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1.
The presence of modern methane seeps at Hydrate Ridge, offshore Oregon, provide an opportunity to study the influence of methane seeps on the ecology and geochemistry of living foraminifera. A series of cores were collected from the southern summit of Hydrate Ridge in 2002. Samples were preserved and stained to determine the δ13C composition of three species of live (stained) and dead benthic foraminifera: Uvigerina peregrina, Cibicidoides mckannai, and Globobulimina auriculata. Specimens were examined under light and Scanning Electron Microscopy (SEM) and exhibit no evidence of diagenesis or authigenic carbonate precipitation. Individual living foraminifera from seep sites recorded δ13C values from −0.4‰ to −21.2‰, indicating the isotopic influence of high methane concentrations. Average δ13C values (calculated from single specimens) range from −1.28 to −5.64‰ at seep sites, and −0.81 to −0.85‰ at a control (off seep) site.Two distinct seep environments, distinguished by the presence of microbial mats or clam fields, were studied to determine environmental influences on δ13C values. Individual foraminifera from microbial mat sites exhibited more depleted δ13C values than those from clam field sites. We interpret these differences as an effect of food source and/or symbiotic microbes on foraminiferal carbon isotopic values, acting to magnify the negative δ13C values recorded via the DIC pool. No statistical difference was found between δ13C values of live vs. dead specimens. This suggests that authigenic carbonate precipitation did not play a dominant role in the observed isotopic compositions. However, a few dead specimens with extremely negative δ13C composition (<-12‰) do indicate potential evidence for an authigenic influence on the recorded δ13C composition.  相似文献   

2.
Li/Ca ratios were measured in planktonic and benthic foraminifera from a variety of hydrographic settings to investigate the factors influencing lithium incorporation into foraminiferal tests including temperature, dissolution, pressure, and interspecies differences. Down-core measurements of planktonic (Orbulina universa, Globigerinoides ruber, and Globigerinoides sacculifer) and benthic foraminifera (calcitic Cibicides wuellerstorfi and aragonitic Hoeglandina elegans) show a systematic variation in Li/Ca with δ18O through the last glacial-interglacial transition. All species examined exhibit an increase in Li/Ca between 14 to 50% from the Holocene to the last glacial maximum. Li/Ca generally increases with decreasing temperature as seen in a latitudinal transect of planktonic O. universa and down-slope benthic species along the Bahama Bank margins. Postdepositional dissolution possibly causes a decrease in planktonic foraminiferal Li/Ca along the Sierra Leone Rise, and increased water depth causes a decrease in benthic foraminiferal Li/Ca in the deep Caribbean. However, none of these effects are sufficient to account for the observed glacial-interglacial changes. Physiological factors such as calcification rate may affect the Li/Ca content of foraminiferal calcite. The calcification rate in turn may be a function of carbonate ion concentration of ambient ocean water. This work shows that incorporation of lithium by foraminifera appears to be influenced by factors other than seawater composition and does not appear to be dominated by changes in temperature, dissolution, or pressure. We hypothesize that the consistent increase in foraminiferal Li/Ca during the last glacial maximum may be linked to changes in seawater carbonate ion concentration. Important parameters to be tested include calcification rate and foraminiferal test size and weight. If foraminiferal Li/Ca is dominantly controlled by calcification rate as a function of seawater carbonate ion concentration, then Li/Ca may act as a proxy of past atmospheric CO2.  相似文献   

3.
Abstract Positive shifts in global seawater δ13CDIC are related to changes in the ratio of organic relative to inorganic carbon burial in oceanic basins, whereas factors such as climatic cooling and the accumulation of polar ice are known to cause positive shifts in δ18O. Here, an alternative model is proposed for the formation of local positive isotope shifts in shallow-marine settings. The model involves geochemically altered platform-top water masses and the effects of early meteoric diagenesis on carbonate isotopic composition. Both mechanisms are active on modern (sub)tropical carbonate platforms and result in low carbonate δ13C and δ18O relative to typical oceanic values. During high-amplitude transgressive events, the impact of isotopically light meteoric fluids on the carbonate geochemistry is much reduced, and 13C-depleted platform-top water mixes with open oceanic water masses having higher isotope values. Both factors are recorded as a transient increase in carbonate 13C and 18O relative to low background values. These processes must be taken into consideration when interpreting the geochemical record of ancient epeiric seas.  相似文献   

4.
We discuss water oxygen isotopes (δ18Ow) and carbon isotopes of dissolved inorganic carbon (δ13CDIC) of brine‐enriched shelf water (BSW) from Storfjorden (southern Svalbard) in comparison to Recent benthic foraminiferal δ18Oc and δ13Cc calcified in the same water. We determined relatively high δ18Ow values of 0.15±0.03‰ VSMOW in BSW below sill depth at temperatures below ?1.8 °C, and high δ18Oc values of 3.90±0.18‰ VPDB. Such high BSW δ18Ow cannot significantly deplete 18Ow contents of Arctic Ocean deep water; furthermore, such high δ18Oc cannot be distinguished from δ18Oc values of 3.82±0.12‰, calcified in warmer Arctic and Nordic seas intermediate and deeper waters. Today, in Storfjorden low benthic δ13Cc and high δ18Oc reflect the low δ13CDIC and relatively high δ18Ow of BSW. High benthic δ18Oc is in contrast to expected low δ18Oc as brine rejection is widely thought to predominantly take place in surface water diluted by meteoric water with very low δ18Ow. Low epibenthic δ13Cc values of 0.50±0.12‰ partly reflect low δ13CDIC caused by enhanced uptake of atmospheric low δ13CCO2 decreased by anthropogenic activities. An adjustment for preindustrial higher values would increase δ13Cc by about 0.6‰. Therefore, in Storfjorden brine formed before the industrial era would be characterized by both high δ13Cc as well as high δ18Oc values of benthic foraminiferal calcite. Our data may cast doubt on scenarios that explain negative excursions in benthic foraminiferal stable isotope records from the Atlantic Ocean during cold stadials in the last glacial period by enhanced brine formation in Nordic seas analogously to modern processes in Storfjorden.  相似文献   

5.
《Geochimica et cosmochimica acta》1999,63(13-14):2001-2007
Stable oxygen isotope ratios of foraminiferal calcite are widely used in paleoceanography to provide a chronology of temperature changes during ocean history. It was recently demonstrated that the stable oxygen isotope ratios in planktonic foraminifera are affected by changes of the seawater chemistry carbonate system: the δ18O of the foraminiferal calcite decreases with increasing CO32− concentration or pH. This paper provides a simple explanation for seawater chemistry dependent stable oxygen isotope variations in the planktonic foraminifera Orbulina universa which is derived from oxygen isotope partitioning during inorganic precipitation. The oxygen isotope fractionation between water and the dissolved carbonate species S = [H2CO3] + [HCO3−] + [CO32−] decreases with increasing pH. Provided that calcium carbonate is formed from a mixture of the carbonate species in proportion to their relative contribution to S, the oxygen isotopic composition of CaCO3 also decreases with increasing pH. The slope of shell δ18O vs. [CO32−] of Orbulina universa observed in culture experiments is −0.0022‰ (μmol kg−1)−1 (Spero et al., 1997), whereas the slope derived from inorganic precipitation is −0.0024‰ (μmol kg−1)−. The theory also provides an explanation of the nonequilibrium fractionation effects in synthetic carbonates described by Kim and O’Neil (1997) which can be understood in terms of equilibrium fractionation at different pH. The results presented here emphasize that the oxygen isotope fractionation between calcium carbonate and water does not only depend on the temperature but also on the pH of the solution from which it is formed.  相似文献   

6.
藏南定日地区Cenomanian/Turonian界线附近的生物古海洋事件   总被引：4，自引：0，他引：4  
赵文金  万晓樵 《地质科学》2003,38(2):155-164
藏南定日地区在白垩纪中期发育一套浅灰色深灰色的以钙质页岩、泥灰岩及微晶灰岩为主的浅海陆棚相沉积,岩石中除含较高的粘土矿物及陆源石英矿物颗粒以外,还含有十分丰富的有孔虫化石。依据浮游有孔虫Helvetoglobotruncana praehelvetica的首次出现将C/T界线置于样品9922及9923之间,该界线位于Whiteinella archaeocretacea化石带之中。通过定量分析,该区有孔虫的丰度、分异度以及浮游与底栖有孔虫比率、具旋脊与不具旋脊有孔虫的比率等指标在剖面纵向上表现出3个明显的演化阶段,即Rotalipora cushmani带上部、W.archaeocretacea带及H.helvetica带下部,有孔虫动物群的变化特征完整地记录了C/T界线附近古海洋事件的全过程。该次事件中,碳稳定同位素δ13C值存在着明显的异常变化——正向偏移,元素地球化学U、Th及K的丰度也表现出明显的异常变化,其丰度值均比标准平均值偏高。所有这些特征均是在全球洋脊迅速扩张这一背景下海平面发生剧烈变化的结果。  相似文献   

7.
The 13C/12C ratios of Upper Holocene benthic foraminiferal tests (genera Cibicides and Uvigerina) of deep sea cores from the various world ocean basins have been compared with those of the modern total carbon dioxide (TCO2) measured during the GEOSECS program. The δ13C difference between benthic foraminifera and TCO2 is 0.07 ± 0.04‰ for Cibicides and ?0.83 ± 0.07‰ for Uvigerina at the 95% confidence level. δ13C analyses of the benthic foraminifera that lived during the last interglaciation (isotopic substage 5e, about 120,000 yr ago) show that the bulk of the TCO2 in the world ocean had a δ13C value 0.15 ± 0.12‰ lower than the modern one at the 95% confidence level, reflecting a depletion, compared to the present value, of the global organic carbon reservoir. Regional differences in δ13C between the various oceanic basins are explained by a pattern of deep water circulation different from the modern one: the Antarctic Bottom Water production was higher than today during the last interglaciation, but the eastward transport in the Circumpolar Deep Water was lower.  相似文献   

8.
Porewaters from a variety of Recent, Pleistocene, and Eocene lithified marine carbonate frameworks displayed similar chemical characteristics: highly depleted concentrations of dissolved oxygen (>20 μM), elevated levels of dissolved methane (25-5000 nM), and near-seawater sulphate levels. These porewaters also had low pH values (7·5-7·9), and contained elevated concentrations of sulphide (4–10 μM), dissolved inorganic carbon (2·05–2·46 mM), and inorganic nutrients. Hydrocarbon composition data indicate that the methane is biogenic, whereas the methane δ13C values (–47·4 ± 2·7%0) suggest that it has been subject to oxidation. The porewater dissolved inorganic carbon δ13C values varied from –0·6 to –39%0, suggesting input of carbon dioxide from organic matter oxidation. We conclude that anaerobic diagenesis involving bacterial degradation of organic matter is a common process in lithified marine carbonates and hypothesize that it may be an important factor controlling their carbonate geochemistry.  相似文献   

9.
Cenomanian/Turonian boundary (upper Sarvak Formation) benthic foraminiferal assemblages were analyzed to reconstruct oxygen level, primary productivity, and water turbulence in the Izeh Zone, Zagros Basin. The interplay between environmental perturbations during the Oceanic Anoxic Event 2 (OAE2) and regional tectonic activities in the Zagros Basin resulted in formation of various benthic foraminiferal assemblages in the study section. The OAE2 interval at the region of study starts with extinction of rotaliporids at the onset of δ13C positive excursion (peak “a”), which is associated with population of infaunal benthic foraminifera (especially Bolivina alata). The following interval at the onset of Whiteinella archaeocretacea Biozone is characterized by the total absence of benthic taxa and dominance of planoheterohelicids (“Heterohelix shift”) in the black shale strata, indicating expansion of oxygen minimum zone and unhospitable conditions for both benthic and planktic foraminifera. The upper part of OAE2 interval (including δ13C peaks “b” and “c”) coincides with harbinger of Neo-Tethys closure in the Arabian Plate, causing a compressional tectonic regime, and creation of uplifted terrains in the basin. The relative sea level started to locally fall in this succession, which was accompanied by a better ventilation of seafloor, lower TOC contents, and reappearance of benthic foraminifera.  相似文献   

10.
The δ13C values of dissolved HCO3? in 75 water samples from 15 oil and gas fields (San Joaquin Valley, Calif., and the Houston-Galveston and Corpus Christi areas of Texas) were determined to study the sources of CO2 of the dissolved species and carbonate cements that modify the porosity and permeability of many petroleum reservoir rocks. The reservoir rocks are sandstones which range in age from Eocene through Miocene. The δ13C values of total HCO3? indicate that the carbon in the dissolved carbonate species and carbonate cements is mainly of organic origin.The range of δ13C values for the HCO3? of these waters is ?20–28 per mil relative to PDB. This wide range of δ13C values is explained by three mechanisms. Microbiological degradation of organic matter appears to be the dominant process controlling the extremely low and high δ13C values of HCO3? in the shallow production zones where the subsurface temperatures are less than 80°C. The extremely low δ13C values (< ?10 per mil) are obtained in waters where concentrations of SO42? are more than 25 mg/l and probably result from the degradation of organic acid anions by sulfate-reducing bacteria (SO42? + CH3COO? → 2HCO3? + HS?). The high δ13C values probably result from the degradation of these anions by methanogenic bacteria (CH3COO? + H2OaiHCO3? + CH4).Thermal decarboxylation of short-chain aliphatic acid anions (principally acetate) to produce CO2 and CH4 is probably the major source of CO2 for production zones with subsurface temperatures greater than 80°C. The δ13C values of HCO3? for waters from zones with temperatures greater than 100°C result from isotopic equilibration between CO2 and CH4. At these high temperatures, δ13C values of HCO3? decrease with increasing temperatures and decreasing concentrations of these acid anions.  相似文献   

11.
《Applied Geochemistry》2002,17(11):1457-1466
Ocean Drilling Program (ODP) Leg 169, which was conducted in 1996 provided an opportunity to study the gas geochemistry in the deeper part of the sediment-rich hydrothermal system in Escanaba Trough. Gas void samples obtained from the core liner were analyzed and their results were compared with analytical data of vent fluid samples collected by a submersible dive program in 1988. The gas geochemistry of the pore fluids consisted mostly of a hydrothermal component and was basically the same as that of the vent fluids. The He isotope ratios (R/RA=5.6–6.6) indicated a significant mantle He contribution and the C isotopic compositions of the hydrocarbons [δ13C(CH4)=−43‰, δ13C(C2H6)=−20‰] were characterized as a thermogenic origin caused by hydrothermal activity. On the other hand, the pore fluids in sedimentary layers away from the hydrothermal fields showed profiles which reflected lateral migration of the hydrothermal hydrocarbons and abundant biogenic CH4. Helium and C isotope systematics were shown to represent a hydrothermal component and useful as indicators for their distribution beneath the seafloor. Similarities in He and hydrocarbon signatures to that of the Escanaba Trough hydrothermal system were found in some terrestrial natural gases, which suggested that seafloor hydrothermal activity in sediment-rich environments would be one of the possible petroleum hydrocarbon generation scenarios in unconventional geological settings.  相似文献   

12.
The Neoproterozoic Doushantuo Formation on the Yangtze Platform, South China, documents a sedimentary succession with different sedimentary facies from carbonate platform to slope and to deep sea basin, and hosts one of the world-class phosphorite deposits. In these strata, exquisitely preserved fossils have been discovered： the Weng＇an biota. This study presents carbon isotope geochemistry which is associated paired carbonate and organic matter from the Weng＇an section of a carbonate platform （shelf of the Yangtze Platform, Guizhou Province） from the Songtao section and Nanming section of a transition belt （slope of the Yangtze Platform, Guizhou Province） and from the Yanwutan section （basin area of the Yangtze Platform, Hunan Province）. Environmental variations and bio-events on the Yangtze Platform during the Late Neoproterozoic and their causal relationship are discussed. Negative carbon isotope values for carbonate and organic carbon （mean δ^13Corg = -35.0‰） from the uppermost Nantuo Formation are followed by an overall increase in δ^13C up-section. Carbon isotope values vary between -9.9‰ and 3.6‰ for carbonate and between -35.6‰ and -21.5‰ for organic carbon, respectively. Heavier δ^13Ccarb values suggest an increase in organic carbon burial, possibly related to increasing productivity （such as the Weng＇an biota）. The δ^13C values of the sediments from the Doushantuo Formation decreased from the platform via the slope to basin, reflecting a reduced environment with minor dissolved inorganic carbon possibly due to a lower primary productivity. It is deduced that the classical upwelling process, the stratification structure and the hydrothermal eruption are principally important mechanisms to interpret the carbon isotopic compositions of the sediments from the Doushantuo Formation.  相似文献   

13.
冷泉区底栖有孔虫研究进展   总被引：1，自引：0，他引：1  
海底冷泉区是海洋能源和生物资源同时富集的一类特殊区域。底栖有孔虫群落及其地球化学组成是海底冷泉区发育的重要指示标志之一。海底冷泉区的底栖有孔虫及其碳同位素研究,对于探讨冷泉演化、评估古冷泉甲烷排放对全球气候变化的影响有重要的研究意义。综述了全球一些主要冷泉区的底栖有孔虫研究方法及其进展,对比了各冷泉区底栖有孔虫群落结构的主要特征及地域差异,评述了冷泉区底栖有孔虫的碳同位素记录特征、影响因素及其对冷泉活动的潜在指示意义,最后提出了南海北部冷泉活动区底栖有孔虫方面的研究展望。  相似文献   

14.
Abstract

The Upper Triassic Chang 8 Member, the eighth member of the Yanchang Formation, is a key reservoir interval in the Jiyuan area of the Ordos Basin. The reservoir quality of the Chang 8 Member tight sandstones is extremely heterogeneous owing to the widespread distribution of carbonate cements. The carbonate cements commonly develop near sandstone–mudstone interfaces and gradually decrease away from the interfaces to the centres of the sand bodies. However, the content of carbonate cements (≤6%) has a positive correlation with the visual porosity in the Chang 8 Member sandstone, revealing that the carbonate cements contribute to the compaction resistance and the residual primary pores of reservoirs during the diagenetic process. Three main types of carbonate cement are identified: type I (calcite), type II (calcite and ferrocalcite), and type III (dolomite and ankerite). The type I calcite is characterised by enriched δ13C (mean –3.41‰) and δ18O (mean –15.17‰) values compared with the type II (mean δ13C?=?–7.33‰, δ18O?=?–18.90‰) and type III (mean δ13C?=?–10.0‰, δ18O?=?–20.2‰) cements. Furthermore, the mean δ18O value (–4.7‰) of the type I pore fluids is 1.5‰ and 0.9‰ lower than the type II (mean –3.2‰) and type III (mean –3.8‰) pore fluids, respectively. This indicates that the evolving pore fluids experienced some relative strong water–rock interactions that provided the original materials (e.g. Ca2+, Fe3+, and Mg2+) for the carbonate cements during the diagenetic process. The highly saline lake water directly provided the primary material for the type I calcite precipitation, which also provided the material necessary for the precipitation of the type II and type III carbonate cements, causing enriched δ18O values of the pore fluids during the precipitation of the type II and type III carbonate cements. Although the earlier dissolved pores were filled with ferrocalcite, dolomite and ankerite in the middle–late diagenetic stages, some residual pores and fractures remained to become the potential reservoir storage spaces for the oil and gas exploration in the Jiyuan area.  相似文献   

15.
C and O isotope composition of Middle-Upper Miocene and Lower Pliocene carbonates from Kerch-Taman Region (Eastern Paratethys) have been studied in order to reconstruct palaeoenvironmental variability and post-sedimentation changes. The δ13C and δ18О values of the Upper Sarmatian to Lower Pliocene organogenic carbonates reflect the desalinization of paleobasins, global Late Miocene Cooling, and increase in seasonal temperature fluctuations. Isotopic composition of the Middle Sarmatian organogenic carbonates was strongly influenced by evaporation processes, high bioproductivity, and local submarine methane emissions. Warm climate and low bioproductivity together with unstable hydrological regime during the Late Chokrakian and the Karaganian times influenced the isotope composition of primary carbonates. Calcite shell of Spiratella sp. (δ13C =–0.4‰ and δ18О =–0.4‰) from Tarkhanian sediments was formed in warm marine environment. Dolomitization prevails over other secondary mineralization in the studied carbonate rocks. Two groups of secondary dolomites that are characterized by negative and positive δ13C values have been recognized. Lowe δ13C values (up to–31.4‰) in dolomites indicate the influence of both dissolved inorganic carbon (DIC) from oxidized organic matter (Сorg) and methane. Dolomites with positive δ13C values (7.0 and 7.8‰) associat with migration of CO2- and CH4-containing saline groundwater.  相似文献   

16.
Carbon and oxygen isotopic analyses have been performed on live-stained aragonitic and calcitic benthic foraminifera and dissolved inorganic carbon (DIC) from the Southern California Borderland to examine carbon isotopic fractionation in foraminifera. Temperature, salinity and pH data have also been collected to permit accurate determination of the δ13C of bicarbonate ion and thus aragonite-HCO3 and calcite-HCO?3 isotopic enrichment factors (?ar-b and ?cl-b, respectively). Only species which precipitate in 18O equilibrium have been considered.?ar-b values based on Hoeglundina elegans range from 1.9%. at 2.7°C to 1.1%. at 9.5°C. Only the lower temperature values agree with a tentative carbon isotope equilibrium equation for aragonite based on the data of Rubinson and Clayton (1969) and Emrich et al. (1970). The temperature dependence of ?ar-b is considerably greater than the equilibrium equation would predict and may be due to a vital effect.The calcitic foraminifera Cassidulina tortuosa, Cassidulina braziliensis, and Cassidulina limbata, Bank and Terrace dwellers, have similar δ13C values and yield an average ?cl-b value of ?0.2 ± .1%. between 8° and 10°C. Calcitic Uvigerina curticosta, Uvigerina peregrina, and megalospheric B. argentea, Slope and Basin dwellers, are ?0.7 ± .1%. enriched relative to ambient bicarbonate for 3 to 9°C. No temperature dependence for ?cl-b was observed for the species in either habitat. The ?cl-b values for Cassidulina species are close (± 0.3%.) to the values given by the tentative equilibrium curve for calcite, while Uvigerina and Bolivina species give values 0.2–0.8%. less. The ?cl-b difference between the Cassidulina species and the Uvigerina and Bolivina species is attributed to the incorporation of 13C-depleted pore water DIC by the latter group rather than to taxonomic or temperature differences.  相似文献   

17.
Methane concentration [CH4] and stable isotope ratio values (δ13C) characterize methanogenic and methanotrophic processes within two contrasting peatland ecosystems of the Fluxnet Canada Research Network: (i) a western Canada peatland fen in northern Alberta (Fen) and (ii) an eastern Canada peatland bog in southeastern Ontario (Bog). We use carbon isotope ratio discrimination of produced methane (δ13CCH4) from the precursor carbon compounds (δ13CpreC) to estimate the relative proportions of archaebacterial acetoclastic methanogenesis (AM) and hydrogenotrophic carbonate reduction methanogenesis (HM) in these terrestrial ecosystems. The [CH4] and δ13CCH4 signatures describe contrasts in the methanogenic and methanotrophic processes between the Fen and the Bog. The differences are substantiated by stable hydrogen isotope ratio (δD) separation between the dissolved δDCH4 and co-existing δDH2O. Methanogenesis at the Fen is dominated by AM, in contrast to the Bog, which is essentially HM. We suggest that this is potentially a result of differences in type/quality of organic substrates. The trajectory of 13C enrichment in δ13CCH4 values with depth at the Bog reflects a closed system, substrate depletion effect. Our Rayleigh distillation model estimates 58-76% depletion in the source dissolved inorganic carbon (DIC).  相似文献   

18.
Bulk carbonate content, planktic and benthic foraminiferal assemblages, stable isotope compositions of bulk carbonate and Nuttallides truempyi (benthic foraminifera), and non-carbonate mineralogy were examined across ∼30 m of carbonate-rich Paleogene sediment at Deep Sea Drilling Project (DSDP) Site 259, on Perth Abyssal Plain off Western Australia. Carbonate content, mostly reflecting nannofossil abundance, ranges from 3 to 80% and generally exceeds 50% between 35 and 57 mbsf. A clay-rich horizon with a carbonate content of about 37% occurs between 55.17 and 55.37 mbsf. The carbonate-rich interval spans planktic foraminiferal zones P4c to P6b (∼57–52 Ma), with the clay-rich horizon near the base of our Zone P5 (upper)—P6b. Throughout the studied interval, benthic species dominate foraminiferal assemblages, with scarce planktic foraminifera usually of poor preservation and limited species diversity. A prominent Benthic Foraminiferal Extinction Event (BFEE) occurs across the clay-rich horizon, with an influx of large Acarinina immediately above. The δ13C records of bulk carbonate and N. truempyi exhibit trends similar to those observed in upper Paleocene–lower Eocene (∼57–52 Ma) sediment from other locations. Two successive decreases in bulk carbonate and N. truempyi δ13C of 0.5 and 1.0‰ characterize the interval at and immediately above the BFEE. Despite major changes in carbonate content, foraminiferal assemblages and carbon isotopes, the mineralogy of the non-carbonate fraction consistently comprises expanding clay, heulandite (zeolite), quartz, feldspar (sodic or calcic), minor mica, and pyrolusite (MnO2). The uniformity of this mineral assemblage suggests that Site 259 received similar non-carbonate sediment before, during and after pelagic carbonate deposition. The carbonate plug at Site 259 probably represents a drop in the CCD from ∼57 to 52–51 Ma, as also recognized at other locations.  相似文献   

19.
A unique dataset from paired low- and high-temperature vents at 9°50′N East Pacific Rise provides insight into the microbiological activity in low-temperature diffuse fluids. The stable carbon isotopic composition of CH4 and CO2 in 9°50′N hydrothermal fluids indicates microbial methane production, perhaps coupled with microbial methane consumption. Diffuse fluids are depleted in 13C by ∼10‰ in values of δ13C of CH4, and by ∼0.55‰ in values of δ13C of CO2, relative to the values of the high-temperature source fluid (δ13C of CH4 =−20.1 ± 1.2‰, δ13C of CO2 =−4.08 ± 0.15‰). Mixing of seawater or thermogenic sources cannot account for the depletions in 13C of both CH4 and CO2 at diffuse vents relative to adjacent high-temperature vents. The substrate utilization and 13C fractionation associated with the microbiological processes of methanogenesis and methane oxidation can explain observed steady-state CH4 and CO2 concentrations and carbon isotopic compositions. A mass-isotope numerical box model of these paired vent systems is consistent with the hypothesis that microbial methane cycling is active at diffuse vents at 9°50′N. The detectable 13C modification of fluid geochemistry by microbial metabolisms may provide a useful tool for detecting active methanogenesis.  相似文献   

20.
Natural gases and associated condensate oils from the Zhongba gas field in the western Sichuan Basin, China were investigated for gas genetic types and origin of H2S by integrating gaseous and light hydrocarbon geochemistry, formation water compositions, S isotopes (δ34S) and geological data. There are two types of natural gas accumulations in the studied area. Gases from the third member of the Middle Triassic Leikoupo Formation (T2l3) are reservoired in a marine carbonate sequence and are characterized by high gas dryness, high H2S and CO2 contents, slightly heavy C isotopic values of CH4 and widely variable C isotopic values of wet gases. They are highly mature thermogenic gases mainly derived from the Permian type II kerogens mixed with a small proportion of the Triassic coal-type gases. Gases from the second member of the Upper Triassic Xujiahe Formation (T3x2) are reservoired in continental sandstones and characterized by low gas dryness, free of H2S, slightly light C isotopic values of CH4, and heavy and less variable C isotopic values of wet gases. They are coal-type gases derived from coal in the Triassic Xujiahe Formation.The H2S from the Leikoupo Formation is most likely formed by thermochemical SO4 reduction (TSR) even though other possibilities cannot be fully ruled out. The proposed TSR origin of H2S is supported by geochemical compositions and geological interpretations. The reservoir in the Leikoupo Formation is dolomite dominated carbonate that contains gypsum and anhydrite. Petroleum compounds dissolved in water react with aqueous SO4 species, which are derived from the dissolution of anhydrite. Burial history analysis reveals that from the temperature at which TSR occurred it was in the Late Jurassic to Early Cretaceous and TSR ceased due to uplift and cooling thereafter. TSR alteration is incomplete and mainly occurs in wet gas components as indicated by near constant CH4 δ13C values, wide range variations of ethane, propane and butane δ13C values, and moderately high gas dryness. The δ34S values in SO4, elemental S and H2S fall within the fractionation scope of TSR-derived H2S. High organo-S compound concentrations together with the occurrence of 2-thiaadamantanes in the T2l reservoir provide supplementary evidence for TSR related alteration.  相似文献   

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