Anoxic Mineralization Research Articles

Offshore Windfarm Footprint of Sediment Organic Matter Mineralization Processes

Offshore windfarms (OWFs) offer part of the solution for the energy transition which is urgently needed to mitigate effects of climate change. Marine life has rapidly exploited the new habitat offered by windfarm structures, resulting in increased opportunities for filter- and suspension feeding organisms. In this study, we investigated the effects of organic matter (OM) deposition in the form of fecal pellets expelled by filtering epifauna in OWFs, on mineralization processes in the sediment. OM deposition fluxes produced in a 3D hydrodynamic model of the Southern Bight of the North Sea were used as input in a model of early diagenesis. Two scenarios of OWF development in the Belgian Part of the North Sea (BPNS) and its surrounding waters were calculated and compared to a no-OWF baseline simulation. The first including constructed OWFs as of 2021, the second containing additional planned OWFs by 2026. Our results show increased total mineralization rates within OWFs (27–30%) in correspondence with increased deposition of reactive organic carbon (OC) encapsulated in the OM. This leads to a buildup of OC in the upper sediment layers (increase by ∼10%) and an increase of anoxic mineralization processes. Similarly, denitrification rates within the OWFs increased, depending on the scenario, by 2–3%. Effects were not limited to the OWF itself: clear changes were noticed in sediments outside of the OWFs, which were mostly opposite to the “within-OWF” effects. This contrast generated relatively small changes when averaging values over the full modeling domain, however, certain changes, such as for example the increased storage of OC in sediments, may be of significant value for national / regional carbon management inventories. Our results add to expectations of ecosystem-wide effects of windfarms in the marine environments, which need to be researched further given the rapid rate of expansion of OWFs.

Faunal and environmental drivers of carbon and nitrogen cycling along a permeability gradient in shallow North Sea sediments

Ecosystem functions are driven by abiotic and biotic factors, but due to high collinearity of both, it is often difficult to disentangle the drivers of these ecosystem functions. We studied sedimentological and faunal controls of benthic organic matter mineralization, a crucial ecosystem process provided for by sediments of shelf seas. Subtidal benthic habitats representative of the wide permeability gradient found in the Belgian Part of the North Sea (Northeast Atlantic Shelf) were characterized in terms of sediment descriptors, macrofauna, and sediment biogeochemistry was estimated. Our results confirmed a strong correlation between sediment characteristics and macrofauna, and estimated sediment biogeochemical process rates were clearly linked to both. Results of variance partitioning and statistical modelling showed that oxic mineralization and nitrification were mainly regulated by faunal activities whereas anoxic mineralization was regulated by sediment properties, with permeability as a decisive factor. Both biotic and abiotic factors were needed to explain variability in oxygen consumption and total mineralization estimates, suggesting that macrofaunal activities have different effects across habitats. The statistical models were a useful tool to interpret the impact of anthropogenic activities in the study area and represent a step towards predicting the effects of human activities on crucial ecosystem functions.

Rapid organic matter cycling in North Sea sediments

Coastal shelf seas are zones of intense nutrient cycling, where a strong coupling between the sediment and the water column enhances primary productivity. To identify factors that control the strength of this benthic-pelagic coupling we measured sediment characteristics, solute fluxes, and porewater nutrient profiles in spring along a south - north transect in the North Sea crossing distinct regions: the shallow Oyster Grounds closest to the Dutch shore, the shallow Dogger Bank, the 80-m deep central North Sea, and the 150-m deep Fladen Grounds between the north of Scotland and Norway. The data were used to constrain rates of different mineralization processes with the 1-D diagenetic model (OMEXDIA). Surprisingly, we found no major differences in the biogeochemical signature along the 670 km long North Sea transect, despite sediments ranging in median grainsize from 25 to 217 μm, and a permeability range >3 orders of magnitude. Total carbon mineralization ranged between 4 and 13.5 mmol C m−2 d−1, and decreased significantly northward. Oxic mineralization was the dominant mineralization process in all studied sites. Finest, least permeable sediments were found in the Fladen Grounds where highest denitrification rates were recorded, linked to high nitrate concentrations in the overlying water. The coarsest, most permeable sediments of the shallow Dogger Bank represented a transition area between the Oyster Grounds, where oxic mineralization was highest (75–90%), and the central North Sea samples, where anoxic mineralization increased relative to oxic mineralization due to higher bioturbation rates (oxic: 59–72%, anoxic: 27–39%). Overall, denitrification rates increased, while phosphorus removal tended to decrease northward. This contrasting behaviour in nitrogen and phosphorus removal was identified as a possible cause for decreasing DIN:DIP ratios in the water column towards the north.

Spatio-temporal dynamics of sedimentary phosphorus along two temperate eutrophic estuaries: A data-modelling approach

Sediments play an important role in dissolved inorganic phosphorus (DIP) recycling, which needs to be precisely quantified in eutrophic estuaries. A coupled field data and diagenetic modelling approach was used to study P dynamics in two estuaries (Elorn and Aulne, Brittany, France).An existing model (OMEXDIA) was extended with phosphorus (P) benthic cycle by adding dissolved inorganic phosphorus (DIP) and particulate P fractions (organic P, Fe-bound P and Ca-bound P). The model was fitted to pore water oxygen, nitrate, ammonium, oxygen demand units (reduced substances that were produced by the anoxic mineralization; Fe2+, Mn2+ and H2S; ODU), DIP, sediment organic P and C, Fe-bound P and Ca-bound P data from four seasons (February, May, July and October 2009) in two eutrophic estuaries (Elorn and Aulne). These two systems were investigated along the salinity gradient with 3 stations per estuary.The model shows that high organic C fluxes deposited in the sediments (23–98 mmol m-2 d-1) caused high organic P mineralization rates, which is the modeled main benthic source of DIP (77%). The DIP recycling fluxes were calculated in Elorn (463 ± 122 μmol m-2 d-1) and in Aulne (320 ± 137 μmol m-2 d-1) estuaries, and overall 85% of DIP produced in the sediment was recycled to the overlying water. A limited but substantial proportion (15%) was precipitated as Ca-bound P except in the upstream reach of the Aulne in February, where the integrated rate of Ca-bound P precipitation constitutes a major sink of DIP (83%).The high DIP recycling fluxes in Elorn and Aulne estuaries, integrated over the entire area, were 3–20 times larger than the river DIP input. This comparison of external and internal DIP loads shows the major role of these sediments as sources of DIP to the estuary and the potential storage of formerly discharged nutrients.

Bacterial chemoautotrophic reoxidation in sub-Arctic sediments: a seasonal study in Kobbefjord, Greenland

Anoxic mineralization of organic matter releases dissolved inorganic carbon and produces reduced mineralization products. The reoxidation of these reduced compounds is essential for biogeochemical cycling in sediments and is mainly performed by chemoautotrophic microbes, which synthesize new organic carbon by dark CO2 fixation. At present however, the biogeochemical importance of chemoautotrophy in high-latitude sediments is largely unknown. Here, we determine the seasonal variation in sedimentary chemoautotrophic production in Kobbefjord (SW Greenland). Intact sediment cores from the fjord were incubated, and dark CO2 fixation was quantified by combining bacterial phospholipid-derived fatty acid analysis with 13C stable isotope probing (PLFA-SIP). Our results reveal a distinct seasonal cycle in chemoautotrophic activity, which increases after the spring bloom and shows lowest activity in the late winter when the fjord is covered by sea ice. The depth distribution of chemoautotrophic activity also varied seasonally, likely due to seasonal variation in the bioturbation activity of sediment infauna. Although chemoautotrophy rates (0.4 ± 0.2 mmol C m-2d-1) were in the low range for coastal sediments, they are comparable to those from intertidal sandflats and brackish tropical lagoons, and scale with the sulfide production through sulfate reduction in the fjord. Chemoautotrophic production in these fjord sediments thus appears to be mainly driven by sulfide oxidation and can re-fix 4% of the CO2 produced by mineralization

Anoxic denitrification of BTEX: Biodegradation kinetics and pollutant interactions

Anoxic mineralization of BTEX represents a promising alternative for their abatement from O2-deprived emissions. However, the kinetics of anoxic BTEX biodegradation and the interactions underlying the treatment of BTEX mixtures are still unknown. An activated sludge inoculum was used for the anoxic abatement of single, dual and quaternary BTEX mixtures, being acclimated prior performing the biodegradation kinetic tests. The Monod model and a Modified Gompertz model were then used for the estimation of the biodegradation kinetic parameters. Results showed that both toluene and ethylbenzene are readily biodegradable under anoxic conditions, whereas the accumulation of toxic metabolites resulted in partial xylene and benzene degradation when present both as single components or in mixtures. Moreover, the supplementation of an additional pollutant always resulted in an inhibitory competition, with xylene inducing the highest degree of inhibition. The Modified Gompertz model provided an accurate fitting for the experimental data for single and dual substrate experiments, satisfactorily representing the antagonistic pollutant interactions. Finally, microbial analysis suggested that the degradation of the most biodegradable compounds required a lower microbial specialization and diversity, while the presence of the recalcitrant compounds resulted in the selection of a specific group of microorganisms.

Carbonate chemistry in sediment porewaters of the Rhône River delta driven by early diagenesis (northwestern Mediterranean)

Abstract. The Rhône River is the largest source of terrestrial organic and inorganic carbon for the Mediterranean Sea. A large fraction of this terrestrial carbon is either buried or mineralized in the sediments close to the river mouth. This mineralization follows aerobic and anaerobic pathways, with a range of impacts on calcium carbonate precipitation and dissolution in the sediment near the sediment–water interface. This study focuses on the production of dissolved inorganic carbon (DIC) and total alkalinity (TA) by early diagenesis, consequential pH variations and the effect on calcium carbonate precipitation or dissolution. The sediment porewater chemistry was investigated along a transect from the Rhône River outlet to the continental shelf. TA and concentrations of DIC, SO42− and Ca2+ were analyzed on bottom waters and extracted sediment porewaters, whereas pH and oxygen concentrations were measured in situ using microelectrodes. The average oxygen penetration depth into the sediment was 1.7 ± 0.4 mm close to the river mouth and 8.2 ± 2.6 mm in the continental shelf sediments, indicating intense respiration rates. Diffusive oxygen fluxes through the sediment–water interface ranged between 3 and 13 mmol O2 m−2 d−1. In the first 30 cm of the sediment, TA and DIC porewater concentrations increased with depth up to 48 mmol L−1 near the river outlet and up to 7 mmol L−1 on the shelf as a result of aerobic and anaerobic mineralization processes. Due to aerobic processes, at all stations pH decreased by 0.6 pH units in the oxic layer of the sediment accompanied by a decrease of the saturation state regarding calcium carbonate. In the anoxic layer of the sediments, sulfate reduction was the dominant mineralization process and was associated with an increase of porewater saturation state regarding calcium carbonate. Ultimately anoxic mineralization of organic matter caused calcium carbonate precipitation demonstrated by a large decrease in Ca2+ concentration with depth in the sediment. Carbonate precipitation decreased in the offshore direction, together with the carbon turnover and sulfate consumption in the sediments. The large production of porewater alkalinity characterizes these sediments as an alkalinity source to the water column, which may increase the CO2 buffering capacity of these coastal waters. Estuarine sediments should therefore receive more attention in future estimations of global carbon fluxes.

Spatial and Temporal Variability of Sediment Organic Matter Recycling in Two Temperate Eutrophicated Estuaries

This paper deals with the spatial and seasonal recycling of organic matter in sediments of two temperate small estuaries (Elorn and Aulne, France). The spatio-temporal distribution of oxygen, nutrient and metal concentrations as well as the organic carbon and nitrogen contents in surficial sediments were determined and diffusive oxygen fluxes were calculated. In order to assess the source of organic carbon (OC) in the two estuaries, the isotopic composition of carbon (δ 13C) was also measured. The temporal variation of organic matter recycling was studied during four seasons in order to understand the driving forces of sediment mineralization and storage in these temperate estuaries. Low spatial variability of vertical profiles of oxygen, nutrient, and metal concentrations and diffusive oxygen fluxes were monitored at the station scale (within meters of the exact location) and cross-section scale. We observed diffusive oxygen fluxes around 15 mmol m−2 day−1 in the Elorn estuary and 10 mmol m−2 day−1 in the Aulne estuary. The outer (marine) stations of the two estuaries displayed similar diffusive O2 fluxes. Suboxic and anoxic mineralization was large in the sediments from the two estuaries as shown by the rapid removal of very high bottom water concentrations of NO x − (>200 μM) and the large NH4 + increase at depth at all stations. OC contents and C/N ratios were high in upstream sediments (11-15 % d.w. and 4-6, respectively) and decreased downstream to values around 2 % d.w. and C/N ≤ 10. δ 13C values show that the organic matter has different origins in the two watersheds as exemplified by lower δ 13C values in the Aulne watershed. A high increase of δ 13C and C/N values was visible in the two estuaries from upstream to downstream indicating a progressive mixing of terrestrial with marine organic matter. The Elorn estuary is influenced by human activities in its watershed (urban area, animal farming) which suggest the input of labile organic matter, whereas the Aulne estuary displays larger river primary production which can be either mineralized in the water column or transferred to the lower estuary, thus leaving a lower mineralization in Aulne than Elorn estuary. This study highlights that (1) meter scale heterogeneity of benthic biogeochemical properties can be low in small and linear macrotidal estuaries, (2) two estuaries that are geographically close can show different pattern of organic matter origin and recycling related to human activities on watersheds, (3) small estuaries can have an important role in recycling and retention of organic matter.

Biogeochemical processes in the groundwater discharge zone of urban streams

The influence of biogeochemical processes on nitrogen and organic matter transformation and transport was investigated for two urban streams receiving groundwater discharge during the dry summer baseflow period. A multiple lines of evidence approach involving catchment-, and stream reach-scale investigations were undertaken to describe the factors that influence pore water biogeochemical processes. At the catchment-scale gaining stream reaches were identified from water table mapping and groundwater discharge estimated to be between 0.1 and 0.8 m3 m−2 d−1 from baseflow analysis. Sediment temperature profiles also suggested that the high groundwater discharge limited stream water infiltration into the sediments. At the stream reach-scale, dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations were higher in stream water than in groundwater. However, DOC and DON concentrations were greatest in sediment pore water. This suggests that biodegradation of sediment organic matter contributes dissolved organic matter (DOM) to the streams along with that delivered with groundwater flow. Pore water ammonium (NH4 +) was closely associated with areas of high pore water DOM concentrations and evidence of sulfate (SO4 2−) reduction (low concentration and SO4:Cl ratio). This indicates that anoxic DOM mineralization was occurring associated with SO4 2− reduction. However the distribution of anoxic mineralization was limited to the center of the streambed, and was not constrained by the distribution of sediment organic matter which was higher along the banks. Lower sediment temperatures measured along the banks compared to the center suggests, at least qualitatively, that groundwater discharge is higher along the banks. Based on this evidence anoxic mineralization is influenced by groundwater residence time, and is only measurable along the center of the stream where groundwater flux rates are lower. This study therefore shows that the distribution of biogeochemical processes in stream sediments, such as anoxic mineralization, is strongly influenced by both the biogeochemical conditions and pore water residence time.

Reinterpreting the Importance of Oxygen‐Based Biodegradation in Chloroethene‐Contaminated Groundwater

Chlororespiration is common in shallow aquifer systems under conditions nominally identified as anoxic. Consequently, chlororespiration is a key component of remediation at many chloroethene‐contaminated sites. In some instances, limited accumulation of reductive dechlorination daughter products is interpreted as evidence that natural attenuation is not adequate for site remediation. This conclusion is justified when evidence for parent compound (tetrachloroethene, PCE, or trichloroethene, TCE) degradation is lacking. For many chloroethene‐contaminated shallow aquifer systems, however, nonconservative losses of the parent compounds are clear but the mass balance between parent compound attenuation and accumulation of reductive dechlorination daughter products is incomplete. Incomplete mass balance indicates a failure to account for important contaminant attenuation mechanisms and is consistent with contaminant degradation to nondiagnostic mineralization products like CO2. While anoxic mineralization of chloroethene compounds has been proposed previously, recent results suggest that oxygen‐based mineralization of chloroethenes also can be significant at dissolved oxygen concentrations below the currently accepted field standard for nominally anoxic conditions. Thus, reassessment of the role and potential importance of low concentrations of oxygen in chloroethene biodegradation are needed, because mischaracterization of operant biodegradation processes can lead to expensive and ineffective remedial actions. A modified interpretive framework is provided for assessing the potential for chloroethene biodegradation under different redox conditions and the probable role of oxygen in chloroethene biodegradation.

Anoxic Mineralization: Environmental Reality or Experimental Artifact?

Anoxic Mineralization: Environmental Reality or Experimental Artifact?

Microscale mineralization pathways in surface sediments: A chemical sensor study in Lake Baikal

We used an array of ion-selective electrodes (oxygen [O2], hydrogen [H+], carbonate [CO32-], calcium [Ca2+], ammonium [NH4+], and nitrate [NO3-]) with a micromanipulator to study mineralization processes in the surface sediments in Lake Baikal. Concentration profiles at submillimeter resolution were measured in sediment cores from four depths (160-1,400 m) in the South Basin. Oxidation rates of organic carbon (C) estimated from O2 and NO3- profiles measured in March and July 2001 ranged between 2.2 and 4.9 mmol C m-2 d-1. The characteristic shape of the O2 profiles allowed separation of oxidation of organic carbon from reoxidation of reduced compounds at the oxic-anoxic boundary. Of the benthic carbon turnover, 60-75% was metabolized through oxic respiration and 11-28% through anoxic mineralization. The remainder (12-14%) was due to denitrification. Carbon dioxide (CO2) profiles calculated from O2 agreed well with those from pH and CO32-, supporting the concept that oxic respiration was the prevailing mineralization pathway. Alkalinity balance calculated from flux rates of reduced compounds and bicarbonate (HCO3-) calculated from pH and CO32- profiles showed that the sediment was a sink for alkalinity. The flux rates in the range of 0.13-1.0 mmol m-2 d-1 were caused by buffering the hydrogen ions (H+) generated from reoxidation processes of reduced compounds. Potential dark CO2 assimilation by chemoautotrophic bacteria in the sediment was 0.03-0.1 mmol C m-2 d-1. Because of the long O2 exposure time of 25-2,500 yr, however, only 3-14% of the initially settled organic carbon was finally buried in the sediments, forming the paleolimnological record of Lake Baikal.

Anoxic degradation of biogenic debris in sediments of eutrophic subalpine Lake Bled (Slovenia)

Anoxic degradation of sedimentary biogenic debris using closed sediment incubation experiments was studied in eutrophic subalpine Lake Bled (NW Slovenia) which, for most of the year, has an anoxic hypolimnion. Production rates of dissolved inorganic carbon (DIC), NH4+, PO43- and dissolved Si, and reduction rates of SO42- were measured and anoxic mineralization rates were modelled using G-model. The depth profiles indicated major mineralization of biogenic debris and SO42- reduction near the sediment surface. A comparison between depth integrated anoxic mineralization rates and diffusive benthic fluxes of DIC, NH4+ and PO43- showed that the anoxic incubation experiments provide a good estimate of N degradation of biogenic debris. The contributions of SO42- reduction and acetate fermentation in NH4+ production are about 30 and 70%, respectively. The DIC production accounted for only 15% of DIC benthic flux, indicating that methanogenesis and oxidation of methane provides 80% of this flux. Only about 30% of PO43- was released because phosphate precipitated in the closed incubation experiments. The depth integrated production of Si accounts for 70–80% of Si benthic fluxes indicating intense dissolution of biogenic Si in the surficial lake sediment.

Beach morphology and food web structure: comparison of an eroding and an accreting sandy shore in the North Sea

Food components and inorganic nutrients were studied on two sandy shores of the adjacent barrier islands of Sylt and Romo in the North Sea, differing in morphodynamics. Implications of high and low wave energy on the food structure were assessed. The Sylt shore represents a dynamic intermediate beach type, while the Romo shore is morphologically stable and dissipative. On the steep-profiled, coarse-grained Sylt shore, strong hydrodynamics resulted in erosion and high fluxes of organic material through the beach, but prevented any storage of food sources. In contrast, the flat-profiled, fine-grained Romo shore, with low wave energy and accretion, accumulated organic carbon from surf waters. At Sylt, oxic nutrient regeneration prevailed, while anoxic mineralization was more important at Romo. Macrofauna on the Sylt shore was impoverished compared with the community at Romo. Correspondingly, abundances of epibenthic predators such as shrimps, crabs, fish, and shorebirds were also lower at Sylt. Meiofauna was abundant on both shores, but differed in taxonomic composition. Several major taxa were represented in fairly equal proportions of individual numbers on the well-oxygenated Sylt shore, while nematodes strongly dominated the assemblage at Romo. Thus, on cold-temperate, highly dynamic intermediate shores with high wave energy and subject to erosion, the small food web dominates. Organisms are agile and quickly exploit fresh organic material. Larger organisms and nematodes abound under stable, dissipative and accreting shore conditions, where some food materials may accumulate and zoomass builds up to support numerous visitors from higher trophic levels.

A Model for Early Diagenetic Processes in Sediments of the Continental Shelf of the Black Sea

A numerical model for early diagenetic processes in the sediments of the north-western continental shelf of the Black Sea is presented. The north-western shelf area is influenced by nutrient, organic matter and reactive iron inputs from major rivers such as the Danube, Dnieper and Dniester. Low oxygen concentrations in the bottom water and high carbon deposition rates have resulted in a dominance of anoxic mineralization processes including manganese, iron and sulphate reduction. The model includes six solid phase variables (labile and less labile organic carbon, iron-and manganese oxides, iron monosulphide and pyrite) and eight solutes (oxygen, nitrate, ammonium, manganese, ferrous iron sulphate, hydrogen sulphide and methane). The cycling of iron and sulphur is explicitly incorporated in the model since these elements play an important role in the mineralization of organic matter on the continental shelf. The model is capable of performing both steady state and dynamic simulations and can be coupled to biogeochemical models for water-column processes. The output of the model is composed of depth profiles of the state variables and their biogeochemical reaction rates and fluxes of the solute substances across the sediment–water interface. The model has been applied to solid-phase and porewater data from a station near the Danube delta. A steady state stimulation reasonably reproduced the main features, but not the distribution of iron and the concentrations of oxygen and total organic carbon in the first 2 cm. A dynamic simulation revealed that a pulse flux of organic matter resulting from the spring bloom could be responsible for the deviation from steady state in the near-surface distribution of total organic carbon and oxygen. The effect of carbon loading on organic matter degradation pathways has been investigated. There is a gradual increase in the relative importance of anoxic mineralization pathways as carbon loadings increase, but the transitions from manganese-based respiration to iron-based respiration and from iron-based respiration to sulphate reduction are rather abrupt. The first jump is due to the positive feedback between manganese reduction coupled to ferrous iron oxidation and iron oxide reduction. The second swap is due to the formation of iron sulphides. This will consume part of the reduced iron, which is consequently not available for re-oxidation to iron oxide, and the iron-reducing bacteria become iron-oxide limited. The cycling efficiency of iron changes abruptly, and a larger part of the iron will be fixed as iron sulphides.

Anoxic mineralization of biogenic debris in near-shore marine sediments (Gulf of Trieste, northern Adriatic)

Anoxic degradation of sedimentary biogenic debris using closed incubation experiments was studied at two sampling stations in the Gulf of Trieste (northern Adriatic). Production rates of dissolved inorganic carbon (DIC), NH 4 +, PO 4 3− and dissolved Si (dSi), and reduction rates of SO 4 2− were measured and anoxic mineralization rates were modeled using a first order G-model and multi-G approach. The depth profiles of these rates revealed an exponential decrease indicating that the largest fraction of mineralization of biogenic debris and SO 4 2− reduction occurs in the surficial sediment layer and on the sediment surface. Comparing the depth-integrated anoxic mineralization rates at both stations with benthic fluxes of DIC, NH 4 +, PO 4 3− and dSi measured at the in situ temperature in the dark, it appears that the DIC and PO 4 3− fluxes are higher because the mineralization mostly occurs at the sediment–water interface, and that besides SO 4 2− reduction, other electron acceptors are involved in the organic matter decomposition pathway in these surficial sediments. The NH 4 + production was higher than the benthic fluxes because of NH 4 + oxidation. The production of dSi was in good agreement with benthic fluxes implying that temperature is the main factor of dSi production and benthic fluxes in these sediments.

Benthic dynamics and carbon fluxes on the NW European continental margin

Across the Goban Spur on the NW European continental margin, laterally directed, intermittent, off-slope transport of particulate matter takes place by intermediate and bottom nepheloid layers (BNLs). These are generated by semidiurnal tidal currents, which on the upper slope reach maximum near-bed speeds of up to 20 cm s−1, and which are directed predominantly off-slope (during 15–20% of the tidal cycle). BNLs are semi-permanently present, increasing in thickness above the seabed in downslope direction but decreasing in particle density. Near-bed currents measured on the upper slope are stronger in autumn than during summer, and both long- and short-term records suggest interannual variability. Aggregate formation in the benthic boundary layer (BBL) is considered the dominant process controlling particle accumulation. The organic fraction has low settling velocities and high residence times within the BBL. The flux of lithogenic material into the sediment on Goban Spur decreases from >44 g m−2 a−1 on the shelf edge to 6.9 and 4.9 g m−2 a−1 on the upper slope, then increases to a maximum of 19.1 g m−2 a−1 on the continental rise. CaCO3 flux increases with depth from about 13 g m−2 a−1 on the shelf edge to a maximum of 30.7 g m−2 a−1 on the continental rise, with minima on the upper slope. Flux values at comparable depths on Meriadzek Terrace are considerably higher. Mineralization of organic carbon on Goban Spur, representing more than 97.7% of the deposition flux, decreases with depth from 19.13 g C m−2 a−1 on the shelf edge, to 4.39 g C m−2 a−1 on the continental rise, and 1.10 g C m−2 a−1 on Porcupine Abyssal Plain. Organic carbon burial fluxes range between 0.05 and >0.16 gC m−2 a−1 on Goban Spur, and up to 0.41 gC m−2 a−1 on Meriadzek Terrace. Over 90% of the organic carbon mineralization at the sediment–water interface and directly below the seabed is driven by oxygen, as shown by pore water modelling and in situ oxygen measurements. Denitrification is of only minor (<5%) importance for the organic carbon mineralization; anoxic mineralization plays a (minor) role on shallow stations. Inventories and fluxes of 210Pbxs in surface sediments on Goban Spur indicate that the slope below 1500 m receives only about half of the amount of relatively young sedimentary material compared to the upper slope and shelf. Yet total sediment fluxes increase from the upper slope downward, indicating a significant contribution of reworked sediment in lower-slope sediments. 210Pb-derived mixing coefficients correlate with macro- and meiofaunal density and biomass, decreasing with increasing depth downslope. Fluxes of lithogenic material, CaCO3 and 210Pbxs on the lower slope agree reasonably well with fluxes recorded in deep-water sediment traps, suggesting that the bulk of the sediment may be supplied via vertical settling through midwater depth. Benthic fluxes of organic carbon, however, are three times higher than deep-water trap fluxes, emphasizing the importance of lateral transport of organic matter over the slope. At present, the NW European continental margin at Goban Spur is not a major carbon depocenter.

The fate of carbon in continental shelf sediments of eastern Canada: a case study

This paper discusses the effects of organic carbon oxidation on the dissolution and precipitation of calcium carbonate (aragonite and calcite) in fine-grained continental shelf and slope sediments, using data obtained during the Canadian Joint Global Ocean Flux Study program in the Gulf of St. Lawrence and on the Scotia shelf. The oxygen-penetration depth in these sediments is on the order of 10–15 mm, indicating that organic carbon is mineralized aerobically within this interval. Below this depth, organic matter degradation proceeds mostly through anoxic mineralization processes. The organic carbon content of these sediments decreases smoothly with depth. At all sites, the bottom water is supersaturated with respect to calcite. However, in most cores, immediately below the sediment–water interface, the acidity produced by the aerobic degradation of organic matter is sufficient to overcome the supersaturation of the overlying waters and induce CaCO 3 dissolution. This is most strongly reflected by an increase in the porewater calcium concentration near the sediment–water interface. Deeper in the cores, the saturation state of the porewaters increases at depth as a result of alkalinity generation by sulfate reduction and CaCO 3 of precipitates. Unlike organic carbon, the inorganic carbon content of the sediments therefore varies little or even increases with depth because what is lost through dissolution near the sediment–water interface is replaced at depth by precipitation. The carbon precipitated as CaCO 3 in the sulfate-reduction zone originates in part from the organic carbon, resulting in the preservation of a fraction of the original organic carbon as inorganic carbon. The precipitation of CaCO 3 in the anoxic zone creates an additional source of CO 2 to the porewaters. The geochemical significance of this source is discussed.

A model of early diagenetic processes from the shelf to abyssal depths

We present a numerical model of sedimentary early diagenetic processes that includes oxic and anoxic mineralization. The model belongs to the new wave of early diagenesis models that account for depth-dependent bioturbation and porosity profiles; it can be used both for calculating steady-state conditions and transient simulation. It was developed to reproduce the cycling of carbon, oxygen, and nitrogen along the ocean margin; it resolves the sediment-depth profiles of carbon, oxygen, nitrate, ammonium, and other reduced substances. Organic carbon is modeled as two degradable fractions with different first-order degradation rates and nitrogen:carbon ratios, to account for the decreasing reactivity and N/C ratio of the organic matter with depth into the sediment. The consumption of oxygen and nitrate as terminal electron acceptors is explicitly modeled, and mineralization is limited both by carbon (first order kinetics) and by oxidant availability (Michaelis-Menten type kinetics). Nitrification and oxic mineralization are decoupled, which allows the description of ammonium profiles. Mineralization processes using other oxidants (manganese oxides, iron oxides, sulphate) are lumped into one process, where degradation is only carbon limited; the terminal electron acceptors are not explicitly modeled, only the production of reduced substances is described. These substances are in part permanently removed (e.g., pyrite formation below the bioturbation zone) and partly diffuse towards the oxic layer where they react with oxygen. The values of several parameters were constrained using literature-derived relationships. The model was calibrated on a dataset obtained from the literature, which relates the magnitude of the different pathways to total organic carbon mineralization. The influence of carbon flux, bioturbation, sedimentation rate, bottomwater concentrations of oxygen, and nitrate and carbon degradability on the different mineralization pathways is examined. The relative contribution of the oxic mineralization in the model is significantly depressed under high organic flux, under low bottomwater oxygen conditions and when the bioturbation increases; higher carbon degradability has only a small positive effect, while sedimentation rate is relatively unimportant. Denitrification is mainly influenced by the nitrate concentration in the overlying bottomwater.

Benthic nutrient regeneration in the ERSEM ecosystem model of the North Sea

In any ecosystem modelling approach to shallow seas an adequate description of the sediment-water interactions to all the essential nutrients is necessary. With this aim a fairly simple concept has been developed and applied for the modelling of the nutrient cycles of N, P and Si which includes the essential diagenetic processes such as vertical transport, oxic and anoxic mineralization, silicate dissolution, adsorption, nitrification and denitrification. All these processes are explicitly formulated for one or more distinct layers in the sediment. The model contains a low number of variables and is efficient with respect to computer time. The model is part of the European Regional Seas Ecosystem Model (ERSEM) of the North Sea, a joint effort of several institutes around the North Sea. The benthic-pelagic coupling of the ERSEM model is discussed with emphasis on the consequences for the benthic nutrient cycling and the interrelations between the different benthic nutrient submodels. The results show that the model is able to describe the seasonal variation of nutrient fluxes including the sediment-water exchanges and the vertical profiles in the pore water. From the model results is concluded that changes in organic matter deposition directly influence the benthic nitrification and denitrification through changes in the oxygen availability to the nitrifiers and, due to this, the supply of nitrate to the denitrifiers. Short fluctuations in organic deposition ( e.g. after the spring bloom) are not reflected in the fluxes due to sorption buffering of phosphate in the oxidized surface layer of the sediment.