Controls on dissolved inorganic carbon and δC in cave waters from DeSoto Caverns: Implications for speleothem δC assessments     

W. Joe Lambert  Paul Aharon 《Geochimica et cosmochimica acta》2011,75(3):753-768

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 δ¹³C_DIC 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 δ¹³C_DIC values of cave waters range from 0.2 to 6.0 mM and 2.7 to −12.9 (‰ VPDB), respectively. [DIC] and δ¹³C_DIC of “seasonal drips” show seasonal, albeit noisy, variability and are inversely related (δ¹³C_DIC = −2.49[DIC] + 0.64, r² = 0.84). A shallow pool fed by multiple drips shows a bimodal δ¹³C_DIC 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 δ¹³C_DIC variability. Measured values of high [DIC] and low δ¹³C_DIC from cave waters collected during the summer/early fall closely approximate isotopic equilibrium conditions. Conversely, low [DIC] and high δ¹³C_DIC 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   

New isotopic evidence for the origin of groundwater from the Nubian Sandstone Aquifer in the Negev,Israel     

Avner Vengosh  Sharona Hening  Jiwchar Ganor  Bernhard Mayer  Constanze E. Weyhenmeyer  Thomas D. Bullen  Adina Paytan 《Applied Geochemistry》2007

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The role of sulfur in chemical weathering and atmospheric CO₂ fluxes: Evidence from major ions, δC_DIC, and δS_SO4 in rivers of the Canadian Cordillera     

Jody Spence  Kevin Telmer 《Geochimica et cosmochimica acta》2005,69(23):5441-5458

Water samples from the Fraser, Skeena and Nass River basins of the Canadian Cordillera were analyzed for dissolved major element concentrations (HCO₃⁻, SO₄²⁻, Cl⁻, Ca²⁺, Mg²⁺, K⁺, Na⁺), δ¹³C of dissolved inorganic carbon (δ¹³C_DIC), and δ³⁴S of dissolved sulfate (δ³⁴S_SO4) to quantify chemical weathering rates and exchanges of CO₂ between the atmosphere, hydrosphere, and lithosphere. Weathering rates of silicates and carbonates were determined from major element mass balance. Combining the major element mass balance with δ³⁴S_SO4 (−8.9 to 14.1‰_CDT) indicates sulfide oxidation (sulfuric acid production) and subsequent weathering of carbonate and to a lesser degree silicate minerals are important processes in the study area. We determine that on average, 81% of the riverine sulfate can be attributed to sulfide oxidation in the Cordilleran rivers, and that 25% of the total weathering cation flux can be attributed to carbonate and silicate dissolution by sulfuric acid. This result is validated by δ¹³C_DIC values (−9.8 to −3.7‰ _VPDB) which represents a mixture of DIC produced by the following weathering pathways: (i) carbonate dissolution by carbonic acid (−8.25‰) > (ii) silicate dissolution by carbonic acid (−17‰) ≈ (iii) carbonate dissolution by sulfuric acid derived from the oxidation of sulfides (coupled sulfide-carbonate weathering) (+0.5‰).δ³⁴S_SO4 is negatively correlated with δ¹³C_DIC in the Cordilleran rivers, which further supports the hypothesis that sulfuric acid produced by sulfide oxidation is primarily neutralized by carbonates, and that sulfide-carbonate weathering impacts the δ¹³C_DIC of rivers. The negative correlation between δ³⁴S_SO4 and δ¹³C_DIC is not observed in the Ottawa and St. Lawrence River basins. This suggests other factors such as landscape age (governed by tectonic uplift) and bedrock geology are important controls on regional sulfide oxidation rates, and therefore also on the magnitude of sulfide-carbonate weathering—i.e., it is more significant in tectonically active areas.Calculated DIC fluxes due to Ca and Mg silicate weathering by carbonic acid (38.3 × 10³ mol C · km⁻² · yr⁻¹) are similar in magnitude to DIC fluxes due to sulfide-carbonate weathering (18.5 × 10³ mol C · km⁻² · yr⁻¹). While Ca and Mg silicate weathering facilitates a transfer of atmospheric CO₂ to carbonate rocks, sulfide-carbonate weathering can liberate CO₂ from carbonate rocks to the atmosphere when sulfide oxidation exceeds sulfide deposition. This implies that in the Canadian Cordillera, sulfide-carbonate weathering can offset up to 48% of the current CO₂ drawdown by silicate weathering in the region.  相似文献   

Seasonal water-column dynamics of dissolved inorganic carbon stable isotopic compositions (δC_DIC) in small hardwater lakes in Minnesota and Montana     

A. Myrbo  M.D. Shapley 《Geochimica et cosmochimica acta》2006,70(11):2699-2714

The carbon stable isotopic value of dissolved inorganic carbon (δ¹³C_DIC) was measured over several years at different depths in the water column in six carbonate-precipitating temperate lakes. δ¹³C_DIC behavior in three of these lakes departed from the conventional model wherein epilimnetic waters are seasonally enriched relative to all hypolimnetic waters, and in general δ¹³C_DIC values in the water column were not readily correlated to parameters such as lake stratification, algal productivity, hydraulic residence time, or water chemistry. Additionally, the processes implicated in generating the δ¹³C_DIC values of individual lakes differ between lakes with similar δ¹³C_DIC compositions. Each lake thus initially appears idiosyncratic, but when the effects of carbonate mineral equilibria, microbial activity, and lake residence time are viewed in terms of the magnitude of distinct DIC pools and fluxes in stratified lakes, generalizations can be made that allow lakes to be grouped by δ¹³C_DIC behavior. We recognize three modes in the relationship between δ¹³C_DIC values and DIC concentration ([DIC]) of individual lakes: (A) δ¹³C_DIC values decreasing with increasing [DIC]; (B) δ¹³C_DIC values increasing with increasing [DIC]; (C) δ¹³C_DIC values decreasing with increasing [DIC] but increasing again at the highest [DIC]. This approach is useful both in understanding δ¹³C_DIC dynamics in modern hardwater lakes and in reconstructing the environmental changes recorded by sedimentary δ¹³C components in the lacustrine paleorecord.  相似文献   

Application of carbon isotopes to detect seepage out of coalbed natural gas produced water impoundments     

Shikha Sharma  Joshua K. Baggett 《Applied Geochemistry》2011,26(8):1423-1432

Coalbed natural gas (CBNG) production from coal bed aquifers requires large volumes of produced water to be pumped from the subsurface. The produced water ranges from high quality that meets state and federal drinking water standards to low quality due to increased salinity and/or sodicity. The Powder River Basin of northeastern Wyoming is a major coalbed natural gas producing region, where water quality generally decreases moving from the southeastern portion of the basin towards the center. Most produced water in Wyoming is disposed into impoundments and other surface drainages, where it may infiltrate into shallow groundwater. Groundwater degradation caused by infiltration of CBNG produced water holding impoundments into arid, soluble salt-rich soils is an issue of immense importance because groundwater is a major source for stock water, irrigation, and drinking water for many small communities in these areas. This study examines the potential of using stable C isotope signatures of dissolved inorganic C (δ¹³C_DIC) to track the fate of CBNG produced water after it is discharged into the impoundments. Other geochemical proxies like the major cations and major anions were used in conjunction with field water quality measurements to understand the geochemical differences between CBNG produced waters and ambient waters in the study area. Samples were collected from the CBNG discharge outfalls, produced water holding impoundments, and monitoring wells from different parts of the Powder River Basin and analyzed for δ¹³C_DIC. The CBNG produced waters from outfalls and impoundments have positive δ¹³C_DIC values that fall within the range of +12‰ to +22‰, distinct from the ambient regional surface and groundwaters with δ¹³C_DIC values ranging from −10‰ to −14‰. The results from the study demonstrate that these contrasting δ¹³C_DIC signatures can be used to trace seepage out of CBNG produced water impoundments into shallow groundwaters.  相似文献   

Indoor and outdoor urban atmospheric CO₂: Stable carbon isotope constraints on mixing and mass balance     

Yurena Yanes  Crayton J. Yapp 《Applied Geochemistry》2010

From July to November 2009, concentrations of CO₂ in 78 samples of ambient air collected in 18 different interior spaces on a university campus in Dallas, Texas (USA) ranged from 386 to 1980 ppm. Corresponding δ¹³C values varied from −8.9‰ to −19.4‰. The CO₂ from 22 samples of outdoor air (also collected on campus) had a more limited range of concentrations from 385 to 447 ppm (avg. = 408 ppm), while δ¹³C values varied from −10.1‰ to −8.4‰ (avg.=-9.0‰). In contrast to ambient indoor and outdoor air, the concentrations of CO₂ exhaled by 38 different individuals ranged from 38,300 to 76,200 ppm (avg. = 55,100 ppm), while δ¹³C values ranged from −24.8‰ to −17.7‰ (avg. = −21.8‰). The residence times of the total air in the interior spaces of this study appear to have been on the order of 10 min with relatively rapid approaches (∼30 min) to steady-state concentrations of ambient CO₂ gas. Collectively, the δ¹³C values of the indoor CO₂ samples were linearly correlated with the reciprocal of CO₂ concentration, exhibiting an intercept of −21.8‰, with r² = 0.99 and p < 0.001 (n = 78). This high degree of linearity for CO₂ data representing 18 interior spaces (with varying numbers of occupants), and the coincidence of the intercept (−21.8‰) with the average δ¹³C value for human-exhaled CO₂ demonstrates simple mixing between two inputs: (1) outdoor CO₂ introduced to the interior spaces by ventilation systems, and (2) CO₂ exhaled by human occupants of those spaces. If such simple binary mixing is a common feature of interior spaces, it suggests that the intercept of a mixing line defined by two data points (CO₂ input from the local ventilation system and CO₂ in the ambient air of the room) could be a reasonable estimate of the average δ¹³C value of the CO₂ exhaled by the human occupants. Thus, such indoor spaces appear to constitute effective “sample vessels” for collection of CO₂ that can be used to determine the average proportions of C₃ and C₄-derived C in the diets of the occupants. For the various groups occupying the rooms sampled in this study, C₄-derived C appears to have constituted ∼40% of the average diet.  相似文献   

Paleowaters in Silurian-Devonian carbonate aquifers: Geochemical evolution of groundwater in the Great Lakes region since the Late Pleistocene   总被引：2，自引：0，他引：2  

J.C. McIntosh  L.M. Walter 《Geochimica et cosmochimica acta》2006,70(10):2454-2479

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 δ¹³C_DIC 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 (δ¹³C_DIC = −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   

Chemical and isotopic tracers of the contribution of microbial gas in Devonian organic-rich shales and reservoir sandstones,northern Appalachian Basin     

Stephen G. Osborn  Jennifer C. McIntosh 《Applied Geochemistry》2010

In this study, the geochemistry and origin of natural gas and formation waters in Devonian age organic-rich shales and reservoir sandstones across the northern Appalachian Basin margin (western New York, eastern Ohio, northwestern Pennsylvania, and eastern Kentucky) were investigated. Additional samples were collected from Mississippian Berea Sandstone, Silurian Medina Sandstone and Ordovician Trenton/Black River Group oil and gas wells for comparison. Dissolved gases in shallow groundwaters in Devonian organic-rich shales along Lake Erie contain detectable CH₄ (0.01–50.55 mol%) with low δ¹³C–CH₄ values (−74.68 to −57.86‰) and no higher chain hydrocarbons, characteristics typical of microbial gas. Nevertheless, these groundwaters have only moderate alkalinity (1.14–8.72 meq/kg) and relatively low δ¹³C values of dissolved inorganic C (DIC) (−24.8 to −0.6‰), suggesting that microbial methanogenesis is limited. The majority of natural gases in Devonian organic-rich shales and sandstones at depth (>168 m) in the northern Appalachian Basin have a low CH₄ to ethane and propane ratios (3–35 mol%; C₁/C₂ + C₃) and high δ¹³C and δD values of CH₄ (−53.35 to −40.24‰, and −315.0 to −174.6‰, respectively), which increase in depth, reservoir age and thermal maturity; the molecular and isotopic signature of these gases show that CH₄ was generated via thermogenic processes. Despite this, the geochemistry of co-produced brines shows evidence for microbial activity. High δ¹³C values of DIC (>+10‰), slightly elevated alkalinity (up to 12.01 meq/kg) and low SO₄ values (<1 mmole/L) in select Devonian organic-rich shale and sandstone formation water samples suggest the presence of methanogenesis, while low δ¹³C–DIC values (<−22‰) and relatively high SO₄ concentrations (up to 12.31 mmole/L) in many brine samples point to SO₄ reduction, which likely limits microbial CH₄ generation in the Appalachian Basin. Together the formation water and gas results suggest that the vast majority of CH₄ in the Devonian organic-rich shales and sandstones across the northern Appalachian Basin margin is thermogenic in origin. Small accumulations of microbial CH₄ are present at shallow depths along Lake Erie and in western NY.  相似文献   

Sulfur and carbon isotope records from 1700 to 800 Ma carbonates of the Jixian section, northern China: Implications for secular isotope variations in Proterozoic seawater and relationships to global supercontinental events   总被引：1，自引：0，他引：1  

Xuelei Chu  Tonggang Zhang  Qirui Zhang 《Geochimica et cosmochimica acta》2007,71(19):4668-4692

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 δ³⁴S_CAS using the same samples analyzed for δ¹³C_carbonate. 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). δ¹³C_carbonate 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 δ³⁴S_CAS 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, δ³⁴S_CAS 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 δ³⁴S_CAS and ∼3‰ increase in δ¹³C_carbonate 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 δ³⁴S_CAS and >3‰ for the δ¹³C_carbonate 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 areδ³⁴S_CAS 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   

Abundance of mass 47 CO₂ in urban air, car exhaust, and human breath     

Hagit P. Affek  John M. Eiler 《Geochimica et cosmochimica acta》2006,70(1):1-12

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 δ¹³C_VPDB = 0, δ¹⁸O_VSMOW = 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   

Br/Cl ratios and O, H, C, and B isotopic constraints on the origin of saline waters from eastern Canada   总被引：1，自引：0，他引：1  

M.I. Leybourne  W.D. Goodfellow 《Geochimica et cosmochimica acta》2007,71(9):2209-2223

Saline groundwaters were recovered from undisturbed (Restigouche deposit) and active (Brunswick #12 mine) Zn-Pb volcanogenic massive sulfide deposits in the Bathurst Mining Camp (BMC), northern New Brunswick, Canada. These groundwaters, along with fresh to brackish meteoric ground and surface waters from the BMC, have been analyzed to determine their major, trace element and stable isotopic (O, H, C, and B) compositions. Saline groundwaters (total dissolved solids = 22-45 g/L) are characterized by relatively high Na/Ca ratios compared to brines from the Canadian Shield and low Na/Cl_molar and δ¹¹B isotopic compositions (−2.5‰ to 11.1‰) compared to seawater. Although saline waters from the Canadian Shield commonly have oxygen and hydrogen isotopic compositions that plot to the left of the global meteoric water line, those from the BMC fall close to the water line. Fracture and vein carbonate minerals at the Restigouche deposit have restricted carbon isotopic compositions of around −5‰ to −6‰. The carbon isotopic compositions of the saline waters at the Restigouche deposit (+12‰ δ¹³C_DIC) are the result of fractionation of dissolved inorganic carbon by methanogenesis. We suggest that, unlike previous models for shield brines, the composition of saline waters in the BMC is best explained by prolonged water-rock reaction, with no requirement of precursor seawater. We suggest that elevated Br/Cl ratios of saline waters compared to seawater may be explained by differential uptake of Br and Cl during groundwater evolution through water-rock reaction.  相似文献   

Late Holocene environmental reconstruction using cave sediments from Belize     

Jason S. Polk  Philip E. van Beynen  Philip P. Reeder 《Quaternary Research》2007,68(1):53-63

Cave sediments collected from Reflection Cave on the Vaca Plateau, Belize show variations in the δ¹³C values of their fulvic acids (FAs), which indicate periods of vegetation change caused by climatic and Maya influences during the late Holocene. The δ¹³C values range from − 27.11‰ to − 21.52‰, a shift of ∼ 5.59‰, which suggests fluctuating contributions of C₃ and C₄ plants throughout the last 2.5 ka, with C₄ plant input reflecting periods of Maya agriculture. Maya activity in the study area occurred at different intensities from ∼ 2600 cal yr BP until ∼ 1500 cal yr BP, after which agricultural practices waned as the Maya depopulated the area. These changes in plant assemblages were in response to changes in available water resources, with increased aridity leading to the eventual abandonment of agricultural areas. The Ix Chel archaeological site, located in the study area, is a highland site that would have been among the first agricultural settlements to be affected during periods of aridity. During these periods, minimal water resources would have been available in this highly karstified, well-drained area, and supplemental groundwater extraction would have been difficult due to the extreme depth of the water table.  相似文献   

Stable isotope geochemistry of ground and surface waters associated with undisturbed massive sulfide deposits; constraints on origin of waters and water-rock reactions     

M.I. Leybourne  I.D. Clark 《Chemical Geology》2006,231(4):300-325

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 δ¹⁸O_VSMOW and δ²H_VSMOW, 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 δ¹⁸O_VSMOW 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 δ¹⁸O_VSMOW 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‰ δ¹³C_VPDB 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‰ δ¹³C_VPDB). The DIC of saline Restigouche deposit groundwater is isotopically heavy (∼+ 12‰ δ¹³C_VPDB), 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   

Multi-proxy approach (H/H, O/O, C/C and Sr/Sr) for the evolution of carbonate-rich groundwater in basalt dominated aquifer of Axum area, northern Ethiopia     

Tewodros Alemayehu  Albrecht Leis  Anton Eisenhauer  Martin Dietzel 《Chemie der Erde / Geochemistry》2011,(2):177-187

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 δ¹³C_DIC values of shallow groundwater between −12 and −7‰ (VPDB) display the uptake of CO₂ from local soil horizons, whereas δ¹³C_DIC of deep groundwater ranges from −5 to +1‰. Considering open system conditions with respect to gaseous CO₂, δ¹³C_DIC = +1‰ of the deep groundwater with highest PCO₂ = 10^−0.9 atm yields δ¹³C_CO2(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   

Note on the isotopic geochemistry of fossil-lacustrine tufas in carbonate plateau—A study from Dungul region (SW Egypt)     

Kamaleldin M. Hassan 《Chemie der Erde / Geochemistry》2014

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: (δ¹³C_mean = −2.49 ± 0.99‰; δ¹⁸O_mean = −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 (δ¹³C_mean = −2.06 ± 0.84‰; δ¹⁸O_mean = −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   

Geochemical features and origin of natural gas in heavy oil area of the Western Slope,Songliao Basin,China     

Guangyou Zhu  Shuichang Zhang  Barry Jay Katz  Jin Su  Xue Wang  Zihui Feng  Jie Cui  Xingwang Liu 《Chemie der Erde / Geochemistry》2014

The Western Slope of the Songliao Basin is rich in heavy oil resources (>70 × 10⁸ bbl), around which there are shallow gas reservoirs (∼1.0 × 10¹² m³). The gas is dominated by methane with a dryness over 0.99, and the non-hydrocarbon component being overwelmingly nitrogen. Carbon isotope composition of methane and its homologs is depleted in ¹³C, with δ¹³C₁ values being in the range of −55‰ to −75‰, δ¹³C₂ being in the range of −40‰ to −53‰ and δ¹³C₃ being in the range of −30‰ to −42‰, respectively. These values differ significantly from those solution gases source in the Daqing oilfield. This study concludes that heavy oils along the Western Slope were derived from mature source rocks in the Qijia-Gulong Depression, that were biodegraded. The low reservoir temperature (30–50 °C) and low salinity of formation water with neutral to alkaline pH (NaHCO₃) appeared ideal for microbial activity and thus biodegradation. Natural gas along the Western Slope appears mainly to have originated from biodegradation and the formation of heavy oil. This origin is suggested by the heavy δ¹³C of CO₂ (−18.78‰ to 0.95‰) which suggests that the methane was produced via fermentation as the terminal decomposition stage of the oil.  相似文献   

Stable carbon and oxygen isotope fractionation in bivalve (Placopecten magellanicus) larval aragonite     

Erin F. Owen  Alan D. Wanamaker Jr.  Bernd R. Schöne 《Geochimica et cosmochimica acta》2008,72(19):4687-4698

The relationship between stable isotope composition (δ¹³C and δ¹⁸O) in seawater and in larval shell aragonite of the sea scallop, Placopecten magellanicus, was investigated in a controlled experiment to determine whether isotopes in larval shell aragonite can be used as a reliable proxy for environmental conditions. The linear relationship between δ¹³C_DIC and δ¹³C_aragonite (r² = 0.97, p < 0.0001, RMSE = 0.18) was: