Selenium (Se) deficiency and excess in agricultural ecosystems are related to many human health problems through the food chain and depend on the spatial variability and bioavailability of Se in soils. This paper examines the spatial variability of Se in soils as affected by geologic and pedogenic processes and its effects on rice grains, groundwater, and human health using Rugao County, Jiangsu Province, an agricultural area in Yangtze River Delta Region, which has a high level of nonagenarians, as a case study. The results showed that total soil Se (T-Se) concentration in the study area (average 0.13 ± 0.02 mg kg^-1, n = 203) was close to the Se Clark value, the Se mean concentration in earth crust, and had narrow spatial variation as a result of consistent geological processes. As a fraction of the potential bio-available Se in ecosystems, soil water soluble Se (WS-Se) (average 2.27 ± 1.09 μg kg^-1, n = 203) had wide spatial variation. The higher concentrations of WS-Se were spatially distributed on loamy and highly weathered soils in the east and north of the county. Spatial heterogeneity of WS-Se was primarily due to changes in soil physicochemical properties caused by pedogenic processes related to cultivation. Average Se concentration in the rice grains and groundwater were 97 ± 69 μg kg^-1 (n = 93) and 0.19 ± 0.21 μg L^-1 (n = 95), respectively. Similar spatial variability and correlation between WS-Se and rice Se, groundwater Se, and longevity ratio (an index of nonagenarians per 100,000 inhabitants), were found, indicating that the soluble fractions of Se in soils play an important role in biogeochemical cycling in the ecosystem and might be an important indicator of Se deficiency or excess in soils. Thorough small-scale surveys of element availability in the soil should be emphasized where geologic settings are simple but pedogenic processes are complex due to intensive human activities. So WS-Se in soils was advised as a better indicator for Se environmental effect than T-Se, especially when dealing with simple geologic settings and complex pedogenic processes on a small scale.

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High As groundwater was found in shallow aquifers of the Hetao Basin, Inner Mongolia, which has caused serious chronic arsenism. Groundwater particles are ubiquitous in natural aquifers, and significantly affect As mobilization and transportation. This study has investigated their effect on element hydrogeochemistry. Groundwater samples were filtered in the field through a progressively decreasing pore size (10, 5, 3, 1, 0.8, 0.45 μm). Unfiltered samples were also taken. Chemical analysis of 43 groundwater samples shows concentrations of most elements (Ba, Co, Mn, Mo, Ni, V, Sr, Mg, Ca, Na, K) in unfiltered samples are well correlated with those in <0.45 μm fraction, and the ratios between them are close to 1.00. Those elements are considered to be present in the form of true dissolved group and are not affected by the filtration. Although saturation indices for calcite and dolomite are greater than 0, they could possibly exist as small size colloids (<0.45 μm) in groundwater. Concentrations of Cd, Pb, Fe, and Al in <0.45 μm fraction are mostly less than those in unfiltered fraction, implying that large-size inorganic-rich (Al/Fe complexing) colloids could be possible carrier for Cd and Pb in the groundwaters. Arsenic, Cu, Zn and Cr are fallen into another group. These elements could mainly be associated with organic-complexing colloids with particle size less than 0.45 μm, although As could partially be carried by large-size colloids. In groundwaters saturated with respect to pyrite and siderite, a small proportion of As (about 15%) is associated with large-size Fe complexing particles (>0.45 μm). Larger amounts of As is trapped with small size organic colloids, which has been confirmed by the ultrafiltration. The study suggests that groundwater particles transported in the aquifers could affect element geochemistry, especially for As expected to be associated in small size complexes.

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We report here high-resolution Mn/Ca ratios of planktonic foraminifer, Globigerinoides sacculifer from ODP Site 1144 in the northern South China Sea (SCS). The variation pattern of the Mn/Ca ratios of G. sacculifer from the top 170 mcd is very similar to those of the generally used paleoclimate records, such as δ¹⁸O of Globigerinoides ruber and SSTs derived from Mg/Ca ratios of G. sacculifer. Robust Milankovitch periodicities are presented in the power spectrum of the Mn/Ca record, as well as in those of the other paleoclimate records, suggesting orbital forcing for the variation of the Mn/Ca ratios. Further spectral analysis indicates that the variations of the Mn/Ca ratios lead those of the δ¹⁸O of G. ruber at all the orbital periodicities, and are roughly in pace with or slightly lead those of the SSTs at eccentricity and obliquity periodicities, but lag behind that of the SSTs at precession periodicity. This suggests that the driven forcing for the variation of the Mn/Ca ratios mainly associate with those for the SSTs. Comparisons between the Mn/Ca ratios and other geochemical proxies for redox condition in sediments show that higher Mn/Ca ratios of G. sacculifer generally correspond to higher authigenic Mn concentrations and lower contents of total sulfur (TS) and total organic carbon (TOC). Thus, the Mn coatings in the G. sacculifer at ODP Site 1144 are mainly associated with Mn oxides, and the variation of the Mn/Ca ratios of G. sacculifer appears to indicate changes of redox condition in sediments. Exception is observed at the top 10 mcd, in which the Mn/Ca ratios show an upward decreasing trend and low core-top values, while other proxies suggest oxic environment in this section. The low Mn/Ca ratios agree with the general low Mn/Ca ratios in foraminifer tests from core-top sediments in the SCS, and seem to attribute to the fact that authigenic Mn precipitation are mainly generated within sediment column rather than at sediment surface and the oxidation states for them may change from Mn(III) oxyhydroxides to Mn(IV) oxides after burial. However, details are not well known yet. Despite of such uncertainties, the response of the variation of the Mn/Ca ratios of G. sacculifer to paleoclimate changes is significant, indicating that it is an informative paleoclimate indicator.

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Few investigations have been made to discuss the influence of water on the yields and carbon and hydrogen isotopic ratios of gas hydrocarbons generated from oil cracking although water occurs in oil reservoirs. In this study, a pyrolysis experiment for a crude oil under hydrous and anhydrous conditions in a gold tube confined system was performed at T = 370°C, P = 21 MPa and times from 1 to 7 days. It is found that water has an obvious inhibition on the gas yields generated from oil cracking and a pronounced influence on the carbon and hydrogen isotopic fractionations of the cracking gas. Compared with the anhydrous pyrolysis, the hydrous pyrolysis has a smaller C_1-5 alkane yields and lighter carbon and hydrogen isotopic ratios of the C_1-3 hydrocarbons within maturity levels investigated. The differences in their C_1-5 alkane yields reach 3-12 ml/g, in their δ¹³CH₄, δ¹³C₂H₆ and δ¹³C₃H₈ values reach 1.0-2.7‰, 0.2-0.8‰ and 0.2-0.3‰, respectively, and in their δD_CH4, δD_C2H6, δD_C3H8 values reach 5-16‰, 22-80‰ and 4-13‰, respectively. Moreover, the hydrous pyrolysis experiment formed a greater amount of H₂ and alkenes and a smaller amount of CO₂ than the anhydrous pyrolysis experiment did. The differences of H₂, ethene, propene and CO₂ between the two experiment conditions are 0.44-2.12 ml/g, 0.03-0.05 ml/g, 0.20-0.34 ml/g and 0.08-0.98 ml/g, respectively. These results indicate that these differences in carbon and hydrogen isotopic ratios of generated gases between the hydrous and anhydrous experiments is possibly because water affects the free radical reactions of oil cracking and inhibits the combination between alkyl and hydrogen to form alkane to some extent, rather than due to the isotopic exchange reactions between water and some pyrolysates as usually believed for kerogen hydrous pyrolysis.

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Low-grade metamorphic Paleozoic sedimentary rocks are exposed along the northern margin of the Qinling-Dabie orogenic belt that formed by the collision of the North and South China blocks during the Early Mesozoic. Geological provenance and tectonic setting of these low-grade sedimentary rocks are strongly debated. Recent studies have shown that they contain Paleozoic, Late Proterozoic and Late Archean detrital zircons, which points to an origin from different sources including rocks from the South and North China blocks. This study presents single-grain mineral Rb-Sr results for detrital muscovites and biotites from Carboniferous sedimentary sequences of the Beihuaiyang low-grade metamorphic zone exposed along the northern margin of the Dabie Mountains. About 15% of detrital muscovite grains are phengitic in composition with high Si/Al ratios of >3.3. Single grain Rb-Sr isotopic analyses show that both muscovites and phengites give a pooled age of about 420 Ma, implying Middle Paleozoic thermal and high-pressure metamorphic events in the source area(s). Rb-Sr isotopic evidence from detrital biotites suggests at least two different sedimentary sources of old crustal rocks and juvenile material. The latter probably comprised magmatic rocks related to the Paleozoic convergence along the southern margin of the North China block. Whole-rock Sm-Nd isotopic composition of the sedimentary rocks also demonstrates a mixture of sedimentary sources of different ages that confirm conclusions proposed previously from evidence of detrital zircon ages. We conclude that the Paleozoic detrital phengites originate from Paleozoic subduction and subsequent collision proposed for the North Qinling terrane. Our study shows that, single-grain detrital mica Rb-Sr isotopic characteristics can be a potential indicator for the provenance of geological complex terranes.

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This article was retracted.

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In order to investigate if pressure can increase the yield of the yield of methane and gaseous hydrocarbon as a whole, both anhydrous and hydrous isothermal pyrolysis experiments were performed on Fushun coal and Kimmeridge shale under pressure of 10 MPa and 50 MPa respectively, and chemical components and stable isotopes of the gaseous products from pyrolysis were analyzed. For both anhydrous and hydrous pyrolysis experiments, Fushun coal and Kimmeridge shale both show lower total gas yields, lower CO₂ yields and lower H₂ yields at high pressure than those at low pressure, but higher total gaseous hydrocarbon yields and higher methane yields at high pressure than those at low pressure. Meanwhile, both Fushun coal and Kimmeridge shale show carbon and hydrogen isotope ratios being heavier at high pressure than at low pressure. However, pyrolysis experiments, theoretical reasoning and carbon and hydrogen isotope analyses proved that pressure can not promote hydrocarbon generation from organic matter, and the increase of hydrocarbon yield is largely due to reaction between carbon dioxide and hydrogen in forming hydrocarbon at high pressure.

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