Biogeochemical cycling in an organic-rich coastal marine basin. 6. Temporal and spatial variations in sulfate reduction rates

Rates of sulfate reduction were measured over a 3 year period in the anoxic nearshore sediments of Cape Lookout Bight, North Carolina, using both a tube incubation method and a ³⁵S-sulfate direct injection technique. The methods yielded similar depth-integrated rates over the upper 30 cm ranging from less than 10 mol SO⁼₄ · m⁻² · y⁻¹ in winter to greater than 50 mol SO⁼₄ · m⁻² · y⁻¹ in summer. There were also seasonal changes in the Arrhenius activation energies for the sulfate reduction rates indicating that the assumption that E_a is constant with temperature is not always valid. The time averaged annual turnover rate for all three years was 20.4 (±11.4) mol SO⁼₄ · m⁻² · y⁻¹. Surface rates ranged seasonally from less than 0.01 to over 3 mM SO⁼₄ · d⁻¹ between winter and summer, respectively. A subsurface rate maximum was observed to develop during the summer months which accounted for 28 percent of the annual depth integrated sulfate reduction rate. Subsurface rate maxima are the result of changes in the chemistry (substrate type and/or concentration) and the microbiology in the sediments. The possibility of the subsurface maximum being an artifact of the ³⁵S method is also discussed. However, the sulfate reduction rates compare well with previous measurements of the carbon sediment-water plus burial fluxes and with a depth integrated CO₂ production rate modelled from a ΣCO₂ concentration profile from the same site.     

Biogeochemical cycling in an organic-rich coastal marine basin—3. Dissolved gas transport in methane-saturated sediments

The transport of dissolved gases in the anoxic sediments of Cape Lookout Bight, North Carolina, is controlled by diffusion and bubble ebullition and exhibits a distinct seasonal cycle. Detailed seasonal profiles of CH₄, N₂ and ²²²Rn and direct gas flux measurements indicate that ebullition dominates the flux of all dissolved gases across the sediment-water interface during summer months, and is of major importance on an annual basis. Transport within the upper 6–8 cm of sediment appears to be controlled by molecular diffusion of gases. Transport at greater depths is controlled by diffusion in winter and ebullition in summer. Rn-222 profiles were used to estimate the rate of stripping of dissolved gases within the CH₄ production zone (10–30 cm); rates averaged 3–5 percent per day and agreed with estimates derived from N₂ profiles. As a result of summer ebullition, the sediments of the bight are never at saturation with respect to the major gases present in seawater. Evidence from other sites suggests that ebullition may be an important transport process in many coastal sediments, and may provide mechanism for the transport of volatile reduced compounds between anoxic sediments and the atmosphere. ²²²Rn is a useful tracer for quantifying this transport.     

Biogeochemical cycling in an organic-rich coastal marine basin—I. Methane sediment-water exchange processes

Methane produced in anoxic organic-rich sediments of Cape Lookout Bight, North Carolina, enters the water column via two seasonally dependent mechanisms: diffusion and bubble ebullition. Diffusive transport measured in situ with benthic chambers averages 49 and 163 μmol · m ^?2 · hr ^?1 during November–May and June–October respectively. High summer sediment methane production causes saturation concentrations and formation of bubbles near the sediment-water interface. Subsequent bubble ebullition is triggered by low-tide hydrostatic pressure release. June–October sediment-water gas fluxes at the surface average 411 ml (377 ml STP: 16.8 mmol) · m^?2 per low tide. Bubbling maintains open bubble tubes which apparently enhance diffusive transport. When tubes are present, apparent sediment diffusivities are 1.2–3.1-fold higher than theoretical molecular values reaching a peak value of 5.2 × 10^?5 cm² · sec^?1. Dissolution of 15% of the rising bubble flux containing 86% methane supplies 170μmol · m^?2 · hr^?1 of methane to the bight water column during summer months; the remainder is lost to the troposphere. Bottom water methane concentration increases observed during bubbling can be predicted using a 5–15 μm stagnant boundary layer dissolution model. Advective transport to surrounding waters is the major dissolved methane sink: aerobic oxidation and diffusive atmospheric evasion losses are minor within the bight.     

Biogeochemical cycling in an organic-rich coastal marine basin: 9. Sources and accumulation rates of vascular plant-derived organic material

The sources, degradation and burial of vascular plant debris deposited over the past several decades in the lagoonal sediments of Cape Lookout Bight, North Carolina, are quantified using alkaline cupric oxide lignin oxidation product (LOP) analysis. Non-woody angiosperms, accounting for 92 ± 32% of the recognizable sedimentary vascular plant debris, are calculated to contribute 23 ± 17% of the total organic carbon buried over the past decade (upper meter of sediment column). When combined with a previously established sedimentary organic carbon budget for this site (Martens and Klump, 1984; Martens et al., 1987, in preparation) a vascular plant derived carbon burial rate of 26 ± 20 mole C m⁻² yr⁻¹ is calculated for this same time interval. The refractory nature and invariant depth distributions of the lignin oxidation products (LOP), when coupled with evidence for constant degradation rates of metabolizable materials, indicate that sediment accumulation at this site has been a steady state process with respect to source and burial of organic carbon since its conversion from an inner-continental shelf to a lagoonal environment during the late 1960's. Thus systematic down-core decreases in labile organic matter result from early diagenetic processes rather than input rate variations.     

Biogeochemical cycling in an organic rich coastal marine basin—II. Nutrient sediment-water exchange processes

The release of remineralized N and P from the organic-rich anoxic sediments of Cape Lookout Bight is controlled by processes occurring within the sediment column and at the sediment-water interface. The relatively rapid rates of temperature dependent microbial degradation of organic matter support seasonally varying nutrient fluxes ranging from 20 to 1200 μmol·m^?2·hr^?1 for dissolved ammonium and from ? 20 to 120 μmol·m^?2·hr^?1 for total dissolved phosphate (measured in situ over the period October, 1976 to October, 1978). Molecular diffusion along steep vertical pore water concentration gradients measured simultaneously cannot explain the high fluxes observed during warmer months. Gradients for ammonium and phosphate ranged from 0.33 to 1.10 and from 0 to 0.29 μmol·cm^?3_pw·cm^?1_s respectively. These high summertime fluxes appear to result from increased sediment-water transport associated with bubble tubes created and maintained by low-tide methane gas bubble ebullition. When these tubes are present, apparent bulk sediment diffusivities calculated from concurrent studies of methane and radon-222 sediment-water exchange are 1.0–3.1 times greater than molecular diffusivities. Nutrient fluxes calculated via Fick's first law taking into account this enhanced transport and the differential diffusive mobilities of dissolved ammonium, phosphate and phosphate ion pairs indicate that removal by aerobic adsorption and/or biological uptake at the sediment-water interface plays an important role in controlling nutrient exchange in these sediments.     

Sulfate reduction and methanogenesis in marine sediments

Methanogenesis and sulfate-reduction were followed in laboratory incubations of sediments taken from tropical seagrass beds. Methanogenesis and sulfate-reduction occurred simultaneously in sediments incubated under N₂, thereby indicating that the two processes are not mutually exclusive. Sediments incubated under an atmosphere of H₂ developed negative pressures due to the oxidation of H₂ by sulfate-respiring bacteria. H₂ also stimulated methanogenesis, but methanogenic bacteria could not compete for H₂ with the sulfate-respiring bacteria.     

Biogeochemical cycling in an organic-rich coastal marine basin. 5. Sedimentary nitrogen and phosphorus budgets based upon kinetic models,mass balances,and the stoichiometry of nutrient regeneration

The rapid rates of sediment accumulation (~ 10–20 cm/yr) in the recently formed Cape Lookout Bight, North Carolina, have resulted in the deposition of approximately 157 moles of carbon, 14 moles of nitrogen and 1.3 moles of phosphorus, per square meter annually. The metabolism of the organic matter in these anoxic sediments is dominated by sulfate reduction and fermentation reactions. Sedimentary nitrogen and phosphorus budgets are estimated using 3 related approaches: 1) a kinetic model of solid phase diagenesis; 2) direct measurements of nutrient burial and regeneration; and 3) nutrient recycling rates estimated from annual rates of sulfate reduction and the SO₄:NH₄ and SO₄:PO₄ stoichiometry of nutrient regeneration. The mass balances derived agree reasonably well and indicate that approximately 30% of the total nitrogen and 15% of the total phosphorus deposited in these sediments are recycled. The kinetics of nutrient regeneration are rapid. The mean residence time for recycled nutrients within the sediment is 4 to 6 months for nitrogen and 1.5 to 2 years for phosphorus. Nearly 60% of the total nitrogen regeneration and 90% of the total phosphorus regeneration occur during the 4 month summer period of June through September. Nitrogen regeneration, like carbon, appears to be controlled by the microbially-mediated metabolism of labile organic matter. The greater asymmetry and lower percent turnover in phosphorus cycling is apparently due to changes in its solubility under oxidized and reduced conditions and selective regeneration prior to deposition.     

Carbohydrate dynamics and contributions to the carbon budget of an organic-rich coastal sediment

Potential hydrolysis rates of three different polysaccharides, pullulan, laminarin, and xylan, were measured in intact sediment cores from Cape Lookout Bight, North Carolina, in order to constrain the rates at which a fraction of the high-molecular-weight sedimentary carbon pool may be hydrolyzed to lower molecular weights. Potential hydrolysis rates of pullulan were somewhat higher than those of laminarin and xylan. Highest potential rates were measured in surface sediments; rates at depths of 5–7 and 14–16 cm differed relatively little from one another. Total dissolved carbohydrates, dissolved organic carbon (DOC), sulfate, and sulfate reduction rates were also measured and compared with data previously collected at Cape Lookout Bight in order to investigate carbohydrate dynamics and establish the relative contribution of carbohydrates to the sedimentary carbon budget. Total porewater carbohydrates constitute a disproportionate fraction of DOC, ranging from a maximum of 85% in near-surface intervals to 24% at depths of 14–16 cm. A comparison of potential hydrolysis rates, dissolved carbohydrate concentrations, DOC, and sulfate reduction rates, along with results from a wide range of studies previously conducted at this site suggests that hydrolysis of high-molecular-weight polysaccharides can potentially be very rapid relative to carbon remineralization rates. Dissolved porewater carbohydrates form a dynamic pool that is likely turned over on short timescales in Cape Lookout Bight sediments.     

Model for the distribution of sulfate reduction and methanogenesis in freshwater sediments

A model, based on the in situ physiological characteristics of methanogens and sulfate reducers, was developed to describe the distribution of methanogenesis and sulfate reduction in freshwater sediments. The model predicted the relative importance of methane production and sulfate reduction in lakes of various trophic status and generated profiles of sulfate, acetate, methanogenesis, and sulfate reduction comparable to the profiles that are expected based on field studies. The model indicated that at sulfate concentrations greater than 30μM a sulfate-reducing zone develops because sulfate reducers maintain acetate concentrations too low for methanogens to grow. At lower sulfate concentrations a methanogenic zone develops because the dual limitations of low sulfate concentrations and acetate consumption by methanogens prevents sulfate reducers from growing. The model and a compilation of previously published field data indicate that, within the reported range of sulfate concentrations, the relative importance of methanogenesis and sulfate reduction in freshwater sediments is primarily dependent upon the rates of organic matter decomposition.     

The anaerobic degradation of organic matter in Danish coastal sediments: iron reduction, manganese reduction, and sulfate reduction

We used a combination of porewater and solid phase analysis, as well as a series of sediment incubations, to quantify organic carbon oxidation by dissimilatory Fe reduction, Mn reduction, and sulfate reduction, in sediments from the Skagerrak (located off the northeast coast of Jutland, Denmark). In the deep portion of the basin, surface Mn enrichments reached 3.5 wt%, and Mn reduction was the only important anaerobic carbon oxidation process in the upper 10 cm of the sediment. In the less Mn-rich sediments from intermediate depths in the basin, Fe reduction ranged from somewhat less, to far more important than sulfate reduction. Most of the Mn reduction in these sediments may have been coupled to the oxidation of acid volatile sulfides (AVS), rather than to dissimilatory reduction. High rates of metal oxide reduction at all sites were driven by active recycling of both Fe and Mn, encouraged by bioturbation. Recycling was so rapid that the residence time of Fe and Mn oxides, with respect to reduction, ranged from 70-250 days. These results require that, on average, an atom of Fe or Mn is oxidized and reduced between 100-300 times before ultimate burial into the sediment. We observed that dissolved Mn2+ was completely removed onto fully oxidized Mn oxides until the oxidation level of the oxides was reduced to about 3.8, presumably reflecting the saturation by Mn2+ of highly reactive surface adsorption sites. Fully oxidized Mn oxides in sediments, then, may act as a cap preventing Mn2+ escape. We speculate that in shallow sediments of the Skagerrak, surface Mn oxides are present in a somewhat reduced oxidation level (< 3.8) allowing Mn2+ to escape, and perhaps providing the Mn2+ which enriches sediments of the deep basin.     

The significance of isotope specific diffusion coefficients for reaction-transport models of sulfate reduction in marine sediments

Modeling isotopic signatures in systems affected by diffusion, advection, and a reaction which modifies the isotopic abundance of a given species, is a discipline in its infancy. Traditionally, much emphasis has been placed on kinetic isotope effects during biochemical reactions, while isotope effects caused by isotope specific diffusion coefficients have been neglected. A recent study by Donahue et al. (2008) suggested that transport related isotope effects may be of similar magnitude as microbially mediated isotope effects. Although it was later shown that the assumed differences in the isotope specific diffusion coefficients were probably overstated by one or two orders of magnitude (Bourg, 2008), this study raises several important issues: (1) Is it possible to directly calculate isotopic enrichment factors from measured concentration data without modeling the respective system? (2) Do changes in porosity and advection velocity modulate the influence of isotope specific diffusion coefficients on the fractionation factor α? (3) If one has no a priori knowledge whether diffusion coefficients are isotope specific or not, what is the nature and magnitude of the error introduced by either assumption? Here we argue (A) That the direct substitution of measured data into a differential equation is problematic and cannot be used as a replacement for a reaction-transport model; (B) That the transport related fractionation scales linearly with the difference between the respective diffusion coefficients of a given isotope system, but depends in a complex non-linear way on the interplay between advection velocity, and downcore changes of temperature and porosity. Last but not least, we argue that the influence of isotope specific diffusion coefficients on microbially mediated sulfate reduction in typical marine sediments is considerably smaller than the error associated with the determination of the fractionation factor.     

Degradation of dissolved organic monomers and short-chain fatty acids in sandy marine sediment by fermentation and sulfate reduction

The decay of a wide range of organic monomers (short-chain volatile fatty acids (VFA’s), amino acids, glucose and a pyrimidine) was studied in marine sediments using experimental plug flow-through reactors. The reactions were followed in the presence and absence of 10 mM SO₄²⁻. Degradation stoichiometry of individual monomers (inflow concentration of 6 mM organic C) was traced by measuring organic (VFA’s, amino acids) and inorganic (CO₂, NH₄⁺, SO₄²⁻) compounds in the outflow. Fermentation of amino acids was efficient and complete during passage through anoxic sediment reactors. Aliphatic amino acids (alanine, serine and glutamate) were primarily recovered as CO₂ (24-34%), formate (3-22%) and acetate (41-83%), whereas only ∼1/3 of the aromatic amino acid (tyrosine) was recovered as CO₂ (13%) and acetate (20%). Fermentation of glucose and cytosine was also efficient (78-86%) with CO₂ (30-35%), formate (3%) and acetate (28-33%) as the primary products. Fermentation of VFA’s (acetate, propionate and butyrate), on the other hand, appeared to be product inhibited. The presence of SO₄²⁻ markedly stimulated VFA degradation (29-45% efficiency), and these compounds were recovered as CO₂ (17% for butyrate to 100% for acetate) and acetate (51% and 82% for propionate and butyrate, respectively). When reaction stoichiometry during fermentation is compared with compound depletion during sulfate reduction, the higher proportion CO₂ recovery is consistent with lower acetate and formate accumulation. Our results therefore suggest that fermentation reactions mediate the initial degradation of added organic compounds, even during active sulfate reduction. Fermentative degradation stoichiometry also suggested significant H₂ production, and >50% of sulfate reduction appeared to be fuelled by H₂. Furthermore, our results suggest that fermentation was the primary deamination step during degradation of the amino acids and cytosine.     

Impact of polychaetes (Nereis spp. and Arenicola marina) on carbon biogeochemistry in coastal marine sediments

Known effects of bioturbation by common polychaetes (Nereis spp. and Arenicola marina) in Northern European coastal waters on sediment carbon diagenesis is summarized and assessed. The physical impact of irrigation and reworking activity of the involved polychaete species is evaluated and related to their basic biology. Based on past and present experimental work, it is concluded that effects of bioturbation on carbon diagenesis from manipulated laboratory experiments cannot be directly extrapolated to in situ conditions. The 45–260% flux (e.g., CO₂ release) enhancement found in the laboratory is much higher than usually observed in the field (10–25%). Thus, the faunal induced enhancement of microbial carbon oxidation in natural sediments instead causes a reduction of the organic matter inventory rather than an increased release of CO₂ across the sediment/water interface. The relative decrease in organic inventory (G _b /G _u) is inversely related to the relative increase in microbial capacity for organic matter decay (k _b /k _u). The equilibrium is controlled by the balance between organic input (deposition of organic matter at the sediment surface) and the intensity of bioturbation. Introduction of oxygen to subsurface sediment and removal of metabolites are considered the two most important underlying mechanisms for the stimulation of carbon oxidation by burrowing fauna. Introduction of oxygen to deep sediment layers of low microbial activity, either by downward irrigation transport of overlying oxic water or by upward reworking transport of sediment to the oxic water column will increase carbon oxidation of anaerobically refractory organic matter. It appears that the irrigation effect is larger than and to a higher degree dependent on animal density than the reworking effect. Enhancement of anaerobic carbon oxidation by removal of metabolites (reduced diffusion scale) may cause a significant increase in total sediment metabolism. This is caused by three possible mechanisms: (i) combined mineralization and biological uptake; (ii) combined mineralization and abiogenic precipitation; and (iii) alleviation of metabolite inhibition. Finally, some suggestions for future work on bioturbation effects are presented, including: (i) experimental verification of metabolite inhibition in bioturbated sediments; (ii) mapping and quantification of the role of metals as electron acceptors in bioturbated sediments; and (iii) identification of microbial community composition by the use of new molecular biological techniques. These three topics are not intended to cover all unresolved aspects of bioturbation, but should rather be considered a list of obvious gaps in our knowledge and present new and appealing approaches.     

Stable carbon isotope evidence for two sources of organic matter in coastal sediments: seagrasses and plankton

Organic carbon from sediments collected in Texas seagrass meadows was enriched in ¹³C by an average of 6.6% relative to organic carbon from offshore sediments. Within the South Texas hay system examined. δ¹³C values became increasingly more typical of offshore sediments with increasing distance from seagrass meadows. This permits the use of carbon isotope data as a measure of the relative contributions of seagrasses and plankton to sedimentary organic matter.     

Determination of organic carbon in soils and sediments in an automatic method

Our automatic digestion device is applied in determining the quantity of organic carbon in the soils/sediments. Its operation process is simple. The reaction conditions are optimized; the complex pretreatments are automated; and a great number of samples can be analyzed at the same time. Comparison shows that the experiment using the device is safer and easier. The correlation coefficiency is greater than 0.999, indicating a good linear relationship. The relative standard deviations of three different concentrations are less than 5%. Standard addition recoveries of high and low concentration range between 94.7% and 100% and between 91.7% and 105% respectively. Method determination limitation (MDL) of this method meets the practical requirements. The device in this paper supports a compositive SOC determination method. Its advantages include improved time and labor efficiency, and accuracy. The device is widely used in the studies of agricultural science, carbon cycle, climate change and environmental protection.     

Spectroscopic and Chromatographic studies of organic carbon in Recent marine sediments from the Peruvian Upwelling Zone

Early diagenesis of organic matter in sediments from two sites in the Peruvian Upwelling Zone (12°05′S, 77°39′W; 15°17′S, 75°24′W) has been studied by observing changes in the total organic carbon and lipid and humin fractions with depth. Transformations of the total carbon and humin fraction have been characterized by conventional and time-dependent solid state NMR techniques, while lipid diagenesis was monitored by measuring the concentration of sterols in the same sedimentary horizons. Both the quantity of total sterols and the relative abundances of individual sterols vary with sampling location, suggesting a difference in the input of biomass to the sediments at the two sites. Total sterol concentrations decrease with depth at both sites, but the loss of sterols occurs much more slowly at the more anoxic northern site, where sedimentation rates and organic carbon contents are approximately twice those at the southern site. ¹³C-NMR spectra of the total organic carbon and the humin fraction suggest that humin-like compounds are an original component of the sedimentary biomass, and dipolar-dephased spectra of the humin residue indicate that diagenetic alterations of the humin fraction are occurring even in these very young sediments. Conventional and time-dependent spectroscopic data support the hypothesis that humin formation results from selective preservation of microbially-resistant biopolymers which are an original component of the sedimentary biomass combined with loss of certain labile compounds.     

Isotopic compositions of tropical East African flora and their potential as source indicators of organic matter in coastal marine sediments

The C and N stable isotope compositions of some flora of East Africa from coastal Tanzania and Amboseli National Park (Kenya) are used to assess if they can be used as a terrestrial end member during the estimation of terrestrial fraction in coastal marine sediments. The results of C isotope composition of various tree leaves, which average −29.3 ± 1.4%, indicate that these tropical higher land plant species follow a Calvin-Benson or non-Kranz (C₃) type of metabolism. The results for grass species, which average −13.2 ± 2.4%, indicate that most of them follow a Hatch-Slack or Kranz (C₄) type of metabolism. However, some of the succulent plants from the Amboseli National Park have δ¹³C values that average −14.7%, an indication that they follow a CAM (Crassulacean Acid Metabolism) type of metabolism. The N isotope values are relatively higher than expected for the terrestrial organic material. The average δ¹⁵N values for both tree and grass samples are higher than 5% and fall within the range normally considered to be marine. The high enrichment in ¹⁵N may be related to the environmental conditions in which plants thrive. Plants growing in sandy, dry and overgrazed environments are expected to be enriched in ¹⁵N owing to full utilisation of all available N species, regardless of their isotopic compositions. Other processes which may cause an enrichment in ¹⁵N include adsorption by various types of clay minerals, supply of ¹⁵N-enriched nitrate through sea-spray, and local denitrification, especially in swampy and lake margins where the input of organic matter may be higher than the rate of decomposition.The stable isotopic composition of organic C and N for surficial organic matter for the coastal marine sediments averages −17.0 ± 0.9% and 5.4 ± 1.1%, respectively. These values indicate a substantial contribution of C₄ plants and sea grasses. However, contribution of C₄ relative to that of sea grasses can not be evaluated owing to the fact that there is no significant difference in the isotopic compositions between the two groups.In the savannah environment, where a contribution from the C₄ types of plants might be substantial, the δ¹³C value for a terrestrial end member needs to be established prior to evaluation of the terrestrially derived organic matter in the marine environment. Owing to a significant contribution of sea grasses to the total organic matter preserved in coastal marine sediments, the stable isotopes of organic C seem to have a limited applicability as source indicators in the East African coastal waters. Furthermore, the results indicate that N stable isotopes seem to have a limited applicability as source indicators in coastal waters of East Africa. However, more work needs to be conducted to determine the terrestrial and sea grass end member values for the coastal areas.     

A model for coupled sulfate reduction and methane oxidation in the sediments of Saanich Inlet

A methane-sulfate coupled reaction diffusion model has been developed to describe the inverse relationship commonly observed between methane and sulfate concentrations in the pore waters of anoxic marine sediments. The sediment column was divided into two zones; an upper zone where diagenetic reaction rates are limited by the concentration of oxidizable organic matter and a lower zone in which reaction rates are limited by the concentration of oxidizing agent—sulfate. For each zone differential equations describing the distribution of methane and sulfate were derived. The boundary conditions used to solve these equations resulted in a set of four coupled equations. When fit to data from Saanich Inlet (B.C., Canada) and Skan Bay (Alaska) the model not only reproduces the observed methane and sulfate pore water concentration profiles but also accurately predicts the methane oxidation and sulfate reduction rates. Maximum methane oxidation rates occur at the transition boundary from the upper to the lower layer. In Saanich Inlet sediments from 23 to 40% of the downward sulfate flux is consumed in methane oxidation while in Skan Bay this value is only about 12%.