Inhibition of dissolution within shallow water carbonate sediments: impacts of terrigenous sediment input on syn-depositional carbonate diagenesis

The early diagenetic chemical dissolution of skeletal carbonates has previously been documented as taking place within bioturbated, shallow water, tropical carbonate sediments. The diagenetic reactions operating within carbonate sediments that fall under the influence of iron‐rich (terrigenous) sediment input are less clearly understood. Such inputs should modify carbonate diagenetic reactions both by minimizing bacterial sulphate reduction in favour of bacterial iron reduction, and by the reaction of any pore‐water sulphide with iron oxides, thereby minimizing sulphide oxidation and associated acidity. To test this hypothesis sediment cores were taken from sites within Discovery Bay (north Jamaica), which exhibit varying levels of Fe‐rich bauxite sediment contamination. At non‐impacted sites sediments are dominated by CaCO₃ (up to 99% by weight). Pore waters from the upper few centimetres of cores show evidence for active sulphate reduction (reduced SO₄/Cl^? ratios) and minor CaCO₃ dissolution (increased Ca²⁺/Cl^? ratios). Petrographic observations of carbonate grains (specifically Halimeda and Amphiroa) show clear morphological evidence for dissolution throughout the sediment column. In contrast, at bauxite‐impacted sites, the sediment is composed of up to 15% non‐carbonate and contains up to 6000 μg g^?1 Fe. Pore waters show no evidence for sulphate reduction, but marked levels of Fe(II), suggesting that bacterial Fe(III) reduction is active. Carbonate grains show little evidence for dissolution, often exhibiting pristine surface morphologies. Samples from the deeper sections of these cores, which pre‐date bauxite influence, commonly exhibit morphological evidence for dissolution implying that this was a significant process prior to bauxite input. Previous studies have suggested that dissolution, driven by sulphate reduction and sulphide oxidation, can account for the loss of as much as 50% of primary carbonate production in localized platform environments. The finding that chemical dissolution is minor in a terrigenous‐impacted carbonate environment, therefore, has significant implications for carbonate budgets and cycling, and the preservation of carbonate grains in such sediment systems.     

Dissolved organic matter in pore water of carbonate sediments from Bermuda

Pore water samples from seven nearshore areas in Bermuda were obtained under in situ conditions and analyzed for dissolved organic carbon, dissolved carbohydrates, dissolved free amino acids and dissolved humic substances. The concentration of dissolved organic carbon is higher than in the overlying nearshore waters indicating significant diagenetic remobilization of carbon in these recently deposited carbonate sediments. Dissolved carbohydrates decrease with depth due to microbial utilization.     

On the relationship between silica and carbonate diagenesis in deep-sea sediments

Silica diagenesis and carbonate diagenesis are interrelated. This is confirmed by observations of DSDP Sites 462, 463, 465, 466, and 577. Carbonate sediments containing chert (1) tend to be more indurated and display more advanced diagenetic alterations, regardless of sub-bottom depth; and (2) microfossil components are more strongly affected (overgrown and/or dissolved), while the amount of micritic particles and larger, euhedral calcite crystals is greater. In addition, mass physical properties, porosity in particular, vary more widely in sediment sections containing chert. Furthermore, in the studied similarly composed sediments recrystallization of biogenic opal is indicated by a significant reduction of the specific surface area, reaching a minimum value when quartz is formed.One possible mechanism involved is the production of «surplus« dissolved carbonate created by the replacement of carbonate material by silica during the process of chert formation and silicification. The «extra« carbonate is then available for precipitation as overgrowths and cement outside the chert nodules and silicified zones. Hence silica diagenesis, if it occurs early enough in the sediment, bears some influence on carbonate diagenesis. It is therefore suggested that silica diagenesis be added to the list of factors included in the «diagenetic potential« equation ofSchlanger &Douglas (1974).

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Zusammenfassung Die Diagenese von biogenem Silikat und Karbonat steht in engem Zusammenhang, wie Beobachtungen an Sedimenten der DSDP Sites 462, 463, 465, 466 und 577 zeigen. Karbonatische Sedimente, die biogenes SiO₂ enthalten, zeichnen sich aus durch (1) größere Verfestigung und stärkere diagenetische Veränderungen — unabhängig von der Tiefe im Sediment, (2) mehr Lösung und Überwuchs an Mikrofossil-Komponenten, (3) höheren Anteil mikritischer Partikel wie auch größerer idiomorpher Kalzitkristalle, (4) stärkere Variation der sedimentphysikalischen Eigenschaften, speziell der Porosität und damit zusammenhängender Parameter.Die Umkristallisation des biogenen SiO₂ führt in ähnlich Zusammengesetzen Sedimenten zu einer drastischen Abnahme der spezifischen Oberfläche. Minimalwerte werden erreicht, wenn sich Quarz bildet.Diagenetisch wichtig ist die Produktion von zusätzlichem Karbonat durch die Silizifizierung von Karbonatschalen. Dieses »Überschuß«-Karbonat wird dann als Überwuchs, Zement oder außen an den »Hornstein«-Aggregaten gefällt. Demzufolge beeinflußt die Diagenese von biogenem SiO₂ auch die Karbonatdiagenese. Daher ist es sinnvoll, die Diagenese von biogenem SiO₂ mit zu den Faktoren zu rechnen, die das »diagenetische Potential« — wie esSchlanger &Douglas (1974) definierten — ausmachen.

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Résumé L'étude de sédiments provenant des sites DSDP 462, 463, 465, 466 et 577 montre qu'il existe une relation entre la diagenèse de la silice et celle du carbonate biogéniques. Les sédiments carbonatés qui renferment de la silice biogémque présentent: 1) une induration plus marquée et des modifications diagénétiques plus poussées — et ce indépendamment de la profondeur sous la surface du fond; 2) une dissolution et/ou un accroissement plus développés des micro-fossiles; 3) une plus grande teneur en particules micritiques et une plus grande taille des calcites idiomorphes; 4) un éventail plus large de leurs propriétés physiques, particulièrement de la porosité et des paramètres qui en dépendent.La cristallisation de l'opale biogénique, dans des sédiments de compositions semblables, se traduit par une réduction drastique de la surface spécifique, qui atteint une valeur minimale lorsque du quartz est formé.Un rôle diagénétique important est joué par l'excès de carbonate dissous engendré par la silicification de coquilles carbonatées; cet excès de carbonate est dès lors disponible pour la précipitation des auréoles d'accroissement et du ciment hors des nodules de chert et des zones silicifiées. Il s'ensuit que la diagenèse du SiO₂ biogénique influence la diagenèse du carbonate. Il conviendrait dès lors d'ajouter la diagenèse de la silice à la liste des facteurs qui interviennent dans l'équation du «potentiel diagénétique» deSchlanger etDouglas (1974).

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462, 463, 465, 466 577, . , , : 1) , ; 2) ; 3) , 4) , . SiO₂ . . . »« , , « ». , .. «, 1974 Schlanger & Douglas.

     

Organic matter in small mesopores in sediments and soils

The three-way correlation among organic matter concentrations, specific surface area and small mesopores observed for many soils and sediments led to the hypothesis that enclosure within the pores might explain the apparent protection of organic matter by minerals. We test this hypothesis by examining whether the bulk of organic matter resides within small mesopores. Pore volumes as a function of pore width were measured before and after organic matter removal, and the volume differences ascribed to organic matter filling of pores. Minor changes in small mesopore size distributions upon treatments such as centrifugation and muffling indicate the robustness of the mineral matrices that form these pores. We developed an additional method to assess organic matter densities using high-resolution pycnometry, and used these densities to convert pore volumes to organic matter contents. Although smaller mesopores are shown to have sufficient volumes to contain significant fractions of the total organic matter, only small fractions of total organic matter were found to reside in them. These results are consistent with preferential association between organic matter and aluminous clay particle edges, rather than the largely siliceous clay faces that contribute most surface area and form pore walls. While simple enclosure within smaller mesopores cannot, therefore, explain protection, network effects working at larger size scales may account for exclusion of digestive agents and hence organic matter protection.     

Thallium diagenesis in lacustrine sediments

Dated sediment cores and porewaters from two Canadian Shield lakes, located 40 km from Québec City (L. Tantaré) and 25 km downwind from an important smelter in northwestern Québec (L. Vose), were analyzed for thallium and other geochemical variables. Atmospheric deposition is the only source of anthropogenic Tl to these lakes. The porewater Tl concentration profiles in L. Tantaré display a peak below the sediment-water interface that suggests post-depositional Tl remobilization and transport to the overlying water and deeper sediments; such a dissolved Tl peak occurs just above the sediment-water interface in L. Vose. Modeling the porewater Tl profiles with a diagenetic reaction-transport equation reveals a zone of dissolved Tl production lying above a zone of Tl consumption in L. Tantaré sediments. In contrast, in L. Vose, Tl diffuses across the sediment-water interface from the anoxic hypolimnion and is fixed to the surficial sediments. The localization of the consumption zones, the shape of the dissolved Tl, sulfide and iron profiles, as well as calculations of saturation states are all consistent with the removal of Tl from porewater by co-precipitation with and/or adsorption to Fe sulfides. The concentrations of Tl removed from or added to sediments after their deposition (i.e., diagenetic Tl) amount to up to 36% of measured sediment Tl concentrations. Comparison of the reconstructed historical Tl records with those of polycyclic aromatic hydrocarbons (PAHs) and of Pb originating from coal burning or from smelting activities indicate that coal combustion is the major source of atmospheric Tl to L. Tantaré and that an additional important Tl source to L. Vose is the nearby smelter.     

Early diagenesis of organic matter in the water column and sediments: Microbial degradation and resynthesis of lipids in Lake Haruna

Particulate matter, sediment trap, and surface sediment samples collected in freshwater Lake Haruna were studied to understand early diageesis of organic materials in the water column and in bottom sediments. The samples were analyzed for biomarkers, including aliphatic and aromatic hydrocarbons, fatty alcohols, saturated and unsaturated fatty acids, β- and ω-hydroxyacids, and α,ω-dicar☐ylic acids. Decreases in concentrations of autochthonous saturated C₁₂–C₁₉ fatty acids and polyunsaturated C₁₈ acids relative to TOC occured with the settling of organic matter ot the lake bottom, whereas the amounts of terrestial saturated C₂₀–C₃₀ acids remained almost constant. Conversely, the concentrations of monosaturated fatty acids, branched chain fatty acids, and β- and ω-hydroxyacids, which are probably produced by microbial activity, increased. These results indicate that preferential degradation of algal lipids accompanies microbial resynthesis of lipids during settling, however, terrigenous lipids are relatively stable.     

Iodine diagenesis in pelagic deep-sea sediments

Nineteen sediment cores from the Madeira, Seine, Tagus and Nares Abyssal Plains and the Alboran Sea have been used to evaluate the speciation, fluxes and diagenesis of iodine in the deep sea. The sediments have surficial molar I/C ratios of 10–30 × 10⁻⁴ in excess of previous reported values for planktonic material (~1 × 10⁻⁴). Solid phase I contents decrease exponentially with depth corresponding to decomposition rate constants of 5–260 × 10⁻⁶ yr⁻¹ which vary with the carbon accumulation rate.Iodine species in the pore waters follow a vertical sequence of four zones: 1. a zone of I⁻ production where total dissolved iodine (∑I) concentrations initially increase at the seawater-sediment interface; 2. a zone of I⁻ oxidation where interconversion of I⁻ to IO⁻₃ occurs; 3. a zone of IO⁻₃ reduction where interconversion of IO⁻₃ back to I⁻ occurs which corresponds to the suboxic part of the sediment column; and 4. a further zone of I⁻ production which is confined to the lower anoxic part of the sediment column. Benthic ∑I fluxes in the Madeira Abyssal Plain measured from shipboard incubation experiments and calculated from porewater gradients are similar, averaging 0.55 and 0.36 × 10⁻⁸ μmol cm⁻² sec⁻, respectively.In the surface sediment the observed I enrichment results from a quasi-closed cycle for iodine initially involving release of I⁻ from decomposing marine organic matter followed by rapid removal onto organic matter at the sediment-seawater interface where I/C regeneration ratios of up to 200 × 10⁻⁴ are found, lodate reduction occurs during suboxic diagenesis, after denitrification and before MnO₂ reduction, consistent with the sequence of reactions predicted from the free energy yields for organic matter oxidation. There is some further I⁻ production in the anoxic section of sediments but at much smaller rates than occur during the interfacial diagenetic cycling.     

Cadmium diagenesis in Laurentian Trough sediments

The depth distributions of Cd, Mn and Fe in pore water and sediment were determined on three replicate box cores collected at a 325 m deep station in the Laurentian Trough. The results reveal a surface layer in which the content of solid-phase Cd first decreases but then increases sharply with depth. In contrast, Mn decreases regularly over the same depth interval. In the oxygenated zone of this layer, Cd, probably associated with organic matter, is released to the pore water, resulting in concentrations that are more than an order of magnitude higher than in the overlying water column. Some of the dissolved Cd is returned to the water column and some migrates downward and is precipitated at depth. This part of the Cd cycle is virtually complete within the surface layer, the base of which corresponds to the base of the zone of Mn enrichment.In the subsurface layer, solid-phase Cd and Mn show little or no concentration change. However, dissolved Cd, which reaches often non-detectable concentrations (<0.05 nM) in this layer, increases once again deep within the anaerobic zone and attains concentrations even higher than in the surface layer. The presence of dissolved Cd complexes (revealed by 1.5 to 8 fold increases in electrochemically active Cd in pore water following UV-treatment) as well as inconsistent distributions of dissolved and solid-phase Cd, indicate complexities in diagenesis that merit further investigation.     

Iodine diagenesis in non-pelagic deep-sea sediments

Measurements of sediment geochemistry and porewater speciation have been made using eight cores containing turbidite sections from the Madeira and Nares Abyssal Plains. The results have been used to evaluate how the diagenetic chemistry of iodine in these sediments compares with that in sediments undergoing steady-state diagenesis. The behaviour of iodine is related to the development of a redox front within the turbidite, between the organic-rich anoxic sediment and its oxic cap, and the downward migration of the front through the turbidite with time. In contrast to the steady-state case, sediment I contents and I/ C ratios increase downwards through the oxidised section reaching a maximum at the redox front (up to ~ 100 μ/g I; molar I/C~ 20 × 10⁻⁴) below which values drop dramatically (I/C ~ 5 × 10⁻⁴). A strong iodate enrichment (up to ~3 μmol kg⁻¹) is observed in the oxidised section of the sediment. At the front interconversion of I⁻ and IO⁻₃ species occur and below the front porewater IO⁻₃ is absent and I~ concentrations increase with depth (as in other cases of anoxic diagenesis) up to ~ 10 μmol kg⁻. In the oxidised section of the sediment the I enrichment has been supplied by upward transport of iodide with the increasing I content, with depth being accounted for by progressive diagenetic enrichment with time.     

Early diagenesis of organic matter in recent Black Sea sediments: characterization and source assessment

The organic matter in 9 recent (not more than 250 years old) and ‘organic-rich’ sediments from the southern Black Sea shelf and upper slope have been characterized semi-quantitatively by Pyrolysis/Gas Chromatography/Mass Spectrometry (PY/GC/MS) and¹³C Cross Polarization Magic Angle Spinning Nuclear Magnetic Resonance (CPMAS-NMR) spectrometry. The organic matter of 7 of the studied sediments was found to be ligno-carbohydrate with a proteinaceous component, one sediment appeared to contain oxidized coal dust and one contained thiophenes in association with pyrite. The ligno component is derived from grasses and soft wood lignin. Material entrapped in an anoxic environment contained the highest proportions of carbohydrate and protein. All the samples had suffered diagenesis as is generally shown by the attachment of carboxyl groups and the removal of methoxyl groups. The evidence suggests that diagenesis occurred whilst the particles traversed the oxic water column.     

碳酸盐成岩作用研究现状与前瞻

碳酸盐成岩作用是沉积学最为活跃的领域之一,与之有关的研究成果会直接影响人们对碳酸盐油气储层形成机制和层控矿床形成机制的理解。近年来,碳酸盐成岩作用在多个领域取得了令人瞩目的研究成果,主要包括以下几个方面:(1)实验室沉淀白云石获得成功:人们模拟海水(泻湖)条件下的细菌硫酸盐还原环境,在常温下沉淀出具有序反射的白云石。(2)热液白云化作为新的主流模式成为关注的热点:人们已认识到热液白云岩储层、沉积-喷流型铅锌矿床和Mississippi河谷型铅锌矿床具有共同的形成机制,它们均受构造(伸展、走滑构造)和热流体流动的强烈控制。(3)深埋藏条件下封闭系统的白云化作用受到关注:一些对油气储集空间和层控矿床有较大意义的白云化作用主要是在相对封闭条件下发生的,在对岩石储集空间的变化的贡献上以及在相应的地球化学指标上不同于开放条件下的白云化作用。(4)混合水白云化模式受到质疑:人们重新研究了作为混合水白云化典型地点的美国威斯康星弧碳酸盐的成岩作用,根据流体包裹体分析、阴极发光分析、偏光显微镜观察及稳定同位素分析等方法并结合有机物成熟度的数据,得出威斯康星弧与白云化有关的水-岩相互作用是由与温度升高有关的浓卤水导致的,白云化作用是热水成岩作用的结果。(5)淡水环境的成岩机制受到挑战:新近研究证明,大范围的低位体系域中可以没有具化学活性的淡水透镜体,海水环境同样可以产生成熟的石灰岩,细菌硫酸盐还原作用可以引起海水对碳酸盐的不饱和。(6)锶同位素在海相碳酸盐研究中的广泛应用:由于锶同位素不像氧、碳同位素那样因温度、压力和微生物作用而分馏,矿物可直接反映流体的同位素组成,地质历史中海水锶同位素组成也具有独特的长期变化趋势,这使得海相碳酸盐的锶同位素组成在沉积期后流体示踪中得到广泛应用。(7)硫酸盐还原作用对碳酸盐成岩作用的影响受到重视:如BSR和TSR可减小SO42-对白云石沉淀(或白云化)的动力学屏障,其产物H2S和CO2可在不同条件下对碳酸盐储层产生影响,BSR和TSR可提高成岩流体锶含量,甚至在形成天青石矿床中也具极大的潜在价值。     

Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis

Pore water profiles of total-CO₂, pH, PO^3?₄, NO^?₃ plus NO^?₂, SO^2?₄, S^2?, Fe²⁺ and Mn²⁺ have been obtained in cores from pelagic sediments of the eastern equatorial Atlantic under waters of moderate to high productivity. These profiles reveal that oxidants are consumed in order of decreasing energy production per mole of organic carbon oxidized (O₂ > manganese oxides ~ nitrate > iron oxides > sulfate). Total CO₂ concentrations reflect organic regeneration and calcite dissolution. Phosphate profiles are consistent with organic regeneration and with the effects of release and uptake during inorganic reactions. Nitrate profiles reflect organic regeneration and nitrate reduction, while dissolved iron and manganese profiles suggest reduction of the solid oxide phases, upward fluxes of dissolved metals and subsequent entrapment in the sediment column. Sulfate values are constant and sulfide is absent, reflecting the absence of strongly anoxic conditions.     

A quantitative reconstruction of organic matter and nutrient diagenesis in Mediterranean Sea sediments over the Holocene

A multicomponent diagenetic model was developed and applied to reconstruct the conditions under which the most recent sapropel, S1, was deposited in the eastern Mediterranean Sea. Simulations demonstrate that bottom waters must have been anoxic and sulphidic during the formation of S1 and that organic matter deposition was approximately three times higher than at present. Nevertheless, most present day sediment and pore water profiles — with the exception of pyrite, iron oxyhydroxides, iron-bound phosphorus and phosphate — can be reproduced under a wide range of redox conditions during formation of S1 by varying the depositional flux of organic carbon. As a result, paleoredox indicators (e.g., C_org:S ratio, C_org:P_org ratio, trace metals) are needed when assessing the contribution of oxygen-depletion and enhanced primary production to the formation of organic-rich layers in the geological record. Furthermore, simulations show that the organic carbon concentration in sediments is a direct proxy for export production under anoxic bottom waters.The model is also used to examine the post-depositional alteration of the organic-rich layer focussing on nitrogen, phosphorus, and organic carbon dynamics. After sapropel formation, remineralisation is dominated by aerobic respiration at a rate that is inversely proportional to the time since bottom waters became oxic once again. A sensitivity analysis was undertaken to identify the most pertinent parameters in regulating the oxidation of sapropels, demonstrating that variations in sedimentation rate, depositional flux of organic carbon during sapropel formation, bottom water oxygen concentration, and porosity have the largest impact. Simulations reveal that sedimentary nutrient cycling was markedly different during the formation of S1, as well as after reoxygenation of bottom waters. Accumulation of organic nitrogen in sediments doubled during sapropel deposition, representing a significant nitrogen sink. Following reventilation of deep waters, N₂ production by denitrification was almost 12 times greater than present day values. Phosphorus cycling also exhibits a strong redox sensitivity. The benthic efflux of phosphate was up to 3.5 times higher during the formation of S1 than at present due to elevated depositional fluxes of organic matter coupled with enhanced remineralisation of organic phosphorus. Reoxygenation of bottom waters leads to a large phosphate pulse to the water column that declines rapidly with time due to rapid oxidation of organic material. The oxidation of pyrite at the redox front forms iron oxyhydroxides that bind phosphorus and, thus, attenuate the benthic phosphate efflux. These results underscore the contrasting effects of oxygen-depletion on sedimentary nitrogen and phosphorus cycling. The simulations also confirm that the current conceptual paradigm of sapropel formation and oxidation is valid and quantitatively coherent.     

Artificial diagenesis in quartz sand and carbonate rocks

Diagenetic changes in carbonates and sand can be produced in the laboratory through injection of sea water. These changes are comparable to those found in the natural environment and are shown on photomicrographs. Artificial sea water of known composition under a partial pressure of carbon dioxide was injected through a composite lithology representing a shallow marine shelf. After passage of water for several days, the diagenetic alterations in the sediments were studied under a petrographic microscope. The study includes (1) the interrelation of the chemistry of the water in sediments and the mineralogy of the sediments, (2) the factors which control the processes of calcite and silica precipitation, (3) the processes responsible for transforming sediments into hard rock and, (4) the sequence of these processes.

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Zusammenfassung Diagenetische Veränderungen in Karbonaten und Sanden können im Laboratorium durch Injektionen von Seewasser experimentell nachgeahmt werden. Die beobachteten Veränderungen sind mit denen natürlicher Vorkommen vergleichbar, was anhand von Mikrophotographien demonstriert wird. Künstliches Seewasser bekannter Komposition und definiertem CO₂-Partialdruck wurde in eine Mischung injiziert, die den Sedimentverhältnissen des Flachschelfbereiches entspricht. Nach tagelangem Wasserdurchfluß wurden die Veränderungen in diesem künstlichen Sediment mikroskopisch untersucht. Die Untersuchungen enthielten (1) die Beziehungen des Chemismus des Wasser im Sediment zur Mineralogie desselben, (2) die Faktoren, die die Ausfällung von Silikaten und Karbonaten kontrollieren, (3) die Prozesse, die für die Umwandlung von Sedimenten in Gesteine verantwortlich sind und (4) die Abfolge dieser Prozesse.

     

Amino acid composition of organic matter associated with carbonate and non-carbonate sediments

Amino acids comprise from 15 to 36% by weight of humic substances from carbonate and non-carbonate sediments. Humic and fulvic acids extracted from carbonate sediments are characterized by an amino acid composition consisting primarily of the acidic amino acids, aspartic and glutamic acid. Humic substances from non-carbonate sediments have a distinctly different amino acid composition consisting primarily of glycine and alanine. Amino acid analyses of various molecular weight fractions of fulvic acids extracted from carbonates show that lower molecular weight fractions have appreciably higher relative abundances of the acidic amino acids compared to higher molecular weight fractions. Based on typical values for carboxyl group content in humic substances, acidic amino acids may be a significant contributor of these functional groups. Carbonate surfaces appear to selectively adsorb aspartic acid-enriched organic matter while non-carbonates do not have this property.     

Organic matter distribution in the modern sediments of the Pearl River Estuary

We determined biomarker concentrations and distributions for surface sediments from 54 sites in the Pearl River Estuary, China. We focus on a suite of four biomarker-based indicators for relative terrestrial to marine organic matter (OM) source: the branched-isoprenoid tetraether (BIT) index, the ratio of high/low molecular weight n-alcohols [(ΣC_26–34/(ΣC₁₆₊₁₈ + ΣC_26–34)], an analogous ratio for n-fatty acids and the ΣC₂₉-steroids/(ΣC₂₉-steroids + brassicasterol) ratio. All four exhibit the same terrestrial to marine transition seen in previous bulk δ¹³C studies, but with an abrupt decrease in the relative terrestrial contribution across the delta front to pro-delta transition. Concentrations of terrestrially-derived biomarkers show no systematic decrease across the transition. Instead, the decrease in the proportion of terrestrial OM is due to a decrease in the sedimentation rate and associated terrestrial OM burial across the delta toe. This suggests that diagenetic controls on the fate of terrestrial OM, such as increased biodegradation where sedimentation rate is low, are subordinate to sedimentological processes. Biomarker-derived temperature values are cooler than expected for the lower Pearl River catchment, suggesting that the dominant component of the terrestrial OM is derived from the cooler upland regions of the catchment. The dominance of input from more distal terrain with greater topographic relief is evidence for the importance of geomorphological control on terrigenous OM transport. Collectively, the results demonstrate the importance of sedimentological processes in the supply, deposition and transport of terrestrial OM.     

Biological diagenesis: dicarboxylic acids in recent sediments

A series of even carbon numbered α, ω-dicarboxylic acids ranging from C₁₆ to C₂₄ has been identified in recent sediments from various environments and sampling localities. A lacustrine sediment did not show detectable quantities of diacids. Consideration of quantitative relationships involving the diacids leads us to propose a dual origin for these diacids: deposition by mangroves is their main source in mangal areas while in situ production by sedimentary organisms is the only important source of diacids in a non-mangal marine environment. A fresh water lagoon shows an intermediate situation between these extremes.     

Carbon and nitrogen diagenesis in deep sea sediments

The sections penetrated on Leg 58 of the Deep Sea Drilling Project represent periods of geologic time during which depositional conditions apparently remained quite constant, thus offering an unusual opportunity to study the effects of diagenesis on organic material.Organic carbon and nitrogen contents decrease monotonically with increasing depth of burial before levelling off at minimum values of about 0.05-0.10 and 0.01%, respectively. The depths at which minima are reached vary from site to site, but the ages of the sediments at the minima are all about 2–5 Myr.These data indicate that diagenetic transformations are responsible for the gradual depletion of organic carbon and nitrogen. If diagenesis is at least partly the result of microbial activity, then the role of bacterial ecosystems in deep water sediments is much greater than has previously been thought.     

Provenance and diagenesis of organic matter in Late Cretaceous and Tertiary sediments from the southern Black Sea margin

The organic matter in the Late Cretaceous and Tertiary sediments from the southern Black Sea margin is assigned to the terrestrial-marine/terrestrial range of organic facies. Within this range, the stratigraphic section yields different organic facies types in response to different accumulation and preservation controlling processes. During the Late Companian-Maastrichtian, organic material from the shelf and slope was re-deposited in the deeper oxic parts of the basin. Rapid transport and sedimentation resulted in a higher degree of preservation of lipid-rich, terrestrial components (sporinite, cutinite, resinite) in comparison to the autochthonous sediments. The increase in organic carbon with increasing silt/clay content together with low carbon concentrations in the allochthonous sediments suggest that the accumulation of organic matter in the source areas was controlled by terrigenous influx and that the accumulation conditions were not favorable.In the Eocene (fore-arc basin), the higher content of marine organic matter can be explained by progressive shallowing of the environment and by reduced oxygen content in the bottom waters (reduced bioturbation).In the Miocene and Pliocene (back-arc basin), the organic fraction of the sediments from the basin margin is purely terrestrial and consists mostly of inertinite and reworked terrigenous liptinite indicating oxidative conditions. The dominance of inertodetrinite in the Miocene and of semifusinite in the Pliocene point to a change in the source area or to a higher energy transport or deposition conditions for the Miocene marginal sediments. In the basin interior, the higher content of marine organic matter is due to an oxygen deficiency or anoxic conditions in the bottom waters. Mineral associations indicate complete sulfate reduction and consequent methanogenesis. This is also implied in the hydrocarbon distributions. Periodic oxic conditions lead to a decrease in the marine liptinitic component. In the basin interior, however, the terrigenous fraction is still dominant, implying a continuous influx from the basin margins.The Late Cretaceous to Pliocene sediments are thermally immature (Rm<0.5%, T_max<435 °C).

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Zusammenfassung Die organische Substanz in den Sedimenten der Oberkreide und des Tertiärs der südlichen Schwarzmeerregion ist dem terrestrischen bis marin-terrestrischen Bereich organischer Fazies zuzuordnen. Innerhalb dieses Bereiches weisen die stratigraphischen Abschnitte unterschiedliche organische Faziestypen auf, die auf unterschiedliche, die Akkumulation und den Erhaltungsgrad der organischen Substanz kontrollierende Prozesse zurückzuführen sind.Während des Obercampan-Maastrichtiums und des Paläozäns (fore-arc Becken) wurde organisches Material des Schelf/-hanges in den tieferen oxischen Beckenbereichen resedimentiert. Die rasche Zufuhr und Ablagerung führte zu einem gegenüber den autochthonen Sedimenten höheren Erhaltungsgrad an lipidreichen, terrestrischen Komponenten (Sporinit, Cutinit, Resinit). Die Zunahme an organischem Kohlenstoff mit steigendem Silt-/Tonanteil bei insgesamt niedrigen Kohlenstoffkonzentrationen in den Resedimenten läßt vermuten, daß die Akkumulation organischer Substanz in den Liefergebieten durch terrigene Zufuhr bestimmt wurde und die Akkumulationsbedingungen ungünstig waren.Für das Eozän ist ein erhöhter Eintrag an marinem organischem Material zu verzeichnen, der mit der zunehmenden Verflachung des Ablagerungsraumes (fore-arc Becken) und einer Reduzierung im Sauerstoffgehalt des Bodenwassers (abnehmende Bioturbation) erklärt wird.Für das Miozän und Pliozän (back-arc Becken) ist die organische Fraktion der Ablagerungen des Beckenrandes rein terrestrisch und besteht zum größten Teil aus Inertinit und wieder aufgearbeitetem terrigenem Liptinit, die oxidative Verhältnisse anzeigen. Die Dominanz von Inertodetrinit im Miozän und Semifusinit im Pliozän indiziert eine Änderung im Liefergebiet oder ein höheres Energieniveau beim Transport bzw. im Ablagerungsraum der miozänen Randsedimente. Im Beckeninneren ist ein erhöhter Anteil an mariner organischer Substanz festzustellen, der auf Sauerstoffverarmung oder anoxische Verhältnisse im Bodenwasser zurückgeführt wird. Die Mineralassoziationen in den Sedimenten weisen auf vollständige Sulfatreduktion und nachfolgende Methanogenese hin, die sich auch mit den Kohlenwasserstoffverteilungen nachvollziehen läßt. Periodisch oxische Bedingungen führen zu einer Reduzierung der marin-liptinitischen Komponente. Im Beckeninneren dominiert jedoch auch die terrigene Fraktion (Huminit/Vitrinit, Inertinit), was auf kontinuierliche Zufuhr vom Beckenrand schließen läßt.Die Sedimente der Oberkreide bis Pliozän sind thermisch unreif (Rm<0.5%, T_max<435 °C).

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Résumé La matière organique contenue dans les sédiments du Crétacé supérieur et du Tertiaire de la partie sud de la Mer Noire est à rapporter au domaine de facies organique terrestre à marin-terrestre. La série stratigraphique présente, dans les limites de ce domaine, divers types de facies organiques qui traduisent les divers processus qui régissent l'accumulation et la préservation.Au cours du Campanian supérieur-Maastrichtien, des matériaux organiques provenant du shelf et du talus continental ont été redéposés dans les parties oxygénées plus profondes du bassin. La rapidité du transport et de la sédimentation a entraîné la préservation de composants terrestres riches en lipides (sporonite, cutinite, résinite), dans une mesure plus élevée que dans les sédiments autochtones. L'augmentation de la teneur en carbone organique corrélative à celle de la fraction fine (boue et silt), de même que la faible concentration en carbone des sédiments allochtones indique que, dans la région-source, l'accumulation de matières organiques était régie par un afflux terrigène et que les conditions d'accumulation n'étaient pas favorables.A l'Eocène (bassin d'avant-arc) le contenu en matière organique marine est plus élevé, ce qui s'explique par la diminution progressive de la profondeur et par la réduction de la teneur en oxygène des eaux du fond (bioturbation réduite).Au Miocène et au Pliocène (bassin d'arrière-arc), la fraction organique des sédiments de la bordure du bassin est purement terrestre et consiste principalement en inertinite et en liptinite terrigène remaniée, ce qui indique des conditions oxydantes. La prédominance d'inertodétrinite au Miocène et de semifusinite au Pliocène indique soit une source différente, soit un transport ou un dépôt dans les conditions de plus haute énergie des sédiments miocènes marginaux. Vers l'intérieur du bassin, le contenu plus élevé en matière organique marine est dû à une déficience en oxygène ou à des conditions anoxiques dans les eaux de fond. Les associations minérales indiquent une réduction complète des sulfates et en conséquence une méthanogenèse, ce qui ressort également de la distribution des hydrocarbures. Des conditions oxydantes périodiques provoquent une diminution du composant liptinitique marin. Dans l'intérieur du bassin, toutefois, la fraction terrigène reste dominante, ce qui implique un afflux continu depuis les marges du bassin.Les sédiments d'âge crétacé supérieur à pliocène sont thermiquement immatures (Rm<0,5%; Tmax<435 °C).

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