Early Diagenetic Transformation Research Articles

Lacustrine microporous micrites of the Madrid Basin (Late Miocene, Spain) as analogues for shallow-marine carbonates of the Mishrif reservoir Formation (Cenomanian to Early Turonian, Middle East)

Shallow-marine microporous limestones account for many carbonate reservoirs. Their formation, however, remains poorly understood. Due to the lack of recent appropriate marine analogues, this study uses a lacustrine counterpart to examine the diagenetic processes controlling the development of intercrystalline microporosity. Late Miocene lacustrine microporous micrites of the Madrid Basin (Spain) have a similar matrix microfabric as Cenomanian to Early Turonian shallow-marine carbonates of the Mishrif reservoir Formation (Middle East). The primary mineralogy of the precursor mud partly explains this resemblance: low-Mg calcites were the main carbonate precipitates in the Cretaceous seawater and in Late Miocene freshwater lakes of the Madrid Basin. Based on hardness and petrophysical properties, two main facies were identified in the lacustrine limestones: a tight facies and a microporous facies. The tight facies evidences strong compaction, whereas the microporous facies does not. The petrotexture, the sedimentological content, and the mineralogical and chemical compositions are identical in both facies. The only difference lies in the presence of calcite overgrowths: they are pervasive in microporous limestones, but almost absent in tight carbonates. Early diagenetic transformations of the sediment inside a fluctuating meteoric phreatic lens are the best explanation for calcite overgrowths precipitation. Inside the lens, the dissolution of the smallest crystals in favor of overgrowths on the largest ones rigidifies the sediment and prevents compaction, while partly preserving the primary microporous network. Two factors appear essential in the genesis of microporous micrites: a precursor mud mostly composed of low-Mg calcite crystals and an early diagenesis rigidifying the microcrystalline framework prior to burial.

Variations of Fatty Acids During the Sulphidization Process in the Authie Bay Sediments (8 pp)

Background, Aim and Scope The sulphidization process in relatively clean sediments sampled in a mudflat of the Authie estuary (located in Northern France) has been studied by coupling geochemical expertise and the use of fatty acids (FAs) as biochemical markers. Materials and Methods: Three sediment cores have been sampled in September 2003, November 2003 and May 2004, and cut every 2 cm in the field under nitrogen atmosphere so as to prevent any oxidation of reduced species. In the solid phase, reduced sulphur compounds, e.g. AVS (Acid Volatile Sulphides) and CRS (Chromium Reducible Sulphur) [including also the calculation of the degree of sulphidization (DOS) and the degree of pyritization (DOP)], and fatty acids have been carried out. Eh, pH, metal species (mostly iron and manganese), dissolved S(-II) and sulphate have also been determined in the porewaters. Results: The sediment cores display a lot of differences due to the high sedimentation rate and the seasonal evolution as well. The presence of Mn2+, Fe2+, S(-II) and the decrease of the redox potential and the concentration of sulphates clearly indicate early diagenetic transformations promoted by the bacterial activity. Acid Volatile Sulphides are produced in the first cm and are stabilized with depth. A rapid decrease of FAs concentrations in September and May has also been pointed out owing to a rapid consumption of the labile organic matter. Several categories of FAs have been separated and most of them belong here to the saturated and monounsaturated groups. In the saturated group, branched chain FAs, iso and anteiso C15:0 are predominant and represent the bacterial imprint in the sediments. Maximum proportions are observed between 5 and 10 cm in September, and between 13 and 17 cm in November and May. Discussion: As sulphate concentrations remain high in the porewater, the limitation of the sulphidization process in our sediments must be due to a lack of labile organic matter input. The presence of pyrite in our sediment is bound to its formation at the water-sediment interface, where partial reoxidation may take place. However, at deeper depths, pyritization processes does not continue any more. Presence of maximum, dissolved S(-II) concentrations have been observed, simultaneously with maximum proportion relative to total FAs of iso and anteiso C15:0, and, in September, with an increase in proportions of C18:1ω7. This indicates the presence of sulphate-reducing bacterial activity at the time when the sediments were sampled. However, no close correspondence between bacterial FAs concentrations and S(-II) concentrations has been found. Conclusions: In each core, the sulphidization process is not complete, and this is probably due to the lack of biodegradable organic matter, which appears as the limiting factor from a qualitative point of view. S(-II) production in porewaters is linked with the activity of sulphate-reducing bacteria. Seasonal effects have also been pointed out and, especially, a more important input of diatom organic matter in May when compared to September and November. Recommendations and Perspectives: Fatty acid analyses represent an original and a useful tool for a better understanding of an early diagenetic process in the first cm of the sediments. More studies should be carried out associating inorganic chemical parameters and chemical biomarkers for pointing out stronger and more reproducible relations. Moreover, the use of microcosms in our group is on the way to take into account the kinetics of the organic matter degradation during the early diagenesis.

Retention and early diagenetic transformation of phosphorus in Lake Arendsee (Germany) - consequences for management strategies

Repeated sediment core investigations over one decade, mass balance calculations, and vertical flux measurements by traps and dialysis samplers, were used to determine P retention rates, release potential, and early diagenetic transformation processes in the sediment of Lake Arendsee (Germany). Sediment cores were dated by varve counting, by 137 cesium, and by a distinct layer originating from a restoration attempt in 1995, which involved the capping of the sediment with calcium rich material from the lake shore. P retention rates and the internal P cycle have not been altered by the sediment capping. The sharp decline of total P content within the first two centimeters of the sediment shows that diagenetic P mobilization is a rapid process. The temporary P pool in the sediment, calculated from core analysis (mean ± SE: 709 ± 82mg m -2 , n = 7), was small compared to the rates of hypolimnetic SRP increase (10.7 ± 0.45 mg m -2 d -1 , 1992-1997) and total P losses in the epilimnion caused by sedimentation (11.7 ± 0.53 mg m -2 d -1 , 1992-1997), both of the latter calculated by mass balances during summer stratification. Without additional supply of freshly settled material, the temporary P pool in the sediment would be exhausted in less than three months. The fast P release of freshly settled material was also demonstrated during summer by the three times higher vertical P sedimentation rates calculated on the basis of mass balance data, compared to rates determined on the basis of cylindrical traps, in which some P was released during four weeks exposure time. The driving process for the rapid P release in Lake Arendsee is the remineralisation of organic P. This study demonstrates that high hypolimnetic P accumulation rates are not always correlated with a large total or potentially mobile P pool in the sediment. In lakes with small temporary P pools in the sediment, a decrease of P in the water body would immediately decrease the hypolimnetic P accumulation in summer; capping or dredging as in-lake measures are ineffective in such lakes.

Factors influencing the biogeochemistry of sedimentary carbon and phosphorus in the Sacramento-San Joaquin Delta

This study characterizes organic carbon (Corganic) and phosphorus (P) geochemistry in surface sediments of the Sacramento-San Joaquin Delta, California. Sediment cores were collected from five sites on a sample transect from the edge of the San Francisco Bay eastward to the freshwater Consumnes River. The top 8 cm of each core were analyzed (in 1-cm intervals) for Corganic, four P fractions, and redox-sensitive trace metals (uranium and manganese). Sedimentary Corganic concentrations and Corganic: P ratios, decreased, while reactive P concentrations increased moving inland in the Delta. The fraction of total P represented by organic P increased inland, while that of authigenic P was higher bayward than inland reflecting increased diagenetic alteration of organic matter toward the bayward end of the transect. The redox indicator metals are consistent with decreasing sedimentary suboxia inland. The distribution of P fractions and C:P ratios, reflect the presence of relatively labile organic matter in upstream surface sediments. Sediment C and P geochemistry is influenced by site-specific particulate organic matter sources, the sorptive power of the sedimentary material present, physical forcing, and early diagenetic transformations presumably driven by Corganic oxidation.

Processes controlling forms of phosphorus in surficial sediments from the eastern Arabian Sea impinged by varying bottom water oxygenation conditions

Abstract The surficial sediments from the upper continental slope of the eastern Arabian Sea, impinged by the oxygen minimum zone (OMZ, 150–1200 m water depth), show varying concentrations of the biogenic element phosphorus (P, 0.1–0.2%) in the northern and southern areas even though total organic carbon concentrations are relatively constant (TOC, 2–5%; Prakash Babu et al., 1999). To understand this discordance, 17 surface sediment samples from shelf, slope and deep sea of the eastern Arabian Sea were investigated using a five-step sequential extraction scheme to delineate the process responsible for P enrichment in OMZ. High fractions of organic phosphorus (Porg 10–26%), biogenic phosphorus (Pbio 36–48%), relatively low molar Corg/Porg ratios (322–447), and Corg/Preactive ratios close to Redfield Ratio in OMZ sediments of the SE Arabian Sea suggest accumulation under high surface production and low residence time of labile forms of P due to high sedimentation rates. Despite higher productivity in surface waters, low fractions of Porg (8–13%; less than deep-sea sediments of the study area 12–13%), Pbio (25–33%), relatively high molar Corg/Porg ratios (341–508), and Corg/Preactive ratios less than Redfield Ratio in OMZ sediments from the NE Arabian Sea may indicate a higher degree of regeneration and diagenetic transformation of labile forms of P to other phases. Authigenic phosphorus (Paut) fraction varies by a factor of 2–8 in sediments from the OMZ when compared to well-oxygenated deep-sea sediments of the study area. While the Ptotal remains constant, significant P transformation seems to occur in NE Arabian Sea, which is suggested by high Paut fraction (∼50%) compared to low Paut fraction (10–39%) in the SE Arabian Sea sediments. Supply rates of phosphorus, variable rates of P dissolution under varying dissolved oxygen contents in the bottom waters, and early diagenetic transformation of P within the sediments seem to influence P geochemistry in sediments overlain by the OMZ in the eastern Arabian Sea.

Early diagenetic transformation of organic matter in a marine-influenced lignite

The polar lipid and macromolecular compositions of three samples from the marine-influenced Heartbreak Ridge lignite deposit (Bremer Basin, Western Australia) were investigated for chemical changes wrought during early diagenesis. The results of Rock-Eval, microscope Fourier transform infrared spectroscopic and thermal desorption/pyrolysis-gas chromatography–mass spectrometric analyses indicate that the composition of the lignite and its extract are typical of immature Type III organic matter, and thus contain a high proportion of aromatic and oxygen-containing functionalities. Down-seam variation in the proportions of these functionalities, together with changes in the carbon preference index (CPI) of the pyrolysate n-alkenes and the wax indices of straight chain n-alkyl pyrolysis products, is consistent with more pronounced degradation of the lowermost lignite horizon. This profile is also reflected in the diagenetic products of higher plant triterpenoids, identified in the lignite extracts by gas chromatography–mass spectrometry. The extent of aromatisation, demethylation, ring opening and defunctionalisation is recorded through this depth profile by the compositions of the free, sulfurised and oxygenated higher plant triterpenoids. In contrast, the extended hopanoid composition is invariant in all three lignite samples. Oxygen-containing lipids are implicated as the primary source of the sulfurised hydrocarbons, although not necessarily via direct sulfurisation of the identified oxygenated lipids, but through the formation of more reactive intermediate species. These results suggest that the onset of marine incursion and associated sulfur fixation inhibits and, through preservation, illuminates the typical biogeochemical transformation of lipids and biopolymers in coal-forming environments.

Recent brachiopods collected during the “SEAMOUNT 1” CRUISE off Portugal and the Ibero-Moroccan Gulf (Northeastern Atlantic) in 1987

The Seamount 1 expedition, on board R.V. Noroit in 1987, explored the Lusitanian seamounts off Portugal and the Ibero-Moroccan Gulf of the Northeast Atlantic. A rich brachiopod fauna of about 15 species has been identified in 52 of the 117 stations sampled. The species living around these seamounts are examined, observed using scanning electron microscopy (SEM) to complete their micro-morphological characteristics, and classified in accordance with the Treatise on Invertebrate Paleontology (Brachiopoda Revised, Treatise on Invertebrate Paleontology, Geological Society of America Inc. and University of Kansas, Boulder, Colorado and Lawrence vol. 1 (1997), vols. 2, 3 (2000) and forthcoming volumes). The location of these seamounts, between the Mediterranean and the Caribbean Seas, allows some faunal comparisons to be made. Comparisons are also made with the brachiopod fauna from Seamount 2, which allows taxonomic assessment of the status of one species, Stenosarina crosnieri (COOPER), found in both cruises. Apart from the species characteristics, and the occurrence of early diagenetic transformations of the shells, this paper describes the depth-ranges, the substrate relationships, and highlights the wide colonisation of these seamounts due to the free larval stage of these benthic species.

Tectonic–palaeoenvironmental forcing of clay-mineral assemblages in nonmarine settings: the Oligocene–Miocene As Pontes Basin (Spain)

Two small, alluvial-lacustrine subbasins developed during the early restraining overstep stages of the Oligocene–Miocene As Pontes strike-slip Basin (NW Spain). Later, the basin evolved into a restraining bend stage and an alluvial-swamp-dominated depositional framework developed. The palaeobiological record demonstrates that the Oligocene–Miocene palaeoclimate in NW Spain was subtropical, warm and humid to subhumid. The metamorphic and igneous basin catchment yielded clay assemblages made up by kaolinite, illite and Al-smectite. Illite occurred as an original mineral in the source rock area, whereas kaolinite and Al-smectite resulted mainly from weathering of feldspar and clinochlore, respectively. This detrital primary clay assemblage remained preserved in the colluvial, alluvial fan and shallow lacustrine facies, whose early diagenesis was influenced by diluted, poorly evolved pore waters with neutral to slightly alkaline pH. The original clay assemblage was mildly to strongly transformed under early diagenetic conditions in the lacustrine and swampy environments where significant hydrochemical and Eh–pH changes took place. A fibrous magnesium-rich clay mineral-dominated assemblage (palygorskite and sepiolite) formed in shallow, saline lakes and palustrine zones under the influence of magnesium-rich, alkaline waters. Moreover, kaolinite-enriched assemblages formed in deep lacustrine, swamp and swamp-related alluvial zones under the influence of slightly to highly acidic pore waters. Pore water acidic conditions, characterising environments with organic matter accumulation, led to early diagenetic transformation of Al-smectite into kaolinite. This process was relatively limited in some environments such as organic matter-rich bottoms in meromictic lacustrine zones, whereas it was pervasive in peat-forming swamp zones. The stratigraphic relationships between the diverse clay mineral assemblages in the As Pontes Basin fill demonstrate the coeval development of rather diverse clay compositions. This clearly reflects the influence on clay assemblages of palaeoenvironmental changes forced by the morphological and tectonic evolution of the catchment–basin system. The interplay between climate and tectonic processes in the source areas did not result in major variations of the clay minerals fed into the basin. Conversely, this tectonic-sedimentation interplay influenced the evolution of the drainage and the water balance in the depositional zones, causing a complex environmental–hydrochemical evolution to occur. As a consequence, drastic early diagenetic changes affected the original clay mineral assemblages and resulted in a variety of early diagenetic assemblages. The As Pontes case study emphasises the major influence of palaeogeographical and tectonosedimentary evolution on the clay mineral record in nonmarine depositional systems. The depositional record in As Pontes Basin demonstrates that morphological and tectonically forced environmental and hydrochemical changes can result in variations similar to those forced by low-order climatic changes and that, in some cases, the role of climate can be negligible.

Early diagenesis of the Upper Devonian Escuminac Formation in the Gaspé Peninsula, Québec: sedimentological and geochemical evidence

Abstract Clay mineral investigations, pyrolysis of organic matter, and microscopic observation of palynomorphs are used to interpret the diagenetic conditions of the Upper Devonian Escuminac Formation, Quebec, Canada. Pyrolysis of kerogen, vitrinite reflectance, and the presence of smectite show that the sediments of this formation have been subjected only to a shallow burial diagenesis; hence, the geochemical signatures are informative on the palaeoenvironment. Concretions are observed in the Escuminac Formation. Sedimentological and geochemical evidence suggest that these concretions were formed during early diagenesis from biodegradation of organic matter. The well-known fish-fauna of the Escuminac Formation show good preservation, due in part to extensive early diagenetic carbonate cementation. The occurrence of calcite cements and fibrous calcite is attributed to the early diagenetic transformation of pre-existing organic matter.

Textural features of Holocene perennial saline lake deposits of the Taoudenni–Agorgott basin, northern Mali

Abstract The Holocene salt lake deposits of the Taoudenni–Agorgott basin, northern Mali, mainly consist of sediments with a high glauberite (Na2Ca(SO4)2) content. The remainder of the deposits largely consists of salt beds with a bloedite (Na2Mg(SO4)2·4H2O), thenardite (Na2SO4) or halite (NaCl) composition. A petrographical study of the deposits demonstrates that they formed in a perennial lake that experienced a gradual decrease in water depth. Textural features of the glauberite-dominated deposits are found to be related to water depth, through the control that this factor exerts on the sensitivity of the lake to changes in water supply and to short-term variations in evaporation rates. In this way, layering — due to variations in glauberite content and crystal size — is inferred to be typical of deposits that formed in shallow water, whereas unstratified deposits are the product of high lake level stages. Halite textures are found to be indicative of the place within the water column where crystal growth occurred (along the lake bottom or higher), which is mainly determined by water depth and partly by evaporation rates. The oldest halite beds are largely unaltered cumulate deposits, whereas the youngest layers developed exclusively through bottom growth. The basal part of one thick halite bed at a level between these two groups of halite layers developed by an alternation of both types of growth, in response to variations in evaporation rates. Variations in mineralogical composition between and within the salt beds that formed during the earliest periods with a higher salinity, up to the first stage with halite formation, record a change in lake water chemistry with time but they are in one instance also determined by an early diagenetic mineral transformation.

Molecular indicators for palaeoenvironmental change in a Messinian evaporitic sequence (Vena del Gesso, Italy). II: High-resolution variations in abundances and 13C contents of free and sulphur-bound carbon skeletons in a single marl bed

The extractable organic matter of 10 immature samples from a marl bed of one evaporitic cycle of the Vena del Gesso sediments (Gessoso-solfifera Fm., Messinian, Italy) was analyzed quantitatively for free hydrocarbons and organic sulphur compounds. Nickel boride was used as a desulphurizing agent to recover sulphur-bound lipids from the polar and asphaltene fractions. Carbon isotopic compositions (δ vs PDB) of free hydrocarbons and of S-bound hydrocarbons were also measured. Relationships between these carbon skeletons, precursor biolipids, and the organisms producing them could then be examined. Concentrations of S-bound lipids and free hydrocarbons and their δ values were plotted vs depth in the marl bed and the profiles were interpreted in terms of variations in source organisms, 13C contents of the carbon source, and environmentally induced changes in isotopic fractionation. The overall range of δ values measured was 24.7‰, from −11.6‰ for a component derived from green sulphur bacteria (Chlorobiaceae) to −36.3‰ for a lipid derived from purple sulphur bacteria (Chromatiaceae). Deconvolution of mixtures of components deriving from multiple sources (green and purple sulphur bacteria, coccolithophorids, microalgae and higher plants) was sometimes possible because both quantitative and isotopic data were available and because either the free or S-bound pool sometimes appeared to contain material from a single source. Several free n-alkanes and S-bound lipids appeared to be specific products of upper-water-column primary producers (i.e. algae and cyanobacteria). Others derived from anaerobic photoautotrophs and from heterotrophic protozoa (ciliates), which apparently fed partly on Chlorobiaceae. Four groups of n-alkanes produced by algae or cyanobacteria were also recognized based on systematic variations of abundance and isotopic composition with depth. For hydrocarbons probably derived from microalgae, isotopic variations are well correlated with those of total organic carbon. A resistant aliphatic biomacromolecule produced by microalgae is, therefore, probably an important component of the kerogen. These variations reflect changes in the depositional environment and early diagenetic transformations. Changes in the concentrations of S-bound lipids induced by variations in conditions favourable for sulphurization were discriminated from those related to variations in primary producer assemblages. The water column of the lagoonal basin was stratified and photic zone anoxia occurred during the early and middle stages of marl deposition. During the last stage of the marl deposition the stratification collapsed due to a significant shallowing of the water column. Contributions from anaerobic photoautotrophs were apparently associated with variations in depth of the chemocline.

Early diagenetic transformation of higher-plant triterpenoids in deep-sea sediments from Baffin Bay

A sediment sequence of early Miocene to Holocene from Ocean Drilling Program Site 645 in central Baffin Bay was found to contain abundant triterpenoids of higher-plant origin including monodesmethyl derivatives and other oxidative degradation products. 24-nor-triterpenoids of the oleanene, ursene, and lupane series were found in the nonaromatic hydrocarbon and alcohol fractions of the sediment extracts. Other diagenetic transformation products included 24,28- dinor-olean-17-ene and 24- nor-urs-12-en-1 1-one as well as their tentatively identified des-A counterparts. The identification of these novel degradation products was confirmed by synthesis of a suite of reference compounds. The simple reaction sequence applied to obtain 24- nor-urs-12-ene from 3α-acetoxyurs-12-en-24-oic acid is likely to happen in a similar manner during early diagenesis of organic matter in nature. These results have implications for the understanding of the diagenetic reaction sequence leading to the formation of other demethylated triterpenoids of terrigenous origin (e.g., 24,28- dinor-lupanes). 28,30- dinor-17α-hopane is sometimes the dominant hydrocarbon in the most deeply buried samples. Its co-occurrence with higher plant triterpenoids and the low abundance of other regular hopanoids is not considered an indication, however, that 28,30 dinor-17α-hopane is derived from a higher plant precursor molecule. It is suggested that two independent sedimentary processes are responsible for the presence of the two types of demethylated triterpenoids observed in Baffin Bay sediments.

Radium isotopes, alkaline earth diagenesis, and age determination of travertine from Mammoth Hot Springs, Wyoming, U.S.A.

Travertine from active springs, former vents, and drill core from Mammoth Hot Springs, Wyoming, was analyzed for Ra isotopes ( 226Ra, 228Ra), other alkaline earth elements (Mg, Sr, Ba), and mineralogical composition. Thermal water also was analyzed. Travertine, presently being deposited, contains 3.0–15.3 pCi/g 226Ra, and has a 228Ra/ 226Ra ratio identical to that in thermal water. Travertine precipitates mostly as aragonite and experiences a complete diagenetic transformation to calcite within 9 a (estimated from 228Ra/ 226Ra value of youngest completely calcite travertine). Systematic compositional changes associated with this diagenetic transformation are enrichment of Mg and depletion of Sr, Ba and Ra. Apparent mineral-water distribution coefficients for Mg, Sr and Ba in aragonite and calcite are within the range of those determined experimentally, implying near-equilibrium conditions and high water-rock ratios during travertine diagenesis. Impure travertine from near the base of the travertine section in the U.S. Geological Survey Y-10 drill hole (at 72.9 m depth) has a 230Th/ 234U isochron age of 7700± 440a. The content of 226Ra in normal, subhorizontally layered, porous travertine decreases with depth in Y-10. The observed 226 Ra vs depth relation is consistent with continuous deposition of travertine at the Y-10 site from 7700 a BP to near present at a mean rate of ∼1.0 cm/a, and indicates minimal exchange of Ra between travertine and pore water after the early diagenetic transformation of aragonite to calcite. These data imply that, under favorable conditions, 226Ra measurements may be useful in determining ages and deposition rates for other travertines.

Hydrocarbons associated with hydrothermal minerals, vent waters and talus on the East Pacific Rise and Mid-Atlantic Ridge

Two active hydrothermal vent sites on the East Pacific Rise (EPR) at 13°N and 21°N have similar biogeochemicals. They are characterized by large animal communities surrounding massive porous mixed mineral (e.g. sulfide) edifices venting hot water (200–350°C) as black smokers. An analogous vent system on the Mid-Atlantic Ridge (MAR) at 26°N has also been studied. Sediment (talus), mound mineral and water samples from these areas were analyzed in terms of biological marker lipids such as normal and isoprenoid alkanes, polynuclear aromatic hydrocarbons, triterpanes, steranes and fatty acids to describe their sources, bacterially-mediated transformation products and alteration by hydrothermal fluids. The mixed interior/exterior of a chimney at 21°N on the EPR shows a series of iso- and Anteiso-alkanes which are known constituents of high wax crude oils. These alkanes, coupled with the absence of triterpanes and steranes seem to indicate a higher temperature, water-soluble fraction from hydrothermal pyrolysis which is unfavorable for the preservation of the biomarkers. Chimney fragments with tube worm casts at 21°N contain products that are characteristic of immature biodegraded organic matter as indicated by microbial markers (branched C 15 and C 17 fatty acids) and low thermal alteration which enhanced formation of cholestene isomers and diacholestenes. In comparisons, a metalliferous sediment and sediment trap particles at 13°N on the EPR contain a biomaker mixture, namely recent biomarkers derived from early diagenetic transformations of biogenic compounds (sterols, squalene) and more mature components [5α(H), 14β(H), 17β(H) steranes (20 R + 20 S), dominant 17α(H)-hopane series], both having the same origin. Hydrocarbons isolated from hydrothermal anhydrite, chalcopyrite and sphalerite-enriched samples from 26°N on the MAR exhibit a high degree of thermal maturation, compared to an Fe-oxide-enriched sample, as evidenced by the high abundance of low molecular weight n-alkanes ranging from C 14 to C 25 with a broad distribution of naphthenes. All these trace hydrothermally derived petroleums contain polycyclic aromatic hydrocarbons (PAH). The Fe-oxide-enriched sample has a lower content of PAH than the other mineral samples; however, benzopyrenes and coronene are present at significant levels and a series of mature triterpanes is also detectable. These trace levels of petroleum with the associated biomarkers and PAH strongly suggest generation from contemporary organic detritus by hydrothermal activity and subsequent association with the minerals, talus, vent fluids, and ambient water.

Steroidal ketones in the early diagenetic transformations of Δ5 sterols in different types of sediments

Incubation experiments of cholesterol labelled with various isotopes ( 2H, 3H and 14C) were performed in two different types of sediment, one highly oxygenated and the other completely anoxic, to ascertain the presence and the role of steroidal ketones in the early diagenesis of unsaturated sterols. Two different pathways for the hydrogenation of cholesterol have been observed, depending on the oxygenation rate of the sediment. If the intermediacy of steroidal ketones is evident in the oxic sediments of Lake Léman (Switzerland), a direct reduction of the Δ 5 double bond also occurs in the anoxic sediments of Motte Lake (France).