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1.
The redox state of Precambrian shallow seas has been linked with material cycle and evolution of the photosynthesis-based ecosystem. Iron is a redox-sensitive element and exists as a soluble Fe(II) species or insoluble Fe(III) species on Earth’s surface. Previous studies have shown that the iron isotopic ratio of marine sedimentary minerals is useful for understanding the ocean redox state, although the redox state of the Archean shallow sea is poorly known. This is partly because the conventional bulk isotope analytical technique has often been used, wherein the iron isotopic record may be dampened by the presence of isotopically different iron-bearing minerals within the same sample. Here we report a microscale iron isotopic ratio of individual pyrite grains in shallow marine stromatolitic carbonates over geological time using a newly developed, near-infrared femtosecond laser ablation multicollector ICP-MS technique (NIR-fs-LA-MC-ICP-MS).We have determined that the grain-scale iron isotopic distribution of pyrite from coeval samples shows a bimodal (2.7 and 2.3 Ga) or unimodal pattern (2.9, 2.6, and 0.7 Ga). In particular, pyrite from the 2.7 Ga Fortescue Group shows a unique bimodal distribution with highly positive (+1.0‰ defined as Type 1) and negative δ56Fe values (−1.8‰ defined as Type 2). Type 1 and 2 pyrites occasionally occur within different siliceous layers in the same rock specimen. Layer-scale iron isotopic heterogeneity indicates that the iron isotopic ratios of the two types of pyrite are not homogenized by diagenesis after deposition. Some cubic pyrites have a core with a positive δ56Fe value (1‰) and a rim with a crustal δ56Fe value (0‰). The observed isotopic zoning suggests that the positive δ56Fe value is a primary signature at the time of stromatolite formation, while secondary pyrite precipitated during diagenesis.The positive δ56Fe value of Type 1 and the large iron isotopic difference between Type 1 and 2 (2.8‰.) suggest partial Fe(II) oxidation in the 2.7-Ga shallow sea, i.e., pyritization of 56Fe-enriched ferric oxyhydroxide (Type 1) and 56Fe depleted Fe2+aq in seawater (Type 2). Type 2 pyrite was probably not produced by microbial iron redox cycling during diagenesis because this scenario requires a higher abundance of pyrite with δ56Fe of 0‰ than of −1.8‰. Consequently, the degree of Fe(II) oxidation in the 2.7-Ga shallow sea can be estimated by a Fe2+aq steady-state model. The model calculation shows that half the Fe2+aq influx was oxidized in the seawater. This implies that O2 produced by photosynthesis would have been completely consumed by oxidation of the Fe2+aq influx. Grain-scale iron isotopic distribution of pyrite could be a useful index for reconstructing the redox state of the Archean shallow sea.  相似文献   

2.
The Callovian-Oxfordian (COx) clayey unit is being studied in the Eastern part of the Paris Basin at depths between 400 and 500 m depth to assess of its suitability for nuclear waste disposal. The present study combines new mineralogical and isotopic data to describe the sedimentary history of the COx unit. Petrologic study provided evidence of the following diagenetic mineral sequence: (1) framboidal pyrite and micritic calcite, (2) iron-rich euhedral carbonates (ankerite, sideroplesite) and glauconite (3) limpid calcite and dolomite and celestite infilling residual porosity in bioclasts and cracks, (4) chalcedony, (5) quartz/calcite. Pyrite in bioturbations shows a wide range of δ34S (−38‰ to +34.5‰), providing evidence of bacterial sulphate reduction processes in changing sedimentation conditions. The most negative values (−38‰ to −22‰), measured in the lower part of the COx unit indicate precipitation of pyrite in a marine environment with a continuous sulphate supply. The most positive pyrite δ34S values (−14‰ up to +34.5‰) in the upper part of the COx unit indicate pyrite precipitation in a closed system. Celestite δ34S values reflect the last evolutionary stage of the system when bacterial activity ended; however its deposition cannot be possible without sulphate supply due to carbonate bioclast dissolution. The 87Sr/86Sr ratio of celestite (0.706872-0.707040) is consistent with deposition from Jurassic marine-derived waters. Carbon and oxygen isotopic compositions of bulk calcite and dolomite are consistent with marine carbonates. Siderite, only present in the maximum clay zone, has chemical composition and δ18O consistent with a marine environment. Its δ13C is however lower than those of marine carbonates, suggesting a contribution of 13C-depleted carbon from degradation of organic matter. δ18O values of diagenetic chalcedony range between +27‰ and +31‰, suggesting precipitation from marine-derived pore waters. Late calcite crosscutting a vein filled with chalcedony and celestite, and late euhedral quartz in a limestone from the top of the formation have lower δ18O values (∼+19‰), suggesting that they precipitated from meteoric fluids, isotopically close to present-day pore waters of the formation. Finally, the study illustrates the transition from very active, biotic diagenesis to abiotic diagenesis. This transition appears to be driven by compaction of the sediment, which inhibited movement of bacterial cells by reduction of porosity and pore sizes, rather than a lack of inorganic carbon or sulphates.  相似文献   

3.
This paper presents the results of an isotopic study of spring waters in ?wi?tokrzyski (Holy Cross Mountain) National Park (?NP), south-central Poland. The δ34SV-CDT and δ18OV-SMOW of soluble sulfates (n = 40) varied from 0.5‰ to 18.1‰ and from 3.5‰ to 12.2‰, respectively. The average δ34S values are closely similar to those of rainwater, soils and rocks (comprising scattered pyrite). This suggests that soluble sulfates in the springs originated from mixing of recent and historic deposition, sulfates derived from pyrite oxidation, and CS-mineralization in soils and debris. An additional anthropogenic sulfur input (inorganic fertilizer) occurs in the water of spring S-61 located in the ?wi?tokrzyski National Park buffer zone. The δ18OV-SMOW of spring waters (n = 4) were in the range of −10.6‰ to −10.2‰ indicating that they are derived from vadose groundwater in ?NP. This was the first isotope study of spring waters in the national parks of Poland. It enabled the determination of sulfur pathways and discrimination between natural and anthropogenic sources of this element in a relatively pristine area.  相似文献   

4.
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.  相似文献   

5.
Stable isotopes (H, O, C) were determined for ground and surface waters collected from two relatively undisturbed massive sulfide deposits (Halfmile Lake and Restigouche) in the Bathurst Mining Camp (BMC), New Brunswick, Canada. Additional waters from active and inactive mines in the BMC were also collected. Oxygen and hydrogen isotopes of surface and shallow groundwaters from both the Halfmile Lake and Restigouche deposits are remarkably uniform (− 13 to − 14‰ and − 85 to − 95‰ for δ18OVSMOW and δ2HVSMOW, respectively). These values are lighter than predicted for northern New Brunswick and, combined with elevated deuterium excess values, suggest that recharge waters are dominated by winter precipitation, recharged during spring melting. Deeper groundwaters from the Restigouche deposit, and from active and inactive mines have heavier δ18OVSMOW ratios (up to − 10.8‰) than shallow groundwaters suggesting recharge under warmer climate or mixing with Shield-type brines. Some of the co-variation in Cl concentrations and δ18OVSMOW ratios can be explained by mixing between saline and shallow recharge water end-members. Carbon isotopic compositions of dissolved inorganic carbon (DIC) are variable, ranging from − 15 to − 5‰ δ13CVPDB for most ground and surface waters. Much of the variation in the carbon isotopes is consistent with closed system groundwater evolution involving soil zone CO2 and fracture zone carbonate minerals (calcite, dolomite and siderite; average = − 6.5‰ δ13CVPDB). The DIC of saline Restigouche deposit groundwater is isotopically heavy (∼+ 12‰ δ13CVPDB), indicating carbon isotopic fractionation from methanogenesis via CO2 reduction, consistent with the lack of dissolved sulfate in these waters and the observation of CH4-degassing during sampling.  相似文献   

6.
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.  相似文献   

7.
Large, correlated, mass-dependent enrichments in the heavier isotopes of O, Cr, Fe, and Ni are observed in type-I (metal/metal oxide) cosmic spherules collected from the deep sea. Limited intraparticle variability of oxygen isotope abundances, typically <5‰ in δ18O, indicates good mixing of the melts and supports the application of the Rayleigh equation for the calculation of fractional evaporative losses during atmospheric entry. Fractional losses for oxygen evaporation from wüstite, assuming a starting isotopic composition equal to that of air (δ18O = 23.5‰; δ17O = 11.8‰), are in the range 55%-77%, and are systematically smaller than evaporative losses calculated for Fe (69%-85%), Cr (81%-95%), and especially Ni (45%-99%). However, as δ18O values increase, fractional losses for oxygen approach those of Fe, Cr, and Ni indicating a shift in the evaporating species from metallic to oxidized forms as the spherules are progressively oxidized during entry heating. The observed unequal fractional losses of O and Fe can be reconciled by allowing for a kinetic isotope mass-dependent fractionation of atmospheric oxygen during the oxidation process and/or that some metallic Fe may have undergone Rayleigh evaporation before oxidation began.In situ measurements of oxygen isotopic abundances were also performed in 14 type-S (silicate) cosmic spherules, 13 from the Antarctic ice and one from the deep sea. Additional bulk Fe and Cr isotopic abundances were determined for two type-S deep-sea spherules. The isotopic fractionation of Cr isotopes suggest appreciable evaporative loss of Cr, perhaps as a sulfide. The oxygen isotopic compositions for the type-S spherules range from δ18O = −2‰ to + 27‰. The intraspherule isotopic variations are typically small, ∼5% relative, except for the less-heated porphyritic spherules which have preserved large isotopic heterogeneities in at least one case. A plot of δ17O vs. δ18O values for these spherules defines a broad parallelogram bounded at higher values of δ17O by the terrestrial fractionation line, and at lower values of δ17O by a line parallel to it and anchored near the isotopic composition of δ18O = −2.5‰ and δ17O = −5‰. Lack of independent evidence for substantial evaporative losses suggests that much of this variation reflects the starting isotopic composition of the precursor materials, which likely resembled CO, CM, or CI chondrites. However, the enrichments in heavy isotopes indicate that some mixing with atmospheric oxygen was probably involved during atmospheric entry for some of the spherules. Isotopic fractionation due to evaporation of incoming grain is not required to explain most of the oxygen isotopic data for type-S spherules. However spherules with barred olivine textures that are thought to have experienced a more intense heating than the porphyritic ones might have undergone some distillation. Two cosmic spherules, one classified as a radial pyroxene type and the other showing a glassy texture, show unfractionated oxygen isotopic abundances. They are probably chondrule fragments that survived atmospheric entry unmelted.Possible reasons type-I spherules show larger degrees of isotopic fractionation than type-S spherules include: a) the short duration of the heating pulse associated with the high volatile content of the type-S spherule precursors compared to type-I spherules; b) higher evaporation temperatures for at least a refractory portion of the silicates compared to that of iron metal or oxide; c) lower duration of heating of type-S spherules compared to type-I spherules as a consequence of their lower densities.  相似文献   

8.
To better understand reaction pathways of pyrite oxidation and biogeochemical controls on δ18O and δ34S values of the generated sulfate in acid mine drainage (AMD) and other natural environments, we conducted a series of pyrite oxidation experiments in the laboratory. Our biological and abiotic experiments were conducted under aerobic conditions by using O2 as an oxidizing agent and under anaerobic conditions by using dissolved Fe(III)aq as an oxidant with varying δ18OH2O values in the presence and absence of Acidithiobacillus ferrooxidans. In addition, aerobic biological experiments were designed as short- and long-term experiments where the final pH was controlled at ∼2.7 and 2.2, respectively. Due to the slower kinetics of abiotic sulfide oxidation, the aerobic abiotic experiments were only conducted as long term with a final pH of ∼2.7. The δ34SSO4 values from both the biological and abiotic anaerobic experiments indicated a small but significant sulfur isotope fractionation (∼−0.7‰) in contrast to no significant fractionation observed from any of the aerobic experiments. Relative percentages of the incorporation of water-derived oxygen and dissolved oxygen (O2) to sulfate were estimated, in addition to the oxygen isotope fractionation between sulfate and water, and dissolved oxygen. As expected, during the biological and abiotic anaerobic experiments all of the sulfate oxygen was derived from water. The percentage incorporation of water-derived oxygen into sulfate during the oxidation experiments by O2 varied with longer incubation and lower pH, but not due to the presence or absence of bacteria. These percentages were estimated as 85%, 92% and 87% from the short-term biological, long-term biological and abiotic control experiments, respectively. An oxygen isotope fractionation effect between sulfate and water (ε18OSO4-H2O) of ∼3.5‰ was determined for the anaerobic (biological and abiotic) experiments. This measured value was then used to estimate the oxygen isotope fractionation effects between sulfate and dissolved oxygen in the aerobic experiments which were −10.0‰, −10.8‰, and −9.8‰ for the short-term biological, long-term biological and abiotic control experiments, respectively. Based on the similarity between δ18OSO4 values in the biological and abiotic experiments, it is suggested that δ18OSO4 values cannot be used to distinguish biological and abiotic mechanisms of pyrite oxidation. The results presented here suggest that Fe(III)aq is the primary oxidant for pyrite at pH < 3, even in the presence of dissolved oxygen, and that the main oxygen source of sulfate is water-oxygen under both aerobic and anaerobic conditions.  相似文献   

9.
Understanding past climate change is critical to the interpretation of earth history. Even though relative temperature change has been readily assessed in the marine record, it has been more difficult in the terrestrial record due to restricted taxonomic distribution and isotopic fractionation. This problem could be overcome by the use of multiple paleoproxies. Therefore, the δ18O isotopic composition of five paleoproxies (rodent tooth enamel, δ18OPhosphate = +17.7 ± 2.0‰ n = 74 (VSMOW); fish scale ganoine δ18OPhosphate = +19.7 ± 0.7‰ n = 20 (VSMOW); gastropod shell δ18OCalcite = −1.7 ± 1.3‰ n = 50 (VPDB); charophyte gyrogonite δ18OCalcite = −2.4 ± 0.5‰ n = 20 (VPDB); fish otolith δ18OAragonite = δ18O = −3.6 ± 0.6‰ n = 20 (VPDB)) from the Late Eocene (Priabonian) Osborne Member (Headon Hill Formation, Solent Group, Hampshire Basin, UK) were determined. Because diagenetic alteration was shown to be minimal the phosphate oxygen component of rodent tooth enamel (as opposed to enamel carbonate oxygen) was used to calculate an initial δ18OLocal water value of 0.0 ± 3.4‰. However, a skewed distribution, most likely as a result of the ingestion of evaporating water, necessitated the calculation of a corrected δ18OLocal water value of −1.3 ± 1.7‰ (n = 62). This δ18OLocal water value corresponds to an approximate mean annual temperature of 18 ± 1°C. Four other mean paleotemperatures can also be calculated by combining the δ18OLocal water value with four independent freshwater paleoproxies. The calculated paleotemperature using the fish scale thermometry equations most likely represents the mean temperature (21 ± 2°C) of the entire length of the growing season. This should be concordant with the paleotemperature calculated using the Lymnaea shell thermometry equation (23 ± 2°C). The lack of concordance is interpreted to be the result of diagenetic alteration of the originally aragonitic Lymnaea shell to calcite. The mean paleotemperature calculated using the charophyte gyrogonite thermometry equation (21 ± 2°C), on the other hand, most likely represents the mean temperature of a single month toward the end of the growing season. The fish otolith mean paleotemperature (28 ± 2°C) most likely represents the mean temperature of the warmest months of the growing season. An approximate mean annual temperature of 18 ± 1°C, in addition to a mean growing season paleotemperature of 21 ± 2°C (using fish scale only) with a warmest month temperature of 28 ± 2°C, and high associated standard deviations suggest that a subtropical to warm temperate seasonal climate existed during the deposition of the Late Eocene Osborne Member.  相似文献   

10.
The Martian meteorite ALH84001 contains ∼1% by weight of carbonate formed by secondary processes on the Martian surface or in the shallow subsurface. The major form of this carbonate is chemically and isotopically zoned rosettes which have been well documented elsewhere. This study concentrates upon carbonate regions ∼200 μm across which possess previously unobserved magnesium rich inner cores, interpreted here as rosette fragments, surrounded by a later stage cement containing rare Ca-rich carbonates (up to Ca81Mg07Fe04Mn07) intimately associated with feldspar. High spatial resolution ion probe analyses of Ca-rich carbonate surrounding rosette fragments have δ18OV-SMOW values as low as −10‰. These values are not compatible with deposition from a global Martian atmosphere invoked to explain ALH84001 rosettes. The range of δ18O values are also incompatible with a fluid that has equilibrated with the Martian crust at high temperature or from remobilisation of carbonate of rosette isotopic composition. At Martian atmospheric temperatures, the small CO2(gas)-CO2(ice) fractionation makes meteoric CO2 an unlikely source for −10‰ carbonates. In contrast, closed system Rayleigh fractionation of H2O can generate δ18OH2O −30‰, as observed at high latitudes on Earth. We suggest that atmospheric transport and precipitation of H2O in a similar fashion to that on Earth provides a source of suitably 18O depleted water for generation of carbonate with δ18OV-SMOW = −10‰.  相似文献   

11.
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.  相似文献   

12.
We measured the δ18O of cellulose (δ18Ocel) extracted from fossil wood collected at 9 sites in the northern and southern hemispheres as a potential source of information about precipitation δ18O (δ18Oppt) in the past and paleotemperatures. The samples had been buried in fluvial sediments for periods of time ranging from ca. 45 million to 250 years. At the oldest localities (high latitude, Eocene- through Pliocene-age sites in Canada and Russia), mean annual temperature (MAT) estimates derived from the modern relationship between MAT and δ18Ocel are 6-16 °C lower than the MAT estimates derived from other biological proxies. Estimates of Pleistocene and Holocene mean annual temperatures are close to the modern values at those sites. These results are consistent with other recent findings that the MAT/δ18Oppt relationship across North America was not constant throughout the Cenozoic. Paleo-δ18Oppt estimates derived from fossil cellulose and the modern North American relationship between δ18Ocel and δ18Oppt are within the current annual range of δ18Oppt values at all locations. The middle Eocene δ18Oppt we determined from arctic cellulose samples (−21.9‰) is consistent with river water δ18O determined in two other studies (−19.1‰ to −22‰). These findings provide some evidence that a precipitation δ18O signal may be retained in wood cellulose during millions of years of burial, and that latitudinal patterns in δ18Oppt may not have changed much during the past 45 Ma. These interpretations depend, of course, on the assumption that the isotopic composition of the cellulose has not changed during burial, an assumption for which it is difficult to gather direct evidence. XRD analysis shows that the crystalline form of the fossil cellulose we used to estimate paleoprecipitation δ18O and paleo-MAT is the same as that of modern cellulose, and that the samples are free of quartz and iron oxide contaminants that result in negative errors in measured δ18Ocel.  相似文献   

13.
Goethite (Ax-2) from Axel Heiberg Island (∼80°N) on the margin of the Arctic Ocean is the dominant mineral in a sample of “petrified” Eocene wood, but U, Th, and He measurements suggest that the goethite (α-FeOOH) crystallized in the latest Miocene/Pliocene (ca. 5.5 to 2.8 Ma). Measured δD and δ18O values of Ax-2 are −221 (±6)‰ and −9.6 (±0.5)‰, respectively. The inferred δD and δ18O values of the ancient water were about −139‰ and −18.6‰, respectively, with a calculated temperature of crystallization of 3 (±5)°C, which compares with the modern summer (J-J-A) temperature of 3 °C and contrasts with a modern MAT of −19 °C. Published results from various biological proxies on nearby Ellesmere Island indicate a Pliocene (∼4 Ma) MAT of either −6 or −0.4 °C and corresponding seasonal amplitudes of about 18 or 13 °C. A conductive heat flow model suggests that a temperature of 3 °C could represent goethite crystallization at depths of ∼100-200 cm (for MAT = −6 °C) or ∼250-450 cm (for MAT = −0.4 °C) over seasonally restricted intervals of time.The δ18O value of the Ax-2 water (−18.6‰) is more positive than the modern J-J-A precipitation (−22‰). In combination, the paleotemperatures and δ18O values of ancient waters (from Ax-2 and published results from three Eocene or Pliocene proxy sites on Axel Heiberg and Ellesmere Islands) are consistent with a warm season bias in those isotopic proxies. The results are also consistent with higher proportions of J-J-A precipitation in the annual total. If so, this emphasizes the importance of seasonality at high latitudes even in times of warmer global climates, and suggests that the Arctic hydrologic cycle, as expressed in the seasonal distribution and isotopic composition of precipitation (perhaps modified by a warmer Arctic Ocean), differed from modern.The δ13C value of the Fe(CO3)OH component in the Ax-2 goethite is +6.6‰, which is much more positive than expected if crystallizing goethite incorporated CO2 derived primarily from oxidation of relict Eocene wood with δ13C values of about −24‰. This apparent paradox may be resolved if the goethite is a product of oxidation of 13C-rich siderite, which had previously replaced wood in an Eocene methanogenic burial environment. Thus, the goethite retains a carbon isotope “memory” of a diagenetic Eocene event, but a δD and δ18O record of the latest Miocene/Pliocene Arctic climate.  相似文献   

14.
Speleothems from Hoti Cave in northern Oman provide a record of continental pluvial periods over the last 330,000 yr. Periods of rapid speleothem deposition occurred from 6000 to 10,500, 78,000 to 82,000, 120,000 to 135,000, 180,000 to 200,000, and 300,000 to 330,000 yr ago, with little or no growth during the intervening periods. During each of these five pluvial periods, δD values of water extracted from speleothem fluid inclusions (δDFI) are between −60 and −20‰ (VSMOW) and δ18O values of speleothem calcite (δ18OC) are between −12 and −4‰ to (VPDB). These values are much more negative than modern rainfall (for δD) or modern stalagmites (for δ18O). Previous work on the isotopic composition of rainfall in Oman has shown that northern and southern moisture sources are isotopically distinct. Combined measurements of the δD values of fluid-inclusion water with calculated δ18O values from peak interglacial speleothems indicate that groundwater was predominantly recharged by the southern (Indian Ocean) moisture source, when the monsoon rainfall belt moved northward and reached Northern Oman during each of these periods.  相似文献   

15.
Evaluation of the extent of volatile element recycling in convergent margin volcanism requires delineating likely source(s) of magmatic volatiles through stable isotopic characterization of sulfur, hydrogen and oxygen in erupted tephra with appropriate assessment of modification by degassing. The climactic eruption of Mt. Mazama ejected approximately 50 km3 of rhyodacitic magma into the atmosphere and resulted in formation of a 10-km diameter caldera now occupied by Crater Lake, Oregon (lat. 43°N, long. 122°W). Isotopic compositions of whole-rocks, matrix glasses and minerals from Mt. Mazama climactic, pre-climactic and postcaldera tephra were determined to identify the likely source(s) of H2O and S. Integration of stable isotopic data with petrologic data from melt inclusions has allowed for estimation of pre-eruptive dissolved volatile concentrations and placed constraints on the extent, conditions and style of degassing.Sulfur isotope analyses of climactic rhyodacitic whole rocks yield δ34S values of 2.8-14.8‰ with corresponding matrix glass values of 2.4-13.2‰. δ34S tends to increase with stratigraphic height through climactic eruptive units, consistent with open-system degassing. Dissolved sulfur concentrations in melt inclusions (MIs) from pre-climactic and climactic rhyodacitic pumices varies from 80 to 330 ppm, with highest concentrations in inclusions with 4.8-5.2 wt% H2O (by FTIR). Up to 50% of the initial S may have been lost through pre-eruptive degassing at depths of 4-5 km. Ion microprobe analyses of pyrrhotite in climactic rhyodacitic tephra and andesitic scoria indicate a range in δ34S from −0.4‰ to 5.8‰ and from −0.1‰ to 3.5‰, respectively. Initial δ34S values of rhyodacitic and andesitic magmas were likely near the mantle value of 0‰. Hydrogen isotope (δD) and total H2O analyses of rhyodacitic obsidian (and vitrophyre) from the climactic fall deposit yielded values οf −103 to −53‰ and 0.23-1.74 wt%, respectively. Values of δD and wt% H2O of obsidian decrease towards the top of the fall deposit. Samples with depleted δD, and mantle δ18O values, have elevated δ34S values consistent with open-system degassing. These results imply that more mantle-derived sulfur is degassed to the Earth’s atmosphere/hydrosphere through convergent margin volcanism than previously attributed. Magmatic degassing can modify initial isotopic compositions of sulfur by >14‰ (to δ34S values of 14‰ or more here) and hydrogen isotopic compositions by 90‰ (to δD values of −127‰ in this case).  相似文献   

16.
The oxygen isotopic micro-distributions within and among minerals in a coarse-grained Ca, Al-rich inclusion (CAI), 7R-19-1 from the Allende meteorite, were measured by in situ using secondary ion mass spectrometry (SIMS). All values of O isotopic ratios in 7R-19-1 minerals fall along the carbonaceous chondrite anhydrous mineral mixing (CCAM) line on a δ17OSMOW vs. δ18OSMOW plot. Major refractory minerals (spinel, fassaite and melilite) in 7R-19-1 showed large negative anomalies of Δ17O in the order, spinel (−21‰) > 16O-rich melilite (∼−18‰) > fassaite (−15 to +1‰) > 16O-poor melilite (−8 to +2‰). However, the lower limit values of Δ17O are similar at about −21‰, a value commonly observed in CAIs. The similarity in the extreme values of the isotope anomaly anomalies suggests that crystallization of all CAIs started from an 16O enrichment of 21‰ (Δ17O) relative to terrestrial values. The order of the O isotopic anomalies observed for 7R-19-1, except for 16O-poor melilite, is parallel to the crystallization sequence determined by experiment from CAI liquid (Stolper, 1982), indicating that the O isotopic exchange in 7R-19-1 occurred between CAI melt and surrounding gas while 7R-19-1 was crystallizing from the 16O enriched CAI liquid (∼−21‰ in Δ17O) in the 16O-poor solar nebula. However, the a single crystallization sequence during the cooling stage cannot explain the existence of 16O-poor melilite. The presence of 16O-poor melilite suggests that multiple heating events occurred during CAI formation. The sharp contact between 16O-rich and 16O-poor melilite crystals and within 16O-rich melilite indicates that these multiple heatings occurred quickly. Based on the O isotopic and chemical compositions, fassaite crystals were aggregates of relic crystals formed from CAI melt whichthat have had various O isotopic compositions from the remelting processes. The results of intra-mineral distributions of O isotopes also support multiple heating events during CAI formation.  相似文献   

17.
We present high-precision measurements of Mg and Fe isotopic compositions of olivine, orthopyroxene (opx), and clinopyroxene (cpx) for 18 lherzolite xenoliths from east central China and provide the first combined Fe and Mg isotopic study of the upper mantle. δ56Fe in olivines varies from 0.18‰ to −0.22‰ with an average of −0.01 ± 0.18‰ (2SD, n = 18), opx from 0.24‰ to −0.22‰ with an average of 0.04 ± 0.20‰, and cpx from 0.24‰ to −0.16‰ with an average of 0.10 ± 0.19‰. δ26Mg of olivines varies from −0.25‰ to −0.42‰ with an average of −0.34 ± 0.10‰ (2SD, n = 18), opx from −0.19‰ to −0.34‰ with an average of −0.25 ± 0.10‰, and cpx from −0.09‰ to −0.43‰ with an average of −0.24 ± 0.18‰. Although current precision (∼±0.06‰ for δ56Fe; ±0.10‰ for δ26Mg, 2SD) limits the ability to analytically distinguish inter-mineral isotopic fractionations, systematic behavior of inter-mineral fractionation for both Fe and Mg is statistically observed: Δ56Feol-cpx = −0.10 ± 0.12‰ (2SD, n = 18); Δ56Feol-opx = −0.05 ± 0.11‰; Δ26Mgol-opx = −0.09 ± 0.12‰; Δ26Mgol-cpx = −0.10 ± 0.15‰. Fe and Mg isotopic composition of bulk rocks were calculated based on the modes of olivine, opx, and cpx. The average δ56Fe of peridotites in this study is 0.01 ± 0.17‰ (2SD, n = 18), similar to the values of chondrites but slightly lower than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). The average δ26Mg is −0.30 ± 0.09‰, indistinguishable from chondrites, MORB, and OIB. Our data support the conclusion that the bulk silicate Earth (BSE) has chondritic δ56Fe and δ26Mg.The origin of inter-mineral fractionations of Fe and Mg isotopic ratios remains debated. δ56Fe between the main peridotite minerals shows positive linear correlations with slopes within error of unity, strongly suggesting intra-sample mineral-mineral Fe and Mg isotopic equilibrium. Because inter-mineral isotopic equilibrium should be reached earlier than major element equilibrium via chemical diffusion at mantle temperatures, Fe and Mg isotope ratios of coexisting minerals could be useful tools for justifying mineral thermometry and barometry on the basis of chemical equilibrium between minerals. Although most peridotites in this study exhibit a narrow range in δ56Fe, the larger deviations from average δ56Fe for three samples likely indicate changes due to metasomatic processes. Two samples show heavy δ56Fe relative to the average and they also have high La/Yb and total Fe content, consistent with metasomatic reaction between peridotite and Fe-rich and isotopically heavy melt. The other sample has light δ56Fe and slightly heavy δ26Mg, which may reflect Fe-Mg inter-diffusion between peridotite and percolating melt.  相似文献   

18.
High-precision measurements of the oxygen isotopic compositions of carbonates (calcite and dolomite) from five CM2 chondrites are presented and put into context of the previously determined mineralogic alteration index (MAI), which places these meteorites into an alteration sequence. The carbonate oxygen isotopic compositions range from +20.0 to +35.7‰ for δ18O, +8.0 to +17.7‰ for δ17O, and −0.7 to −2.7‰ for Δ17O. Carbonate Δ17O values are inversely correlated with MAI and track the evolution of fluid composition from higher to lower Δ17O values with increasing alteration on the CM parent body. Similar Δ17O values for calcite and dolomite fractions from the same splits of the same meteorites indicate that calcite and dolomite in each split precipitated from a single fluid reservoir. However, reversed calcite dolomite fractionations (δ18Odol − δ18Occ) indicate that the fluid was subject to processes, such as freeze-thaw or evaporation, that fractionated isotopes in a mass-dependent way. Consideration of the carbonate isotopic data in the context of previously proposed models for aqueous alteration of carbonaceous chondrites has provided important insights into both the evolving alteration conditions and the utility of the models themselves. The data as a whole indicate that the isotopic evolution of the fluid was similar to that predicted by the closed-system, two-reservoir models, but that a slightly larger matrix-water fractionation factor may apply. In the context of this model, more altered samples largely reflect greater reaction progress and thus probably indicate more extended times of fluid exposure. Petrographic observations of carbonates reveal a trend of variable carbonate morphology correlated with alteration that is also consistent with changes in the duration of fluid-rock interaction. The data can also be reconciled with fluid-flow models in a restricted region of the parent body, which is consistent with assertions that the different types of carbonaceous chondrites derive from different regions of their parent bodies. In this case, the model results for a 9-km-radius body, and our data place the location of the CM chondrite formation in a 100-m-thick zone 1 km from the surface. The size of this zone could be increased if the model parameters were adjusted.  相似文献   

19.
Applications of speleothem calcite geochemistry in climate change studies require the evaluation of the accuracy and sensitivity of speleothem proxies to correctly infer paleoclimatic information. The present study of Harrison’s Cave, Barbados, uses the analysis of the modern climatology and groundwater system to evaluate controls on the C and O isotopic composition of modern speleothems. This new approach directly compares the δ18O and δ13C values of modern speleothems with the values for their corresponding drip waters in order to assess the degree to which isotopic equilibrium is achieved during calcite precipitation. If modern speleothems can be demonstrated to precipitate in isotopic equilibrium, then ancient speleothems, suitable for paleoclimatic studies, from the same cave environment may also have been precipitated in isotopic equilibrium. If modern speleothems are precipitated out of isotopic equilibrium, then the magnitude and direction of the C and O isotopic offsets may allow specific kinetic and/or equilibrium isotopic fractionation mechanisms to be identified.Carbon isotope values for the majority of modern speleothem samples from Harrison’s Cave fall within the range of equilibrium values predicted from the combined use of (1) calcite-water fractionation factors from the literature, (2) measured temperatures, and (3) measured δ13C values of the dissolved inorganic carbon of drip waters. Calcite samples range from ∼0.8‰ higher to ∼1.1‰ lower than predicted values. The 13C depletions are likely caused by kinetically driven departures in the fractionation between HCO3− (aq) and CaCO3 from equilibrium conditions, caused by rapid calcite growth. 13C enrichments can be accounted for by Rayleigh distillation of the HCO3− (aq) reservoir during degassing of 13C-depleted CO2.Modern speleothems from Harrison’s Cave are not in O isotopic equilibrium with their corresponding drip waters and are 0.2‰ to 2.3‰ enriched in 18O relative to equilibrium values. δ18O variations in modern calcite are likely controlled by kinetically driven changes in the fractionation between HCO3− (aq) and CaCO3 from equilibrium conditions to nonequilibrium conditions, consistent with rapid calcite growth. In contrast to δ13C, δ18O values of modern calcite may not be affected by Rayleigh distillation during degassing because CO2 hydration and hydroxylation reactions will buffer the O isotopic composition of the HCO3− (aq) reservoir. If the effects of Rayleigh distillation manifest themselves in the O isotopic system, they will result in 18O enrichment in the HCO3− (aq) reservoir and ultimately in the precipitated CaCO3.  相似文献   

20.
In order to understand spatial variations of stable isotope geochemistry in the Quruqtagh basin (northwestern China) in the aftermath of an Ediacaran glaciation, we analyzed carbonate carbon isotopes (δ13Ccarb), carbonate oxygen isotopes (δ18Ocarb), carbonate associated sulfate sulfur (δ34SCAS) and oxygen isotopes (δ18OCAS), and pyrite sulfur isotopes (δ34Spy) of a cap dolostone atop the Ediacaran Hankalchough glacial diamictite at four sections. The four studied sections (YKG, MK, H and ZBS) represent an onshore-offshore transect in the Quruqtagh basin. Our data show a strong paleobathymetry-dependent isotopic gradient. From the onshore to offshore sections, δ13Ccarb values decrease from −2‰ to −16‰ (VPDB), whereas δ18Ocarb values increase from −4‰ to −1‰ (VPDB). Both δ34SCAS and δ34Spy show stratigraphic variations in the two onshore sections (MK and YKG), but are more stable in the two offshore sections (H and ZBS). δ18OCAS values of onshore samples are consistent with terrestrial oxidative weathering of pyrite. We propose that following the Hankalchough glaciation seawater in the Quruqtagh basin was characterized by a strong isotopic gradient. The isotopic data may be interpreted using a three-component mixing model that involves three reservoirs: deep-basin water, surface water, and terrestrial weathering input. In this model, the negative δ13Ccarb values in the offshore sections are related to the upwelling of deep-basin water (where anaerobic oxidation of dissolved organic carbon resulted in 13C-depleted DIC), whereas sulfur isotope variations are strongly controlled by surface water sulfate and terrestrial weathering input derived from oxidative weathering of pyrite. The new data provide evidence for the oceanic oxidation following the Hankalchough glaciation.  相似文献   

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