首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 85 毫秒
1.
We use the evolution of river sediment characteristics and sedimentary Corg from the Himalayan range to the delta to study the transport of Corg in the Ganga-Brahmaputra system and especially its fate during floodplain transit.A detailed characterisation of both mineral and organic particles for a sampling set of river sediments allows taking into account the sediment heterogeneity characteristic of such large rivers. We study the relationships between sediment characteristics (mineralogy, grain size, specific area) and Corg content in order to evaluate the controls on Corg loading. Contributions of C3 and C4 plants are estimated from Corg stable isotopic composition (δ13Corg). We use the evolution of δ13Corg values from the Himalayan range to the delta in order to study the fate of Corg during floodplain transit.Ganga and Brahmaputra sediments define two distinct linear relations with specific area. In spite of 4-5 times higher specific area, Ganga sediments have similar Corg content, grain size and mineralogy as Brahmaputra sediments, indicating that specific area does not exert a primary control on Corg loading. The general correlation between the total Corg content and Al/Si ratio indicates that Corg loading is mainly related to: (1) segregation of organic particles under hydrodynamic forces in the river, and (2) the ability of mineral particles to form organo-mineral aggregates.Bed and suspended sediments have distinct δ13Corg values. In bed sediments, δ13Corg values are compatible with a dominant proportion of fossil Corg derived from Himalayan rocks erosion. Suspended sediments from Himalayan tributaries at the outflow of the range have low δ13Corg values (−24.8‰ average) indicating a dominant proportion of C3 plant inputs. In the Brahmaputra basin, δ13Corg values of suspended sediments are constant along the river course in the plain. On the contrary, suspended sediments of the Ganga in Bangladesh have higher δ13Corg values (−22.4‰ to −20.0‰), consistent with a significant contribution of C4 plant derived from the floodplain. Our data indicate that, during the plain transit, more than 50% of the recent biogenic Corg coming from the Himalaya is oxidised and replaced by floodplain Corg. This renewal process likely occurs during successive deposition-erosion cycles and river course avulsions in the plain.  相似文献   

2.
We analyzed the isotopic patterns found in the tooth enamel of modern feral horses from Shackleford Banks, North Carolina (USA), which has a temperate climate and supports primarily C4 grasslands. Enamel δ13C values averaged −4.1‰ with a standard deviation (1σ) of 1.7‰, which corresponds to an average diet of 66 ± 12% C4 plants. Our results differ from dietary reconstructions from 1978 to 1981, which found that horses consumed 91% C4 plants. This suggests that horses have increased their consumption of C3 forbs, likely as a result of the removal of cattle, sheep, and goats from the island. Shackleford surface waters had δ18O values that averaged −3.3 ± 0.5‰ and −1.3 ± 1.8‰ on the western and eastern ends of the island, respectively. Tooth enamel samples averaged 27.3 ± 1.5‰ and displayed the same range of δ18O values as surface waters. The variability of both δ18O and the δ13C values among individuals within this population demonstrates that horses from relatively homogenous temperate environments can display a wide range of isotopic values. Given the observed range of isotopic values for modern horses, we suggest that researchers use the mean values of multiple (≥9) equids when attempting to reconstruct average paleodiets and/or paleoenvironmental conditions.  相似文献   

3.
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   

4.
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   

5.
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. δ13CDIC values in the Murray River vary between −9.5 and −4.7‰ with a range of <3‰ within any sampling round. δ13CDIC 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 δ13CDIC 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 δ13CDIC 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 δ13CDIC 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   

6.
Fatty acid (FA) composition and stable isotope (δ13C, δ15N) signatures of four aquatic plants, plankton, sediment, soil and suspended particulate organic matter (SPOM) collected from open floodplain lakes (Várzea) and rivers of the central Brazilian Amazon basin were gathered during high and low water stages in 2009. SPOM from Várzea had a major contribution of autochthonous material from phytoplankton and C3 aquatic plants. As shown from stable isotope composition of SPOM (δ13C −31.3 ± 3.2‰; δ15N 3.6 ± 1.5‰), the C4 aquatic phanerogam (δ13C −13.1 ± 0.5‰; δ15N 4.1 ± 1.7‰) contribution appeared to be weak, although these plants were the most abundant macrophyte in the Várzea. During low water season, increasing concentration of 18:3ω3 was recorded in the SPOM of lakes. This FA, abundant mainly in the Várzea plants (up to 49% of total FAs), was due to the accumulation of their detritus in the ecosystem. This dry season, when connectivity with the river mainstem was restricted, was also characterized by a high concentration in the SPOM of the cyanobacteria marker 16:1ω7 (up to 21% of total FAs). The FA compositions of SPOM from the Amazon River also exhibited significant seasonal differences, in particular a higher concentration of 16:1ω7 and 18:3ω3 during the dry season. This suggests a seasonal contribution of autochthonous material produced in Várzea to the Amazon River SPOM.  相似文献   

7.
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   

8.
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   

9.
A suite of natural gases from the northern Songliao Basin in NE China were characterized for their molecular and carbon isotopic composition. Gases from shallow reservoirs display clear geochemical evidence of alteration by biodegradation, with very high dryness (C1/C2+ > 100), high C2/C3 and i-C4/n-C4 ratios, high nitrogen content and variable carbon dioxide content. Isotopic values show wide range variations (δ13CCH4 from −79.5‰ to −45.0‰, δ13CC2H6 from −53.7‰ to −32.2‰, δ13CC3H8 from −36.5‰ to −20.1‰, δ13CnC4H10 from −32.7‰ to −24.5‰, and δ13CCO2 from −21.6‰ to +10.5‰). A variety of genetic types can be recognized on the basis of chemical and isotopic composition together with their geological occurrence. Secondary microbial gas generation was masked by primary microbial gas and the mixing of newly generated methane with thermogenic methane already in place in the reservoir can cause very complicated isotopic signatures. System openness also was considered for shallow biodegraded gas accumulations. Gases from the Daqing Anticline are relatively wet with 13C enriched methane and 13C depleted CO2, representing typically thermogenic origin. Gases within the Longhupao-Da’an Terrace have variable dryness, 13C enriched methane and variable δ13C of CO2, suggesting dominant thermogenic origin and minor secondary microbial methane augment. The Puqian-Ao’nan Uplift contains relatively dry gas with 13C depleted methane and 13C enriched CO2, typical for secondary microbial gas with a minor part of thermogenic methane. Gas accumulations in the Western Slope are very dry with low carbon dioxide concentrations. Some gases contain 13C depleted methane, ethane and propane, indicating low maturity/primary microbial origin. Recognition of varying genetic gas types in the Songliao Basin helps explain the observed dominance of gas in the shallow reservoir and could serve as an analogue for other similar shallow gas systems.  相似文献   

10.
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   

11.
Pollen grains from grasses using the C3 and C4 photosynthetic pathways have distinct ranges of δ13C values that may be used to estimate their relative abundance in paleorecords. We evaluated a spooling-wire microcombustion device interfaced with an isotope-ratio mass spectrometer (SWiM-IRMS) for δ13C analysis of individual grass-pollen grains. Pollen from four C3 and four C4 grass species was isolated through micromanipulation and analyzed as single grains suspended in water. A carbon yield greater than the 2σ range of the carbon content of blanks containing only water was used to distinguish samples containing pollen (“pollen present”) from those not containing pollen. This criterion resulted in the exclusion of ∼45% of the 946 samples applied to the wire. The average δ13C values (±1σ) of the remaining samples were −26.9‰ (±6.3‰) and −11.5‰ (±9.6‰) for C3 grasses and C4 grasses, respectively, after blank-correcting the δ13C data. These results suggest that the SWiM-IRMS system can be used to distinguish C3 from C4 grass pollen. The high variability in measured δ13C values is likely caused by a combination of factors. These include natural isotopic variability among individual pollen grains; the relatively poor precision that can be obtained when determining δ13C values of such small samples; and the uncertainty in the magnitude, isotopic composition, and stability of the analytical blank. Nonetheless, high percentages of individual pollen grains were correctly classified as being of either C3 or C4 origin. On average, 90% (range = 78-100%) of pollen grains from C3 grasses had δ13C values more negative than the cutoff threshold of −19.2‰; while 84% (range = 77-90%) of pollen grains from C4 grasses had δ13C values more positive than −19.2‰. Compared with analysis using an elemental analyzer interfaced with an IRMS (EA-IRMS), the number of pollen grains required for δ13C-based evaluation of C3/C4 grass composition is many times lower with the SWiM-IRMS. Additionally, δ13C data from the SWiM-IRMS does not need to be incorporated into a mixing model to derive estimates of the abundance of C3 and C4 grass pollen. Carbon-isotopic analysis of individual grass-pollen grains using the SWiM-IRMS system may help improve our understanding of the evolutionary and ecological significance of grass taxa in the paleorecord.  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.
Oceania supplies ∼40% of the global riverine flux of organic carbon, approximately half of which is injected onto broad continental shelves and processed in shallow deltaic systems. The Gulf of Papua, on the south coast of the large island of New Guinea, is one such deltaic clinoform complex. It receives ∼4 Mt yr−1 particulate terrestrial organic carbon with initial particle Corg loading ∼0.7 mg m−2. Corg loading is reduced to ∼0.3 mg m−2 in the topset-upper foreset zones of the delta despite additional inputs of mangrove and planktonic detritus, and high net sediment accumulation rates of 1-4 cm yr−1. Carbon isotopic analyses (δ13C, Δ14C) of ΣCO2 and Corg demonstrate rapid (<100 yr) remineralization of both terrestrial (δ13C <−28.6) and marine Corg (δ13C ∼−20.5) ranging in average age from modern (bomb) (Δ14C ∼60) to ∼1000 yr (Δ14C ∼−140). Efficient and rapid remineralization in the topset-upper foreset zone is promoted by frequent physical reworking, bioturbation, exposure, and reoxidation of deposits. The seafloor in these regions, particularly <20 m, apparently functions as a periodically mixed, suboxic batch reactor dominated by microbial biomass. Although terrestrial sources can be the primary metabolic substrates at inshore sites, relatively young marine Corg often preferentially dominates pore water ΣCO2 relative to bulk Corg in the upper foreset. Thus a small quantity of young, rapidly recycled marine organic material is often superimposed on a generally older, less reactive terrestrial background. Whereas the pore water ΣCO2 reflects both rapidly cycled marine and terrestrial sources, terrestrial material dominates the slower overall net loss of Corg from particles in the topset-upper foreset zone (i.e. recycled marine Corg leaves little residue). Preferential utilization of Corg subpools and diagenetic fractionation of C isotopes supports the reactive continuum model as a conceptual basis for net decomposition kinetics. Early diagenetic fractionation of C isotopes relative to the bulk sedimentary Corg composition can produce changes in 14C activity independent of radioactive decay. In the Gulf of Papua topset-upper foreset, Δ14C of pore water ΣCO2 averaged ∼ 300‰ greater than Corg sediment between ∼1-3 m depth in deposits. Diagenetic fractionation and decomposition aging of sedimentary Corg compromises simple application of 14C for determination of sediment accumulation rates in diagenetically reactive deposits.  相似文献   

14.
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   

15.
Silica phytoliths, which are deposits of opal-A that precipitate in the intra- and intercellular spaces of plant tissues during transpiration, commonly contain small amounts of occluded organic matter. In this paper, we investigate whether the δ13C values of phytoliths from a C4 grass, Calamovilfa longifolia, vary in response to climatic variables that can affect the carbon-isotope composition of plant tissues. There is no significant correlation (r2 < 0.3) between climate variables and the δ13C values of C. longifolia tissues (average δ13Ctissue = −13.1 ± 0.6 ‰; n = 70) across the North American prairies. However, plant tissue δ13C values are lower for grasses collected in populated areas where the δ13C value of atmospheric CO2 is expected to be lower because of fossil fuel burning. Phytolith δ13C values are more variable (δ13C = −27.3 to −23.0‰; average = −25.1 ± 1.3‰; n = 34) and more sensitive to changes in aridity than whole tissue δ13C values. The strongest correlations are obtained between the δ13C values of stem or sheath phytoliths and humidity (r2 = 0.3), latitude (r2 = 0.4) and amount of precipitation (r2 = 0.5). However, use of these relationships is limited by the wide spread in δ13C values of phytoliths from different plant tissues at the same location. We have been unable to infer any relationship between δ13C values of phytoliths and expected variations in the δ13C values of atmospheric CO2. The C. longifolia phytoliths are depleted of 13C relative to tissue carbon by 10-14‰. This means that the phytoliths examined in this study have carbon isotopic compositions within the range reported previously for phytoliths from C3 plants. This observation may further limit the usefulness of soil-phytolith assemblage δ13C values for identifying shifts in grassland C3:C4 ratios.  相似文献   

16.
To constrain seasonal changes in the long-range atmospheric transport of land-derived lipid biomarker compounds, we investigated the compound-specific stable isotopic composition of marine aerosol n-alkanes collected from 1990 to 1993 at a remote island, Chichi-Jima (27°04′N, 142°13′E), in the western North Pacific. Compound-specific isotope analysis revealed, in particular, strong seasonal changes in the δ13C values of the C29 and C31n-alkanes (biomarkers for higher plants). Lighter δ13C values were observed in winter (typically −32 to −34‰), with a transition to heavier values in summer (typically −28 to −31‰). Using a mixing equation and typical end members for C3 and C4 plants, we found that this is due to relative increases in the contributions from C4 plants in the summer season. Using backward air-mass trajectory analyses, it was shown that the Asian continent was the major source region for C3 plant material during winter/spring, whereas Indonesia/Australia and possibly the Americas were the major source regions for C4 material during the summer/autumn. Also observed was an enhanced atmospheric transport of n-alkanes from C4 plants in 1991 summer/autumn during a strong El Nino event, which was associated with forest and bushfires in Indonesia and Australia. In addition to providing information on contemporary processes, this study also provides a base for future paleoclimatological work in ocean sediments.  相似文献   

17.
The contribution of soil organic matter (SOM) to continental margins is largely ignored in studies on the carbon budget of marine sediments. Detailed geochemical investigations of late Quaternary sediments (245-0 ka) from the Niger and Congo deep-sea fans, however, reveal that Corg/Ntot ratios and isotopic signatures of bulk organic matter (δ13Corg) in both fans are essentially determined by the supply of various types of SOM from the river catchments thus providing a fundamentally different interpretation of established proxies in marine sciences. On the Niger fan, increased Corg/Ntot and δ13Corg (up to −17‰) were driven by generally nitrogen-poor but 13C-enriched terrigenous plant debris and SOM from C4/C3 vegetation/Entisol domains (grass- and tree-savannah on young, sandy soils) supplied during arid climate conditions. Opposite, humid climates supported drainage of C3/C4 vegetation/Alfisol/Ultisol domains (forest and tree-savannah on older/developed, clay-bearing soils) that resulted in lower Corg/Ntot and δ13Corg (< −20‰) in the Niger fan record. Sediments from the Congo fan contain a thermally stable organic fraction that is absent on the Niger fan. This distinct organic fraction relates to strongly degraded SOM of old and highly developed, kaolinite-rich ferallitic soils (Oxisols) that cover large areas of the Congo River basin. Reduced supply of this nitrogen-rich and 12C-depleted SOM during arid climates is compensated by an elevated input of marine OM from the high-productive Congo up-welling area. This climate-driven interplay of marine productivity and fluvial SOM supply explains the significantly smaller variability and generally lower values of Corg/Ntot and δ13Corg for the Congo fan records. This study emphasizes that ignoring the presence of SOM results in a severe underestimation of the terrigenous organic fraction leading to erroneous paleoenvironmental interpretations at least for continental margin records. Furthermore, burial of SOM in marine sediments needs more systematic investigation combining marine and continental sciences to assess its global relevance for long-term sequestration of atmospheric CO2.  相似文献   

18.
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   

19.
  相似文献   

20.
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   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号