Decomposition Of Sedimentary Organic Matter Research Articles

Extreme13 C enrichments in a shallow hypereutrophic lake: Implications for carbon cycling

An analysis of stable carbon isotope (δ13C) ratios in Lake Apopka, Florida, reveals extreme 13C enrichments of dissolved inorganic carbon (DIC) pools in the water column and sediment pore water of this shallow polymictic and hypereutrophic lake. The sediment pore water had high averageδ13C of DIC (26.4‰), DIC (8.37 mmol L−1), and methane (CH4) concentrations (1.23 mmol L−1). The extreme 13C enrichment in the sediment pore‐water DIC pool is attributed to methanogenesis, which produces 13C‐rich carbon dioxide (CO2) and 13C‐poor CH4 during the bacterial fermentation of organic matter. The δ13C in the water‐column DIC pool ranged from 5‰ to 13‰ with an average of 9.0‰. The flux‐weighted δ13C from the DIC due to external loading and sediment respiration was estimated as −12‰, whereas the δ13C from particulate organic carbon (POC) due to water‐column production was −13‰. The 13C enrichment in the water column is attributed directly to the diffusion and advection of isotopically heavy DIC from the sediment and to the isotopic fractionation by phytoplankton photosynthesis and is attributed indirectly to the removal of isotopically light CH4 by ebullition and organic matter by sedimentation and outflow. Atmospheric invasion and sedimentation were the most important source and sink, respectively, in the carbon mass balance. CH4 oxidation, atmospheric invasion, anaerobic respiration, and sedimentation are the important flux terms affecting the isotopic mass balance. A combination of shallow water depth, frequent wind mixing, anoxic sediments with high rates of methanogenesis, high phytoplankton productivity, and lack of external loading dominated by terrestrial carbon led to the 13C enrichment of the water‐column DIC pool in Lake Apopka.

Nutrient fluxes and sulfur cycling in the organic-rich sediment of Makirina Bay (Central Dalmatia, Croatia)

Fluxes of dissolved nutrients in the organic-rich sediment of Makirina Bay (Croatia) were estimated in in-situ incubation experiments and compared to those calculated from a diagenetic model based on the concentration vs. depth profiles in the pore water extracted from a sediment core at the same sampling site. Benthic fluxes observed in the in-situ experiment were strongly influenced by the decomposition of sedimentary organic matter by bacterial sulfate reduction and precipitation of authigenic pyrite and calcite. Assimilation of dissolved nitrate and ammonia within the sediment resulted in peculiar nitrate and ammonia concentration profiles. Differences between the benthic and diffusive fluxes indicated that the effects of bioturbation and irrigation, which were not taken into account in the model calculation of diffusive fluxes, should not be considered as negligible. They are, however, extremely difficult to estimate because of the heterogeneity of the sediment. Sulfate reduction in the pore water could not be detected, although it was obvious from pyrite precipitation and the isotopic compositions of different sedimentary sulfur species. It is suggested that reoxidation of sulfide and upward diffusion of sulfate from the sediment layer accumulated during past salt production can account for the scattered sulfate vs. depth profile. The distribution and stable isotope compositions of sedimentary sulfur species were studied in more detail. The influx of sulfate was explained by rapid sulfate reduction in the sediment. The sulfate reduction rate was highest in the upper part of the sediment column, as concluded from δ 34S values. In lower horizons, the concentration of organic sulfur increased, while concentrations of sulfide decreased. The mirror-imaged concentration vs. depth profiles of sulfide and organic sulfur, as well as their stable isotopic compositions, show that they were formed simultaneously, most probably in competitive processes.

Methane generation from coal during open system pyrolysis investigated by isotope specific, Gaussian distributed reaction kinetics

A reaction kinetic model is presented to differentiate individual complexes of reactions of the thermal decomposition of sedimentary organic matter during pyrolysis experiments. This model is applied to methane generation from coal during open system pyrolysis experiments with linear heating. The reaction kinetic model is based on the concept of a certain number of reaction complexes contributing to the generation of the product methane, each reaction complex distinguished by an initial pool of precursor sites with a defined stable carbon isotope signature and a common reaction rate determining step. For each reaction the mathematical model defines Gaussian distributed activation energies, a single pre-exponential factor, an initial isotope ratio of the precursor and a single difference in activation energy between the heavy and the light isotope species. Based on this concept, stable carbon isotopes and the dynamics of thermal methane generation from a Carboniferous coal with 0.72% vitrinite reflectance are explained with a set of four independent reactions. It is suggested that these reactions represent the following pathways (listed in the order of occurrence): (I) reactions involving the cleavage of thermally unstable C–O and C–S bonds, (II) demethylation, (III) release of groups which cross link ring structures and/or secondary cracking of long chain hydrocarbons within the molecular network of the coal, and (IV) polymerisation and condensation reactions. Reactions II and III contribute the major part to total methane potential. However, the addition of the minor reactions I and IV has significant influence on the stable carbon isotope trend of methane.

Sediment Quality in Arkansas Bait Minnow Ponds

AbstractSediment cores were collected from 7‐yr‐old, 20‐ to 25‐yr‐old, and 30‐ to 35‐yr‐old ponds at a bait minnow farm at Lonoke, Arkansas, USA. Average depths of soft sediment (S and M horizons) were 8 cm in young ponds, 12 em in intermediate‐age ponds, and 26 cm in old ponds. Organic carbon concentrations in sediment were low to moderate (1–2%) and carbon to nitrogen ratios were wide (20–50). Phosphorus and sulfur concentrations increased as ponds aged. Most of the phosphorus (78.9%) was in organic form, but sulfur was primarily inorganic in form (presumably iron sulfide). There appears to be two major problems associated with sediment accumulation over time. Deep, soft sediment interferes with pond management and especially with harvest. High phosphorus concentration in old sediment may contribute to dense phytoplankton blooms by supplying phosphorus to the water. Sodium nitrate treatment did not increase the rate of sediment organic matter decomposition in laboratory trials and would not be expected to enhance the degradation of sediment organic matter in ponds. The best method for improving the condition of bottoms in older bait minnow ponds probably is to remove the sediment.

Late Generation of Methane from Mature Kerogens

The thermal decomposition of sedimentary organic matter, or kerogen, within the metagenesis zone (T > 160 °C) leads to the formation of large amounts of late gas, mainly composed by methane. With the purpose of understanding and quantifying the mechanisms of late methane generation, artificial maturation experiments were performed in closed system on natural samples of type II and type III mature kerogens (Ro > 1.3%, H/C < 0.65). For each experiment, mass and atomic (C, H, and O) balances were obtained by recovering, fractionating and quantifying the entire pyrolysis effluents. These data were interpreted by means of a kinetic schema based on three bulk reactions: one corresponding to the decomposition of short chain alkylated polyaromatics and diarenylalkanes that constitute partly mature kerogens, another one related to demethylation processes, a last one based on the autohydrogenation of the remaining pyrolysis residue. Kinetic parameters derived from these experiments confirmed that late methane is g...

Decomposition of sedimentary organic matter and methane formation in the recent sediment of Lake Bled (Slovenia)

Decomposition of sedimentary organic matter and methane formation in a eutrophic dimictic subalpine lake (Lake Bled, NW Slovenia) were investigated by measurement of in situ concentrations of dissolved inorganic carbon (DIC) in the pore water and its stable carbon isotopic composition, as well as by incubation experiments. The isotopic composition of carbon in the methane produced during incubation experiments was also studied. The ratio between methane produced by acetate fermentation and CO2/H2 reduction was estimated from δ13C-DIC vs. DIC concentration profiles in sediment pore water using a mathematical model developed by LaZerte [LaZerte, 1981. The relationship between dissolved carbon dioxide and its stable isotope ratio in aquatic sediments. Geochim. Cosmochim. Acta 45, 647–656]. It was found that in the spring acetate fermentation is dominant when about 65% of methane is produced by this reaction pathway, whereas in the autumn, over 90% of the methane results from CO2 reduction. δ13C of methane released during a 6-week incubation experiment with sediment taken in the autumn varied between −70 to −85‰ V-PDB, i.e., values typical of methane produced by CO2/H2 reduction, in spite of methane oxidation evident from the low δ13C-CO2 values, thus confirming the results obtained by mathematical modelling.

The influence of stand age on benthic decomposition and recycling of organic matter in managed mangrove forests of Malaysia

Decomposition of sediment organic matter was examined in relation to forest age in 2-, 15- and 60-year old, managed Rhizophora apiculata (Blume) stands in the Matang Mangrove Forest Reserve of peninsular Malaysia. Rates of O 2 consumption (range: 11.5–21.4 mmol m −2 d −1) and CO 2 production (range: 8.9–20.9 mmol m −2 d −1) were equivalent among the forests indicating that early diagenesis is not linked to stand age and age-related differences in rates of forest production. There were, however, site differences in the dominance of specific diagenetic pathways. Rates of sulfate reduction (to 40 cm depth) averaged 8.9±3.1 mmol S m −2 d −1 and 7.2±0.3 mmol S m −2 d −1 in the 15- and 60-year old forests, respectively, accounting for most (75–125%) of the total mineralization. In contrast, sulfate reduction (3.0±0.5 mmol S m −2 d −1) constituted a considerably smaller proportion (42%) of total organic matter oxidation at the 2-year old forest. Rates of solute efflux across the sediment-water interface and vertical profiles of dissolved Mn and NO 2 −+NO 3 − suggest that manganese reduction and denitrification–nitrification, coupled with aerobic respiration, account for most oxidation of organic matter at the 2-year old forest. The loss of particulate organic matter and the increased importance of aerobic and suboxic processes in the 2 year-old forest suggest some impact from disturbance of tree removal. A shift to proportionally less sulfate reduction in sediments of regenerating forests may result in greater availability of dissolved nutrients and some trace metals, and serve to reduce exposure of R. apiculata seedlings to anoxic, toxic solutes (e.g., sulfides). This diagenetic shift may facilitate rapid seedling growth and regeneration of forests.

Porewater geochemistry and solute flux from bottom sediments, Devils Lake, North Dakota

Sediment cores were collected for pore-water analysis from the eastern end of Devils Lake, located in northeastern North Dakota, to determine diagenetic reactions occurring in surficial bottom sediments and to evaluate the impact of these reactions on chemical concentrations in the overlying lake water. Sediment pore waters are enriched in major ions and nutrients relative to lake water. The principal sources of major ions to pore water are saline sediments located in the upper 1 m of bottom sediment. The principal source of titration alkalinity and nutrients to pore water is microbial decomposition of sedimentary organic matter by sulfate reduction. Sediment pore waters in the eastern part of Devils Lake have higher major-ion concentrations and solute-flux rates than the sediment pore waters in the central part of the lake. In contrast, sediment pore waters in the central part of Devils Lake have significantly higher nutrient concentrations and solute-flux rates. Major-ion concentrations and solute-flux rates in sediment pore water increase from west to east. These trends indicate that bottom-sediment diagenetic processes are, in part, responsible for the observed concentration gradient in the lake. The higher nutrient concentrations and the higher nutrient diffusional-flux rates in Main Bay are the result of more labile sedimentary organic matter and the occurrence of sulfate reduction. Environmentally-reactive trace-metal concentrations (Cu, Pb, Zn, and Fe) in bottom sediments decrease from west to east with distance from the surface-water sources and with increasing surface-water salinity.

Nutrient Transformations in Sediments as Influenced by Oxygen Supply

AbstractThe oxidation status of sediments greatly affects biogeochemical processes. The objective of this study was to determine the influence of various levels of O2 on nutrient transformations in lake sediments. Decomposition of sediment organic matter was evaluated under varying levels of O2. Bulk sediment samples obtained from a hypereutrophic lake in central Florida were incubated in stirred microcosms at 25°C for 100 d at four O2 levels. The O2 levels were accomplished by bubbling N2 containing 0, 0.2, 2.0, and 20.0% O2 through the stirred suspensions. Redox potential (Eh) measurements indicated that the two highest O2 levels (i.e., 2.0 and 20.0% O2) resulted in oxidizing conditions (Eh &gt; 400 mV), whereas the lowest O2 levels (0 and 0.2% O2) resulted in reduced conditions (Eh &lt; −300 mV). First‐order rate constants calculated based on CO2 evolution were 0.00010, 0.00027, 0.00062, and 0.00068 d−1 for the 0, 0.20, 2.0, and 20.0% O2 levels, respectively. Methane was detected in the sediments incubated under 0 and 0.2% O2 level, with CO2/CH4 ratios of 3.5 and 160, respectively. Accumulation of soluble organic C (SOC) was greater under reduced conditions than oxidized conditions. Ammonium concentrations decreased rapidly in the oxidized treatments, with concurrent increases in NO3‐N concentrations, whereas under reducing conditions NH4‐N concentrations slowly increased. Rates of N mineralization were slightly higher under oxidized conditions. Although the ratio of N mineralized to CO2 evolved was higher at low O2 levels, the amount of N mineralized vs. the sum of C metabolites (CO2 + CH4 + SOC) was relatively constant under all O2 levels (6.7:1). Reducing conditions resulted in a depletion of SO2−4, whereas under oxidizing conditions SO2−4 concentrations increased, probably due to sulfide oxidation. Soluble reactive P concentrations were lower under oxidizing conditions.