应用氢氧同位素研究矿床成因的一些问题探讨

成矿热液的氢、氧同位素组成与其水的类型、水/岩交换的岩石成分和同位素组成、水/岩交换时的温度及水/岩交换程度(W/R比值大小)等诸多因素有关,微生物和有机质也对其有一定的影响。因此,仅通过简单投影的方法将成矿热液的氢、氧同位素值与一些所谓的标准值进行类比,由此就推断出热液中水的来源,这种方法是不可取的;尤其当成矿热液的氢、氧同位素值介于大气降水和岩浆水的值之间时,切忌滥用两种水混合成矿模式,因为实际情况往往并不是这样。本文以胶东乳山金矿床为例,展开了这方面的讨论。     

New data on variations of stable isotopes in the pingo ice core in the southern part of the Tazovsky Peninsula

The aim of this work is to obtain the vertical isotopic profile of the thick Pestsovoe pingo ice core in the southern part of the Tazovsky Peninsula, to determine the oxygen and hydrogen isotopic composition of the ice, and to reveal its formation conditions. Two trends were identified for the isotopic profile of the pingo ice: an insignificant increase of the δ¹⁸O (~1.5‰) and δD (~9‰) values at depths of 12–15 m and a gradual decrease of isotopic values by 3.8 and 23‰ for δ¹⁸O and δD, respectively, at a depth of 15–26 m. The formation of the pingo ice core in the semiclosed system resulted in fractionation of the isotopic composition of oxygen and hydrogen by more than 4 and 20‰, respectively.     

Oxygen isotopic signature of 4.4-3.9 Ga zircons as a monitor of differentiation processes on the Moon

We report oxygen isotopic compositions for 14 zircon grains from a sample of sawdust from lunar breccia 14321. The zircons range in age from ∼4.4 to 3.9 Ga and in U and Th content from a few to several hundred ppm. As such these grains represent a range of possible source rocks, from granophyric to mafic composition, and cover the total age range of the major initial lunar bombardment. Nevertheless, results show that the oxygen isotopic compositions of the zircons fall within a narrow range of δ¹⁸O of about 1 per mil and have δ¹⁸O values indistinguishable from those observed for terrestrial mid-ocean ridge basalts confirming the coincidence of lunar and Earth oxygen isotopic compositions. In the δ¹⁷O vs. δ¹⁸O, coordinates data form a tight group with a limited trend on the terrestrial fractionation line. The zircon oxygen isotopes show minimal evidence of the extreme and variable mineral differentiation and element fractionation that have contributed to the formation of their parent rocks.     

Dashuigou Tellurium Deposit in Sichuan Province, China: S, C, O, and H Isotope Data and Their Implications on Hydrothermal Mineralization

Abstract: Dashuigou, a unique tellurium‐dominated deposit over the world, is located in the western margin of the Yangtze cra‐ton in southwestern China. It is characterized by high‐grade tellurium accompanied by bismuth, gold, silver, and sulfur, and occurs in the area of less than one km². The mineralization is divided into three stages, i.e. (1) tellurium‐bearing pyrrhotite–pyrite stage, (2) tetradymite stage, and (3) auriferous quartz veins stage. Tellurium mineralization coexisting with bismuth, silver, selenium, and gold predominantly develops in the stage 2, while the stage 1 is enriched only in sulfur and iron, and the stage 3 is very weakly mineralized with gold. The δ³⁴S values of sulfides in the ore of the deposit vary in a narrow range of –3.1 ‐ +2.8 per mil with –3.1 ‐ +2.8 per mil for the stage 1 and –0.5 ‐ +2.1 for the stage 2, showing the isotopic characteristics of mantle derived sulfur. The δ¹³C values of vein dolomites vary from –5.3 to –7.2 per mil, with –5.3 ‐ –6.6 per mil for the stage 1 and –5.3 ‐ –7.2 per mil for the stage 2, which are significantly different from those of surrounding Triassic marble with δ¹³C values of –0.3 ‐ +2.8 per mil, and show characteristics of mantle derived carbon. The δ¹⁸O values of vein dolomites range from +10.2 to +13.1 per mil, which are higher than those of carbonatite, but lower than those of the marble. Their corresponding δ¹⁸O_water values are +0.6 ‐ +3.9 per mil, with +2.7 ‐ +3.8 per mil for the stage 1 and +0.6 ‐ +3.9 per mil for the stage 2. The data implies that these vein carbonates were formed by the mixing fluids of magmatic or mantle source with meteoric or formation water. The δ¹⁸O values of ore‐forming fluids responsible for the formation of vein quartz are estimated to be +3.2 to +6.8, the δD values of inclusion fluids of the quartz are measured to be –54 to –82 per mil. All those stable isotopic data suggest the involvement of the fluids from mantle and/or mantle‐derived magmas through fault system in the forming process of the Dashuigou tellurium deposit.     

Hydrogen and oxygen in brine shrimp chitin reflect environmental water and dietary isotopic composition

Hydrogen and oxygen isotope ratios of the common structural biopolymer chitin are a potential recorder of ecological and environmental information, but our understanding of the mechanisms of incorporation of H and O from environmental substrates into chitin is limited. We report the results of a set of experiments in which the isotopic compositions of environmental water and diet were varied independently in order to assess the contribution of these variables to the H and O isotopic composition of Artemia franciscana chitin. Hydrogen isotope ratios of chitin were strongly linearly correlated with both food and water, with approximately 26% of the hydrogen signal reflecting food and approximately 38% reflecting water. Oxygen isotopes were also strongly correlated with the isotopic composition of water and food, but whereas 69% of oxygen in chitin exchanged with environmental water, only 10% was derived from food. We propose that these observations reflect the position-specific, partial exchange of H and O atoms with brine shrimp body water during the processes of digestion and chitin biosynthesis. Comparison of culture experiments with a set of natural samples collected from the Great Salt Lake, UT in 2006 shows that, with some exceptions, oxygen isotope compositions of chitin track those of water, whereas hydrogen isotopes vary inversely with those of lake water. The different behavior of the two isotopic systems can be explained in terms of a dietary shift from allochthonous particulate matter with relatively higher δ²H values in the early spring to autochthonous particulate matter with significantly lower δ²H values in the late summer to autumn. These results suggest oxygen in chitin may be a valuable proxy for the oxygen isotopic composition of environmental water, whereas hydrogen isotope values from the same molecule may reveal ecological and biogeochemical changes within lakes.     

USGS46 Greenland Ice Core Water – A New Isotopic Reference Material for δ2H and δ18O Measurements of Water

Ice core from Greenland was melted, filtered, homogenised, loaded into glass ampoules, sealed, autoclaved to eliminate biological activity, and calibrated by dual‐inlet isotope‐ratio mass spectrometry. This isotopic reference material (RM), USGS46, is intended as one of two secondary isotopic reference waters for daily normalisation of stable hydrogen (δ²H) and stable oxygen (δ¹⁸O) isotopic analysis of water with a mass spectrometer or a laser absorption spectrometer. The measured δ²H and δ¹⁸O values of this reference water were ?235.8 ± 0.7‰ and ?29.80 ± 0.03‰, respectively, relative to VSMOW on scales normalised such that the δ²H and δ¹⁸O values of SLAP reference water are, respectively, ?428 and ?55.5‰. Each uncertainty is an estimated expanded uncertainty (U = 2u_c) about the reference value that provides an interval that has about a 95‐percent probability of encompassing the true value. This reference water is available in cases containing 144 glass ampoules that are filled with either 4 ml or 5 ml of water per ampoule.     

Antiphase hydrogen and oxygen isotope periodicity in chert nodules

Oxygen and hydrogen isotope analyses were made of Jurassic-age chert nodules from the Holy Cross Mountains, SE Poland, along radial transects at high spatial resolution. There is a radial “sigmoidal” periodicity for both isotope ratios, but the two are out of phase, with high δD values corresponding to low δ¹⁸O values. Periodicity for a 100- to 120-mm diameter nodule is approximately 16 mm, increasing slightly toward the rim, with amplitudes approaching 20 and 3.0‰ for hydrogen and oxygen, respectively. The combined hydrogen-oxygen isotope data for one nodule fall on a published curve for chert forming in equilibrium with seawater (Knauth and Epstein, 1976); the range of delta values corresponds to temperature variations of ∼10°C. Data for a second chert fall on a subparallel δD-δ¹⁸O line with δD values that are almost 50‰ lower. The δD-δ¹⁸O patterns for the nodules cannot be explained by periodic mixing of meteoric and ocean water because the hydrogen and oxygen isotope data are out of phase. Two possible explanations for the antiphase periodicity are (a) cyclical temperature variations, perhaps related to an unstable convection system (e.g., Bolton et al., 1999), and (b) self-organizing catalytic precipitation (e.g., Wang and Merino, 1990). The systematic isotopic variations are difficult to explain by diagenesis and strongly suggest that primary isotopic compositions are preserved. The isotopic data provide important information on the thermal history of the sedimentary basin, if temperature variations are the cause of the isotopic periodicity.     

A model for O/O variations in CO2 evolved from goethite during the solid-state α-FeOOH to α-Fe2O3 phase transition

“Plateau” δ¹⁸O values of CO₂ that evolved from the Fe(CO₃)OH component during isothermal vacuum dehydrations (200-230 °C) of 18 natural goethites range from 8.2 to 28.1‰. In contrast, the measured δ¹⁸O values of the goethite structural oxygen range from −11.3 to 1.7‰. The results of this study indicate that the apparent oxygen isotope fractionation factor (¹⁸α_app) between plateau CO₂ and initial goethite is systematically related to the rate of isothermal vacuum dehydration. The nonlinear correlation and the magnitudes of the ¹⁸α_app values are predicted by a relatively simple mass balance model with the following assumptions: (1) the rate of isothermal vacuum dehydration of goethite (for the interval from 0 to ∼60 to 80% loss of structural hydroxyl hydrogen) can be reasonably well represented by first-order kinetics and (2) isotopic exchange between evolving H₂O vapor and solid occurs only in successive, local transition states. The generally good correspondence between the model predictions and the experimental data seems to validate these assumptions. Thus, the ¹⁸O/¹⁶O ratios of the evolved CO₂ can act as probes into the transient processes operating at the molecular level during the solid-state goethite-to-hematite phase transition. For example, the activation energy for the rate constant associated with the transition state, oxygen isotopic exchange between solid and H₂O vapor, is tentatively estimated as 28 ± 11 KJ/mol. Such knowledge may be of consequence in understanding the significance of ¹⁸O/¹⁶O ratios in hematites from some natural environments (e.g., Mars?).Kinetic data and δ¹⁸O values of CO₂ are routinely obtained in the course of measurements of the abundance and δ¹³C values of the Fe(CO₃)OH in goethite. The observed correlation between ¹⁸α_app and dehydration rates suggests that plateau δ¹⁸O values of evolved CO₂ may provide complementary estimates of the δ¹⁸O values of total goethite structural oxygen (O, OH, CO₂) with an overall precision of about ±1‰. However, because of isotopic exchange during the dehydration process, δ¹⁸O values of the evolved CO₂ do not reflect the original δ¹⁸O values of the CO₂ that was occluded as Fe(CO₃)OH in goethite.     

O/O and D/H in goethite from a North American Oxisol of the Early Eocene climatic optimum

An Early Eocene Oxisol in the Ione Formation of California formed in a coastal continental weathering environment at a paleolatitude of ∼38°N. The dominant minerals in the Oxisol are goethite, quartz, and kaolinite. Material balance calculations were applied to new measurements of chemical composition, D/H, and ¹⁸O/¹⁶O ratios of Oxisol samples to determine the δD (−150 ± 3‰) and δ¹⁸O (−2.4 ± 0.3‰) values of the goethite (α-FeOOH). These data, in combination with the global meteoric water line (MWL), yielded an isotopic temperature of 21(±4) °C. The nominal value of 21 °C contrasts with the modern mean annual temperature (MAT) of 16 °C in that area. The warmer temperature is consistent with formation of the goethite during the Early Eocene climatic optimum. The isotopic composition of the goethite and a temperature of 21 °C imply ancient water with a δD value of −61(±4)‰ and a δ¹⁸O value of −8.9(±0.5)‰. This Early Eocene δ¹⁸O (or δD) value is more negative than values in the range of isotopic scatter observed for modern global precipitation at sites with a MAT of 21 °C.At times of warm global climates, the location of a near-surface atmospheric isotherm would generally shift relative to its location under modern climatic conditions. A simple Rayleigh-type condensation model indicates that, if one “follows the isotherm”, the associated scatter in δD and δ¹⁸O of precipitation in very warm global climates should shift (for a given isotherm) to more negative values that may be detectable in proxy records. The isotopic results from the goethite of the Early Eocene Oxisol appear to add to evidence in support of this idea.     

Oxygen and hydrogen isotope ratios in freshwater chert as indicators of ancient climate and hydrologic regime

The oxygen and hydrogen isotopic composition of Eocene and Miocene freshwater cherts in the western United States records regional climatic variation in the Cenozoic. Here, we present isotopic measurements of 47 freshwater cherts of Eocene and Miocene age from the Great Basin of the western United States at two different sites and interpret them in light of regional climatic and tectonic history. The large range of δ¹⁸O of terrestrial cherts measured in this study, from 11.2‰ to 31.2‰ (SMOW: Standard Mean Ocean), is shown to be primarily the result of variations in δ¹⁸O of surface water. The following trends and patterns are recognized within this range of δ¹⁸O values. First, in Cenozoic rocks of northern Nevada, chert δ¹⁸O records the same shift observed in authigenic calcite between the Eocene and Miocene that has been attributed to regional surface uplift. The consistent covariation of proxies suggests that chert reliably records and retains a signal of ancient meteoric water isotopic composition, even though our analyses show that chert formed from warmer waters (40°C) than coexisting calcite (20°C). Second, there is a strong positive correlation between δ¹⁸O and δD in Eocene age chert from Elko, Nevada and Salina, Utah that suggests large changes in lake water isotopic composition due to evaporation. Evaporative effects on lake water isotopic composition, rather than surface temperature, exert the primary control on the isotopic composition of chert, accounting for 10‰ of the 16‰ range in δ¹⁸O measured in Eocene cherts. From authigenic mineral data, we calculate a range in isotopic composition of Eocene precipitation in the north-central Great Basin of −10 to −14‰ for δ¹⁸O and −70 to −100‰ for δD, which is in agreement with previous estimates for Eocene basins of the western United States. Due to its resistance to alteration and record of variations in both δ¹⁸O and δD of water, chert has the potential to corroborate and constrain the cause of variations in isotope stratigraphies.     

Ion microprobe measurements of O/O ratios of phosphate minerals in the Martian meteorites ALH84001 and Los Angeles

Oxygen isotope ratios of merrillite and chlorapatite in the Martian meteorites ALH84001 and Los Angeles have been measured by ion microprobe in multicollector mode. δ¹⁸O values of phosphate minerals measured in situ range from ∼3 to 6‰, and are similar to Martian meteorite whole-rock values, as well as the δ¹⁸O of igneous phosphate on Earth. These results suggest that the primary, abiotic, igneous phosphate reservoir on Mars is similar in oxygen isotopic composition to the basaltic phosphate reservoir on Earth. This is an important first step in the characterization of Martian phosphate reservoirs for the use of δ¹⁸O of phosphate minerals as a biomarker for life on Mars. Cumulative textural, major-element, and isotopic evidence presented here suggest a primary, igneous origin for the phosphates in Los Angeles and ALH84001; textural and chemical evidence suggests that phosphates in ALH84001 were subsequently shock-melted in a later event.     

The oxygen and carbon isotope composition of Carboniferous fossil components: sea-water effects

The aragonitic molluscs and lime-mud of the Pennsylvanian Buckhorn asphalt (Deese Group) of southern Oklahoma precipitated calcium carbonate in oxygen and carbon isotopic equilibrium with ambient sea-water. In addition, δ¹⁸O values indicate that the pelecypods precipitated their shells during the warmer months of the year. The coiled nautiloids probably precipitated their shells in the warm surface water and throughout the year. For the orthocone nautiloids, the δ¹⁸O values suggest that they precipitated their shells in deeper/cooler water. The low-Mg calcite brachiopods of the Mississippian Lake Valley Formation of New Mexico precipitated shells in oxygen and carbon isotopic equilibrium with ambient sea-water. The δ¹⁸O and δ¹³C values of the Buckhorn and Lake Valley faunas, in conjunction with other published results, suggest that Carboniferous sea-water was, on a average, depleted in δ¹⁸O by 1·5 ± 2‰, PDB, relative to Recent sea-water. However, the δ¹³C value of +2.6 ± 2‰, PDB, for average Carboniferous sea-water is similar to that of Recent ocean water. Early diagenetic alteration of metastable carbonates probably occurs in a meteoric-sea-water mixing zone. In this zone the oxygen and carbon isotopic compositions of these components are increased by about 2-4‰, PDB over their marine composition.     

The relationship between phytolith- and plant-water δO values in grasses

Information regarding climatic conditions during plant growth is preserved by the oxygen-isotope composition of biogenic silica (phytoliths) deposited in grasses. The O-isotope compositions of phytoliths and the plant water from which they precipitate are dependent on soil-water δ¹⁸O values, relative humidity, evapotranspiration rates, and temperature. Plant water and phytoliths from two grass species, Ammophila breviligulata (C₃) and Calamovilfa longifolia (C₄) at Pinery Provincial Park in southwestern Ontario, Canada, were examined to determine the variability in their δ¹⁸O values. Stem water was unfractionated from soil-water in oxygen isotopic composition and the δ¹⁸O values of stem silica provide a good proxy for the soil water available to roots during the growing season. Greater spatial and temporal variation in the δ¹⁸O values of water in the top 5 cm of the soil, and their enhanced sensitivity to evaporative ¹⁸O enrichment, are reflected in the generally higher δ¹⁸O values of water in the shallow roots and rhizomes of these grasses. Water within the sheath and lower and upper leaf tissues experiences continual evaporation, becoming progressively enriched in ¹⁸O as it moves towards the tip of the leaf. However, the water from which leaf silica precipitates has not acquired the extreme ¹⁸O enrichment predicted using steady-state models, or measured for midday or average daily leaf water. Possible explanations for this behaviour include preferential deposition of silica at night; the existence of a secluded water fraction within the leaf, which experiences smaller diurnal variations in isotopic composition than leaf water at sites of evaporation; kinetic isotope effects during rapid precipitation of leaf silica; and incomplete exchange between the oxygen in the silicic acid and the leaf water.     

Isotopic variations of oxygen and hydrogen in groundwater of carbonate aquifer in an arid environment

The stable isotopic characteristics were used together with the total chloride to assess changes in groundwater from recharge zones into the carbonate aquifer in an arid environment. The aquifer under study represents a major source of groundwater and thermal springs in Al-Ain city, which are located at the northern part of Jabal Hafit in the United Arab Emirates (UAE). The relationship between oxygen and hydrogen isotopic composition of groundwater is established and is described by δD?=?2.2δ¹⁸O???9.96. The lower slope and y-intercept of groundwater samples relative to the local meteoric waterline suggests that the isotopic enrichment is due to the evaporation of shallow groundwater after recharge occurs. The majority of the shallow groundwater samples have a negative deuterium excess (d-excess) which might be ascribed to high a degree of evaporation, while most of the groundwater samples from deep wells, have a positive value of d-excess which may be related to a low degree of evaporation. The δ¹⁸O values of the thermal waters suggest enrichment towards δ¹⁸O of the carbonate rocks because of the exchange with oxygen at higher temperatures. A possible mixing between thermal or hot water and shallow groundwater is evident in some samples as reflected by δD vs. Cl and d-excess vs. δ¹⁸O plots.     

Diagenesis and the reconstruction of paleoenvironments: A method to restore original δO values of carbonate and phosphate from fossil tooth enamel

Intra-tooth δ¹⁸O variations within the carbonate (δ¹⁸Oc) and phosphate (δ¹⁸Op) components of tooth apatite were measured for Miocene and Pliocene hypsodont mammals from Afghanistan, Greece and Chad in order to evaluate the resistance of enamel to diagenetic alteration. Application of water-apatite interaction models suggest that the different kinetic behaviours of the phosphate-water and carbonate-water systems can be used to detect subtle oxygen isotope disequilibria in fossil enamel when intra-individual variations are considered. Selective alteration of the oxygen isotope composition from the carbonate component of Afghan and Greek enamels suggests inorganic isotopic exchange processes. Microbially-induced isotopic exchange for phosphate is demonstrated for the first time in enamel samples from Chad, in association with extensive recrystallization. In Chad, δ¹⁸Op values were derived from partial isotopic exchange with fossil groundwater during early diagenesis. Mass balance calculations using average carbonate content in enamel as a proxy for recrystallization, and the lowest δ¹⁸Op value of dentine as a proxy for the isotopic composition of the diagenetic fluid, indicate that diagenesis can alter δ¹⁸Op by as much as 3‰ in some enamel samples. This diagenetic alteration is also responsible for a decrease in intra-individual variations of up to 1‰ in affected specimens. The effects of diagenesis on δ¹⁸Op values of fossil enamel are not systematic, however, and can only be estimated if sequential δ¹⁸Op and δ¹⁸Oc analyses are performed on fossil enamel and dentine. Reconstruction of large temporal- or spatial-scale paleoclimates based on δ¹⁸Op analyses from mammalian teeth cannot be considered valid if enamel has been affected by bacterial activity or if the data cannot be corrected for diagenetic effects.     

Variations of the Ca/Ca ratio in seawater during the past 24 million years: evidence from δCa and δO values of Miocene phosphates

The Ca isotope variation of 11 Miocene and Pleistocene, authigenic, marine phosphates is rather small compared to the corresponding variation in δ¹⁸O values. The δ⁴⁴Ca values are not correlated with the δ¹⁸O values and, therefore, they are not temperature controlled. It is likely that the δ⁴⁴Ca values of the phosphorites reflect the variation in the δ⁴⁴Ca values of paleo-seawater but, in contrast to Sr, not the isotopic composition of seawater itself. Furthermore, Ca and Sr isotopic compositions are decoupled with decreasing stratigraphic age of the phosphate peloids with Sr isotopic compositions changing to more radiogenic values while Ca isotopic compositions remain rather stable. All samples have δ⁴⁴Ca values below present-day seawater values, suggesting that phosphate formation discriminates against heavy Ca isotopes.Phosphorites and carbonaceous sediments have a similar Ca isotopic variation during the Miocene. A systematic and more or less constant shift between marine carbonates and phosphates is observed: the phosphate samples are slightly less enriched in ⁴⁰Ca compared to carbonates. This shift has been related to a mineral-dependent kinetic mass fractionation during precipitation from seawater. The rather stable δ⁴⁴Ca value for the 19 to 9 Ma old phosphorites points to a constant δ⁴⁴Ca fractionation of about 1.1 between seawater and phosphorites during the past and suggests steady-state conditions for the Mid-Miocene seawater (sedimentation flux equals erosion flux).     

Early diagenetic processes during Mn-carbonate formation: evidence from the isotopic composition of authigenic Ca-rhodochrosites of the Baltic Sea

The formation of authigenic Ca-rich rhodochrosite (ACR) in sapropelic sediments of the Gotland Basin, Baltic Sea, is governed by deepwater renewal processes whereby saline water from the North Atlantic flushes the brackish anoxic Baltic Deeps. The carbon and oxygen isotopic compositions of these Mn-carbonates suggest that ACR formation takes place just below the sediment surface and that dissolved compounds from the deepwater column, such as water and bicarbonate molecules, were incorporated in ACR during authigenesis. Porewaters near the sediment surface display δ¹⁸O values of −5.4‰ (VSMOW) and are generally depleted in ¹⁸O, compared to the oxygen isotopic composition of water in equilibrium with Mn-carbonate solid solutions (ACR δ¹⁸O values are −4.6‰). This suggests that early burial diagenetic processes significantly modify the initial isotopic composition of water during Mn-carbonate formation. The reduction of sulfate having δ¹⁸O values of +8.4‰ accounts for a permanent enrichment of porewater ¹⁸O and observed δ¹⁸O values at depth equal to −4.6‰. However, this process does not explain the observed disequilibrium in the oxygen isotopic composition between water and ACR close to the sediment surface where Mn-carbonate formation takes place. Based on isotopic mass balance calculations, we suggest that MnO₂ with δ¹⁸O values of +8.9‰ released oxygen enriched in ¹⁸O into the anoxic porewaters close below the sediment surface. This process should occur after oxygenation events during deepwater renewal when MnO₂ accumulates at the surface of anoxic sediments. Manganese carbonates formed in these waters display δ¹⁸O values of ∼1.0‰ heavier than values expected solely from the initial deepwater composition. This quantitatively explains the discrepancy between paleosalinities calculated from ACR δ¹⁸O based on Mn-carbonate/water isotopic equilibrium fractionation and direct observations for the same period. Our results emphasize the important role of microbial MnO₂ reduction during rhodochrosite authigenesis and suggest that Mn(II) activity, rather than alkalinity, is the limiting component for sedimentary Mn-carbonate formation.     

Relative humidity recorded in tree rings: A study along a precipitation gradient in the Olympic Mountains, Washington, USA

Humidity is one of the fundamental variables controlling the energy balance of the climate, and thus its reconstruction represents a significant aspect of paleoclimate studies. We here report a study of oxygen and hydrogen isotopic compositions of tree-ring cellulose along a precipitation gradient in the Olympic Mountains of Washington, USA, to examine whether and how the relative humidity is recorded in the cellulose. According to the physically-based model described in this paper, the relationship between δD and δ¹⁸O of cellulose should have the same slope as that in the relationship between δD and δ¹⁸O in the source water (∼8.8 for the local meteoric water) if there is no systematic variation in the mean relative humidity among study sites and no physiologic differences among trees. However, our isotopic analyses of cellulose yielded a slope of 17.4, significantly greater than the slope of the Local Meteoric Water Line. We show that to produce such a slope, a positive covariation between the relative humidity and the δD and δ¹⁸O in the source water is required across the precipitation gradient. This work suggests that the δD vs. δ¹⁸O relationship in tree rings can be a useful tool for paleohumidity reconstruction.     

海南岛玻璃陨石和第四纪玄武岩氧同位素特征

海南岛北部(琼北地区)广泛分布着第四纪玄武岩,根据地质和地貌特征,风化壳发育程度,玄武岩和沉积地层之间接触关系以及玄武岩古地磁测量结果,可以分成四期(图1)。