Fossil Organic Matter Research Articles

An integrated model for soil organic carbon and CO2: Implications for paleosol carbonate pCO2 paleobarometry

The geologic history of atmospheric CO2 concentration (CAtm) is relevant to studies of paleoclimate, paleobiology, and elemental cycling at the Earth's surface. Here we present a model that represents processes controlling the concentration, stable carbon isotope composition, and vertical distribution of organic matter and CO2 within soils. In the model we include a volumetric scaling term necessary to calculate the soil pore CO2 concentration and δ13CO2 from soil respiration rate per unit area, which has been omitted in previous work. Our model provides several new insights into the commonly used method for estimating paleo‐CAtm based on the δ13C of paleosol carbonates. We show that because refractory, microbially processed organic matter is increasingly abundant toward the base of the soil, isotope effects associated with organic matter decay can potentially lead to significant offset between the δ13C of soil‐respired CO2 (δφ) and that of soil organic matter at depth. The magnitude of this effect depends on the production and decay characteristics of the individual soil, and can lead to substantial underestimation of paleo‐CAtm if organic matter preserved in subsurface paleosol horizons is used to approximate δφ. Paleo‐CAtm reconstruction will be more accurate if surface‐layer fossil organic matter is used as a proxy for δφ. In this case, however, there is a small (200–300 ppm), consistent, positive bias in CAtm estimates resulting from the decay of 13C‐enriched organic matter deep within the soil. Re‐examination of earlier studies using the soil carbonate approach to estimate CAtm identifies examples where each bias may be present.

Aromatic compounds in molecular phase of Baltic amber—synchronous luminescence analysis

Synchronous luminescence analysis was performed in order to identify aromatic compounds in solvent extracts of Baltic amber. The investigated extracts were obtained, for comparisons, as products of extraction by various techniques and solvents. Methylene chloride and ethanol were applied independently for extraction at the ambient temperature (conservative extraction), as well as at the temperature of solvent boiling (extraction in Soxhlet apparatus). Ethanol, as the solvent, was also used for extraction in an ultrasonic bath and for the decoction process. The extraction, by techniques mentioned, of the analysed amber has resulted in products generally containing the same groups of aromatics: mainly naphthalenes, phenanthrenes and anthracenes. Among phenanthrenes, in all samples the retene was also identified, being one of the characteristic links of the diagenetic chain of chemical transformations of vegetal precursors. The identification of a series of individual compounds made, using the synchronous luminescence technique, was verified by the record of conventional emission and excitation spectra. Presented identified compounds were also confirmed by the results of GC-MS analysis. The luminescence analysis was also performed comparatively for fossil resin from Galicia, Spain ( Cretaceous) older than Baltic amber ( Tertiary, Eocene). The obtained preliminary results of synchronous luminescence analysis suggest the possibility of diversification in this manner of fossil resins of various ages by characterisation of aromatisation degree and alkyl substitution of aromatic rings. It is since well known that aromatisation progress is an indicator of a natural process of maturation of fossil organic matter. However, a greater number of samples should be taken to further testify to the investigations.

Estimation of the annual yield of organic carbon released from carbonates and shales by chemical weathering

The aim of this paper is to propose an initial estimation of the annual organic matter yield induced by chemical weathering of carbonates and shales, considering their global surface at outcrop and their organic matter content. The calculation also uses data on river fluxes resulting from carbonate rocks and shales weathering in major world watersheds, published by numerous authors. The results obtained from the studied watersheds have then been extrapolated to a global scale. Despite rather large uncertainty to such an approach, the calculated value of ca. 0.1 Gt implies that the annual organic carbon yield related to carbonates and shales chemical weathering might be a non-negligible component of the global carbon cycle. The individual contributions of different watersheds necessarily depend on the organic matter content of altered rocks. They are also obviously controlled by climatic parameters. The calculated yields do not constitute a direct supply to soils and rivers because of mineralisation when organic carbon is brought in contact with the atmosphere. Even so, the release of fossil organic matter would have implications for the global carbon cycle through the efficiency of the global chemical weathering as a carbon sink. Whatever the chosen hypothesis, the results of this study suggest that the recycled organic yield is a neglected component in the global organic carbon cycle assessment. Because it exists and, in addition, because it might represent a non-negligible carbon pool, fossil organic carbon deserves to be taken into account for a better evaluation of the organic stocks in soils and rivers presently only based on climatic data and current vegetal production.

Direct and continuous methodological approach to study the ageing of fossil organic material by infrared microspectrometry imaging: application to polymer modified bitumen

A new simulation method was developed for studying the ageing of polymer modified bitumens (PmBs). An ageing cell specially was designed and fitted to a Fourier transform infrared (FTIR) microscope to continually and directly study the oxidation of PmB by FTIR microscopy imaging. The method was applied to a styrene–butadiene–styrene (SBS) modified bitumen. This PmB features a two-phase structure with polymer-rich areas along with polymer poor regions. Changes in the chemical properties were observed during ageing. This structural and functional evolution was monitored in both phases simultaneously to determine the kinetics of appearance and disappearance of chemical species. The method also allows the study of ageing by kinetic mapping. Moreover, this methodology allows the analysis of PmB samples without destroying their internal equilibrium. This paper presents some of the possibilities of this new ageing cell.

00/00326 Molecular characterization of fossil organic matter in Glyptostrobus europaeus remains from the Orawa basin (Poland). Comparison of pyrolytic techniques

00/00326 Molecular characterization of fossil organic matter in Glyptostrobus europaeus remains from the Orawa basin (Poland). Comparison of pyrolytic techniques

Induction of aromatic catabolic activity in Sphingomonas aromaticivorans strain F199.

Enzyme induction studies with Sphingomonas aromaticivorans F199 demonstrated that both toluene and naphthalene induced expression of both naphthalene and toluene catabolic enzymes. However, neither aromatic compound induced expression of all the enzymes required for complete mineralization of either naphthalene or toluene. Activity measurements in combination with gene sequence analyses indicate that growth on either aromatic substrate in the absence of the other is, therefore, sub-optimal and is predicted to lead to the build-up of metabolites due to imbalance in toluene or naphthalene catabolic enzyme activities. Growth on toluene may be further inhibited by the co-expression of two toluene catabolic pathways, as predicted from gene sequence analyses. One of these pathways may potentially result in the formation of a dead-end intermediate, possibly benzaldehyde. In contrast, either p-cresol or benzoate can support high levels of growth. Analyses of promoter region sequences on the F199 aromatic catabolic plasmid, pNL1, suggest that additional regulatory events are modulated through the interaction of BphR with Sigma54 type promoters and through the binding of a regulator upstream of p-cresol catabolic genes and xylM. We hypothesize that the unusual gene clustering in strain F199 is optimized for simultaneous degradation of multiple aromatic compound classes, possibly in response to the heterogeneous composition of aromatic structures in the fossil organic matter present in the deep Atlantic Coastal Plain sediments from which this bacterium was isolated.

The Stable Isotopic Geochemistry of the Sulfur and Carbon Cycles in a Modern Karst Environment

The stable isotopic geochemistry of the sulfur and carbon cycles in a modern karst environment at the southwest border of the Harz Mountains (Germany) has been studied during a 7-year period. Beside major, minor and trace elements, river and sulfate-carbonate ground water were analyzed for the 13C/12C ratios of the dissolved carbonate species, the 18O/16O ratio of water, and the 34S/32S and 18O/16O ratios of dissolved sulfate. Additionally, the carbon, sulfur and oxygen isotopic composition of the major karst aquifers (carbonate and gypsum anhydrite rocks) of the Permian Zechstein formation was measured. Nowadays, the stable isotope biogeochemistry of the river and karst water in the studied area results from the complex interactions between dissolution of biogenic CO2 in the water-unsaturated zone, (minor) subterrestrial microbial decomposition reactions of organic matter (recent DOC or fossil organic matter), interactions with carbonate and sulfate aquifer minerals, and input of acids from atmospheric pollution (e.g., sulfuric acid). Subsurface precipitation of secondary calcite due to the incongruent dissolution of gypsum and/or dolomite is deduced from the hydrogeochemical composition of selected ground waters and the isotopical composition of calcite found in gypsum aquifer material. Spring and river waters were additionally influenced by the liberation of carbon dioxide into the Earth's atmosphere, a process which is accompanied by the preferrential desorbtion of 12CO2. The sulfur isotopic composition of dissolved sulfate from ground and spring waters indicates a mixture of sulfate derived from surface waters (mainly originating from atmospheric pollution) and geogenic sulfate from the subsurface dissolution of gypsum/anhydrite of the Zechstein formation. The oxygen isotopic composition of dissolved sulfate is generally far away from the exchange equilibrium with water, but consistent with the two-component mixing model derived from the sulfur isotope ratios.

Molecular characterization of fossil organic matter in Glyptostrobus europaeus remains from the Orawa basin (Poland). Comparison of pyrolytic techniques

Pyrolytic methods (standard Curie-point pyrolysis and pyrolysis in the presence of tetramethylammonium hydroxide (TMAH), followed by gas chromatography–mass spectrometry, were used to analyze the organic composition of Glyptostrobus (Taxodiaceae) remains from the Miocene deposits of Lipnica Mala (Poland), consisting of branches with their leaves. The pyrolytic analysis revealed a series of aromatic compounds with a large proportion of guaiacyl-type lignin markers (including intact C 3-methoxyphenols), and small quantities of polycyclic aromatic hydrocarbons. The alkyl compounds included fatty acid series where the C 14–C 18 homologues dominated as well as additional amounts of alkanes and alkenes (maximum ca. C 21). The results suggest that lignin as well as protective epicuticular lipid polymers (including cutin and other long chain-based polyalkyl structures) selectively show the greatest degree of molecular preservation in the Glyptostrobus remains. The comparison between pyrolytic methods shows that the lignin-derived aromatic assemblages predominate in the chromatograms after conventional pyrolysis whereas thermochemolysis with TMAH leads to an “aliphatic enhancement”. This latter technique was found to be the best for analysis of the lipid signature in fossil samples with condensed polymethylene networks.

Geochemical and microfacies characterization of a Holocene depositional sequence in northwest Germany

Holocene depositional sequences in the coastal zone of the North Sea reflect climate-driven sea-level changes that led to lateral interfingering of marine, brackish, lagoonal and limnic sediments and peat layers. This succession is reflected in an 18 m sediment core drilled in the marsh lands of the river Weser. The core spanning the entire Holocene was analyzed at high stratigraphic resolution for major and minor elements, bulk and molecular constituents and microfacies characteristics. Abundant triterpenoids in sediments of the basal organic unit indicate nutrient-rich limnic paleoconditions not yet influenced by the rising sea level. Lithological description of several sedimentary units overlying the basal sequence initially suggested a mainly lagoonal setting with occasional influence by tidal channel activities; geochemical and microfacies analyses confirmed repeated marine ingression. Marine conditions are indicated by pyrite enrichment, the presence of sterathiols formed by the reaction of sterols with reduced sulfur species, and the occurrence of marine pelagic diatoms. Towards the top of the sedimentary sequence, estuarine conditions become more and more pronounced as the river Weser established its modern course whereas it had been quite variable previously. Estuarine conditions are reflected by sterol and amino acid distributions and by limnic diatom species that are mixed with marine pelagic forms. Although the majority of the clastic units consist of silty to clayey sediments, some intercalated quartz sand layers reflect the increase of hydrodynamic energy characteristic of lower tidal flats and channels. Lumps of reworked peat within fine clastic deposits also reflect high energy events. Even a dark layer within the upper clastic unit, formerly considered a fossil soil horizon, is not the direct result of regression-induced soil development, but originates mainly from eroded fossil organic material from the basal unit. This interpretation was established by biomarker analysis and radiocarbon dating.

Variations in [formula omitted] ratios in recent marine sediments: evidence for a fossil organic component

The value of the pre-bomb 129 I/ 127 I ratio in the hydrosphere is important for the use of this isotope system for dating and tracing purposes. In order to determine this value and to examine spatial variability, 129 I/ 127 I ratios were measured in 25 sediment samples from five cores taken at geographically distinct coastal areas. The results indicate a pre-anthropogenic 129 I/ 127 I ratio of 1500×10 −15, in good agreement with values derived from previous sediment and groundwater studies. Results from shallow sediments show that the layer of bioturbation is dominated by the addition of anthropogenic 129 I . The good agreement between results for this layer from different cores supports the understanding that marine iodine is isotopically homogeneous and associated with organic material which is actively remineralized at the sediment–water interface. Lower ratios (down to 354×10 −15, corresponding to an age of 32.5 Ma) and larger variations than expected were found in sections of the cores below the layer of bioturbation. These observations can be explained by the presence of refractory fossil organic material, transported by rivers to the sediments from uplifted, continental shales. The results suggest that marine iodine is present in two forms, a labile component which is isotopically homogeneous, and a refractory component which is associated with kerogen (or other fossil organic compounds) whose 129 I/ 127 I ratio reflects the age of these compounds. 129 I/ 127 I ratios can be used to identify and date fossil organic material in Recent sediments, the presence of which can alter the assumptions underlying models of the global carbon cycle.

Appraisal of pyrolytic techniques on different forms of organic matter from a Cretaceous basement in Central Spain

Pyrolytic techniques were used to characterize three different forms of fossil organic matter in Cerro de la Mesa (an important Cretaceous paleobotanic basement in Central Spain): two kerogens isolated from carbonaceous marls, and from lignitiferous layers, and well-preserved remains of the fossil Gymnosperm Frenelopsis oligostomata. By conventional flash pyrolysis the Frenelopsis remains released mainly alkyl-substituted phenols and polyaromatic compounds, whereas the organic matter from the lignitiferous layer is dominated by phenols and that from the carbonaceous marls by polycyclic and aliphatic compounds. Pyrolysis-thermochemolysis alkylation in the presence of tetramethylammonium hydroxide (TMAH) allowed the detection of additional diagnostic compounds, such as series of methylated fatty acids, benzenecarboxylic and phenolic acids, which revealed further differences between the three materials studied. The chemical characterization of the organic matter in addition to the micropalaeontological study suggests that the Frenelopsis remains consist mainly of heavily altered lignin and cuticular lipid biomacromolecules, whereas the materials from the lignitiferous layers and the carbonaceous marls correspond to intermediate coalification stages of woody plants, the latter showing the greatest degree of selective preservation of aliphatic structures. The comparison of the above results with those obtained by thermochemolysis suggests that the latter technique produces independent and complementary reactions to conventional pyrolysis, providing additional information to an extent which depends mainly on the polyalkyl and oxidized character of the sample.

Changes in land plant function over the Phanerozoic: reconstructions based on the fossil record

Major fluctuations in the concentrations of atmospheric CO2and O2are predicted by historical long-term carbon and oxygen cycle models of atmospheric evolution and will have impacted directly on past climates, plant function and evolutionary processes. Here, palaeobotanical evidence is presented from the stomatal density record of fossil leaves spanning the past 400 Myr supporting the predicted changes in atsmopheric CO2. Evidence from experiments on plants exposed to long-term high CO2environments and the newly assembled fossil data indicate the potential for genetic modification of stomatal characters. The influence of the changes in fossil stomatal characteristics and atmospheric composition on the rates of leaf gas exchange over the course of land plant evolution has been investigated through modelling. Three contrasting eras of plant water economies emerge in the Devonian (high), Carboniferous (low) and from the Upper Jurassic to the present-day (high but declining). These patterns of change result from structural changes of the leaves and the impact of atmospheric CO2and O2concentrations on RuBisCo function and are consistent with the fossil evidence of sequential appearances of novel plant anatomical changes. The modelling approach is tested by comparing predicted leaf stable carbon isotope ratios with those measured on fossil plant and organic material. Viewed in a geological context, current and future increases in the concentration of atmospheric CO2might be considered as restoring plant function to that more typically experienced by plants over the majority of their evolutionary history.

Separation and Artificial Maturation of Macerals from Type II Kerogen

Immature Type II kerogen (HI = 660 mg/g) from the Lower Toarcian of the Paris Basin was separated into an alginite concentrate (HI = 952 mg/g) and an amorphous organic matter (AOM) concentrate (HI = 573 mg/g) by density centrifugation. The flash pyrolysate of the alginite is characterized by high relative concentrations of several series of n-alkanones and n-alkenones (including mid-chain alkyl ketones), in addition to n-alkanes, n-alk-1-enes, and n-alkadienes. To our knowledge, this Toarcian alginite is the oldest example of marine organic matter whose pyrolysate contains mid-chain alkanones in such high relative concentrations. In sharp contrast, the AOM produced predominantly alkylbenzenes, alkylthiophenes, n-alkanes, and n-alk-1-enes upon pyrolysis. Micro-FTIR spectroscopy indicated that the alginite was enriched in aliphatic C-H (particularly CH2) and depleted in aromatic C=C, relative to the AOM, consistent with the pyrolysis results. Aliquots of the concentrates were heated separately in gold tubes (24 h, 70 MPa) at fixed temperatures ranging between 250 and 375 °C. Yields of liquid products as a function of temperature were initially greater for the AOM, reaching a maximum at 325 °C. In contrast, the alginite yielded little liquid product at low temperatures, attaining its maximum at 350 °C, at which temperature its yield greatly surpassed that of the AOM. This kerogen is a heterogeneous assemblage of fossil organic matter, exhibiting different degrees of preservation and petroleum potential. The alginite is fossilized marine algaenans with alkyl chains cross-linked by ether bridges, while the AOM component is at least in part a geopolymer with thioether linkages, the thermally labile nature of which is responsible for its lower temperature peak liquid generation. It is evident that the alginite concentrate is chemically distinct from its companion AOM in this kerogen and that the full extent of its uniqueness would not have been revealed without the density separation step.

Ancient polycyclic aromatic hydrocarbons in modern soils: 13C, 14C and biomarker evidence

Polycyclic aromatic hydrocarbons (PAHs) of soils from two maize growing fields were analyzed for molecular structure by gas chromatography-mass spectrometry, for 13C/ 12C ratios by isotope ratio mass spectrometry and for 14C age by accelerator mass spectrometry. These investigations indicate that the aromatic fractions are dominated by pyrolytic inputs. Calculation of molecular maturity parameters based on hopanes and PAHs gives values typical of high thermal stress such as exhibited by high rank coals and petroleums with equivalent vitrinite reflectances of ∼0.6–1.5%. The interpretations based on molecular compositions are corroborated by isotopic data. Specifically, both the absence of 13C/ 12C variations in the aromatic fractions during 23 years of maize cropping, and a 14C age greater than 9820 years demonstrate that these substances are not derived by the aromatization of modern plant material or soil biomass. Together, these results show that soil PAHs are mainly of pyrolytic origin, e.g. exogenous (airborne) contributions of fossil fuel combustion products and (old) vegetation fires, with a small contribution of uncombusted fossil fuels. This approach using molecular and isotope information represents a promising way to study the fate of ancient, reworked, fossil organic matter in modern media such as soils, sediments, plants, air and food.

Cl 36 — A potential paleodating tool

The need for an isotopic tool capable of extending our ability to date fossil organic materials beyond the 50000 year range of 14C has been recognized for some time. Chlorine 36, a cosmogenic nuclide with a half-life of 301000 years, appears well-suited to this role. It exchanges readily with stable chloride present in ppm quantities in soil pore water, and can be measured at extremely low levels by Accelerator Mass Spectrometry (AMS).

Laboratory Oxidation of Fossil Organic Matter Studiedby in situ Infrared Spectroscopy, Rock-Eval Pyrolysis and Pyrolysis-Gas Chromatography-Mass Spectrometry

Fossil organic matter in Miocene and Silurian sediments was subjected to experimental oxidation, which was investigated by Rock-Eval pyrolysis, the "off-line" pyrolysis-gas chromatography-mass spectrometry combination, and continuous FTIR monitoring. The pyrolysate yield decreased during the oxidation particularly in the low-matured, predominantly aliphatic organic matter from the Miocene sediments (type I kerogen, algae-type kerogen). This suggests that aliphatic chains are preferentially oxidized, which is in agreement with the marked decrease in the intensity of the v(CH2) and v(CH3) IR bands. The n-alkane distribution in the chromatographic profile was unaffected by the oxidation; hence, the oxidation of the alkane chains was not selective. At the same time, the bands within the 1 900-1 550 cm-1 range grew in intensity for both the aliphatic and mixed-type (type II kerogen) organic matter. The oxidation of the aromatic (humic) type of organic fossil matter in the Miocene sediments (type III kerogen, coal-type kerogen) was only accompanied by very small changes in the FTIR spectra. The results of the "of-line" pyrolysis are consistent with those of the Rock-Eval pyrolysis. For all samples, the oxidation was accompanied by a gradual decrease in the hydrogen index (HI) as well as in the pyrolysis temperature maximum (Tmax). The changes in the S2/S1 ratio ("bound"-to-"free" hydrocarbons) indicate that the "free" hydrocarbons in the rocks are preferentially oxidized during the first 8-16 h of the experimental run. Subsequently, hydrocarbon chains involved in the kerogen macromolecule are attacked.

Gas Chromatographic and Mass Spectrometric Characterization of Pyrolysis Products of Fossil Organic Matter from Localities of the Czech Republic

The composition of fossil organic matter of different origin and thermal maturation was studied based on the gas chromatographic-mass spectrometric characterization of their pyrolysis products. Increase in the pyrolysis temperature from 300 to 500 °C was found to cause the cracking of aliphatic hydrocarbons resulting in a shortening of the n-alkane chains. High yields of pyrolysis products can be obtained at temperatures above 350 °C; the cracking reactions can be suppressed by applying relatively short duration of pyrolysis. The chromatographic patterns then clearly reflect the origin of the organic matter and degree of its thermal maturation in rocks.

Impact of heterogeneous aquifer materials on sorption capacities and sorption dynamics of organic contaminants

Abstract Physico-chemical properties as well as sorption and long term sorption kinetics of phenanthrene were measured in various size fractions and characteristic lithocomponents of heterogeneous aquifer material from the Horkheim Aquifer Test Site in southwest Germany. High contents of organic carbon as well as high equilibrium sorption capacities were found in the silt and the gravel fraction. The relatively high sorption capacities in the gravel sized fraction is due to high amounts of Triassic and Jurassic carbonate lithofragments which contain a significant amount of fossil organic matter (presumably kerogen). However, since sorption kinetics is slow in the coarse fraction, sorption equilibrium is not reached. Therefore under nonequilibrium conditions retardation factors will be orders of magnitudes lower than expected. INTRODUCTION Aquifer sediments are often a heterogeneous mixture of various lithocomponents which differ in size and properties. Fate and transport of contaminants depend on the relevant chemical and physical properties of specific lithocomponents of the aquifer sediments. These reactive lithocomponents may constitute only a relatively small fraction of the bulk material. Weber et al. (1992), for example, report that shale-like materials in soil samples showed sorption capacities of 1,2,4-trichlorobenzene approximately 100 times higher than the parent bulk material. The paramagnetic fraction of aquifer sediments from the Cape Cod Test Site in Massachusetts showed higher sorption of chlorobenzene (factor 3) and a much higher organic carbon content (factor 5) than the nonmagnetic fraction (Barber et al., 1992). Studies on the sorption kinetics of hydrophobic organic contaminants in various aquifer materials show that diffusive mass transfer was limiting the sorptive uptake and the desorption rates (Karickhoff & Morris, 1985; Wu & Gschwend, 1986; Steinberg et al., 1987; Ball & Roberts, 1991; Brusseau, 1991). In many cases very low rates were observed (Grathwohl & Reinhard, 1993 ; Farrell & Reinhard, 1994; Harmon & Roberts, 1994). The time periods required in order to reach sorption equilibrium or to desorb the pollutants from contaminated material ranged from several months to many years (Steinberg et al, 1987; Pignatello et al, 1993; Schuth & Grathwohl, 1994). If the sorption kinetics are slow compared to the advective contaminant transport in groundwater, retardation factors become velocity or time dependent (Roberts et al, 1986; Schuth & Grathwohl, 1994) and extended tailing in aquifer remediation

Isotopengeochemische Charakterisierung kohlenwasserstoffbildender organischer Substanzen

Abstract Within the framework of a larger research project on contributions of isotope investigations to natural gas and crude oil exploration, the following publication is a first part of research reports from the author's laboratory. The isotopic composition of carbon in fossil organic matter is primarily determined by the type of biological precursors. The main constituents forming organic matter in sediments are higher plants and microorganisms (algae, bacteria). These precursors contain a great number of organic compounds with corresponding isotope signatures. Hydrocarbons and lignin e.g., are enriched in carbon 13, while lipids in most cases are depleted. Semi-empirical calculations on a thermodynamic basis are used to precalculate isotopic compositions of components or single compounds, applying the so-called β-factor method. The results are compared with numerous data from references about isotopic compositions of higher plants, algae and plant components. Also environmental conditions of plant gr...

Experimental field study of the effects of crude oil, drill cuttings and natural biodeposits on microphyto- and macrozoobenthic communities in a Mediterranean area

Effects of an experimental pollution by neogenous and fossil organic matter on microphyto- and macrozoobenthic communities were studied in situ over a 1 yr period (July 1989 to May 1990) in a shallow microtidal bay (Gulf of Fos, south coast of France). Three experimental enclosures of 1 m2 non-defaunated sediments were covered with 1 cm of polluted defaunated sediments. The sediment in one enclosure contained natural biodeposits with a high organic matter content (BD), that in a second enclosure contained Arabian light crude oil (BAL), and the sediment in the third enclosure contained diesel oil-based cuttings (CUT). Pollution by contaminants did not prevent microphytobenthos from colouizing sediments. Population changes over time were quite similar in all enclosures, except in CUT, where a four times higher chlorophyll a content appeared to be related to a decreased number of grazers and consequently lower grazing rates of animals. Toxicity to the fauna was immediate in the case of BAL and occurred within 3 mo at CUT. Opportunistic species settled in all contaminated sediments; this occurred quite rapidly in BD and BAL which recovered within 3 mo to levels comparable with control sediments. In CUT, natural populations had not recovered after 1 yr, whereas a quasi-monospecific population of Capitella capitata was still present. During the first 3 mo, the oxygen demand of the sediment was higher in oil-contaminated sediments than in controls. On the whole, the changes in fluxes and organism assemblages in our weakly tidal area appear to be consistent with other findings in macrotidal seas.