湖相沉积物有机地球化学在古环境研究中的应用

湖相沉积物中的有机质蕴含着丰富的地球化学信息,是古环境研究中必要的基础资料之一。文章综述了近年来湖相沉积物有机地球化学在古环境研究中的应用,结果表明:1在应用有机地球化学信息探讨古环境时,首先要根据有机质组成特征区分其来源,不同来源的有机质,其地球化学特征存在差异;2有机质碳同位素(δ13Corg)是研究古气候变化的常用指标,在区分暖湿、暖干、冷湿、冷干气候类型时需结合沉积物中总有机质丰度(TOC)、总有机氮丰度(TN)、自生碳酸盐氧同位素(δ18O)等资料;3脂肪酸不饱和度可作为恢复古环境温度的定性指标,UK37和UK37'在定量恢复古湖水表层温度中有较好的应用效果,TEX86有望应用在古湖水表层温度的重建;4生物标志化合物可较好地区分沉积物中有机质的来源、沉积水体的盐度及氧化还原性。可见,有机地球化学是研究古环境常用且有效的技术手段,多项有机地球化学参数相互验证能够更精确地重建古环境并预测其演化趋势。     

Organic geochemistry of Walvis Bay diatomaceous ooze—I. Occurrence and significance of the fatty acids

Diatomaceous ooze of the shelf off Walvis Bay (S.W. Africa) was analysed for lipid material. The lipids from the sediment consist of a mixture of hydrocarbons, alcohols, ketones, fatty acids, sterols, carotenoid pigments and chlorophylls. The total fatty acid mixture has been analysed by capillary gas chromatography mass spectrometry and shown to consist of straight chain, iso-, anteiso- and isoprenoid acids. The environmental setting of the S.W. African shelf (Walvis Bay) makes it possible to discuss these fatty acids as markers for the fate of organic matter. The acids in the sediment point to a generation during passage of algal lipids through levels of microbial activity either on or slightly above the burial surface.     

Evolution of organic matter degradation in Cretaceous black shales inferred from authigenic barite: A reaction-transport model

A reaction-transport model was used to infer the long-term evolution of anaerobic organic matter degradation in Cretaceous black shales from the distribution of authigenic barite in sediments drilled at Demerara Rise (ODP Leg 207, Site 1258). In these sediments, sulfate-reduction and methanogenesis are the major pathways of organic matter decomposition and the depth-distribution of authigenic barite serves as an indicator for the temporal evolution of the sulfate-methane transition zone (SMTZ), the strength of the biogenic methane flux and, ultimately, the organic matter reactivity in the black shales over geological timescales. Organic matter degradation is described according to the reactive continuum model approach and parameters values are determined by inverse modeling, based on present-day porewater and authigenic barite profiles. Fully transient simulations were performed over a period of 100 Myrs and indicate that important features of the biogeochemical dynamics are associated to changes in the boundary forcing. Hiatuses in sediment accumulation rate result in quasi-steady-state conditions and lead to distinct accumulations of authigenic barites in the SMTZ. The inversely determined parameters reveal that the reactivity of the organic matter was already low (apparent first order rate constant ) at the time of its deposition in the Cretaceous. The geochemical characteristics of sediments drilled at Demerara Rise, as well as the presence of specific biomarkers, suggest that this low reactivity is most likely due to the euxinic palaeo-conditions which favored the sulfurization of the organic matter. Simulation results predict average initial organic carbon contents between 8.1 and 9.5 wt%, implying a high preservation efficiency of the organic matter (between 79% and 89%). Calculated mass accumulation rates (between 0.43 and 0.5 ) compare well with estimations for the western basin of the Cretaceous southern North Atlantic. Simulation results thus indicate that the enhanced preservation of organic matter under euxinic conditions may have been the main cause for the formation of organic-rich Cretaceous black shales at Demerara Rise.     

The sedimentary structures of barite: examples from the Chaudfontaine ore deposit, Belgium

The Chaudfontaine ore deposit is composed mainly of barite and hosted in Frasnian shale and carbonate formations of Belgium. Thirteen sedimentary structures involving barite crystals are described and discussed. Many of these structures appear to have developed under gravity control. The role of mechanical and chemical ore apposition processes are detailed in each case. The study shows that two populations of barite were formed: (1) crystals which grew in brine above the water-sediment interface and (2) crystals which developed in the sediment during early diagenesis. The two modes of formation have specific sizes, habits and structures. Both populations crystallized under the control of the prevailing physico-chemical conditions, and the size and abundance of the barite crystals may be expressed as a function of the degree of barium supersaturation. High-energy currents and unstable sediment caused previously deposited crystals to be reworked. Morphological similarities were found between Chaudfontaine barite and gypsum from evaporites.     

Precambrian sedimentary carbonates: carbon and oxygen isotope geochemistry and implications for the terrestrial oxygen budget

Carbon isotope values of 260 Precambrian limestones and dolomites (most of them being substantially unaltered) have yielded an overall mean of $\text{δ}{}^{13}\text{C}\text{= +0.4 ± 2.7‰}$ vs. PDB; the corresponding oxygen values average at $\text{δ}{}^{13}\text{O = +20.0 ± 4.2‰}$ vs. SMOW. Like the overall mean, the δ ¹³C values furnished by individual carbonate occurrences are, as a rule, fairly “modern” and almost constant as from the very beginning of the sedimentary record. A remarkable exception are the “heavy” dolomites of the Middle Precambrian Lomagundi Group, Rhodesia, with $\text{δ}{}^{13}\text{C}\text{= +9.4 ± 2.0‰}$ vs. PDB. As a result of our measurements, the sporadic occurrence in the geological past of anomalously heavy carbonates seems to be established.The approximate constancy around zero per mill of the δ ¹³C values of marine carbonates through geologic time would imply a corresponding constancy of the relative proportion of organic carbon in the total sedimentary carbon reservoir since about 3.3 · 10⁹ y ago (with C_org/C_total ? 0.2). Utilizing this ratio and current models for the accumulation of the sedimentary mass as a function of time, we get a reasonable approximation for the absolute quantity of organic carbon buried in sediments and, accordingly, of photosynthetic oxygen released. Within the constraints of our model (based on a terrestrial degassing constant λ = 1.16 · 10^?9 y^?1) close to 80% of the amount of oxygen contained in the present oxygen budget should have been released prior to 3 · 10⁹ y ago. Since geological evidence indicates an O₂-deficient environment during the Early and most parts of the Middle Precambrian, there is reason to believe that the distribution of this oxygen between the “bound” and the “molecular” reservoir was different from that of today (with effective O₂-consuming reactions bringing about an instantaneous transfer to the crust of any molecular oxygen released). Accordingly, the amount of C_org in the ancient sedimentary reservoir as derived from our isotope data is just a measure of the gross amount of photosynthetic oxygen produced, withholding any information as to how this oxygen was partitioned between the principal geochemical reservoirs. As a whole, the carbon isotope data accrued provide evidence of an extremely early origin of life on Earth since the impact of organic carbon on the geochemical carbon cycle can be traced back to almost 3.5 · 10⁹y.     

The geochemistry of cadmium in some sedimentary rocks

The cadmium contents of 11 shales, 11 sandstones, 7 limestones, 14 metamorphic rocks and 28 stream sediments, all from Pennsylvania, were determined by flameless atomic absorption spectrometry. On the basis of these cadmium values and those in the literature, cadmium is found to be enriched in dark shales and soils, depleted in red shales, sandstones and limestones, and about the same in stream sediments as in most igneous and metamorphic rocks and the crust. In stream sediments, cadmium correlates most significantly with zinc, followed by carbon, weight loss on ignition, cobalt, readily extractable lead and manganese. Examination of the correlation plots and of an Eh-pH diagram indicates that the primary cause of cadmium enrichment in the sedimentary environment is the adsorption and/or complexation of cadmium with organic matter followed by the accumulation of organic debris in a reducing depositional environment.     

The organic geochemistry of torbanite precursors

The generally accepted hypothesis of the composition of torbanite precursors has been shown to be incorrect. Contrary to previous thinking, the monomers largely responsible for coorongite, and torbanite, are not fatty acids or branched chain hydrocarbons but straight chain alkadienes arising from the green form of the alga.The rubbery deposit of the orange form of the alga is composed of polymerised botryococcenes.     

Molecular and stable carbon isotopic compositions of saturated fatty acids within one sedimentary profile in the Shenhu,northern South China Sea: Source implications

This study examined the distributions and stable carbon isotopic compositions of saturated fatty acids (SaFAs) in one 300 cm long sedimentary profile, which was named as Site4B in Shenhu, northern South China Sea. The concentrations of total SaFAs in sediments ranged from 1.80 to 10.16 μg/g (μg FA/g dry sediment) and showed an even-over-odd predominance in the carbon chain of C₁₂ to C₃₂, mostly with n-C₁₆ and n-C₁₈ being the two major components. The short-chain fatty acids (ScFAs; n-C₁₂ to n-C₁₈) mainly from marine microorganisms had average δ¹³C values of −26.7‰ to −28.2‰, whereas some terrigenous-sourced long-chain fatty acids (LcFAs; n-C₂₁ to n-C₃₂) had average δ¹³C values of −29.6‰ to −34.1‰. The other LcFAs (n-C₂₄ & n-C₂₆ ∼ n-C₂₈; average δ¹³C values are −26.1‰ to −28.0‰) as well as n-C₁₉ and n-C₂₀ SaFAs (average δ¹³C values are −29.1‰ and −29.3‰, respectively) showed a mixed signal of carbon isotope compositions.The relative bioproductivity calculation (marine vs. terrigenous) demonstrated that most of organic carbon accumulation throughout the sedimentary profile was contributed by marine organism. The high marine productivity in Shenhu, South China Sea may be related to the hydrocarbon seepage which evidenced by diapiric structures. Interestingly, there is a sever fluctuation of terrigenous inputs around the depth of 97 cm below the seafloor (bsf), probably resulting from the influence of the Dansgaard–Oeschger events and the Younger Dryas event as revealed by ¹⁴C age measurements.     

Mineralogy and geochemistry of clay fractions from different saprolites,Egypt: implications for the source of sedimentary kaolin deposits

The clay fractions of saprolites from granites, basalt, and schists in Egypt were subjected to mineralogical and geochemical investigations to examine the effect of source rock on the composition of the saprolites and the possibilities of these saprolites as a source of the nearby sedimentary kaolin deposits. The clay fractions of the studied saprolites show mineralogical and geochemical variations. Saprolites from the granites consist of kaolinite, while saprolites from the basalts are composed entirely of smectite. Schists-derived saprolites are composed of kaolinite in some cases and of a mixture of kaolinite, illite, and chlorite in the other. Saprolite from the basalt is characterized by relatively higher contents of TiO₂ and Ni compared to the saprolites from granites. Saprolites from granites have higher contents of Ba, Li, Pb, Sr, Th, Y, and Zr compared to those of the saprolites from the basalts and schists. Saprolites from different schists show variations in the distributions of many constituents, such as TiO₂, Cr, Ni, Ba, Y, and Zr. Although chondrite-normalized rare earth elements (REE) patterns are characterized by relative enrichments in the light rare earth elements (LREE) compared to the heavy rare earth elements (HREE) in all saprolites, granitic saprolites show negative Eu anomalies, while saprolite from basalt has no Eu anomaly. REE patterns of the saprolites from schists exhibit slight positive Ce anomalies and slight to moderate negative Eu anomalies. Weathering of saprolites from the basalt and metasediments is classified as the bisiallitization type, while weathering of saprolite from the granite is allitization type. Saprolites from schists vary from the bisiallitization (Aswan and Abu Natash) and allitization (Khaboba) types. Saprolites from the Khaboba schist can be considered the possible source of the Carboniferous kaolin deposits in the Hasber and Khaboba areas of Sinai, based on the similarity in the mineralogy and geochemistry of major, trace, and REE between the saprolites and the deposits. On the other hand, Carboniferous sedimentary kaolin deposits in the Abu Natash area, as well as the Cretaceous kaolin deposits in all areas of Sinai, might have been derived from the nearby schist saprolites, based on the similarity in the mineralogy and geochemistry between the saprolites and the kaolin deposits. Granites from the Arabian-Nubian Shield (ANS) and East Sahara Craton (ESC) are the possible sources of the pisolitic and plastic kaolin deposits in the Kalabsha area (Aswan), as indicated by the similarity in the mineralogy and geochemistry of the granitic saprolites and the kaolin deposits.     

The geochemistry of mercury in sedimentary rocks and soils in Pennsylvania

The mercury contents of 11 sandstone, 11 shale and 6 limestone samples from Pennsylvania average 7, 23 and 9 ppb Hg, respectively, which is lower than the values for sedimentary rocks reported in the literature. The differences may arise because many of the reported high values are from regions characterized by more mineralization and volcanism than is present in central Pennsylvania. The lowest values found for shale and sandstone in Pennsylvania (0.4 and 0.5 ppb Hg, respectively) are lower by an order of magnitude than the lowest previously reported values. The mercury content of sedimentary rocks varies markedly due to the effects of volcanism, organic material and sulfur in reducing environments, iron and manganese oxides in oxidizing environments, diagenesis, hydrothermal processes, and the thermal history of the rock.Soils in Pennsylvania have much greater amounts of mercury than their parent rocks even after taking into account possible residual concentration, suggesting that mercury is added to the soils from an outside source. Rain is the major source of mercury absorbed by the soil. A portion of the absorbed mercury returns to the atmosphere, establishing a rain-soil-atmosphere mercury cycle. The general enrichment of mercury in soils and sediments compared to rocks is supported by the observation that the mercury content of rain is greater than freshwater. The much higher values of mercury in unconsolidated sediments compared to sedimentary rocks suggest that mercury is lost during diagenesis.Man's contribution of mercury to the surface environment is nearly equal to the natural contribution. Industrial loss contributes more than 65 per cent of man's total, and the contribution of fossil fuel consumption is small, although it may be locally important. The implications of mercury loss and absorption by soils may be an important factor in concentrating mercury in crops and other living matter, especially near industrialized areas.     

Influencing factors and significance of organic and inorganic nitrogen isotopic compositions in lacustrine sedimentary rocks

Comprehensive nitrogen biogeochemical cycle has been reconstructed for representative lacustrine organic-rich sedimentary rock in China, namely the Triassic Yanchang Formation (YF, 199–230 Ma) in Ordos and the Cretaceous Qingshankou Formation (QF, 86–92 Ma) in Songliao basins, by evaluating the organic and inorganic nitrogen isotopic compositions rather than only organic or bulk nitrogen isotopic compositions. The results indicate that the nitrogen isotope values of bulk rock (δ¹⁵N_bulk) in the non-metamorphic stage are significantly different from that of kerogen, which challenge the conceptual framework of sedimentary nitrogen isotope interpretation. The δ¹⁵N_bulk from the YF and QF were lower than their respective the nitrogen isotope values of kerogen (δ¹⁵N_ker), with offsets up to ～5.1‰, which have the inverse relationship for the metamorphosed rock. Thermal evolution did not significantly modify the δ¹⁵N of bulk rock and kerogen. The δ¹⁵N of sediments from the YF (δ¹⁵N_bulk, 1.6‰–5.6‰) were lower than that of rock from the QF (δ¹⁵N_bulk, 10.2‰–15.3‰). The nitrogen isotope values of silicate incorporated nitrogen (δ¹⁵N_sil) were slightly lower than those of the δ¹⁵N_ker in the YF and obviously lower for the QF. The fact that different nitrogen cycles occur in the YF and QF due to the different depositional redox conditions leads to different isotopic results. The YF water environment dominated by oxic conditions is not conducive to the occurrence of denitrification and anammox, and no abundant N₂ loss leads to the relatively light δ¹⁵N_bulk. In the stratified water for the QF, redox transition zone promotes denitrification and anammox, resulting in the heavy δ¹⁵N_bulk of rock and promotes the DNRA, resulting in heavy δ¹⁵N_ker and low δ¹⁵N_sil.     

黔北镇远牛蹄塘组黑色页岩沉积环境与有机质富集关系

通过矿物组分、主量元素、微量元素、碳氧同位素等特征,分析了黔北地区镇远县ZX井牛蹄塘组黑色页岩的岩相类型和不同岩相页岩的沉积环境及其与有机质富集间的关系。结果表明,牛蹄塘组下段以硅质页岩为主,TOC含量4.96%～10.10%;上段以富泥硅质页岩为主,TOC含量1.43%～9.04%。下段硅质页岩沉积于水流停滞的深水陆棚环境,沉积古水体为贫氧的还原状态;上段富泥硅质页岩沉积时期,仍为贫氧的还原环境,但水体滞留程度、还原性较下段硅质页岩沉积期弱。贫氧的还原环境是影响牛蹄塘组黑色页岩有机质富集的主要因素,热液作用和陆源碎屑对牛蹄塘组黑色页岩有机质富集的影响较小。     

The mechanism of fatty acid adsorption in the presence of fluorite,calcite and barite

The interaction of oleic acid with fluorite, calcite and barite has been studied using solubility, oleate abstraction, electrophoretic mobility and Hallimond-tube flotation measurements. Abstraction of oleate from aqueous solution corresponds to the precipitation of the metal oleate. Multilayers of metal oleate inhibits the dissolution of the minerals and prevents true equilibrium from being obtained. Flotation is not only dependent on the amount of oleate abstracted but also on the strength of adhesion of the precipitated metal oleate to the minerals. Selectivity between the flotation of calcite, fluorite and barite is unlikely to be obtained by varying the pH because similar responses are observed.     

天柱大河边重晶石矿床铅同位素特征及来源探讨

对取自天柱大河边重晶石矿床的重晶石样品和含方铅矿黑色页岩样品进行了铅同位素测定与研究.结果表明,在 Zartman铅构造模式图中,重晶石矿石、含方铅矿黑色页岩和黑色页岩有大致相同的铅同位素分布区,均沿上地壳铅演化线分布,表明它们之间具有同源关系;重晶石矿石及其赋矿黑色页岩的铅同位素组成与基底地层的铅同位素组成在 Zartman铅构造模式图中具有完全不同的分布范围,表明重晶石矿石中的铅不大可能来源于基底地层;结合重晶石矿石及其赋矿黑色页岩的铅同位素组成在 Zartman铅构造模式图和Δγ-Δβ铅来源分类图的分布,天柱大河边重晶石矿床的铅主要具上地壳铅、壳幔混合的俯冲带铅和热水沉积作用铅的混合来源特点,这为天柱大河边重晶石矿床的热水沉积成因提供了铅同位素证据.     

Recent vegetation history from a swampy environment to a pond based on macromolecular organic matter (lignin and fatty acids) and pollen sedimentary records

Evidence of environmental and vegetation changes in the catchment area of a small pond created artificially during the Middle Ages from a swampy area was provided by coupling palynology data and a detailed study of macromolecular organic matter (OM). Tetramethylammonium hydroxide (TMAH) thermochemolysis after solvent extraction was used to characterize macromolecular OM and particularly lignin and fatty acids (FAs). Molecular ratios such as TAR_(FA), C_16:1/C_16:0 and C_18:1/C_18:0 were calculated to characterize the FAs incorporated into the macromolecular OM. Moreover, syringyl vs. vanillyl phenols (S/V) and cinnamyl vs. vanillyl phenols (C/V) molecular ratios were investigated to fingerprint the lignin sources. These different markers suggested that the swampy area was dominated by a non-woody angiosperm input and gave rise to pond status over a period of several centuries. Indeed, TAR_(FA) correlated with the aquatic contribution, which became predominant after creation of the pond. The use of ratios of total acid phenols to the total aldehyde phenols of syringyl or vanillyl units (Ad/Al_(s), Ad/Al_(v)), which are related to the degree of oxidation of lignin, revealed the irregular operation of oxidative conditions.     

The significance of rubidium-strontium age of sedimentary rock

The isotopic composition of strontium and the rubidium and strontium contents were measured for samples of the Lower Permian Havensville Shale collected at four different localities across a distance of approximately 200 miles in Kansas and northern Oklahoma. The Rb-Sr dates ranged from 320 m.y. to 395 m.y. The initial Sr⁸⁷/Sr⁸⁶ ratios were between 0.709 and 0.717. The ages are interpreted to be intermediate between the age of the source material and the time of sedimentation and, therefore, are not indicative of any geologic event. A model has been proposed to explain the colinearity of the Rb-Sr data and the evolution of different Rb-Sr dates for the same stratigraphic unit at different localities.     

Dissolution and transport of manganese by organic acids and their role in sedimentary Mn ore formation

Dissolution experiments of Mn and Fe under natural conditions from fresh basalt, weathered basalt and Mn laterite by different organic acids show that Mn is highly enriched over Fe in solutions from the weathered rocks but that more Fe than Mn is dissolved from the fresh basalt. The enrichment of Mn is caused by sparingly soluble Fe-oxides and hydroxides and more soluble Mn-oxides. In addition from the weathered rocks the Mn concentration dissolved by the organic acids is up to 1000 times higher than in inorganic solutions. Mn enrichment is caused by acid attack, organic reduction of Mn⁴+ to Mn²+ and complexing by the organic acids. The complexed Mn is not attacked as easily by oxidation as free Mn ions. Higher concentrations of manganese in the organic dissolved stage can therefore be transported by rivers over greater distances. Organic complexed Mn, derived from lateritic weathered rocks may therefore contribute to the formation of low iron marine sedimentary Mn deposits.     

The persistence of memory: the fate of ancient sedimentary organic carbon in a modern sedimentary system

The cycle of organic carbon burial and exhumation moderates atmospheric chemistry and global climate over geologic timescales. The burial of organic carbon occurs predominantly at sea in association with clay-sized particles derived from the erosion of uplifted continental rocks. It follows that the history of the fine-grained particles on land may bear on the nature of the organic carbon buried. In this study, the evolution of clay-associated organic matter was followed from bedrock source to the seabed in the Eel River sedimentary system of northern California using natural abundance ¹³C and ¹⁴C tracers. Approximately half of the fine-grained organic carbon delivered to the shelf is derived from ancient sedimentary organic carbon found in the uplifted Mesozoic-Tertiary Franciscan Complex of the watershed. The short residence time of friable soils on steep hill slopes, coupled with rapid sediment accumulation rates on the shelf-slope, act to preserve the ancient organic carbon. A comparable quantity of modern organic carbon is added to particles in the watershed and on the shelf and slope. The bimodal mixture of ancient and modern C in soils and sediments may be characteristic of many short, mountainous rivers. If the Eel River chemistry is typical of such rivers, more than 40 Tg of ancient organic C may be delivered to the world’s oceans each year. A flux of that magnitude would have a significant influence on marine and global C-cycles.     

Groundwater in-situ generation of aquatic humic and fulvic acids and the mineralization of sedimentary organic carbon

In this paper the groundwater in-situ generation of dissolved organic carbon (DOC) is discussed based on the origin of groundwaters, their physico-chemical and isotopic properties, chemical composition and the dissolved inorganic carbon (DIC) concentration and its ¹³C content. Three aquifer systems are investigated. Two of these have relatively well defined hydrological and geochemical conditions (Fuhrberg and Munich) and are used as reference systems. The third aquifer (Gorleben) is a complex system containing DOC concentrations up to 200 mg C/L in deep groundwaters. From this aquifer system 19 groundwaters from different hydrogeochemical conditions are analyzed. The in-situ generation of DOC is found to occur in conjunction with the microbiologically mediated mineralization of sedimentary organic carbon (SOC). Thereby, SO₄ is reduced and phosphate is released into the groundwater. Where SO₄ is depleted, the mineralization of SOC occurs via fermentation, resulting in CH₄ generation.     

The significance of microbial processes in hydrogeology and geochemistry

2 (methanogenesis). In aquifers contaminated by anthropogenic contaminants, an excess of available organic carbon often exists, and microbial metabolism is limited by the availability of electron acceptors. In addition to changes in groundwater chemistry, the solid matrix of the aquifer is affected by microbial processes. The production of carbon dioxide and organic acids can lead to increased mineral solubility, which can lead to the development of secondary porosity and permeability. Conversely, microbial production of carbonate, ferrous iron, and sulfide can result in the precipitation of secondary calcite or pyrite cements that reduce primary porosity and permeability in groundwater systems. Received, January 1999/Revised, July 1999, August 1999/Accepted, October 1999