Pore Water Concentration Gradients Research Articles

Longevity and efficacy of lanthanum-based P remediation under changing dissolved oxygen availability in a small eutrophic lake.

We set out to study the seasonal variations in porewater phosphorus and lanthanum concentrations in the dated sediment cores from a small eutrophic lake that has been treated with Phoslock, a lanthanum-modified bentonite (LMB) amendment. Three sites were sampled when the hypolimnion was either oxygenated or anoxic: (i) the lake's deepest point, (ii) a littoral site receiving inflows from the catchment, and (iii) a littoral site influenced by nearby septic tanks. Phosphate (PO43--P), lanthanum (La), iron (Fe), dissolved organic carbon (DOC) and sulfate (SO42-) were measured in porewater samples. An inverse diagenetic model was used to quantify fluxes of dissolved elements across the sediment-water interface as well as the net rate of their reactions along the porewater concentration gradients. Results show that porewater P and Fe underwent strong seasonal dynamics, while La did not. P fluxes, 20-fold higher at the deepest site than elsewhere in the basin, were influenced by anoxic conditions in the hypolimnion during summer and winter, suggesting that P mobility remained sensitive to redox fluctuations despite the addition of La. At the deepest site, fluxes of P across the sediment-water interface increased from 1 to 9 × 10-9 μmol cm-2 s-1 between spring and summer, while the rate of P production to the porewater also increased a hundredfold. These increases were concurrent with Fe mobilization. Finally, sediment dating shows that the fraction of P sequestered by La is buried under freshly deposited sediment at a rate of 2-3 mm per year. These results indicate that external P fluxes and erosion control remain crucial to maintain the longevity of the LMB treatment.

Uranium reduction and isotopic fractionation in reducing sediments: Insights from reactive transport modeling

Uranium concentrations and isotopic ratios (238U/235U, denoted as δ238U) have been used to provide quantitative information about the degree of oxygenation and de-oxygenation of past oceans. The potential to constrain changes in global redox conditions, in contrast to many other proxies that reflect local conditions, is a particular strength of the uranium isotope approach. Because uranium reduction primarily occurs in sediments underlying anoxic water columns rather than in the anoxic water column itself, the removal of uranium in organic-rich shales is the largest lever on seawater δ238U. Accordingly, accumulation and isotopic fractionation are modulated by local variations in productivity, basin connectivity, sedimentation rate, and bottom-water redox conditions. To isolate the processes at the sediment-water interface that control δ238U and uranium accumulation in reducing sediments, we constructed a reactive transport model that couples biogeochemical reactions to diffusive transport and the burial of solutes and minerals.Using the model framework, we test the sensitivity of authigenic uranium isotopic fractionation and accumulation to oxygenation, permeability, sedimentation rate, organic carbon delivery, and basin restriction. Our results demonstrate that these external forcings produce diagnostic patterns in isotopic fractionation. Specifically, the model predicts that authigenic δ238U is sensitive to productivity because of the associated organic carbon burial rate. Moreover, our results suggest that the isotopic offset does not vary significantly due to changing bottom-water O2 concentrations, but the amount of accumulation does—a result that differs from previous estimates. Water column uranium reduction adds additional complexity to the ultimate δ238U value. The predictive patterns derived from model results can offer insight into local depositional conditions, such as sedimentation patterns. Collectively, these effects—including bottom-water redox conditions and related reducing sediments—alter the isotopic signature of the overlying water column according to the authigenic δ238U value and the diffusive fluxes arising from porewater concentration gradients. More broadly, this work provides important new constraints on the major controls on the δ238U of sediments while also supporting its use as a proxy for global marine redox conditions.

Estimates of the remineralization and burial of organic carbon in Lake Baikal sediments

Sediment cores collected from several stations throughout Lake Baikal in water depths from 100 m off the Selenga River delta to the deepest basin of the lake (~1640 m), have been analyzed for sedimentary organic carbon, nitrogen, and the remineralized components in pore water. The organic carbon content of surface sediments generally varied from 2.3 to 3.2% by weight, and profiles typically showed an exponential decrease in both organic carbon and nitrogen in the upper 20–30 cm of the sediment column. Steady state models of organic matter diagenesis yield first order decomposition rate constants which range from 0.0009 to 0.022 y−1. The calculated residence times for the metabolizable fraction of the organic matter in these sediments increases roughly with increasing water depth and is on the order of 50–300 years. Pore water concentration profiles were determined for dissolved inorganic carbon, dissolved organic carbon (DOC), methane, and dissolved ammonium. At depth (25–30 cm) methane concentrations ranged from 50 to 800 μmol Lpw−1 and DOC from 400 to 900 μmol Lpw−1. Estimation of carbon recycling rates based upon diffusion along pore water concentration gradients at the sediment-water interface, indicate that combined DOC and methane fluxes generally contribute <15% of the overall turnover of sedimentary organic carbon. Comparisons to Laurentian Great Lakes environments show trends in sediment deposition, organic matter remineralization, and the time scales of carbon recycling across nearly two orders of magnitude with the fraction of organic content buried generally decreasing with decreasing sedimentation rates.

Beyond the Black Sea paradigm: The sedimentary fingerprint of an open-marine iron shuttle

We present iron (Fe) concentration and Fe isotope data for a sediment core transect across the Peru upwelling area, which hosts one of the ocean’s most pronounced oxygen minimum zones (OMZs). The lateral progression of total Fe to aluminum ratios (FeT/Al) across the continental margin indicates that sediments within the OMZ are depleted in Fe whereas sediments below the OMZ are enriched in Fe relative to the lithogenic background. Rates of Fe loss within the OMZ, as inferred from FeT/Al ratios and sedimentation rates, are in agreement with benthic flux data that were calculated from pore water concentration gradients. The mass of Fe lost from sediments within the OMZ is within the same order of magnitude as the mass of Fe accumulating below the OMZ. Taken together, our data are in agreement with a shuttle scenario where Fe is reductively remobilized from sediments within the OMZ, laterally transported within the anoxic water column and re-precipitated within the more oxic water below the OMZ. Sediments within the OMZ have increased 56Fe/54Fe isotope ratios relative to the lithogenic background, which is consistent with the general notion of benthic release of dissolved Fe with a relatively low 56Fe/54Fe isotope ratio. The Fe isotope ratios increase across the margin and the highest values coincide with the greatest Fe enrichment in sediments below the OMZ. The apparent mismatch in isotope composition between the Fe that is released within the OMZ and Fe that is re-precipitated below the OMZ implies that only a fraction of the sediment-derived Fe is retained near-shore whereas another fraction is transported further offshore. We suggest that a similar open-marine shuttle is likely to operate along many ocean margins. The observed sedimentary fingerprint of the open-marine Fe shuttle differs from a related transport mechanism in isolated euxinic basins (e.g., the Black Sea) where the laterally supplied, reactive Fe is quantitatively captured within the basin sediments. We suggest that our findings are useful to identify OMZ-type Fe cycling in the geological record.

Effects of temperature on silicate weathering: Solute fluxes and chemical weathering in a temperate rain forest watershed, Jamieson Creek, British Columbia

Chemical weathering of silicate minerals has long been known as a sink for atmospheric CO 2, and feedbacks between weathering and climate are believed to affect global climate. While warmer temperatures are believed to increase rates of weathering, weathering in cool climates can be accelerated by increased mineral exposure due to mechanical weathering by ice. In this study, chemical weathering of silicate minerals is investigated in a small temperate watershed. The Jamieson Creek watershed is covered by mature coniferous forest and receives high annual precipitation (4000 mm), mostly in the form of rainfall, and is underlain by quartz diorite bedrock and glacial till. Analysis of pore water concentration gradients indicates that weathering in hydraulically unsaturated ablation till is dominated by dissolution of plagioclase and hornblende. However, a watershed scale solute mass balance indicates high relative fluxes of K and Ca, indicating preferential leaching of these solutes possibly from the relatively unweathered lodgement till. Weathering rates for plagioclase and hornblende calculated from a watershed scale solute mass balance are similar in magnitude to rates determined using pore water concentration gradients. When compared to the Rio Icacos basin in Puerto Rico, a pristine tropical watershed with similar annual precipitation and bedrock, but with dissimilar regolith properties, fluxes of weathering products in stream discharge from the warmer site are 1.8 to 16.2-fold higher, respectively, and regolith profile-averaged plagioclase weathering rates are 3.8 to 9.0-fold higher. This suggests that the Arrhenius effect, which predicts a 3.5- to 9-fold increase in the dissolution rate of plagioclase as temperature is increased from 3.4° to 22 °C, may explain the greater weathering fluxes and rates at the Rio Icacos site. However, more modest differences in K and Ca fluxes between the two sites are attributed to accelerated leaching of those solutes from glacial till at Jamieson Creek. Our findings suggest that under conditions of high rainfall and favorable topography, weathering rates of silicate minerals in warm tropical systems will tend to be higher than in cool temperate systems, even if the temperate system is has been perturbed by an episode of glaciation that deposits regolith high in fresh mineral surface area.

The cycling and oxidation pathways of organic carbon in a shallow estuary along the Texas Gulf Coast

The cycling and oxidation pathways of organic carbon were investigated at a single shallow water estuarine site in Trinity Bay, Texas, the uppermost lobe of Galveston Bay, during November 2000. Radio-isotopes were used to estimate sediment mixing and accumulation rates, and benthic chamber and pore water measurements were used to determine sediment-water exchange fluxes of oxygen, nutrients and metals, and infer carbon oxidation rates. Using 7Be and 234Th XS, the sediment-mixing coefficient ( D b) was 4.3 ± 1.8 cm 2 y −1, a value that lies at the lower limit for marine environments, indicating that mixing was not important in these sediments at this time. Sediment accumulation rates ( S a), estimated using 137Cs and 210Pb XS, were 0.16 ± 0.02 g cm −2 y −1. The supply rate of organic carbon to the sediment-water interface was 30 ± 3.9 mmol C m −2 d −1, of which ∼10% or 2.9 ± 0.44 mmol C m −2 d −1was lost from the system through burial below the 1-cm thick surface mixed layer. Measured fluxes of O 2 were 26 ± 3.8 mmol m −2 d −1 and equated to a carbon oxidation rate of 20 ± 3.3 mmol C m −2 d −1, which is an upper limit due to the potential for oxidation of additional reduced species. Using organic carbon gradients in the surface mixed layer, carbon oxidation was estimated at 2.6 ± 1.1 mmol C m −2 d −1. Independent estimates made using pore water concentration gradients of ammonium and C:N stoichiometry, equaled 2.8 ± 0.46 mmol C m −2 d −1. The flux of DOC out of the sediments (DOC efflux) was 5.6 ± 1.3 mmol C m −2 d −1. In general, while mass balance was achieved indicating the sediments were at steady state during this time, changes in environmental conditions within the bay and the surrounding area, mean this conclusion might not always hold. These results show that the majority of carbon oxidation occurred at the sediment-water interface, via O 2 reduction. This likely results from the high frequency of sediment resuspension events combined with the shallow sediment mixing zone, leaving anaerobic oxidants responsible for only ∼10–15% of the carbon oxidized in these sediments.

Iron monosulfide formation and oxidation in drain-bottom sediments of an acid sulfate soil environment

Iron monosulfide formation and oxidation processes were studied in the extensively drained acid sulfate soil environment of the Tweed River floodplain in eastern Australia. Porewater profiles of pH, Eh, SO 4 2−, Fe 2+, Fe 3+, Cl −, HCO 3 −, and metals (Cd, Co, Cr, Cu, Ni, Pb and Zn) were obtained using in situ dialysis membrane samplers (`peepers'). Concentrations of acid volatile S (AVS), pyrite, total S, reactive Fe, total and organic C, simultaneously extracted metals (SEMs) and total elemental composition by X-ray fluorescence, were determined on sediment samples. The oxidation of pyrite in the surrounding landscape provides a source of acidity, Fe, Al, SO 4 and metals, which are exported into the drainage system where they accumulate in the sediments and porewaters. Negative porewater concentration gradients of SO 4 2− and Fe 2+, and large AVS concentrations in the sediments, indicate Fe monosulfides form rapidly under reducing conditions and consume acidity and metals. Oxidation of the sediments during previous drought episodes has resulted in the conversion of monosulfides and pyrite to oxidised Fe minerals and the release of acidity, SO 4 2−, Fe 3+, and metals to the surface waters. These formation and oxidation cycles show that Fe monosulfides play an important role in controlling water quality in the drainage system.

Biogeochemical cycles of Org-C, Tot-N and Tot-S in the sediment of the Ghar El Melh Lagoon (north of Tunisia)

Pore water analysis of chloride (Cl −), alkalinity, ammonium (NH 4 +), and sulfate (SO 4 2−) in the Lagoon of Ghar El Melh and the evolution of their relative concentrations indicate that these concentrations had changed over time reflecting the non-steady state conditions prevailing in this lagoon. Cl −, which is considered in this study as a non-conservative tracer element, was used to determine the effective diffusion rates, including bioirrigation or wind action in lagoon sediment, indicating that the effective diffusion coefficient ( D eff) varied between 10 and 20 dm 2 year −1 in summer and was equal to 50 dm 2 year −1 in autumn. The so-calculated diffusion rates clearly expressed the particular role of bioirrigation and wind action as major factors which control the biogeochemical processes in this lagoon. The examination of the 210Pb profile along the sediment column indicated that bioturbation was low in this lagoon, and did play only a very limited role. On the other hand, based on the excess of 210Pb, the sedimentation rate ( w) had been estimated to be less than 0.1 cm year −1 in this lagoon. The alkalinity and NH 4 + modeling combined with calculations for Cl − diffusion have led to the estimation of the following production rates ( R p) at the surface of the lagoon sediment: alkalinity=52–60 mmol l −1 year −1 and NH 4 +=8 mmol l −1 year −1. In spring, D eff and R p were fitted simultaneously showing the following results: D eff alk=10–50 dm 2 year −1; R p alk=57 mmol l −1 year −1; D eff NH 4 + =150 dm 2 year −1 and R p NH 4 + =8 mmol l −1 year −1. Such particularly high values of D eff reflect one more time the crucial impact of bioirrigation or wind action on the biogeochemical processes developing within the pore-water sediment. This result is however supported by the increase of micro-organisms in the sediment and by the typical windy periods occurring in spring. Fluxes calculated using the concentration gradients in pore water within the undisturbed sediment, ranged from 4.48 to 11.13 mmol m −2 day −1 for alkalinity, 0.011–0.351 mmol m −2 day −1 for NH 4 + and 4.64–11.92 mmol m −2 day −1 for T–CO 2. Nevertheless, the calculated NO 3 − fluxes were very low. Seasonal fluxes of SO 4 2−, estimated by the alkalinity budget, are found to range from 1.18 and 3.43 mmol m −2 day −1 in the undisturbed sediment. Combination of fluxes determined using pore water concentration gradients and solid phases have led to a better understanding of the biogeochemical cycles of organic carbon (Org-C), total nitrogen (Tot-N) and total sulfur (Tot-S) developing in the undisturbed sediment of the lagoon. The results showed that Org-C regeneration remained important in the deep sediment, while most of the Tot-N regeneration occurred mainly near the sediment–water interface. As regards the Tot-S, this element appeared to be mainly governed either by the SO 4 2− reduction within the undisturbed sediment and/or by the FeS oxidation/reduction processes occurring at the upper part of the sediment.

Fluid and geochemical transport through oceanic crust: a transect across the eastern flank of the Juan de Fuca Ridge

The geochemical implications of thermally driven flow of seawater through oceanic crust on the mid-ocean ridge flank have been examined on a well-studied 80 km transect across the eastern flank of the Juan de Fuca Ridge at 48–N, using porewater and basement fluid samples obtained on ODP Leg 168. Fluid flow is recognised by near-basement reversals in porewater concentration gradients from altered values in the sediment section to seawater-like values in basaltic basement. In general, the basement fluids become more geochemically evolved with distance from the ridge and broadly follow basement temperature which ranges from ~16° to 63°C. Although thermal effects of advective heat exchange are only seen within 20 km east of where basement is exposed near the ridge crest, chemical reactivity extends to all sites. Seawater passing through oceanic crust has reacted with basement rocks leading to increases in Ca 2+ and decreases in alkalinity, Mg 2+, Na +, K +, SO 2- and δ 18O. Sr isotope exchange between seawater and oceanic crust off axis is unequivocally demonstrated with endmember 87Sr/ 86Sr ~ 0.707. Evidence of more evolved fluids is seen at sites where rapid upwelling of fluids through sediments occurs. Chlorinities of the basement fluids are consistent with post-glacial seawater and thus a short residence time in the crust. Rates of lateral flow have been by estimated by modelling porewater sulphate gradients, using Cl as a glacial chronometer, and from radiocarbon dating of basal fluids. All three methods reveal fluid flow with 14C ages less than 10,000 yr and particle velocities of ~1–5 m/yr, in agreement with thermally constrained volumetric flow rates through a ~600 m thick permeable layer of ~10% porosity. Δ element/Δ heat extraction ratios are similar to values for ridge-crest hydrothermal systems.

Fluxes of dissolved organic carbon from California continental margin sediments

Fluxes of dissolved organic carbon (DOC) from marine sediments represent a poorly constrained component of the oceanic carbon cycle that may affect the concentration and composition of DOC in the ocean. Here we report the first in situ measurements of DOC fluxes from continental margin sediments (water depths ranging from 95 to 3,700 m), and compare these fluxes with measured benthic fluxes from 20 other coastal and continental margin sediments. With this combined data set data we have estimated that benthic DOC fluxes are less than ∼10% of sediment carbon oxidation rates, and that the integrated DOC flux from sediments in water depths less than 2,000 m is ∼180 Tg C/yr. These fluxes are roughly equivalent to the riverine DOC flux, and the organic carbon burial rate in marine sediments. Benthic DOC fluxes therefore represent an important net source of DOC to the oceans. We also note that: (1) benthic DOC fluxes represent a loss of organic carbon from sediments; (2) in many sediments these fluxes appear to be controlled by molecular diffusion (i.e., by pore water concentration gradients); (3) pore water DOC may be an important intermediate in sediment carbon burial and preservation. These observations therefore suggest a linkage between benthic DOC fluxes and sediment carbon preservation that may be mediated by pore water DOC concentrations and cycling. The magnitude and fate of DOC effluxing from marine sediments is thus important to understanding carbon cycles and budgets in the marine environment.

Manganese and Iron Oxidation During Benthic Oxygenic Photosynthesis

The effect of benthic oxygenic photosynthesis on sediment-water fluxes of manganese and iron was studied for an intertidal sediment. Undisturbed sediments were incubated at an incident surface irradiance of 250μEm−2s−1at 26°C. Oxygenic photosynthesis was selectively inhibited by adding [3-(3,4-dichloro)-1,1-dimethyl-urea] (DCMU). Benthic fluxes were determined experimentally from the change in manganese and iron concentrations in the overlying water, and were predicted from the pore water concentration gradients at the sediment-water interface assuming molecular diffusion as the transport mechanism. The experimental fluxes of manganese and iron in DCMU-treated cores amounted to −0·84 and −0·59mmolm−2day−1, respectively, and were directed from the sediment towards the overlying water. In the control cores, showing high rates of benthic oxygenic photosynthesis, the fluxes of manganese and iron were directed towards the sediment, 0·06 and 0·01mmolm−2day−1, respectively. Mass balances for the 0·1–0·14cm thick oxic zone, calculated from the experimental fluxes and the predicted fluxes, suggest a minimum areal reoxidation of 0·6mmolm−2day−1for manganese and of 0·48mmolm−2day−1for iron in cores showing benthic photosynthesis. The estimated turnover times for dissolved Mn2+and dissolved Fe2+in the oxic surface layer during benthic photosynthesis were 0·8 and 0·25h, respectively. Sediment oxygen microprofiles and the sediment pH profiles suggest that chemical precipitation and reoxidation dominates the retention of manganese and iron during benthic oxygenic photosynthesis in shallow intertidal sediments.

Iron and sulfate reduction in the sediments of acidic mine lake 116 (Brandenburg, Germany): Rates and geochemical evaluation

A combination of rate measurements of iron(III)oxide and sulfate reduction, thermodynamic data, and pore-water and solid phase analyses was used to evaluate the relative significance of iron and sulfate reduction in the sediments of an acidic strip mining lake (Lake 116, Brandenburg, Germany). The rate of sulfate reduction was determined using a 35S-radiotracer method. Rates of iron turnover were quantified by mass balances based on pore-water concentration profiles. The differences in Gibbs free energy yield from reduction of iron and sulfate and from methanogenesis were calculated from individual redox couples and concentrations of reactants to account for the influence of high Fe2+ concentrations and differing mineral phases. Integrated (O-20 cm) mean rates of sulfate reduction were 1.2 (pelagial), respectively 5.2 (littoral) mmol (m2d)-1. Based on electron equivalents, the estimated iron reduction rates reached between about 50 % (pelagial) and 75 % (littoral) of the sulfate reduction rates. Compared to conditions usually assumed in the literature, in the sediments Gibbs free energy advantage of iron reduction over sulfate reduction was reduced frmm +11 KJeq-1 to a range of about +7 KJeq-1 (ferrihydrite, "reactive iron") to -6 KJeq-1 (goethite). This indicates that iron reduction was thermodynamically favored to sulfate reduction only if amorphous iron(III)oxides were available and is in accordance to the high competitiveness of sulfate reducers in the sediment. While total iron concentration in the sediments was high (up to 80% of the dryweight), reactive iron only accounted for 11-38% and was absolutely and relatively diminished in the zone of iron reduction. Pore-water concentration gradients and 137CS profiles indicated that little or no bioturbation occurred in the sediments, probably inhibiting the renewal of reactive iron. We further hypothesize that the reactivity of the iron oxide surfaces was reduced due to adsorption of DOM, suggested by IR spectra of the DOM and by a surface coverage estimate using literature data. Pelagial and littoral sediments displayed different dynamics. At the littoral relative iron reduction rate estimates were higher, iron sulfides were not accumulated and residence times of iron oxides were short compared to the pelagial. At the littoral site reoxidation of iron sulfides probably resulted in the renewal of reactive iron(III)oxides, possibly allowing for higher relative rates of iron reduction.

The activity ofNereis Diversicolor(Polychaeta) and its impact on nutrient fluxes in estuarine waters

Abstract Seasonally and spatially varying populations of the polychaete Nereis diversicolor have been studied at three sites in the estuary of the River Tamar, south west England. Undisturbed cores from each of these sites, and from a fourth site devoid of this species, were incubated in the laboratory on each of six dates between December 1990 and May 1992 for sediment-water exchange studies. The fluxes of soluble ammonium and silicate were shown to be up to 100 times greater than those calculated from pore-water concentration gradients, and to be related to the extent of burrow surfaces as estimated from the population parameters of the worms recovered from the cores. Cores with the most worms (up to 5000 per m2) had burrow surfaces areas equivalent to between 200 and 300% of the unburrowed interface. This scale of increase was insufficient to fully explain the magnitude of the fluxes which are further enhanced by irrigation of the burrow surfaces, causing nutrient-enriched pore waters diffusing through...

Benthic lead fluxes in San Francisco Bay, California, USA

Porewater concentration gradients indicate relatively large benthic fluxes of Pb from sediments in the San Francisco Bay estuary. Gradients in total dissolved (<0.45 μm) Pb concentrations in sediment porewaters, which range from 0.07–19.2 nM, parallel gradients in ammonia and dissolved Fe in sediment cores from the bay. Corresponding Fickian diffusive fluxes range from 2.6 × 10 −9 moles m −2 d −1 to 3.1 × 10 −8 moles m −2 d −2 in anoxic surface (<2 cm) sediments along the periphery of the estuary. These indicate the net diffusive benthic flux of Pb from sediments in San Francisco Bay (3–31 moles d −1) is at least an order of magnitude greater than the fluvial input of dissolved Pb to the estuary (0.2 moles d −1) during low flow periods. Moreover, estimates of the total benthic Pb flux, which were based on Hammond et al. (1985) irrigation benthic flux model, are two- to six-fold greater (6–186 moles d −1) than the estimates of diffusive fluxes. Therefore, the total benthic flux of Pb from the bay's sediments may be within an order of magnitude of the total anthropogenic flux of Pb to the San Francisco Bay estuary (965–8,410 moles d −1).

A laboratory study to quantify the influence of Nereis Diversicolor O.F. Müller in the exchange of phosphate between sediment and water

Fluxes of phosphate and oxygen across the sediment-water interface have been measured under different abundances of Nereis diversicolor in order to quantify their influence in this process. Results show that an increase of Nereis abundance results in a decrease in the gradient of dissolved phosphate in the interstitial water, and in the ratio calculated flux ( J o ) in the interface/ diffusive flux, based on the pore water concentration gradients ( J d ) increase as well. The stoichiometry of oxygen consumption by sediment and phosphate release decreases when Nereis was present in the sediment. Mass balance calculated for total carbon, nitrogen and phosphorus revealed a loss of nitrogen from the sediment and some net accumulation of carbon and phosphorus.

Benthic fluxes of nutrients and some trace metals in the Tamar estuary, SW England

Pore water concentration gradients and fluxes of chemical components have been studied in sediments from six intertidal sites in the Tamar Estuary, SW England over the course of a year. Fluxes of nutrients (ammonia, nitrate, nitrite, phosphate and silicate) and trace metals (iron, manganese, zinc, copper and cadmium) were determined using a laboratory microcosm incorporatingin situ pore water samplers. Nutrients (except nitrate) were transported out of the sediment throughout the year, but nitrate fluxes were directed into the sediment in the summer (denitrification) and out of the sediment in the winter (nitrification). The activities of benthic macrofauna resulted in enhanced fluxes but these differed between sites depending on population structure and density and whether irrigation or sediment reworking predominated. Fluxes of trace metals were seasonally and spatially variable and specific differences were observed that could be attributed to both chemical and biological activity.

Seasonal changes in pore water concentrations of nutrients and their diffusive fluxes at the sediment-water interface

Seasonal variations of dissolved NH 4 + , NO 3 − , NO 2 − and Si(OH) 4 within an oyster bed sediment from the Bay of Morlaix, Brittany, France, were studied from February 1985 to January 1986. They were compared to the annual pattern of sedimentary organic C and N. Nutrient fluxes at the sediment-water interface were measured using sediment cores and compared to those estimated from solute gradients between pore water and the water column. Organic matter showed three accumulation periods. The highest one occurred during summer. Humic compounds analysis confirmed a marine origin (seaweeds, oyster feces) while they displayed a terrigenous origin in January and April. Vertical profiles of organic matter are characteristic for a strong burial activity. One example of this activity is described through the good correlation which is found between the organic C : N ratios at 1 and 10 cm. Gradients of ammonium and nitrate between the first centimetre of the sediment and the water column ranged from 36.6 to 202.4 and from − 15.2 to 130.2 μmol · 1 −1 · cm −1. For nitrite and silicate, they ranged from 0.8 to 9.2 and from - 3.6 to 152.4 μmol · 1 −1 · cm −1. Whatever the organic matter source was, the degradation pattern in the sediment followed the same processes. Sedimentary organic matter was oxidized to NH 4 + which was partially transformed to NO 2 −. NO 3 − production occurred only during spring when redox conditions were appropriate. Pore water NO 3 − profiles were greatly influenced by endofauna activity. These oxidation processes appeared slower than in the water column but organic and nutrient concentrations encountered were generally much higher. Molecular diffusion calculated from vertical pore water concentration gradients could not entirely explain fluxes observed throughout the year. The annual pattern of the exchanges of the remineralized compounds Si(OH) 4, NH 4 + , NO 3 − and NO 2 − at the sediment-water interface was studied under laboratory conditions using a core incubation method. These nutrient fluxes ranged over the year from − 24.8 to 47.7 μmol · m −2 · h −1 for ammonium, from − 15.6 to 94.1 μmol · m −2 · h −1 for nitrate, from − 10. to 9.3 μmol · m −2 · h −1 for nitrite and from −18.9 to 94.5 μmol · m −2 · h −1 for silicate. The annual variations of nitrate and nitrite fluxes have been found to be well correlated with the C : N ratio of the organic matter within the first centimetre of the sediment. Annual schedules of observed nutrient exchanges supplied to water column enrichment.

The sedimentary flux of nutrients at a Delaware salt marsh site: A geochemical perspective

A basic geochemical approach has been used to study the diagenetic cycling and sediment-water exchange rates of essential nutrients at a site in a Delaware salt marsh. The pore water nutrients Si (OH)4, NH4+, PO4−3, and NO3− were analyzed and compared seasonally, and potential diffusive fluxes were calculated from the corresponding pore water concentration gradients. Concurrent direct flux measurements were also made using closed chambers deployed on the salt marsh surface under ambient and controlled conditions. The differences in these two approaches allows for estimation of nutrient production and consumption in the root zone and at the salt marsh surface. Silicate and phosphate appear to be primarily geochemically controlled, while the nitrogen species undergo dynamic seasonal redox fluctuations resulting from microbial mediation (e.g., nitrification and denitrification).

Laboratory studies of the diagenesis and mobility of 239,240pu and 137Cs in nearshore sediments

Controlled laboratory experiments have been used to study the diagenetic chemistry of 239,240Pu 137Cs, and 55Fe. Experiments using Buzzards Bay sediments in small tanks show that sulfate reduction is accompanied by the production of large pore water concentration gradients of alkalinity, phosphate, ammonia and dissolved organic carbon and the formation of subsurface maxima in Fe and Mn. These pore water profiles demonstrate that bacterially-mediated processes of organic matter degradation and redox reactions can be simulated in the laboratory. A vertical profile of 55Fe in pore waters is reported for the first time: it follows the profile of stable Fe and as such has a large (200 dpm/100 kg) subsurface maximum between 2–4 cm depth. Comparison of 55Fe/Fe ratios in sediments and pore waters shows that there is preferential solubilization of 55Fe over stable Fe. The pore water activities of 239,240Pu show no gradients within the large uncertainties of the counting statistics, but are two to four times higher than Buzzards Bay seawater (0.05 dpm/100 kg). The activity of 137Cs in the pore water profile is constant (40 dpm/100 kg) within the large counting uncertainties and is twice that of Buzzards Bay seawater. Cs-137 does not appear to be involved in diagenetic chemistry but may increase in pore waters as a result of ion exchange reactions. Flux estimates based on the pore water data show that remobilization and transport of 239,240 Pu in coastal sediments are not significant processes while the transport of l37Cs may be.

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.