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1.
Radiocarbon analyses of bulk carbon and individual organic compounds are presented for the hydrothermal environment of the Rebecca’s Roost vent in the southern trough of the Guaymas Basin hydrothermal field. The Δ14C values of CO2 and CH4in the hottest hydrothermal fluids (317°C) are nearly “radiocarbon dead” (−944‰ and −923‰, respectively). In contrast, the Δ14C values of sediments and individual fatty acids (−418‰ to −227‰) obtained from a bacterial mat located south of the vent site are similar to values previously reported for hydrothermal petroleum in this environment and are more depleted in 14C than overlying waters. Hydrothermal fluids moving through the sediments appear to supply 14C of intermediate age to the bacteria. This carbon may take the form of, or may be supplied by processes similar to, the generation of hydrothermal petroleum. Although the bacterial mat visibly was dominated by Beggiatoa spp., such mats are known to include numerous other species. Individual compound data show that preaged carbon is being consumed by the integrated bacterial assemblage. Values of δ13C and Δ14C indicate that petroleum-derived carbon is incorporated directly into fresh bacterial biomass. Subsequently, some of this newly synthesized material also is consumed by heterotrophs, as eukaryotic sterols from the same sample also have 14C-depleted values (Δ14C = −136‰ to −110‰). Therefore, the entire system may operate as a complex consortium to transform relict carbon back into biomass. Bacterial consumption of relict carbon occurs despite the ample supply of fresh carbon delivered from the productive, overlying water column.  相似文献   

2.
Small rivers draining mountain islands are important in the transfer of terrestrial particulate organic carbon (POC) to the oceans. This input has implications for the geochemical stratigraphic record. We have investigated the stable isotopic composition of POC (δ13Corg) in rivers draining the mountains of Taiwan. In 15 rivers, the suspended load has a mean δ13Corg that ranges from −28.1±0.8‰ to −22.0±0.2‰ (on average 37 samples per river) over the interval of our study. To investigate this variability we have supplemented suspended load data with measurements of POC in bedrock and river bed materials, and constraints on the composition of the terrestrial biomass. Fossil POC in bedrock has a range in δ13Corg from −25.4±1.5‰ to −19.7±2.3‰ between the major geological formations. Using coupled δ13Corg and N/C we have found evidence in the suspended load for mixing of fossil POC with non-fossil POC from the biosphere. In two rivers outside the Taiwan Central Range anthropogenic land use appears to influence δ13Corg, resulting in more variable and lower values than elsewhere. In all other catchments, we have found that 5‰ variability in δ13Corg is not controlled by the variable composition of the biomass, but instead by heterogeneous fossil POC.In order to quantify the fraction of suspended load POC derived from non-fossil sources (Fnf) as well as the isotopic composition of fossil POC (δ13Cfossil) carried by rivers, we adapt an end-member mixing model. River suspended sediments and bed sediments indicate that mixing of fossil POC results in a negative trend between N/C and δ13Corg that is distinct from the addition of non-fossil POC, collapsing multiple fossil POC end-members onto a single mixing trend. As an independent test of the model, Fnf reproduces the fraction modern (Fmod) in our samples, determined from 14C measurements, to within 0.09 at the 95% confidence level. Over the sampling period, the mean Fnf of suspended load POC was low (0.29 ± 0.02, n = 459), in agreement with observations from other mountain rivers where physical erosion rates are high and fossil POC enters river channels. The mean δ13Cfossil in suspended POC varied between −25.2±0.5‰ and −20.2±0.6‰ from catchment to catchment. This variability is primarily controlled by the distribution of the major geological formations. It also covers entirely the range of δ13Corg found in marine sediments which is commonly thought to derive from mixing between marine and terrigenous POC. If land-sourced POC is preserved in marine sediments, then changes in the bulk δ13Corg observed offshore Taiwan could instead be explained by changes in the onshore provenance of sediment. The range in δ13Corg of fossil organic matter in sedimentary rocks exposed at the surface is large and given the importance of these rocks as a source of clastic sediment to the oceans, care should be taken in accounting for fossil POC in marine deposits supplied by active mountain belts.  相似文献   

3.
We examined oxygen three-isotope ratios of 48 extraterrestrial chromite (EC) grains extracted from mid-Ordovician sediments from two different locations in Sweden, and one location in south-central China. The ages of the sediments (∼470 Ma) coincide with the breakup event of the L chondrite parent asteroid. Elemental compositions of the chromite grains are generally consistent with their origin from L or LL chondrite parent bodies. The average Δ17O (‰-deviation from the terrestrial mass-fractionation line, measured in situ from 15 μm spots by secondary ion mass spectrometry; SIMS) of EC grains extracted from fossil meteorites from Thorsberg and Brunflo are 1.17 ± 0.09‰ (2σ) and 1.25 ± 0.16‰, respectively, and those of fossil micrometeorites from Thorsberg and Puxi River are 1.10 ± 0.09‰, and 1.11 ± 0.12‰, respectively. Within uncertainty these values are all the same and consistent with the L chondrite group average Δ17O = 1.07 ± 0.18‰, but also with the LL chondrite group average Δ17O = 1.26 ± 0.24‰ (Clayton et al., 1991). We conclude that the studied EC grains from correlated sediments from Sweden and China are related, and most likely originated in the same event, the L chondrite parent body breakup. We also analyzed chromites of modern H, L and LL chondrites and show that their Δ17O values coincide with averages of Δ17O of bulk analyses of H, L and LL chondrites. This study demonstrates that in situ oxygen isotope data measured by SIMS are accurate and precise if carefully standardized, and can be used to classify individual extraterrestrial chromite grains found in sediments.  相似文献   

4.
Ocean drilling has revealed the existence of vast microbial populations in the deep subseafloor, but to date little is known about their metabolic activities. To better understand the biogeochemical processes in the deep biosphere, we investigate the stable carbon isotope chemistry of acetate and other carbon-bearing metabolites in sediment pore-waters. Acetate is a key metabolite in the cycling of carbon in anoxic sediments. Its stable carbon isotopic composition provides information on the metabolic processes dominating acetate turnover in situ. This study reports our findings for a methane-rich site at the northern Cascadia Margin (NE Pacific) where Expedition 311 of the Integrated Ocean Drilling Program (IODP) sampled the upper 190 m of sediment. At Site U1329, δ13C values of acetate span a wide range from −46.0‰ to −11.0‰ vs. VPDB and change systematically with sediment depth. In contrast, δ13C values of both the bulk dissolved organic carbon (DOC) (−21.6 ± 1.3‰ vs. VPDB) and the low-molecular-weight compound lactate (−20.9 ± 1.8‰ vs. VPDB) show little variability. These species are interpreted to represent the carbon isotopic composition of fermentation products. Relative to DOC, acetate is up to 23.1‰ depleted and up to 9.1‰ enriched in 13C. Broadly, 13C-depletions of acetate relative to DOC indicate flux of carbon from acetogenesis into the acetate pool while 13C-enrichments of pore-water acetate relative to DOC suggest consumption of acetate by acetoclastic methanogenesis. Isotopic relationships between acetate and lactate or DOC provide new information on the carbon flow and the presence and activity of specific functional microbial communities in distinct biogeochemical horizons of the sediment. In particular, they suggest that acetogenic CO2-reduction can coexist with methanogenic CO2-reduction, a notion contrary to the hypothesis that hydrogen levels are controlled by the thermodynamically most favorable electron-accepting process. Further, the isotopic relationship suggests a relative increase in acetate flow to acetoclastic methanogenesis with depth although its contribution to total methanogenesis is probably small. Our study demonstrates how the stable carbon isotope biogeochemistry of acetate can be used to identify pathways of microbial carbon turnover in subsurface environments. Our observations also raise new questions regarding the factors controlling acetate turnover in marine sediments.  相似文献   

5.
Molybdenum (Mo) isotope studies in black shales can provide information about the redox evolution of the Earth’s oceans, provided the isotopic consequences of Mo burial into its major sinks are well understood. Previous applications of the Mo isotope paleo-ocean redox proxy assumed quantitative scavenging of Mo when buried into sulfidic sediments. This paper contains the first complete suite of Mo isotope fractionation observations in a sulfidic water column and sediment system, the meromictic Lake Cadagno, Switzerland, a small alpine lake with a pronounced oxygen-sulfide transition reaching up to H2S ∼ 200 μM in the bottom waters (or about 300 μM total sulfide: ΣS2− = H2S + HS− + S2−). We find that Mo behaves conservatively in the oxic zone and non-conservatively in the sulfidic zone, where dissolved Mo concentrations decrease from 14 nM to 2-8 nM across this transition. Dissolved Mo in the upper oxic waters has a δ98Mooxic = 0.9 ± 0.1‰, which matches that of the riverine input, δ98Moriver = 0.9 ± 0.1‰. In the deeper sulfidic waters, a subaquatic source delivers Mo at 1.55 ± 0.1‰, but the dissolved Mo is even heavier at δ98Mosulfidic = 1.8‰. Sediment traps in the sulfidic zone of the lake collect particles increasingly enriched in Mo with depth, with δ98Mo values significantly fractionated at −0.8‰ to −1.2‰ both near the chemocline and in the deepest trap. Suspended particulates in the sulfidic waters carry lighter Mo than the ambient dissolved Mo pool by ∼0.3-1.5‰. Sedimentary Mo concentrations correlate with total organic carbon and yield Mo levels which are two orders of magnitude higher than typical crustal values found in rocks from the catchment area. Solid-phase Mo in the sediment shows a slightly positive δ98Mo trend with depth, from δ98Mo = 1.2‰ to 1.4‰ while the pore waters show dramatic enrichments of Mo (>2000 nM) with a relatively light isotope signature of δ98Mo = 0.9-1.0‰.These data are explained if Mo is converted to particle-reactive oxythiomolybdates in the sulfidic waters and is fractionated during removal from solution onto particles. Isotope fractionation is expressed in the water column, despite the high sulfide concentrations, because the rate of Mo removal is fast compared to the slow reaction kinetics of thiomolybdate formation. However, elemental and isotopic mass balances show that Mo is indeed quantitatively removed to the lake sediments and thus the isotopic composition of the sediments reflects sources to the sulfidic water. This efficient Mo drawdown is expected to occur in settings where H2S is very much in excess over Mo or in a restricted setting where the water renewal rate is slow compared to the Mo burial rate. We present a model for the Mo isotope fractionation in sulfidic systems associated with the slow reaction kinetics and conclude that quantitative removal will occur in highly sulfidic and restricted marine systems.  相似文献   

6.
The Po River (Italy) experienced a 100-year flood in October 2000. Surface sediments (0-1 cm) from cross-shelf transects were collected in the Po prodelta area (Adriatic Sea) in December 2000, in order to describe the distribution of organic matter (OM) along the main sediment dispersal system immediately after the flood event. Stations were subsequently reoccupied in October 2001 and April 2002. This sampling program provided a special opportunity to characterize the initial surficial flood deposit and the evolution of its associated OM over the course of 2 years. CuO oxidation, elemental, δ13C, Δ14C, and grain-size analyses were carried out to characterize the source, age, and spatial variability of sedimentary OM. Statistical analysis (PERMANOVA) was then applied to investigate temporal changes in different portions of the Po prodelta area. Isotopic and biomarker data suggest that the sedimentary OM in the flood deposit was initially dominated by aged (Δ14CDec-00 = −298.7 ± 56.3‰), lignin-poor OM (ΛDec-00 = 1.96 ± 0.33 mg/100 mg OC), adsorbed on the fine material (clayDec-00 = 72.1 ± 4.8%) delivered by the flood. In the 2 years following the flood, post-depositional processes significantly increased the content of lignin (ΛOct-01 = 2.19 ± 0.51 mg/100 mg OC; ΛApr-02 = 2.61 ± 0.63 mg/100 mg OC); and coarse material (silt and sand), while decreasing the contributions from aged OC (Δ14COct-01 = −255.7 ± 32.8‰; Δ14CApr-02 = −213.2 ± 30.4‰) and fine fraction (clayOct-01 = 54.8 ± 9.5%; clayApr-02 = 44.6 ± 13.3%). The major changes were observed in the northern and central portions of the prodelta.  相似文献   

7.
Holocene sediments from the Gotland Deep basin in the Baltic Sea were investigated for their Fe isotopic composition in order to assess the impact of changes in redox conditions and a transition from freshwater to brackish water on the isotope signature of iron. The sediments display variations in δ56Fe (differences in the 56Fe/54Fe ratio relative to the IRMM-14 standard) from −0.27 ± 0.09‰ to +0.21 ± 0.08‰. Samples deposited in a mainly limnic environment with oxygenated bottom water have a mean δ56Fe of +0.08 ± 0.13‰, which is identical to the mean Fe isotopic composition of igneous rocks and oxic marine sediments. In contrast, sediments that formed in brackish water under periodically euxinic conditions display significantly lighter Fe isotope signatures with a mean δ56Fe of −0.14 ± 0.19‰. Negative correlations of the δ56Fe values with the Fe/Al ratio and S content of the samples suggest that the isotopically light Fe in the periodically euxinic samples is associated with reactive Fe enrichments and sulfides. This is supported by analyses of pyrite separates from this unit that have a mean Fe isotopic composition of −1.06 ± 0.20‰ for δ56Fe. The supply of additional Fe with a light Fe isotopic signature can be explained with the shelf to basin Fe shuttle model. According to the Fe shuttle model, oxides and benthic ferrous Fe that is derived from dissimilatory iron reduction from shelves is transported and accumulated in euxinic basins. The data furthermore suggest that the euxinic water has a negative dissolved δ56Fe value of about −1.4‰ to −0.9‰. If negative Fe isotopic signatures are characteristic for euxinic sediment formation, widespread euxinia in the past might have shifted the Fe isotopic composition of dissolved Fe in the ocean towards more positive δ56Fe values.  相似文献   

8.
We examined stable carbon isotope fractionation in biosynthesis of fatty acids of a piezophilic bacterium Moritella japonica strain DSK1. The bacterium was grown to stationary phase at pressures of 0.1, 10, 20, and 50 MPa in media prepared using sterile-filtered natural seawater supplied with glucose as the sole carbon source. Strain DSK1 synthesized typical bacterial fatty acids (C14-19 saturated, monounsaturated, and cyclopropane fatty acids) as well as long-chain polyunsaturated fatty acids (PUFA) (20:6ω3). Bacterial cell biomass and individual fatty acids exhibited consistent pressure-dependent carbon isotope fractionations relative to glucose. The observed ΔδFA-glucose (−1.0‰ to −11.9‰) at 0.1 MPa was comparable to or slightly higher than fractionations reported in surface bacteria. However, bulk biomass and fatty acids became more depleted in 13C with pressure. Average carbon isotope fractionation (ΔδFA-glucose) at high pressures was much higher than that for surface bacteria: −15.7‰, −15.3‰, and −18.3‰ at 10, 20, and 50 MPa, respectively. PUFA were more 13C depleted than saturated and monounsaturated fatty acids at all pressures. The observed isotope effects may be ascribed to the kinetics of enzymatic reactions that are affected by hydrostatic pressure and to biosynthetic pathways that are different for short-chain and long-chain fatty acids. A simple quantitative calculation suggests that in situ piezophilic bacterial contribution of polyunsaturated fatty acids to marine sediments is nearly two orders of magnitude higher than that of marine phytoplankton and that the carbon isotope imprint of piezophilic bacteria can override that of surface phytoplankton. Our results have important implications for marine biogeochemistry. Depleted fatty acids reported in marine sediments and the water column may be derived simply from piezophilic bacteria resynthesis of organic matter, not from bacterial utilization of a 13C-depleted carbon source (i.e., methane). The interpretation of carbon isotope signatures of marine lipids must be based on principles derived from piezophilic bacteria.  相似文献   

9.
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   

10.
An 18 million year record of the Ca isotopic composition (δ44/42Ca) of planktonic foraminiferans from ODP site 925, in the Atlantic, on the Ceara Rise, provides the opportunity for critical analysis of Ca isotope-based reconstructions of the Ca cycle. δ44/42Ca in this record averages +0.37 ± 0.05 (1σ SD) and ranges from +0.21‰ to +0.52‰. The record is a good match to previously published Neogene Ca isotope records based on foraminiferans, but is not similar to the record based on bulk carbonates, which has values that are as much as 0.25‰ lower. Bulk carbonate and planktonic foraminiferans from core tops differ slightly in their δ44/42Ca (i.e., by 0.06 ± 0.06‰ (n = 5)), while the difference between bulk carbonate and foraminiferan values further back in time is markedly larger, leaving open the question of the cause of the difference. Modeling the global Ca cycle from downcore variations in δ44/42Ca by assuming fixed values for the isotopic composition of weathering inputs (δ44/42Caw) and for isotope fractionation associated with the production of carbonate sediments (Δsed) results in unrealistically large variations in the total mass of Ca2+ in the oceans over the Neogene. Alternatively, variations of ±0.05‰ in the Ca isotope composition of weathering inputs or in the extent of fractionation of Ca isotopes during calcareous sediment formation could entirely account for variations in the Ca isotopic composition of marine carbonates. Ca isotope fractionation during continental weathering, such as has been recently observed, could easily result in variations in δ44/42Caw of a few tenths of permil. Likewise a difference in the fractionation factors associated with aragonite versus calcite formation could drive shifts in Δsed of tenths of permil with shifts in the relative output of calcite and aragonite from the ocean. Until better constraints on variations in δ44/42Caw and Δsed have been established, modeling the Ca2+ content of seawater from Ca isotope curves should be approached cautiously.  相似文献   

11.
Iron isotope compositions in marine pore fluids and sedimentary solid phases were measured at two sites along the California continental margin, where isotope compositions range from δ56Fe = −3.0‰ to +0.4‰. At one site near Monterey Canyon off central California, organic matter oxidation likely proceeds through a number of diagenetic pathways that include significant dissimilatory iron reduction (DIR) and bacterial sulfate reduction, whereas at our other site in the Santa Barbara basin DIR appears to be comparatively small, and production of sulfides (FeS and pyrite) was extensive. The largest range in Fe isotope compositions is observed for Fe(II)aq in porewaters, which generally have the lowest δ56Fe values (minimum: −3.0‰) near the sediment surface, and increase with burial depth. δ56Fe values for FeS inferred from HCl extractions vary between ∼−0.4‰ and +0.4‰, but pyrite is similar at both stations, where an average δ56Fe value of −0.8 ± 0.2‰ was measured. We interpret variations in dissolved Fe isotope compositions to be best explained by open-system behavior that involves extensive recycling of Feflux. This study is the first to examine Fe isotope variations in modern marine sediments, and the results show that Fe isotopes in the various reactive Fe pools undergo isotopic fractionation during early diagenesis. Importantly, processes dominated by sulfide formation produce high-δ56Fe values for porewaters, whereas the opposite occurs when Fe(III)-oxides are present and DIR is a major pathway of organic carbon respiration. Because shelf pore fluids may carry a negative δ56Fe signature it is possible that the Fe isotope composition of ocean water reflects a significant contribution of shelf-derived iron to the open ocean. Such a signature would be an important means for tracing iron sources to the ocean and water mass circulation.  相似文献   

12.
The concentrations of mercury (Hg) and other trace metals (Ni, Cu, Zn, Mo, Ba, Re, U) and the Hg isotopic composition were examined across a dramatic redox and productivity transition in a mid-Pleistocene Mediterranean Sea sapropel sequence. Characteristic trace metal enrichment in organic-rich layers was observed, with organic-rich sapropel layers ranging in Hg concentration from 314 to 488 ng/g (avg = 385), with an average enrichment in Hg by a factor of 5.9 compared to organic-poor background sediments, which range from 39 to 94 ng/g Hg (avg = 66). Comparison of seawater concentrations and sapropel accumulations of trace metals suggests that organic matter quantitatively delivers Hg to the seafloor. Near complete scavenging of Hg from the water column renders the sapropel Hg isotopic composition representative of mid-Pleistocene Mediterranean seawater. Sapropels have an average δ202Hg value of −0.91‰ ± 0.15‰ (n = 5, 1 SD) and Δ199Hg value of 0.11‰ ± 0.03‰ (n = 5, 1 SD). Background sediments have an average δ202Hg of −0.76‰ ± 0.16‰ (n = 5, 1 SD) and Δ199Hg of 0.05‰ ± 0.01‰ (n = 5, 1 SD), which is indistinguishable from the sapropel values. We suggest that the sapropel isotopic composition is most representative of the mid-Pleistocene Tyrrhenian Sea.  相似文献   

13.
Microorganisms are ubiquitous in deep subsurface environments, but their role in the global carbon cycle is not well-understood. The natural abundance δ13C and Δ14C values of microbial membrane phospholipid fatty acids (PLFAs) were measured and used to assess the carbon sources of bacteria in sedimentary and granitic groundwaters sampled from three boreholes in the vicinity of the Tono Uranium Mine, Gifu, Japan. Sample storage experiments were performed and drill waters analyzed to characterize potential sources of microbial contamination. The most abundant PLFA structures in all waters sampled were 16:0, 16:1ω7c, cy17:0, and 18:1ω7c. A PLFA biomarker for type II methanotrophs, 18:1ω8c, comprised 3% and 18% of total PLFAs in anoxic sedimentary and granitic waters, respectively, sampled from the KNA-6 borehole. The presence of this biomarker was unexpected given that type II methanotrophs are considered obligate aerobes. However, a bacterium that grows aerobically with CH4 as the sole energy source and which also produces 56% of its total PLFAs as 18:1ω8c was isolated from both waters, providing additional evidence for the presence of type II methanotrophs. The Δ14C values determined for type II methanotroph PLFAs in the sedimentary (−861‰) and granite (−867‰) waters were very similar to the Δ14C values of dissolved inorganic carbon (DIC) in each water (∼−850‰). This suggests that type II methanotrophs ultimately derive all their carbon from inorganic sources, whether directly from DIC and/or from CH4 produced by the reduction of DIC. In contrast, δ13C values of type II PLFAs in the sedimentary (−93‰) and granite (−60‰) waters indicate that these organisms use different carbon assimilation schemes in each environment despite very similar δ13CCH4 values (∼−95‰) for each water. The δ13CPLFA values (−28‰ to −45‰) of non-methanotrophic bacteria in the KNA-6 LTL water do not clearly distinguish between heterotrophic and autotrophic metabolisms, but Δ14CPLFA values indicate that >65% of total bacteria filtered from the KNA-6 LTL water are heterotrophs. Ancient Δ14C values (∼−1000‰) of some PLFAs suggest that many heterotrophs utilize ancient organic matter, perhaps from lignite seams within the sedimentary rocks. The more negative range of δ13CPLFA values determined for the KNA-6 granitic water (−42‰ to −66‰) are likely the result of a microbial ecosystem dominated by chemolithoautotrophy, perhaps fuelled by abiogenic H2. Results of sample storage experiments showed substantial shifts in microbial community composition and δ13CPLFA values (as much as 5‰) during 2-4 days of dark, refrigerated, aseptic storage. However, water samples collected and immediately filtered back in the lab from freshly drilled MSB-2 borehole appeared to maintain the same relative relationships between δ13CPLFA values for sedimentary and granitic host rocks as observed for samples directly filtered under artesian flow from the KNA-6 borehole of the Tono Uranium Mine.  相似文献   

14.
We analyzed the deuterium composition of individual plant-waxes in lake sediments from 28 watersheds that span a range of precipitation D/H, vegetation types and climates. The apparent isotopic fractionation (εa) between plant-wax n-alkanes and precipitation differs with watershed ecosystem type and structure, and decreases with increasing regional aridity as measured by enrichment of 2H and 18O associated with evaporation of lake waters. The most negative εa values represent signatures least affected by aridity; these values were −125 ± 5‰ for tropical evergreen and dry forests, −130‰ for a temperate broadleaf forest, −120 ± 9‰ for the high-altitude tropical páramo (herbs, shrubs and grasses), and −98 ± 6‰ for North American montane gymnosperm forests. Minimum εa values reflect ecosystem-dependent differences in leaf water enrichment and soil evaporation. Slopes of lipid/lake water isotopic enrichments differ slightly with ecosystem structure (i.e. open shrublands versus forests) and overall are quite small (slopes = 0-2), indicating low sensitivity of lipid δD variations to aridity compared with coexisting lake waters. This finding provides an approach for reconstructing ancient precipitation signatures based on plant-wax δD measurements and independent proxies for lake water changes with regional aridity. To illustrate this approach, we employ paired plant-wax δD and carbonate-δ18O measurements on lake sediments to estimate the isotopic composition of Miocene precipitation on the Tibetan plateau.  相似文献   

15.
Copper isotope fractionation in acid mine drainage   总被引：4，自引：0，他引：4  
We measured the Cu isotopic composition of primary minerals and stream water affected by acid mine drainage in a mineralized watershed (Colorado, USA). The δ65Cu values (based on 65Cu/63Cu) of enargite (δ65Cu = −0.01 ± 0.10‰; 2σ) and chalcopyrite (δ65Cu = 0.16 ± 0.10‰) are within the range of reported values for terrestrial primary Cu sulfides (−1‰ < δ65Cu < 1‰). These mineral samples show lower δ65Cu values than stream waters (1.38‰ ? δ65Cu ? 1.69‰). The average isotopic fractionation (Δaq-min = δ65Cuaq − δ65Cumin, where the latter is measured on mineral samples from the field system), equals 1.43 ± 0.14‰ and 1.60 ± 0.14‰ for chalcopyrite and enargite, respectively. To interpret this field survey, we leached chalcopyrite and enargite in batch experiments and found that, as in the field, the leachate is enriched in 65Cu relative to chalcopyrite (1.37 ± 0.14‰) and enargite (0.98 ± 0.14‰) when microorganisms are absent. Leaching of minerals in the presence of Acidithiobacillus ferrooxidans results in smaller average fractionation in the opposite direction for chalcopyrite (Δaq-mino=-0.57±0.14‰, where mino refers to the starting mineral) and no apparent fractionation for enargite (Δaq-mino=0.14±0.14‰). Abiotic fractionation is attributed to preferential oxidation of 65Cu+ at the interface of the isotopically homogeneous mineral and the surface oxidized layer, followed by solubilization. When microorganisms are present, the abiotic fractionation is most likely not seen due to preferential association of 65Cuaq with A. ferrooxidans cells and related precipitates. In the biotic experiments, Cu was observed under TEM to occur in precipitates around bacteria and in intracellular polyphosphate granules. Thus, the values of δ65Cu in the field and laboratory systems are presumably determined by the balance of Cu released abiotically and Cu that interacts with cells and related precipitates. Such isotopic signatures resulting from Cu sulfide dissolution should be useful for acid mine drainage remediation and ore prospecting purposes.  相似文献   

16.
The equilibrium Mg isotope fractionation factor between epsomite and aqueous MgSO4 solution has been measured using the three isotope method in recrystallization experiments conducted at 7, 20, and 40 °C. Complete or near-complete isotopic exchange was achieved within 14 days in all experiments. The Mg isotope exchange rate between epsomite and MgSO4 solution is dependent on the temperature, epsomite seed crystal grain size, and experimental agitation method. The Mg isotope fractionation factors (Δ26Mgeps-sol) at 7, 20, and 40 °C are 0.63 ± 0.07‰, 0.58 ± 0.16‰, and 0.56 ± 0.03‰, respectively. These values are indistinguishable within error, indicating that the Mg isotope composition of epsomite is relatively insensitive to temperature. The magnitude of the isotope fractionation factor (Δ26Mgeps-sol = ca. 0.6‰ between 7 and 40 °C) indicates that significant Mg isotope variations can be produced in evaporite sequences, and Mg isotopes may therefore, constrain the degree of closed-system behavior, paleo-humidity, and hydrological history of evaporative environments.  相似文献   

17.
Unraveling the factors controlling the carbon chemistry and transport of carbon within extant karst systems has important implications concerning the assessment of time-series δ13C records of speleothems. Here we report the results of a 3-year study of total dissolved inorganic carbon [DIC] and δ13CDIC from cave waters at DeSoto Caverns (Southeastern USA) that offer valuable insight on carbon transport and the accompanied isotope fractionations from end-member sources to speleothems.[DIC] and δ13CDIC values of cave waters range from 0.2 to 6.0 mM and 2.7 to −12.9 (‰ VPDB), respectively. [DIC] and δ13CDIC of “seasonal drips” show seasonal, albeit noisy, variability and are inversely related (δ13CDIC = −2.49[DIC] + 0.64, r2 = 0.84). A shallow pool fed by multiple drips shows a bimodal δ13CDIC distribution with an isotopically heavier mode during winter (−4‰ to −5‰ VPDB) relative to summer months (−9‰ to −10‰ VPDB). A multi-year trend of decreasing water availability during the study period is not reflected in a response of cave water carbon chemistry suggesting that rainfall amount may not be a significant controlling factor of the carbon chemistry. Coupled cave air winter ventilation/summer stagnation and varying CO2 fluxes through the soil horizon and epikarst exert the strongest influence on seasonal [DIC] and δ13CDIC variability. Measured values of high [DIC] and low δ13CDIC from cave waters collected during the summer/early fall closely approximate isotopic equilibrium conditions. Conversely, low [DIC] and high δ13CDIC values during winter/early months indicate kinetically enhanced isotopic fractionations within the cave waters. The kinetically enhanced isotopic fractionation of partitioned between degassed CO2 and precipitated CaCO3(1000lnα[(CO2-HCO3)+(CaCO3(AR)-HCO3)]/2) is greater by about a factor of two (−6.7 ± 0.3‰) relative to the same isotopic fractionation under equilibrium conditions (−3.1‰).On the basis of 14C mass balance and paired 14C-U/Th measurements we estimate that on average about ∼23% of C delivered annually by the drips to the aragonite stalagmites is derived from 14C-dead dolomite cap while the remainder of ∼77% is derived from 14C-live biomass. δ13C measurements of aragonite (n = 12) sampled from the tips of active speleothems during the summer months are consistent with theoretical aragonite δ13C values calculated using the shallow pool summer/early fall data thus confirming the δ13C seasonality in both drips and coeval aragonite. δ13C values of an active stalagmite section spanning the last 200 years show a normal distribution with a mean of −7.1 ± 1.2‰ (n = 81) and a mode of −7‰ to −8‰ that are statistically indistinguishable from the annual mean and mode of all dripwaters. Thus secular time-series δ13C records of stalagmites at DeSoto Caverns with resolving power >10−1 year will likely carry the imprints of drip annual means that record climate-driven δ13C seasonal biases.  相似文献   

18.
Silica phytoliths, which are deposits of opal-A that precipitate in the intra- and intercellular spaces of plant tissues during transpiration, commonly contain small amounts of occluded organic matter. In this paper, we investigate whether the δ13C values of phytoliths from a C4 grass, Calamovilfa longifolia, vary in response to climatic variables that can affect the carbon-isotope composition of plant tissues. There is no significant correlation (r2 < 0.3) between climate variables and the δ13C values of C. longifolia tissues (average δ13Ctissue = −13.1 ± 0.6 ‰; n = 70) across the North American prairies. However, plant tissue δ13C values are lower for grasses collected in populated areas where the δ13C value of atmospheric CO2 is expected to be lower because of fossil fuel burning. Phytolith δ13C values are more variable (δ13C = −27.3 to −23.0‰; average = −25.1 ± 1.3‰; n = 34) and more sensitive to changes in aridity than whole tissue δ13C values. The strongest correlations are obtained between the δ13C values of stem or sheath phytoliths and humidity (r2 = 0.3), latitude (r2 = 0.4) and amount of precipitation (r2 = 0.5). However, use of these relationships is limited by the wide spread in δ13C values of phytoliths from different plant tissues at the same location. We have been unable to infer any relationship between δ13C values of phytoliths and expected variations in the δ13C values of atmospheric CO2. The C. longifolia phytoliths are depleted of 13C relative to tissue carbon by 10-14‰. This means that the phytoliths examined in this study have carbon isotopic compositions within the range reported previously for phytoliths from C3 plants. This observation may further limit the usefulness of soil-phytolith assemblage δ13C values for identifying shifts in grassland C3:C4 ratios.  相似文献   

19.
Boron isotope compositions (δ11B) and B concentrations of rains and snows were studied in order to characterize the sources and fractionation processes during the boron atmospheric cycle. The 11B/10B ratios of instantaneous and cumulative rains and snows from coastal and continental sites show a large range of variations, from −1.5 ± 0.4 to +26.0 ± 0.5‰ and from −10.2 ± 0.5 to +34.4 ± 0.2‰, respectively. Boron concentrations in rains and snows vary between 0.1 and 3.0 ppb. All these precipitation samples are enriched in 10B compared to the ocean value (δ11B = +39.5‰). An empirical rain-vapour isotopic fractionation of +31‰ is estimated from three largely independent methods. The deduced seawater-vapour fractionation is +25.5‰, with the difference between the rain and seawater fractionations principally reflecting changes in the speciation of boron in the liquid with ∼100% B(OH)3 present in precipitations. A boron meteoric water line, δD = 2.6δ11B − 133, is proposed which describes the relationship between δD and δ11B in many, but not all, precipitations. Boron isotopic compositions of precipitations can be related to that of the seawater reservoir by the seawater-vapour fractionation and one or more of (1) the rain-vapour isotopic fractionation, (2) evolution of the δ11B value of the atmospheric vapour reservoir via condensation-precipitation processes (Rayleigh distillation process), (3) any contribution of vapour from the evaporation of seawater aerosols, and (4) any contribution from particulate matter, principally sea salt, continental dust and, perhaps more regionally, anthropogenic sources (burning of biomass and fossil fuels). From the δ11B values of continental precipitations, a sea salt contribution cannot be more than a percent or so of the total B in precipitation over these areas.  相似文献   

20.
Atmospheric carbon dioxide is widely studied using records of CO2 mixing ratio, δ13C and δ18O. However, the number and variability of sources and sinks prevents these alone from uniquely defining the budget. Carbon dioxide having a mass of 47 u (principally 13C18O16O) provides an additional constraint. In particular, the mass 47 anomaly (Δ47) can distinguish between CO2 produced by high temperature combustion processes vs. low temperature respiratory processes. Δ47 is defined as the abundance of mass 47 isotopologues in excess of that expected for a random distribution of isotopes, where random distribution means that the abundance of an isotopologue is the product of abundances of the isotopes it is composed of and is calculated based on the measured 13C and 18O values. In this study, we estimate the δ13C (vs. VPDB), δ18O (vs. VSMOW), δ47, and Δ47 values of CO2 from car exhaust and from human breath, by constructing ‘Keeling plots’ using samples that are mixtures of ambient air and CO2 from these sources. δ47 is defined as , where is the R47 value for a hypothetical CO2 whose δ13CVPDB = 0, δ18OVSMOW = 0, and Δ47 = 0. Ambient air in Pasadena, CA, where this study was conducted, varied in [CO2] from 383 to 404 μmol mol−1, in δ13C and δ18O from −9.2 to −10.2‰ and from 40.6 to 41.9‰, respectively, in δ47 from 32.5 to 33.9‰, and in Δ47 from 0.73 to 0.96‰. Air sampled at varying distances from a car exhaust pipe was enriched in a combustion source having a composition, as determined by a ‘Keeling plot’ intercept, of −24.4 ± 0.2‰ for δ13C (similar to the δ13C of local gasoline), δ18O of 29.9 ± 0.4‰, δ47 of 6.6 ± 0.6‰, and Δ47 of 0.41 ± 0.03‰. Both δ18O and Δ47 values of the car exhaust end-member are consistent with that expected for thermodynamic equilibrium at∼200 °C between CO2 and water generated by combustion of gasoline-air mixtures. Samples of CO2 from human breath were found to have δ13C and δ18O values broadly similar to those of car exhaust-air mixtures, −22.3 ± 0.2 and 34.3 ± 0.3‰, respectively, and δ47 of 13.4 ± 0.4‰. Δ47 in human breath was 0.76  ± 0.03‰, similar to that of ambient Pasadena air and higher than that of the car exhaust signature.  相似文献   

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