Geochemical characteristics and organic carbon sources within the upper reaches of the Xi River,southwest China during high flow

Carbon (POC, DOC) and carbon isotopes (δ¹³C) within two headwater tributaries to the Xi River Basin, southwest China were analyzed to document the geochemical characteristics and sources of organic carbon (C) within basins characterized by a monsoonal climate and karst landforms. δ¹³ C _POC value and C/N ratio data indicate that suspended soil organic carbon (SOC) was an important source of POC in both the Nanpan and Beipan rivers (i.e., the studied tributaries). However, differences in C sources exist between the Nanpan and Beipan River Basins. Higher terrestrial plants supplied a portion of the POC within the Beipan River. In contrast, the Nanpan River was characterized by an inverse correlation between POC and DOC, and a positive relationship between the δ¹³C values. These trends indicate that DOC within the Nanpan River was partly derived from the degradation of soil C within the water column. In addition, the interception of C by hydrological projects (e.g., dams) positioned along the Nanpan River led to higher DOC/POC ratios. In contrast, within the Beipan River δ¹³C _DOC values range from ?20 to ?25.2 ‰ and are consistent with ratios associated with soil C, suggesting that leaching of C from catchment soil was the dominant source of DOC. Organic C in tributaries to the Beipan River may also have been derived from intense upland soil erosion, a process that resulted in the lowest DOC/POC ratios. The collected data indicate that land-use changes have potentially influenced regional- to local-scale organic C budgets within subtropical basins subjected to karstification.     

Organic carbon isotope ratios in Quaternary cores from the Gulf of Mexico

All of the major deep-water sedimentary provinces of the Gulf of Mexico were sampled with 48 piston cores, representative of the late Quaternary. The amount (per cent) and δ5C¹³ of the organic carbon in the sediment was measured at intervals within each core.Graphs of δC¹³ versus depth for each core give an indication of the sedimentological history of the Gulf. They show the extent of terrestrial influence on the Gulf during the late Pleistocene.Changes in δC¹³ of up to 6.0%.(from ~ ?19%.to ?25%. vs NBS-20) were measured across the Pleistocene-Holocene boundary in cores from the abyssal plain. These changes are consistent with a model wherein varying amounts of land-derived organic carbon were transported to the Gulf basin during glacial periods.By comparing graphs for cores from different areas, it was concluded that the major parameter affecting the δC¹³ values of organic carbon from marine sediments is the relative amount of terrestrial material present in the sediment. The maximum possible effect of the Pleistocene-Holocene temperature change in the Gulf was determined to be ~1.0%, if such an effect occurs at all.     

Organic carbon isotope ratios of recent sediments from coastal lagoons of the Gulf of Mexico,Mexico

The stable carbon isotope composition sedimentary organic carbon was determined in the sediments of seven coastal lagoons of the Gulf of Mexico, Mexico. For most of the lagoons the δ¹³C values for sediments ranged from ?20.1 to ?23.9%. Anomalously low values, ?26.8 to 29.3%. were determined in sediments of two of the studied lagoons, probably due to the presence of organic carbon from anthropogenic sources, naturally absent in these environments. The δ¹³C values determined in the tissues of oysters collected at the same time in the different lagoons were very similar to those recorded in the sediments.     

Carbon isotope composition of organic seston and sediments in a Georgia salt marsh estuary

The distribution of δ¹³C values for organic seston and sediment was determined in three sounds in the Spartina marsh estuaries along the Georgia coast, which had high, moderate, and low inputs of freshwater. Organic matter in all three sounds had similar carbon isotope compositions, for the most part within the range of marine values (δ¹³C of ?18%. to ?24%.). It appears that river flow does not introduce significant quantities of particulate C₃ plant material (δ¹³C of ?25%. to ?28%.) to Georgia estuaries. Evaluation of δ¹³C values of estuarine seston and three size fractions of sediment indicated that while Spartina carbon (δ¹³C of ?13%.) can be an important component of organic matter in intertidal sediments (mean δ¹³C of ?14.3%. to ?20.0%.), it is less so in subtidal sediments (mean δ¹³C of ?18.8%. to ?21.2%.), and it is hardly present at all in the seston (mean δ¹³C of ?24.5%.). δ¹³C values of dissolved inorganic carbon (DIC) in several water samples ranged between ?2.5%. and ?5.6%., suggesting that the isotope composition of estuarine DIC is influenced by respiratory CO₂ derived from metabolism of ¹³C-depleted plant carbon. Phytoplankton production utilizing this comparatively light DIC could be a source of relatively negative δ¹³C carbon in the estuary. Additional origins of estuarine organic matter greatly depleted in ¹³C compared to Spartina carbon remain to be identified.     

Stable carbon isotopic evidence for sources of particulate organic carbon found in sea foam

Particulate organic carbon found in sea foam and water samples from North Inlet, South Carolina, were examined for their δ¹³C isotopic composition. Sea foam particulate organic carbon (POC) δ¹³C values ranged from ?20.4 to ?24.6‰ (mean=?22.3‰) and water POC δ¹³C values ranged from ?21.0 to ?28.5‰ (mean= ?24.4‰). Temporal trends in sea foam and water POC indicate that δ¹³C values for both POC components are depleted in the colder months and enriched in the warmer months. Measurement of δ¹³C from potential sources for organic matter found in sea foam, combined with data on macroalgae productivity and phytoplankton biomass, indicates that macroalgae are the principal source of POC for sea foam in the colder months. In the warmer months, phytoplankton appear to be more important contributors. The observed water POC δ¹³C values were always depleted relative to foam POC δ¹³C values. This isotopic difference may result from chemical segregation during sea foam formation or may reflect DOC δ¹³C values from terrestrial origins.     

Carbon,oxygen and sulphur isotope variations in concretions from the Upper Lias of N.E. England

Carbon, oxygen and sulphur isotope data for transects across two pyrite-bearmg carbonate concretions, and their host sediments, from the Upper Lias of N.E. England show symmetrical zonation. δ¹³C_PDB values of the calcite cement (?12.9 to ?15.4%.) indicate that most of it originated from organic matter by bacterial reduction of sulphate, augmented with marine and, to a lesser extent, fermentation derived carbonate. Organic carbon (δ¹³C_PDB = ?26.1 to ?37.0%.). reflects the admixture of allochtho-nous terrestrial organic matter with marine material and the selective preservation of isotopically light organic material through microbiological degradation.Two phases of pyrite are present in each concretion. The earlier framboidal pyrite formed throughout the sediment prior to concretionary growth and has δ³⁴S_CD values of ?22 to ?26%. indicating formation by open system sulphate reduction. The later euhedral phase is more abundant and reaches values of ? 2.5 to ? 5.5%. at concretion margins. This phase of sulphate reduction provided the carbonate source for concretionary growth and occurred in a partially closed system. The δ¹³C and δ³⁴S data are consistent with mineralogical and chemical evidence which suggest that both concretions formed close to the sediment surface. The δ₁₈O values of the calcite in one concretion (δ¹⁸O_PDB = 2.3 to ?4.8%.) indicate precipitation in pore waters whose temperature and isotopic composition was close to that of overlying seawater. The other concretion is isotopically much lighter (δ¹⁸O_PDB?8.9 to ?9.9%.) and large δ¹⁸O differences between concretions in closely-spaced horizons imply that local factors control the isotopic composition of pore waters.     

The isotopic composition of particulate organic carbon in mountain rivers of Taiwan

Small rivers draining mountain islands are important in the transfer of terrestrial particulate organic carbon (POC) to the oceans. This input has implications for the geochemical stratigraphic record. We have investigated the stable isotopic composition of POC (δ¹³C_org) in rivers draining the mountains of Taiwan. In 15 rivers, the suspended load has a mean δ¹³C_org that ranges from −28.1±0.8‰ to −22.0±0.2‰ (on average 37 samples per river) over the interval of our study. To investigate this variability we have supplemented suspended load data with measurements of POC in bedrock and river bed materials, and constraints on the composition of the terrestrial biomass. Fossil POC in bedrock has a range in δ¹³C_org from −25.4±1.5‰ to −19.7±2.3‰ between the major geological formations. Using coupled δ¹³C_org and N/C we have found evidence in the suspended load for mixing of fossil POC with non-fossil POC from the biosphere. In two rivers outside the Taiwan Central Range anthropogenic land use appears to influence δ¹³C_org, resulting in more variable and lower values than elsewhere. In all other catchments, we have found that 5‰ variability in δ¹³C_org is not controlled by the variable composition of the biomass, but instead by heterogeneous fossil POC.In order to quantify the fraction of suspended load POC derived from non-fossil sources (F_nf) as well as the isotopic composition of fossil POC (δ¹³C_fossil) carried by rivers, we adapt an end-member mixing model. River suspended sediments and bed sediments indicate that mixing of fossil POC results in a negative trend between N/C and δ¹³C_org that is distinct from the addition of non-fossil POC, collapsing multiple fossil POC end-members onto a single mixing trend. As an independent test of the model, F_nf reproduces the fraction modern (F_mod) in our samples, determined from ¹⁴C measurements, to within 0.09 at the 95% confidence level. Over the sampling period, the mean F_nf of suspended load POC was low (0.29 ± 0.02, n = 459), in agreement with observations from other mountain rivers where physical erosion rates are high and fossil POC enters river channels. The mean δ¹³C_fossil in suspended POC varied between −25.2±0.5‰ and −20.2±0.6‰ from catchment to catchment. This variability is primarily controlled by the distribution of the major geological formations. It also covers entirely the range of δ¹³C_org found in marine sediments which is commonly thought to derive from mixing between marine and terrigenous POC. If land-sourced POC is preserved in marine sediments, then changes in the bulk δ¹³C_org observed offshore Taiwan could instead be explained by changes in the onshore provenance of sediment. The range in δ¹³C_org of fossil organic matter in sedimentary rocks exposed at the surface is large and given the importance of these rocks as a source of clastic sediment to the oceans, care should be taken in accounting for fossil POC in marine deposits supplied by active mountain belts.     

Temporal variability in terrestrially-derived sources of particulate organic carbon in the lower Mississippi River and its upper tributaries

In this study, we examined the temporal changes of terrestrially-derived particulate organic carbon (POC) in the lower Mississippi River (MR) and in a very limited account, the upper tributaries (Upper MR, Ohio River, and Missouri River). We used for the first time a combination of lignin-phenols, bulk stable carbon isotopes, and compound-specific isotope analyses (CSIA) to examine POC in the lower MR and upper tributaries.A lack of correlation between POC and lignin phenol abundances (Λ₈) was likely due to dilution effects from autochthonous production in the river, which has been shown to be considerably higher than previously expected. The range of δ¹³C values for p-hydroxycinnamic and ferulic acids in POC in the lower river do support that POM in the lower river does have a significant component of C₄ in addition to C₃ source materials. A strong correlation between δ¹³C values of p-hydroxycinnamic, ferulic, and vanillyl phenols suggests a consistent input of C₃ and C₄ carbon to POC lignin while a lack of correlation between these same phenols and POC bulk δ¹³C further indicates the considerable role of autochthonous carbon in the lower MR POC budget. Our estimates indicate an annual flux of POC of 9.3 × 10⁸ kg y⁻¹ to the Gulf of Mexico. Total lignin fluxes, based on Λ₈ values of POC, were estimated to be 1.2 × 10⁵ kg y⁻¹. If we include the total dissolved organic carbon (DOC) flux (3.1 × 10⁹ kg y⁻¹) reported by [Bianchi T. S., Filley T., Dria K. and Hatcher, P. (2004) Temporal variability in sources of dissolved organic carbon in the lower Mississippi River. Geochim. Cosmochim. Acta68, 959-967.], we get a total organic carbon flux of 4.0 × 10⁹ kg y⁻¹. This represents 0.82% of the annual total organic carbon supplied to the oceans by rivers (4.9 × 10¹¹ kg).     

Tracing iron-fueled microbial carbon production within the hydrothermal plume at the Loihi seamount

The Loihi hydrothermal plume provides an opportunity to investigate iron (Fe) oxidation and microbial processes in a system that is truly Fe dominated and distinct from mid-ocean ridge spreading centers. The lack of hydrogen sulfide within the Loihi hydrothermal fluids and the presence of an oxygen minimum zone at this submarine volcano’s summit, results in a prolonged presence of reduced Fe within the dispersing non-buoyant plume. In this study, we have investigated the potential for microbial carbon fixation within the Loihi plume. We sampled for both particulate and dissolved organic carbon in hydrothermal fluids, microbial mats growing around vents, and the dispersing plume, and carried out stable carbon isotope analysis on the particulate fraction. The δ¹³C values of the microbial mats ranged from −23‰ to −28‰, and are distinct from those of deep-ocean particulate organic carbon (POC). The mats and hydrothermal fluids were also elevated in dissolved organic carbon (DOC) compared to background seawater. Within the hydrothermal plume, DOC and POC concentrations were elevated and the isotopic composition of POC within the plume suggests mixing between background seawater POC and a ¹³C-depleted hydrothermal component. The combination of both DOC and POC increasing in the dispersing plume that cannot solely be the result of entrainment and DOC adsorption, provides strong evidence for in-situ microbial productivity by chemolithoautotrophs, including a likelihood for iron-oxidizing microorganisms.     

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

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.     

Dissolved and particulate organic carbon in Chesapeake Bay

We measured dissolved and particulate organic carbon (DOC and POC) in samples collected along 13 transects of the salinity gradient of Chesapeake Bay. Riverine DOC and POC end-members averaged 232±19 μM and 151±53 μM, respectively, and coastal DOC and POC end-members averaged 172±19 μM and 43±6 μM, respectively. Within the chlorophyll maximum, POC accumulated to concentrations 50–150 μM above those expected from conservative mixing and it was significantly correlated with chlorophylla, indicating phytoplankton origin. POC accumulated primarily in bottom waters in spring, and primarily in surface waters in summer. Net DOC accumulation (60–120 μM) was observed within and downstream of the chlorophyll maximum, primarily during spring and summer in both surface and bottom waters, and it also appeared to be derived from phytoplankton. In the turbidity maximum, there were also net decreases in chlorophylla (?3 μg l^?1 to ?22 μg l^?1) and POC concentrations (?2 μM to ?89 μM) and transient DOC increases (9–88 μM), primarily in summer. These occurred as freshwater plankton blooms mixed with turbid, low salinity seawater, and we attribute the observed POC and DOC changes to lysis and sedimentation of freshwater plankton. DOC accumulation in both regions of Chesapeake Bay was estimated to be greater than atmospheric or terrestrial organic carbon inputs and was equivalent to ≈10% of estuarine primary production.     

Organic carbon 13C12C variations in estuarine sediments

The δC¹³ value for sedimentary organic carbon in four estuaries of the Gulf of Mexico increases with radial distance from the river mouth. Mass balance calculations indicate that terrestrial organic carbon is limited to sediments within a relatively short distance from the river mouth. This distance is a function of the discharge rate of the river. For the Mississippi River, terrestrial organic carbon is limited to sediments within 69 km of the mouth of Pass à Loutre and 61 km of South Pass. These data indicate that the low δC¹³ (< ?22%.) values reported for Pleistocene sediments in the Gulf of Mexico may be the result of factors in addition to the postulated large influx of terrestrial organic carbon.     

Isotopic fractionation between coexisting organic carbon—carbonate pairs in Precambrian sediments

δ¹³C_org and δ¹³C_carb values of 58 coexisting organic carbon-carbonate pairs covering the whole Precambrian have yielded means of ?24.7 ± 6.0%. [PDB] and +0.9 ± 2.7%. [PDB], respectively. Accordingly, isotopic fractionation between inorganic and organic carbon in Precambrian sediments is about the same as in geologically younger rocks (Δδ ? 25%.), a slight increase displayed by the Early Precambrian pairs (Δδ ? 28%.) being probably biassed by an over-representation in this age group of samples from one single locality (nevertheless, this value still lies within the range permitted for a possible deviation). It is reasonable to assume, therefore, that the overall isotope fractionation factor governing biological fixation of inorganic carbon has been virtually constant since some 3.3 × 10⁹ yr ago.     

Significance of hypoburrow nodule formation associated with large biogenic sedimentary structures in open-marine bay siliciclastics of the Upper Eocene Birket Qarun Formation,Wadi El-Hitan,Fayum, Egypt

Unusually large biogenic sedimentary structures from the shallow quiescent-marine siliciclastics of the Upper Eocene Birket Qarun Formation in the Fayum area of Egypt display pronounced concretion formation around the trace fossils. The structures are massive, and vary morphologically, forming branched pillars (up to dm-scale), vertical (up to 180 cm height) amphora-like masses, and 3-D box-work “maze”. Bioturbation, mainly Thalassinoides attributable to the Glossifungites ichnofacies, mediated and modified the physical and chemical microenvironments influencing early diagenesis; i.e., burrows promote the precipitation of pervasive calcite-dominated cement. The inferred paragenesis, combined with the negative (light) carbon and oxygen stable-isotopic values of the bulk calcite (δ¹³C_PDB from ?0.94 to ?4.98‰ and δ¹⁸O_PDB from ?4.63 to ?7.22‰) and bulk dolomite (δ¹³C_PDB from ?2.05 to ?8.23‰ and δ¹⁸O_PDB from ?1.41 to ?11.20‰), imply that the pore-water carbon was derived directly from seawater and dissolution of metastable carbonate, which was mediated by bacterial decomposition of organic matter and mixing of meteoric ground water. Thereby, the carbonate cement precipitated mostly under eodiagenetic conditions near the sediment/water interface (<~3 m in depth). The distribution of these structures is confined to parasequence-bounding flooding surfaces (generally expressed as transgressive surfaces of erosion). Notably, sedimentological, ichnological and paragenetic data can be related to stratigraphic evolution such that geochemical and textural evidence is distinctly associated with (1) early cementation of the host sandstone during highstands of relative sea level, (2) the formation of firmgrounds during low relative sea level, (3) the development of a Glossifungites-demarcated discontinuity during initial relative sea-level rise, and (4) continued cementation with rising relative sea level. This was followed by burial diagenesis, evidence for which is derived from petrographic and isotopic data.     

Tracing the decomposition of dissolved organic carbon in artificial groundwater recharge using carbon isotope ratios

Reducing the concentration of dissolved organic C (DOC) in water is one of the main challenges in the process of artificial groundwater recharge. At the Tuusula waterworks in southern Finland, surface water is artificially recharged into an esker by pond infiltration and an equal amount of groundwater is daily pumped from the aquifer. This groundwater study was conducted to consider the role of redox processes in the decomposition of DOC. The isotopic composition of dissolved inorganic C (δ¹³C_DIC) in the recharged water was used as a tracer for redox reactions. The isotopic composition of O and H in water was determined in order to calculate mixing ratios between the local groundwater and the infiltrated surface water. Three distinct processes in the reduction of the DOC content were traced using isotopic methods and concentration analyses of DIC and DOC: (1) the decomposition of DOC, (2) adsorption of DOC on mineral matter, and (3) the dilution of artificially recharged water by mixing with local groundwater. The largest decrease (44%) in the DOC content occurred during the early stage of subsurface flow, within 350 m of the infiltration ponds. The reduction of DOC was accompanied by an equal increase in DIC and a significant drop in δ¹³C_DIC. This change is attributed to the oxidative decomposition of DOC. A further 23% decrease in DOC is attributed to adsorption and a final drop of 14% to dilution with local groundwater.     

The concentration and isotopic composition of hydrogen,carbon and nitrogen in carbonaceous meteorites

Concentrations and isotopic compositions were determined for H₂, N₂ and C extracted by stepwise pyrolysis from powdered meteorites, from residues of meteorites partially dissolved with aqueous HF, and from residues of meteorites reacted with HF-HCl solutions. The meteorites treated were the carbonaceous chondrites, Orgueil, Murray, Murchison, Renazzo and Cold Bokkeveld. Data determined for whole rock samples are in approximate agreement with previously published data. Acidification of the meteorites removed the inorganic sources of H₂, so that H₂ in the HF-HCl acid residues came primarily from insoluble organic matter, which makes up 70–80% fraction of the total carbon in carbonaceous meteorites. The δD in the organic matter differs markedly from previously determined values in organic matter in meteorites. The δD values of organic matter from acid residues of C1 and C2 carbonaceous chondrites range from +650 to + 1150%. The acid residues of the Renazzo meteorite, whose total H₂ has a δD of +930‰, gave a δD value of +2500‰. Oxidation of the HF-HCl residue with H₂O₂ solution removes the high δD and the low δ¹⁵N components. The δ¹³C values range between ?10 and ?21 and δ¹⁵N values range between +40 and ?11. The δ¹⁵N of Renazzo is unusual; its values range between +150 and ?190.There is good correlation between δD and the concentration of H₂ in the acid residues, but no correlation exists between δD, δ¹³C and δ¹⁵N in them. A simple model is proposed to explain the high δD values, and the relationships between δD values and the concentration of H₂. This model depends on the irradiation of gaseous molecules facilitating reaction between ionic molecules, and indicates that an increase in the rate of polymerization and accumulation of organic matter on grains would produce an increase in the deuterium concentration in organic matter.     

Hydrogen and carbon isotopes of petroleum and related organic matter

$\text{D}\text{H}$ and ${}^{13}\text{C}{}^{12}\text{C}$ ratios were measured for 114 petroleum samples and for several samples of related organic matter. δD of crude oil ranges from ?85 to ?181‰, except for one distillate (?250‰) from the Kenai gas field; δ¹³C of crude oil ranges from ?23.3 to ?32.5‰, Variation in δD and δ¹³C values of compound-grouped fractions of a crude oil is small, 3 and 1.1%., respectively, and the difference in δD and δ¹³C between oil and coeval wax is slight. Gas fractions are 53–70 and 22.6–23.2‰ depleted in D and ¹³C, respectively, relative to the coexisting oil fractions.The δD and δ¹³C values of the crude oils appear to be largely determined by the isotopic compositions of their organic precursors. The contribution of terrestrial organic debris to the organic precursors of most marine crude oils may be significant.     

Carbon and oxygen isotopes suggesting deep-water basin deposition associated with hydrothermal events （Shangsi Section,Northwest Sichuan Basin-South China）

Carbon and oxygen isotope records for Shangsi Section in Northwest Sichuan Basin, South China can help investigating depositional environments and processes, including the burial rate and possible contribution of hydrothermal events. Samples from the lower Chihsian Formation show δ13CPDB and δ18OPDB values close to those of typical marine limestone. However, the overlying Permian middle-upper Chihsian, Wujiaping, and Maokou Formation samples reveal negative δ18OSMOW values and strong positive δ13CPDB values. These indicate high biological productivity and rapid burial of organic carbon. Samples from the Dalong Formation present both negative δ13CPDB and negative δ18OPDB values, which are quite different from the underlying Permian strata. These abnormal carbon and oxygen isotope characteristics in the Dalong Formation may suggest that hydrothermal processes contributed to deposition.     

Stable carbon isotopes of alkane gases from the Xujiahe coal measures and implication for gas-source correlation in the Sichuan Basin,SW China

The Upper Triassic Xujiahe Formation in the Sichuan Basin, SW China consists of a series of coal measures. The first, third and fifth members of this formation are dominated by gas prone dark mudstones and coals. The mudstones contain Type II and III kerogens with average organic carbon contents around 1.96%. These source rocks are mature in the central Sichuan and highly mature in the western Sichuan Basin, characterized by gas generation with subordinate amounts of light oil or condensate oils. The source rocks are intercalated with the sandstone dominated second, fourth and sixth members of the Xujiahe Formation, thus leading to three separate self contained petroleum systems in the region. The proven gas reserves in the Xujiahe Formation are only less than that of the Triassic Feixianguan Formation and the Xujiahe Formation has the second largest gas field (Guang’an gas field) in the basin. Gases derived from the Xujiahe Formation coals generally show a normal stable carbon isotopic trend for C₁–C₄ n-alkanes, with the highest δ¹³C₂ values among the nine gas pay zones in the basin (?20.7‰ to ?28.3‰), and δ¹³C₁ values as low as ?43.0‰ in the central Sichuan. Gas accumulations with an oil leg have also been found in the eastern and southern Sichuan where the thickness of the Xujiahe Formation is significantly reduced. Gases in these accumulations tend to show low δ¹³C₂ values (?30.0‰ to ?36.3‰), characteristic of oil prone source rocks.     

Distribution and diagenesis of organic compounds in JOIDES sediment from Gulf of Mexico and western Atlantic

Fifteen sediment samples were studied from five drill sites recovered by the ‘Glomar Challenger’ on Legs I and IV in the Gulf of Mexico and western Atlantic. This study concentrated on compounds derived from biogenic precursors, namely: (1) hydrocarbons, (2) fatty acids, (3) pigments and (4) amino acids.Carbon isotope (δC¹³) data (values < ? 26%, relative to PDB), long-chain n-alkyl hydrocarbons ( ?C₂₇7) with odd carbon numbered molecules dominating even carbon numbered species, and presence of perylene proved useful as possible indicators for terrigenous contributions to the organic matter in some samples. Apparently land-derived organic matter can be transported for distances over 1000 km into the ocean and their source still recognized.The study was primarily designed to investigate: (i) the sources of the organic matter present in the sediment, (ii) their stability with time of accumulation and (iii) the conditions necessary for in situ formation of new compounds.