The effect of early diagenesis on the Fe isotope compositions of porewaters and authigenic minerals in continental margin sediments

Iron isotope compositions in marine pore fluids and sedimentary solid phases were measured at two sites along the California continental margin, where isotope compositions range from δ⁵⁶Fe = −3.0‰ to +0.4‰. At one site near Monterey Canyon off central California, organic matter oxidation likely proceeds through a number of diagenetic pathways that include significant dissimilatory iron reduction (DIR) and bacterial sulfate reduction, whereas at our other site in the Santa Barbara basin DIR appears to be comparatively small, and production of sulfides (FeS and pyrite) was extensive. The largest range in Fe isotope compositions is observed for Fe(II)_aq in porewaters, which generally have the lowest δ⁵⁶Fe values (minimum: −3.0‰) near the sediment surface, and increase with burial depth. δ⁵⁶Fe values for FeS inferred from HCl extractions vary between ∼−0.4‰ and +0.4‰, but pyrite is similar at both stations, where an average δ⁵⁶Fe value of −0.8 ± 0.2‰ was measured. We interpret variations in dissolved Fe isotope compositions to be best explained by open-system behavior that involves extensive recycling of Feflux. This study is the first to examine Fe isotope variations in modern marine sediments, and the results show that Fe isotopes in the various reactive Fe pools undergo isotopic fractionation during early diagenesis. Importantly, processes dominated by sulfide formation produce high-δ⁵⁶Fe values for porewaters, whereas the opposite occurs when Fe(III)-oxides are present and DIR is a major pathway of organic carbon respiration. Because shelf pore fluids may carry a negative δ⁵⁶Fe signature it is possible that the Fe isotope composition of ocean water reflects a significant contribution of shelf-derived iron to the open ocean. Such a signature would be an important means for tracing iron sources to the ocean and water mass circulation.     

氧化作用对气态烃组成和碳同位素组成的影响

通过加水模拟实验,揭示了气态烃在被矿物氧化过程中分子组成和碳同位素组成的变化.实验结果表明,随着反应时间的延长,气态烃、非烃 (H2、 CO2、 H2S)组成及碳同位素组成发生了明显且很有规律的变化.气态烃碳数越高,氧化速率越快,碳同位素变化 (增重 )越大.当氧化剂为赤铁矿或赤铁矿 硫酸镁时, CH4含量没有明显降低,δ 13C值增重 1‰～ 2‰; C2H6含量最后 (288h)降低了约 20%,δ 13C值增重约 3‰; C3H8含量最后降低了约 50%,δ 13C值增重约 5‰; Ic4h10含量在 72 h时即降低了约 80%,氧化速率远高于 Nc4h10.非烃 CO2的含量增加了 1.26～ 1.71倍.当氧化剂为硫酸镁时, CH4含量明显增高,最多时增加了 34.7%,δ 13C值增重约 8‰; C2H6含量在 72 h时降低了约 14%,在 144 h时降低了约 85%,δ 13C值增重约 24‰; C3H8含量在 72 h时降低了约 65%,在 144 h时降低了 98%以上; Ic4h10和 Nc4h10在 72 h时即降低了 90%以上.非烃 CO2含量最多增加了 1.86倍, H2S最多增加了 9.62倍.这些实验结果对认识天然气藏在矿物氧化过程中分子组成和碳同位素组成的变化具有重要意义.     

Kaolinite and dickite formation during shale diagenesis: isotopic data

X-ray diffraction, scanning electron microscopy and O-isotope geochemistry have been used to investigate the origin and possible controls on polymorphic transformation of kaolin minerals filling veins in Cretaceous shales from the Gibraltar Strait area (southern Spain).The mineralogy of the enclosing shales indicates that kaolin minerals formed from smectite dissolution, a process that silmultaneously originated I/S mixed-layers and quartz. Kaolinite and dickite δ¹⁸O values suggest that an increase in the water isotopic composition, from Cretaceous sea water values (−1%) to values of about 3%, occurred parallel to smectite dissolution, the intensity of this process being the main factor controlling the isotopic composition of kaolin minerals. The minimum formation temperature ranges from 62°C for kaolinite to 86–96°C for dickite, indicating that the depth of burial was the main control on polymorph formation. This temperature range agrees with that deduced for illite/smectite ordering. The passage from kaolinite- to dickite-rich veins was accompanied, as deduced from SEM examination, by a morphologic evolution characterized by the division of large vermiculae, dominant in kaolinite samples, and the formation of short stacks and platy crystals, which are predominant in dickite. The mechanism of dickite formation, however, remains uncertain.     

Remobilization of Iodine in Marine Sediments During Early Diagenesis

Based on the sedimentary geochemical studies of the Antarctic Ocean and the various geochemical parameters available,this paper deals with the process of emobilization of iodine in marine sediments during early diagenesis.The results showed that the process is not always controlled completely by organic matter as was expected previously.On average the adsorption and oxide phases of iodine account respectively for 23% and 32% of the total in continental-shelf and hemipelagic surficial sediments.Chemical analysis has revealed that the upward diffusion flux and redox conditions would play an important role in the concentration of iodine in the surface sediments.And the species of iodine in the surfial sediments characteristic of high I/Corg ratios would bepredominated by the oxide and adsorption phases.As experimentally evidenced,it is the early diagenetic remoibilization of iodine associated with the oxide and adsorption phases that led to the decrease of I/Corg with increasing depth.Calculations suggested that the diffusion flux of iodine from the deep parts of te sedimentary columum upwards is on the same order of magnitude as the deposition flux of it from sea water.This may be one of the important factors leading to the depletion of iodine in sedimentary rocks.On the basis of the above discussion and calculations the author has proposed a model for the remobilization of iodine in marine sediments during early diagenesis.     

The early diagenesis of aliphatic hydrocarbons and organic matter in sedimentary particulates from Dabob Bay,Washington

Aliphatic hydrocarbon compositions were quantitatively characterized in plankton, sediment trap-collected particulate materials and sediments from Dabob Bay using high resolution glass capillary gas chromatography. The average net accumulation of individual hydrocarbons measured in a 1-yr series of sediment traps was compared with the net accumulation of corresponding compounds measured in three depth intervals of ²¹⁰Pb-dated bottom sediments. Systematic and rapid decreases in the net accumulation of individual hydrocarbons were observed from the sediment traps to the sediments. Most pronounced decreases were measured for planktonically derived hydrocarbon constituents (e.g. pristane and two unsaturated compounds) which are rapidly remineralized at or near the sediment-water interface. Consequently, the amount of each compound measured in deposited sediments is not necessarily a quantitative indication of its initial flux to the sediments. The n-alkanes (C_25,27,29,31). characteristic of terrestrial plant waxes, are the predominant hydrocarbons measured by 4–6 cm depth in these sediments and show reasonably constant net accumulation below this interval.Significant diagenetic alteration of the bulk organic matter contained in the average sediment trap particulate material is also noted through comparison with bottom sediments on the basis of organic C/N and δ¹³C measurements. Organic matter elementally similar to marine plankton is preferentially remineralized upon deposition of the sedimentary particulates. The residual organic matter remaining and buried in the bottom sediments closely resembles terrestrial organic matter.     

早期成岩过程中铁元素地球化学循环研究进展

铁是海洋沉积物中重要的氧化还原敏感性元素之一,是早期成岩过程中地球化学循环变化的重要动力因素。早期成岩过程中,表层沉积物中铁氧化物的赋存形态主要可分无定形(弱晶型)铁氧化物和晶型铁氧化物,且前者的含量主要决定着沉积物中铁氧化物的还原活性;铁氧化物可以通过与硫酸盐还原产生的硫化物反应进行还原,还能在铁还原菌的参与下被表层沉积物中的有机质还原,沉积物中活性铁含量、有机质含量、沉积速率、植物根系导氧作用及底栖生物的扰动均能对铁还原率造成影响。早期成岩过程中可以形成黄铁矿,形成机理主要有:1)沉积物中先前形成的硫复铁矿(Fe_3S_4)等前体物质通过加硫反应形成;2)硫过饱和的球粒胶体通过脱水、成核、结晶以及聚合作用而成单个草莓状黄铁矿或初始自行黄铁矿微晶成核、生长、聚集、固化的小型黄铁矿微球团并入更大的胶体状黄铁矿结核、草莓状黄铁矿分组,从而形成黄铁矿集合体;黄铁矿化度(DOP)可作为区分古海洋氧化还原环境的指标。对铁同位素的研究表明,异化还原作用(DIR)过程中产生的铁同位素值偏低;页岩中黄铁矿的铁同位素在2.3Ga附近发生的突变反映了第一次大气氧气增高事件。磁学参数对铁相变化具有良好的反应,环境磁学在早期成岩过程研究中的应用,有助于快速划分铁还原带、研究环境中重金属循环行为。     

不同类型球粒陨石钙同位素组成特征及对比研究

准确限定球粒陨石的Ca同位素组成对于研究太阳星云物质演化和行星形成都具有重要意义。选取7块典型的球粒陨石,包括3块CV3型陨石(Leoville、Allende和Vigarano)、1块CM2型陨石(Murchison)、1块CO3.2型陨石(Kainsaz)、1块EH4型陨石(Indarch)以及1块H4型陨石(LaPaz Icefield 03601),进行了Ca同位素组成的研究。其中, Kainsaz、Leoville和LaPaz Icefield 03601共3块陨石的Ca同位素组成是首次报道。结果显示:(1)在增大样品量以规避"样品量效应"的情况下,我们对CV群球粒陨石Ca同位素组成进行更加精确的制约,δ^44/40Ca的平均值为0.45‰±0.04‰(n=3, 2SE);(2)碳质球粒陨石的Ca同位素组成相对于硅酸盐地球偏轻,从CV群(0.45‰±0.04‰, 2SE)、CM群(0.73‰±0.04‰, 2SE)到CO群(0.78‰±0.03‰, 2SE)逐渐变重,可能与不同化学群陨石中富钙铝难熔包体(CAIs)丰度的变化有关;(3)顽火辉石球粒陨石和普...     

生物降解稠油沥青质热解产物中生物标志化合物与单体烃碳同位素组成研究

本文运用热解技术对采自准噶尔盆地风城地区6个重度生物降解稠油进行沥青质热解实验。通过分析沥青质热解产物中生物标志化合物组成和单体正构烷烃碳同位素组成,探索其在降解稠油油源研究中的应用。研究结果显示,沥青质热解产物中生物标志化合物组成特征易受热解温度的影响,指示母质沉积水体环境的参数 Pr/Ph比值随热解温度升高逐渐增大,在较高的温度下,高碳数、高环数的生物标志化合物易裂解生成低碳数、低环数的化合物,导致C20三环萜烷/C30 藿烷和孕甾烷/αααC29R甾烷两个参数的比值随热解温度升高逐渐增大,表明利用沥青质热解产物生物标志化合物组成进行降解稠油油源判识易出现偏差。而沥青质热解产物单体正构烷烃碳同位素组成随热解温度升高并没有明显的变化, 6个降解稠油沥青质热解产物单体正构烷烃碳同位素分布曲线呈水平分布,与来自风城组烃源岩的典型原油基本相似,说明沥青质热解产物中单体烃碳同位素组成特征可用于重度降解稠油油源分析。另外,通过对比稠油与其沥青质热解产物中单体正构烷烃单体碳同位素组成差异,可用于研究多油源以及后期混入的问题。     

Carbon isotope fractionation of sapropelic organic matter during early diagenesis

Study of an algal, sapropelic sediment from Mangrove Lake, Bermuda shows that the mass balance of carbon and stable carbon isotopes in the major organic constituents is accounted for by a relatively straightforward model of selective preservation during diagenesis. The loss of ¹³C-enriched carbohydrates is the principal factor controlling the intermolecular mass balance of ¹³C in the sapropel. Results indicate that labile components are decomposed leaving as a residual concentrate in the sediment an insoluble humic substance that may be an original biochemical component of algae and associated bacteria. An overall decrease of up to about 4‰ in the δ ¹³C values of the organic matter is observed as a result of early diagenesis.     

Formation of clay minerals during early diagenesis of a calcareous ooze

Deep-sea sediments from a basin in the Southeast Pacific are described, in which strong carbonate dissolution and strong diagenetic migration of metals can be observed. Clay minerals of smectite type are formed in large quantities during the early diagenesis of this sediment, in a first stage as an X-ray amorphous phase, which later crystallizes to smectite. Rb, Sc, Na and K become enriched during the process of clay-mineral formation, probably due to uptake of these elements from sea-water to interlayer positions of smectite. The unsolved question is what sort of mechanism connects the calcite dissolution with the formation of smectite? The source of the Al, necessary for the formation of smectite is also unknown.     

Characterisation of chlorinated hydrocarbons from chlorine and carbon isotopic compositions: scope of application to environmental problems

A set of chlorinated hydrocarbons (TCE, PCE, DCM, 1,1,1-TCA, chloroform) provided by four manufacturers has been isotopically characterised for both C and Cl, using a new sensitive method. A very large range of δ¹³C (from −51.66 to −24.07‰/PDB) associated with a very large range of δ³⁷Cl (from −2.7 to +3.4‰/SMOC) was obtained. This range of δ³⁷Cl is much larger than that of inorganic Cl (±1‰ SMOC) and most individual solvents show a very distinct δ³⁷Cl compared to inorganic Cl isotopic signatures. Moreover, δ³⁷Cl/δ¹³C pairs are distinct from one solvent/manufacturer to another. In a δ¹³C versus δ³⁷Cl diagram, δ³⁷Cl / δ¹³C pairs show different trends for the products of a single manufacturer compared to another. This suggests that Cl isotopic compositions are probably highly fractionated during organic synthesis. The δ³⁷Cl values can be interpreted in terms of the probable manufacturing processes. Unlike the data published previously, with one exception, all the new results for samples reported here have positive δ³⁷Cl values which might differentiate natural Cl from that derived from degradation. This method has significant potential as a tool for investigating environmental pollution problems; in particular, it offers the possibility for validating models of transport and fate of pollutants.     

Elucidating hydrological complexity and climate-influenced diagenesis during the early Carboniferous of South Wales

A revised age provided by conodonts from the Gilwern Oolite of the Clydach area of South Wales allows a clearer understanding of the palaeohydrology, palaeoclimatic history and diagenesis of previously correlated oolitic units. Earlier uncertainty over the apparent sub-regional differences in climate during the early Visean (Chadian—Arundian) lowstand is resolved. Previously a humid interval evidenced by prominent palaeo-epikarst capping the Gilwern Oolite along the northern outcrops of the South Wales synclinorium, was not recognized in what were regarded as correlative outcrops in the Chadian Gully Oolite in the southern part of the synclinorium. Conodont dating now shows that the Gilwern Oolite is much older (Courceyan) than the Chadian Gully Oolite, and during the prolonged exposure of the former there was an interval of weathering under a humid climate. This also explains the contrast between the diagenesis seen between the Gilwern and Gully oolites, which are no longer seen as correlatives.     

贵州乌江水系枯水期河水硫同位素组成研究

对枯水期乌江及其主要支流河水的硫同位素组成进行了研究。河水SO42-的δ34S值在-15.7‰～18.9‰之间,干流δ34S值介于-3.7‰～0.0‰之间。主要支流河水的SO42-浓度和δ34S值具有明显的区域性差异:上游碳酸盐岩地区支流河水SO42-浓度较高而δ34S值较低,河水中的SO24-来源于煤中还原态硫的氧化、矿床硫化物氧化和大气降水;下游碳酸盐岩夹碎屑岩地区支流河水则相反,具有较低SO24-浓度和较高δ34S值,河水中的SO24-来源于硫酸盐蒸发岩溶解、大气降水以及煤中还原态硫的氧化。干流的硫同位素组成显示枯水期河水中的硫酸盐主要来源于碳酸盐岩地区。     

Oxygen isotopic compositions of Allende Type C CAIs: Evidence for isotopic exchange during nebular melting and asteroidal metamorphism

In situ oxygen isotopic measurements of primary and secondary minerals in Type C CAIs from the Allende CV3 chondrite reveal that the pattern of relative enrichments and depletions of ¹⁶O in the primary minerals within each individual CAI are similar to the patterns observed in Types A and B CAIs from the same meteorite. Spinel is consistently the most ¹⁶O-rich (Δ¹⁷O = −25‰ to −15‰), followed by Al,Ti-dioside (Δ¹⁷O = −20‰ to −5‰) and anorthite (Δ¹⁷O = −15‰ to 0‰). Melilite is the most ¹⁶O-depleted primary mineral (Δ¹⁷O = −5‰ to −3‰). We conclude that the original melting event that formed Type C CAIs occurred in a ¹⁶O-rich (Δ¹⁷O  −20‰) nebular gas and they subsequently experienced oxygen isotopic exchange in a ¹⁶O-poor reservoir. At least three of these (ABC, TS26F1 and 93) experienced remelting at the time and place where chondrules were forming, trapping and partially assimilating ¹⁶O-poor chondrule fragments. The observation that the pyroxene is ¹⁶O-rich relative to the feldspar, even though the feldspar preceded it in the igneous crystallization sequence, disproves the class of CAI isotopic exchange models in which partial melting of a ¹⁶O-rich solid in a ¹⁶O-poor gas is followed by slow crystallization in that gas. For the typical (not associated with chondrule materials) Type C CAIs as well for as the Types A and B CAIs, the exchange that produced internal isotopic heterogeneity within each CAI must have occurred largely in the solid state. The secondary phases grossular, monticellite and forsterite commonly have similar oxygen isotopic compositions to the melilite and anorthite they replace, but in one case (CAI 160) grossular is ¹⁶O-enriched (Δ¹⁷O = −10‰ to −6‰) relative to melilite (Δ¹⁷O = −5‰ to −3‰), meaning that the melilite and anorthite must have exchanged its oxygen subsequent to secondary alteration. This isotopic exchange in melilite and anorthite likely occurred on the CV parent asteroid, possibly during fluid-assisted thermal metamorphism.     

Stable carbon isotopic compositions of individual aromatic hydrocarbons as source and age indicators in oils from western Australian basins

The present study aims to establish the factors controlling the stable carbon isotopic compositions (δ¹³C) of individual aromatic hydrocarbons analysed by compound specific isotope analysis (CSIA) in crude oils from western Australian petroleum basins of varying age and facies type. This paper reports δ¹³C values of individual aromatic hydrocarbons, like alkylbenzenes, alkylnaphthalenes, alkylphenanthrenes and methylated biphenyls. The main aims are to confirm the origin (source) and age of these oils based on CSIA of selected aromatic compounds and to understand why the Sofer plot is ineffective in establishing the source of western Australian petroleum systems. The bulk δ¹³C of saturated and aromatic hydrocarbon fractions of crude oils have been previously used to differentiate sources, however, many Australian crude oils are not classified correctly using this method. The oils were classified as marine by the δ¹³C values of individual aromatic compounds and as terrigenous based on the bulk δ¹³C data (Sofer plot).The oils where the δ¹³C values of 1,6-DMN and 1,2,5-TMN isomers are most negative are indicative of a marine source, whereas oils with a less negative values for the 1,6-DMN and 1,2,5-TMN isomers are derived from marine source rocks that contain a significant terrigenous component. Similarly, oils with the least negative δ¹³C values for the 1-MP and 1,9-DMP isomers reflect varying inputs of terrigenous organic matter to the their marine source rocks. Plots of P/DBT and Pr/Ph concentration ratios versus δ¹³C values of DMP, 1,6-DMN, 1,2,5-TMN, 1-MP and 1,9-MP are constructed to establish the relative amount of terrigenous organic matter contributing to the source rock of a series of marine oils. The ratios of P/DBT and Pr/Ph plotted against the δ¹³C values of the aromatic isomers (such as 1,6-DMN, 1,2,5-TMN, 1-MP and 1,9-MP) provide a novel and convenient way to discriminate crude oils derived from different source rocks that contain varying amounts of marine and terrigenous organic matter.     

The anatomy of an early Dinantian terraced floodplain: palaeo-environment and early diagenesis

The lowermost Carboniferous rocks in the Cockburnspath area of east Berwickshire (southern Scotland) are interpreted as coastal floodplain sediments. A lower mudstone-dominated unit is composed of silty mudstones and shales with subordinate sandstones and argillaceous ferroan dolomites (cementstones). These are interpreted as distal floodplain sediments with periodic crevasse-splay deposition. The dark grey colour of the mudrocks suggests deposition in reducing conditions, probably in floodplain lakes. Most of the cementstones are concretionary, some with septarian cracks, suggesting an early diagenetic origin. An immature palaeosol suggests periodic pedogenesis under improved drainage. A synsedimentary erosion surface indicates incision of a valley into the floodplain, presumably in response to base-level lowering. An upper sandstone-dominated unit starts with fine-grained rippled sandstones, cut by small channel sandstones. These are interpreted as floodplain lake deposits fed by crevasse channels. A distinctive conglomerate with cementstone clasts, wood fragments and fish remains is interpreted as a major overbank deposit, dumped into a pre-existing floodplain lake. A bivalve fauna was established in the overlying mudstones, followed by a thin limestone with a restricted marine fossil assemblage, showing that seawater flooding of the lakes occurred at times. Mudrocks throughout the sequence contain no pyrite, except for the marine band which has an organic-carbon/sulphur ratio and degree of pyritization value typical of marine sediments. The concretionary cementstones have δ¹³C values around —4 to —6%^0/00 PDB which are interpreted as indicative of anaerobic oxidation of organic matter. The combined geochemical data suggest a significant involvement of iron reduction in cementstone formation, although the δ¹³C values are ambiguous in assessing the relative involvement of methanogenesis and methane oxidation. Limited seawater inundation of the floodplains might have supplied magnesium and calcium ions for dolomite formation assuming that any H₂S derived as a result of sulphate reduction was oxidized by iron reduction. Alternatively a weathering source for solutes might have been involved.