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1.
The early diagenesis of trace elements (V, Cr, Co, Cu, Zn, As, Cd, Ba, U) in anoxic sediments of the Achterwasser, a shallow lagoon in the non-tidal Oder estuary in the Baltic Sea, was investigated in the context of pyrite formation. The dissolved major redox parameters show a two-tier distribution with transient signals in the occasionally re-suspended fluid mud layer (FM) and a permanently established diagenetic sequence in the sediment below. Intense microbial respiration leads to rapid depletion of O2 within the uppermost mm of the FM. The reduction zones of Mn, Fe and sulfate overlap in the FM and in the permanently anoxic sediment section which appears to be a typical feature of estuarine sediments, under low-sulfate conditions. Degrees of pyritization (DOP) range from 50% in the FM to remarkably high values > 90% at 50 cm depth. Pyrite formation at the sediment surface is attributed to the reaction of Fe-monosulfides with intermediate sulfur species via the polysulfide pathway. By contrast, intense pyritization in the permanently anoxic sediment below is attributed to mineral growth via adsorption of aqueous Fe-sulfide complexes onto pyrite crystals which had originally formed in the surface layer.The studied trace elements show differential behavior patterns which are closely coupled to the diagenetic processes described above: (i) Zn, Cu and Cd are liberated from organic matter in the thin oxic layer of the sediment and diffuse both upwards across the sediment/water boundary and downwards to be trapped as monosulfides, (ii) V, Cr, Co and As are released during reductive dissolution of Mn- and Fe-oxyhydroxides, (iii) U removal from pore water occurs concomitantly to Fe reduction in the FM and is attributed to reduction of U(VI) to U(IV), (iv) the Ba distribution is controlled by reductive dissolution of authigenic barite in the sulfate reduction zone coupled with upward diffusion and re-precipitation. The incorporation of trace elements into pyrite is most intense for Co, Mn and As, intermediate for Cu and Cr and little to negligible for U, Zn, Cd, V and Ba. The observed trend is largely in agreement with previous studies and may be explained with differing rates for ligand exchange. Slow and fast ligand exchange and thus precipitation kinetics are also displayed by downcore increasing (Mn, Cr, Co and As) or constantly low (Zn, Cu, Cd) pore water concentrations. The downward increasing degrees of trace metal pyritization (DTMP) for Co, Cu, Zn and As are, in analogy to pyrite growth, assigned to adsorption of sulfide complexes or As oxyanions onto preexisting pyrite minerals.  相似文献   

2.
The speciation of sedimentary sulfur (pyrite, acid volatile sulfides (AVS), S0, H2S, and sulfate) was analyzed in surface sediments recovered at different water depths from the northwestern margin of the Black Sea. Additionally, dissolved and dithionite-extractable iron were quantified, and the sulfur isotope ratios in pyrite were measured. Sulfur and iron cycling in surface sediments of the northwestern part of the Black Sea is largely influenced by (1) organic matter supply to the sediment, (2) availability of reactive iron compounds and (3) oxygen concentrations in the bottom waters. Biologically active, accumulating sediments just in front of the river deltas were characterized by high AVS contents and a fast depletion of sulfate concentration with depth, most likely due to high sulfate reduction rates (SRR). The δ34S values of pyrite in these sediments were relatively heavy (−8‰ to −21‰ vs. V-CDT). On the central shelf, where benthic mineralization rates are lower, re-oxidation processes may become more important and result in pyrite extremely depleted in δ34S (−39‰ to −46‰ vs. V-CDT). A high variability in δ34S values of pyrite in sediments from the shelf-edge (−6‰ to −46‰ vs. V-CDT) reflects characteristic fluctuations in the oxygen concentrations of bottom waters or varying sediment accumulation rates. During periods of oxic conditions or low sediment accumulation rates, re-oxidation processes became important resulting in low AVS concentrations and light δ34S values. Anoxic conditions in the bottom waters overlying shelf-edge sediments or periods of high accumulation rates are reflected in enhanced AVS contents and heavier sulfur isotope values. The sulfur and iron contents and the light and uniform pyrite isotopic composition (−37‰ to −39‰ vs. V-CDT) of sediments in the permanently anoxic deep sea (1494 m water depth) reflect the formation of pyrite in the upper part of the sulfidic water column and the anoxic surface sediment. The present study demonstrates that pyrite, which is extremely depleted in 34S, can be found in the Black Sea surface sediments that are positioned both above and below the chemocline, despite differences in biogeochemical and microbial controlling factors.  相似文献   

3.
Seasonal dynamics of elemental sulfur in two coastal sediments   总被引：1，自引：0，他引：1  
A spectrophotometric method for elemental sulfur (S0) analysis without interference from other reduced sulfur compounds was adapted for the use in reducing sediments. The S0 distribution in two coastal sediments was studied regularly from summer to winter and compared to factors regulating the S0 accumulation, such as redox potentials, the rate of bacterial sulfide production and the general sulfur chemistry. Dense coatings of sulfur bacteria developed on the sediment surface of a sulfuretum which had an S0 concentration of up to 41 μmol S cm?3. The 2·5-mm thick bacterial coating contained 40% of all S0 in the sediment. A more typical marine sediment with a few cm thick oxidized surface layer had an S0 maximum of 1–3 μmol S cm?3 at 2–4 cm depth. The S0 maximum in both sediments increased from summer to winter as the sediments gradually became more oxidized. The deeper layers maintained a low S0 concentration. Most of the S0 in the upper few mm of a laboratory sulfuretum was present inside sulfur bacteria and actively migrated up and down with the bacteria depending upon the changing light and oxygen conditions.  相似文献   

4.
The northern slope of the South China Sea is a gas-hydrate-bearing region related to a high deposition rate of organic-rich sediments co-occurring with intense methanogenesis in subseafloor environments.Anaerobic oxidation of methane(AOM) coupled with bacterial sulfate reduction results in the precipitation of solid phase minerals in seepage sediment,including pyrite and gypsum.Abundant aggregates of pyrites and gypsums are observed between the depth of 667 and 850 cm below the seafloor(cmbsf) in the entire core sediment of HS328 from the northern South China Sea.Most pyrites are tubes consisting of framboidal cores and outer crusts.Gypsum aggregates occur as rosettes and spheroids consisting of plates.Some of them grow over pyrite,indicating that gypsum precipitation postdates pyrite formation.The sulfur isotopic values(δ~(34) S) of pyrite vary greatly(from –46.6‰ to –12.3‰ V-CDT) and increase with depth.Thus,the pyrite in the shallow sediments resulted from organoclastic sulfate reduction(OSR) and is influenced by AOM with depth.The relative high abundance and δ~(34) S values of pyrite in sediments at depths from 580 to 810 cmbsf indicate that this interval is the location of a paleo-sulfate methane transition zone(SMTZ).The sulfur isotopic composition of gypsum(from–25‰ to –20.7‰) is much lower than that of the seawater sulfate,indicating the existence of a 34 S-depletion source of sulfur species that most likely are products of the oxidation of pyrites formed in OSR.Pyrite oxidation is controlled by ambient electron acceptors such as MnO_2,iron(Ⅲ) and oxygen driven by the SMTZ location shift to great depths.The δ~(34) S values of gypsum at greater depth are lower than those of the associated pyrite,revealing downward diffusion of 34 S-depleted sulfate from the mixture of oxidation of pyrite derived by OSR and the seawater sulfate.These sulfates also lead to an increase of calcium ions from the dissolution of calcium carbonate mineral,which will be favor to the formation of gypsum.Overall,the mineralogy and sulfur isotopic composition of the pyrite and gypsum suggest variable redox conditions caused by reduced seepage intensities,and the pyrite and gypsum can be a recorder of the intensity evolution of methane seepage.  相似文献   

5.
程俊  王淑红  黄怡  颜文 《海洋科学》2019,43(5):110-122
综述了天然气水合物赋存区甲烷渗漏活动的地球化学响应指标的研究进展,分析了应用单一指标识别甲烷渗漏活动各自所存在的问题,包括浅表层沉积物孔隙水中CH_4、SO_4~(2–)、Cl~–等离子浓度随深度的变化;浅层沉积物全岩W_(TOC)(W表示质量分数,TOC表示总有机碳)和W_(TS)(TS表示总硫)之间的相关性及比值;自生碳酸盐岩δ~(13)C和δ~(18)O;自生矿物重晶石、黄铁矿、自生石膏的δ~(34)S;有孔虫壳体和生物标志化合物的δ~(13)C等。结果表明孔隙水中的CH_4、SO4_~(2–)浓度及溶解无机碳的碳同位素组成可以用来识别目前正在发生的甲烷渗漏活动;而沉积物中的WTS、自生矿物的δ~(34)S、钡含量及其异常峰值和生物标志化合物的δ~(13)C等指标的联合使用可以更真实准确地反映地质历史时期天然气水合物赋存区的甲烷渗漏活动。因此,在实际研究过程中,可将孔隙水和沉积物两种介质的多种指标相结合。随着非传统稳定同位素(Fe、Ca、Mg等)和沉积物氧化还原敏感元素(Mo、V、U等)等研究的发展,甲烷渗漏活动地球化学响应指标的研究也将得到拓展,而多种地球化学指标的联合使用将为天然气水合物勘探及其形成分解过程识别研究提供重要的科学依据。  相似文献   

6.
Concentrations of dissolved sulfate and sulfur isotopic ratios of dissolved sulfide in surface sediments of the Peru shelf and upper slope indicate that the sediments can be divided into two depth intervals based on the dominant biogeochemical reactions. Although rates of bacterial sulfate reduction are high throughout Peru surface sediments, chemistry of the upper interval (<10–20 cm) is dominated by chemoautotrophic oxidation of dissolved sulfide and elemental sulfur, while the lower interval (>10–20 cm) is dominated by dissimilatory sulfate reduction. In three of the four cores examined here, pore water concentrations of sulfate in the top 10 cm of the sediment are significantly higher than those of the overlying seawater. Peak sulfate concentrations in pore water (37–53 mmol/l) are ∼1.3–1.9 times that of seawater sulfate and are located 1–6 cm below the sediment/water interface (SWI). The excess sulfate is most likely produced by oxidation of elemental sulfur coupled to reduction of nitrate, a reaction mediated by a facultative chemoautotrophic sulfide-oxidizing bacterium, Thioploca spp. Numerical simulations demonstrate that the anomalously high concentrations of dissolved sulfate can be produced by steady-state or non-steady-state processes involving high rates of bacterial oxidation of elemental sulfur. If bacterial sulfur oxidation is a transient phenomenon, then it is probably triggered by seasonal or El Niño-induced changes in water-column chemistry of the Peru undercurrent.  相似文献   

7.
Sediment cores were taken from the Gulf of California, and pore waters recovered by mechanical squeezing. The chemistry and isotopic abundance of sulfur in these pore fluids were compared with coexisting solid phases to deduce the mechanisms involved in pyrite formation. The results suggest that burrowing activities of benthonic organisms supply sulfate sulfur to sediments to depths of approximately 0.5 m from the surface. This is inferred from essentially constant pore water concentration profiles of dissolved ions in horizons where sulfate reduction is demonstrated by the presence of iron sulfides.For a core from Pescadero Basin, it is estimated that beneath the mixed zone, diffusion adds 0.4% sulfur by dry weight of sediment, whereas burial of sulfate adds less than 0.1% sulfur. It is shown that diffusion can add isotopically light sulfur to sediments, due to more rapid relative addition of 32SO42? compared to 34SO42? down a concentration gradient maintained by bacterial processes. The overall net isotopic value of the sulfate so added is δ34S = ?4.5‰. The depth distribution of S-isotope in sulfur is controlled by the balance between a bacterial kinetic isotope effect preferentially removing 32S relative to 34S, and the supply of sulfate by diffusion. The isotopic fractionation factor, α, calculated by a mathematical formulation which takes diffusion into account, is larger (1.060±0.010) than when sulfate reduction is assumed to occur in a closed system (1.035). The larger value is supported by the sulfur isotope distribution in metastable iron sulfide. Essentially, the same open-system α was calculated for a core from Carmen Basin.  相似文献   

8.
Salt marsh sediment volume decreases from organic decomposition, compaction of solids, and de-watering, and each of these processes may change with age. Variability in the vertical accretion rate within the upper 2 m was determined by assembling results from concurrent application of the 137Cs and 210Pb dating techniques used to estimate sediment age since 1963/1964, and 0 to ca 100+ years before present (yBP), respectively. The relationship between 210Pb and the 137Cs dated accretion rates (Sed210 and Sed137, respectively) was linear for 45 salt marsh and mangrove environments. Sed210 averaged 75% of Sed137 suggesting that vertical accretion over the last 100+ years is driven by soil organic matter accumulation, as shown for the pre 137Cs dated horizon. The ratio of Sed210/Sed137 declines with increasing mineral content. A linear multiple regression equation that includes bulk density and Sed137 to predict Sed210 described 97% of the variance in Sed210. Sediments from Connecticut, Delaware and Louisiana coastal environments dated with 14C indicate a relatively constant sediment accretion rate of 0.13 cm year−1 for 1000–7000 yBP, which occurs within 2 m of today's marsh surface and equals modern sea level rise rates. Soil subsidence is not shown to be distinctly different in these vastly different coastal settings. The major reason why the Sed137 measurements indicate higher accretion rates than do the Sed210 measurements is because the former apply to younger sediments where the effects of root growth and decomposition are greater than in the latter. The most intense rates of change in soil volume in organic-rich salt marshes sediments is, therefore, neither in deep or old sediments (>4 m; >1000 years), but within the first several hundreds of years after accumulation. The average changes in organic and inorganic constituents downcore are nearly equal for 58 dated sediment cores from the northern Gulf of Mexico. These parallel changes downcore are best described as resulting from compaction, rather than from organic matter decomposition. Thus most of the volumetric changes in these salt marsh sediments occurs in the upper 2 m, and declines quickly with depth. Extrapolation forwards or backwards, using results from the 210Pb and the 137Cs dating technique appear to be warranted for the types of samples from the environments described here.  相似文献   

9.
《Marine Geology》2001,172(3-4):167-180
The distribution of reactive iron in sediments of the northwestern shelf, the shelf edge and the abyssal part of the Black Sea has been studied. In the euxinic Black Sea, iron sulfides (pyrite and iron monosulfide) are formed in the upper part of the anoxic water column and sink to the deep-sea floor where they are buried in the sediment. This flux of iron sulfides from the water column is reflected in enhanced concentrations of highly reactive iron and a high degree of pyritization (0.57–0.80) for the deep-water sediments of the Black Sea. The iron enrichment of deep-water sediments is balanced by a loss of highly reactive iron from the oxic continental shelf. Calculations from a numerical diagenetic model and reported in situ flux measurements indicate that the dissolved iron flux out of the shelf sediments is more than sufficient to balance the enrichment in reactive iron in deep-sea sediments, and that the majority of the dissolved iron efflux is redeposited on the continental shelf. This iron mobilization mechanism likely operates in most shelf areas, but its net effect becomes only apparent when reactive iron is trapped in sulfidic water bodies as iron sulfides or when iron is incompletely oxidized in low oxygen zones of the ocean and transported over long distances.  相似文献   

10.
Direct measurements of CO2 fluxes were made in salt, brackish and freshwater marshes and parallel adjacent open water areas in Barataria Basin, Louisiana. Vertical flux density was determined by monitoring the accumulation of CO2 in aluminum chambers placed over the water or sediment surfaces. Annual CO2 fluxes were 418, 180 and 618 g Cm?2 from the salt, brackish and freshwater marsh, respectively. Water bodies adjacent to the marsh evolved 103, 54 and 242 g CO2-Cm?2yr?1 to the atmosphere from saline, brackish and freshwater lakes, respectively. The role these marshes play in serving as a major carbon sink was determined from the carbon content of the sediment, vertical accretion rates and the bulk density of the sediment. Accretion rates were calculated from the depth in the sediment of the 1963 horizon, the year of peak 137Cs fallout. Net carbon accumulation was essentially the same in all three marshes; 183, 296 and 224 g Cm?2yr?1 from the salt, brackish and fresh marsh, respectively. Data presented suggest a limited net export of carbon from these coastal marshes. A large percentage of fixed carbon remained on the marsh, being immobilized in accretionary processes or lost to the atmosphere as CO2.  相似文献   

11.
对珠江口淇澳岛海岸带3个站位(QA-11,QA-9和QA-14)的沉积物中不同形态的还原硫(酸可挥发性硫,黄铁矿和有机硫)、总有机碳(TOC)和孔隙水中SO42-,甲烷浓度进行了测定,并且利用稳态扩散模型计算其中2个站位(QA-9和QA-14)硫酸盐还原通量[1.74和1.14 mmol/(m2.d)]和甲烷厌氧氧化通量[0.34和0.29 mmol/(m2.d)]。研究结果表明由于潮间带沉积物受到SO42-供给的限制,因此位于潮间带的QA-11站位硫酸盐还原带较浅(约16 cm);在潮下带的QA-9和QA-14站位,随离海岸距离和水深的增加,硫酸盐还原通量呈现减小的趋势,并且硫酸还原逐渐受到可利用活性有机质的限制;甲烷厌氧氧化对硫酸盐还原的贡献表现出增加的趋势,由19.2%增加至25.5%。三个站位沉积物中按不同形态还原硫含量由大到小列出,它们是有机硫(OS)、黄铁矿(DS)、酸可挥发性硫(AVS)。沉积物中AVS的空间分布与硫酸盐还原通量有正相关性。QA-11和QA-14站位的黄铁矿与AVS硫的含量比值大于3,分别为7.9和3.6,表明两个站位的黄铁矿形成可能受硫酸盐还原作用的控制;QA-9站位黄铁矿与AVS硫的含量比值为2.2,暗示AVS向黄铁矿转化受到可利用活性铁的限制。  相似文献   

12.
Elemental (TOC, TN, C/N) and stable carbon isotopic (δ13C) compositions and n-alkane (nC16–38) concentrations were measured for Spartina alterniflora, a C4 marsh grass, Typha latifolia, a C3 marsh grass, and three sediment cores collected from middle and upper estuarine sites from the Plum Island salt marshes. Our results indicated that the organic matter preserved in the sediments was highly affected by the marsh plants that dominated the sampling sites. δ13C values of organic matter preserved in the upper fresh water site sediment were more negative (−23.0±0.3‰) as affected by the C3 plants than the values of organic matter preserved in the sediments of middle (−18.9±0.8‰) and mud flat sites (−19.4±0.1‰) as influenced mainly by the C4 marsh plants. The distribution of n-alkanes measured in all sediments showed similar patterns as those determined in the marsh grasses S. alterniflora and T. latifolia, and nC21 to nC33 long-chain n-alkanes were the major compounds determined in all sediment samples. The strong odd-to-even carbon numbered n-alkane predominance was found in all three sediments and nC29 was the most abundant homologue in all samples measured. Both δ13C compositions of organic matter and n-alkane distributions in these sediments indicate that the marsh plants could contribute significant amount of organic matter preserved in Plum Island salt marsh sediments. This suggests that salt marshes play an important role in the cycling of nutrients and organic carbon in the estuary and adjacent coastal waters.  相似文献   

13.
The relationship between particulate organic carbon (POC) concentrations measured in modern sediment and fluxes of exported POC to the sediment surface needs to be understood in order to use POC content as a proxy of paleo-environmental conditions. The objective of our study was to compare POC concentrations, POC mineralization rates calculated from O2 consumption and POC burial rates. Benthic O2 distributions were determined in 58 fine-grained sediment cores collected at different periods at 14 stations in the southeastern part of the Bay of Biscay with depths ranging from 140 to 2800 m. Depth-dependent volume-specific oxygen consumption rates were used to assess rates of aerobic oxidation of organic matter (OM), assuming that O2 consumption solely was related to heterotrophic activity at the sediment–water interface. Heterogeneity of benthic O2 fluxes denoted changes in time and space of fresh organic material sedimentation. The most labile fraction of exported POC engendered a steep decrease in concentration in the upper 5 mm of vertical O2 profiles. The rupture in the gradient of O2 microprofile may be related to the bioturbation-induced mixing depth of fast-decaying carbon. Average diffusive O2 fluxes showed that this fast-decaying OM flux was much higher than buried POC, although diffusive O2 fluxes underestimated the total sediment oxygen demand, and thus the fast-decaying OM flux to the sediment surface. Sedimentary POC burial was calculated from sediment mass accumulation rate and the organic carbon content measured at the top of the sediment. The proportion of buried POC relative to total exported POC ranged at the most between 50% and 10%, depending on station location. Therefore, for a narrow geographic area like the Bay of Biscay, burial efficiency of POC was variable. A fraction of buried POC consisted of slow-decaying OM that was mineralized within the upper decimetres of sediment through oxic and anoxic processes. This fraction was deduced from the decrease with depth in POC concentration. At sites located below 500 m water depth, where the fast-decaying carbon did not reach the anoxic sediment, the slow-decaying pool may control the O2 penetration depth. Only refractory organic material was fossilized in sedimentary records at locations where labile OM did not reach the anoxic portion of the sediment.  相似文献   

14.
The products of 35S-sulphate reduction by sedimentary bacteria were measured at two sites in a salt marsh on the east coast of England. Non-acid-volatile products were measured, after acid-volatile sulphide was removed, by their reduction to sulphide by digestion with tin. The proportion of the sulphate reduced to tin-reducible products varied between 25% in a salt marsh pan and 61% in creek sediment, over a 0–25 cm depth profile. There were also variations with depth at each site in the proportions of sulphate reduced to tin-reducible products. Further examination revealed differences in the proportions of sulphate which were reduced to free sulphide, acid-volatile sulphide, sulphur or pyrite at the two sites. The data suggest that previous work which did not measure non-acid-volatile products underestimated sulphate reduction rates by three-fold in the creek site, but by only one third in the pan.  相似文献   

15.
New data are reported on the sulfur isotope composition and concentration of sulfide and sulfate in the upper part of the Black Sea anoxic zone as a function of the potential water density. The observations were performed at a station with the coordinates 44.489° N and 37.869° E three times a week every two days. A local negative deficiency in sulfate concentration up to 1.7% related to the sulfate reduction processes was recorded. This anomaly in sulfate concentration was short-lived and did not affect the sulfur isotope composition. In the upper part of the anaerobic zone, the δ34S(SO4) value varied from 21.2 to 21.5‰, which could have occurred from mixing of water masses from the oxic zone (21.1‰) and the Bottom Convective Layer (23.0 ± 0.2‰). The sulfur isotope composition of sulfide ranged from ?40.8% at a depth of 250 m to ?39.4‰ at the upper boundary of the anoxic zone with a H2S content of only 2.7 μM. Two models (mass balance and fractionation of sulfur isotopes using the Rayleigh equation) are considered to explain the differences in δ34S(H2S) values observed.  相似文献   

16.
长江口潮沼植物对动力沉积过程的影响   总被引：36，自引：5，他引：36  
为探讨潮沼植物对动力沉积过程的影响,在长江口南汇东滩沼泽和邻近的光滩上进行了近底层流速、波浪、悬沙浓度、沉积物粒径和滩面淤积速率等对比实验,并在九段沙潮沼中做了两种先锋植物黏附悬沙的实验.结果表明,沼泽和同高程的邻近光滩近底层平均流速的比值为0.16～0.84;两种环境平均波周期、平均波高和平均悬沙浓度的比值分别为1.4,0.43,和0.71.波浪传入沼泽后,一方面波高大大降低,另一方面变得规则有序.海三棱草和草两种先锋植物群落黏附的悬沙分别达到298和40g/m2.由于水动力降低和植物的捕沙功能,细颗粒泥沙易于在沼泽中沉积下来,导致沼泽环境沉积物明显较相邻光滩细;沼泽中沉积速率加快,侵蚀受到抑制.  相似文献   

17.
磁性矿物的早期成岩作用是沉积物埋藏后的重要过程,辨别早期成岩作用,才能更好地解释地层的矿物磁性变化。本研究对珠江三角洲顺德平原全新世钻孔MZ孔进行沉积相和室温磁性分析,并辅以热磁分析鉴定磁性矿物,以探讨钻孔不同深度和沉积相的早期成岩作用阶段。结果表明,MZ孔全新世地层自下而上包括感潮河道、河口湾和三角洲相。室温磁性特征与沉积相缺乏明显关联,表现出强烈的早期成岩作用。此外,全新世晚期岩芯磁性特征还受人类活动影响。该孔早期成岩作用以磁性矿物溶解和形成自生黄铁矿为主。在三角洲前缘相的上部和河口湾相底部保存了硫复铁矿。根据矿物组合推测以4.51～4.56 m和30.4～30.5 m两个深度为代表的硫复铁矿形成机制不同,即三角洲前缘相中硫复铁矿可能形成于早期成岩作用的硫酸盐还原阶段,而河口湾相的硫复铁矿形成于甲烷厌氧氧化阶段,后者的含量随着深度增加逐渐增多。上述现象说明,沉积环境可以通过影响有机质和硫酸根离子的供应量,决定磁性矿物所达到的早期成岩作用阶段。  相似文献   

18.
The performance of extractants commonly used in the determination of acid volatile sulfide minerals (e.g., ‘amorphous-FeS’, mackinawite and greigite) and pyrite has been evaluated using pure mineral phases and anoxic sediments. ‘Amorphous-FeS’ and mackinawite are quantitatively recovered by most cold acid extractions, but greigite recovery is incomplete. Harsher extractants with reducing agents are necessary for the complete recovery of greigite, but dissolution of fine-grained synthetic pyrite occurs under such conditions. A quantitative separation between greigite and pyrite is not possible using these techniques, but the use of ‘acid volatile sulfide’ and ‘pyrite’ as operational categories is adequate for most field studies.  相似文献   

19.
In the paper (Wang and Morse, 1996) that preceded this study, we presented results of experiments performed using a silica gel crystal growth technique to produce pyrite under conditions approximating those commonly occurring in anoxic marine sediments. The primary focus of that study was on the chemical pathways that pyrite formation follows and how differing conditions influenced reaction kinetics and morphology of pyrite crystals. In this paper, we present results of further long-term (up to 1 y) studies of pyrite formation, using the silica gel experimental technique, in which we investigated the role that different precursor iron (hydr)oxide minerals and marine organic matter play in pyrite formation. The minerals studied were akaganeite (β-FeOOH), ferrihydrite (Fe5HO8 · 4H2O), goethite (α-FeOOH), hematite (α-Fe2O3), lepidocrocite (γ-FeOOH), and magnetite (Fe3O4). Marine organic matter used in this study was freeze-dried plankton collected from near-surface water in the Gulf of Mexico. The influence of precursor iron (hydr)oxide mineralogy, although important for initial iron sulfidization rates, was relatively minor compared to other variables, such as solution pH and sulfide concentration, in controlling the rate of pyrite formation. Consequently, major variations in the observed rate of pyritization of different iron (hydr)oxide minerals in sediments (e.g., Canfield and Berner, 1987) may reflect large differences in surface areas of the minerals rather than their intrinsic reactivity and is a confirmation of the estimates of Canfield et al. (1992) that most iron oxides have similar reactivity. The presence of marine organic matter (freeze-dried plankton) caused an increase in the sulfidization rate of goethite and a major (about 20 ×) decrease in the rate of pyrite formation. This can be interpreted as indicating that organic matter-iron interactions are important in both iron (hydr)oxide dissolution, and pyrite nucleation and growth. A possible explanation for this behavior is that dissolved organic matter produced during the long experiments (up to 1 year) increased the rate of goethite dissolution while inhibiting pyrite nucleation and growth by complexing iron. The lessons learned in the study of other mineral reaction kinetics (e.g., calcite and aragonite), that rates determined in pure inorganic systems, may not always be reliably applied to natural systems where organic matter can significantly influence mineral dissolution and growth rates, are, alas, repeated here for pyrite.  相似文献   

20.
对南海北部陆坡柱状沉积物样品总有机碳、总硫含量,以及其中的自生黄铁矿形貌、含量进行分析．结果显示,沉积物中黄铁矿（FeS2）、总有机碳（TOC）、总硫（TS）的质量分数分别为0～0,71％、0．37％～1．18％、0．04％～0．81％;黄铁矿和总有机碳、总硫的含量随深度加深逐渐增大,达到峰值后不断减少,三者的分布趋势基本一致;扫描电镜下观察到黄铁矿主要以莓球状集合体和八面体微晶形貌产出,局部层位亦发现管状、生物内膜状和立方体状黄铁矿晶体．表明该区浅表层环境为缺氧环境,硫化物主要以黄铁矿形式产出,其成因与有机质的厌氧氧化作用有关,而甲烷的厌氧氧化作用也可能促使自生黄铁矿的加速形成．莓球状黄铁矿占主导亦指示一种强还原性的缺氧微环境．黄铁矿富集的缺氧环境与下伏地层中天然气水合物分解释放的甲烷有关,为天然气水合物在该区的勘探提供一定的科荤依据．  相似文献   

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