Combined organic and inorganic geochemical reconstruction of paleodepositional conditions of a Pliocene sapropel from the eastern Mediterranean Sea

Layers of organic-carbon-rich sapropels in the sediment record of the Mediterranean Sea give evidence of repetitive changes in regional Plio-Pleistocene climate. Results from biomarker molecule and major and trace element analyses of closely spaced samples are used to reconstruct the conditions leading to deposition of a Pliocene sapropel at Ocean Drilling Program (ODP) Site 969 on the Mediterranean Ridge. Organic carbon concentrations increase from 0.2% outside the sapropel and peak to more than 30% within it. Major and trace elemental composition and biomarker-derived parameters indicate elevated productivity, depletion of water-column dissolved-oxygen content, and changes in sediment provenance in response to climatic changes. Budgets of rhenium, thallium, and other trace metals indicate that deep-water exchange between the Mediterranean subbasins and the Atlantic Ocean was not completely interrupted during sapropel formation. Enrichment factors of redox-sensitive and sulfide-forming trace metals as well as the presence of isorenieratene derivatives and high stanol/sterol ratios point to an extended zone of anoxic water masses. Depth profiles of biomarker compositions (sterols, long-chain alkenones, alkandiols and -ketols, fatty acids) indicate great floral diversity during deposition of a single sapropel and highlight the sensitive response of the marine community to variable environmental conditions. Changes in water mass circulation and eolian transport can be reconstructed by use of both lithogenic elements and average chain lengths of n-alkanes (ACL index).     

Late Quaternary sapropel sediments in the eastern Mediterranean Sea: Faunal variations and chronology

Distinctive planktonic foraminiferal assemblages which characterize particular late Quaternary sapropel layers in deep basin sediments from the eastern Mediterranean Sea have been identified using cluster analysis. Three distinct clusters allow for identification and intercore correlation of the nine sapropels deposited during the last 250,000 yr. Cluster 1, representing sapropel layers S1 and S9, exhibits low abundances of Neogloboquadrina dutertrei and high abundances of Globigerinoides ruber; Cluster 2, which groups S3, S5, and S7, contains high abundances of G. ruber, N. dutertrei, and Globigerina bulloides, and Cluster 3, which includes samples from S4, S6, and S8, is marked by extremely abundant N. dutertrei and G. bulloides, and rare G. ruber. Analysis of sedimentation rates in 14 cores reveals the following approximate ages for the sapropel layers: S2 = 52,000 yr B.P.; S3 = 81,000–78,000 yr B.P.; S4 = 100,000–98,000 yr B.P.; and S5 = 125,000–116,000 yr B.P. As previously suggested, sedimentation rates on the Mediterranean Ridge were determined to be relatively constant during the last 127,000 yr. In contrast, basin sedimentation rates have fluctuated markedly from lower rates during interglacial stage 5 to higher rates during the last glacial episode. These glacial/interglacial differences are most pronounced in the northern Ionian Basin, because of increased terrigenous sediment deposition during glacial episodes. Unusually high biogenic sedimentation rates occurred in an arc south of Crete during the deposition of sapropel S5, probably due to higher productivity in this region.     

Development of climatic and vegetation conditions and the geochemical and isotopic composition in the Franconian Albvorland aquifer system

The impact of climatic and vegetation conditions over the past 15 Ka on the chemical composition and ¹⁴C dating of groundwater from the Franconian Albvorland aquifer system is discussed. Seven groundwaters over a flow distance of 25.5 km are investigated. Groundwater dating is made by ¹⁴C of dissolved inorganic carbon (DIC) and aqueous fulvic acid as well as ¹⁸O. ¹⁴C dating via fulvic acid gives groundwater ages consistent with climatic and vegetation records and variations in the groundwater composition. No correction for geochemical processes is required, since under these geochemical conditions fulvic acid remains stable over this time period and flow-distance. On the other hand, ¹⁴C dating via DIC requires correction of the ¹⁴C value due to perturbation by different geochemical processes. Up to a groundwater flow distance of approximately 17 km and an age of about 10 Ka, the ¹⁴C dating by DIC shows considerable dependence on the ¹⁴C-correction model applied. Beyond this groundwater age, ¹⁴C-DIC dating results in an overestimation by two to three ¹⁴C half-lives (T_1/2=5730 a). This deviation may result from different groundwater recharge conditions at the end of the past glaciation and geochemical processes acting on DIC that cannot be adequately characterized. The present study has implications for humic substance mediated transport of pollutants in natural aquatic systems over long time periods.     

The isotopic composition of plant sulfur

The isotopic composition of sulfur has been studied in plants representative of various regions of the U.S.S.R., two oceanic islands, and atmospheric precipitations on land and in marine areas. In soils, the isotopic composition of sulfur in the atmospheric water varies as a result of sulfate reduction (increase of δ³⁴S of the soil sulfate) and sulfate regeneration from hydrogen sulfide. The sulfur in plants from the oceanic islands has characteristically higher values of δ³⁴S than the sulfur in the plants and in the atmospheric water of the continents. Compared to sea water, the sulfur from the island plants that were studied contains a considerably lesser proportion of the ³⁴S isotope. This can be explained by the significant role in such plants of the sulfur of the atmospheric air masses coming from the continents.     

The geochemical and isotopic composition of aquifer systems in the deltaic region of the Po River plain (northern Italy)

The chemical and hydrodynamic characteristics of groundwater in deltaic regions are strongly influenced by the complex stratigraphy of these areas, caused by the continuously varying depositional environments associated with their recent hydrographic evolution. As a case study, the eastern sector of the Po River plain, northern Italy, has been investigated to understand the quality of the available groundwater resources. Based on the analysis of hydrochemical and isotopic data, the recharge characteristics, the groundwater residence time and the aquifer vulnerability are defined. The results show significant qualitative degradation of the unconfined aquifer due to the shallow depth to water, while in the underlying confined aquifer, a hydrochemical facies of Ca–HCO₃ type prevails. The spatial variation and relationship between oxygen-18 and deuterium determine: firstly, hydraulic separation of the two hydrogeological units; secondly, direct infiltration of local precipitation to the unconfined aquifer; thirdly, the occurrence of waters originating in the Alps and locally from the Apennines, pervading the confined aquifer. The tritium results suggest local mixing between the superficial waters and the confined aquifer, occurring along the palaeo-river channels. This increases the pollution vulnerability of the confined hydrogeological unit within the plain, which is the only natural groundwater resource exploited for water supply.     

滇西地区腾冲地块东侧混合岩锆石年龄和Sr-Nd-Hf同位素组成

滇西地区特提斯构造带，是东特提斯构造带的重要组成部分之一，由多个微陆块和陆块相间的造山带组成。陆块内部基底出露局限并经历多期构造作用的叠加，因而其基底属性和演化历史难于恢复。本文报道腾冲地块东侧混合岩的锆石年龄、地球化学组成和锶-钕-铪同位素特征，探讨原岩的形成时代和背景。混合岩原岩可能源自中元古代花岗闪长岩．花岗岩质地壳岩石，晚中生代重熔再造，可能经历后期混合岩化。亏损地幔钕模式年龄为1．9—1．5Ga，锆石铪模式年龄集中于1．8—1．5Ga，与出露于保山地块的早古生代和晚中生代花岗岩存在明显差别。原岩主要表现轻稀土富集、轻度的中／重稀土分馏、显著的负铕、铌、钛、钡和锶异常。初始8Nd值（1．0Ga，-1．7—3．5）和锆石8Hf值（1．0Ga，平均值10．5—13．1）及地球化学特征可能暗示腾冲地块基底东侧在中元古代发育与岛弧有关的岩浆作用。     

The isotopic composition of carbonatite and kimberlite carbonates and their bearing on the isotopic composition of deep-seated carbon

New carbon and oxygen isotopic compositions of carbonates from 14 carbonatite and 11 kimberlite occurrences are reported. A review of the available data on the carbon isotopic composition ranges of carbonatite and kimberlite carbonates shows that they are similar and overlap that of diamonds. The mean carbon isotopic composition of carbonates from 22 selected carbonatite complexes (?5.1%., s = ±l.4%.vsPDB) is indistinguishable from that of 13 kimberlite pipes (?4.7%. s = ±1.2%.) as well as that of 60 individual diamond analyses (?5.8%., s = 1.8%.). The oxygen isotopic compositions of kimberlite carbonates, however, are enriched in O¹⁸ by several permil with respect to those of carbonates from the subvolcanic type of carbonatite.The data suggest that not all carbonatite, kimberlite and diamond occurrences have the same average carbon isotopic composition and that significant differences exist between them. Carbon isotopic composition measurements available for the East African Rift system suggest geographic and/or tectonic groupings e.g. carbonate lavas, tuffs and intusive carbonatites associated with the Eastern Rift yield a range of δC¹³ values from ?5.8 to ?7.4%., similar to that of the carbonate rocks associated with the Western Rift volcanism (?5.8 to ?7.9%.). In contrast the interrift area encompassing Lakes Victoria, Malawi (Nyasa) and Chilwa, apparently are characterized by carbonatitic carbonates of higher C¹³ content (?2.4 to ?4.4%.).If carbonatite and kimberlite carbonates as well as diamonds represent deep seated carbon, the mean isotopic composition of this carbon is estimated as ?5.2%. and the range is ?2 to ?8%. The selection of any particular value within this range to be used as a criterion of deep-seated origin is at the moment not warranted. Indeed, the choice of any specific composition for such carbon may be meaningless, as the source of kimberlite, carbonatite and diamond carbon may not be isotopically uniform.     

Carbon isotopic composition of fossil leaves from the Early Cretaceous sediments of western India

Stable carbon isotope analysis of fossil leaves from the Bhuj Formation, western India was carried out to infer the prevailing environmental conditions. Compression fossil leaves such as Pachypteris indica, Otozamite kachchhensis, Brachyphyllum royii and Dictyozamites sp. were recovered from three sedimentary successions of the Bhuj Formation, Early Cretaceous in age. A chronology was established based on faunal assemblage and palyno-stratigraphy and further constrained by carbon isotope stratigraphy. The three sampling sites were the Karawadi river bank near Dharesi; the Chawad river bank near Mathal; and the Pur river section near Trambau village in Gujarat. The Dharesi sample was also analyzed to investigate intra-leaf δ ¹³C variability. The mean δ ¹³C of the leaf was − 24.6 ± 0.4‰ which implied negligible systematic change along the leaf axis. The Mathal sample was fragmented in nature and showed considerable variation in carbon isotopic composition. The Trambau sample considered to be the oldest, dating to the middle of Aptian (ca. 116 Ma), shows the most depleted value in δ ¹³C among all of them. The overall δ ¹³C trend ranging from mid Aptian (ca. 116 Ma) to early Albian (ca. 110 Ma) shows a progressive increase in δ ¹³C from −26.8 to −20.5‰. Based on these measurements the carbon isotopic composition of atmospheric carbon dioxide of the Aptian–Albian period is estimated to be between −7.4 and −1.7‰. The ratio of the partial pressure of carbon dioxide in leaf to that of the ambient atmosphere calculated based on a model is estimated to be similar to that of the modern plants. This indicates that the Early-Cretaceous plants adapted to the prevailing high carbon dioxide regime by increasing their photosynthetic uptake.     

The Hf-Nd isotopic composition of marine sediments

This paper presents new major and trace-element data and Lu-Hf and Sm-Nd isotopic compositions for representative suites of marine sediment samples from 14 drill sites outboard of the world’s major subduction zones. These suites and samples were chosen to represent the global range in lithology, Lu/Hf ratios, and sediment flux in subducting sediments worldwide. The data reported here represent the most comprehensive data set on subducting sediments and define the Hf-Nd isotopic variations that occur in oceanic sediments and constrain the processes that caused them.Using new marine sediment data presented here, in conjunction with published data, we derive a new Terrestrial Array given by the equation, ε_Hf = 1.55 × ε_Nd + 1.21. This array was calculated using >3400 present-day Hf and Nd isotope values. The steeper slope and smaller y-intercept of this array, compared to the original expression (ε_Hf = 1.36 × ε_Nd + 2.89; Vervoort et al., 1999) reflects the use of present day values and the unradiogenic Hf of old continental samples included in the array.In order to examine the Hf-Nd isotopic variations in marine sediments, we have classified our samples into 5 groups based on lithology and major and trace-element geochemical compositions: turbidites, terrigenous clays, and volcaniclastic, hydrothermal and hydrogenetic sediments. Compositions along the Terrestrial Array are largely controlled by terrigenous material derived from the continents and delivered to the ocean basins via turbidites, volcaniclastic sediments, and volcanic inputs from magmatic arcs. Compositions below the Terrestrial Array derive from unradiogenic Hf in zircon-rich turbidites. The anomalous compositions above the Terrestrial Array largely reflect the decoupled behavior of Hf and Nd during continental weathering and delivery to the ocean. Both terrigenous and hydrogenetic clays possess anomalously radiogenic Hf, reflecting terrestrial sedimentary and weathering processes on the one hand and marine inheritance on the other. This probably occurs during complementary processes involving preferential retention of unradiogenic Hf on the continents in the form of zircon and release of radiogenic Hf from the breakdown of easily weathered, high Lu-Hf phases such as apatite.     

Tracing salinization processes in coastal aquifers using an isotopic and geochemical approach: comparative studies in western Morocco and southwest Portugal

Environmental stable and radioactive isotopes (δ²H, δ¹³C, δ¹⁸O; ³H and ¹⁴C), together with physical and geochemical data, were used in the determination of the origins of groundwater salinization and geochemical evolution processes in coastal regions. Two case studies on the Atlantic Coast are discussed, one located in the Essaouira sedimentary basin, western Morocco, and the second, in the Lower Tagus–Sado sedimentary basin, southwest Portugal. In both regions, groundwater degradation occurs by salinization increase to different concentrations and in relation to different origins. The main quality issues for the groundwater resources are related to seawater intrusion, dissolution of diapiric structures intruding the aquifer layers, brine dissolution at depth, and/or evaporation of irrigation water. Anthropogenic pollution ascribed to agricultural activities is another source for groundwater degradation, affecting mainly the shallow aquifers. The apparent ¹⁴C age of the analysed samples ranges from 2.9?±?0.3 up to 45.6?±?0.6 pmC in the Miocene groundwater samples from the basin in Portugal; at the Essaouira basin in Morocco, the ¹⁴C content varies from 60 to 86 pmC. In most of the water samples, the ³H concentration is below the detection limit. In both basins, the isotopic results together with the geochemical data provided an effective label for tracing the mineralization origin and groundwater degradation processes. Further, the isotopic signatures were used in the identification of a paleoclimate (colder period), recorded in the stable isotopic composition and corroborated with the ¹⁴C data.     

元素的地球化学性质与关键金属成矿:前言

正本专辑重点从元素地球化学性质入手,结合地质过程和区域地质及一些具体矿床实例的研究,探讨关键金属的成矿规律。包括22篇文章,内容涵盖钨、锡、铌、钽、钛、锑、铀、铼、硼、钼、铜等多种关键金属的成矿规律。矿床是元素异常富集的地质体,因此,元素在不同地质     

钼的地球化学性质与成矿

钼在硅酸盐地球中的丰度很低,作为中度不相容元素,简单的岩浆形成、演化过程很难使之从地壳丰度直接富集到工业品位。我们研究结果显示钼的成矿主要通过两阶段富集过程来实现:一是与风化、沉积有关的表生过程;二是与板块俯冲或深埋熔融有关的岩浆过程。钼是变价元素,在寒武纪大气氧再次升高以后,钼在地表化学风化过程中容易被氧化为水溶性的Mo O42-,进入地表径流和海洋湖泊中,并且在还原条件下进入富含有机质的黑色页岩等沉积物(岩)中。富钼沉积物(岩)在随板块俯冲到地幔或深埋等条件下可以因升温变质,进而发生部分熔融,形成富钼的原始岩浆,在俯冲条件下往往形成斑岩铜(金)钼矿床,氧逸度低于斑岩铜矿,而斑岩钼矿的氧逸度更低。具有高化学风化速率、大的流域面积的封闭、半封闭型水域和河口地区是表生过程中钼富集的最佳区域,是大型钼矿形成的基础。     

The triple isotopic composition of oxygen in leaf water

The isotopic composition of atmospheric O₂ depends on the rates of oxygen cycling in photosynthesis, respiration, photochemical reactions in the stratosphere and on δ¹⁷O and δ¹⁸O of ocean and leaf water. While most of the factors affecting δ¹⁷O and δ¹⁸O of air O₂ have been studied extensively in recent years, δ¹⁷O of leaf water—the substrate for all terrestrial photosynthesis—remained unknown. In order to understand the isotopic composition of atmospheric O₂ at present and in fossil air in ice cores, we studied leaf water in field experiments in Israel and in a European survey. We measured the difference in δ¹⁷O and δ¹⁸O between stem and leaf water, which is the result of isotope enrichment during transpiration. We calculated the slopes of the lines linking the isotopic compositions of stem and leaf water. The obtained slopes in ln(δ¹⁷O + 1) vs. ln(δ¹⁸O + 1) plots are characterized by very high precision (∼0.001) despite of relatively large differences between duplicates in both δ¹⁷O and δ¹⁸O (0.02-0.05‰). This is so because the errors in δ¹⁸O and δ¹⁷O are mass-dependent. The slope of the leaf transpiration process varied between 0.5111 ± 0.0013 and 0.5204 ± 0.0005, which is considerably smaller than the slope linking liquid water and vapor at equilibrium (0.529). We further found that the slope of the transpiration process decreases with atmospheric relative humidity (h) as 0.522-0.008 × h, for h in the range 0.3-1. This slope is neither influenced by the plant species, nor by the environmental conditions where plants grow nor does it show strong variations along long leaves.     

Calcarenite and sapropel deposition in the Mediterranean Pliocene: shallow- and deep-water record of astronomically driven climatic events

ABSTRACT Calcarenitic bodies punctuate the shallow-water deposits of Plio-Pleistocene Mediterranean basins. Their rhythmic stacking pattern and stratigraphic distribution suggest a close relationship with deep-water sapropel cycles, whose development is controlled by periodic changes in the Earth's orbital parameters. Calcarenitic bodies occur as eccentricity-controlled clusters (over periods of 100–400 kyr) showing a time-correlation with sapropel clusters, starting from 3.1 Ma. Formation of individual calcarenites is possibly driven by obliquity and/or precession cyclicity. This has important implications both for an improved understanding of Mediterranean palaeoceanographic events and correlation of shallow- and deep-water successions. The appearance of sapropel and calcarenitic clusters starting from 3.1 Ma suggests a direct link with the onset of Northern Hemisphere glaciation, which could be responsible for the amplification of oceanographic events within the Mediterranean.     

土壤中Hg离子地球化学行为模拟实验 ——以山西的典型土壤为例

土壤样品分别采于山西省大同、太原和临汾3个地区。实验分成吸附解吸实验和运移实验2个部分。其中吸附实验分成砂粒土壤和粘粒土壤2个粒级分别进行。结果表明：3种土壤对Hg的吸附量随着加入Hg离子溶液浓度的增加而增加。用4种吸附模式来拟合3种土样对Hg的吸附可以得出,Freundlich模式和Langmuir模式能较好地拟合3种土壤对Hg吸附的实验数据。3种土壤对Hg都有一定的吸持固定作用。解吸率由大到小依次为大同土壤〉临汾土壤〉太原土壤。当Hg^2＋的浓度为300mg/L时,Hg在大同土壤中的穿透速率最快,在太原土壤中穿透速率最慢。从而得出Hg^2＋在壤质砂土中运移速度最快、在粉壤土中运移速度最慢的结论。     

Late Paleozoic gabbroids of western Transbaikalia: U-Pb and Ar-Ar isotopic ages,composition, and petrogenesis

We provide new isotope-geochronological evidence for the synchronous occurrence of Late Paleozoic basic and granitoid magmatism in western Transbaikalia; this is a strong argument for the contribution of mantle magmas to granitoid petrogenesis. The Late Paleozoic basic rocks originated from the phlogopite-garnet-bearing lherzolitic mantle, which melted under “hydration conditions.” The specific features of Late Paleozoic magmatism in western Transbaikalia were determined by the combination of the activity of a low-energy mantle plume with the final stage of the Hercynian orogeny in space and time. At the early stage of magmatism, during the formation of the Barguzin granites,the plume had only a thermal influence on the crustal rocks heated as a result of Hercynian fold-thrust deformations. The mixing of mantle basic and crustal salic magmas at different levels marked the transition from crustal to mixed (mantle-crustal) granites, which include all post-Barguzin complexes (probably, except for alkali granites). In the geologic evolution of Transbaikalia, the Late Paleozoic magmatism was postorogenic, but it was initiated and influenced by the mantle plume.     

The Hemlo gold deposit,Ontario: A geochemical and isotopic study

The Hemlo deposit, near Marathon, Ontario, is one of the largest gold deposits in North America. It is stratiform within Archean metamorphosed volcano-sedimentary rocks. The main ore zone is composed of pyritic, sericitic schist, and massive barite. This is the first report of stratiform barite in the Archean of North America, but other occurrences have since been found west of Hemlo. The mineralization is substantially enriched in Au, Mo, Sb, Hg, Tl and V and lacks carbonate. Because of metamorphism and deformation of the body its genesis is uncertain.${}^{87}\text{Sr}{}^{86}\text{Sr}$ of .7017 for barite from the deposit is similar to that of the sedimentary barite west of Hemlo and to initial ratios of contemporaneous volcanic rocks. At the base of the main ore zone, barite with δ^34S of +8 to +12%. was deposited with ~0%. pyrite. Upward, both barite and pyrite get isotopically lighter, with minimum values for pyrite, to ?17.5%, in non-baritic schist forming the upper part of the ore zone. In drill section, Au grades correlate with the isotopic composition of pyrite. This, and the association of fractionated sulphide with sulphate, suggests that Au, pyrite and barite were deposited contemporaneously. The linked, asymmetric distributions of S minerals and isotopic distributions, which are continuous from section to section, and the isotopic similarity of the Hemlo and western barites are consistent with a syngenetic depositional model.Two sources for the S minerals are considered. In the first, exogenous sulphate from a restricted basin were partially reduced in a geothermal system to form ³⁴S-depleted sulphide. In the second, the sulphate and sulphide are of magmatic-hydrothermal origin. Sulphate and fractionated sulphide are uncommon in Archean rocks, but one or both occur with unusual frequency in major Archean gold deposits. Hydrothermal fluids of moderately high , containing sulphate and permitting isotopic fractionation between oxidized and reduced S species, may have favoured the dissolution, transport and precipitation of Au.     

The isotopic composition of nitrate produced from nitrification in a hardwood forest floor

Dual isotopic analysis of nitrate (¹⁵N/¹⁴N and ¹⁸O/¹⁶O) is increasingly used to investigate the environmental impacts of human-induced elevated atmospheric nitrate deposition. In forested ecosystems, the nitrate found in surface water and groundwater can originate from two sources: (1) atmospheric deposition, and (2) nitrate produced from nitrification in forest soils (microbial nitrate). Application of the dual nitrate isotope technique for determining the relative importance of nitrate sources in forested catchments requires knowledge of the isotopic composition of microbial nitrate. We excluded precipitation inputs to three zero-tension lysimeters installed below the F-horizon (Oe) at the Turkey Lakes Watershed (TLW) in order to measure the isotopic composition of microbial nitrate produced in situ. To our knowledge, this is the first in situ study of the isotopic composition of microbial nitrate in forest soils. Over a 2-week period, nitrate produced by nitrification was periodically flushed to the lysimeters by watering the area with a nitrogen-free solution. Nitrate produced in the forest floor had δ¹⁸O values ranging from +3.1‰ to +10.1‰ with a mean of +5.2‰. These values were only slightly higher than from the expected value of +1.0‰ calculated for chemolithoautotrophic nitrification, which depends on the δ¹⁸O of available O₂ and H₂O. In addition to nitrate, we also collected soil gas to determine if soil respiration and O₂ diffusion affected soil gas δ¹⁸O-O₂, which is typically assumed to be identical to atmospheric O₂ (+23.5‰) when calculating microbial nitrate δ¹⁸O values. No significant difference in δ¹⁸O-O₂ from atmospheric O₂ was found in forest soils to a depth of 55 cm, and therefore ¹⁸O-enrichment of soil gas O₂ could not explain the modest enrichment of nitrate ¹⁸O. Evaporative ¹⁸O-enrichment of soil water available to nitrifiers in the forest floor is a plausible mechanism for slightly elevated nitrate δ¹⁸O values. However, the observed nitrate δ¹⁸O values could also be explained by a minor contribution of nitrate from heterotrophic nitrifiers. The δ¹⁵N of nitrate produced ranged from −10.4 to −7.3‰ and, as expected, was depleted in ¹⁵N relative to soil organic nitrogen. Microbial nitrate produced in the forest floor was also significantly depleted in ¹⁵N relative to microbial nitrate exported in groundwater and headwater streams at the TLW. We hypothesize that ¹⁵N-depleted forest floor nitrate is not detected in groundwaters largely because of: (1) the immobilization of forest floor nitrate in the mineral soil and (2) the mixing of the remaining forest floor nitrate with nitrate generated in the mineral soil, which is expected to have higher δ¹⁵N values. This study demonstrates that current methods of calculating a priori the δ¹⁸O of microbial nitrate provide a reasonable value for nitrate produced by nitrification at the TLW.     

The vanadium isotopic composition of L ordinary chondrites

Stable isotopic data of meteorites are critical for understanding the evolution of terrestrial planets. In this study, we report high-precision vanadium (V) isotopic compositions of 11 unequilibrated and equilibrated L chondrites. Our samples show an average δ⁵¹V of ??1.25‰?±?0.38‰ (2SD, n?=?11), which is ~?0.5‰ lighter than that of the bulk silicate Earth constrained by mantle peridotites. Isotopic fractionation in type 3 ordinary chondrites vary from ??1.76‰ to ??1.29‰, whereas the δ⁵¹V of equilibrated chondrites vary from ??1.37‰ to ??1.08‰. δ⁵¹V of L chondrites do not correlate with thermal metamorphism, shock stage, or weathering degree. Future studies are required to explore the reason for V isotope variation in the solar system.