Diagenesis Of Organic Matter Research Articles

Early diagenesis of marine organic-matter and magnetic properties of sedimentary rocks: the role of iron limitation and organic-matter source organisms

Abstract The magnetic parameters of sedimentary rocks can record accurately tectonic and climatic influences upon sedimentary processes, when they are not altered during diagenesis. This paper is focused on the possible alteration of the primary magnetic-susceptibility signal during early diagenesis of marine organic matter (OM). In the late Kimmeridgian-Tithonian Argiles de Châtillon Formation (Fm.) of the Boulonnais area (northern France – lateral time equivalent of the distal organic-rich sediments of the Kimmeridge Clay Fm. of Dorset), the organic content is dominated by amorphous OM (AOM), either brown or orange, originating from selective preservation or sulphurisation, respectively. Total sulphur content correlates well with orange + brown AOM abundance, and organic S content correlates well with orange AOM abundance. The magnetic signal of these claystones and paper shales is dominantly carried by clay minerals. The magnetic susceptibility vs. brown-AOM abundance relationship shows a clear correlation. Furthermore, if the part of the magnetic-susceptibility signal linked to the lithoclastic fraction of the sediments is removed from the total signal, a positive correlation is also drawn between the brown-AOM abundance and the ‘excess’ magnetic susceptibility. The cause for this resides in the iron-sulphide abundance. The presence of both types of AOM implies that intense sulphate reduction took place and that the HS−/H2S released reacted with either organic molecules (orange AOM dominating the palynofacies) or reactive iron (brown AOM dominating). When reactive iron was abundant enough and when sulphate reduction-induced sulphide ions could react with it, iron sulphides could form and this sediment component (present now as pyrite) influences the magnetic-susceptibility signal. The type of dominating source organisms (phytoplankton) may condition the reactivity of OM towards sulphide ions. This may influence iron-sulphide formation, and in turn the sediment magnetic signal.

Organic matter diagenesis in the sediments of the San Pedro Shelf along a transect affected by sewage effluent

A study was conducted to examine the rate of organic matter degradation within the sediments adjacent to the Whites Point Los Angeles County sewage outfall system on the San Pedro Shelf. Benthic chamber deployments were made at three stations in a transect away from the outfall pipe during three cruise periods (October 1990, February and October 1991). The fluxes of phosphate, silicate and radon-222 showed the most significant difference (factor of 3) between stations proximal and distal to the sewage effluent outfall pipe; the fluxes of nitrate, ammonia, alkalinity and TCO 2 showed some gradient (a factor of 1.5–2) and the uptake of oxygen showed no variability between sites. Carbon oxidation in these sediments is driven primarily by net oxygen consumption and secondarily by net sulfate reduction. Net sulfate reduction accounts for about 30% of the carbon oxidation near the outfall pipe and 10–15% at the distal sites. Measurements of radon-222 fluxes and radon emanation rates from sediments indicate that the intensity of bio-irrigation is greater by a factor of 2 at sites away from the outfall pipe, but also shows that bio-irrigation does take place at the site adjacent to the pipe. The total amount of particulate organic carbon (POC) oxidized in the sediments around the outfall is ∼2×10 7 g C/d. An estimate of carbon burial in this region is 4×10 7 g C/d. The outfall system could supply ∼3×10 7 g C/d of which 12–20% is estimated to have been deposited in the region. Hence, effluent-derived POC, although it may be a part of the total organic carbon pool undergoing diagenesis and burial on the sea floor, is not likely the only source of carbon to this system. Primary productivity in the surface ocean could supply 9–13×10 7 g C/d to the sea floor. A balanced carbon budget requires that half the primary production is exported to the sea floor, although this budget does not account for other sources of POC to the region.

Nature of sedimentary organic matter in the lower reaches of the Godavari River basin, India

The distribution and nature of sedimentary organic matter (OM) have been examined in sediment cores collected from the lower reaches, including estuary, of the Godavari River in order to understand sedimentation patterns, sources, and diagenesis of OM. The samples were analyzed for organic carbon (Corg), total nitrogen, amino acids and hexosamines. The observed irregular trends in Corg distribution with depth indicate the unstable nature of bed sediment in the lower reaches of the river. Yet, in the lower estuarine region, regular trends in Corg distribution with depth reflect the deposition of sediment. The atomic ratio of Corg and total nitrogen (C/N; 10.5–16.1) also supported this observation. The distribution of amino acids and diagenetic indicators (β-ala+γ-aba mol.%, AA/HA and Glc-NH2/Gal-NH2) in individual cores revealed the post-depositional changes in the OM. In the core sediment from the lower reaches, there was no clear cut trend in amino acid content with depth. In the estuarine region, however, amino acid content was very low (50.5 and 186.5μgg−1) in the upper layers compared to that in the lower layers (558.5 and 1099.3μgg−1). Reactivity index (range 0.3–3.7) revealed that OM in the upper few centimeters, especially in the lower estuarine region, was more reactive relative to that in the deeper layers.

A hypothesis for the origin of perylene based on its low abundance in sediments of Green Bay, Wisconsin

Perylene, a polycyclic aromatic hydrocarbon (PAH) that is common in sediments, is believed to originate principally from anaerobic diagenesis of organic matter, but its precursor material remains enigmatic. We have investigated the formation of perylene in a dated sediment core from Green Bay, WI. Comparisons of sedimentary profiles of perylene, anthropogenic PAHs, land-plant-derived hydrocarbons, and aquatic contributions of organic matter rule out a specific industrial, terrigenous, or aquatic precursor for perylene. Instead, perylene seems to be formed from any kind of organic matter as a consequence of a specific type of microbial activity in sub-bottom sediments. Despite an abundance of organic matter, the amount of perylene is low in Green Bay and in other organic-carbon-rich lake sediments. The abundant availability of organic matter that stimulates microbial activity in Green Bay sediments would seem to favor perylene formation in such sediments. We therefore hypothesize that the microorganisms responsible for perylene formation do not compete successfully with those that flourish in sediments rich in organic matter. Perylene formation consequently does not become significant until deeper in sediments after the more-reactive types of organic matter have been consumed.

Early diagenesis of organic matter from sediments of the eastern subtropical Atlantic: evidence from stable nitrogen and carbon isotopes

Stable isotopes of sedimentary nitrogen and organic carbon are widely used as proxy variables for biogeochemical parameters and processes in the water column. In order to investigate alterations of the primary isotopic signal by sedimentary diagenetic processes, we determined concentrations and isotopic compositions of inorganic nitrogen (IN), organic nitrogen (ON), total nitrogen (TN), and total organic carbon (TOC) on one short core recovered from sediments of the eastern subtropical Atlantic, between the Canary Islands and the Moroccan coast. Changes with depth in concentration and isotopic composition of the different fractions were related to early diagenetic conditions indicated by pore water concentrations of oxygen, nitrate, and ammonium. Additionally, the nature of the organic matter was investigated by Rock-Eval pyrolysis and microscopic analysis. A decrease in ON during aerobic organic matter degradation is accompanied by an increase of the 15N/ 14N ratio. Changes in the isotopic composition of ON can be described by Rayleigh fractionation kinetics which are probably related to microbial metabolism. The influence of IN depleted in 15N on the bulk sedimentary (TN) isotope signal increases due to organic matter degradation, compensating partly the isotopic changes in ON. In anoxic sediments, fixation of ammonium between clay lattices results in a decrease of stable nitrogen isotope ratio of IN and TN. Changes in the carbon isotopic composition of TOC have to be explained by Rayleigh fractionation in combination with different remineralization kinetics of organic compounds with different isotopic composition. We have found no evidence for preferential preservation of terrestrial organic carbon. Instead, both TOC and refractory organic carbon are dominated by marine organic matter. Refractory organic carbon is depleted in 13C compared to TOC.

Sulphate reduction and sulphur cycling in lake sediments: a review

1. The concentration of sulphate is low in lakes and sulphur cycling has often been neglected in studies of organic matter diagenesis in lake sediments. The cycling of sulphur is, however, both spatially and temporally dynamic and strongly influences many biogeochemical reactions in sediments, such as the binding of phosphorus. This review examines the control of sulphate reduction and sulphur cycling in sediments of lakes with different trophic status.2. The factors that control the rate of sulphate reduction have not been identified with certainty in the various environments because many factors are involved, e.g. oxygen and sulphate concentrations, temperature and organic matter availability.3. Sulphate reduction is less significant under oligotrophic conditions, where mineralization is dominated by oxic decomposition. The supply of organic matter may not be sufficient to support sulphate reduction in the anoxic parts of sediments and, also, sulphate availability may control the rate as the concentration is generally low in oligotrophic lakes.4. There is a potential for significant sulphate reduction in eutrophic lakes, as both the availability of organic matter and sulphate concentration are often higher than in oligotrophic lakes. Sulphate is rapidly depleted with sediment depth, however, and methanogenesis is generally the most important process in overall carbon mineralization. Sulphate reduction is generally low in acidic lakes because of low sulphate availability and reduced microbial activity.5. It is still unclear which of the forms of sulphur deposits are the most important and under which conditions burial occurs. Sulphur deposition is controlled by the rate of sulphate reduction and reoxidation. Reoxidation of sulphides occurs rapidly through several pathways, both under oxic and anoxic conditions. Only a few studies have been able to examine the importance of reoxidation, but it is hypothesized that most of the reoxidation takes place under anoxic conditions and that disproportionation is often involved. The presence of sulphide oxidizing bacteria, benthic fauna and rooted macrophytes may substantially enhance oxic reoxidation. Deposition of sulphur is generally higher in eutrophic than in oligotrophic lakes because of a number of factors: a higher rate of sulphate reduction, enhanced sedimentation of organic sulphur and less reoxidation as a result of reduced penetration of oxygen into the sediments, a lack of faunal activity and rooted macrophytes.

A distributed reactivity model for sorption by soils and sediments 13. Simulated diagenesis of natural sediment organic matter and its impact on sorption/desorption equilibria.

Subcritical water treatment was used to effect rapid compositional and functional changes to peat organic matter that mimic those of the natural diagenesis process. Elemental, solid state 13C NMR, FTIR, and calorimetry analyses all indicated that the organic matter of the artificially aged peat was chemically similar to that of geologically mature coal kerogens. This paper extends the work of the previous paper in this series, which investigated the effects of subcritical water treatment of humic topsoil on subsequent phenanthrene sorption and desorption equilibria. As opposed to the previous study, however, changes in sorptive reactivity herein were unequivocally related to changes in organic matter rather than other soil constituents, and organic matter functional changes due to the simulated diagenesis were more accurately characterized. Phenanthrene sorption capacity and isotherm nonlinearity both increased with increasing degrees of artificial aging, supporting the viewpoint that hydrophobic organic contaminant sorption equilibrium properties can be directly related to the degree of diagenesis of geosorbent organic matter. In addition, this work investigated effects of subcritical water treatment of a geologically mature, kerogen-containing shale sample. In contrast to the peat, the functional characteristics of the shale were unchanged by this treatment, and subsequent phenanthrene sorption equilibria were altered far less.

Transport and diagenesis of trace metals and organic matter in Palos Verdes shelf sediments affected by a wastewater outfall

Abstract Particles from the Whites Point/JWPCP outfalls operated by the Los Angeles County Sanitation District (LACSD) have been discharged onto the Palos Verdes (PV) shelf, Southern California, since the late 1930s. Since the early 1950s, they have made a significant contribution to the sedimentary deposits on the shelf. In order to study the transport and diagenesis of organic carbon (OC) and associated trace metals, replicate sediment cores were collected during 1996 and 1997 at four different sites at the ∼60 m isobath on the PV shelf, and analyzed for OC, Ag, Al, Cd, Cr, Cu, Mn, Ni, Pb, and Zn. We conclude from these results that a significant fraction of OC and associated heavy metals were transported laterally on silt particles from shallower environments. Cross-shelf transport of sediments caused multiple peaks in measured profiles of OC and trace metals at site 6C, 2 km away from the outfall. The same mechanism is likely to contribute to a concentration decrease that is smaller than that expected from decreases from the Whites Point outfall emissions. Based on Pb/OC ratios in sediments, deposited in 1971, and comparisons to the outfall from the same year, we estimate that 50±10% of the OC deposited in the early 1970s, now buried at 30–50 cm depth, had oxidized since that time, implying a half-life of about 26 years for the outfall-OC, as an upper limit. The average OC oxidation rate at peak depth (about 2 mg C cm −2 year −1 ) is, however, only about 10% of the present-day OC accumulation rate (20 mg C cm −2 year −1 ), which itself is adding not much more than 1% per year to the post-1950s OC inventory (∼1500 mg cm −2 ). We furthermore estimate that the OC inventory in PV shelf sediments in 1971 was equivalent to about 35% of that emitted by the outfall. OC and trace metal inventories did not decrease in the period 1981 to 1997, contrary to those of other contaminants such as DDTs and PCBs.

Origin of phosphatic stromatolites in the Upper Cretaceous condensed sequence of the Polish Jura Chain

The Turonian stromatolite-bearing condensed sequence in the Polish Jura Chain (the European epicontinental basin) provides good insight into the environment of formation of Cretaceous phosphatic stromatolites, owing to their purely phosphatic development and negligible post-depositional alteration. The sequence developed as a result of slow pelagic sedimentation and microbial mat phosphatization on a submarine swell surrounded by local basins with non-condensed carbonate deposition. Diagenesis of organic matter and dissolution of biogenic apatite were the major sources of reactive phosphorus for the microbial mat phosphatization. Stromatolite growth occurred due to pulses of amorphous or poorly ordered calcium phosphate precipitation followed by crystallization of carbonate fluorapatite (CFA). The phosphogenic environment left an imprint on the isotopic composition of limestone carbon and lattice-bound carbon and sulphur in CFA, and on the light rare-earth element (LREE) distribution in CFA. The δ13C of the stromatolite-bearing sequence shows a negative excursion (−1 to −3‰), standing in marked contrast to positive carbon values of the surrounding basinal carbonate. Most of the δ34S values of CFA (+20 to +21‰) fit the value range of the coeval seawater sulphate, and the LREE distribution shows a well-defined seawater pattern. This geochemical signature is indicative of intense diagenesis of organic matter at the seafloor, pelagic carbonate dissolution, and prolonged exposure of the deposited phosphate towards the water column. The enhanced deposition and diagenesis of organic phosphorus in the stromatolitic environment reflects elevated levels of the epicontinental basin nutrification related to sea-level rises and the associated oceanographic and geochemical changes.

Sources and diagenesis of organic matter in tidal flat sediments from the German Wadden Sea

The sources and diagenesis of organic matter in a sediment core from the Swinnplate backbarrier area near Spiekeroog Island in the northwest German Wadden Sea have been examined using stable carbon isotopes, 14 C -ages and lipid biomarker data. Twenty-two core sections were analysed from the surface to a depth of 90 cm, representing sedimentation over the past approximately 200 years. Total organic carbon (TOC) contents were highly variable (0.1–1%), as was the grain size with some core sections containing up to 50% of clay and silt (mud fraction). These data indicate a highly variable depositional regime in which organic matter is extensively degraded both before and after incorporation into the sediments. The TOC content was strongly correlated with the abundance of the mud fraction, indicating the importance of organic matter sorption onto particles for preservation of both marine and terrestrial organic matter. Sediments near the top of the core were enriched in marine organic matter, but terrestrial organic matter predominated in most core sections. Some samples showed higher TOC contents than might be predicted from the TOC-grain size relationship. Isotope and biomarker studies showed that these contained additional terrestrial organic matter from peats, possibly eroded from areas to the west of the investigated area. The organic matter in these layers had the lightest values of δ 13 C (about −26‰ compared with a more typical mixed marine-terrestrial value of −24‰). Most of the n-alkane distributions show a strong predominance of odd-carbon-number alkanes typical of the distributions found in higher plant waxes. All core sections contained abundant long-chain alcohols and triterpenoid alcohols such as α-amyrin, β-amyrin, lupeol, taraxerol, taraxerone and friedelin from higher plants. The dihydroxy triterpenoid betulin was particularly abundant confirming that eroded peats are a major source of the lipids. Further confirmation was obtained from AMS-determined 14 C -ages of fatty acid and neutral lipid fractions from 46.5 to 60 cm deep core sections which were 5570 and 6240 yr BP, respectively. These data attest to the importance of recycled older organic material (eroded peat) in the carbon cycle of this coastal environment.

Density-driven convection during flooding of warm, permeable intertidal sediments: the ecological importance of the convective turnover pump

Vertical temperature profiles during exposure and flooding of a sandy tidal flat were investigated at a single site in the Sado estuary (Portugal), and compared with the change in benthic nitrogen pools. Data from April 1994 (night and day flood periods), July and November 1994 were analysed in order to study possible convective transport induced by the changing thermal regime on a tidal time scale. An ecological consequence of the flooding of warm, permeable sediment beds by cool water was the reversal of porewater density gradients, leading to a quick exchange of porewater for flood water, in what constitutes a rapid, powerful pathway for benthic solute removal in tidal ecosystems. The matrix-averaged interstitial velocity of up- and down-draught plumes of water ranged between 10 −7 and 10 −6 m s −1 over a depth scale of 6–10 cm. The Peclet number ranged from 1.4 to 28 in heavily bioturbated environments ( D Bs for N solutes=5×10 −9 m 2 s −1 ), and from 70 to 1400 in non-bioturbated environments ( D Bs for N solutes ≈10 −10 m 2 s −1). The results indicate that convective turnover of porewaters in permeable tidal flats is abrupt, may occur with daily frequency, and may have three orders of magnitude more impact on sediment-water fluxes than diffusion alone. Convective flow is a major component of sediment-water fluxes in tidal areas and crucial to accurate budget studies on the sandy intertidal. On the basis of these results and recent literature, the potential ecological importance of the Convective Turnover Pump is discussed, showing it to be a very powerful potential accelerator of organic matter diagenesis in tidal systems due to its frequency and range of action.

Organic matter diagenesis at the oxic/anoxic interface in coastal marine sediments, with emphasis on the role of burrowing animals

The present paper reviews the current knowledge on diagenetic carbon transformations at the oxic/anoxic interface in coastal marine sediments. Oxygen microelectrodes have revealed that most coastal sediments are covered only by a thin oxic surface layer. The penetration depth of oxygen into sediments is controlled by the balance between downward transport and consumption processes. Consumption of oxygen is directly or indirectly caused by respiration of benthic organisms. Aerobic organisms have the enzymatic capacity for complete oxidation of organic carbon. Anaerobic decay occurs stepwise, involving several types of bacteria. Large organic molecules are first fermented into small moieties. These are then oxidized completely by anaerobic respirers using a sequence of electron acceptors: Mn4+, NO3-, Fe3+, SO42- and CO2. The quantitative role of each electron acceptor depends on the sediment type and water depth. Since most of the sediment oxygen uptake is due to reoxidation of reduced metabolites, aerobic respiration is of limited importance. It has been suggested that sediments contain three major organic fractions: (1) fresh material that is oxidized regardless of oxygen conditions; (2) oxygen sensitive material that is only degraded in the presence of oxygen; and (3) totally refractory organic matter. Processes occurring at the oxic/anoxic boundaries are controlled by a number of factors. The most important are: (1) temperature, (2) organic supply, (3) light, (4) water currents, and (5) bioturbation. The role of bioturbation is important because the infauna creates a three-dimensional mosaic of oxic/anoxic interfaces in sediments. The volume of oxic burrow walls may be several times the volume of oxic surface sediment. The infauna increases the capacity, but not the overall organic matter decay in sediments, thus decreasing the pool of reactive organic matter. The increase in decay capacity is partly caused by injection of oxygen into the sediment, and thereby enhancing the decay of old, oxygen sensitive organic matter several fold. Finally, some future research directions to improve our understanding of diagenetic processes at the oxic/anoxic interface are suggested.

Soluble humic substances from in situ precipitated microcrystalline calcium carbonate, internal sediment, and spar cement in a Cretaceous carbonate mud-mound

Albian carbonate mud-mound limestones exposed near Iraneta, northern Spain, show a fabric- and particle-specific fluorescence. Intense fluorescence is restricted to in situ precipitated microcrystalline (automicritic) fabrics, calcified demosponges, and coralline sponges. Intermediate intensity derives from micritized bioclasts, pellets, and a rim of marine bladed cement. Most invertebrate skeletons, late-diagenetic equant cement, and crosscutting zones of dolomitization are weakly to nonfluorescent. Internal microcrystalline sediment (allomicrite) and red algae debris have variable fluorescence. Correlation between rock fluorescence and soluble humic substances was evaluated from 3 g of automicrite, allomicrite, and cement. Time-resolved laser-induced fluorescence spectroscopy (TRLFS) with ultra-short pulses on two extracrystalline fractions (NaOH-soluble) and two intracrystalline fractions (HCl-soluble and NaOH-soluble) showed that most of the soluble humic substances of automicrite are within the crystals; but conversely, are significantly enriched on outer surfaces of allomicrite. Spar cement is close to detection limits. Fluorescence lifetimes are in the range of 0.5–2 ns and 3.5–6 ns. We conclude that precipitation of automicrite took place during oxidative organic matter diagenesis, i.e., during condensation reactions of degradation products of marine biopolymers. By contrast, allomicrite formed by skeletal breakdown followed by ingestion, organic coating, and reingestion during deposit feeding. A humic-substance–based model of marine polymer gels represents a new approach for the understanding of ancient polygenetic carbonate muds, so typical of Phanerozoic mud-mounds in deeper water settings.

Organic matter diagenesis in sediments on the continental shelf and slope of the Eastern Tropical and temperate North Pacific

Carbon and nitrogen mineralization were determined in sediments from about 100 to 1000 m depth on the continental shelf and slope off the Pacific coast of Mexico and Washington State. Vertical down core profiles of carbon mineralization were measured both as sulfate reduction using short-term 35 S incubations and as net TCO 2 production using long-term anaerobic incubations (`jar’ technique). Nitrogen mineralization was measured as net NH + 4 production using only the latter technique. The down core attenuation of jar rates was described adequately by a double exponential decay model. By subtraction of 35 S rates from jar rates in the upper 3 cm of the sediment, oxic plus suboxic respiration were roughly estimated to account for at least 5–28% (Mexico) and 10–19% (Washington) of the 0–30 cm integrated carbon mineralization. The agreement between measured porewater profiles of TCO 2 and NH + 4 and those estimated by diagenetic modelling using rates obtained from jar incubations in the oxygen deficient zone off Mexico indicated no infaunal irrigation. At all other stations the benthic fauna appeared responsible for considerably lower measured porewater concentrations than predicted by a simple diffusion-based diagenetic model. Total carbon mineralization in sediments off Mexico was reduced 40% with water depth from 100–200 m to 1000 m, while the reduction in the same depth interval off Washington was about 80%. Water depth attenuation of total sediment nitrogen mineralization at the two locations, on the other hand, was almost similar with a reduction of 70–80%. The overall molar C : N ratio of mineralization products was generally higher off Washington than in the oxygen deficient zone off Mexico. It is suggested that the difference in sediment mineralization patterns are associated with changes in water column processes related to the different strength of the oxygen minimum at the two locations.

A Distributed Reactivity Model for Sorption by Soils and Sediments. 12. Effects of Subcritical Water Extraction and Alterations of Soil Organic Matter on Sorption Equilibria

Subcritical water extraction was used as a tool to remove the carboxylic, aliphatic, and carbohydrate types of organic carbon from a humic soil. The rates and extents of soil organic carbon removal were quantified as functions of superheated water temperature, phase, and exposure time. The experimental data suggest that superheated water effects deoxygenation/aromatization reactions of soil organic matter that mimic those of geologically slow, natural diagenesis processes. Phenanthrene sorption and desorption equilibrium isotherms for the altered soils were measured. The sorption isotherms were characterized by increasing capacity and nonlinearity with increasing degree of polar functionality removal and simulated diagenesis of the soil organic matter.

Diagenesis of planktonic fatty acids and sterols in Long Island Sound sediments: Influences of a phytoplankton bloom and bottom water oxygen content

Diagenesis of organic matter in coastal sediments from Long Island Sound (LIS) was investigated by measuring fatty acids and sterols in (1) a time-series of surface sediment samples over a spring phytoplankton bloom; and (2) sediment cores collected during and after a bloom at two sites with distinctively different bottom-water oxygen contents. Time-dependent distributions of sedimentary fatty acids and sterols in LIS were strongly affected by pulsed inputs from the overlying water column, variations in benthic community, and redox-related degradation processes. The phytoplankton bloom delivered an intense pulse of unsaturated fatty acids (e.g., 16:1(ω7) and 20:5) to the surface sediments. Continuous increases of cholesterol and diunsaturated sterols after the bloom were related to zooplankton grazing processes and increase in benthic faunal abundance. High inventories of planktonic fatty acids and sterols in the upper 5 cm sediments were observed at the low oxygen site during summer, probably caused by a combination of higher input, reduced degradation rates and lower macrofaunal activity under anoxic conditions compared to oxic conditions.

Molecular taphonomy of arthropod and plant cuticles from the Carboniferous of North America: implications for the origin of kerogen

Analyses of identifiable organic fossil remains of animals and plants have considerable potential to resolve conflicting models of organic matter diagenesis and kerogen formation (e.g. selective preservation versus random polymerization). Fossil cuticles of arthropods (scorpion, eurypterid) and plants (cordaite, pteridosperm) from Upper Carboniferous strata of Lone Star Lake, Kansas, USA and Joggins, Nova Scotia, Canada were analysed by pyrolysis–gas chromatography/mass spectrometry and examined by electron microscopy. Recent Pandinus (scorpion) and Araucaria (conifer) provided a basis for comparison. Pyrolysis of Recent dewaxed scorpion cuticle yielded products derived from chitin and proteins. These products were absent in the fossil arthropod cuticles, however, which yielded an homologous series of alkanes and alkenes, together with phenolic and other aromatic constituents. Recent dewaxed plant cuticle yielded fatty acids, phenols and carbohydrate-derived compounds indicative of cutin polyester and associated lignocellulose. The pyrolysates of the fossil plant cuticles, on the other hand, were dominated by alkane–alkene doublets, with minor phenolic and other benzenoid components. There is no evidence that the preservation of these cuticles as particulate organic matter in kerogen is simply a result of selective preservation. Nonetheless, the chemistry and morphology remain characteristic of a particular taxon, thereby eliminating the possibility of incorporation of randomly repolymerized materials or the transfer of material between plant and animal residues. The aliphatic moieties in the fossil cuticles are thought to be the result of polymerization of the associated epicuticular, cuticular and/or tissue lipids during diagenesis.

Total and hydrolizable particulate organic matter (carbohydrates, proteins and lipids) at a coastal station in Terra Nova Bay (Ross Sea, Antarctica)

We analysed quantity and quality of particulate organic matter during the austral summer 1994/1995 at a coastal station in Terra Nova Bay (Ross Sea, Antarctica). Our main aims were to investigate the origin and biochemical composition of particulate organic matter (POM), to measure its availability for consumers through the study of its digestible fraction (measured by using different enzymes separately) and to highlight the role of hydrolizable compounds in the organic matter diagenesis in the coastal waters at Terra Nova Bay. Temporal and spatial patterns of chlorophyll-a concentrations were reflected by the particulate organic carbon, nitrogen and total biopolymeric carbon concentrations, suggesting that most POM originated directly from phytoplankton. The most evident feature of POM in the coastal waters at Terra Nova Bay was the dominance of proteins (on average 57% of total biopolymeric particulate carbon), followed by carbohydrates (25%) and lipids (18%). We found that about 30% of the refractory particulate organic carbon (assumed to be present only after the complete exploitation of particulate organic nitrogen) did not originate from biopolymeric carbon (as sum of carbohydrate, protein and lipid carbon). This allows us to suggest the use of the digestible fraction of particulate biopolymeric carbon as a more accurate measure of the food availability of POM for consumers. In Terra Nova Bay coastal waters, most of the particulate protein pool was associated with large phytoplankton cells or phytodetritus. As a result, the protein pool appeared less available (i.e. less digestible) than the one present in oligotrophic waters where, conversely, most particulate organic nitrogen is sequestered into bacteria. The relative low availability of the protein pool, together with the rapid sinking of POM and the low remineralization rates of benthic heterotrophic microbes, are suggested as possible factors in determining the “inefficiency” in organic matter recycling of coastal waters at Terra Nova Bay, which behaves as a “loss type” system.

The effect of the Water Column Oxygen Minimum Zone on Sedimentary Organic Matter Diagenesis

Early organic matter diagenesis in continental margin sediments is fueled by the rain of carbon from the surface layer. Previous workers have found that sediment trap carbon fluxes are proportional to primary productivity and decrease as a power function of water column depth, e.g. Flux = Flux o x depth -~ Martin, et al., 1987). The water column decrease results from progressively more organic matter oxidation with time as particles settle. As sediment trap flux is a measure of the carbon rain rate at that depth, a similar depth relationship would be expected for sedimentary rate processes. To test this hypothesis, we have occupied two transects: one off the coast of northwest Mexico (~20 ~ N) and a second off the northwest coast of Washington State, U.S.A. (-47 ~ N: Devol and Christensen, 1994). Off Mexico, the sediments in the 150-600 m depth interval are in contact with the oxygen deficient waters of the eastern tropical North Pacific, while off Washington State oxygen is present in the water column at all depths.

Early Diagenesis of Organic Matter from Higher Plants in a Malagasy Peaty Marsh. Application to Environmental Reconstruction During the Sub-Atlantic

The purposes of the present work were to examinate the morphological features and chemical composition of organic matter (OM) from a peaty marsh, in order to derive information on biological sources and on the changes in depositional conditions that occurred during the last 2300 years. The studied site, Tritrivakely, is a maar (a former volcanic crater lake) about 600 m wide and 50 m deep, located on Ankaratra Plateau. The maar is presently occupied by a peaty marsh with abundant Cyperaceae. The waterbody is shallow, fluctuating from 0 to 2 m. The study was concerned with the uppermost meter of peat which has accumulated for 2300 years B.P. in the maar. The Cyperaceae living in the marsh were also studied for comparison.