Benthic Fluxes Research Articles

Investigation on depth-dependent properties and benthic effluxes of dissolved organic matter (DOM) in pore water from plateau lake sediments

Dissolved organic matter (DOM) in sediment pore water plays a key role in lake water quality, complexation of the contaminants, biogeochemical cycles, and even climate. To better understand the vertical dynamics and characteristics of DOM among pore water profiles, we investigated concentrations, chemical structures, and benthic fluxes of DOM in plateau lake sediments via stoichiometric analysis, ultraviolet–visible (UV–vis) absorption, three-dimensional fluorescence matrix spectroscopy (EEMs), and parallel factor model (PARAFAC). The results revealed that dissolved organic carbon (DOC), concurrent with chromophoric DOM (CDOM) and fluorescent DOM (FDOM), tended to accumulate in the anaerobic deeper layers. These trends also demonstrated a good agreement with the ammonium nitrogen (NH4+-N) concentration, implying that the pathway of DOM dynamics was closely related to the redox state. The EEMs-PARAFAC model revealed that DOM in pore water mostly consisted of two terrestrial humic-like components (average contribution:> 60%) followed by microbial and tryptophan-like components, respectively. Terrestrial humic-like components were relatively stable, and their abundance was positively correlated with the FDOM level (p < 0.05). The changes of aromatic, molecular weight, and humification degree of DOM were possibly driven by microbial activities at different depths. Moreover, benthic flux estimations indicated that DOM and the total dissolved nitrogen (TDN) in inland sediments were potential sources for the overlying water and possibly affected the quality and ecological safety of lake water bodies.

Dispersal of persistent organic pollutants from fiber-contaminated sediments: biotic and abiotic pathways

PurposeNumerous sites contaminated with fiber emissions from pulp and paper industries are found in coastal areas of the Baltic Sea, but there is limited knowledge about the magnitude of dispersal of persistent organic pollutants (POPs) from these anthropogenic, organic-rich sediments called fiberbanks. The aim of this study was to quantify and compare different POP dispersal pathways from such fiberbanks. Dispersal mechanisms studied included abiotic and biotic routes (dissolved in water, particle-bound, and bioaccumulation).Materials and methodsContaminated fibrous sediments located in Ångermanälven River estuary in north-eastern Sweden were studied in sediment types representing different fiber content (i.e., fiberbanks, fiber-rich sediments, and less fiber impacted sediments). Sediment-to-water fluxes of dissolved contaminants (polychlorinated biphenyls (PCBs), hexachlorobenzene (HCB), and dichlorodiphenyltrichloroethane (DDT)) were measured in situ using benthic flux chambers. Particle resuspension was measured by sampling bottom water, before and after disturbing the sediment surface. Benthic biota was collected to determine the body burden of contaminants and to determine biota-pore water accumulation factors (BAFPW) and biota-sediment accumulation factors (BSAFs). In addition, concentrations of dissolved POPs in the water column were measured in field using passive samplers. Instrumental analysis was performed using gas chromatography coupled to a triple quadrupole mass spectrometer (GC-MS/MS).Results and discussionThe flux of dissolved Σ20PCBs was approximately two times higher from one of the investigated fiberbanks (3.4 ng m− 2 day− 1) compared to the other. The average particle burden of PCBs was also higher at this fiberbank after artificial disturbance (15 ng g−1 particle), which indicates that larger amounts of contaminants are likely to disperse via particle resuspension from this site compared to the other fiberbank (4.8 ng g− 1 particle). The difference might be associated with a layer of recently settled minerogenic material that covers one of the fiberbanks, which probably functions as a protective barrier. The lack of benthic biota implies that contaminant release by bioturbation is negligible in the studied fiberbanks. However, benthic biota from fiber-rich sediment showed bioaccumulation and biomagnification of contaminants.ConclusionsThe importance of diffusive flux from fiberbanks under undisturbed conditions became apparent when the different dispersal pathways were quantified. However, no dispersal pathway could be judged as irrelevant, since even under undisturbed conditions, advective particle transport was significant. Additionally, the uptake by biota and trophic transfer can be considerable. Quantification of dispersal routes and understanding of the relative importance of various pathways is critical for proper risk assessment and management of contaminated sediments.

Global continental and marine detrital εNd: An updated compilation for use in understanding marine Nd cycling

Understanding the role of sediment-water interactions in the oceanic cycling of neodymium (Nd) isotopes is essential for its reliable use as a modern and palaeoceanographic tracer of ocean circulation. However, the exact processes that control Nd cycling in the ocean are poorly defined and require an up-to-date knowledge of the sources, sinks and transformation of this tracer to and within the ocean (e.g. as per the GEOTRACES core mission). We propose a considerable improvement of Nd-source identification by providing an extensive and up-to-date compilation of published terrestrial and marine sedimentary Nd isotopic measurements. From this database, we construct high resolution, gridded, global maps that characterise the Nd-isotopic signature of the continental margins and seafloor sediment. Here, we present the database, interpolation methods, and final data products. Consistent with the previous studies that inform our compilation, our global results show unradiogenic detrital Nd isotopic values (εNd ≈ -20) in the North Atlantic, εNd values of ≈ -12 to -7 in the Indian and Southern Ocean, and radiogenic values (εNd ≈ -3 to +4) in the Pacific. The new, high-resolution interpolation is useful for improving conceptual knowledge of Nd sources and sinks and enables the application of isotope-enabled ocean models to understand targeted Nd behaviour in the oceans. Such applications may include: examining the strength and distribution of a possible benthic flux required to reconcile global Nd budgets, establishing the potential use of Nd isotopes as a kinematic tracer of ocean circulation, and a general quantification of the non-conservative sedimentary processes that may contribute to marine Nd cycling.

Defining a biogeochemical baseline for sediments at Carbon Capture and Storage (CCS) sites: An example from the North Sea (Goldeneye)

Injection of carbon dioxide (CO2) into subseafloor reservoirs is gaining traction as a strategy for mitigating anthropogenic CO2 emissions to the atmosphere. Yet, potential leakage, migration and dissolution of externally-supplied CO2 from such reservoirs are a cause for concern. The potential impact of CO2 leakage on the biogeochemistry of sediments and overlying waters in the North Sea was studied during a controlled subsurface CO2 release experiment in 2019 at a potential carbon capture and storage site (Goldeneye). This study describes the natural (unperturbed) biogeochemistry of sediments. They are classified as muddy sand to sandy mud with low organic carbon content (∼0.6 %). Distributions of dissolved inorganic carbon (DIC) and total alkalinity (TA) in sediment porewaters are reported in addition to in situ benthic fluxes of dissolved nutrients and oxygen between the sediments and the overlying water. Oxygen fluxes into the sediment, measured using benthic chambers and eddy covariance, were 6.18 ± 0.58 and 5.73 ± 2.03 mmol m−2 d-1, respectively. Diagnostic indicators are discussed that could be used to detect CO2 enrichment of sediments due to reservoir leakage at CCS sites. These include the ratio TA and ammonium to sulfate in sediment porewaters, benthic fluxes and chloride-normalized cation distributions. These indicators currently suggest that the organic carbon at Goldeneye has an oxidation state below zero and is mainly degraded via sulfate reduction. Carbonate precipitation is apparently negligible, whereas decreases in Mg2+ and K+ point toward ongoing alteration of lithogenic sediments by reverse weathering processes.

Submarine mud volcanoes as a source of chromophoric dissolved organic matter to the deep waters of the Gulf of C\xe1diz

Seafloor structures related to the emission of different fluids, such as submarine mud volcanoes (MVs), have been recently reported to largely contribute with dissolved organic matter (DOM) into the oceans. Submarine MVs are common structures in the Gulf of Cádiz. However, little is known about the biogeochemical processes that occur in these peculiar environments, especially those involving DOM. Here, we report DOM characterization in the sediment pore water of three MVs of the Gulf of Cádiz. Estimated benthic fluxes of dissolved organic carbon (DOC) and chromophoric DOM (CDOM) were higher than in other marine sediments with an average of 0.11 ± 0.04 mmol m−2 d−1 for DOC and ranging between 0.11 and 2.86 m−1 L m−2 d−1, for CDOM. Protein-like components represented ~ 70% of the total fluorescent DOM (FDOM). We found that deep fluids migration from MVs (cold seeps) and anaerobic production via sulfate-reducing bacteria represent a source of DOC and FDOM to the overlying water column. Our results also indicate that fluorescent components can have many diverse sources not captured by common classifications. Overall, MVs act as a source of DOC, CDOM, and FDOM to the deep waters of the Gulf of Cádiz, providing energy to the microbial communities living there.

Eddy correlation measurements of benthic oxygen fluxes in a stratified and operated reservoir

Benthic oxygen fluxes at the sediment-water interface (SWI) regulate the process and even direction of biochemical interfacial reactions; thus, they are crucial to the water quality. However, many medium- and large-sized reservoirs suffer from thermal stratification and human operations, lead to a significant impact on SWI oxygen exchanges. A 3-month-long noninvasive measurement of SWI oxygen fluxes was conducted in the Daheiting Reservoir using the eddy correlation method. Its result shows that reservoir stratification markedly affects the benthic boundary layer environment factors such as temperature, dissolved oxygen (DO) concentration, and hydrodynamics. In contrast, reservoir operations determine the benthic flow directions and its correlation to velocities in the mixed stage. A further discussion suggests that benthic temperatures mainly participate in the thermal structure rather than directly affecting the benthic fluxes. Conversely, it is DO concentrations coupled with hydrodynamics that control SWI oxygen fluxes. The flux is inhibited by DO shortage in anoxic conditions, but it is mainly controlled by hydrodynamics with redundant DO. Likewise, the hydrodynamics affects the fluxes by changing the diffusive boundary layer thickness, but this effect is also restrained by the DO field. These results indicate that a long-term and in-situ measurement of SWI oxygen fluxes in drinking reservoirs is of considerable significance to forecast water quality disasters, optimize operation plans, and ensure water security.

Potential linkage between sedimentary oxygen consumption and benthic flux of biogenic elements in a coastal scallop farming area, North Yellow Sea

Surface sediments were collected from a mariculture area adjacent to the Yangma Island suffering from hypoxia in summer, and a laboratory static incubation was conducted to study the sedimentary oxygen consumption (SOC) and the benthic fluxes of nutrients and fluorescent dissolved organic matter (FDOM). Compared with some coastal areas, the SOC of the studied area was relatively low in summer with the value of 2.34–6.03 mmol m−2 d−1. Sediment acted as an important source of nutrients (except for nitrate) and FDOM for the overlying water. Dissolved oxygen (DO) in the overlying water could affect the decomposition mode of sedimentary organic matter (SOM), i.e. aerobic and anaerobic decomposition and subsequently dominated the release of nutrients and FDOM. When DO > 50 μmol l−1, it was beneficial to the release of ammonium, silicate and FDOM. In contrast, low oxygen conditions, i.e. DO < 100 μmol l−1, stimulated sediment phosphate efflux. In addition, scallop farming activities also affected the SOC and benthic flux of nutrients and FDOM mainly through biological deposition.

Benthic oxygen fluxes in a coastal upwelling system (Ria de Vigo, NW Iberia) measured by aquatic eddy covariance

Organic carbon mineralization and nutrient cycling in benthic environments are critically important for their biogeochemical functioning, but are poorly understood in coastal upwelling systems. The main objective of this study was to determine benthic oxygen fluxes in a muddy sediment in the Ria de Vigo (NW Iberian coastal upwelling), by applying the aquatic eddy covariance (AEC) technique during 2 campaigns in different seasons (June and October 2017). The main drivers of benthic fluxes were studied and compared among days in each season and between seasons. The 2 campaigns were characterized by an upwelling-relaxation period followed by a downwelling event, the last of which was due to the extratropical cyclone Ophelia in October. The mean (±SD) seasonal benthic oxygen fluxes were not significantly different for the 2 campaigns despite differences in hydrodynamic and biogeochemical conditions (June: -20.9 ± 7.1 mmol m-2 d-1 vs. October: -26.5 ± 3.1 mmol m-2 d-1). Benthic fluxes were controlled by different drivers depending on the season. June was characterized by sinking labile organic material, which enhanced benthic fluxes in the downwelling event, whereas October had a significantly higher bottom velocity that stimulated the benthic fluxes. Finally, a comparison with a large benthic chamber (0.50 m2) was made during October. Despite methodological differences between AEC and chamber measurements, concurrent fluxes agreed within an acceptable margin (AEC:benthic chamber ratio = 0.78 ± 0.13; mean ± SD). Bottle incubations of water sampled from the chamber interior indicated that mineralization could explain this difference. These results show the importance of using non-invasive techniques such as AEC to resolve benthic flux dynamics.

Sedimentary Nutrient Supply in Productive Hot Spots off the West Antarctic Peninsula Revealed by Silicon Isotopes

In this study we evaluate the benthic fluxes of silicic acid along the West Antarctic Peninsula (WAP). Silicic acid (DSi) is one of the macronutrients essential in fuelling biological hot spots of diatom‐dominated primary production along the WAP. Here we measure the concentration and stable silicon isotopic composition of DSi in porewater profiles, biogenic silica content (BSi), and diatom abundance from sediment cores collected along the WAP. We couple these measurements with reaction‐transport modeling, to assess the DSi flux and the processes that release this key nutrient from the sediment into the overlying waters. Our results show that the benthic DSi flux is dominated by the diffusive flux, which is estimated to be equivalent to 26.7 ± 2.7 Gmol yr−1 for the WAP continental shelf. The DSi isotope profiles reveal the important impact of sedimentary processes on porewater DSi and suggest that biogenic silica dissolution is the main source of DSi in porewaters and consequently of the benthic fluxes. Our integrated data‐model assessment highlights the impact of surface productivity on sedimentary processes and the dynamic environment of core‐top sediments where dissolution and reverse weathering reactions control DSi exchanges.

An Invasive Mussel (Arcuatula senhousia, Benson 1842) Interacts with Resident Biota in Controlling Benthic Ecosystem Functioning

The invasive mussel Arcuatula senhousia has successfully colonized shallow soft sediments worldwide. This filter feeding mussel modifies sedimentary habitats while forming dense populations and efficiently contributes to nutrient cycling. In the present study, the density of A. senhousia was manipulated in intact sediment cores taken within an intertidal Zostera noltei seagrass meadow in Arcachon Bay (French Atlantic coast), where the species currently occurs at levels corresponding to an early invasion stage. It aimed at testing the effects of a future invasion on (1) bioturbation (bioirrigation and sediment mixing) as well as on (2) total benthic solute fluxes across the sediment–water interface. Results showed that increasing densities of A. senhousia clearly enhanced phosphate and ammonium effluxes, but conversely did not significantly affect community bioturbation rates, highlighting the ability of A. senhousia to control nutrient cycling through strong excretion rates with potential important consequences for nutrient cycling and benthic–pelagic coupling at a broader scale. However, it appears that the variability in the different measured solute fluxes were underpinned by different interactions between the manipulated density of A. senhousia and several faunal and/or environmental drivers, therefore underlining the complexity of anticipating the effects of an invasion process on ecosystem functioning within a realistic context.

Contrasting Effects of Bioturbation Studied in Intact and Reconstructed Estuarine Sediments

Macrofauna can produce contrasting biogeochemical effects in intact and reconstructed sediments. We measured benthic fluxes of oxygen, inorganic carbon, and nitrogen and denitrification rates in intact sediments dominated by a filter and a deposit feeder and in reconstructed sediments added with increasing densities of the same organisms. Measurements in reconstructed sediments were carried out 5 days after macrofauna addition. The degree of stimulation of the measured fluxes in the intact and reconstructed sediments was then compared. Results confirmed that high densities of bioturbating macrofauna produce profound effects on sediment biogeochemistry, enhancing benthic respiration and ammonium recycling by up to a factor of ~3 and ~9, respectively, as compared to control sediments. The deposit feeder also increased total denitrification by a factor of ~2, whereas the filter feeder activity did not stimulate nitrogen removal. Moreover, the effects of deposit feeders on benthic fluxes were significantly higher (e.g., on respiration and ammonium recycling) or different (e.g., on denitrification) when measured in intact and reconstructed sediments. In intact sediments, deposit feeders enhanced the denitrification coupled to nitrification and had no effects on the denitrification of water column nitrate, whereas in reconstructed sediments, the opposite was true. This may reflect active burrowing in reconstructed sediments and the long time needed for slow growing nitrifiers to develop within burrows. Results suggest that, in bioturbation studies, oversimplified experimental approaches and insufficient preincubation time might lead to wrong interpretation of the role of macrofauna in sediment biogeochemistry, far from that occurring in nature.

In situ incubations with the Gothenburg benthic chamber landers: Applications and quality control

Abstract In situ incubations of sediment with overlying water provide valuable and consistent information about benthic fluxes and processes at the sediment-water interface. In this paper, we describe our experiences and a variety of applications from the last 14 years and 308 deployments with the Gothenburg benthic chamber lander systems. We give examples of how we use sensor measurements for chamber leakage control, in situ chamber volume determination, control of syringe sampling times, sediment resuspension and stirring quality. We present examples of incubation data for in situ measurements of benthic fluxes of oxygen, dissolved inorganic carbon, nutrients, metals and gases made with our chamber landers, as well as manipulative injection experiments to study nitrogen cycling (injections of 15N nitrate), phosphate retention (injections of marl suspension) and targeted sediment resuspension. Our main goal is to demonstrate the possibilities that benthic chamber lander systems offer to measure solute fluxes and study processes at the sediment-water interface. Based on our experience, we recommend procedures to be used in order to obtain high quality data with benthic chamber landers.

Benthic Respiration in Hypoxic Waters Enhances Bottom Water Acidification in the Northern Gulf of Mexico

AbstractIt is known that surface water eutrophication enhances bottom water ocean acidification via respiration in coastal oceans. However, the role of benthic processes in influencing bottom water acidification has not been sufficiently explored. We examined this issue by analyzing a 10‐year summer carbonate chemistry dataset in bottom water together with recent benthic flux measurements and literature benthic flux data in the northern Gulf of Mexico. The difference between the observed and estimated pH (Ω) values calculated from anthropogenic CO2 increase and water column aerobic respiration were defined as ΔpH (ΔΩ). We found that ΔpH and ΔΩ values in hypoxic condition were −0.03 ± 0.04 (mean ± standard deviation) and −0.15 ± 0.39, respectively. Both ΔpH and ΔΩ values in hypoxic conditions were significantly lower than zero (p &lt; 0.05). The net results of anaerobic respiration, oxidation of reduced chemcials, burial of iron sulfide minerals, and possible CaCO3 dissolution may have led to an alkalinity to DIC production ratio of less than 1 in porewater. This caused the ratio of alkalinity to dissolved inorganic carbon fluxes from sediment to bottom water to be less than 1, which led to additional bottom water acidification. Our analysis and model simulations demonstrate that severe hypoxic and anoxic conditions, which correspond to less water movement, favor the accumulation of benthic respiration products, leading to additional pH and Ω reductions. The findings on sediment processes contributing to acidification in bottom waters provide new insights into the sensitivity of coastal ocean acidification to low‐oxygen conditions under current and future climates and anthropogenic nutrient loading scenarios.

Fueling of a marine-terrestrial ecosystem by a major seabird colony

Seabirds redistribute nutrients between different ecosystem compartments and over vast geographical areas. This nutrient transfer may impact both local ecosystems on seabird breeding islands and regional biogeochemical cycling, but these processes are seldom considered in local conservation plans or biogeochemical models. The island of Stora Karlsö in the Baltic Sea hosts the largest concentration of piscivorous seabirds in the region, and also hosts a large colony of insectivorous House martins Delichon urbicum adjacent to the breeding seabirds. We show that a previously reported unusually high insectivore abundance was explained by large amounts of chironomids—highly enriched in δ15N—that feed on seabird residues as larvae along rocky shores to eventually emerge as flying adults. Benthic ammonium and phosphate fluxes were up to 163% and 153% higher close to the colony (1,300 m distance) than further away (2,700 m) and the estimated nutrient release from the seabirds at were in the same order of magnitude as the loads from the largest waste-water treatment plants in the region. The trophic cascade impacting insectivorous passerines and the substantial redistribution of nutrients suggest that seabird nutrient transfer should be increasingly considered in local conservation plans and regional nutrient cycling models.

Technical note: Measurements and data analysis of sediment–water oxygen flux using a new dual-optode eddy covariance instrument

Abstract. Sediment–water oxygen fluxes are widely used as a proxy for organic carbon production and mineralization at the seafloor. In situ fluxes can be measured non-invasively with the aquatic eddy covariance technique, but a critical requirement is that the sensors of the instrument are able to correctly capture the high-frequency variations in dissolved oxygen concentration and vertical velocity. Even small changes in sensor characteristics during deployment as caused, e.g. by biofouling can result in erroneous flux data. Here we present a dual-optode eddy covariance instrument (2OEC) with two fast oxygen fibre sensors and document how erroneous flux interpretations and data loss can effectively be reduced by this hardware and a new data analysis approach. With deployments over a carbonate sandy sediment in the Florida Keys and comparison with parallel benthic advection chamber incubations, we demonstrate the improved data quality and data reliability facilitated by the instrument and associated data processing. Short-term changes in flux that are dubious in measurements with single oxygen sensor instruments can be confirmed or rejected with the 2OEC and in our deployments provided new insights into the temporal dynamics of benthic oxygen flux in permeable carbonate sands. Under steady conditions, representative benthic flux data can be generated with the 2OEC within a couple of hours, making this technique suitable for mapping sediment–water, intra-water column, or atmosphere–water fluxes.

The relation between temperature and silica benthic exchange rates and implications for near-seabed formation of diagenetic opal

This study calculated the dissolution rates of biogenic silica deposited on the seafloor and the silicic acid benthic flux for 22 Ocean Drilling Program sites. Simple models developed for two host sediment types—siliciclastic and carbonate—were used to explain the variability of biogenic silica dissolution and recycling under present-day low (−0.3 to 2.14 °C) bottom-water temperatures. The kinetic constants describing silicic acid release and silica saturation concentration increased systematically with increasing bottom-water temperatures. When these temperature effects were incorporated into the diagenetic models, the prediction of dissolution rates and diffusive fluxes was more robust. This demonstrates that temperature acts as a primary control that decreases the relative degree of pore-water saturation with biogenic opal while increasing the silica concentration. The correlation between the dissolution rate and benthic flux with temperature was pronounced at sites where biogenic silica is hosted in surficial sediments mostly composed of biogenic carbonates. This association is because the dissolution of carbonates provides the alkalinity necessary for both silica dissolution (also its reprecipitation as opal-CT) and clay formation; thus strongly reducing the retarding influence of clays on biogenic opal dissolution. Conversely, the silica exchange rates were modified by presence of aluminosilicates, which led to a higher burial efficiency for biogenic opal in detrital- than in carbonate-dominated benthic layers. Though model prediction of first-order silica early transformation suggests likely effects from surface temperatures (0–4 °C) on opal-CT precipitation over short geological times (< 4 Ma) near seabed in the Antarctic Site 751, the relationship between silica solubility and surface-area variability in time is a more critical control. Since silica solubility and surface area decrease with time, the < 4 Ma elapsed time aged opal-A to the point that changes in specific surface area caused minor effects on solubility, allowing for formation of opal-CT at low temperature settings near the seabed.

Technical note: Estimating light-use efficiency of benthic habitats using underwater O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; eddy covariance

Abstract. Light-use efficiency defines the ability of primary producers to convert sunlight energy to primary production and is computed as the ratio between the gross primary production and the intercepted photosynthetic active radiation. While this measure has been applied broadly within terrestrial ecology to investigate habitat resource-use efficiency, it remains underused within the aquatic realm. This report provides a conceptual framework to compute hourly and daily light-use efficiency using underwater O2 eddy covariance, a recent technological development that produces habitat-scale rates of primary production under unaltered in situ conditions. The analysis, tested on two benthic flux datasets, documents that hourly light-use efficiency may approach the theoretical limit of 0.125 O2 per photon under low-light conditions, but it decreases rapidly towards the middle of the day and is typically 10-fold lower on a 24 h basis. Overall, light-use efficiency provides a useful measure of habitat functioning and facilitates site comparison in time and space.

Benthic fluxes of oxygen and nutrients under the influence of macrobenthic fauna on the periphery of the intermittently hypoxic zone in the Baltic Sea

Understanding the role of benthic organisms in marine sediments is becoming increasingly important with the growing problem of eutrophication of marine ecosystems around the world, including the Baltic Sea. Therefore, we have conducted a series of incubation experiments on sediment cores collected from sites characterized by varying oxygen conditions and measured the influx (uptake by sediment) of oxygen as well as the sediment–water exchange of phosphate, ammonia and silicate. The research involved macrozoobenthic communities that clearly reflect oxygen deficiency episodes in the past. We also used Fick's first law to calculate diffusive fluxes of nutrients.The size and direction of benthic fluxes largely depend on physicochemical conditions. Silicate fluxes were affected by the biomass of macrozoobenthos in the surface layer of sediments, while those of phosphate – effluxes – by the biomass of the Baltic tellin Limecola balthica and Marenzelleria polychaetes. Despite oxic conditions in all incubated cores, we observed a phosphate release at most sites. The estimation of substance fluxes at the sediment–water interface based on the incubation experiments gave comparable values – to those of diffusive fluxes – only in areas without benthic communities. We have demonstrated how benthic macrofauna affects the exchange of substances at the sediment–water interface when bottom-water oxygen conditions fluctuate between normoxic and hypoxic.

Towards balancing the oceanic Ni budget

Nickel isotopes are a novel and promising tracer of the chemistry of past ocean environments, but realisation of this tracer's potential requires a comprehensive understanding of the controls on Ni burial in the marine sedimentary archive. An outstanding puzzle in the marine budget of Ni, first recognised in the 1970s, is a major imbalance in the known inputs and outputs to and from the ocean: the sedimentary outputs of Ni are much larger than the inputs (rivers, dust). Much more recently, it has also been recognised that the outputs are also considerably isotopically heavier than the inputs. In this study, we find light Ni isotope compositions (δ60NiNIST SRM986 = −0.2 to −0.8‰) for Mn-rich sediments from the eastern Pacific compared to Fe-Mn crusts (at about +1.6‰). These data suggest that diagenetic remobilisation of isotopically heavy Ni leads to a significant benthic Ni flux (estimated at 0.6−2.3×108 mol/yr), similar in magnitude to the riverine flux, to the ocean. Diagenetic remobilisation of Ni may occur either via cycles of Mn-oxide dissolution and precipitation, with associated Ni sorption and release, or during mineralogical transformation of birnessite to todorokite. A minor role for retention of isotopically light Ni by Fe oxides or Fe-rich authigenic clays is also proposed. Overall, a benthic flux of isotopically heavy Ni (at about +3‰) can balance the marine Ni budget, pinpointing diagenesis as a key missing piece of the Ni puzzle.

The control of hydrogen sulfide on benthic iron and cadmium fluxes in the oxygen minimum zone off Peru

Abstract. Sediments in oxygen-depleted marine environments can be an important sink or source of bio-essential trace metals in the ocean. However, the key mechanisms controlling the release from or burial of trace metals in sediments are not exactly understood. Here, we investigate the benthic biogeochemical cycling of iron (Fe) and cadmium (Cd) in the oxygen minimum zone off Peru. We combine bottom water and pore water concentrations, as well as benthic fluxes determined from pore water profiles and from in situ benthic chamber incubations, along a depth transect at 12∘ S. In agreement with previous studies, both concentration–depth profiles and in situ benthic fluxes indicate a release of Fe from sediments to the bottom water. Diffusive Fe fluxes and Fe fluxes from benthic chamber incubations (−0.3 to −17.5 mmol m−2 yr−1) are broadly consistent at stations within the oxygen minimum zone, where the flux magnitude is highest, indicating that diffusion is the main transport mechanism of dissolved Fe across the sediment–water interface. The occurrence of mats of sulfur-oxidizing bacteria on the seafloor represents an important control on the spatial distribution of Fe fluxes by regulating hydrogen sulfide (H2S) concentrations and, potentially, Fe sulfide precipitation within the surface sediment. Rapid removal of dissolved Fe after its release to anoxic bottom waters hints at oxidative removal by nitrite and interactions with particles in the near-bottom water column. Benthic flux estimates of Cd suggest a flux into the sediment within the oxygen minimum zone. Fluxes from benthic chamber incubations (up to 22.6 µmol m−2 yr−1) exceed diffusive fluxes (&lt;1 µmol m−2 yr−1) by a factor of more than 25, indicating that downward diffusion of Cd across the sediment–water interface is of subordinate importance for Cd removal from benthic chambers. As Cd removal in benthic chambers covaries with H2S concentrations in the pore water of surface sediments, we argue that Cd removal is mediated by precipitation of cadmium sulfide (CdS) within the chamber water or directly at the sediment–water interface. A mass balance approach, taking the contributions of diffusive and chamber fluxes as well as Cd delivery with organic material into account, suggests that CdS precipitation in the near-bottom water could make an important contribution to the overall Cd mass accumulation in the sediment solid phase. According to our results, the solubility of trace metal sulfide minerals (Cd ≪ Fe) is a key factor controlling trace metal removal and, consequently, the magnitude and the temporal and spatial heterogeneity of sedimentary fluxes. We argue that, depending on their sulfide solubility, sedimentary source or sink fluxes of trace metals will change differentially as a result of declining oxygen concentrations and the associated expansion of sulfidic surface sediments. Such a trend could cause a change in the trace metal stoichiometry of upwelling water masses with potential consequences for marine ecosystems in the surface ocean.