A large metabolic carbon contribution to the δC record in marine aragonitic bivalve shells     

David P. Gillikin  Anne Lorrain  Frank Dehairs 《Geochimica et cosmochimica acta》2007,71(12):2936-2946

It is well known that the incorporation of isotopically light metabolic carbon (C_M) significantly affects the stable carbon isotope (δ¹³C) signal recorded in biogenic carbonates. This can obscure the record of δ¹³C of seawater dissolved inorganic carbon (δ¹³C_DIC) potentially archived in the shell carbonate. To assess the C_M contribution to Mercenaria mercenaria shells collected in North Carolina, USA, we sampled seawater δ¹³C_DIC, tissue, hemolymph and shell δ¹³C. All shells showed an ontogenic decrease in shell δ¹³C, with as much as a 4‰ decrease over the lifespan of the clam. There was no apparent ontogenic change in food source indicated by soft tissue δ¹³C values, therefore a change in the respired δ¹³C value cannot be the cause of this decrease. Hemolymph δ¹³C, on the other hand, did exhibit a negative relationship with shell height indicating that respired CO₂ does influence the δ¹³C value of internal fluids and that the amount of respired CO₂ is related to the size or age of the bivalve. The percent metabolic C incorporated into the shell (%C_M) was significantly higher (up to 37%, with a range from 5% to 37%) than has been found in other bivalve shells, which usually contain less than 10%C_M. Interestingly, the hemolymph did contain less than 10%C_M, suggesting that complex fractionation might occur between hemolymph and calcifying fluids. Simple shell biometrics explained nearly 60% of the observed variability in %C_M, however, this is not robust enough to predict %C_M for fossil shells. Thus, the metabolic effect on shell δ¹³C cannot easily be accounted for to allow reliable δ¹³C_DIC reconstructions. However, there does seem to be a common effect of size, as all sites had indistinguishable slopes between the %C_M and shell height (+0.19% per mm of shell height).  相似文献   

The origins and behaviour of carbon in a major semi-arid river,the Murray River,Australia, as constrained by carbon isotopes and hydrochemistry     

Ian Cartwright 《Applied Geochemistry》2010

δ¹³C values of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate organic C (POC) together with δ¹⁸O and δ²H values of water, δ³⁴S values of dissolved SO₄, and major ion concentrations were measured in the Murray River and its tributaries between November 2005 and April 2007 to constrain the origins and behaviour of riverine C. δ¹³C_DIC values in the Murray River vary between −9.5 and −4.7‰ with a range of <3‰ within any sampling round. δ¹³C_DIC values of the tributaries are −11.0‰ to −5.1‰. DIC concentrations of the Murray River increase from ∼25 mg/L in the middle and upper reaches of the river to 45–55 mg/L in the lower reaches. However, the mass ratio of DIC as a proportion of the total dissolved solids (TDS) decreases from ∼0.6–0.7 in the headwaters to ∼0.2–0.3 in the lower reaches of the river, with similar downstream changes in DIC/Cl ratios. This precludes simple evaporative concentration of DIC and is interpreted as the river evading CO₂; this interpretation is consistent with pCO₂ values that are in the range 550–11,200 ppm volume (ppmv), which are far higher than those in equilibrium with the atmosphere (∼360 ppmv). The δ¹³C_DIC values are similar to those that would be produced by the weathering of marine limestone (δ¹³C ∼ 0‰). However, the lack of marine limestones cropping out in the Murray–Darling Basin and the relatively uniform δ¹³C_DIC values of the Murray River (even in upland reaches where the dominant rock types are metamorphosed silicates and granites) make this unlikely. Rather the high pCO₂ values and δ¹³C_DIC values are best explained by a combination of mineralisation of low δ¹³C organic C and evasion to the atmosphere. The rate of these two processes may attain near steady state and control both DIC concentrations and δ¹³C values.  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   

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   

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   

Soil n-alkane δD vs. altitude gradients along Mount Gongga, China   总被引：1，自引：0，他引：1  

Guodong Jia  Kai Wei  Fajin Chen  Ping’an Peng 《Geochimica et cosmochimica acta》2008,72(21):5165-5174

The altitude effect on the isotopic composition of precipitation and its application to paleoelevation reconstruction using authigenic or pedogenic minerals have been intensively studied. However, there are still no studies on variations in biomarker δD along altitude transects to investigate its potential as a paleoelevation indicator, although it has been observed that δD of higher plant lipid may record changes in precipitation δD (δD_p). Here, we present δD values of higher plant-derived C₂₇, C₂₉, and C₃₁n-alkanes from surface soil along the eastern slope of Mount Gongga, China with great changes in physical variables and vegetation over a range from 1000 to 4000 m above sea level. The weighted-mean δD values of these n-alkanes (δD_wax) show significant linear correlations with predicted δD_p values (R² = 0.76) with an apparent isotopic enrichment (ε_wax-p) of −137 ± 9‰, indicating that soil δD_wax values track overall δD_p variation along the entire altitudinal transect. Leaf δD_wax is also highly correlated with mountain altitude by a significant quadratic relationship (R² = 0.80). Evapotranspiration is found declining with altitude, potentially lowering δD_wax values at higher elevations. However, this evapotranspiration effect is believed to be largely compensated by the opposing effect of vegetation changes, resulting in less varied ε_wax-p values over the slope transect. This study therefore confirms the potential of using leaf δD_wax to infer paleoelevations, and more generally, to infer the δD of precipitation.  相似文献   

Oxygen and carbon stable isotopes of modern land snail shells as environmental indicators from a low-latitude oceanic island     

Yurena Yanes  Christopher S. Romanek  Heather A. Brant  María R. Alonso 《Geochimica et cosmochimica acta》2009,73(14):4077-6869

Land snails provide a unique opportunity to study terrestrial paleoenvironments because their shells, which are generally highly abundant and well-preserved in the fossil record, contain a temporal record of environmental change in the form of isotope codes. To evaluate the utility of this approach for a low-latitude oceanic setting, 207 modern shells of 18 species of land snail were analyzed for their oxygen and carbon isotope composition along a north and south facing altitudinal gradient (10-2160 m a.s.l.) in Tenerife Island (∼28°N) of the Canary Archipelago.Shells collected at each locality showed a relatively large range in isotope composition which was greater along the south facing transect (drier and hotter), suggesting that the variance in shell isotope values may be related to water-stress. Although pooled isotope values did not generally show strong relationships with environmental variables (i.e., altitude, temperature and precipitation), mean isotope values were strongly associated with some climatic factors when grouped by site. The mean δ¹⁸O value of the shell (δ¹⁸O_shell) by site displayed a negative correlation with elevation, which is consistent with the positive relationship observed between temperature and the δ¹⁸O value of rain (δ¹⁸O_rain). Calculated δ¹⁸O values of the snail body water (δ¹⁸O_body) derived from observed temperatures and δ¹⁸O_shell values (using the equation of Grossman and Ku [Grossman E. L. and Ku T. L. (1986) Oxygen and carbon isotope fractionation in biogenic aragonite. Chem. Geol. (Isotope Geosci. Sec.)59, 59-74]) displayed a trend with respect to altitude that was similar to measured and hypothetical δ¹⁸O values for local rain water. The calculated δ¹⁸O_body values from the shell declined 0.17‰ (VSMOW) per 100 m, which is consistent with the “altitude effect” observed for tropical rains in Western Africa, and it correlated negatively with rainfall amount. Accordingly, lower δ¹⁸O_shell values indicate lower temperatures, lower δ¹⁸O_rain values and possibly, higher rainfall totals. A positive correlation between the mean δ¹³C values of shells (δ¹³C_shell) and plants by site suggests that shells potentially record information about the surrounding vegetation. The δ¹³C_shell values varied between −15.7 and −0.6‰ (VPDB), indicating that snails consumed C₃ and C₄/CAM plants, where more negative δ¹³C_shell values probably reflects the preferential consumption of C₃ plants which are favored under wetter conditions. Individuals with more positive δ¹³C_shell values consumed a larger percentage of C₄ plants (other potential factors such as carbonate ingestion or atmospheric CO₂ contribution were unlikely) that were more common at lower elevations of the hotter and drier south facing transect. The relatively wide range of shell isotope values within a single site requires the analysis of numerous shells for meaningful paleoclimatic studies. Although small differences were observed in isotope composition among snail species collected at a single sampling site, they were not significant, suggesting that isotope signatures extracted from multi-taxa snail data sets may be used to infer environmental conditions over a broad range of habitats.  相似文献   

Oxygen isotope fractionation between synthetic aragonite and water: Influence of temperature and Mg concentration     

Sang-Tae Kim  James R. O’Neil  Alfonso Mucci 《Geochimica et cosmochimica acta》2007,71(19):4704-4715

Aragonite was precipitated in the laboratory at 0, 5, 10, 25, and 40 °C to determine the temperature dependence of the equilibrium oxygen isotope fractionation between aragonite and water. Forced CO₂ degassing, passive CO₂ degassing, and constant addition methods were employed to precipitate aragonite from supersaturated solutions, but the resulting aragonite-water oxygen isotope fractionation was independent of the precipitation method. In addition, under the experimental conditions of this study, the effect of precipitation rate on the oxygen isotope fractionation between aragonite and water was almost within the analytical error of ±∼0.13‰ and thus insignificant. Because the presence of Mg²⁺ ions is required to nucleate and precipitate aragonite from Na-Ca-Cl-HCO₃ solutions under these experimental conditions, the influence of the total Mg²⁺ concentration (up to ∼0.9 molal) on the aragonite-water oxygen isotope fractionation was examined at 25 °C. No significant Mg²⁺ ion effect, or oxygen isotope salt effect, was detected up to 100 mmolal total Mg²⁺ but a noticeable isotope salt effect was observed at ∼0.9 molal total Mg²⁺.On the basis of results of the laboratory synthesis experiments, a new expression for the aragonite-water fractionation is proposed over the temperature range of 0-40 °C: