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1.
Lithological, chemical, and stable isotope data are used to characterize lacustrine tufas dating back to pre-late Miocene and later unknown times, capping different surfaces of a Tertiary carbonate (Sinn el-Kedab) plateau in Dungul region in the currently hyperarid south-western Egypt. These deposits are composed mostly of calcium carbonate, some magnesium carbonate and clastic particles plus minor amounts of organic matter. They have a wide range of (Mg/Ca)molar ratios, from 0.03 to 0.3. The bulk-tufa carbonate has characteristic isotope compositions: (δ13Cmean = −2.49 ± 0.99‰; δ18Omean = −9.43 ± 1.40‰). The δ13C values are consistent with a small input from C4 vegetation or thinner soils in the recharge area of the tufa-depositing systems. The δ18O values are typical of fresh water carbonates. Covariation between δ13C and δ18O values probably is a reflection of climatic conditions such as aridity. The tufas studied are isotopically similar to the underlying diagenetic marine chalks, marls and limestones (δ13Cmean = −2.06 ± 0.84‰; δ18Omean = −10.06 ± 1.39‰). The similarity has been attributed to common meteoric water signatures. This raises large uncertainties in using tufas (Mg/Ca)molar, δ13C and δ18O records as proxies of paleoclimatic change and suggests that intrinsic compositional differences in material sources within the plateau may mask climatic changes in the records.  相似文献   

2.
The toxodont megaherbivores Toxodon and Mixotoxodon were endemic to South and Central America during the late Quaternary. Isotopic signatures of 47 toxodont teeth were analyzed to reconstruct diet and ancient habitat. Tooth enamel carbon isotope data from six regions of South and Central America indicate significant differences in toxodont diet and local vegetation during the late Quaternary. Toxodonts ranged ecologically from C3 forest browsers in the Amazon (mean δ13C = −13.4‰), to mixed C3 grazers and/or browsers living either in C3 grasslands, or mixed C3 forested and grassland habitats in Honduras (mean δ13C = −9.3‰), Buenos Aires province, Argentina (δ13C = −8.7‰), and Bahia, Brazil (mean δ13C = −8.6‰), to predominantly C4 grazers in northern Argentina (δ13C = −4.4‰), to specialized C4 grazers in the Chaco of Bolivia (δ13C = −0.1‰). Although these toxodonts had very high-crowned teeth classically interpreted for grazing, the isotopic data indicate that these megaherbivores had the evolutionary capacity to feed on a variety of dominant local vegetation. In the ancient Amazon region, carbon isotope data for the toxodonts indicate a C3-based tropical rainforest habitat with no evidence for grasslands as would be predicted from the Neotropical forest refugia hypothesis.  相似文献   

3.
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   

4.
The carbon, oxygen, and strontium isotope compositions of carbonate rocks from the upper Miocene Kudankulam Formation, southern India, were measured to understand palaeoenvironment and carbonate diagenesis of this formation. Both carbon and oxygen isotope ratios of various carbonate phases including whole rocks, ooids, molluscan mold-fill and sparry pore-fill calcite cements are depleted in 18O and 13C compared to those of contemporaneous seawater, indicating that the Kudankulam carbonates underwent extensive meteoric diagenesis. Based on δ13C and δ18O values for sparry calcite cements (pore-fill and molluscan mold-fill) formed in the meteoric diagenetic realm (δ13C from −7.8‰ to −6.0‰ and −9.0‰ to −7.0‰; δ18O from −9.2‰ to −6.5‰ and −9.4‰ to −2.6‰, respectively), it is interpreted that the diagenetic system was open and was proximal to the vadose water recharge zone. The negative δ18O values of various carbonate components (about −9.4‰ to −4.1‰ for whole rocks; about −8.4‰ to −2.6‰ for ooids) suggest that during the late Miocene the paleoclimate of the study area was humid, unlike today, probably due to the intense Indian monsoon system. The carbon isotope compositions (−7.9‰ to −3.6‰ for whole rocks; −4.9‰ to −1.5‰ for ooids) are consistent with the interpretation that the paleo-ecosystem comprised a significant proportion of C4 type plants, supporting a scenario of expansion of C4 plants during the late Miocene in the Indian subcontinent as far south as the southern tip of India. The 87Sr/86Sr ratios of the Kudankulam carbonates (0.70920 to 0.72130) are much greater than those of the contemporaneous or modern seawater (between 0.7089 and 0.7091) and show a general decrease up-sequence. Such high Sr isotope ratios indicate significant radiogenic 87Sr influx to the system from the Archean rocks exposed in the drainage area, implying that the deep-seated Archean rocks were already exposed in southern India by the late Miocene.  相似文献   

5.
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   

6.
Isotopic and chemical composition of groundwater from wells and springs, and surface water from the basalt-dominated Axum area (northern Ethiopia) provides evidence for the origin of water and dissolved species. Shallow (depth < 40 m) and deep groundwater are distinguished by both chemical and isotopic composition. Deep groundwater is significantly enriched in dissolved inorganic carbon up to 40 mmol l−1 and in concentrations of Ca2+, Mg2+, Na+ and Si(OH)4 compared to the shallow type.The δ2H and δ18O values of all solutions clearly indicate meteoric origin. Shifts from the local meteoric water line are attributed to evaporation of surface and spring water, and to strong water–rock interaction. The δ13CDIC values of shallow groundwater between −12 and −7‰ (VPDB) display the uptake of CO2 from local soil horizons, whereas δ13CDIC of deep groundwater ranges from −5 to +1‰. Considering open system conditions with respect to gaseous CO2, δ13CDIC = +1‰ of the deep groundwater with highest PCO2 = 10−0.9 atm yields δ13CCO2(gas) ≈ −5‰, which is close to the stable carbon isotopic composition of magmatic CO2. Accordingly, stable carbon isotope ratios within the above range are referred to individual proportions of CO2 from soil and magmatic origin. The uptake of magmatic CO2 results in elevated cations and Si(OH)4 concentrations. Weathering of local basalts is documented by 87Sr/86Sr ratios of the groundwater from 0.7038 to 0.7059. Highest values indicate Sr release from the basement rocks. Besides weathering of silicates, neoformation of solids has to be considered, which results in the formation of, e.g., kaolinite and montmorillonite. In several solutions supersaturation with respect to calcite is reached by outgassing of CO2 from the solution leading to secondary calcite formation.  相似文献   

7.
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   

8.
We present the first systematic study of Ca isotopes (δ44/40Ca) in Late Triassic to Late Cretaceous dinosaur bones and teeth (enamel and dentin) from sympatric herbivorous and carnivorous dinosaurs. The samples derive from five different localities, and data from embedding sediments are also presented. Additional δ44/40Ca in skeletal tissues from modern reptiles and birds (avian dinosaurs) were measured for comparison in order to examine whether the original Ca isotopic composition in dinosaur skeletal apatite was preserved or might have changed during the diagenesis and fossilization process.δ44/40Ca of fossil skeletal tissues range from −1.62‰ (Tyrannosaurus rex enamel) to +1.08‰ (Brachiosaurus brancai bone), while values in modern archosaur bones and teeth range from −1.63‰ (caiman enamel) to −0.37‰ (ostrich bone). The average δ44/40Ca of the three types of fossil skeletal tissue analyzed - bone, dentin and enamel - show some systematic differences: while δ44/40Ca in bone exhibits the highest values, while δ44/40Ca in enamel has the lowest values, and dentin δ44/40Ca falls in between. Values of δ44/40Ca in the remains of herbivorous dinosaurs (0.1-1.1‰) are generally higher than those of bones of modern mammalian herbivores (−2.6‰ to −0.8‰) and from modern herbivorous archosaurs, which exhibit intermediate δ44/40Ca (−0.8‰ to −0.4‰). These systematic isotopic shifts may reflect physiological differences between dinosaurs, mammals and reptiles representing different taxonomic groups of vertebrates.Systematic offsets in skeletal apatite δ44/40Ca between herbivorous and carnivorous dinosaurs are not obvious, indicating a lack of a clear-cut Trophic Level Effect (TLE) shift between herbivores and carnivores in dinosaurs. This observation can be explained if the carnivorous dinosaurs in this study fed mainly on soft tissues from their prey and did not ingest hard (calcified) tissue to much extent. The most striking indication that the primary δ44/40Ca is actually preserved in most of the fossil teeth is a difference in δ44/40Ca of about 0.35 ± 0.10‰ (1SD) between dentin and enamel, based upon 11 of 16 analyzed dentin-enamel pairs. This difference is close to that found in modern reptiles (0.28 ± 0.05‰), and strongly suggests that this tell-tale signature is a primary feature of the fossilized dinosaur material as well. Furthermore, simple mass balance calculations show that changes of the original δ44/40Ca in bones and teeth by diagenetically-formed calcium-bearing minerals are either small or would require implausible high original δ44/40Ca values in the skeletal apatite.  相似文献   

9.
This study is a comprehensive, stable isotope survey of the marine carbonate-dominated, upper Paleo- to lower Neoproterozoic stratigraphy of Jixian County, China. Carbonate-associated sulfate (CAS) was extracted and measured for δ34SCAS using the same samples analyzed for δ13Ccarbonate. This integrated proxy approach is a step towards a more comprehensive picture of secular variation in the composition of Proterozoic seawater. We specifically sampled marine carbonate intervals from the lower section of the Chuanlinggou Formation, Changcheng Group (ca. 1700 Ma) to the top of the Jingeryu Formation, Qingbaikou Group (ca. 800 Ma). δ13Ccarbonate values are mostly negative in the upper Paleoproterozoic Changcheng Group, with an ascending trend from −3‰ to 0‰. We observed variation of approximately 0 ± 1‰ in the Mesoproterozoic Jixian Group, and positive values of +2 ± 2‰ characterize the lower Neoproterozoic Qingbaikou Group. Stratigraphic variations in δ34SCAS are more remarkable in their ranges and magnitudes, including conspicuously high values exceeding +30‰ in the three intervals at ca. 1700 Ma, 1300-1100 Ma, and 1000-900 Ma. In the Changcheng Group, δ34SCAS values are typically higher than +25‰, with only a few values of less than +15‰. In contrast, most of the data spanning from the Mesoproterozoic Tieling Formation of the Jixian Group to the lower Neoproterozoic Jingeryu Formation of the Qingbaikou Group are highly variable between +10‰ and +25‰, with some values exceeding +25‰.In the late Paleoproterozoic (1700-1600 Ma), a >10‰ decrease in δ34SCAS and ∼3‰ increase in δ13Ccarbonate are coincident with, and likely related to, the breakup of Columbia, a supercontinent that predated Rodinia. Carbon and sulfur isotope data from the Mesoproterozoic, when global tectonic activity was comparatively weaker, fall mostly in the ranges of +15 ± 10‰ and 0 ± 1‰, respectively, but fluctuations of >20‰ for δ34SCAS and >3‰ for the δ13Ccarbonate at ca. 1450-1400 Ma may reflect subduction and large-scale magmatic activity in island arcs marking the end of Columbia breakup. From the late Mesoproterozoic (ca. 1300-1100 Ma) to the early Neoproterozoic (ca. 800 Ma), the δ13C and δ34S of seawater increased gradually with increasing variability. Most impressive areδ34SCAS values that exceed +30‰ in two intervals at ca. 1300-1100 Ma and ca. 1000-900 Ma, which may reflect the assembly and early breakup of Rodinia. Although gaps in the record remain, and studies of even higher resolution are warranted, our results suggest that changes in paleoceanographic conditions linked to global tectonics strongly influenced the biogeochemical cycles of C and S. Furthermore, periods of the Proterozoic previously noted for their isotopic invariability show clear isotopic expressions of this tectonic activity.  相似文献   

10.
The δ18O of ground water (−13.54 ± 0.05 ‰) and inorganically precipitated Holocene vein calcite (+14.56 ± 0.03 ‰) from Devils Hole cave #2 in southcentral Nevada yield an oxygen isotopic fractionation factor between calcite and water at 33.7 °C of 1.02849 ± 0.00013 (1000 ln αcalcite-water = 28.09 ± 0.13). Using the commonly accepted value of ∂(αcalcite-water)/∂T of −0.00020 K−1, this corresponds to a 1000 ln αcalcite-water value at 25 °C of 29.80, which differs substantially from the current accepted value of 28.3. Use of previously published oxygen isotopic fractionation factors would yield a calcite precipitation temperature in Devils Hole that is 8 °C lower than the measured ground water temperature. Alternatively, previously published fractionation factors would yield a δ18O of water, from which the calcite precipitated, that is too negative by 1.5 ‰ using a temperature of 33.7 °C. Several lines of evidence indicate that the geochemical environment of Devils Hole has been remarkably constant for at least 10 ka. Accordingly, a re-evaluation of calcite-water oxygen isotopic fractionation factor may be in order.Assuming the Devils Hole oxygen isotopic value of αcalcite-water represents thermodynamic equilibrium, many marine carbonates are precipitated with a δ18O value that is too low, apparently due to a kinetic isotopic fractionation that preferentially enriches 16O in the solid carbonate over 18O, feigning oxygen isotopic equilibrium.  相似文献   

11.
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   

12.
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   

13.
Sulfide mineralization in the Voisey’s Bay Intrusion, Labrador, Canada, is closely associated with country rock xenoliths that have extensively reacted with basaltic magma. In order to better understand the processes that control the assimilation of country rocks by mafic magma, a detailed study of oxygen isotope systematics related to magma-country rock interaction in the Voisey’s Bay area was undertaken. Protracted interaction of the xenoliths with magma produced refractory mineral assemblages in the xenoliths (2-10 cm in diameter) composed of Ca-rich plagioclase, corundum, hercynite, and minor magnetite. Overgrowth rims of plagioclase and biotite that surround most xenoliths separate the restites from the enclosing igneous matrix. The δ18O values of minerals from regionally metamorphosed pelitic and quartzofeldspathic protoliths are: plagioclase (8.7-12.3‰), orthoclase (9.5-9.8‰), biotite (5.2-8.7‰), garnet (8.3-10.8‰), pyroxene (8.0-10.1‰), and quartz (9.6-14.0). The δ18O values of minerals from the hornfels in the contact aureole of the intrusion are consistent with modeling which indicates that as a result of essentially closed system contact metamorphism oxygen isotope values should differ only slightly from those of the protoliths. Hercynite, plagioclase, and corundum separates from the xenoliths have δ18O values that vary from 2.9‰ to 10.5‰, 5.6‰ to 10.9‰, and 2.0‰ to 6.8‰, respectively. Although a siliceous 18O-enriched melt has been lost from the xenoliths, corundum, and feldspar δ18O values are significantly lower than expected through melt loss alone. The relatively low δ18O values of minerals from the xenoliths may be a function of incomplete isotopic exchange with surrounding mafic magma which had a δ18O value of ∼5.5‰ to 6.0‰. The high-18O melt that was released from the xenoliths is partially recorded in the plagioclase overgrowth on the margin of the xenoliths (δ18O values from 6.2‰ to 10.7‰), and in hercynite that replaced corundum. However, mass balance calculations indicate that a portion of the partial melt must have been transferred to magma that was moving through the conduit system. δ18O and δD values of biotite surrounding the plagioclase overgrowth range from 5.0‰ to 6.2‰ and −58‰ to −80‰, respectively. These data suggest that the outermost rim associated with many xenoliths has closely approached isotopic equilibrium with uncontaminated mafic magma. The current gabbroic to troctolitic matrix of the xenoliths shows no evidence for contamination by the high-18O partial melt from the xenoliths. The feldspar and biotite overgrowths on the xenoliths that formed after the motion of the xenoliths relative to the magma had stopped prevented further isotopic exchange between the xenoliths and final magma. The minerals within the xenoliths are not in oxygen isotopic equilibrium with each other, due in part to rapid thermal equilibration, partial melting, and partial exchange with flow through magma.  相似文献   

14.
Eight DSDP/ODP cores were analyzed for major ion concentrations and δ37Cl values of water-soluble chloride (δ37ClWSC) and structurally bound chloride (δ37ClSBC) in serpentinized ultramafic rocks. This diverse set of cores spans a wide range in age, temperature of serpentinization, tectonic setting, and geographic location of drilled serpentinized oceanic crust. Three of the cores were sampled at closely spaced intervals to investigate downhole variation in Cl concentration and chlorine isotope composition.The average total Cl content of all 86 samples is 0.26 ± 0.16 wt.% (0.19 ± 0.10 wt.% as water-soluble Cl (XWSC) and 0.09 ± 0.09 wt.% as structurally bound Cl (XSBC)). Structurally bound Cl concentration nearly doubles with depth in all cores; there is no consistent trend in water-soluble Cl content among the cores. Chlorine isotope fractionation between the structurally bound Cl site and the water-soluble Cl site varies from − 1.08‰ to + 1.16‰, averaging to + 0.21‰. Samples with negative fractionations may be related to reequilibration of the water-soluble chloride with seawater post-serpentinite formation. Six of the cores have positive bulk δ37Cl values (+ 0.05‰ to + 0.36‰); the other two cores (173-1068A (Leg-Hole) and 84-570) have negative bulk δ37Cl values (− 1.26‰ and − 0.54‰). The cores with negative δ37Cl values also have variable Cl / SO42 ratios, in contrast to all other cores. The isotopically positive cores (153-920D and 147-895E) show no isotopic variation with depth; the isotopically negative core (173-1068A) decreases by ∼1‰ with depth for both the water-soluble and structurally bound Cl fractions.Non-zero bulk δ37Cl values indicate Cl in serpentinites was incorporated during original hydration and is not an artifact of seawater infiltration during drilling. Cores with positive δ37Cl values are most likely explained by open system fractionation during hydrothermal alteration, with preferential incorporation of 37Cl from seawater into the serpentinite and loss of residual light Cl back to the ocean. Fluid / rock ratios were probably low as evidenced by the presence of water-soluble salts. The two isotopically negative cores are characterized by a thick overlying sedimentary package that was in place prior to serpentinization. We believe the low δ37Cl values of these cores are a result of hydration of ultramafic rock by infiltrating aqueous pore fluids from the overlying sediments. The resulting serpentinites inherit the characteristic negative δ37Cl values of the pore waters. Chlorine stable isotopes can be used to identify the source of the serpentinizing fluid and ultimately discern chemical and tectonic processes involved in serpentinization.  相似文献   

15.
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   

16.
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   

17.
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   

18.
Groundwater is the most important source of water supply in southern Tunisia. Previous hydrogeologic and isotopic studies carried out in this region revealed the existence of two major aquifer systems: the “Complex Terminal” (CT) and the “Continental Intercalaire” (CI). Turonian carbonates constitute one of the major aquifer levels of the CT multilayered aquifer. It extends over most of southern Tunisia, and its hydrodynamic regime is largely influenced by tectonics, lithology and recharge conditions. Forty-eight groundwater samples from the CI and Turonian aquifers were collected between January and April 2004 for chemical and isotopic analyses. Hydrochemistry and isotopic tools were combined to get an insight into the processes controlling chemical composition of groundwater and wide-scale interaction of these two aquifer systems. Analysis of the dissolved constituents revealed that several processes control the observed chemical composition: (i) incongruent dissolution of carbonate minerals, (ii) dissolution of evaporitic minerals, and (iii) cation exchange. Dissolution alone cannot account for the observed high supersaturation states of groundwater with respect to calcite and dolomite. The observed supersaturation is most probably linked to geogenic CO2 entering water-bearing horizons of the CT and CI aquifers via deep tectonic faults and discontinuities and subsequent degassing in the exploitation wells. Presence of geogenic CO2 in the investigated region was confirmed by C isotope data of the DIC reservoir. The radiocarbon content of the Turonian samples varied between 9.5 and 43 pmc. For CI samples generally lower values were recorded, between 3.8 and 22.5 pmc. Stable isotope composition of Turonian groundwater samples varied from −8.3 to −5.3‰ for δ18O and from −60 to −25‰ for δ2H. The corresponding ranges of δ values for the Continental Intercalaire samples were from −8.9‰ to −6.9‰ for δ18O and from −68.2‰ to −45.7‰ for δ2H. Stable isotope composition of groundwater representing CT and CI aquifers provide strong evidence for regional interaction between both systems.  相似文献   

19.
The experiments were conducted in the open CO2 system to find out the equilibrium fractionation between the carbonate ion and CO2(g). The existence of isotopic equilibrium was checked using the two-direction approach by passing the CO2−N2 gases with different δ13C compositions (− 1.5‰ and − 23‰) through the carbonate solution with δ13C = − 4.2‰. The ΔCO3T2−−CO2(g) equilibrium fractionation is given as 6.03 ± 0.17‰ at 25 °C. Discussion is provided about the significance of carbonate complexing in determination of ΔCO3T2−−CO2(g) and ΔHCO3T−CO2(g) fractionations. Finally, an isotope numerical model of flow and kinetics of hydration and dehydroxylation is built to predict the isotopic behaviour of the system with time.  相似文献   

20.
Changes in the climatic conditions during the Late Quaternary and Holocene greatly impacted the hydrology and geochemical evolution of groundwaters in the Great Lakes region. Increased hydraulic gradients from melting of kilometer-thick Pleistocene ice sheets reorganized regional-scale groundwater flow in Paleozoic aquifers in underlying intracratonic basins. Here, we present new elemental and isotopic analyses of 134 groundwaters from Silurian-Devonian carbonate and overlying glacial drift aquifers, along the margins of the Illinois and Michigan basins, to evaluate the paleohydrology, age distribution, and geochemical evolution of confined aquifer systems. This study significantly extends the spatial coverage of previously published groundwaters in carbonate and drift aquifers across the Midcontinent region, and extends into deeper portions of the Illinois and Michigan basins, focused on the freshwater-saline water mixing zones. In addition, the hydrogeochemical data from Silurian-Devonian aquifers were integrated with deeper basinal fluids, and brines in Upper Devonian black shales and underlying Cambrian-Ordovician aquifers to reveal a regionally extensive recharge system of Pleistocene-age waters in glaciated sedimentary basins. Elemental and isotope geochemistry of confined groundwaters in Silurian-Devonian carbonate and glacial drift aquifers show that they have been extensively altered by incongruent dissolution of carbonate minerals, dissolution of halite and anhydrite, cation exchange, microbial processes, and mixing with basinal brines. Carbon isotope values of dissolved inorganic carbon (DIC) range from −10 to −2‰, 87Sr/86Sr ratios range from 0.7080 to 0.7090, and δ34S-SO4 values range from +10 to 30‰. A few waters have elevated δ13CDIC values (>15‰) from microbial methanogenesis in adjacent organic-rich Upper Devonian shales. Radiocarbon ages and δ18O and δD values of confined groundwaters indicate they originated as subglacial recharge beneath the Laurentide Ice Sheet (14-50 ka BP, −15 to −13‰ δ18O). These paleowaters are isolated from shallow flow systems in overlying glacial drift aquifers by lake-bed clays and/or shales. The presence of isotopically depleted waters in Paleozoic aquifers at relatively shallow depths illustrates the importance of continental glaciation on regional-scale groundwater flow. Modern groundwater flow in the Great Lakes region is primarily restricted to shallow unconfined glacial drift aquifers. Recharge waters in Silurian-Devonian and unconfined drift aquifers have δ18O values within the range of Holocene precipitation: −11 to −8‰ and −7 to −4.5‰ for northern Michigan and northern Indiana/Ohio, respectively. Carbon and Sr isotope systematics indicate shallow groundwaters evolved through congruent dissolution of carbonate minerals under open and closed system conditions (δ13CDIC = −14.7 to−11.1‰ and 87Sr/86Sr = 0.7080-0.7103). The distinct elemental and isotope geochemistry of Pleistocene- versus Holocene-age waters further confirms that surficial flow systems are out of contact with the deeper basinal-scale flow systems. These results provide improved understanding of the effects of past climate change on groundwater flow and geochemical processes, which are important for determining the sustainability of present-day water resources and stability of saline fluids in sedimentary basins.  相似文献   

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