Hanjie Wen

Molybdenum isotopic records across the Precambrian-Cambrian boundary

Molybdenum (Mo) isotopic data from early Cambrian formations in southern China demonstrate the importance of non-euxinic sediments as a promising archive for coeval seawater Mo records. For the first time, in analogue to noneuxinic, nonskeletal carbonates, we report that the pristine phosphorites also can preserve the ambient fluid, which make the pristine phosphorites a new potential fingerprinting tool to record the coeval paleoseawater. Pristine phosphorite and dolomite samples of two early Cambrian formations in southern China suggest that early Cambrian seawater may have had δ 97/95 Mo values of at least 1.4‰, similar to modern oceans. Low δ 97/95 Mo (as low as −0.5‰) values were obtained from reworked phosphorite, but evidence suggests that these are reflecting Mo related to Fe oxyhydroxide minerals rather than the coeval seawater. In contrast, late Precambrian dolomite just below the transition to the Cambrian would indicate a δ 97/95 Mo of coeval seawater slightly heavier than that of the Mesoproterozoic (0.9‰–1‰ versus 0.8‰), suggesting that oceanic circulation patterns may have been thoroughly reorganized by that time, and may have triggered biological diversification from the Ediacaran to the early Cambrian.

Isotopic delta values of molybdenum standard reference and prepared solutions measured by MC-ICP-MS: Proposition for delta zero and secondary references

We report the isotopic composition of five molybdenum (Mo) standard reference solutions and four fractions from one of these solutions eluted through anion resin column relative to a sixth reference solution. The reference solutions were National Institute of Standards and Technology (NIST) SRM 3134 (lot#891307), Johnson Matthey Specpure (JMC)-Mo Sie (lot #602332B), JMC-Mo Wen (lot#13989C), Merck (lot#170334), Sigma-Aldrich (lot#207306) and Prolabo. Measurements were conducted using Isoprobe multi collector inductively coupled plasma mass spectrometer (MC-ICP-MS) at the Centre de Recherches Petrographiques et Geochimiques (France) and Nu Plasma MC-ICP-MS at either the Ecole Normale Superieure de Lyon (France) or the Laboratory of Isotope Geology in the Ministry of Land and Resources (China). The sample-standard bracketing method was employed to correct the mass bias for Mo isotopes during instrumental measurement. Except for the Merck Mo solution, all the Mo solutions were identical in isotopic composition within error. Although the JMC Mo solution has been used as the internal reference material by various groups, uncertainty may still occur with different lot numbers and availability might be limited. Here, we propose the NIST 3134 Mo solution as a new candidate for delta zero reference material, used for reporting Mo isotopic composition of natural samples. Isotopic compositions for four eluted fractions of the Sigma-Aldrich Mo solution were 2.18‰, 0.98‰, −1.10‰ and −1.95‰ for δ97/95Mo relative to the NIST Mo standard. These values span the range of reported isotopic compositions for natural terrestrial and experimental samples (approximately −0.5‰ to 1.6‰ for δ97/95Mo). We propose these eluted fractions to be used as a secondary reference for Mo isotope measurements. Mo solutions are available at CRPG upon request.

Characteristics of Cd isotopic compositions and their genetic significance in the lead-zinc deposits of SW China

Up to now, the evaporation and condensation, as well as the biological absorption and inorganic absorptions, have been proved to be major factors in Cd isotope fractionation. And Cd isotopes have been widely applied in studies on the universal evolution and marine environment and so on. However, only a few researches have been conducted in applying Cd isotopes to trace the source of metallogenic material and the evolution of the ore-forming fluid in a complex mineralization environment, especially in a hydrothermal ore-formation system. We measured the Cd isotopic compositions of sphalerite, galena, and ores from five lead-zinc deposits in SW China, and found that the δ114/110Cd values varied from −1.53‰ to 0.34‰, with a total range of 1.87‰, which is greater than most of measured geological samples. Meanwhile, through contrasting the Cd content with Cd isotopic compositions of different deposits, it may be concluded that different genetic lead-zinc deposits have different Cd content and isotopic compositions, which could be a tool for the studies on the origin of ore deposits. Also, the biomineralization and crystal fractionation may also result in Cd isotope fractionation. In a word, although the research of Cd isotopes is presently at the preliminary stage (especially in hydrothermal ore-formation system), this study demonstrated that Cd isotopes can give a clue in tracing the evolution of ore-forming fluid and metallogenic environment.

Zn/Cd ratios and cadmium isotope evidence for the classification of lead-zinc deposits

Lead-zinc deposits are often difficult to classify because clear criteria are lacking. In recent years, new tools, such as Cd and Zn isotopes, have been used to better understand the ore-formation processes and to classify Pb-Zn deposits. Herein, we investigate Cd concentrations, Cd isotope systematics and Zn/Cd ratios in sphalerite from nine Pb-Zn deposits divided into high-temperature systems (e.g., porphyry), low-temperature systems (e.g., Mississippi Valley type [MVT]) and exhalative systems (e.g., sedimentary exhalative [SEDEX]). Our results showed little evidence of fractionation in the high-temperature systems. In the low-temperature systems, Cd concentrations were the highest, but were also highly variable, a result consistent with the higher fractionation of Cd at low temperatures. The δ114/110Cd values in low-temperature systems were enriched in heavier isotopes (mean of 0.32 ± 0.31‰). Exhalative systems had the lowest Cd concentrations, with a mean δ114/110Cd value of 0.12 ± 0.50‰. We thus conclude that different ore-formation systems result in different characteristic Cd concentrations and fraction levels and that low-temperature processes lead to the most significant fractionation of Cd. Therefore, Cd distribution and isotopic studies can support better understanding of the geochemistry of ore-formation processes and the classification of Pb-Zn deposits.

A new look at saponite formation and its implications for early animal records in the Ediacaran of South China

Acanthomorphic acritarch fossils, including some interpreted to be the fossils of the earliest animal embryos, first appear in the lower Doushantuo Formation of the Yangtze Gorges area (YGA). Further, the complete paleontological and geochemical record for the YGA has played a central role in defining the global biological and geochemical backdrop that presaged and witnessed the dawn of diverse animal life. Despite the importance of the YGA in our understanding of Neoproterozoic Earth history, basic aspects about its depositional history remain debated. Foremost among the controversies, extensively studied sections in the YGA were recently tied to deposition in an alkaline lake, casting new but contentious light on the environments of early animal evolution and the broader significance of geochemical records from the YGA. Arguments for a lacustrine setting hinged on the presence of trioctahedral clays (saponite-corrensite). However, this clay type commonly forms in other environments, including the weathering profiles of mafic and ultramafic volcanics. Using a coupled geochemical and sedimentological approach, we argue that the trioctahedral clays in the lower Doushantuo of the YGA are better explained as weathering products from a regional mafic-to-ultramafic hinterland delivered by rivers to a shelf or lagoon in the Yangtze Gorges Basin. These novel provenance relationships for YGA sediments and associated clays are consistent with a marine setting for the early animal records and must factor in our current understanding of the broader geochemical fabric of the Doushantuo Formation.

Geology and Geochemistry of Se-Bearing Formations in Central China

It is widely believed that selenium, a typical dispersed element, does not form independent deposits. However, this has been challenged recently by discoveries and investigations of Se-bearing formations in China. In this paper we present our findings with a systematic discussion on the genesis of these Se-bearing formations. The Se-bearing formations typically are developed in tensile tectonic settings, mainly along the margins of the Yangtze and the Tarim cratons, or within the Yangtze craton. They are mostly Cambrian, but a few are Permian in age. Lithologically, they largely consist of carbonaceous cherts and carbonaceous shales (slates) that contain abundant organic matter. In general, the Se-bearing formations are characterized by exceptional associations of elements. In particular, they are characterized by elements with multiple oxidation states, such as V, U, P, Ni, Mo, etc., which are typical for the black shale series, and by elements like As, Ba, and Sb, which are considered as indicative elements for hydrothermal sedimentation. In some cases, they also contain Au and platinum-group elements (PGE), which are quite unusual for ordinary depositions. Organic geochemical studies and biomarker investigations revealed that the Corg in rocks are relatively high in these Se-bearing formations, ranging from 1.3% to 17.8%. The fraction of soluble organic matter extracted from the samples is commonly less than 50 ppm. Gas chromatographic (GC) analyses suggest that saturated hydrocarbons in these formations consist mainly of low-carbon-number alkane, with the highest peak of C15-C18 and with low ratios of Pr/Ph (0.62-1.03). Using gas chromatographic-mass spectrometric (GC-MS) analyses, many biomarkers have been identified, including the relatively complete series of long-chain tricyclic terpanes and hopanes in m/z 191 mass chromatogram and the steroids in m/z 217 mass chromatogram. Evidence suggests that the source or precursors of the organic matter might have been some sort of bacteria and/or algae. The reducing sedimentary environments that hosted these bacteria and algae favored the growth and preservation of such organisms. It was these organisms that acted as the carrier of Se, leading to Se enrichment and deposition. Finally, some evidence also indicates that the source of the Se was also closely related to the formation of hydrothermal sedimentation of cherts.

Ocean to continent transfer of atmospheric Se as revealed by epiphytic lichens

There is still a long-term debate concerning the relative contributions of naturally emitted and anthropogenic Se at the regional and local scales. Here, Se and heavy metal concentrations are reported for epiphytic lichens collected in coastal and inland areas from the USA, Canada and France for assessing atmospheric Se source. Correlations found between Se and Cl in lichens confirmed the major marine biogenic source for atmospheric Se. Continental samples do not show systematic relationships between Se and other metal (Pb, Cu, In ...) contents, even for lichens collected in the vicinity of smelters or close to urban areas. Our results suggest that, although anthropogenic Se may be present, the marine biogenic Se source is a major contributor to atmospheric Se for our sampling locations. The contribution of naturally emitted atmospheric Se may be significant in urban and industrial areas and should be taken into account for further studies.

An analytical method for precise determination of the cadmium isotopic composition in plant samples using multiple collector inductively coupled plasma mass spectrometry

Isotope techniques can be applied to discover the migration and transformation of metal elements in plants. However, only a few studies on Cd isotopes in plants have been carried out so far. In this study, an optimized analytical method consisting of digestion, purification and determination of Cd isotopes in plants was developed. Three Cd standard solutions as well as four plant species (Solanum nigrum, Ricinus communis, Cyperus alternifolius and Pteris vittata), which were grown in soil or hydroponic cultures, were repeatedly analyzed for Cd isotopes using Multiple Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS). The factors that affected the accuracy of Cd isotopic determination, such as isobaric interference and instrumental mass fractionation, have been carefully evaluated and corrected. The purification procedure yielded a Cd recovery of not less than 95% and effectively eliminated the spectral interference of Pd, In and Sn as well. The analysis of pure Cd standard materials showed accurate isotope values, which matched with the results of previously published methods. This technique provided an average long-term external reproducibility of ±0.09‰ for δ114/110Cd (2SD). The overall δ114/110Cd values of four plant species ranged from −0.39‰ to −0.08‰ and provided direct evidence for Cd isotopic fractionation in herbaceous plants.

Fractionation of Stable Cadmium Isotopes in the Cadmium Tolerant Ricinus communis and Hyperaccumulator Solanum nigrum.

Cadmium (Cd) isotopes provide new insights into Cd uptake, transport and storage mechanisms in plants. Therefore, the present study adopted the Cd-tolerant Ricinus communis and Cd-hyperaccumulator Solanum nigrum, which were cultured under controlled conditions in a nutrient solution with variable Cd supply, to test the isotopic fractionation of Cd during plant uptake. The Cd isotope compositions of nutrient solutions and organs of the plants were measured by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). The mass balance of Cd isotope yields isotope fractionations between plant and Cd source (δ114/110Cdorgans-solution) of −0.70‰ to −0.22‰ in Ricinus communis and −0.51‰ to −0.33‰ in Solanum nigrum. Moreover, Cd isotope fractionation during Cd transport from stem to leaf differs between the Cd-tolerant and -hyperaccumulator species. Based on these results, the processes (diffusion, adsorption, uptake or complexation), which may induce Cd isotope fractionation in plants, have been discussed. Overall, the present study indicates potential applications of Cd isotopes for investigating plant physiology.

Marine Redox Conditions in the Early Cambrian Ocean: Insights from the Lower Cambrian Phosphorite Deposits, South China

It is generally considered that a significant change in oceanic redox conditions occurred during the Ediacaran–Cambrian transition. However, there are currently two major conflicting views on the degree of oxygenation of deep water (oxic vs. ferruginous) during this interval. To date, the oxygenation conditions of the Early Cambrian ocean have not been well constrained. The oxygenation magnitude and mechanism of the Early Cambrian ocean could be critical to the significant biological evolution of the “Cambrian Explosion”. To constrain the Early Cambrian oceanic redox environment, we conducted an integrated study on iron and sulfur isotopes and redox-sensitive elements (Mo, U, and V) of Lower Cambrian phosphorite deposits from two shallow sections (Meishucun and Gezhongwu) and a deeper water section (Zunyi) from the Yangtze Platform, South China. The near zero δ56Fe values from the two shallow sections studied here reflect oxic conditions in the lower phosphorite deposition. An obvious positive shift in δ56Fe and redox-sensitive element content was observed in the middle parts of the two shallow water sections, which might reflect loss of light iron by dissimilatory iron reduction during early diagenesis under suboxic shallow water in the platform. However, the highly positive δ56Fe values in the deep section could reflect a lower oxidation degree of dissolved Fe(II) under anoxic deep water. The data suggest redox-stratified oceanic conditions during the Early Cambrian, in which completely oxygenated shallow water (platform) coexisted with anoxic deep water (slope). We propose that prolonged upwelling of dissolved organic carbon (DOC)-, Fe(II)- and phosphorus-rich anoxic deep water in a redox-stratified ocean could have increased exchange with the open ocean, resulting in major phosphorite deposition in oxic-suboxic conditions. The progressive oxygenation of the ocean may have facilitated the Early Cambrian biotic diversification.