Diffusive Equilibration In Thin Films Research Articles

Combined Gel Probes for the In Situ Determination of Dissolved Reactive Phosphorus in Porewaters and Characterization of Sediment Reactivity

Diffusive equilibrium in thin films (DET) and diffusive gradients in thin films (DGT) were applied in situ to obtain high spatial resolution dissolved reactive phosphorus (DRP) sediment porewater profiles in two lagoons of the Gippsland Lakes (SE Australia) during summer. Although the DRP depth profiles were different in each lake, highlighting the sensitivity of DRP to the redox state of the sediment spatial DRP variations obtained from DET and DGT showed striking similarities with depth in each lake. Comparison of DRP concentrations obtained from DGT and DET allows an assessment of the reactivity of the sediment. A dynamic numerical model of DGT-induced flux in sediments (DIFS) quantified reactivity kinetics. Sediment response time (Tc) related to sedimentary phosphorus resupply resulting from DGT-induced lowering of the porewater DRP concentration was calculated. Values of Tc ranged from 4128 to 183 400 s and from 55 to 149 400 s for Lakes Victoria and Wellington, respectively. These results indicate the limited capacity of the sediment to quickly resupply DRP to the porewater, especially in surface sediment which was always characterized by the highest Tc. Adsorption and desorption rate constants were also calculated from DIFS with values ranging from 2 x 10(-3) to 2 x 10(-2) day(-1) and from 0.3 to 21 day(-1) for Lake Victoria and from 2 x 10(-4) to 0.8 day(-1) and from 0.6 to 1558 day(-1) for Lake Wellington, respectively. Diffusive fluxes estimated from the in situ DRP gradient at the sediment-water interface by DET were 558 and 1.2 micromol m(-2) day(-1) in lakes Victoria and Wellington, respectively. Despite only a single measurement, these fluxes highlight a substantial contribution of P from the sediment to the water column in summer for Lake Victoria which is likely to contribute to the recurring blooms of blue-green algae. These calculated fluxes obtained with minimal sediment disturbance and high spatial resolution were substantially different (by a factor of 6-180) from other reported values using more conventional quantitative assessments such as diffusive fluxes (from core slicing) and benthic chambers.

Measurement of soluble reactive phosphorus concentration profiles and fluxes in river-bed sediments using DET gel probes

DET (diffusive equilibrium in thin films) gel probes were used for sampling river-bed sediment porewaters, to characterise in situ soluble reactive phosphorus (SRP) concentration profiles and fluxes. DET probes were deployed in three contrasting rural streams: (1) a headwater ‘pristine’ stream, with minimal P inputs from low intensity grassland and no point sources, (2) an intensively cultivated arable catchment, and (3) a stream subject to high P loadings from sewage effluent and intensive arable farming. The DET results showed highly enriched porewater SRP concentrations of between ca. 400 and 5000 ug-P/1 in the sewage-impacted stream. In contrast, the arable and pristine streams had porewater SRP concentrations <70 ug-P/1 and <20 ug-P/1, respectively. Porewater SRP concentration profiles in both the sewage-impacted and arable-impacted streams showed well-defined vertical structure, indicating internal sources and sinks of SRP within the sediment. However, there was little variability in porewater SRP concentrations in the pristine stream. The DET porewater profiles indicated net diffusion of SRP (a) from the overlying river water into the surface sediment and (b) from subsurface sediment upwards towards the sediment–water interface. A mass balance for the sewage-impacted site showed that the influx of SRP into the surface sediments from the overlying river water was small (ca. 1% of the daily river SRP load). The DET results indicated that, in the arable and sewage-impacted streams, the surface ‘cap’ of fine sediment may play an important role in inhibiting upward movement of SRP from subsurface porewaters into the overlying river water, under steady-state, low-flow conditions.

Insights into redox cycling of sulfur and iron in peatlands using high-resolution diffusive equilibrium thin film (DET) gel probe sampling

High spatial resolution vertical profiles of pore-water chemistry have been obtained for a peatland using diffusive equilibrium in thin films (DET) gel probes. Comparison of DET pore-water data with more traditional depth-specific sampling shows good agreement and the DET profiling method is less invasive and less likely to induce mixing of pore-waters. Chloride mass balances as water tables fell in the early summer indicate that evaporative concentration dominates and there is negligible lateral flow in the peat. Lack of lateral flow allows element budgets for the same site at different times to be compared. The high spatial resolution of sampling also enables gradients to be observed that permit calculations of vertical fluxes. Sulfate concentrations fall at two sites with net rates of 1.5 and 5.0nmol cm − 3 day − 1, likely due to a dominance of bacterial sulfate reduction, while a third site showed a net gain in sulfate due to oxidation of sulfur over the study period at an average rate of 3.4nmol cm − 3 day − 1. Behaviour of iron is closely coupled to that of sulfur; there is net removal of iron at the two sites where sulfate reduction dominates and addition of iron where oxidation dominates. The profiles demonstrate that, in addition to strong vertical redox related chemical changes, there is significant spatial heterogeneity. Whilst overall there is evidence for net reduction of sulfate within the peatland pore-waters, this can be reversed, at least temporarily, during periods of drought when sulfide oxidation with resulting acid production predominates.

Two-dimensional determination of dissolved iron and sulfur species in marine sediment pore-waters by thin-film based imaging. Thau lagoon (France)

A device composed of a polyacrylamide gel thin-film (18 cm high, 5 cm wide and 0.4 mm thick) and a PVC (polyvinyl chloride) film was used as a sediment probe to obtain iron and S(−II) sulfur dissolved species' distributions in sediment pore waters. A porous protective membrane was set on top of hydrogel layer. Probes were deployed in May 2003 for 24–48 h in the superficial sediment of Thau lagoon (France), in a shellfish farming area. The polyacrylamide gel layer was used as a DET (Diffusive Equilibration in Thin-films) device for 2D Fe(II) concentration determination, and as the diffusive layer of a DGT-like (Diffusive Gradients in Thin-films) device for sulfur species study. The accumulation layer of the DGT device consisted in a PVC film layer underneath the polyacrylamide layer. Iron determinations were performed by colorimetric methods with Ferrozine and imaging technique. Image acquisitions were performed with a flatbed scanner. Fe(II) concentrations were deducted from densitometry analysis of the magenta zones (ImageJ software). The calibration curve was obtained by densitometry analysis of polyacrylamide gel pieces which were equilibrated in known iron (II) concentration solutions. ΣFe distribution was performed but not quantified. Analysis of gray zones on the PVC layer provided a qualitative distribution of unidentified S(−II) dissolved compounds, related to H 2S zone for which information is obtained by classical methods (peeper and colorimetric measurements). EDX (X-ray energy dispersive spectrometry) and GIXR (Grazing-Incidence X-ray Diffraction) analyses of gray zone of this PVC layer provide evidence for FeS 2 catalyzed precipitation onto this film. Dissolved Fe(II) was mainly located near the sediment–water interface (SWI), showing a nonhomogeneous layer about 10 mm thick. Small Fe rich domains appeared deeper in the sediment and likely confirm newest paradigms in the field of sediment biogeochemistry. S(−II) species are detected from 3 to 4 cm below the SWI, with a heterogeneous spatial distribution showing a burrow-like structure.

Influence of diagenetic processes in Thau lagoon on cadmium behavior and benthic fluxes

DGT (Diffusive Gradient in Thin-films) and DET (Diffusive Equilibration in Thin-films) combined probes were used in Thau lagoon sediments to describe variations of dissolved concentrations of metals such as cadmium, manganese and iron, through the sediment–water interface. Two contrasted stations regarding organic carbon fluxes were studied from December 2001 to May 2003 during four field campaigns: station C4 in the middle of the lagoon, and station C5 in a shellfish-farming zone. Laboratory experiments and field deployments in such environment showed that DGT sampled pore water labile cadmium whereas iron and manganese concentrations were underestimated. These results suggest that no steady state in the flux of metals onto the gel was established for Fe and Mn. Kinetics of metal sulfide dissolution–precipitation may control metal fluxes onto the gel probe in marine sedimentary environments. Analysis of sediment and water column samples showed cadmium concentrations above natural background (3.3 and 7.6 nmol kg −1 for station C4 and station C5 sediments, respectively; between 40 and 800 pmol L −1 for the water column), suggesting contamination. Spatial and temporal patterns of cadmium behavior were observed. The sediment at station C4 was generally a source of cadmium whereas at station C5 it was a sink. The vertical extension of the diagenetic series was more important at station C4 with deeper oxygen penetration and lack of dissolved sulfide whereas station C5 showed steep ∑H 2S gradients at the same depths. The data suggested that cadmium source was more likely organic matter. Cadmium mobility was probably controlled by aerobic mineralization at station C4 and by dissolution–precipitation of sulfides at station C5. Seasonal variations were observed in the depth of oxygen penetration and sulfide diffusion generating important remobilization of cadmium during December 2001. Conversely in May 2003 at station C5, bottom water suboxic conditions (i.e. %O 2 = 60) enhanced reductive conditions in the sediment favoring uptake of cadmium by the sediment from the water column.

High resolution profiles of thallium, manganese and iron assessed by DET and DGT techniques in riverine sediment pore waters

High resolution profiles of Mn, Tl and Fe concentrations have been assessed in the pore waters of river Leie sediments at Warneton and Menen (at the border of Belgium and France) by DET (Diffusive Equilibrium in Thin Films) and DGT (Diffusive Gradients in Thin Films) techniques. The oxidized, solid Mn (IV), Tl (III) and Fe (III) compounds were reduced in the suboxic (+255 to -20 mV versus Standard Hydrogen Electrode (SHE)) riverine sediments and since these reduced species are much more soluble also they are released into the pore waters. The highest DET (total dissolved) concentrations of Fe (76 mg l(-1)), Mn (2 mg l(-1)) were observed at the station of Menen, while Tl maxima differed only slightly between the 3 surveys (21 to 27 microg l(-1)). The average ratios of Fe/Mn/Tl in the pore waters at the 3 sampling stations are fairly constant for both the DET and DGT samplings. However, the results indicate that compared to Fe and Tl a greater proportion of the Mn measured by DET is accumulated by DGT, reflecting the ready supply of Mn from solid phase to solution.

Fine scale remobilisation of Fe, Mn, Co, Ni, Cu and Cd in contaminated marine sediment

Contaminated sediment from a marine harbour was maintained for 16 months in two flumes that continuously re-circulated the overlying water, sustaining a smooth flow at the sediment surface. The sediment was placed in one flume intact, while for the other it was pre-homogenised. The concentrations of trace metals in the porewaters were measured at a vertical resolution of 2 mm using the technique of diffusive equilibration in thin-films (DET) and microelectrodes were used to measure concentration profiles of oxygen and pH. Separate experiments showed good agreement between metals measured by DET and those measured in porewaters extracted by centrifugation and filtration. Local mobilisation of metals was measured in 2-dimensional arrays by deploying DGT (diffusive gradients in thin-films) probes. There were high concentrations of Fe in the porewaters, which limited the concentration of sulphide to less than 0.3 μM. With both DET and DGT measurements, there were sharply defined maxima of Cu and Cd within 2 mm of the sediment water interface, consistent with their release from organic material as it is oxidised. There was a Co maximum about 5–8 mm lower than the Cu and Cd maxima, apparently coincidental with Mn mobilisation. While there were clear Ni maxima, their location appeared to vary from being coincident with Co to a few mm above the Co maxima. The remobilisation of metals could not be explained by the pH gradients in the near-surface sediments. As sulphate reduction rates were appreciable, the apparent lack of metal mobilisation at depth was attributed to the formation of metal sulphides. The DGT measurements in the same probe were well replicated horizontally. This, the replication of the same features between flumes and the reasonable correspondence between DGT and DET, showed that the localised mobilisation of metals was associated with recent diagenetic processes, rather than the depositional history. There were substantial fluxes of Cu and Cd to the overlying water. Even though there were steep gradients of Fe, Mn, Ni and Co within 1 cm of the sediment–water interface, there was no clear evidence for any substantial metal fluxes across the interface. Adsorption of Mn to Fe oxides, rather than oxidation, may be responsible for its removal from solution at the same depth as Fe.

High-resolution profiles of trace metals in the pore waters of riverine sediment assessed by DET and DGT

The techniques of DET (diffusive equilibrium in thin films) and DGT (diffusive gradients in thin films) were applied to obtain high-resolution vertical profiles of trace metals in freshwater sediments. In the framework of the EU-Interreg project Stardust ( http://www.vliz.be/projects/stardust/) between France and Belgium, in which the mobility of sediment bound metals is investigated, sediment samples were collected from the Upper Scheldt River (at Helkijn, Belgium) and the Leie River (at Warneton, located at the Belgian–French border). Intra- and inter-laboratory comparisons of the gel techniques were carried out between the two laboratories involved. In general, a good agreement was observed, taking sediment heterogeneity into account. At both stations, metal pore water profiles show more or less similar tendencies although the sediment at Warneton was more anoxic than at Helkijn. A strong correlation between Fe and Co was found at Helkijn as well as at Warneton. The metal gradients at the water/sediment interface were calculated from the high resolution profiles and the conventional, low resolution profiles. Significant differences were observed.

Determination of High Resolution Pore Water Profiles of Trace Metals in Sediments of the Rupel River (Belgium) using Det (Diffusive Equilibrium in Thin Films) and DGT (Diffusive Gradients in Thin Films) Techniques

The techniques of diffusional equilibrium in thin films (DET) and diffusional gradients in thin films (DGT) were used to obtain high resolution pore water profiles of total dissolved and labile trace (mobilizable) metals in the sediments of the Rupel River, Belgium. DGT measures labile metal species in situ by immobilizing them on a resin gel after diffusion through a diffusive gel whereas for DET an equilibrium is established between the DET gel and the pore water. Concentrations of Pb and Zn obtained by DGT were in good agreement with the results obtained by centrifugation, and thus were well buffered by rapid equilibrium with the solid phase, whereas Fe, Mn and Cd were very tightly bound to the sediment phase and large differences were observed between the labile and the total metal concentrations. Cu, Zn, Co and Ni show intermediated behavior. Good correlations were found between the profiles of As and Fe and Mn and Co for DET as well as DGT showing a close link between the geochemical behavior of these elements. Cu, Zn, Pb and Cd are also influenced by the reductive mobilization of Fe and Mn oxides but are also closely linked to the bacterial degradation of organic matter in the surface sediments as to the precipitation of metal sulfides in the deeper layers.

Biogeochemistry of the sediment–water interface in the littoral of an acidic mining lake studied with microsensors and gel-probes

Acidic mining lakes (pH<3) represent a big environmental issue in former lignite mining areas. The acidity status of such lakes is mainly governed by the fluxes and the turnover of iron. No data are known about the iron cycling at the sediment–water boundary in the littoral of acidic mining lakes. In order to study the microstratification of iron cycling at the sediment–water interface, existing microsensor and gel-probe techniques (diffusive equilibration in thin-films, DET) were adapted to the extreme conditions of a mining lake. Microprofiles of O 2 and pH measured in situ by a profiler were compared with profiles measured in sediment cores immediately after sampling. The results indicate that microprofile measurements in cores from the littoral underestimate oxygen penetration into the sediment. Both advective pore water movement and pore water displacement during coring did influence concentration profiles. Measurement of sulfate and both ferric and ferrous iron was possible on a millimetre scale using DET. Based on different test measurements, a protocol for the application of gel probes for the analysis of iron and sulfate in acidic mining lakes was developed. The combination of microsensor and DET data allowed the characterisation and localisation of iron turnover on a millimetre scale. As an example, results from the littoral of Mining Lake 111 (Lusatia, Germany) are presented. Pore water gradients indicated a flux of ferric iron and sulfate from the water into the sediment. There were no indications of iron reduction or oxidation in the uppermost 6 cm of the sediment. A relative comparison of the fluxes revealed that the iron and sulfur fluxes were probably driven by the formation of jarosite directly in the sediment.

Use of constrained DET probe for a high-resolution determination of metals and anions distribution in the sediment pore water

The constrained DET (diffusive equilibrium in thin films) probe filled with agarose gel was successfully used for measuring depth profiles of metals and anions in estuarine sediments at high-resolution (mm). The agarose gel was tested for equilibration times and recoveries. For manganese (10 mg l −1), the equilibration time between an aqueous solution and the agarose gel was reached after 60 min. In the presence of humic acids it was five times longer. As a consequence, for field applications a deployment time of 5–24 h was applied. The subsequent elution of manganese into nitric acid was completed in less than 10 min even without mixing. For agarose gel in contact with solutions of 10 mg l −1 ions (cadmium, copper and manganese, bromide, chloride, nitrate and sulfate), recoveries obtained were close to 100%. DET probes were applied in test laboratory sediment and in real sediment in the Authie (France) estuary. Depth profiles for 19 elements (Ag, As, Ba, Bi, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Se, Sr, Tl, V, and Zn) and four anions (bromide, chloride, nitrate and sulfate) were established for the test laboratory sediment. In the real sediment, the pore water concentrations of iron and manganese using gel probes were comparable with those obtained by a conventional method (box core-anoxic slicing followed by centrifugation).

Evidence for suboxic nitrification in recent marine sediments

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 236:31-35 (2002) - doi:10.3354/meps236031 Evidence for suboxic nitrification in recent marine sediments Robert J. G. Mortimer1,*, Michael D. Krom1, Sansha J. Harris1, Peter J. Hayes2, Ian M. Davies2 , William Davison3, Hao Zhang3 1School of Earth Sciences, University of Leeds, Leeds LS2 9JT, United Kingdom 2Fisheries Research Services, Marine Laboratory, PO Box 101, Victoria Road, Aberdeen AB11 9DB, United Kingdom 3Environmental Science Department, Institute of Environmental and Natural Sciences (IENS), Lancaster University, Lancaster LA1 4YQ, United Kingdom *E-mail: r.mortimer@earth.leeds.ac.uk ABSTRACT: The classical scheme of biogeochemical zones (BGZ) is known to be an oversimplification of the microbial processes that occur in organic-rich marine sediments. Results from a coupled deployment of pore-water gel probes in Loch Duich, Scotland, provide direct evidence for rapid recycling within the iron reduction (FeR) and sulphate reduction (SR) zones. High resolution pore-water profiles obtained using diffusive equilibrium in thin films (DET) gel probes found a nitrate peak at the boundary between the FeR and SR zones. This non-steady state feature is consistent with recycling of reduced N occurring throughout the FeR zone. Both conventional pore-water iron profiles and results from diffusive gradient in thin films (DGT) probes indicate that iron is solubilised and precipitated in rapid Fe/S recycling reactions throughout the SR zone. The presence of such complex recycling reactions confirms the oversimplification of the classical BGZ scheme. KEY WORDS: Biogeochemical recycling · Suboxic nitrification · DET · DGT Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 236. Online publication date: July 03, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

Determination of Nitrate in Small Water Samples (100 µL) by the Cadmium-Copper Reduction Method: A Manual Technique with Application to the Interstitial Waters of Marine Sediments

The standard technique for the analysis of $ {\\rm NO}_3^ - $ in seawater and the interstitial waters of marine sediments is the cadmium-copper reduction method. The manual $ {\\rm NO}_3^ - $ method presented here provides a technique that is simple, cost-effective and capable of analysing small volumes (100 µL). The technique is also suitable for analysing $ {\\rm NO}_3^ - $ in DET (diffusive equilibration in thin films) gel samples. Reduction of $ {\\rm NO}_3^ - $ to $ {\\rm NO}_2^ - $ is achieved by drawing the sample into contact with a copper-coated cadmium wire using a micrometer burette. The reduced sample is then expelled and $ {\\rm NO}_2^ - $ is determined colorimetrically. The method has a concentration range of 0-100 µM $ {\\rm NO}_3^ - $ and a detection limit (2 s b ) of 0.4 µM $ {\\rm NO}_3^ - $ . Eight samples can be processed per hour.

High‐resolution metal gradients measured by in situ DGT/DET deployment in Black Sea sediments using an autonomous benthic lander

DET (Diffusive equilibration in thin films) and DGT (diffusive gradients in thin films) have been deployed in situ using an autonomous benthic lander to measure concentrations and induced fluxes of Fe and Mn (DET/DGT) and trace metals (DGT) in pore waters at millimeter spatial resolutions. The newly developed deployment system is described, and based on these first results, its strengths and weaknesses are discussed. Deployments were made in the Western Black Sea in shelf sediments overlain by well‐oxygenated water at a water depth of 77 m. Maxima of the redox‐sensitive metals at 4 and 8 cm deep within the sediment indicated that two zones of reduction dominated the geochemistry. Sharp, but systematic, features were superimposed on this general picture and were well replicated in the profiles of Mn, Co, and Cd, but the sharp features in the Fe profile were offset from those of the others elements by several millimeters. Detection of this functional discrimination between Fe and Mn as regulators of trace metals would not have been possible using more conventional sampling procedures.

Two-Dimensional and Fine Structure in the Concentrations of Iron and Manganese in Sediment Pore-Waters

The technique of diffusive equilibration in thin films (DET) was used to measure sediment pore-water concentrations of dissolved iron and manganese at high-spatial resolution (1 mm). Measurements in a laboratory microcosm of homogeneously mixed sediment gave reproducible concentration-depth profiles with no fine scale features. By contrast, measurements in undisturbed surface sediments showed fine structure that was only occasionally reproduced in profiles from several replicate field deployments. Although redox-related features in the profiles, indicative of processes traditionally considered to occur with lateral uniformity, were generally present, the details observed in the replicate deployments were different. DET was modified to allow horizontal as well as vertical measurement of pore-water solutes. The resulting 2D images of the distribution of iron and manganese concentrations in pore-waters showed systematic vertical and horizontal structure. The individual component vertical profiles of the 2D measurement were only 3 mm apart and yet they were sometimes quite different from one another. These findings suggest that (i) microniches play a major role in metal remobilization, (ii) results from single vertical profiles may not be representative, and (iii) caution should be exercised when estimating exchange fluxes from vertical concentration gradients.

Remobilisation of major ions in freshly deposited lacustrine sediment at overturn

The technique of diffusive equilibration in thin-films (DET) was used to measure pore water concentrations of Ca2+, Mg2+, K+, SO42-, Fe2+ and Mn2+ on the mm scale in surface sediments of a seasonally anoxic lake. Cores were collected at overturn when a 2cm thick layer of unconsolidation freshly recruited material was present at the sediment surface. Comparison with pore water measurements made on samples extracted using syringes agreed well above the interface and at 8cm depth within the sediment. Differences at intermediate depths showed that the syringe-extraction procedure introduced artifacts due to mixing of pore waters with water overlying the sediment. There was evidence of small scale (<1 mm) microniche remobilisation of Fe and Mn within the freshly recruited sediment material. Potassium shows a sharp maxima at the interface, clearly separated from the zone of Fe and Mn resupply, and consistent with remobilisation due to decomposition of freshly recruited organic material. There was evidence of Mg mobilisation at the interface and at 1-2cm depth where Fe and Mn release was occurring.

Use of gel probes for the determination of high resolution solute distributions in marine and estuarine pore waters

Pore water profiles were obtained at high resolution (millimeter scale) in marine sediments using a DET (diffusive equilibration in thin films) gel probe. A plastic probe which holds a 1–2 mm thick polyacrylamide gel covered by a 0.45 μm Millipore filter is inserted into the sediment. The gel was prehydrated in water of similar salinity to the in situ sampling conditions. Chloride and sulphate needed 24 and 48 h for complete front and back equilibration, respectively, while 6–8 h (front) and 4–6 h (back) were used for calcium and alkalinity. Ammonia-N and total CO2 were back-equilibrated for 2 h. Probes were sectioned immediately after sampling, stored for up to 1–2 days (NH4+, ∑CO2) or for up to 1–2 weeks (Ca, alkalinity, Cl, SO4, NO3), before they were back-equilibrated into Milli-Q water or 0.7 M NaCl (calcium and alkalinity). The gel retains between 3–7% of the total sulphate. A simple procedure has been developed to correct for this incomplete recovery. Recovery tests using seawater and 50% seawater spiked to concentrations found in nearshore pore waters, showed recoveries of 101.4±0.5% (chloride), 101.4±0.7% (bromide), 100.3±0.2% (nitrate), 96.6±0.7% (sulphate), 99.7±0.8% (ammonia-N), 99.1±1.2% (∑CO2), 96.9±0.8% (calcium) and 96.8±1.4% (alkalinity) (mean±standard error). Pore water profiles obtained simultaneously using gel probes and conventional techniques (box core-anoxic slicing followed by centrifugation) showed excellent comparability at cm resolution though features which required higher resolution which could only be seen in the gel profiles.

Development of constrained DET for measurements of dissolved iron in surface sediments at sub-mm resolution

The technique of diffusive equilibration in thin-films (DET) was used to measure pore-water concentrations of dissolved Fe at high (1 mm) and ultra-high (400 μm) spatial resolution in the surface sediments (0–12 mm) and immediate overlying water of Esthwaite Water, UK. DET measurements were made using an unconstrained DET probe and a new constrained DET probe with 200-μm wide compartments in a 400-μm thick ceramic sheet. Good agreement between profiles obtained by conventional and constrained DET measurements confirmed the performance of the new procedure, which in principle can be used to measure most major components of pore waters. Single point maxima observed with conventional DET were revealed as systematic peaks with constrained DET. All Fe pore-water profiles showed tightly defined maxima in the surface sediments. Double maxima observed for both spring and winter were probably due to tightly defined zones of bacterial oxidation of Fe(II) although sampling artifacts could not be completely ruled out. Differences in ultra-high resolution vertical concentration profiles measured only 2 mm apart provided evidence of small scale horizontal heterogeneity. Such observations suggest that fluxes through the sediment water interface calculated from concentration profiles should be treated as spatially specific values. Average fluxes can only be obtained from several replicate measurements.