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1.
Paola Ridge, along the NW Calabrian margin (southern Tyrrhenian Sea), is one of the few reported deep sea sites of precipitation of authigenic carbonates in the Tyrrhenian Sea. Here, the changing composition of the seeping fluids and the dynamic nature of the seepage induced the precipitation of pyrite, siderite and other carbonate phases. The occurrence of this array of authigenic precipitates is thought to be related to fluctuation of the sulfate-methane transition zone (SMTZ).Concretions of authigenic minerals formed in the near sub-bottom sediments of the Paola Ridge were investigated for their geochemical and isotopic composition. These concretions were collected in an area characterized by the presence of two alleged mud volcanoes and three mud diapirs. The mud diapirs are dotted by pockmarks and dissected by normal faults, and are known for having been a site of fluid seepage for at least the past 40 kyrs. Present-day venting activity occurs alongside the two alleged mud volcanoes and is dominated by CO2-rich discharging fluids. This discover led us to question the hypothesis of the mud volcanoes and investigate the origin of the fluids in each different domed structure of the study area.In this study, we used stable isotopes (carbon and oxygen) of carbonates coupled with rare earth element (REE) composition of different carbonate and non-carbonate phases for tracing fluid composition and early diagenesis of authigenic precipitates. The analyses on authigenic precipitates were coupled with chemical investigation of venting gas and sea-water.Authigenic calcite/aragonite concretions, from surficial sediments on diapiric structures, have depleted 13C isotopic composition and slightly positive δ18O values. By contrast, siderite concretions, generally found within the first 6 m of sediments on the alleged mud volcanoes, yielded positive δ13C and δ18O values. The siderite REE pattern shows consistent LREE (light REE) fractionation, MREE (medium REE) enrichment and positive Gd and La anomalies. As shown by the REE distribution, the 13C-depleted composition and their association with chemosymbiotic fauna, calcite/aragonite precipitated at time of moderate to high methane flux close to the seafloor, under the influence of bottom seawater. Authigenic siderite, on the other hand, formed in the subseafloor, during periods of lower gas discharges under prolonged anoxic conditions within sediments in equilibrium with 13C-rich dissolved inorganic carbon (DIC) and 18O-rich water, likely related to methanogenesis and intermittent venting of deep-sourced CO2.  相似文献   

2.
The distribution of diagenetic alterations in Triassic fluvio-deltaic, quartzarenitic to sublitharenitic, lowstand systems tract (LST) sandstones of the Grès á Voltzia Formation, anastomosing fluvial, quartzarenitic transgressive systems tract (TST) sandstones of the Grès á Roseaux Formation, and shallow marine, quartzarenitic to sublitharenitic, TST sandstones of the Grès Coquiller Formation, the Paris Basin (France), can be linked to transgression and regression events, and thus to the sequence stratigraphic context. Near-surface eogenetic alterations, which display a fairly systematic link to the depositional facies and sequence stratigraphic framework, include: (i) cementation by meteoric water calcite (δ18O=−8.9‰ and δ13C=−9.1‰) in the fluvio-deltaic, LST sandstones, (ii) cementation by mixed marine–meteoric calcite (δ18O=−5.3‰ to −2.6‰ and δ13C=−3.9‰ to −1.3‰) and dolomite (δ18O=−4.6‰ to −2.6‰ and δ13C=−2.9‰ to −2.3‰) in the foreshore, TST sandstones and below parasequence boundaries (PB), and transgressive surface (TS), and in the shoreface, TST sandstones below maximum flooding surfaces (MFS), being facilitated by the presence of carbonate bioclasts, (iii) dissolution of detrital silicates and precipitation of K-feldspar overgrowths and kaolinite, particularly in the fluvio-deltaic, LST sandstones owing to effective meteoric water circulation, and (vi) formation of autochthonous glauconite, which is increases in abundance towards the top of the fluvio-deltaic, LST sandstones, and along TS, and in the shoreface, TST sandstones, by alteration of micas owing to the flux of seawaters into the sandstones during transgression, whereas parautochthonous glauconite is restricted to the TS sandstones owing to marine reworking. Mesogenetic alterations, which include cementation by quartz overgrowths and illite, display fairly systematic link to fluvio-deltaic, LST sandstones. This study has revealed that linking of diagenesis to transgression and regression events enables a better understanding of the parameters that control the spatial and temporal distribution of diagenetic alterations in sandstones and of their impact on reservoir quality evolution.  相似文献   

3.
Authigenic carbonates are common at cold seep sites as a result of microbial oxidation of hydrocarbons. Seep carbonate samples were collected from the surface of the Bush Hill (Green Canyon Block 185, Gulf of Mexico), a mound containing gas hydrate. The carbonates consisted of oily, porous limestone slabs and blocks containing bioclasts and matrix. Analysis by X-ray diffraction shows that aragonite is the dominant mineral (89–99 wt% with an average of 94 wt%) in the matrix of seep carbonate. This cement occurs in microcrystalline, microspar, and sparite forms. The moderate 13C depletion of the seep carbonate (the most depleted one has δ13C value of −29.4‰, and 26 of 38 subsamples have δ13C values >−20.0‰) indicates that the non-methane hydrocarbons was incorporated during seep carbonate precipitation. Relative enrichment of 18O may be related to localized destabilization of gas hydrate or derived from 18O-enriched pore water originated from smectite–illite transition in the deep sediments. The total content of rare earth elements (REE) of the 5% HNO3-treated solution of the carbonates is from 0.40 ppm to 30.9 ppm. The shale-normalized REE patterns show varied Ce anomalies from significantly negative, slightly negative, and no to positive Ce anomalies. Variable content of trace elements, total REE, and Ce anomalies in different samples and even in the different carbonate mineral forms (microcrystalline, microspar and sparite) of the same sample suggest that the formation condition of the Bush Hill seep carbonate is variable and complex, which is possibly controlled by the rate of fluid flux.  相似文献   

4.
Authigenic carbonates from active methane seeps offshore southwest Africa   总被引：2，自引：1，他引：1  
The southwest African continental margin is well known for occurrences of active methane-rich fluid seeps associated with seafloor pockmarks at water depths ranging broadly from the shelf to the deep basins, as well as with high gas flares in the water column, gas hydrate accumulations, diagenetic carbonate crusts and highly diverse benthic faunal communities. During the M76/3a expedition of R/V METEOR in 2008, gravity cores recovered abundant authigenic carbonate concretions from three known pockmark sites—Hydrate Hole, Worm Hole, the Regab pockmark—and two sites newly discovered during that cruise, the so-called Deep Hole and Baboon Cluster. The carbonate concretions were commonly associated with seep-benthic macrofauna and occurred within sediments bearing shallow gas hydrates. This study presents selected results from a comprehensive analysis of the mineralogy and isotope geochemistry of diagenetic carbonates sampled at these five pockmark sites. The oxygen isotope stratigraphy obtained from three cores of 2–5?m length indicates a maximum age of about 60,000–80,000?years for these sediments. The authigenic carbonates comprise mostly magnesian calcite and aragonite, associated occasionally with dolomite. Their very low carbon isotopic compositions (–61.0?<?δ13C ‰ V-PDB?<?–40.1) suggest anaerobic oxidation of methane (AOM) as the main process controlling carbonate precipitation. The oxygen isotopic signatures (+2.4?<?δ18O ‰ V-PDB?<?+6.2) lie within the range in equilibrium under present-day/interglacial to glacial conditions of bottom seawater; alternatively, the most positive δ18O values might reflect the contribution of 18O-rich water from gas hydrate decomposition. The frequent occurrence of diagenetic gypsum crystals suggests that reduced sulphur (hydrogen sulphide, pyrite) from sub-seafloor sediments has been oxidized by oxygenated bottom water. The acidity released during this process can potentially induce the dissolution of carbonate, thereby providing enough Ca2+ ions for pore solutions to reach gypsum saturation; this is thought to be promoted by the bio-irrigation and burrowing activity of benthic fauna. The δ18O–δ13C patterns identified in the authigenic carbonates are interpreted to reflect variations in the rate of AOM during the last glacial–interglacial cycle, in turn controlled by variably strong methane fluxes through the pockmarks. These results complement the conclusions of Kasten et al. in this special issue, based on authigenic barite trends at the Hydrate Hole and Worm Hole pockmarks which were interpreted to reflect spatiotemporal variations in AOM related to subsurface gas hydrate formation–decomposition.  相似文献   

5.
Multibeam swath bathymetric data collected in 95–120 m water depth on Australia’s North West Shelf revealed two distinct populations of sand waves: a laterally extensive, low-amplitude composite form comprising superimposed dunes and ripples, and a laterally restricted form which has unusually high bedform heights and slopes. These large subaqueous sand waves comprise bioclastic ooid/peloid sand. Significantly, evidence of seabed fluid flow was detected in association with the high-amplitude sand waves. This evidence includes seabed pockmarks approximately 2–15 m in diameter imaged with side-scan sonar, tubular and massive carbonate concretions dredged from the seabed, and potential active venting of a fluid plume from the seabed observed during an underwater camera tow. Molecular and isotopic analyses of carbonate concretions collected from within pockmarks associated with the high-amplitude sand waves indicate that the fluids from which they precipitated comprise modern seawater and are not related to thermogenic fluids or microbial gases. The fluid flow is interpreted to be driven by macrotidal currents flowing over the relatively steep slopes of the high-amplitude sand waves. Pockmarks and carbonate concretions then develop where the interstitial flows are confined and focused by subsurface ‘mounds’ in a shallow seismic reflector.  相似文献   

6.
The aim of this study was to distinguish between sources of the complex variety of Marennes-Oléron Bay suspended particulate organic matter (SPOM) contributing to the tropho-dynamics of the Marennes-Oléron oyster farming bay. Basic biomarkers (Chl a, C/N and POC/Chl a ratios), carbon and nitrogen stable isotopes from SPOM were analyzed and the microalgae community was characterized. The sampling strategy was bimonthly from March 2002 to December 2003; samples were taken from an intertidal mudflat. Four main sources contributed to the SPOM pool: terrigenous input from rivers, neritic phytoplankton, resuspended microphytobenthos and periodic inputs from intertidal Zostera noltii meadows. Seasonal fluctuations were observed in both years of the study period: (1) SPOM collected in the spring of 2002 (δ13C = −25‰ to −23‰) was mainly composed of fresh estuarine inputs; (2) SPOM from the summer and fall of 2002 and 2003 was predominantly neritic phytoplankton (δ13C = − 22‰ to −19‰); (3) SPOM from the winter of 2002, spring of 2003 and winter of 2003 (δ13C = −21 to −23‰) was composed of a mixture of decayed terrigenous river inputs and pelagic phytoplankton, which was predominantly resuspended microphytobenthos. In the summer of 2003—the warmest summer on record in southern France and Europe—SPOM was particularly enriched for 13C, with δ13C values ranging from −14‰ to −12‰. Pulses in δ13C values, indicative of 13C-enriched decaying materials, extended into the fall. These were attributed to benthic intertidal inputs, including both resuspended microphytobenthos and Z. noltii detritus. Changes in SPOM sources in Marennes-Oléron Bay may lead to differences in the quality of the trophic environment available for reared oysters.  相似文献   

7.
To assess the potential of stable isotope ratios as an indicator of fish migration within estuaries, stable isotope ratios in important zooplankton species were analyzed in relation to estuarine salinity gradients. Gut contents from migratory juveniles of the euryhaline marine fish Lateolabrax japonicus were examined along the Chikugo River estuary of the Ariake Sea, which has the most developed estuarine turbidity maximum (ETM) in Japan. Early juveniles in March and April preyed primarily on two copepod species; Sinocalanus sinensis at lower salinities and Acartia omorii at higher salinities. Late juveniles (standard length > 40 mm) at lower salinities preyed exclusively on the mysid Acanthomysis longirostris until July and complementarily on the decapod Acetes japonicus in August. These prey species were collected along the estuary during the spring–summer seasons of 2003 and 2004, and their carbon and nitrogen stable isotope ratios (δ13C and δ15N) were evaluated. The δ13C values of prey species were distinct from each other and were primarily depleted within and in close proximity to the ETM (salinity < 10); S. sinensis (−26.6‰) < Acanthomysis longirostris (−23.3‰) < Acartia omorii (−21.1‰) < Acetes japonicus (−18.5‰). The overall gradient of δ13C with salinity occurred for all prey species and showed minor temporal fluctuations, while it was not directly influenced by the δ13C values in particulate organic matter along the estuary. In contrast to δ13C, the δ15N values of prey species did not exhibit any clear relationship with salinity. The present study demonstrated that δ13C has the potential for application as a tracer of fish migration into lower salinity areas including the ETM.  相似文献   

8.
We simultaneously followed stable carbon (δ13C) and nitrogen (δ15N) isotopes in a two-source food web model to determine trophic levels and the relative importance of open water- and ice-associated food sources (phytoplankton vs. ice algae) in the lower marine food web in the European Arctic during four seasons. The model is based upon extensive seasonal data from 1995 to 2001.Phytoplankton, represented by samples of particulate organic matter from open water (Pelagic-POM) and ice algae, represented by samples from the underside of the ice (Ice-POM), were isotopically different. Ice-POM was generally dominated by the typical ice diatoms Nitzschia frigida and Melosira arctica and was more enriched than Pelagic-POM in 13C (δ13C = −20‰ vs. −24‰), but less enriched in 15N (δ15N = 1.8‰ vs. 4.0‰). However, when dominated by pelagic algae, Ice-POM was enriched in 13C and 15N similarly to Pelagic-POM.The derived trophic enrichment factors for δ15N (ΔN = 3.4‰) and δ13C (ΔC = 0.6‰) were similar in both pelagic and sympagic (ice-associated) systems, although the ΔC for the sympagic system was variable.Trophic level (TL) range for zooplankton (TL = 1.8-3.8) was similar to that of ice fauna (TL = 1.9-3.7), but ice amphipods were generally less enriched in δ15N than zooplankton, reflecting lower δ15N in Ice-POM compared to Pelagic-POM. For bulk zooplankton, TLs and carbon sources changed little seasonally, but the proportion of herbivores was higher during May-September than in October and March. Overall, we found that the primary carbon source for zooplankton was Pelagic-POM (mean 74%), but depending on species, season and TL, substantial carbon (up to 50%) was supplied from the sympagic system. For bulk ice fauna, no major changes were found in TLs or carbon sources from summer to autumn. The primary carbon source for ice fauna was Ice-POM (mean 67%), although ice fauna with TL > 3 (adult Onisimus nanseni and juvenile polar cod) primarily utilized a pelagic food source.  相似文献   

9.
Authigenic carbonates were collected from methane seeps at Hydrate Hole at 3113 m water depth and Diapir Field at 2417 m water depth on the northern Congo deep-sea fan during RV Meteor cruise M56. The carbonate samples analyzed here are nodules, mainly composed of aragonite and high-Mg calcite. Abundant putative microbial carbonate rods and associated pyrite framboids were recognized within the carbonate matrix. The δ13C values of the Hydrate Hole carbonates range from ?62.5‰ to ?46.3‰ PDB, while the δ13C values of the Diapir Field carbonate are somewhat higher, ranging from ?40.7‰ to ?30.7‰ PDB, indicating that methane is the predominant carbon source at both locations. Relative enrichment of 18O (δ18O values as high as 5.2‰ PDB) are probably related to localized destabilization of gas hydrate. The total content of rare earth elements (REE) of 5% HNO3-treated solutions derived from carbonate samples varies from 1.6 ppm to 42.5 ppm. The shale-normalized REE patterns all display positive Ce anomalies (Ce/Ce* > 1.3), revealing that the carbonates precipitated under anoxic conditions. A sample from Hydrate Hole shows a concentric lamination, corresponding to fluctuations in δ13C values as well as trace elements contents. These fluctuations are presumed to reflect changes of seepage flux.  相似文献   

10.
Authigenic minerals were studied in Holocene shelf sediments of the Laptev Sea (cold methane seep site, water depth 71 m). The study presents the first finds of large hard carbonate concretions with Mg-calcite cement in recent sediments of the Arctic shelf seas. These concretions differ from previously reported glendonites and concretions from bottom sediments of the White Sea, Kara Sea, Sea of Okhotsk, etc. A study of the morphology, microstructure, and composition of these newly reported concretions revealed the multistage formation of carbonates (structural varieties of Mg-calcite and aragonite). It was shown that organic matter played an important role in the formation of authigenic carbonates, i.e., in the formation of sedimentary–diagenetic Mg-calcite. The role of methane as a possible source for authigenic carbonate formation was estimated. It was found that methane-derived Mg-calcite accounts for 17–35% of concretion materials. Mg-calcite had δ13С-Сcarb values between–24 and–23‰ and δ13С-Сorg values between–44.5 and–88.5‰.  相似文献   

11.
Vodyanitskii mud volcano is located at a depth of about 2070 m in the Sorokin Trough, Black sea. It is a 500-m wide and 20-m high cone surrounded by a depression, which is typical of many mud volcanoes in the Black Sea. 75 kHz sidescan sonar show different generations of mud flows that include mud breccia, authigenic carbonates, and gas hydrates that were sampled by gravity coring. The fluids that flow through or erupt with the mud are enriched in chloride (up to ∼650 mmol L−1 at ∼150-cm sediment depth) suggesting a deep source, which is similar to the fluids of the close-by Dvurechenskii mud volcano. Direct observation with the remotely operated vehicle Quest revealed gas bubbles emanating at two distinct sites at the crest of the mud volcano, which confirms earlier observations of bubble-induced hydroacoustic anomalies in echosounder records. The sediments at the main bubble emission site show a thermal anomaly with temperatures at ∼60 cm sediment depth that were 0.9 °C warmer than the bottom water. Chemical and isotopic analyses of the emanated gas revealed that it consisted primarily of methane (99.8%) and was of microbial origin (δD-CH4 = −170.8‰ (SMOW), δ13C-CH4 = −61.0‰ (V-PDB), δ13C-C2H6 = −44.0‰ (V-PDB)). The gas flux was estimated using the video observations of the ROV. Assuming that the flux is constant with time, about 0.9 ± 0.5 × 106 mol of methane is released every year. This value is of the same order-of-magnitude as reported fluxes of dissolved methane released with pore water at other mud volcanoes. This suggests that bubble emanation is a significant pathway transporting methane from the sediments into the water column.  相似文献   

12.
Isotopic analyses of nitrate by the denitrifier method, and indeed by many other analytical methods, do not discriminate between nitrate and nitrite. For samples containing both chemical species, accurate isotopic analysis of nitrate requires either removal of nitrite or independent isotopic analysis of nitrite and subtraction of its contribution to the mixed isotopic signal. This study evaluates the application of a variety of available analytical approaches to the isotopic analysis of mixed nitrate and nitrite solutions, with the goal of producing accurate coupled isotopic analyses of both nitrate and nitrite. These methods are tested on mixtures of standard solutions of nitrate and nitrite, and then applied to the coupled δ15N and δ18O analyses of nitrate and nitrite in waters of the Eastern Tropical North Pacific (ETNP). Results from standard mixtures show that even for extreme values of nitrate and nitrite δ15N and δ18O, both nitrite removal by ascorbate and nitrite isotopic analysis and subtraction from the mixed isotopic signal yield nitrate δ15N and δ18O values that are close to the expected values. Application of these analyses to samples from the ETNP yielded δ15NNO3 and δ18ONO3 values as high as 21‰ vs. AIR and 19‰ vs. VSMOW, respectively. Conversely, very low δ15N values were observed in nitrite, with values ranging from − 7.2 to − 18.5‰ vs. AIR. Removal of nitrite from ETNP samples thus revealed differences of up to 5‰ between NO3- and NO2- + NO3- for both δ15N and δ18O. Moreover, the δ15N offset between co-occurring nitrate and nitrite is greater than expected from the action of denitrification alone and may provide a unique constraint on the processes involved in the cycling of nitrite in and around oxygen deficient zones. Finally, subtraction of the nitrite δ15N and δ18O from ETNP samples allows the extension of the Δ(15,18) tracer into suboxic regions containing nitrite. The magnitude and distribution of Δ(15,18) in these samples suggests an important role for nitrite reoxidation in nitrate isotope variations.  相似文献   

13.
Systematic analyses have been carried out on two gas hydrate-bearing sediment core samples, HYPV4, which was preserved by CH4 gas pressurization, and HYLN7, which was preserved in liquid-nitrogen, recovered from the BPXA-DOE-USGS Mount Elbert Stratigraphic Test Well. Gas hydrate in the studied core samples was found by observation to have developed in sediment pores, and the distribution of hydrate saturation in the cores imply that gas hydrate had experienced stepwise dissociation before it was stabilized by either liquid nitrogen or pressurizing gas. The gas hydrates were determined to be structure Type I hydrate with hydration numbers of approximately 6.1 by instrumentation methods such as powder X-ray diffraction, Raman spectroscopy and solid state 13C NMR. The hydrate gas composition was predominantly methane, and isotopic analysis showed that the methane was of thermogenic origin (mean δ13C = −48.6‰ and δD = −248‰ for sample HYLN7). Isotopic analysis of methane from sample HYPV4 revealed secondary hydrate formation from the pressurizing methane gas during storage.  相似文献   

14.
The 13C signature of forty-five macroalgal species from intertidal zones at Cádiz Bay was analysed in order to research the extension of diffusive vs. non-diffusive utilisation of dissolved inorganic carbon (DIC) and to perform a comparison with data published for other bio-geographic regions. The ∂13C values ranged from −6.8‰ to −33‰, although the span of variation was different depending on the taxa. Thus, ∂13C for Chlorophyta varied from −7‰ (Codium adhaerens) to −29.6‰ (Flabellia petiolata), while all the Phaeophyceae (excepting Padina pavonica with ∂13C higher than −10‰) had values between −10‰, and −20‰. The widest variation range was recorded in Rhodophyta, from values above −10‰ (Liagora viscida) to values lower than −30‰ obtained in three species belonging to the subclass Rhodymeniophycidae. Accordingly, the mean ∂13C value calculated for red algae (−20.2‰) was significantly lower than that for brown (−15.9‰) and green algae (−15.6‰). Most of the analysed red algae were species inhabiting crevices and the low intertidal fringe which explains that, on average, the shaded-habitat species had a ∂13C value lower than those growing fully exposed to sun (i.e. in rockpools or at the upper intertidal zone). The comparison between the capacity for non-diffusive use of DIC (i.e. active or facilitated transport of HCO3− and/or CO2) and the ∂13C values reveals that values more negative than −30‰ indicate that photosynthesis is dependent on CO2 diffusive entry, whereas values above this threshold would not indicate necessary the operation of a non-diffusive DIC transport mechanism. Furthermore, external carbonic anhydrase activity (extCA) and ∂13C values were negatively correlated indicating that the higher the dependence of the photosynthesis on the CO2 supplied from HCO3− via extCA, the lower the ∂13C in the algal material. The comparison between the ∂13C values obtained for the analysed species and those published for species inhabiting other bio-geographic areas (warm-temperate, cold and polar) suggests that globally (at least for the red and brown algae) the non-diffusive entry of DIC is more widely spread among the species from Cádiz Bay than among those of polar regions. If it is assumed that non-diffusive use of DIC implies saturation of photosynthesis at the present-day CO2 concentration in seawater, our data indicate that the potential impact of the acidification on photosynthesis in the seaweed communities will be different depending on the latitude.  相似文献   

15.
Preservation of organic matter in estuarine and coastal areas is an important process in the global carbon cycle. This paper presents bulk δ13C and C/N of organic matter from source to sink in the Pearl River catchment, delta and estuary, and discusses the applicability of δ13C and C/N as indicators for sources of organic matter in deltaic and estuarine sediments. In addition to the 91 surface sediment samples, other materials collected in this study cover the main sources of organic material to estuarine sediment. These are: terrestrial organic matter (TOM), including plants and soil samples from the catchment; estuarine and marine suspended particulate organic carbon (POC) from both summer and winter. Results show that the average δ13C of estuarine surface sediment increases from −25.0 ± 1.3‰ in the freshwater environment to −21.0 ± 0.2‰ in the marine environment, with C/N decreasing from 15.2 ± 3.3 to 6.8 ± 0.2. In the source areas, C3 plants have lower δ13C than C4 plants (−29.0 ± 1.8‰ and −13.1 ± 0.5‰ respectively). δ13C increases from −28.3 ± 0.8‰ in the forest soil to around −24.1‰ in both riverbank soil and mangrove soil due to increasing proportion of C4 grasses. The δ13CPOC increases from −27.6 ± 0.8‰ in the freshwater areas to −22.4 ± 0.5‰ in the marine-brackish-water areas in winter, and ranges between −24.0‰ in freshwater areas and −25.4‰ in brackish-water areas in summer. Comparison of the δ13C and C/N between the sources and sink indicates a weakening TOM and freshwater POC input in the surface sedimentary organic matter seawards, and a strengthening contribution from the marine organic matter. Thus we suggest that bulk organic δ13C and C/N analysis can be used to indicate sources of sedimentary organic matter in estuarine environments. Organic carbon in surface sediments derived from anthropogenic sources such as human waste and organic pollutants from industrial and agricultural activities accounts for less than 10% of the total organic carbon (TOC). Although results also indicate elevated δ13C of sedimentary organic matter due to some agricultural products such as sugarcane, C3 plants are still the dominant vegetation type in this area, and the bulk organic δ13C and C/N is still an effective indicator for sources of organic matter in estuarine sediments.  相似文献   

16.
One hundred and ten carbon and nineteen strontium isotopic values of outcropping Cambrian Series 2 and Series 3 carbonate rocks in the Penglaiba section of the Keping area were analyzed. Effective isotopic data with little influence of diagenesis were used to address the global correlation. The δ13C values exhibit two major positive excursions (peaking at 3.1‰ and 3.3‰) and three major negative excursions (peaking at −3.0‰, −4.2‰ and −3.2‰). The carbon isotope excursions (peaking at −3.0‰ and −4.2‰) across the Cambrian Series 2-Series 3 boundary show good correlations with similar variations reported in Siberia, Laurentia, North China and South China. In contrast, the other three carbon isotope excursions (peaking at 3.1‰, 3.3‰ and −3.2‰) do not have a fairly good global correlation because of the lack of biostratigraphic data. With respect to the 87Sr/86Sr ratios, they show good correlation with those reported for Laurentia, and further support the global δ13C comparison. On the basis of these new data, it is showed that the combination of δ13C curves and 87Sr/86Sr variations serves as a powerful tool for correlation and subdivision of Cambrian strata in the Tarim Basin of northwest China, and provides new data for global correlation.  相似文献   

17.
About 120 gas seepage vents were documented along the west and southwest coast of the Hainan Island, South China Sea, in water depths usually less than 50 m. The principal seepage areas include the Lingtou Promontory, the Yinggehai Rivulet Mouth, Yazhou Bay, the Nanshan Promontory and the Tianya Promontory. They occur along three major zones, reflecting the control by faults and lateral conduits within the basement. It is estimated that the total gas emission from these seepage vents is 294–956 m3/year. The seepage gases are characterized by a high CH4 content (76%), heavy δ13C1 values (−38 to −33‰) and high C1/C1–5 ratios (0.95–1.0), resembling the thermogenic gases from the diapiric gas fields of the Yinggehai Basin. Hydrocarbon–source correlation shows that the hydrocarbons in the sediments from seepage areas can be correlated with the deeply buried Miocene source rocks and sandstone reservoirs in the central depression. The 2D basin modeling results based on a section from the source rock center to the gas seepage sites indicate that the gas-bearing fluids migrated from the source rocks upward through faults or weak zones encompassed by shale diapirism or in up-dip direction along the sandstone-rich strata of Huangliu Formation to arrive to seabed and form the nearshore gas seepages. It is suggested that the seepage gases are sourced from the Miocene source rocks in the central depression of the Yinggehai Basin. This migration model implies that the eastern slope zone between the gas source area of the central depression and the seepage zone is also favorable place for gas accumulation.  相似文献   

18.
We estimated the composition of two food sources for the cultured pearl oyster Pinctada fucata martensii using stable isotopes and stomach content analysis in the coastal areas of the Uwa Sea, Japan. The δ13C values of oysters (−17.5 to −16.8‰) were intermediate between that of particulate organic matter (POM, −20.2 to −19.1‰) and attached microalgae on pearl cages (−13.0‰). An isotope mixing model suggested that oysters were consuming 78% POM (mainly phytoplankton) and 22% attached microalgae. The attached microalgal composition of the stomach content showed a strong resemblance to the composition of that estimated through the isotope mixing model, suggesting preferential utilization of specific components is unlikely in this species. These results indicate that P. fucata martensii feed on a mixture of phytoplankton and attached microalgae, and that the attached microalgae on pearl cages can serve as an important additional food source.  相似文献   

19.
Multidisciplinary study of seep-related structures on Southern Vøring Plateau has been performed during several UNESCO/IOC TTR cruises on R/V Professor Logachev. High-resolution sidescan sonar and subbottom profiler data suggest that most of the studied fluid discharge structures have a positive relief at their central part surrounded by depression. Our data shows that the present day fluid activity is concentrated on the top of these “seep mounds”. Number of high hydrocarbon (HC) gas saturated sediment cores and 5 cores with gas hydrate presence have been recovered from these structures. δ13C of methane (between −68 and −94.6‰ VPDB) and dry composition of the gas points to its biogenic origin. The sulfate depletion generally occurs within the upper 30–200 cm bsf and usually coincides with an increase of methane concentration. Pore water δ18O ranges from 0.29 to 1.14‰ showing an overall gradual increase from bottom water values (δ18O ∼ 0.35‰). Although no obvious evidence of fluid seepage was observed during the TV surveys, coring data revealed a broad distribution of living Pogonophora and bacterial colonies on sea bottom inside seep structures. These evidences point to ongoing fluid activity (continuous seepage of methane) through these structures. From other side, considerable number and variety of chemosynthetic macro fauna with complete absence of living species suggest that present day level of fluid activity is significantly lower than it was in past. Dead and subfossil fauna recovered from various seep sites consist of solemyid (Acharax sp.), thyasirid and vesicomyid (cf. Calyptogena sp.) bivalves belonging to chemosymbiotic families. Significant variations in δ13C (−31.6‰ to −59.2‰) and δ18O (0.42‰ and 6.4‰) of methane-derived carbonates collected from these structures most probably related to changes in gas composition and bottom water temperature between periods of their precipitation. This led us to ideas that: (1) seep activity on the Southern Vøring Plateau was started with large input of the deep thermogenic gas and gradually decries in time with increasing of biogenic constituent; (2) authigenic carbonate precipitation started at the near normal deep sea environments with bottom water temperature around +5 °C and continues with gradual cooling up to negative temperatures recording at present time.  相似文献   

20.
Carbon and nitrogen stable isotope composition of a range of organisms collected from two intermittently connected floodplain pools in the Ross River estuary were analysed to assess the extent to which carbon fixed by terrestrial wetland producers is incorporated into adjacent aquatic food webs. The two pools differed in surrounding vegetation with one surrounded by mangroves and the other by the salt couch Sporobolus virginicus. At both pools, animals showed differences in δ13C, indicating differences in sources of carbon. Since δ13C values of C3 mangroves (−29.7 to −26.3‰) were very different from those of the C4 salt couch (−16.3 to −15.4‰), it was possible to determine the importance of terrestrial wetland producers by comparing isotope values of consumers between sites, in a species by species approach. Most animal species collected showed lower δ13C at the mangrove pool than at the Sporobolus pool, which indicates a greater incorporation of mangrove carbon at the mangrove pool. However, the animals’ isotopic shifts were also similar to that shown by epiphytes, and hence the differences in animal δ13C could also be a result of a dependence on these producers. The IsoSource model was useful to clarify this question, indicating that mangrove and salt marsh material was a crucial contributor to the diet of several fish and invertebrate species at both sites, indicating that carbon of terrestrial origin is incorporated in the estuarine food web.  相似文献   

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