Fe-Cu-Zn-Mo同位素示踪氧化还原过程

非传统稳定同位素（Fe-Cu-Zn-Mo）理论与数据相结合提高了科研工作者对地质体系氧化还原过程的理解。本文对这一相对较新的领域进行了综述,包括与氧化还原过程相关的同位素分馏理论和实验约束、时空尺度下的氧逸度以及同位素示踪氧化还原过程。稳定同位素理论预测,Fe-Cu-Zn-Mo同位素应该对氧化还原状态的变化能够做出响应。结果表明,Fe同位素作为岩浆过程、表生过程、俯冲带流体性质"氧逸度计"应用前景广阔;Cu同位素在岩浆、热液、陆地系统可以很好地示踪氧化还原过程;Zn同位素由于络合过程分馏已经被用在许多不同环境中作为含硫/碳流体迁移的敏感示踪剂;Mo同位素作为古氧逸度计可有效重建古海洋-大气氧化还原状态。     

High-precision osmium isotopes in enstatite and Rumuruti chondrites

Isotopic heterogeneity within the solar nebula has been a long-standing issue. Studies on primitive chondrites and chondrite components for Ba, Sm, Nd, Mo, Ru, Hf, Ti, and Os yielded conflicting results, with some studies suggesting large-scale heterogeneity. Low-grade enstatite and Rumuruti chondrites represent the most extreme ends of the chondrite meteorites in terms of oxidation state, and might thus also present extremes if there is significant isotopic heterogeneity across the region of chondrite formation. Osmium is an ideal tracer because of its multiple isotopes generated by a combination of p-, r-, and s-process and, as a refractory element; it records the earliest stages of condensation.Some grade 3-4 enstatite and Rumuruti chondrites show similar deficits of s-process components as revealed by high-precision Os isotope studies in some low-grade carbonaceous and ordinary chondrites. Enstatite chondrites of grades 5-6 have Os isotopic composition identical within error to terrestrial and solar composition. This supports the view of digestion-resistant presolar grains, most likely SiC, as the major carrier of these anomalies. Destruction of presolar grains during parent body processing, which all high-grade enstatite chondrites, but also some low-grade chondrites seemingly underwent, makes the isotopically anomalous Os accessible for analysis. The magnitude of the anomalies is consistent with the presence of a few ppm of presolar SiC with a highly unusual isotopic composition, produced in a different stellar environment like asymptotic giant branch stars (AGB) and injected into the solar nebula. The presence of similar Os isotopic anomalies throughout all major chondrite groups implies that carriers of Os isotopic anomalies were homogeneously distributed in the solar nebula, at least across the formation region of chondrites.     

Himalayan uplift and osmium isotopes in oceans and rivers

Previous studies have shown that ¹⁸⁷Os/¹⁸⁸Os in seawater has become increasingly radiogenic over the last 40 Ma in a manner analogous to strontium. This rapid rise in the marine ¹⁸⁷Os/¹⁸⁸Os over the last 17 Ma has been attributed to an increase in the bulk silicate weathering rates resulting from the rise of the Himalayas and/or selective weathering and erosion of highly radiogenic organic rich ancient sediments. The key test of this hypothesis is the ¹⁸⁷Os/¹⁸⁸Os and the total osmium concentration of the Himalayan rivers. We report the concentration and isotopic composition of osmium in the Ganges, the Brahmaputra, and the Indus rivers. The ¹⁸⁷Os/¹⁸⁸Os of the Ganges close to its source (at Kaudiyal, 30°05′N, 78°50′E) is 2.65 and [Os] = 45 fM/kg. A second sample of the lower reaches of the Ganges at Patna (25°30′N, 85°10′E) gives ¹⁸⁷Os/¹⁸⁸Os =1.59 and [Os] = 171 fM/kg. The ¹⁸⁷Os/¹⁸⁸Os of the Brahmaputra at Guwahati (26°10′N, 91°58′E) is 1.07 and [Os] = 52 fM/kg. A sample of the Indus (Besham, 34°55′N, 72°51′E) has a ¹⁸⁷Os/¹⁸⁸Os of 1.2 and [Os] = 59 fM/kg. We infer that the Himalayas do not provide either a high flow of osmium or a highly radiogenic osmium component to the oceans. The overall trend for osmium and strontium could be explained by a regularly increasing input of global continental weathering sources but the Himalayas themselves appear not to be the dominant source.     

Lead isotopes as tracers of anthropogenic pollution in urban topsoils of Mexico City

A survey was performed to trace the main source of anthropogenic Pb pollution in Mexico City through Pb isotopic signatures (²⁰⁸Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁷Pb, and ²⁰⁸Pb/²⁰⁷Pb) from 103 urban topsoil (0–5 cm) samples. Those were collected in the metropolitan area of Mexico City and compared with isotopic compositions of leaded gasoline (LG), domestic Pb ores (DLO) and parent rock (PR). The isotope ratios (IRs) of Pb were determined by inductively coupled plasma quadrupole mass spectrometry (ICP-QMS) and total Pb concentration analyzed by wavelength dispersive X-ray fluorescence (WDXRF). The range of Pb concentrations levels in urban topsoil samples was 15–473 mg/kg. The IR values obtained for these samples were 37.965–39.718 (²⁰⁸Pb/²⁰⁴Pb), 18.375–19.204 (²⁰⁶Pb/²⁰⁴Pb), 1.177–1.218 (²⁰⁶Pb/²⁰⁷Pb) and 2.443–2.496 (²⁰⁸Pb/²⁰⁷Pb). Analyzed topsoil samples with low Pb content (<50 mg/kg) displayed high dispersion in ²⁰⁸Pb/²⁰⁴Pb values, which are determined by different natural sources. Samples with 51–200 mg/kg Pb content, shown low dispersion that revealed the mixing between the natural Pb and anthropogenic Pb. The assessment of the IR values shown that, as Pb concentration increases, a trend toward gasoline IR data has been observed. The results obtained by this research suggest that although the use of leaded petrol had been banned in Mexico since 1997, the Pb pollution in the urban topsoils due to the historical use of Pb in petrol is still significant.     

Oxygen and nitrogen isotopes as tracers of fluid activities in serpentinites and metasediments during subduction

Summary N and O isotope systematics of a suite of high-pressure (HP) and ultrahigh-pressure (UHP) metasediments of the Schistes Lustrés nappe and metaperidotites of the Erro Tobbio Massif from the Alpine-Appennine system are compared with their unmetamorphosed or hydrothermally-altered equivalent from the same localities and from the South West Indian Ridge (SWIR). The HP and UHP rocks studied represent a sequence of pelagic sediments and altered ultramafic rocks subducted to different depths of down to 90 km along a cold geothermal gradient (8 °C/km). Unmetamorphosed and HP metasediments show the same range in δ¹⁵N values irrespective of their metamorphic grade and bulk nitrogen concentrations. Together with several other geochemical features (K, Rb and Cs contents, δD), this indicates that δ¹⁵N values were unaffected by metamorphism and N was not released during subduction. N isotope analysis of serpentinites coupled with δ¹⁸O systematics suggests the involvement of a mafic (crustal) component during partial deserpentinization of the subducted oceanic mantle at the depth locus of island arc magmatism. This does not imply large-scale fluxes as the metagabbros are spatially associated with the analyzed serpentinites. It rather indicates preservation of presubduction chemical and isotopic heterogeneities on a local scale as documented for the metasediments.     

Strontium isotopes as tracers of airborne fly ash from coal-fired power plants

Fly ash generated by coal-fired power plants is in part collected by filters in the emission stacks while a small portion is vented into the atmosphere. Since many of the coalfired power plants in the western United States are located in the desnrt, the ability to monitor fly ash emissions requires a chemical tracer that utilizes desert soil and plant interactions with the fly ash deposited in the desert environment. This investigation presents the results of a controlled greenhouse experiment in which a native desert plant, the brittlebush (Encelia farinosa), was grown on admixtures of desert soils and fly ash. The fly ash is strongly enriched in Sr and the brittlebush is a Sr accumulator. The data demonstrate that (1) the brittlebush isotopically equilibrates with desert soils whose fly ash components are as low as 0.25% by weight, (2) the fly ash Sr is apparently more available to the plant, than Sr derived from the soils, and (3) the difference between the⁸⁷Sr/⁸⁶Sr ratio of the fly ash (0.70807) and soils (0.71097 to 0.71117) warrants further investigations in the natural environment to determine the practicality of this method as a natural tracer of fly ash in the environment.     

Rare earth elements in seep carbonates as tracers of variable redox conditions at ancient hydrocarbon seeps

Marine hydrocarbon seeps are dynamic systems, which are typically reflected in complex lithologies of limestones forming at these sites. In order to test if seep limestones with their multiple generations of carbonate phases reflect variable redox conditions, the rare earth element (REE) patterns of limestones from five ancient hydrocarbon-seep localities, Hollard Mound (Middle Devonian), Iberg (Mississippian), Beauvoisin (Oxfordian), Canyon and Satsop River (Oligocene), and Marmorito (Miocene) have been investigated. The total REE contents vary widely (0.3–43.7 p.p.m.). Early microcrystalline phases tend to show higher contents than spatially associated later phases (microspar, sparite, and blocky cement). The shale-normalized REE patterns of the carbonates show both negative and positive Ce anomalies even for individual deposits, suggesting that the redox conditions varied significantly. It is apparent that in addition to anoxic conditions, oxic conditions were common in some ancient seep environments. Intermittent oxygenation is presumed to have resulted from abrupt changes in fluid flow.     

Tungsten isotopes as tracers of core-mantle interactions: The influence of subducted sediments

The postulated difference in W isotopic composition of the Earth’s core of ∼2 ε_W units, compared to the bulk silicate earth (BSE) has previously been used to search for evidence of core-mantle interaction (CMI) in ocean island basalts (OIB). The absence of W isotope anomalies has thus been taken as evidence that CMI does not occur. However, the addition of subducted sediment with high W to the sources of OIB could obscure a core signature. This possibility brings into question the utility of W isotopes as tracers for CMI. To accurately consider the effects of sediment addition to mantle sources of OIB with respect to W requires improved constraints on the abundances of W in subducting sediment. Here, we present high-precision W abundance data (and other HFSE) for a suite of sediments from the Banda subduction regime in East Indonesia. Subducting East Indonesian sediments have trace element concentrations that resemble those of average upper continental crust (UCC), making these sediments valuable to consider as typical of subducted sediments. Average W abundances of 2.1 ppm, corrected for carbon content coupled with current models of 0.5% core addition and 1% sediment addition to EM1 or HIMU plume, suggest that a model hybrid source should exhibit values of ε_W = −0.24 with ∼25 ppb W. Prior studies have not reported such low W isotopic compositions or high estimated W concentrations present in the sources of either Hawaiian or French Polynesian lavas, so such large additions of core material to these plume sources seems unlikely. Given these constraints, core contributions to these source, if present, can be no more than ∼0.1%.     

B and Li isotopes as intrinsic tracers for injection tests in aquifer storage and recovery systems

Boron and Li isotopes have been tested as environmental tracers of treated sewage injected into a sandy aquifer (Shafdan reclamation project, Israel). During a 38 days injection test in a newly dug injection well, a conservative artificial tracer (Br⁻) was monitored together with δ¹¹B and δ⁷Li in the injectate, in the unsaturated soil zone (porous cup) and an observation well in the aquifer. In spite of B and Li concentrations in the injectate close to background values, significant shifts of the isotope signatures could be observed over the duration of the injection test. Boron isotope ratios show a breakthrough curve delayed with respect to Br⁻ breakthrough due to some reversible sorption on the aquifer material. No isotope fractionation was observed in the unsaturated or the saturated zone so that B isotopes can be considered as conservative in the investigated part of the aquifer system. Lithium isotopes are strongly fractionated, probably due to sorption processes. Lithium concentrations point to a Li sink in the system, δ⁷Li values vary strongly with a tendency of ⁷Li depletion in the liquid phase over the duration of the experiment. This is opposite to the expected preferential sorption of ⁶Li onto clay minerals. Boron isotopes reveals a valuable tracer of artificial recharge of freshwaters derived from treated sewage, both for short term tracer tests and for long-term monitoring of artificial recharge, even if in aquifers with higher clay contents, sorption-linked isotope fractionation cannot be excluded. More data are needed on Li isotope fractionation in natural groundwater systems to assess the potential of this tracer as monitoring tool.     

Modelling carbon isotopes of carbonates in cave drip water

C isotopes in cave drip water are affected by both the C isotope composition of soil air and host rock carbonate. Furthermore, the C isotope composition of cave drip water strongly depends on the calcite dissolution system, i.e., open, closed and intermediate conditions. Here, we present a calcite dissolution model, which calculates the ¹⁴C activity and δ¹³C value of the dissolved inorganic carbon of the drip water. The model is based on the chemical equations describing calcite dissolution (). The most important improvement, relative to previous models, is the combination of the open and closed system conditions in order to simulate the C isotope composition during intermediate states of calcite dissolution and the application to carbon isotope measurements on cave drip waters from Grotta di Ernesto, Italy. The major changes in the C isotope composition of the drip water occur in response to variations in the open-closed system ratio. Additionally, the ¹⁴C activity and the δ¹³C value of the drip water depend on changes in the partial pressure of soil CO₂. Radiocarbon and δ¹³C values of the Grotta di Ernesto drip water are well reproduced by the model.     

Evaluation of cold-water carbonates as a possible paleoclimatic indicator

The common belief currently shared by many geoscientists concerning the climatic interpretation of limestones is that a warm-water environment is essential. This concept is not necessarily true because the rate and extent of terrigenous sediment dilution, rather than water temperature, is the primary factor determining whether or not a limestone forms at nearshore or continental shelf depths. Because carbonate productivity is lowest in cold climates, however, CaCO₃ abundance and the thickness of carbonate accumulations tend to be least at high latitudes. In this regard present-day continental shelves and beaches offer a poor model for comparing cold-water and warm-water carbonates because of the generally emergent continental tectonic framework, recent eustatic sea-level changes, and the presence of ice caps at the modern poles.Typically, the influence of climate on non-reef continental shelf and beach environments cannot be clearly distinguished by the presence or absence of major taxonomic groups. Faunal diversity and equitability are more sensitive in this regard. The absence of shelf-depth inorganic carbonate precipitates, micrite envelopes, and peloids may also point to the cold-water origin of a rock. Skeletal mineralogy and oxygen isotopes of certain unrecrystallized carbonates may be good paleoclimatic indicators; however, trace elements and physical-textural attributes of the carbonate fraction are probably temperature insensitive.Previous studies of high-latitude continental shelves have concentrated merely on the abundance of calcareous material and there is seemingly a disproportionate amount of information with respect to low-latitude carbonate studies. Further research on cold-water carbonates may open up new avenues for alternative paleoenvironmental and paleoclimatic interpretations.     

Stable lead isotopes as tracers of groundwater pollution in the water supply for a small village

High lead (Pb) concentration has been measured in the incoming water to the water supply for a small Swedish village since the 1990s. There are several sources of the contamination and the objective of this study was to identify these by analysis of Pb isotopes. Lead has four stable isotopes in nature (²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb) and the relative proportions of these vary according to their geological source. The study showed that two anthropogenic sources of Pb, a glassworks deposit and a highway, had similar Pb isotope ratios and thus it was not possible to separate them. However, the very high Pb concentration in the glassworks deposit suggested that this is the main source of the very high concentrations observed occasionally in low flow conditions. The soil in the recharge area of the most important well for the water supply had elevated Pb concentrations compared with background values in soils. Moreover, the Pb ratios in this soil differed from those in the anthropogenic sources. Several sites of mineralisation or natural enrichment have been identified in outcrops about 14?km northwest of the site and several anomalies in Pb exist in the glacial till. The conclusion was that Pb originating from the soil in the recharge area generally dominates and leads to Pb concentrations in water of 1?C2???g?L^?1. However, at higher concentrations, e.g. around 10???g?L^?1, water transported in cracks and fissure from the glassworks deposit becomes more important.     

Copper stable isotopes as tracers of metal-sulphide segregation and fractional crystallisation processes on iron meteorite parent bodies

We report high precision Cu isotope data coupled with Cu concentration measurements for metal, troilite and silicate fractions separated from magmatic and non-magmatic iron meteorites, analysed for Fe isotopes (δ⁵⁷Fe; permil deviation in ⁵⁷Fe/⁵⁴Fe relative to the pure iron standard IRMM-014) in an earlier study (Williams et al., 2006). The Cu isotope compositions (δ⁶⁵Cu; permil deviation in ⁶⁵Cu/⁶³Cu relative to the pure copper standard NIST 976) of both metals (δ⁶⁵Cu_M) and sulphides (δ⁶⁵Cu_FeS) span much wider ranges (−9.30 to 0.99‰ and −8.90 to 0.63‰, respectively) than reported previously. Metal-troilite fractionation factors (Δ⁶⁵Cu_M-FeS = δ⁶⁵Cu_M − δ⁶⁵Cu_FeS) are variable, ranging from −0.07 to 5.28‰, and cannot be explained by equilibrium stable isotope fractionation coupled with either mixing or reservoir effects, i.e. differences in the relative proportions of metal and sulphide in the meteorites. Strong negative correlations exist between troilite Cu and Fe (δ⁵⁷Fe_FeS) isotope compositions and between metal-troilite Cu and Fe (Δ⁵⁷Fe_M-FeS) isotope fractionation factors, for both magmatic and non-magmatic irons, which suggests that similar processes control isotopic variations in both systems. Clear linear arrays between δ⁶⁵Cu_FeS and δ⁵⁷Fe_FeS and calculated Cu metal-sulphide partition coefficients (D_Cu = [Cu]_metal/[Cu]_FeS) are also present. A strong negative correlation exists between Δ⁵⁷Fe_M-FeS and D_Cu; a more diffuse positive array is defined by Δ⁶⁵Cu_M-FeS and D_Cu. The value of D_Cu can be used to approximate the degree of Cu concentration equilibrium as experimental studies constrain the range of D_Cu between Fe metal and FeS at equilibrium to be in the range of 0.05-0.2; D_Cu values for the magmatic and non-magmatic irons studied here range from 0.34 to 1.11 and from 0.04 to 0.87, respectively. The irons with low D_Cu values (closer to Cu concentration equilibrium) display the largest Δ⁵⁷Fe_M-FeS and the lowest Δ⁶⁵Cu_M-FeS values, whereas the converse is observed in the irons with large values D_Cu that deviate most from Cu concentration equilibrium. The magnitudes of Cu and Fe isotope fractionation between metal and FeS in the most equilibrated samples are similar: 0.25 and 0.32‰/amu, respectively. As proposed in an earlier study (Williams et al., 2006) the range in Δ⁵⁷Fe_M-FeS values can be explained by incomplete Fe isotope equilibrium between metal and sulphide during cooling, where the most rapidly-cooled samples are furthest from isotopic equilibrium and display the smallest Δ⁵⁷Fe_M-FeS and largest D_Cu values. The range in Δ⁶⁵Cu_M-FeS, however, reflects the combined effects of partial isotopic equilibrium overprinting an initial kinetic signature produced by the diffusion of Cu from metal into exsolving sulphides and the faster diffusion of the lighter isotope. In this scenario, newly-exsolved sulphides initially have low Cu contents (i.e. high D_Cu) and extremely light δ⁶⁵Cu_FeS values; with progressive equilibrium and fractional crystallisation the Cu contents of the sulphides increase as their isotopic composition becomes less extreme and closer to the metal value. The correlation between Δ⁶⁵Cu_M-FeS and Δ⁵⁷Fe_M-FeS is therefore a product of the superimposed effects of kinetic fractionation of Cu and incomplete equilibrium between metal and sulphide for both isotope systems during cooling. The correlations between Δ⁶⁵Cu_M-FeS and Δ⁵⁷Fe_M-FeS are defined by both magmatic and non-magmatic irons record fractional crystallisation and cooling of metallic melts on their respective parent bodies as sulphur and chalcophile elements become excluded from crystallised solid iron and concentrated in the residual melt. Fractional crystallisation processes at shallow levels have been implicated in the two main classes of models for the origin of the non-magmatic iron meteorites; at (i) shallow levels in impact melt models and (ii) at much deeper levels in models where the non-magmatic irons represent metallic melts that crystallised within the interior of a disrupted and re-aggregated parent body. The presence of non-magmatic irons with a range of Fe and Cu isotope compositions, some of which record near-complete isotopic equilibrium implies crystallisation at a range of cooling rates and depths, which is most consistent with cooling within the interior of a meteorite parent body. Our data therefore lend support to models where the non-magmatic irons are metallic melts that crystallised in the interior of re-aggregated, partially differentiated parent bodies.     

Black shales and carbon isotopes in pelagic sediments from the Tethyan Lower Jurassic

Detailed sampling and analysis of Jurassic pelagic limestones and marls from Italy, Hungary and Switzerland have enabled construction of an isotope stratigraphy across the Pliensbachian-Toarcian boundary with resolution to the zonal level. The oxygen-isotope record is unremarkable. The carbon isotopes, however, show two positive excursions: one, relatively minor, during the Pliensbachian, margaritatus Zone, subnodosus Subzone, the other, more major, during the Toarcian. early falciferum Zone, where a maximum δ¹³C value of 4·52%_PDB is attained. These intervals are known to be favoured periods of organic-rich sedimentation in diverse parts of the globe and the isotopic excursions are interpreted as a response to abnormally high rates of storage of organic carbon in the sedimentary record. A comparable phenomenon has been documented from the Cenomanian-Turonian boundary in the Cretaceous where it has been referred to the influence of an ‘Oceanic Anoxic Event’. Some Italian sections spanning this Lower Jurassic interval contain organic-rich shales in the falciferum Zone; the isotopic signatures from their included, locally manganiferous carbonate betray a considerable diagenetic overprint and they cannot therefore be incorporated in a composite isotopic curve. Carbon isotopes from the organic carbon itself are extremely negative, falling to –33δ_PDB and, in one section examined in detail, correlate with the calcium-carbonate content of the shales; they may reflect a partial change to a non-calcified planktonic biota during deposition of this lime-poor interval, possibly responding to upwelling and increased fertility of near-surface waters. The onset of upwelling may have been as early as spinatum-tenuicostatum Zone time, that is, at the Pliensbachian-Toarcian boundary.     

徐州地区下古生界碳酸盐岩的碳、氧同位素研究

在已有的对徐州地区下古生界碳酸盐岩详细岩石学研究基础上，系统测试了碳酸盐岩方解石和白云石的碳、氧同位素组成特征；地球化学和岩石学研究相结合，综合提取了碳酸盐岩碳、氧同位素组成的沉积、成岩地质信息；初步探讨了同位素组成的地层变化及其意义。研究结果表明：碳酸盐岩碳同位素组成特征可以指示沉积环境及其演化，开阔陆表海、灰质障壁砂坝、局限陆表海、潮下带、潮渠、藻坪诸沉积相间碳酸盐岩的δ^13C值表现出规律性变化，碳同位素组成变化曲线上的显著正偏移与海侵有关；碳酸盐岩氧同位素组成对成岩环境及其演化有明显响应，淡水成岩环境、潮上成岩环境、潮间成岩环境、潮下成岩环境和埋藏成岩环境具有不同的碳酸盐岩氧同位素组成特征；徐州地区大北望寒武系剖面地层界线附近都有碳、氧同位素组成变化的特征显示，研究工作证实了徐州地区下古生界同位素地层划分的可行性。     

A new method for analysis of osmium isotopes and concentrations in surface and subsurface water samples

We describe a new method for the determination of osmium (Os) content and ¹⁸⁷Os/¹⁸⁸Os isotopic ratio in water samples, particularly adapted to analysis of reducing waters such as groundwaters. The critical feature of the method is the improved oxidation step. We use a high pressure asher (HPA-S) to achieve oxidation of all Os species at high pressure (~ 130 bars) and high temperature (250 °C). A series of tests was performed on two groundwater samples in order to study the general behavior of Os during oxidation and to optimize the various parameters of the method. At high temperature (≥ 250 °C), using hydrogen peroxide as an oxidant, complete spike-sample equilibration and oxidation of Os species to the highest oxidation state (OsO₄) are obtained in less than 10 h. A strong contrast in oxidation behavior was observed between tests performed at high (250 °C) and low temperature (100 °C). The results for both samples indicate a difference in the oxidation kinetics between spike and sample. They also demonstrate that a non-negligible proportion of each sample is easily oxidized at low temperature whereas high temperatures are required to oxidize the rest of the sample. This last observation could provide evidence for the existence of several species of osmium in groundwater samples. Our results underscore the need for high temperatures to assure spike-sample equilibration and complete oxidation of all Os species potentially present in water. The method presented here, which is adapted to various matrices, opens the possibility of analyzing Os concentrations and isotopic compositions in groundwater and thus to better constrain the potential sources of osmium delivered to the ocean.     

Geochemistry of the late Cenomanian-early Turonian chalks of the Paris Basin: Manganese and carbon isotopes in carbonates as paleooceanographic indicators

In the North Atlantic DSDP/IPOD cores, carbon isotope data on the bulk carbonates show significant fluctuations. In sediments now exposed on land coeval fluctuations in the carbon isotope concentrations are also recorded in pelagic and epeiric facies. For instance, in the Upper Cretaceous chalks of the Paris Basin, there is a major break at the Cenomanian-Turonian boundary. At this time, the manganese content of the chalks was also at a maximum and consequently a positive relation can be demonstrated between δ¹³C and manganese concentrations. The same positive correlation is also recorded in many pelagic limestones.In the North Atlantic cores, carbon isotope events are related to the black shale facies and to global oceanic anoxic events and one can suppose that in sediments deposited on the continental margins they are also related to mildly anoxic conditions. Considering the manganese geochemistry in carbonate rocks, a high manganese content in such a reducing environment can be found in the sediments only if the Mn concentration of the interstitial solutions are abnormally high. As a high Mn content in marine pore waters is believed to originate from hydrothermal process, Mn and δ¹³C positive excursions are ultimately related to mid-oceanic ridge activity and to a closely connected phenomenon, the great transgressive pulses during which mid-depth waters may have been anoxic. Consequently, major Mn and carbon isotope events would seem to be useful tools in paleooceanographic reconstructions.     

Dissolved noble gases and stable isotopes as tracers of preferential fluid flow along faults in the Lower Rhine Embayment,Germany

Groundwater in shallow unconsolidated sedimentary aquifers close to the Bornheim fault in the Lower Rhine Embayment (LRE), Germany, has relatively low δ²H and δ¹⁸O values in comparison to regional modern groundwater recharge, and ⁴He concentrations up to 1.7?×?10^?4 cm³ (STP) g^–1?±?2.2 % which is approximately four orders of magnitude higher than expected due to solubility equilibrium with the atmosphere. Groundwater age dating based on estimated in situ production and terrigenic flux of helium provides a groundwater residence time of ～10⁷ years. Although fluid exchange between the deep basal aquifer system and the upper aquifer layers is generally impeded by confining clay layers and lignite, this study’s geochemical data suggest, for the first time, that deep circulating fluids penetrate shallow aquifers in the locality of fault zones, implying  that sub-vertical fluid flow occurs along faults in the LRE. However, large hydraulic-head gradients observed across many faults suggest that they act as barriers to lateral groundwater flow. Therefore, the geochemical data reported here also substantiate a conduit-barrier model of fault-zone hydrogeology in unconsolidated sedimentary deposits, as well as corroborating the concept that faults in unconsolidated aquifer systems can act as loci for hydraulic connectivity between deep and shallow aquifers. The implications of fluid flow along faults in sedimentary basins worldwide are far reaching and of particular concern for carbon capture and storage (CCS) programmes, impacts of deep shale gas recovery for shallow groundwater aquifers, and nuclear waste storage sites where fault zones could act as potential leakage pathways for hazardous fluids.     

Sm-Nd in marine carbonates and phosphates: Implications for Nd isotopes in seawater and crustal ages

This study explores the possibility of establishing Nd isotopic variations in seawater over geologic time. Calcite, aragonite and apatite are examined as possible phases recording seawater values of ?_Nd. Modern, biogenic and inorganically precipitated calcite and aragonite from marine environments were found to have Nd concentrations of from 0.2 to 70 ppb, showing that primary marine CaCO₃ contains little REE and that Nd/Ca is not greatly enhanced relative to seawater during carbonate precipitation. Very young marine limestone and dolomite containing no continental detritus have ~200 ppb Nd. All the carbonates are LREE enriched ($\text{?0.16 ≤f}{}^{\mathrm{<mtext>Sm</mtext><mtext>Nd</mtext>}}\text{≤?0.45}$). Modern and very young Atlantic and Pacific carbonates have ?_Nd in the range of shallow Atlantic and Pacific seawater respectively, implying that they derive their REE from local seawater. The Nd in well preserved carbonate fossils is ≤4 × 10⁴ ppb, much greater than in their modern counterparts but like the high values found for carbonates in other studies. We believe the high REE contents (at the 500 ppb level) in some detritusfree carbonates are due to REE-rich Fe-hydroxide in/on the carbonate. In favorable cases, such material may record seawater ?_Nd values, however introduction of extraneous REE may obscure the original isotopic composition of pure CaCO₃ because of its very low intrinsic primary REE abundance.Modern biogenic apatite is also shown to have very low REE content (<150 ppb Nd) but appears to quickly scavenge REE from seawater. Inorganically precipitated apatite from phosphorites has high concentrations of seawater-derived REE. Young phosphorite apatite from the Atlantic and Pacific oceans has ?_Nd in the range of the seawater from these oceans. Older apatite samples of similar age from different localities bordering common oceans record similar values of ?_Nd(T). Sedimentary apatite has ?_Sr(T) values in good agreement with the curves for ${}^{87}\text{Sr}{}^{86}\text{Sr}$ of seawater as a function of time. Individual conodonts from a single formation yield the same ?_Sr(T) and ?_Nd(T). Other workers have shown that sedimentary apatite preserves seawater REE patterns. These characteristics suggest that sedimentary apatite can be used to determine ?_Nd(T) in ancient seawater. The seawater values so inferred range between ?1.7 and ?8.9 over the last 700 my and lie in the range of modern seawater, showing no evidence for drastic changes. High values of seawater ?_Nd(T) in the Triassic and latest Precambrian may correlate with the breakup of large continental landmasses. The initial ?_Nd(T) =?15.0 of a 2 AE old phosphorite implies the presence of ~ 1.5 AE old continental crust at 2 AE ago. The approach outlined here can be used to constrain the age of the exposed crust as a function of time.