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1.
Data on abundance and isotopic composition of porewater and sedimentary sulfur species are reported for relatively uncontaminated and highly contaminated fine-grained anoxic sediments of St. Andrew Bay, Florida. A strong contrast in amount and composition of sedimentary organic matter at the two sites allows a comparative study of the historical effects of increased organic loading on sulfur cycling and sulfur isotopic fractionation. In the contaminated sediments, an increase in organic loading caused increased sedimentary carbon/sulfur ratios and resulted in higher rates of bacterial sulfate reduction, but a lower efficiency of sulfide oxidation. These differences are well reflected in the isotopic composition of dissolved sulfate, sulfide, and sedimentary pyrite. Concentration and isotopic profiles of dissolved sulfate, organic carbon, and total sulfur suggest that the anaerobic decomposition of organic matter is most active in the upper 8cm but proceeds at very slow rates below this depth. The rapid formation of more than 90% of pyrite in the uppermost 2 cm which corresponds to about 3 years of sediment deposition allows the use of pyrite isotopic composition for tracing changing diagenetic conditions. Sediment profiles of the sulfur isotopic composition of pyrite reflect present-day higher rates of bacterial sulfate reduction and lower rates of sulfide oxidation, and record a profound change in the diagenetic cycling of sulfur in the contaminated sediments coincident with urban and industrial development of the St. Andrew Bay area.  相似文献   

2.
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.  相似文献   

3.
Organic matter was isolated from the water columns and sediments of two pond systems in the south-eastern United States. Water column material was ultrafiltered to provide three fractions, i.e. <0.45 μm, but > 50,000 daltons; <50,000 daltons, but > 5000 daltons; and <5000 daltons. Sedimentary organic matter was separated into humic acid and fulvic acid fractions based on solubility criteria and the humic acid fraction was ultrafiltered to provide the same fractions as the water column isolates. All fractions were analysed for organic carbon, Al, Ca, Cu, Fe, Mg and Mn. Infra-red spectra were also measured for the sedimentary organic fractions. Organic matter isolated from the water column of the two ponds had similar organic carbon and elemental distributions, as did the organic matter isolated from the two sediments. However, significant differences in the organic carbon and elemental distributions were observed for water column and sedimentary organic matter isolated from the same pond. These studies have relevance to diagenetic alterations of organic matter and geochemical cycles of elements within lakes.  相似文献   

4.
Organic-rich sediment samples collected from a transect within, and below, the Oman Margin oxygen minimum zone (OMZ) were analysed using a sequential leaching technique to characterise the diagenetic behaviour and speciation of Mn and Fe in operationally defined sediment host fractions. Trace metals showed distinct diagenetic behaviour in the two contrasting environments that were sampled. The absence of non-detrital Mn in the cores below the OMZ site is attributed to the lack of easily reducible oxides in surficial sediments and to the reduction and export of any moderately reducible aged oxides. The reactive form of solid phase Mn showed a classic feature of enrichment in the upper layer of the sediments at the abyssal site, reflecting the presence of an oxidising sedimentary layer which acts as a Mn trap during its recycling. The diagenetic Mn enrichment was inferred from typical downcore colour changes and an upward-increasing Mn content in the upper core sections. An easily reducible Fe oxide layer was observed in the abyssal sediments at an identical depth to the Mn enrichment suggesting that Fe associated with Mn oxides also has undergone sub-oxic diagenesis. However, the association of Fe with organic matter did not indicate diagenetic modification; i.e., the binding strength of the metal with organic materials appears to be sufficiently strong to preserve the trace metal. The speciation signature of non-detrital Fe differed from that of Mn. The association of Fe with organic matter suggests that this metal does not undergo diagenetic modification and is preserved in abyssal sediments. The contrasting behaviour of Mn and Fe observed between cores within the OMZ were particularly interesting. Another interesting observation was that, for cores below the OMZ, the iron oxides were associated with the Mn-oxide peak, rather than deeper in the sediments as observed by earlier studies in the Atlantic [Froelich et al. (1979). Geochim. et Cosmochim Acta 43, 1075–1090].This revised version was published online in May 2005 with corrections to the article title.  相似文献   

5.
Numerous studies of marine environments show that dissolved organic carbon (DOC) concentrations in sediments are typically tenfold higher than in the overlying water. Large concentration gradients near the sediment–water interface suggest that there may be a significant flux of organic carbon from sediments to the water column. Furthermore, accumulation of DOC in the porewater may influence the burial and preservation of organic matter by promoting geopolymerization and/or adsorption reactions. We measured DOC concentration profiles (for porewater collected by centrifugation and “sipping”) and benthic fluxes (with in situ and shipboard chambers) at two sites on the North Carolina continental slope to better understand the controls on porewater DOC concentrations and quantify sediment–water exchange rates. We also measured a suite of sediment properties (e.g., sediment accumulation and bioturbation rates, organic carbon content, and mineral surface area) that allow us to examine the relationship between porewater DOC concentrations and organic carbon preservation. Sediment depth-distributions of DOC from a downslope transect (300–1000 m water depth) follow a trend consistent with other porewater constituents (ΣCO2 and SO42−) and a tracer of modern, fine-grained sediment (fallout Pu), suggesting that DOC levels are regulated by organic matter remineralization. However, remineralization rates appear to be relatively uniform across the sediment transect. A simple diagenetic model illustrates that variations in DOC profiles at this site may be due to differences in the depth of the active remineralization zone, which in turn is largely controlled by the intensity of bioturbation. Comparison of porewater DOC concentrations, organic carbon burial efficiency, and organic matter sorption suggest that DOC levels are not a major factor in promoting organic matter preservation or loading on grain surfaces. The DOC benthic fluxes are difficult to detect, but suggest that only 2% of the dissolved organic carbon escapes remineralization in the sediments by transport across the sediment-water interface.  相似文献   

6.
The geochemistry of Upper Quaternary organic-rich (Corg 3.7–10.0%) sediments recovered by the 40-m-long Core MD02-2508 from the upper continental slope of Baja California is compared to the chemical composition of sedimentary rocks from the Miocene Monterey Formation, California. It is ascertained that concentrations of most macroelements (Al, Ti, Mg, K, Fe, M, S, Corg) and many microelements, including chalcophiles Ag, As, Mo, Sb, Se, Zn, along with some others (U, Co, Ni, Y, and REE) are nearly equal in both types of sediments. In addition, concentrations of most microelements in both types of sediments are close, to the average values common for the worldwide carbonaceous black shales irrespective of their lithology and age, indicating a stable influence of organic matter on their concentration during sedimentation and early digenesis.  相似文献   

7.
近代盐湖沉积物中的生物标志化合物   总被引：2，自引：1，他引：2  
    生物标志化合物是在沉积有机质地质演化过程中可保留其原碳骨架的生物分子，具记录沉积环境和沉积有机质热演化历史的功能。用有机地球化学方法（气相色谱、气相色谱/质谱、热解色谱和同位素质谱）对采自我国西部盐湖的近代沉积物进行了大量研究，而且对盐湖沉积环境地球化学特征、有机质组成与早期成岩作用的关系和各种生物标志化合物指标进行了更加深入细致的探索。研究结果表明干酪根碳同位素组成（δ13C PDB）和盐湖类型有关系，例如，硫酸盐型湖的干酪根δ13C 值较重，而碳酸盐型盐湖的δ13C值则较轻。姥鲛烷/植烷比（Pr/Ph）、总三环萜烷/总萜烷（%）、孕甾烷/甾烷比（%）、伽玛蜡烷/αβ—藿烷和三芴系列化合物组成三角图等生物标志化合物指标可以用来判识沉积环境的盐度。通常，在盐湖或咸水沉积环境中，除额吉卓尔盐湖样品因采自边缘，Pr/Ph比值较大而外，其余各盐湖样品的Pr/Ph比值一般≤1.0，而其余各比值均明显大于在淡水湖中的比值；三芴系列化合物三角图中，各盐湖样品均集中分布在硫芴含量高的区域。与我国各典型沉积环境中的原油及古代岩样的分析结果作了对比，发现同样表现出上述规律，故认为是良好的识别标志。正构烷烃分布、甾烷相对含量及某些特殊生物标志化合物的含量如三环二萜烷可以用来划分可溶有机质类型。察汉卓和查干诺尔盐湖可溶有机质中，正烷烃以低碳数占优势，显示其组成主要来源于低等水生生物；额吉卓尔盐湖则以高碳数为主，显然与边缘相陆生高等植物的输入有关；而小柴旦盐湖正烷烃高低碳数分布均衡，说明高等植物与低等水生生物同时输入。C27～C29正常甾烷及三环二萜烷的组成与分布也显示了相似特征。
    另外，还发现了多种非常规生物标志化合物，报导了其 m/z 217、m/z 231、m/z 245、m/z 259离子色谱特征。这些特征离子大多由三芳甾烷产生。三芳甾烷是由生物分子（甾体）在地质条件下演化而成的。盐湖沉积物中大量该类化合物的发现意味着含盐沉积环境及其继后的早期成岩作用在甾体的这种芳化过程中起着积极作用。  相似文献   

8.
Despite its location on sediment-free basalt, vent fluids from the Main Endeavour Field (MEF) contain chemical species that indicate fluids have interacted with sediments during circulation. We report on the distribution and isotopic abundances of organic compounds (C1-C3 alkanes and alkenes, benzene and toluene) in fluids collected from the Main Endeavour Field (MEF) in July, 2000, to understand the processes that regulate their abundances and characterize fluid sources. Aqueous organic compounds are derived from the thermal alteration of sedimentary organic matter and subsequently undergo further oxidation reactions during fluid flow. Fluid:sediment mass ratios calculated using ΣNH4 concentrations indicate that the sediments are distal to the MEF, resulting in a common reservoir of fluids for all of the vents. Following the generation from sediment alteration, aqueous organic compounds undergo secondary alteration reactions via a stepwise oxidation reaction mechanism. Alkane distributions and isotopic compositions indicate that organic compounds in MEF fluids have undergone a greater extent of alteration as compared to Middle Valley fluids, either due to differences in subsurface redox conditions or the residence time of fluids at subsurface conditions. The distributions of the aromatic compounds benzene and toluene are qualitatively consistent with the subsurface conditions indicated by equilibration of aqueous alkanes and alkanes. However, benzene and toluene do not achieve chemical equilibrium in the subsurface. Methane and CO2 also do not equilibrate chemically or isotopically at reaction zone temperatures, a likely result of an insufficient reaction time after addition of CO2 from magmatic sources during upflow. The organic geochemistry supports the assumption that the sediments with which MEF fluids interact has the same composition as sediments present in Middle Valley itself, and highlight differences in subsurface reaction zone conditions and fluid flow pathways at these two sites.  相似文献   

9.
Isotopic compositions of organic (δ13C-Corg) and carbonate (δ13C-Ccarb) carbon were analyzed in the particulate matter (hereafter, particulates) and sediments from the North and Middle Caspian basins. Isotopic composition of Corg was used for assessing proportions of the allochthonous and autochthonous organic matter in the particulates. Difference between the δ13C-Corg values in surface sediments and particulates is explained by the aerobic and anaerobic diagenetic transformations. Isotopic composition of Corg in sediments may be used as a tool for reconstructing the Quaternary transgressive-regressive history of the Caspian Sea.  相似文献   

10.
We present inorganic geochemical analyses of pore waters and sediments of two Late Quaternary sediment cores from the western Arctic Ocean (southern Mendeleev Ridge, RV Polarstern Expedition ARK-XXIII/3), focussing on the composition and origin of distinct, brown-colored, Mn-rich sediment layers. Carbonate enrichments occur in association with these layers as peaks in Ca/Al, Mg/Al, Sr/Al and Sr/Mg, suggesting enhanced input of both ice-rafted and biogenic carbonate. For the first time, we show that the Mn-rich layers layers are also consistently enriched in the scavenged trace metals Co, Cu, Mo and Ni. Distinct bioturbation patterns, specifically well-defined brown burrows into the underlying sediments, suggest these metal enrichments formed close to the sediment-water interface. The geochemical signature of these metal- and carbonate-rich layers most probably documents formation under warmer climate conditions with an intensified continental hydrological cycle and only seasonal sea ice cover. Both rivers and sea ice delivered trace metals to the Arctic Ocean, while enhanced seasonal productivity exported reactive organic matter to the sea floor. The coeval deposition of organic matter, Mn (oxyhydr)oxides and trace metals triggered intense diagenetic Mn cycling at the sediment-water interface. These processes resulted in the formation of Mn and trace metal enrichments, and the degradation of labile organic matter. With the onset of cooler conditions, reduced riverine runoff and/or a solid sea ice cover terminated the input of riverine trace metal and fresh organic matter, resulting in deposition of grayish-yellowish, metal-poor sediments. Oxygen depletion of Arctic bottom waters under these cooler conditions is not supported by our data, and did not cause the sedimentary Mn distribution. While the original composition and texture of the brown layers resulted from specific climatic conditions and corresponding diagenetic processes, pore water data show that diagenetic Mn redistribution is still affecting the organic-poor deeper sediments. Given persistent steady state conditions, purely authigenic Mn-rich brown layers may form, while others may be partly or completely dissolved. The degree of diagenetic Mn redistribution largely depends on the depositional environment, the Mn and organic matter availability, and apparently affected the Co/Mo ratios of Mn-rich layers. Thus, brown Arctic layers are not necessarily synchronous features, and should not be correlated across the Arctic Ocean without additional age control.  相似文献   

11.
Geochemical analyses of lakebed and core sediments from Lake Sambe on the outskirts of Oda City in Shimane prefecture in southwestern Japan were carried out in order to assess the water quality and the concentration and distribution patterns of sixteen elements. The lake water showed a stratified condition with respect to dissolved O2, and As, Fe, and Mn concentrations in the bottom layers which increased in the summer. The chemical composition of the sediments, as measured by X-ray fluorescence, included major and trace elements (P, Ca, Sc, Ti, V, Cr, Fe, Ni, Cu, Zn, As, Sr, Zr, Pb, and Th), and total sulfur (TS). Elevated values of As, Zn, V, Fe, P, and TS were present in several layers of the upper cores (from 0 to 5 cm) and other surface sediments. Increases in the abundances of these metals in lake sediments are probably related to the reducing condition of the sediments, fine-grained organic rich sediments, and post-depositional diagenetic remobilization. Moreover, correlations between the concentrations of trace metals and iron in the sediments suggest their adsorption onto Fe (oxy)hydroxides, whereas correlations with sulfur indicate that they were precipitated as Fe-sulfides. The average abundances of As, Pb, Zn, and Cu exceeded the lowest effect level and Interim Sediment Quality Guideline values that the New York State Department of Environmental Conservation and the Canadian Council of Ministers of the Environment determined to have moderate impact on aquatic organisms. In addition, concentrations of As and Zn exceeded the Coastal Ocean Sediment Database threshold value, indicating potentially toxic levels. Therefore, the presence of trace metals in the lake sediments may result in adverse effects on biota health.  相似文献   

12.
We investigate the distributions of several key diagenetic reactants (C, S, Fe) and redox-sensitive trace metals (Mo, Cd, Re, U) in sediments from Lake Tanganyika, East Africa. This study includes modern sediments from a chemocline transect, which spans oxygenated shallow waters to sulfidic conditions at depth, as well as ancient sediments from a longer core (∼2 m) taken at ∼900 m water depth. Modern sediments from depths spanning ∼70-335 m are generally characterized by increasing enrichments of C, S, Mo, Cd, and U with increasing water depth but static Fe distributions. It appears that the sedimentary enrichments of these elements are, to varying degrees, influenced by a combination of organic carbon cycling and sulfur cycling. These modern lake characteristics contrast with a period of high total organic carbon (Corg), total sulfur (STot), and trace metal concentrations observed in the 900 m core, a period which follows the most recent deglaciation (∼18-11 ky). This interval is followed abruptly by an interval (∼11-6 ky) that is characterized by lower C, S, U, and Mo. Consistent with other work we suspect that the low concentrations of S, Mo, and U may indicate a period of intense lake mixing, during which time the lake may have been less productive and less reducing as compared to the present. An alternative, but not mutually exclusive, hypothesis is that changes in the lake’s chemical inventory, driven by significant hydrological changes, could be influencing the distribution of sedimentary trace elements through time.  相似文献   

13.
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 (C25,27,29,31). 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.  相似文献   

14.
沿海表层沉积物中重金属的有效结合态   总被引：13，自引：0，他引：13       下载免费PDF全文
以浙江沿海表层沉积物为研究对象,系统地研究了重金属有效结合态与沉积环境、矿物组成和人为排放等环境因素之间的相互关系.结果表明:沿海表层沉积物中重金属的积累数量,呈现出潮汐河口沉积物>沿海沉积物>强潮汐河口沉积物的趋势,这反映了不同沉积类型对重金属富集作用的差异;重金属的有效结合态,总体上以铁锰氧化物结合态>碳酸盐结合态>硫化物及有机结合态>可交换态的顺序存在,但是不同的重金属或同种重金属在不同的采点,主要有效结合态的比例存在明显的差异,这不仅与重金属的地球化学性质、沉积环境和沉积物中的粘土矿物组成有关,更受到重金属污染物的人为排放量的影响.  相似文献   

15.
In the past several decades, the techniques used to discern the different sedimentary fractions of P have been refined. This has allowed for a better understanding of P burial of the different P fractions and diagenetic reactions and, ultimately, the constraining of P residence time in the oceans. P sequential extraction was performed on eight sediment cores (between 16 and 24 cm deep) collected along a salinity gradient from the Ojo de Liebre Lagoon and the salt evaporation saltern of Guerrero Negro, Baja California Sur, Mexico in order to determine, under purely diagenetic conditions (in the absence of anthropogenic activities and biogenic sediment reworking), the fractionation and flux of P to the sediments. The majority of P was found in the authigenic fraction (37 ± 5.4% to 53 ± 8.9%), with P associated to organic matter comprising the overall smallest percentage (0.25 ± 0.43% to 21 ± 6.0%) relative to total P. The average flux of total P to the sediments for all the sites was found to be (451 ± 127) × 10−4 mol m−2 year−1, up to several orders of magnitude greater than those found in other studies. It is concluded that P is most likely transformed from P associated to organic matter to the authigenic mineral phase and that P was retained in the sediments in its mineral form rather than in reactive forms. This particular study area has the ability to retain large quantities of P in the sediments.  相似文献   

16.
The lithology and geochemistry of upper Oxfordian sedimentary rocks enriched in marine organic matter (OM) have been studied. These rocks occur as a persistent unit of Upper Jurassic rocks exposed on the right bank of the Unzha River in the Kostroma district. The OM was investigated in detail in both the carbon-rich rocks and their hosts. It was established that the OMs from the Oxfordian rocks are characterized by a low degree of thermal (catagenetic) maturity and their geochemical signature reflects specific features of synsedimentary and early diagenetic processes. Kerogen in the carbon-rich sedimentary rocks is markedly enriched in Sorg, and its formation was related to the early diagenetic sulfate reduction (sulfurization of the lipid fraction of the initial OM). The composition of kerogen from the host clay is sharply distinct in many parameters. No derivatives of isorenieratene were revealed in the aromatic fraction of bitumen in the carbon-rich rocks. The Oxfordian carbonaceous rocks are distinguished by slightly enriched in S, Mo, V, and Ni. Anoxic conditions were unstable in the water column during the deposition of carbon-rich sediments (such conditions were probably episodic). The Corg-rich unit formed due to a short-term abrupt increase in the productivity of phytoplankton related to eutrophication of water, probably, as a result of the recycling and redistribution of biophile elements.  相似文献   

17.
Localized zones or lenses of naturally reduced sediments have the potential to play a significant role in the fate and transport of redox-sensitive metals and metalloids in aquifers. To assess the mineralogy, microbiology and redox processes that occur in these zones, several cores from a region of naturally occurring reducing conditions in a U-contaminated aquifer (Rifle, CO) were examined. Sediment samples from a transect of cores ranging from oxic/suboxic Rifle aquifer sediment to naturally reduced sediment were analyzed for U and Fe content, oxidation state, and mineralogy; reduced S phases; and solid-phase organic C content using a suite of analytical and spectroscopic techniques on bulk sediment and size fractions. Solid-phase U concentrations were higher in the naturally reduced zone, with a high proportion of the U present as U(IV). The sediments were also elevated in reduced S phases and Fe(II), indicating it is very likely that U(VI), Fe(III), and SO4 reduction has occurred or is occurring in the sediment. The microbial community was assessed using lipid- and DNA-based techniques, and statistical redundancy analysis was performed to determine correlations between the microbial community and the geochemistry. Increased concentrations of solid-phase organic C and biomass in the naturally reduced sediment suggests that natural bioreduction is stimulated by a zone of increased organic C concentration associated with fine-grained material and lower permeability to groundwater flow. Characterization of the naturally bioreduced sediment provides an understanding of the natural processes that occur in the sediment under reducing conditions and how they may impact natural attenuation of radionuclides and other redox sensitive materials. Results also suggest the importance of recalcitrant organic C for maintaining reducing conditions and U immobilization.  相似文献   

18.
The Changjiang and the Jiulong Estuaries, located in eastern and southeastern China, respectively, have different geomorphologic and tidal processes as well as anthropogenic development in their associated watersheds. Sediments in the Changjiang estuary mostly consist of SiO2, CaO and MgO (mean percentages of 63.9, 4.34 and 2.35%), whereas sediments from the Jiulong estuary mostly consist of Al2O3, Fe2O3 and organic matter (mean percentages 19.2, 6.82 and 4.14%). The Jiulong estuarine sediments contain more than twice the concentrations of Pb, Zn, Cu, than those from the Changjiang estuary. In the Jiulong estuary, these heavy metals are associated with carbonates and organic matter, whereas in the Changjiang estuary, they are associated with residual fractions or clay. Sediments from the Changjiang estuary, mostly sediments with little organic matter, do not efficiently sequester anthropogenic-derived trace metals. In contrast, sediments from the Jiulong estuary consist of a mixture of fluvial and marine matter which can sequester heavy metals contributed by larger landscapes with industrial and municipal wastewater.  相似文献   

19.
Biomarker and n-alkane compound specific stable carbon isotope analyses (CSIA) were carried out on 58 crude oil samples from shallow water and deepwater fields of the Niger Delta in order to predict the depositional environment and organic matter characteristics of their potential source rocks. Using a source organofacies prediction approach from oil geochemistry, the presence in the western deepwater oils relatively abundant C27 steranes, C30 24-n-propyl cholestane, low oleanane index, relatively low pr/ph ratios, gammacerane, and positive to nearly flat C12–C30 n-alkane compound specific stable carbon isotope profiles, suggests that the source facies that expelled these oils contain significant marine derived organic matter deposited under sub-oxic and stratified water column conditions. This contrasts with the terrigenous organic matter dominated source rocks accepted for shallow water Niger Delta oils. Oils in the shallow water accumulations can be separated into terrigenous and mixed marine-terrigenous families. The terrigenous family indicates expulsion from source rock(s) containing overwhelmingly higher plant source organic matter (average oleanane index = 0.48, high C29 steranes) as well as having negative sloping n-alkane isotope profiles. Oxic source depositional conditions (pr/ph > 2.5) and non-stratified conditions (absence to low gammacerane content) are inferred for the terrigenous family. The mixed marine-terrigenous family has biomarker properties that are a combination of the deepwater and terrigenous shallow water oils. Bitumen extracts of the sub-delta Late Cretaceous Araromi Formation shale in the Dahomey Basin are comparable both molecularly and isotopically to the studied western deepwater oil set, but with an over all poor geochemical correlation. This poor geochemical match between Araromi shale and the western deepwater oils does not downgrade the potential of sub-delta Cretaceous source rock contribution to the regional oil charge in the deepwater Niger Delta.  相似文献   

20.
Surface sediment samples were collected from the Squamish River Delta, British Columbia, in order to determine the role of sediment surface area in the preservation of organic matter (OM) in a paralic sedimentary environment. The Squamish Delta is an actively prograding delta, located at the head of Howe Sound.Bulk total organic carbon (TOC) values across the Squamish Delta are low, ranging from 0.1 to 1.0 wt.%. The carbon/total nitrogen ratio (Corg/N) ranges from 6 to 17, which is attributed to changes in OM type and facies variations. The <25-μm fraction has TOC concentrations up to 2.0 wt.%, and a Corg/N ratio that ranges from 14 to 16. The 53–106-μm fraction has higher TOC concentrations and Corg/N ratios relative to the 25–53-μm fraction. The Corg/N ratio ranges from 9 to 18 in the 53–106-μm fraction and 5.5–10.5 in the 25–53-μm fraction. Surface area values for bulk sediments are low (0.5–3.0 m2/g) due to the large proportion of silt size material. Good correlation between surface area and TOC in bulk samples suggests that OM is adsorbed to mineral surfaces. Similar relationships between surface area and TOC were observed in size-fractionated samples. Mineralogy and elemental composition did not correlate with TOC concentration.The relationships between surface area, TOC and total nitrogen (TN) can be linked to the hydrodynamic and sedimentological conditions of the Squamish Delta. As a result, the Squamish Delta is a useful modern analogue for the formation of petroleum source rocks in ancient deltaic environments, where TOC concentrations are often significantly lower than those in source rocks formed in other geological settings.  相似文献   

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