Guangzhou Institute of Geochemistry
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Change of soil bacterial communities in chemically stabilized chromium contaminated soils in an accelerated aging experiment
No full textChemical stabilization is a common strategy to clean Cr(VI) contaminated soils. Although many studies have reported both short-term and long-term performance of chemical stabilization, there is no work addressing the impacts of long-term Cr stability on soil bacterial communities after stabilization. In this study, an accelerated aging system that synchronized simulation acid rain leaching and freeze-thaw cycles was established for the first time. We investigated the change of Cr content, environmental variables and bacterial community structure after chemical stabilization by 50-year acid rain leaching and freeze-thaw cycle simulation. Chemical stabilization effectively decreased the content of soil Cr(VI), total Cr in leachate and Cr(VI) in leachate. Different aging conditions altered soil Cr(VI), total Cr in leachate and Cr(VI) in leachate to different extents, hinting at distinct mechanisms of long-term Cr instability between acid rain leaching and freeze-thaw cycle. Cr(VI) and chemical stabilization were identified as the most influential factors affecting soil bacterial community structure, and acid rain leaching and freeze-thaw cycle exhibited considerable but distinct impacts. Our findings provided deeper insights into the long-term restoration of bacterial community in chemically stabilized Cr(VI) contaminated soil and the long-term risk assessment of contaminated soil remediation
Formation of nanophase metallic iron through charge disproportionation of ferrous iron during micrometeoroid impact-induced splash melting
No full textCharge disproportionation of ferrous iron has been considered as one of the mechanisms for the formation of metallic iron on the lunar surface. However, the detailed mechanism of the disproportionation reaction on the Moon is yet to be elucidated. We provide direct evidence for the ferrous disproportionation reaction that produces nano phase metallic iron (npFe(0)) during a rapid cooling process after splash melting from a lunar sample returned by China's Chang'e-5 mission. Space weathering processes have resulted in the formation of three distinct zones at the rim of a pyroxene fragment, as observed through transmission electron microscopy. These zones, made up of splashed melts, newly formed melts from the substrate, and the mineral, are distinguished as I, II, and III. Quantitative analyses of the iron valence state by electron energy loss spectroscopy show that disproportionation reactions occurred in zone II at a low temperature of <570 degrees C during a rapid cooling process. The reaction led to the production of alpha-structure npFe(0) and Fe3+ reserve in the glass phase. The npFe(0) produced by the disproportionation reaction has a larger grain size than those formed from solar wind irradiation, implying that micrometeoroid impacts mainly contribute to the darkening of visible and near-infrared reflectance. These findings reveal a novel rim structure by repeated space weathering and a universal formation mechanism of npFe(0) during micrometeoroid impacts, suggesting that the disproportionation reaction could be widespread on airless bodies with impact-induced splash processes
Contribution of Asian dust into the sediment budget of the Parece Vela Basin over the last 1 Ma: Implications for the climate change in East Asia
No full textBy quantifying the deposition of Asia-derived dust in the Western Pacific Ocean it is possible to constrain the consequences of climate change in East Asia. However, the rates of eolian dust deposition and its transport mechanism in the low-latitude Pacific remain uncertain. Core CP19 recovered sediment dating from similar to 1 Ma to the present in the Parece Vela Basin (PVB). In this study we present bulk-rock major and trace element geochemistry, and Sr and Nd isotopic compositions from this sediment. The results reveal that these clastic sediments are a mixture of volcanic materials derived from the adjacent island arcs and eolian dust material originating from the arid regions of East Asia. Within the PVB, the proportion of Asian dust in the detrital sediment ranges from 30% to 50%, with an average of 44%. The majority of the dust originated from Eastern Asian deserts. Sr and Nd isotopes indicate that there is a significant increase in the overall proportion of eolian dust in the PVB starting at similar to 0.64 Ma, influenced by an enhanced East Asian winter monsoon. In contrast, strengthening of the East Asian summer monsoon led to a decrease in the proportion of the eolian dust from the Ordos deserts. The notable alteration in eolian dust flux to the PVB corresponds to an abrupt intensification of aridity and monsoon climate in the East Asian continent at similar to 0.64 Ma, caused by the Middle Pleistocene Climate Transition
Alternation between terrestrial and aquatic plants dominated organic matter sources in the Tiaoshu wetland (south China) and its response to late Pleistocene environmental changes
No full textWidely distributed wetlands in China play an important role in the global carbon cycle. However, the long-term response of wetland evolution and carbon storage in tropical southern China to climate change remains unclear. Here, we present a sediment core record from the Tiaoshu Wetland in the northern Leizhou Peninsula and multiple proxy indicators are investigated, with the aim of better understanding past vegetation and environmental changes. The following conclusions were drawn: (i) The organic matter deposited in the Tiaoshu Wetland was mainly derived from terrestrial C3 plants and aquatic plants growing in and/or around the catchment. The temporal variability in the relative proportions of organic matter contributed by various sources revealed paleoenvironmental changes in the study area, generally consistent with regional and global climate records. (ii) The peat formation pattern in the Tiaoshu Wetland is in contrast to some individual sites reported in subtropical China, suggesting that there may be different peat formation patterns in the tropical and subtropical regions of China, which may be closely related to specific hydrothermal conditions in different regions. In addition, by comparison with atmospheric CH 4 concentrations, our results suggest potential relationships between the evolution of wetlands in tropical China and global atmospheric CH 4 cycles
Tracing Subducted Carbonates in Earth's Mantle Using Zinc and Molybdenum Isotopes
No full textAlthough carbonates are the primary form of carbon subducted into the mantle, their fate during recycling is debated. Here we report the first coupled high-precision Zn and Mo isotope data for Cenozoic intraplate basalts from western China. The exceptionally high delta 66Zn values (+0.39 to +0.50 parts per thousand) of these lavas require involvement of recycled carbonates in the mantle source. Variable delta 98Mo compositions (-0.39 to +0.27 parts per thousand) are positively correlated with Mo/Ce, best interpreted as mixing between isotopically light Mo from dehydrated oceanic crust and heavy Mo from recycled carbonates, which is also supported by positive coupling between delta 66Zn and delta 98Mo. Modeling reveals that involvement of <= 5% carbonate-bearing oceanic crust fully resolves the observed delta 66Zn-delta 98Mo mantle heterogeneity probed by intracontinental basalts. Our study demonstrates that combined delta 66Zn-delta 98Mo data sets for mantle-derived magmas can track recycled surficial carbonates in Earth's interior, providing a powerful geochemical tool for deep carbon science.
Carbon is an element of life and studying its geological cycle is crucial for understanding Earth's evolution including formation of a life-supporting atmosphere. Here we report the first combined high-precision Zn and Mo isotope data for Cenozoic intraplate lavas from western China, showing that the basalts record <= 5% carbonate-bearing oceanic crust components in their mantle source. Our results provide new evidence for surficial carbonates being delivered into the deep upper mantle, which adds to the debate about the deepest extent of the terrestrial carbon cycle.
First combined zinc (Zn) and molybdenum (Mo) isotope data for mantle-derived magmas to track the fate of subducted carbonates Zn-Mo isotopic compositions of Cenozoic Tarim basalts suggest surficial carbonates being delivered into the deep upper mantle We highlight the utility of combined Zn-Mo isotope data as a powerful tool in deep carbon scienc
Halogenated aromatic pollutants in routine animal-derived food of south China: Occurrence, sources, and dietary intake risks
No full textHalogenated aromatic pollutants (HAPs) including polychlorinated dibenzo- p -dioxins/furans (PCDD/Fs), polychlorinated biphenyls (PCBs), polybrominated dibenzo- p- dioxins/furans (PBDD/Fs), and polybrominated diphenyl ethers (PBDEs) exhibit diverse toxicities and bio-accumulation in animals, thereby imposing risks on human via animal -derived food (ADF) consumption. Here we examined these HAPs in routine ADFs from South China and observed that PBDEs and PCBs showed statistically higher concentrations than PCDD/Fs and PBDD/ Fs. PCDD/Fs and PCBs in these ADFs were mainly from the polluted feed and habitat of animals, except PCDD/Fs in egg, which additionally underwent selective biotransformation/progeny transfer after the maternal intake of PCDD/F-polluted stuff. PBDEs and PBDD/Fs were mostly derived from the extensive use of deca -BDE and their polluted environments. Significant interspecific differences were mainly observed for DL -PCBs and partly for PBDD/Fs and PBDEs, which might be caused by their distinct transferability/biodegradability in animals and the different living habit and habitat of animals. The dietary intake doses (DIDs) of these HAPs via ADF consumption were all highest for toddlers, then teenagers and adults. Milk, egg, and fish contributed most to the DIDs and risks for toddlers and teenagers, which results of several cities exceeded the recommended thresholds and illustrated noteworthy risks. Pork, fish, and egg were the top three risk contributors for adults, which carcinogenic and noncarcinogenic risks were both acceptable. Notably, PBDD/Fs showed the lowest concentrations but highest contributions to the total risks of these HAPs, thereby meriting continuous attention
Dimethylcyclosiloxanes in Mobile Smart Terminal Devices: Concentrations, Distributions, Profiles, and Environmental Emissions
No full textDimethylcyclosiloxanes (DMCs) are utilized as vital monomers in the synthesis of organosilicon compounds, integral to the manufacture of mobile smart terminal devices. Toxicological studies have revealed potential endocrine-disrupting activity, reproductive toxicity, neurotoxicity, and other toxicities of the DMCs. This study investigated the concentrations and composition profiles of seven DMCs, including hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), dodecamethylcyclohexasiloxane (D6), and tetradecamethylcycloheptasiloxane (D7), hexadecamethylcyclooctasiloxane (D8), and octadecamethylcyclononasiloxane (D9) in three types of mobile smart terminal device components (silicone rubber, adhesive, and plastics). Environmental emissions of DMCs from silicone rubber materials were also estimated to improve the recognition of their potential fate within the environment. D5-D9 were widely present in silicone rubber and adhesives with detection rates ranging from 91-95.5% and 50-100%, respectively, while D3 and D4 were more frequently detected in plastics, both showing a detection rate of 61.1%. Silicone rubber had the highest total DMCs (& sum;7DMCs) and a concentration of 802.2 mg/kg, which were dominated by D7, D8, and D9. DMCs detected in adhesives were dominated by D4, D5, and D6. The estimated emission of & sum;DMCs released into the environment in China from silicone rubber used in mobile smart terminal devices exceeds 5000 tons per year. Further studies are needed on the presence of DMCs in various commodities and environmental media to assess their ecological and human health impacts, as well as the toxicological effects of D7-D9 for the appropriate regulation of these chemicals
Fractionation Mechanism and Flux Estimation of Strontium Isotopes During Basalt Weathering
No full textThe fluxes of metal cations and isotopes released by weathering of silicate rocks are crucial and a prerequisite for constraining geochemical fluxes to rivers and oceans. This study presents mineral and elemental compositions along with 87Sr/86Sr and d88Sr data from a basaltic weathering regolith on Hainan Island, South China to elucidate Sr isotope fractionation and weathering fluxes. The 87Sr/86Sr ratios vary from 0.703936 to 0.706338 as a result of differential weathering of the minerals. d88Sr values in the weathering regolith range from 0.29 to 0.37%, with the majority of the weathering regolith having lower d88Sr values than the parent rock. Sr is leached into the soil solution during plagioclase decomposition, while 86Sr is preferentially adsorbed on the surface of secondary minerals. As weathering progresses, smectite decomposes and kaolinite desorbs under weakly acidic conditions, releasing the previously adsorbed 86Sr into the soil solution. The differential weathering of kaolinite and smectite controls the d88Sr values of the weathering regolith, with pH being an important determinant of isotope fractionation. Furthermore, Sr elemental fluxes (SrFlux) and Sr isotopic fluxes (d88SrFlux) of this weathering regolith were calculated using a mass balance model, yielding mean values of 0.20 (mg cm 3 Myr 1) and 0.052 (% (mg cm 3 Myr 1)), respectively. The d88SrFlux exhibits a nonlinear positive correlation with the Chemical Index of Alteration (CIA), indicating that enhanced weathering leads to significant stable Sr isotope fractionation at CIA values below 95%. Our research promotes the understanding of Sr recycling and the fractionation behavior of stable Sr isotopes during consecutive weathering.
Plain Language Summary The stable Sr isotopic composition is sensitive to continental weathering processes and offers insights into weathering mechanisms. However, the fractionation mechanism of stable Sr isotopes remains elusive. Significant stable Sr isotope fractionation was observed in a basaltic weathering regolith on Hainan Island, South China. Plagioclase decomposition releases Sr, with isotopically light Sr adsorbed onto the surfaces of secondary minerals during incipient weathering. Smectite decomposition and kaolinite desorption control the stable Sr isotopic composition of the weathering regolith during advanced weathering. The variable pH of the regolith is an important external control factor. We focus on modeling Sr isotopic fluxes and Sr elemental fluxes from weathering regolith. Our results show that within a certain range, an increase in weathering intensity leads to a nonlinear increase in weathering fluxes. The d88 Sr values of the loss component are consistent with those in silicate-dominated river catchments, suggesting that metal cations from the weathering of silicate rocks have a significant influence on the hydrochemical and isotopic composition of river waters in these catchments. Stable Sr isotope fractionation thus provides quantitative constraints on continental weathering fluxes
Andesitic arc magmas derived from two contrasting melange origins: Evidence from central Tibetan dioritic porphyries
No full textNumerous studies have argued that the andesitic model (i.e., the arc directly produces andesitic magmas) rather than the basaltic-input model (i.e., andesitic magmas were derived by fractionation from basaltic magmas) better explains the composition of continental crust. However, it remains controversial as to how andesitic magmas are directly produced in subduction zones, and this weakens the basis of the andesitic model. Recently, the m & eacute;lange-diapir model has been increasingly considered to be an important mechanism for the direct generation of arc andesites, and would thus support the andesitic model for crustal growth. Moreover, melting experiments on m & eacute;lange rocks show that varying the composition of m & eacute;lange rocks (e.g., serpentinite-dominated and sediment-dominated m & eacute;lange) can produce a range of primary andesitic arc magmas, from tholeiitic, calc-alkaline, to high-K calc-alkaline and shoshonitic magma types. However, it remains challenging to determine the different m & eacute;lange sources for low-K to high-K arc andesites. Here we verify these experimental results using data from early Cretaceous dioritic porphyries in the southern Qiangtang block of central Tibetan plateau. The generation of these dioritic rocks cannot be linked to any coeval basaltic rocks in this region and so they may represent primary andesitic magmas. These dioritic porphyries are divided into low-K type-1 and high-K type-2. Type-1 has higher delta Mg-26 values (-0.16 to -0.08 parts per thousand) than MORBs (mid-ocean ridge basalt, delta Mg-26 = -0.25 +/- 0.06 parts per thousand), which can be ascribed to the contribution of subducted bulk/un-differentiated serpentinite in their source. Combined with their low K contents and La/Sm ratios, and high NdHf isotope ratios (epsilon Nd(t) = 3.36-5.03; epsilon Hf(t) = 12.88-13.76), we suggest a serpentine-dominated m & eacute;lange for their origin. The type-2 and type-1 rocks have similar trace-element distribution patterns, the same ages (similar to 124 Ma), and are indistinguishable in field outcrops, indicating their common petrogenesis. However, compared to type-1, type-2 has higher K and Th contents, and La/Sm and Th/Nd ratios with lower NdHf isotope values (epsilon Nd(t) = -1.61 to +0.30; epsilon Hf(t) = 3.78-4.39), and mantle-like delta Mg-26 values (-0.28 to -0.17 parts per thousand). This likely indicates derivation from a sediment-dominated m & eacute;lange for type-2. This study thus verifies the experimental results of melting of different m & eacute;lange from natural rock record and shows that the m & eacute;lange model provides an important mechanism for generating variable andesitic arc magmas and so the formation of the andesitic continental crust
Crustal recycling and growth via mélange diapir in subduction zones: Insights from two episodes of magmatism in the Northern Yili Block, NW China
No full textArc magmatism can provide crucial information about crustal recycling and growth in subduction zones. However, the crustal material transfer process and mechanism from subducting slab to overlying mantle wedge are controversial. Here, we present detailed petrological, geochronological, geochemical, and Sr-Nd-Hf isotopic data, and previously published data for two episodes of subduction-related magmatism in the Northern Yili Block, NW China, to study these issues in relation to the recycling of continental crust and the growth of two separate magmatic episodes at 400-350 Ma and 350-300 Ma, respectively. The Nd-Sr isotope modeling results, low Nd/Sr, and variable Hf/Nd ratios demonstrate that the arc magmatism of these two episodes could be derived from two distinct melange sources that formed at the slab -mantle interface during subduction of the Junggar oceanic plate. In the first episode, magmatic rocks exhibit high Th/Yb and (La/Sm)N ratios and the decoupling of Nd-Hf isotopes, which leads to an interpretation that the primary magmas were derived from sources mainly mixed by sediments and mantle -wedge peridotites. In contrast, the magmas of the second episode exhibit high Ba/La and Ba/Th ratios and the coupling of Nd-Hf isotopes, which implies that these magmas were likely produced from melange diapirs dominated by the mixing of midoceanic -ridge basalts and mantle peridotites. The episodes of Nd-Hf isotopic decoupling and coupling coincided with crustal material recycling and crustal growth in subduction zones, respectively. Considering that the Northern Yili Block at 400-350 Ma was characterized mainly by enriched Nd-Hf isotopes of magmatic rocks and no development of the forearc accretionary complex, while the Northern Yili Block at 350-300 Ma featured high epsilon Hf(t)-epsilon Nd(t) values of magmatic rocks, occurrences of extension -related magmatism, and distribution of the forearc accretionary complex and immature back -arc basin, we propose that these two different melange sources probably developed in response to subduction transition. That is, one resulted from advancing and the other was relevant to retreating regime. The partial melting of these melanges in the mantle wedge generated intermediate felsic magmas, which might have acted as a leading mechanism for crustal recycling and growth of the accretionary orogenic belt