Lithium (Li) and its isotopes are potential tracers of silicate weathering in river basins. However, the relationship between the Li isotopic composition (δ
7
Li) in rivers and silicate weathering ...intensity remains unclear. This study analyzed the Li concentration and isotopic composition in river waters from the Niyang River, southern Tibetan Plateau. The results show that these samples have significantly variable Li concentrations (0.31-7.4 μg/L) and δ
7
Li values (+7.0-+20.7%o,
n
=28), and high δ
7
Li values are found in several tributaries. Calculations indicate that dissolved Li in river water is predominantly derived from silicate weathering and geothermal water. With the exception of certain tributaries, geothermal water contributes 68% to 85% of the dissolved Li. Geothermal waters have low δ
7
Li values (−0.9-+2.9%o) in the Tibetan Plateau. Differences in the proportional contribution of dissolved Li in river samples from silicate weathering and geothermal water may be the main reason for the spatiotemporal variation in riverine δ
7
Li values. The samples have higher δ
7
Li values when the dissolved Li in the water samples is mainly derived from silicate weathering contributions, and lower values when the contribution from geothermal waters is high. Furthermore, the interaction of dissolved Li from geothermal water with secondary minerals results in Li isotopic fractionation, which may contribute to variations in river water δ
7
Li. It is accepted that the lower weathering intensity in orogenic (or mountainous) belts compared to floodplains is the main controlling factor for lower δ
7
Li values in the rivers. This study indicates that geothermal water input may cover the Li isotope signal of silicate weathering in river water, which in turn, affects the accurate understanding of the relationship between riverine δ
7
Li values and the silicate weathering intensity. Therefore, whether the lower δ
7
Li values of river waters in hydrothermal-rich orogenic belts are mainly controlled by the regional weathering intensity or the input of hot springs (or both) requires in-depth study, and this is the key to accurately establishing the relationship between the Li isotopic composition and silicate weathering intensity in the river basin.
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