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The Black Sea is the world’s largest anoxic basin and a model system for studying processes across redox gradients. In between the oxic surface and the deeper sulfidic waters there is an unusually broad layer of 10–40 m, where neither oxygen nor sulfide are detectable. In this suboxic zone, dissolved phosphate profiles display a pronounced minimum at the upper and a maximum at the lower boundary, with a peak of particulate phosphorus in between, which was suggested to be caused by the sorption of phosphate on sinking particles of metal oxides. Here we show that bacterial polyphosphate inclusions within large magnetotactic bacteria related to the genus Magnetococcus contribute substantially to the observed phosphorus peak, as they contain 26–34% phosphorus compared to only 1–5% in metal-rich particles. Furthermore, we found increased gene expression for polyphosphate kinases by several groups of bacteria including Magnetococcaceae at the phosphate maximum, indicating active bacterial polyphosphate degradation. We propose that large magnetotactic bacteria shuttle up and down within the suboxic zone, scavenging phosphate at the upper and releasing it at the lower boundary. In contrast to a passive transport via metal oxides, this bacterial transport can quantitatively explain the observed phosphate profiles. We are grateful for the competent technical assistance of Ronny Baaske, Christian Burmeister, Christin Laudan and Christian Meeske. We are greatly indebted to Cindy Lee and Bo Barker Jørgensen for providing extremely helpful comments on an earlier version of the manuscript. Horst D. Schulz and René Friedland are acknowledged for stimulating discussions on the modeling approach. We thank the captain and the crew of the R/V “Maria S. Merian” for the excellent support on board and the DFG (MSM33) and BMBF (01DK12043) for financing the cruise. The particle analysis was funded by the BMBF (03F0663A). S.B. was funded by a BONUS BLUEPRINT project (03F0679A awarded to KJ; http://blueprint- project.org), supported by BONUS (Art 185), funded jointly by the EU and the German Federal Ministry of Education and Research (BMBF). T. S. was funded by the German research foundation (DFG) (awarded to K.J., JU 367/16-1). Metagenome sequencing was done at the Swedish National Genomics Infrastructure (NGI) at SciLifeLab (Sweden). We are grateful for the competent technical assistance of Ronny Baaske, Christian Burmeister, Christin Laudan and Christian Meeske. We are greatly indebted to Cindy Lee and Bo Barker Jørgensen for providing extremely helpful comments on an earlier version of the manuscript. Horst D. Schulz and René Friedland are acknowledged for stimulating discussions on the modeling approach. We thank the captain and the crew of the R/V “Maria S. Merian” for the excellent support on board and the DFG (MSM33) and BMBF (01DK12043) for financing the cruise. The particle analysis was funded by the BMBF (03F0663A). S.B. was funded by a BONUS BLUEPRINT project (03F0679A awarded to KJ; http://blueprint- project.org), supported by BONUS (Art 185), funded jointly by the EU and the German Federal Ministry of Education and Research (BMBF). T. S. was funded by the German research foundation (DFG) (awarded to K.J., JU 367/16-1). Metagenome sequencing was done at the Swedish National Genomics Infrastructure (NGI) at SciLifeLab (Sweden).

The reuse of effluent waters and sediments from African catfish (Clarias gariepinus) recirculation aquaculture systems requires a deeper understanding of the nutrient and energy flows and material pathways. Three semi-commercial systems, differing in stocking density, were sampled for nutritive and pollutant elements of the input- (tap water, feed) and output pathways (fillet, carcass, process water, sediments) by ICP-OES/MS and calorimetry. Highly water-soluble elements, e.g., potassium, accumulated in the water, whereas iron, copper, chromium and uranium where found in the solids. Feed derived phosphorous was accounted for, 58.3&ndash, 64.2% inside the fish, 9.7&ndash, 19.3% in sediments, and small amounts 9.6&ndash, 15.5% in the process waters. A total of 7.1&ndash, 9.9% of the feed accumulated as dry matter in the sediments, comprising 5.5&ndash, 8.7% total organic carbon and 3.7&ndash, 5.2% nitrogen. A total of 44.5&ndash, 47.1% of the feed energy was found in the fish and 5.7&ndash, 7.7% in the sediments. For reuse of water and nutrients in hydroponics, the macro-nutrients potassium, nitrate, phosphorus and the micro-nutrient iron were deficient when compared with generalized recommendations for plant nutrition. Low energy contents and C/N-ratio restrict the solely use of African catfish solids for biogas production or vermiculture. Using the outputs both for biogas supplement and general fertilizer in aquaponics farming (s.l.) (combined with additional nutrients) appears possible.

The Eurasian inland propagation of temperature anomalies during glacial millennial-scale climate variability is poorly understood, but this knowledge is crucial to understanding hemisphere-wide atmospheric teleconnection patterns and climate mechanisms. Based on biomarkers and geochemical paleothermometers, a pronounced continental temperature variability between 64,000 and 20,000 years ago, coinciding with the Greenland Dansgaard-Oeschger cycles, was determined in a well-dated sediment record from the formerly enclosed Black Sea. Cooling during Heinrich events was not stronger than during other stadials in the Black Sea. This is corroborated by modeling results showing that regular Dansgaard-Oeschger cycles penetrated deeper into the Eurasian continent than Heinrich events. The pattern of coastal ice-rafted detritus suggests a strong dependence on the climate background state, with significantly milder winters during periods of reduced Eurasian ice sheets and an intensified meridional atmospheric circulation.

The anthropogenically forced expansion of coastal hypoxia is a major environmental problem affecting coastal ecosystems and biogeochemical cycles throughout the world. The Baltic Sea is a semi-enclosed shelf sea whose central deep basins have been highly prone to deoxygenation during its Holocene history, as shown previously by numerous paleoenvironmental studies. However, long-term data on past fluctuations in the intensity of hypoxia in the coastal zone of the Baltic Sea are largely lacking, despite the significant role of these areas in retaining nutrients derived from the catchment. Here we present a 1500-year multiproxy record of near-bottom water redox changes from the coastal zone of the northern Baltic Sea, encompassing the climatic phases of the Medieval Climate Anomaly (MCA), the Little Ice Age (LIA), and the Modern Warm Period (MoWP). Our reconstruction shows that although multicentennial climate variability has modulated depositional conditions and delivery of organic matter (OM) to the basin the modern aggravation of coastal hypoxia is unprecedented, and besides gradual changes in the basin configuration, it must have been forced by excess human-induced nutrient loading. The progressive deoxygenation since the beginning of 1900s was originally triggered by the combined effects of gradual shoaling of the basin and warming climate, which amplified sediment focusing and increased the vulnerability to hypoxia. Importantly, the anthropogenic eutrophication of coastal waters in our study area began decades earlier than previously thought, leading to a marked aggravation of hypoxia in the 1950s through fueling primary productivity, while we find no evidence of anthropogenic forcing during the MCA. These results have implications for the assessment of reference conditions for coastal water quality. Furthermore, this study highlights the need for combined use of sedimentological, ichnological, and geochemical proxies in order to robustly reconstruct subtle redox shifts especially in dynamic, non-euxinic coastal settings with strong seasonal contrasts in the bottom water quality.

Salinity has a strong impact on bacterial community composition such that freshwater bacterial communities are very different from those in seawater. By contrast, little is known about the composition and diversity of the bacterial community in the sediments (bacteriobenthos) at the freshwater-seawater transition (mesohaline conditions). In this study, partial 16S-rRNA sequences were used to investigate the bacterial community at five stations, representing almost freshwater (oligohaline) to marine conditions, in the Baltic Sea. Samples were obtained from the silty, top-layer (0–2.5 cm) sediments with mostly oxygenated conditions. The long water residence time characteristic of the Baltic Sea, was predicted to enable the development of autochthonous bacteriobenthos at mesohaline conditions. Our results showed that, similar to the water column, salinity is a major factor in structuring the bacteriobenthos and that there is no loss of bacterial richness at intermediate salinities. The bacterial communities of marine, mesohaline, and oligohaline sediments differed in terms of the relative rRNA abundances of the major bacterial phyla/classes. At mesohaline conditions typical marine and oligohaline operational taxonomic units (OTUs) were abundant. Putative unique OTUs in mesohaline sediments were present only at low abundances, suggesting that the mesohaline environment consists mainly of marine and oligohaline bacteria with a broad salinity tolerance. Our study provides a first overview of the diversity patterns and composition of bacteria in the sediments along the Baltic Sea salinity gradient as well as new insights into the bacteriobenthos at mesohaline conditions. The study was funded by the Leibniz Institute for Baltic Sea Research (IOW), KJ were funded by the DFG project MicroFun (JU367/15-1), JK by the Leibniz Association project TemBi (SAW-2011-IGB-2). The study was funded by the Leibniz Institute for Baltic Sea Research (IOW), KJ were funded by the DFG project MicroFun (JU367/15-1), JK by the Leibniz Association project TemBi (SAW-2011-IGB-2).

Recent investigations have revealed significant fractionation of 238U/235U between organic-rich sediments of anoxic marginal seas and seawater, indicating redox-dependent U isotope fractionation. This study explores the conditions controlling U isotope fractionation in selected modern anoxic basins (Baltic Sea: Landsort and Gotland Deeps and the Kyllaren fjord in Norway) and compares U with Mo isotope fractionation. Therefore, the concentrations and isotope compositions of dissolved U and Mo from the water column and organic-rich sediments from three currently anoxic basins have been analysed. The water column samples from the Kyllaren fjord display a moderate depletion of U and a strong depletion of Mo with increasing depth. These variations are associated with a decrease in δ238U and an increase in δ98Mo with depth, from − 0.35 ‰ to − 0.70 ‰ and from 2.4 ‰ to 2.6 ‰, respectively. From the U isotope composition of the deep euxinic water column of the Kyllaren fjord, a minimum value of Δ238Ured ≈ 0.7 ‰ for the U isotope fractionation during reduction is inferred. Due to the high sedimentation rate in the Kyllaren fjord, surface sediment samples are only moderately enriched in U and Mo (~ 4 μg/g and 6 μg/g to 37 μg/g, respectively) and display δ238U and δ98Mo averages of − 0.26 ‰ ± 0.10 ‰ and 2.18 ‰ ± 0.21 ‰, respectively. Water column samples from the Baltic Sea display a minor decrease in the U and Mo concentrations and only negligible U and Mo isotope fractionation compared to open seawater. The sediment layers from the Baltic Sea (Landsort Deep), which were deposited under mostly anoxic conditions, are moderately enriched in U (up to 8 μg/g) and highly enriched in Mo (up to 222 μg/g). However, U and Mo isotopic compositions are significantly lighter (with δ238U of − 0.36 ‰ ± 0.08 ‰ and δ98Mo of − 0.04 ‰ ± 0.11 ‰) than those of typical organic-rich sediments from anoxic basins. These findings reveal that compared to Mo, the U isotopic composition of sediments in restricted and strongly euxinic basins like the Kyllaren fjord is significantly more dependent on (1) the extent of U removal from the water column, which is lower than that of Mo, and on (2) the sedimentation rate (i.e., the fraction of authigenic U relative to detrital U in the sediment), which is also lower compared to that of Mo. In more open and only temporarily euxinic basins such as the investigated basins of the Baltic Sea, strong Mo is coupled with weak U isotope fractionation between water and sediment. These signatures were likely the result of isotope fractionation under weakly sulfidic conditions and the generation of isotopically light Mo during frequently occurring flushing events with oxygen-rich water from the open sea. This implies that Mo and U isotope signatures of sediments only record paleo-water column redox conditions of restricted basins if the water column was permanently stratified.

The redox-sensitive trace metal manganese (Mn) is an important electron donor and acceptor in aquatic environments. In stratified basins, the transformation between dissolved Mn2 +, 3 + and particulate Mn(III/IV) oxides constitutes the “manganese pump” at the redoxcline, which separates the oxic surface from anoxic bottom waters. In addition to fluxes of dissolved components and reaction rates, accurate estimates of the sinking velocities of authigenic Mn oxides (MnOx) are an important prerequisite to balance and model such euxinic systems. However, models describing biogeochemical cycles suffer from the lack of measured particle sinking velocities and instead must rely on values calculated according to Stokes' law or on estimates derived from budget calculations. Here we present the first stationary sinking velocities of authigenic MnOx particles from the pelagic redoxclines of the currently anoxic Landsort and Gotland Deeps (Baltic Sea). Sinking velocities were determined on board ship by using high-resolution backlit imaging (shadowgraphy) and manual particle tracking. Within the typical size range of MnOx particles (2–20 μm), the average sinking velocity of 1972 particles was determined to be 0.76 m d− 1, distinctly lower than the values calculated according to Stokes' law and those used in various models. This difference could be attributed to the observed star-shaped structure of the MnOx particles, which was far away from a spherical shape, and to the embedment of the particles in a complex organic matrix, which significantly modified their density and consequently their sinking velocity.

The Gotland Deep Environmental Sampling Station (GODESS) operating between 30 m and 185 m was used to assess the variation of in situ redox potential (Eh) in the stratified water column of the central Baltic Sea, for a period of 56 days (November 2010–January 2011). The aim of this study was to acquire in-depth understanding of redox variations with the overall objective to identify the processes controlling Eh. At an interval of 8 h basic hydro-physical parameters were registered, including the oxidation-reduction potential, dissolved oxygen, chlorophyll a fluorescence, turbidity, temperature and conductivity. In total 170 profiles of all parameters were obtained. The measured Eh (with respect to standard hydrogen electrode, SHE) ranged from −0.055 V to 0.167 V. After temperature and pH correction of the standard reduction potentials, a comparison of the measured Eh with that calculated for the particular redox couples was carried out by applying the Nernst-equation. Furthermore, the concentrations of the most important redox elements such as Fe, Mn, N, O, C and S used for comparison were estimated by using empirical functions or were based on measurements of discrete water samples, taken at the time of deployment and recovery of the mooring. The obtained results reveal that the reduction of Fe(III) and O2 may be the main processes controlling the Eh potential in the Gotland basin. Below the redoxcline the reduction of hematite and then ferric oxyhydroxide could be related to the redox potential and somewhat deeper Fe(III)/Fe(II) was found to be the dominant redox couple. Although mixed potential theory could have been applied, the Fe couples appear to dominate over many other possible redox equilibria. This may be explained by the relatively high exchange current of Fe and by high Fe concentrations (up to 1.5 μM). Finally, a close correspondence to the Eh potentials measured in the oxic/hypoxic part of the water column was found considering the O2/ O 2 − · and the O 2 − · /H2O2 couples. This suggests that one-electron transfer reactions determined the Eh under oxic/hypoxic conditions.

The discharge of groundwater into the sea affects surrounding environments by changing the salinity, temperature and nutrient regimes. This work reports the spatial effects of a submarine groundwater discharge (SGD) on the abundance and structure of the meiofaunal community in the shallow area of Puck Bay (Baltic Sea). Several field expeditions in the years 2009 and 2010 found that low-saline groundwater escapes into the bay from permeable, sandy, near-shore sediments. The SGD literature has grown rapidly during the current decade; however, the effects of this type of disturbance on the shallow sandy bottom fauna have thus far been little studied. We provide evidence that the discharge of groundwater has a clear effect on meiofaunal assemblages in the research area. This effect was reflected in a significant decline of certain meiofaunal taxa, mainly nematodes and harpacticoids, as well as in altered patterns of temporal distribution and small-scale (vertical) zonation of meiofaunal assemblages. Overlooking submarine groundwater discharge processes may lead to serious misinterpretations of ecological data. It is clear that groundwater discharge phenomena should be considered in future scientific studies.

The seasonal dynamics of molybdenum (Mo) were studied in the water column of two tidal basins of the German Wadden Sea (Sylt-Romo and Spiekeroog) between 2007 and 2011. In contrast to its conservative behaviour in the open ocean, both, losses of more than 50% of the usual concentration level of Mo in seawater and enrichments up to 20% were observed repeatedly in the water column of the study areas. During early summer, Mo removal by adsorption on algae-derived organic matter (e.g. after Phaeocystis blooms) is postulated to be a possible mechanism. Mo bound to organic aggregates is likely transferred to the surface sediment where microbial decomposition enriches Mo in the pore water. First δ98/95Mo data of the study area disclose residual Mo in the open water column being isotopically heavier than MOMo (Mean Ocean Molybdenum) during a negative Mo concentration anomaly, whereas suspended particulate matter shows distinctly lighter values. Based on field observations a Mo isotope enrichment factor of e = −0.3‰ has been determined which was used to argue against sorption on metal oxide surfaces. It is suggested here that isotope fractionation is caused by biological activity and association to organic matter. Pelagic Mo concentration anomalies exceeding the theoretical salinity-based concentration level, on the other hand, cannot be explained by replenishment via North Sea waters alone and require a supply of excess Mo. Laboratory experiments with natural anoxic tidal flat sediments and modelled sediment displacement during storm events suggest fast and effective Mo release during the resuspension of anoxic sediments in oxic seawater as an important process for a recycling of sedimentary sulphide bound Mo into the water column.

Bauer, A.; Radziejewska, T.; Liang, K.; Kowalski, N.; Dellwig, O.; Bosselmann, K.; Stark, A.; Xia, Z.; Harff, J.; Bottcher, M.E.; Schulz-Bull, D.E., and Waniek, J.J., 2013. Regional differences of hydrographical and sedimentological properties in the Beibu Gulf, South China Sea. In: Harff, J., Leipe, T., Waniek, J.J., and Zhou, D. (eds.), Depositional Environments and Multiple Forcing Factors at the South China Sea's Northern Shelf, Journal of Coastal Research, Special Issue, No. 66, pp. 49–71. Coconut Creek (Florida), ISSN 0749-0208. Analyzing the Beibu Gulf's hydrography and sediment properties is crucial for the understanding of naturally and anthropogenically induced matter and energy fluxes in the South China Sea's north-western coastal regions. For this reason, the present study combines hydrographical (T, S, σt, chlorophyll, nutrients, suspended particulate matter) and sedimentological (grain size, pore water properties, phosphate speciation, foraminifera, plant pigment contents) investiga...

Marine ecosystems worldwide are threatened by aquatic pollution; however, there is a paucity in data from the Caribbean region. As such, five heavy metals (arsenic, cadmium, copper, zinc, mercury) were measured in tissues of the scleractinian corals Porites furcata and Agaricia tenuifolia and in adjacent sediments in the Bocas del Toro Archipelago, Panama. Samples were collected from five reef sites along a gradient of distance from an international shipping port and were analysed using inductively coupled plasma optical emission spectrometry and atomic absorption spectrophotometry for mercury. Copper and zinc were the most abundant metals and ranged from 11 to 63 mg kg(-1) and from 31 to 185 mg kg(-1) in coral tissues, respectively. The highest concentration of each metal was measured in P. furcata tissues, with copper and mercury concentrations significantly higher in P. furcata than in A. tenuifolia at every site. These results suggest that P. furcata has a higher affinity for metal accumulation and storage than A. tenuifolia. With the exception of cadmium, metal concentrations in coral tissues were generally elevated at coral reefs in closer proximity to the port; however, this pattern was not observed in sediments. Hard coral cover was lowest at reefs in closest proximity to the port, suggesting that metal pollution from port-related activities is influencing hard coral abundance at nearby coral reefs.

Numerous small watercourses are draining the hinterland of the NW German coast. The waters mainly originate from marsh and fen areas and have yellow to deep brownish color. During their flow path, the waters exhibit gradients in salinity (0.2–3), pH (6.2–8.8), particulate organic carbon (5–25%), and iron oxides (7–12%), which alter the concentrations of most dissolved and particulate trace metals. For example, dissolved Fe is rapidly removed from solution at increasing salinities by flocculation, whereas dissolved U is removed in the very low-salinity zone by Fe- and organic-rich colloids. The waters at the flood-gate of Neuharlingersiel, where a composite sample of the entire study area is collected before the freshwater is discharged into the marine-dominated tidal flat area in front of the mainland dike, have the following average trace metal concentrations: dissolved Fe 11 μM, Mn 4 μM, Mo 10 μM, U 1.8 μM, V 75 μM and particulate Fe 7 mg kg −1 , Mn 1200 mg kg −1 , Mo 2 mg kg −1 , U 1.7 mg kg −1 , V 140 mg kg −1 . After passing the flood-gate most Mo- and U-salinity distributions fit well to the conservative mixing line that connects the seawater of the Wadden Sea to the low-salinity river water entering the tidal flat area. In contrast, dissolved Fe, Mn, and V are removed from solution, especially at intermediate salinities, which may be due to mixing of freshwater with seawater depleted in these elements, flocculation and/or scavenging by particulate matter. In general, processes similar to those in estuarine systems alter the geochemical signatures on transects from land to sea. Compared to average river water, the watercourses are enriched in dissolved and particulate Fe, Mn, Mo, U, and V. Due to the fast removal of dissolved Fe in the salinity gradient and the high concentrations of Mo and U in seawater, the watercourses only form a source for Mn, V, terrestrial organic carbon, and particulate Fe for the Wadden Sea.

The dynamics of the redox-sensitive trace metal manganese (Mn) and its response to biological activity were investigated in the water column and shallow pore waters of the German Wadden Sea (southern North Sea) in 2008 and 2009. Two systems, one from the southern (backbarrier area of Spiekeroog Island) and one from the eastern part (Sylt-Romo Wadden Sea) of the German Bight were compared. The major aim was to examine to which extent biogeochemical findings are specific for a particular tidal basin or transferable to other basins of the German Wadden Sea. Although both study areas reveal hydrodynamical, sedimentological, and ecological differences, qualitatively pronounced similarities in Mn dynamics are observed. Thus, complex cycling of dissolved Mn with increasing values in spring and late summer due to elevated biological activity as well as a depletion period in early summer were observed in both study areas. This finding suggests a seasonal behaviour of Mn being generally representative for the tidal basins of the southern North Sea. Quantitative differences are significant as the backbarrier area of Spiekeroog Island shows much higher concentrations of dissolved Mn in the water column especially during spring. We suggest that this difference is due to a larger sediment area/water volume ratio in the Spiekeroog backbarrier area combined with a higher release of dissolved Mn from the tidal flat sediments as seen in pore water profiles from surface sediments. Site-specific differences are also seen in further tidal systems of the North Frisian Wadden Sea, which are characterised by individual hydrodynamical and sedimentological conditions. Several transects leading from offshore locations towards the coast also revealed a highly variable Mn level. Overall, the North Frisian Wadden Sea most likely represents a less important source for dissolved Mn than the East Frisian Wadden Sea. Furthermore, our data suggest that site-specific properties of the different tidal basins have to be considered in budget calculations for the entire Wadden Sea.

Dissolved reactive Mn (dMn react ) has been determined at the redoxclines of two anoxic deeps from the Baltic Sea (Landsort Deep and Gotland Basin) and two seasonally anoxic freshwater lakes (Lake Dagow and Fuchskuhle, Germany). This dMn react fraction is rapidly oxidised under oxygen atmosphere and is assumed to consist predominantly of Mn(III). There is a distinct increase of dMn react from the outer regions towards the central part of the Landsort Deep. Although the presence of MnO x particles in the Gotland Basin is evidence of ongoing oxidation of reduced Mn species, almost no dMn react was detected. Since completely different processes of Mn oxidation appear rather unlikely, we suggest oceanographic properties are responsible. Lateral currents and intrusions in the Gotland Basin seem to prevent the formation of a stable suboxic zone, a prerequisite necessary for accumulation of dMn react . Such perturbations supply trace amounts of O 2 and H 2 S, causing either immediate oxidation/reduction of dMn react or deterioration of its stabilising ligands. dMn react has also been determined in Lake Dagow with values significantly exceeding the level of the Landsort Deep due to stable stratification of this lake. In contrast, H 2 S appearance throughout the entire water column and a pH react in Lake Fuchskuhle.

Experiments with water samples from the redoxclines of the Black Sea and the Baltic Sea identified a fraction of dissolved Mn which is completely oxidised to solid MnO x within less than 48 h at laboratory conditions. Disproportionation of this dissolved reactive Mn (dMn react ) into Mn (II) and Mn (IV) did not occur. Our data suggest that bacteria using oxygen are responsible for the fast oxidation of dMn react . The operational definition of dMn react is a Mn phase that passes a 0.45 μm filter, but can be separated from remaining dissolved Mn (II) by filtration 48 h after exposure to atmospheric oxygen. The application of this method to water samples from the redoxcline of the Black Sea reveals dMn react profiles comparable to published Mn (III) profiles analysed by polarography thus identifying Mn (III) as the dominating constituent of dMn react . As the degree of autocatalytic oxidation of dissolved Mn (II) by readily produced MnO x and microbial Mn (II) oxidation within the applied oxidation period is unknown, dMn react is at least a semi-quantitative measure of dissolved Mn (III). Furthermore, the present method helps to assess the full potential for oxidation of dissolved Mn within aquatic ecosystems. This method has the advantage that sample preparation can be easily done on site, followed by analysis of dissolved Mn by conventional methods.

Continuous flow analysis for soluble Fe(II) species in seawater was performed with a colorimetric method using ferene as a spectrophotometric reagent. The method is based on the measurement of absorbance of the [FeII(fer)3]4 --complex at 594 nm. No preconcentration-steps are required. Samples prepared in the laboratory in line with external calibration experiments have been successfully analyzed in the nanomolar range (R2 = 0.959). A detection limit of 20 nM was obtained. Furthermore, investigations were performed in situ with a wet chemical analyzer in the water column of the central Baltic Sea. Finally the results were consistent with measurements made by an independent laboratory based method (ICP-OES).

A combined study of lithological, geochemical and physical sediment properties is reported from a completely laminated S5 sapropel, recovered in three gravity cores (M40-4 SL67, M51-3 SL103, M51-3 SL104) from the Pliny Trench region of the eastern Mediterranean. The thickness of the studied sapropel S5 varies between 85 and 91 cm and tops most S5-sapropels in the Mediterranean. Based on optical features like color and thickness of laminae, the sapropels were subdivided into thirteen distinct lithostratigraphic zones. These zones, as well as the finer layering pattern within them, could be followed exactly among the three cores, indicating that the processes responsible for this variation acted at least on a regional scale. The sapropel sediment is characterized by exceptionally high porosity, which is strongly correlated with Si/Ca. This relationship implies that the sapropel is in essence an organic-matter rich diatomite and its exceptional thickness can be explained by preservation of diatoms forming a loosely packed sediment fabric. Compared to other S5 sapropels, the preservation of diatoms has apparently led to a twofold increase in the thickness of the sapropel layer. Relative abundances of 10 elements were determined at ultra-high resolution (0.2 mm) by XRF-scanner over the complete length of each sapropel including several cm of enclosing marl. An analysis of the chemical data indicates that the lowermost 13 cm of the sapropel is chemically more similar to the underlying marl and that the sediment chemistry shows different signals at different scales. The strongest pattern is the contrast between the sapropel and the surrounding marl, which is accentuated in elements indicative for redox conditions as well as terrigenous sediment input and productivity. Within the sapropel, a mm- to cm-scale layering is observed. The abundances of many elements are systematically linked to the pattern of these layers, indicating a common origin, related to productivity and/or terrigenous sediment and/or redox conditions. This pattern indicates a link to a regional climatic process, making the S5 sapropel horizon in M40-4 SL67, M51-3 SL103 and M51-3 SL104 a potential high-resolution archive of climatic variability during the last interglacial in the Mediterranean Sea and its adjacent landmasses.

Following a tentative evidence for the occurrence of low-temperature barium manganese(II) carbonates in brackish sediments of the Baltic Sea, a stoichiometric double carbonate, BaMn[CO 3 ] 2 , was synthesized from aqueous solutions at ambient temperature for the first time. Here we report the results of a multi-method approach, including scanning electron microscopy (SEM–EDX) investigations, the chemical composition, and, in particular, the diagnostic powder X-ray diffraction pattern, as well as diagnostic parts of the FT infrared absorption and Raman spectra for future identification of this new carbonate in low-temperature environments, like brackish sediments.

A decade of studies of metal and nutrient inputs to the back-barrier area of Spiekeroog Island, NW German Wadden Sea, have concluded that pore water discharge provides a significant source of the enrichments of many components measured in the tidal channels during low tide. In this paper we add studies of radium isotopes to help quantify fluxes into and out of this system. Activities of radium isotopes in surface water from tidal channels in the back-barrier area exhibit pronounced changes in concert with the tide, with highest activities occurring near low tide. Other dissolved components: silica, total alkalinity (TA), manganese, and dissolved organic carbon (DOC) exhibit similar changes, with patterns matching the Ra isotopes. Uranium follows a reverse pattern with highest concentrations at high tide. Here we use radium isotope measurements in water column and pore water samples to estimate the fluxes of pore waters that enter the tidal channels during low tide. Using a flushing time of 4 days and the average activities of 224Ra, 223Ra, and 228Ra measured in the back-barrier surface and pore waters, we construct a balance of these isotopes, which is sustained by a deep pore water flux of (2–4) × 108 L per tidal cycle. This flux transports Ra and the other enriched components to the tidal channels and causes the observed low tide enrichments. An independent estimate of pore water recharge is based on the depletion of U in the tidal channels. The U-based recharge is about two times greater than the Ra-based discharge; however, other sinks of U could reduce the recharge estimate. The pore waters have wide ranges of enrichment in silica, alkalinity, manganese, DOC, and depletion of U with depth. We estimate concentrations of these components in pore water from the depth expected to contribute the majority of the pore water flux, 3.5 m, to determine fluxes of these components to the tidal channels. Samples from this depth have minimum concentrations of silica, alkalinity, manganese, and DOC. We also estimate the exports of these components (and import of U) due to mixing based on average measured concentrations in the tidal creeks and the 4-day flushing time. A comparison of these estimates reveals that the exports (negative in the case of U) equal or exceed the pore water fluxes. By using values slightly higher than the minimum concentrations at 3.5 m to calculate inputs, the two estimates could be forced to match. We conclude that pore water drainage is the major factor regulating fluxes of Ra isotopes, silica, alkalinity, manganese, DOC, and uranium in this system.

Here we present stable isotope data for vertical profiles of dissolved molybdenum of the modern euxinic water columns of the Black Sea and two deeps of the Baltic Sea. Dissolved molybdenum in all water samples is depleted in salinity-normalized concentration and enriched in the heavy isotope (δ98Mo values up to + 2.9‰) compared to previously published isotope data of sedimentary molybdenum from the same range of water depths. Furthermore, δ98Mo values of all water samples from the Black Sea and anoxic deeps of the Baltic Sea are heavier than open ocean water. The observed isotope fractionation between sediments and the anoxic water column of the Black Sea are in line with the model of thiomolybdates that scavenge to particles under reducing conditions. An extrapolation to a theoretical pure MoS42− solution indicates a fractionation constant between MoS42− and authigenic solid Mo of 0.5 ± 0.3‰. Measured waters with all thiomolybdates coexisting in various proportions show larger but non-linear fractionation. The best explanation for our field observations is Mo scavenging by the thiomolybdates, dominantly — but not exclusively — present in the form of MoS42−. The Mo isotopic compositions of samples from the sediments and anoxic water column of the Baltic Sea are in overall agreement with those of the Black Sea at intermediate depth and corresponding sulphide concentrations. The more dynamic changes of redox conditions in the Baltic deeps complicate the Black Sea-derived relationship between thiomolybdates and Mo isotopic composition. In particular, the occasional flushing/mixing, of the deep waters, affects the corresponding water column and sedimentary data. δ98Mo values of the upper oxic waters of both basins are higher than predicted by mixing models based on salinity variations. The results can be explained by non-conservative behaviour of Mo under suboxic to anoxic conditions in the shallow bottom parts of the basin, most pronounced on the NW shelf of the Black Sea.

De-embankment in the salt marshes of the island of Langeoog was carried out in 2004, thereby inducing an artificial transgression within an area of 2.2 km2. Material from three suspended matter traps (SMTs) located along a N–S transect was collected monthly between January 2006 and February 2007. Besides geochemical (major and trace elements) and grain-size analyses, the duration and height of water cover were continuously measured by pressure gauges during the sampling period at two sites, thus revealing inundation frequency (max. 280 year−1) and level (max. 2.4 m). Generally, the silt-dominated SMT material exhibits a geochemical composition similar to that of suspended particulate matter from the adjacent Wadden Sea. However, distinctly increasing enrichments of TOC, P, Mn and Mo from the shoreline towards the higher salt marsh clearly indicate fractionation processes during material transport. Geochemical comparison with older Holocene coastal deposits reveals a mixture of brackish and tidal flat sediments, thus reflecting an early stage of sea-level rise and the development from a terrestrial towards a marine-dominated system. Sedimentation rates are higher than the local sea-level rise, as revealed by vertical salt marsh growth. Storm surges deliver the highest amounts of sediment and play an important role in salt marsh accumulation within the study area. Average accumulation rates of TOC (780 t year−1), P (54 t year−1) and Mn (5.2 t year−1) in the de-embanked area suggest that the former sand-dominated sediments currently receive significant amounts of reactive organic-rich material, thus fostering biogeochemical cycling.

The barrier islands of the southern North Sea were formed during the Holocenesea-level rise. These islands form part of a highly dynamic environment whoseevolution continues today. Subjected to sea-level changes, tides and stormevents, the sedimentary record reflects processes occurring under varyingenergy conditions. This article presents geochemical, mineralogical anddiatom investigations carried out in the salt marsh of the East Frisian barrierisland of Langeoog, which is re-exposed to a rising sea-level due tode-embankment. The major aim of this study is to improve the knowledge ofthe sedimentological and geochemical development of these deposits underthe influence of sea-level rise, with a special focus on the geochemistry anddistribution of heavy mineral-associated elements. Correlation diagramsbetween FeO, TiO 2 and MnO, as well as ternary plots (Al 2 O 3 –SiO 2 –Zr orTiO 2 ), clearly indicate the variable appearance of heavy minerals in differentlithological facies, comprising marsh soil, mixed and sand flat, and relocatedbeach sands. A dominating abundance of ilmenite followed by zircon, garnetsand some other heavy minerals is evidenced by Scanning Electron Microscope-Energy Dispersive X-ray measurements. The data presented here suggests thatthese geochemical proxies are useful tools for characterizing depositionalenergy conditions. Increasing depositional energy is evident for the lithologicalunits in the following order: marsh soil, mixed flat, sand flat and relocatedbeach sand. The energetic conditions during sediment deposition, as well asthe sedimentary history, are confirmed by diatom analyses as an additionalindependent indicator. Depending on source rock composition, thegeochemical parameters used in this study may also help to investigatedepositional energy regimes of other siliciclastic sedimentary systems.Keywords Depositional energy, diatoms, heavy minerals, inorganicgeochemistry, North Sea, salt marsh, tidal flat.INTRODUCTIONThe depositional history of the barrier islands andadjacent tidal flat system of the southern NorthSea coast was directly related to climate changesduring the Holocene sea-level rise. The lasttransgressive cycle started in the WeichselianLate Glacial and continues today, as evidenced bySedimentology (2012) 59, 337–355

Pelagic redoxclines of anoxic basins and deeps form the suboxic transition between oxygenated surface and anoxic or even sulfidic bottom waters. Intense element cycling, favoured by elevated microbial activity, causes steep gradients of physico-chemical parameters, nutrients and redox-sensitive trace metals. This study presents a conceptual model for authigenic particle formation at pelagic redoxclines, which is based on the tight coupling of Mn, Fe, and P cycles. Besides the well-known occurrence of Mn-oxides, textural (SEM-EDX) and geochemical (ICP-OES, ICP-MS) analyses of particles from the redoxclines of the Black Sea and the Baltic Sea (Gotland Basin, Landsort Deep) evidence the existence of earlier postulated Fe-oxyhydroxo-phosphates and emphasize mixed phases consisting of Mn-oxides and Fe-oxyhydroxo-phosphates as a new solid species. Most of the analyzed particles are star-shaped, of about 5 μm in size, and occur as single particles or aggregates without any morphological differences between Mn-oxides, Fe-oxyhydroxo-phosphates, and mixed phases. Throughout the redoxcline, these minerals show a general succession with maximum abundance of Mn-oxides above the redoxcline followed by mixed phases and almost pure Fe-phosphates within and below the redoxcline, respectively. Molar Fe/P ratios of single particles argue against the formation of known pure Fe-phosphates like vivianite or strengite at the lower end of the redox transition zone, but are consistent with recent experimental findings for colloidal P-bearing hydrous ferric oxides. Moreover, morphological similarities suggest the formation of irregular Fe-oxyhydroxo coatings due to oxidation of upward diffusing Fe 2+ by oxygen and stepwise replacement of Mn(IV) by Fe(III) on sinking MnO x particles followed by immediate adsorption or even co-precipitation of phosphate. Batch-type experiments using biogenic MnO x particles demonstrate the efficient potential of Fe 2+ oxidation by sinking MnO x particles. When entering sulfidic waters MnO x particles are progressively reduced leading to an increasing relative abundance of Fe- and P-rich particles. In deeper parts of the water column these particles are also reductively dissolved, thereby releasing Fe 2+ and phosphate to the water column. This Mn–Fe–P-shuttle likely affects phosphate transport throughout the water column and thus impacts primary production at least over longer time scales. Furthermore, the particulate Mn–Fe–P-shuttle must have played an important role for the cycling of P and certain trace metals in ancient ocean basins, e.g., during certain periods of Cretaceous black shale formation and should be considered in future mass balances and modeling approaches dealing with oxic/anoxic interfaces of aquatic ecosystems.

In the tidal inlet of the back barrier area of Spiekeroog Island (Southern North Sea), nutrient concentrations (silica, phosphate, and nitrite plus nitrate) were determined hourly by an autonomously analysing system on a permanently installed time-series station from April 2006 to December 2008. Based on the high frequency of analyses we studied nutrient dynamics on annual, seasonal, and tidal time scales. By comparing the nutrient input to the tidal flat area via freshwater through a flood-gate and pore water discharge from tidal flat sediments, we conclude that nutrients are primarily supplied to the water column by pore water advection, while the freshwater contribution is negligible. To assess the annual nutrient contribution of our study area to the German Bight, we used a numerical Euler–Lagrangian model (EcoTiM) to calculate annual budgets of silica, phosphate, and nitrite plus nitrate. The model results indicate that the back barrier area of Spiekeroog Island exports inorganic silica (128 ⁎ 106 mol a− 1), phosphate (3 ⁎ 106 mol a− 1), and nitrite plus nitrate (29 ⁎ 106 mol a− 1) to the North Sea. Extrapolation of these data to the entire Wadden Sea along the Southern North Sea reveals that the back barrier areas export silica and phosphate in the same order of magnitude and nitrite plus nitrate one order of magnitude lower than the combined rivers Elbe, Weser, and Ems.

Salt marshes form part of the widely distributed intertidal landscape. The salt marshes of the East Frisian barrier island Langeoog (NW Germany) belong to the barrier-connected salt marsh type and were protected by a summer dike that was removed in 2004. In this study, pore water and sediment data were combined to investigate the effects of the de-embankment along a transect including sites on seawater-influenced grassland, high and low salt marsh, and transition zone tidal flat–low salt marsh. Pore waters were sampled with in situ pore water samplers for 13 mo and analysed for trace metals (Fe, Mn), nutrients (NH4+, PO43−), dissolved organic carbon, and sulphate. Additionally, on site measurements of pH and salinity were carried out. Pore water ultrafiltration experiments with 5000 Da MWCO (molecular weight cut off) complemented the water analyses. Sediment samples were taken from hammer corings and were analysed for bulk parameters (total carbon [TC], total inorganic carbon [TIC], total organic...

Tidal and seasonal behaviour of the redox-sensitive trace metals Mn, Fe, Mo, U, and V have been investigated in the open-water column and shallow pore waters of the backbarrier tidal flats of the island of Spiekeroog (Southern North Sea) in 2002 and 2007. The purpose was to study the response of trace metal cycles on algae blooms, which are assumed to cause significant changes in the redox state of the entire ecosystem. Trace metal data were complemented by measurements of nutrients and enumeration of algae cells in 2007. Generally, Mn and V show a tidal cyclicity in the water column with maximum values during low tide which is most pronounced in summer due to elevated microbial activity in the sediments. Mo and U behave almost conservatively throughout the year with slightly increasing levels towards high tide. Exceptions are observed for both metals after summer algae blooms. Thus, the seasonal behaviour of the trace metals appear to be significantly influenced by productivity in the water column as the occurrence of algae blooms is associated with an intense release of organic matter (e.g. transparent exopolymer particles, TEP) thereby forming larger organic-rich aggregates. Along with elevated temperatures in summer, the deposition of such aggregates favours microbial activity within the surface sediments and release of DOC, nutrients and trace metals (Mn, Mo and V) during the degradation of the aggregates. Additionally, pronounced reducing conditions lead to the reduction of Mn(IV)-oxides and Fe(III)-(oxihydr)oxides, thereby releasing formerly scavenged compounds as V and phosphate. Therefore, pore-water profiles show significant enrichments in trace metals especially from July to September. Finally, the trace metals are released to the open water column via draining pore waters (esp. Mo, Mn, and V) and/or fixed in the sediment as sulphides (Fe, Mo) and bound to organic matter (U). Non-conservative behaviour of Mo in oxygenated seawater, first observed in the investigation area by Dellwig et al. (Geochim Cosmochim Acta 71:2745–2761, 2007a), was shown to be a recurrent phenomenon which is closely coupled to bacterial activity after the breakdown of algae blooms. In addition to the postulated fixation of Mo in oxygen-depleted micro-zones of the aggregates or by freshly formed organic matter, a direct removal of Mo from the water column by reduced sediment surfaces may also play an important role.

Back-barrier tidal flat systems are characterized by basins and inlets through which water is exchanged with the coastal sea by tidal water movements. The hydrographic and morphometric properties at the inlets and in the basins vary considerably, but there is little information available how biogeochemical properties in the water column at these different sites respond to these differences. Therefore, we investigated tidal dynamics of suspended particulate matter (SPM), particulate and dissolved organic carbon (DOC), chlorophyll a, phaeopigments, numbers of particle-associated (PA) and free-living bacteria (FL), bacterial biomass production, and concentrations of dissolved manganese (Mn). Samples were taken at the surface, a mid-depth and 1 m above the bottom at a fixed station at the inlet and in the basin of the Spiekeroog back-barrier tidal flat system in the German Wadden Sea. Five tidal cycles representative for typical seasonal situations, January (winter), April and May (late spring bloom), July (summer), and November (late fall) were studied in 2005 and 2006. In July, processes related to phytoplankton dynamics and bacterial decomposition were much more enhanced in the basin, whereas in April, these processes were enhanced at the inlet but were particularly low within the basin itself. The low values within the basin were a result of the settled phytoplankton spring bloom and represent a rather short period at the decline of this bloom. In November and January, differences were much less pronounced than during the growing season and restricted mainly to SPM and PA bacteria, exhibiting higher values in the basin. FL bacteria, DOC, and dissolved Mn exhibited different patterns and much less differences between the two stations, indicating that biogeochemical processes in the dissolved phase were controlled by different factors than PA biogeochemical processes. These differences reflect the retentive properties of the basin for particles and PA biogeochemical processes, particularly during the growing season, and in general emphasize the high productivity of back-barrier tidal flat systems.

The spatial and temporal distributions of chromophoric dissolved organic matter (CDOM) and dissolved organic carbon (DOC) was studied in the East-Frisian Wadden Sea (Southern North Sea) during several cruises between 2002 and 2005. The spatial distribution of CDOM in the German Bight shows a strong gradient towards the coast. Tidal and seasonal variations of dissolved organic matter (DOM) identify freshwater discharge via flood-gates at the coastline and pore water efflux from tidal flat sediments as the most important CDOM sources within the backbarrier area of the Island of Spiekeroog. However, the amount and pattern of CDOM and DOC is strongly affected by various parameters, e.g. changes in the amount of terrestrial run-off, precipitation, evaporation, biological activity and photooxidation. A decoupling of CDOM and DOC, especially during periods of pronounced biological activity (algae blooms and microbial activity), is observed in spring and especially in summer. Mixing of the endmembers freshwater, pore water, and open sea water results in the formation of a coastal transition zone. Whilst an almost conservative behaviour during mixing is observed in winter, summer data point towards non-conservative mixing.

Measurements of methane (CH 4 ) so far have always shown supersaturation in the entire North Sea relative to the atmospheric partial pressure and the distribution of surface CH 4 reveals a distinct increase towards the shore. Since North Sea sediments presumably are an insignificant source for CH 4 the coastal contribution via rivers and tidal flats gains in importance. In this work, CH 4 data from the River Weser, the back barrier tidal flats of Spiekeroog Island (NW Germany), and the German Bight are presented. Results from the River Weser are compared to other rivers draining into the German Bight. Measurements in the tidal flat area of Spiekeroog Island highlight this ecosystem as an additional contributor to the overall CH 4 budget of the southern North Sea. A tidally driven CH 4 pattern is observed for the water column with maximum values during low tide. Tidal flat sediments turn out to be the dominating source because pore waters discharged during low tide are highly enriched in CH 4 . In contrast, the freshwater contribution to the tidal flats by small coastal tributaries has almost no impact on water column CH 4 concentrations. The CH 4 level seems to be disturbed irregularly by wind forcing due to elevated degassing and prevention of advective flow when tidal flats remain covered by water. Based on our data, two model calculations were used to estimate the impact of tidal flats on the CH 4 budget in the German Bight. Our results demonstrate that the back barrier tidal flats of the east Frisian Wadden Sea contribute CH 4 in an order of magnitude between the Wash estuary and River Elbe and thus have to be considered in budget calculations.

Spatial and seasonal variations of sulphate, dissolved organic carbon (DOC), nutrients and metabolic products were determined down to 5 m sediment depth in pore waters of intertidal flats located in NW Germany. The impact of sediment permeability, pore water flow, and organic matter supply on deep pore water biogeochemistry was evaluated. Low sediment permeability leads to an enrichment of remineralisation products in pore waters of clay-rich sediments. In permeable sandy sediments pore water biogeochemistry differs depending on whether tidal flat margins or central parts of the tidal flat are studied. Pore water flow in tidal flat margins increases organic matter input. Substrate availability and enhanced temperatures in summer stimulate sulphate reducers down to 3.5 m sediment depth. Sulphate, DOC, and nutrient concentrations exhibit seasonal variations in deep permeable sediments of the tidal flat margin. In contrast, seasonal variations are small in deep pore waters of central parts of the sand flat. This study shows for the first time that seasonal variations in pore water chemistry are not limited to surface sediments, but may be observed down to some metres depth in permeable tidal flat margin sediments. In such systems more organic matter seems to be remineralised than deduced from surface sediment studies.

This study addresses deep pore water chemistry in a permeable intertidal sand flat at the NW German coast. Sulphate, dissolved organic carbon (DOC), nutrients, and several terminal metabolic products were studied down to 5 m sediment depth. By extending the depth domain to several meters, insights into the functioning of deep sandy tidal flats were gained. Despite the dynamic sedimentological conditions in the study area, the general depth profiles obtained in the relatively young intertidal flat sediments of some metres depth are comparable to those determined in deep marine surface sediments. Besides diffusion and lithology which control pore water profiles in most marine surface sediments, biogeochemical processes are influenced by advection in the studied permeable intertidal flat sediments. This is supported by the model setup in which advection has to be implemented to reproduce pore water profiles. Water exchange at the sediment surface and in deeper sediment layers converts these permeable intertidal sediments into a “bio-reactor” where organic matter is recycled, and nutrients and DOC are released. At tidal flat margins, a hydraulic gradient is generated, which leads to water flow towards the creekbank. Deep nutrient-rich pore waters escaping at tidal flat margins during low tide presumably form a source of nutrients for the overlying water column in the study area. Significant correlations between the inorganic products of terminal metabolism (NH4 + and PO4 3−) and sulphate depletion suggest sulphate reduction to be the dominant pathway of anaerobic carbon remineralisation. Pore water concentrations of sulphate, ammonium, and phosphate were used to elucidate the composition of organic matter degraded in the sediment. Calculated C:N and C:P ratios were supported by model results.

The Marine Isotope Stage (MIS) 3 stands out due to its abrupt changes from cold and dry stadials to warm and humid interstadials, the so-called Dansgaard-Oeschger cycles that also affected temperature and rainfall in the Black Sea region. This study is based on a gravity core from the southeastern (SE) Black Sea that covers the last glacial lake stage from 64 to 20 ka BP. By using the composition of major and trace elements in the sediments, terrestrial plant-derived n -alkane flux, and Sr/Ca from benthic ostracods, we reconstruct the variability of riverine and aeolian input, salinity, and productivity in the SE Black Sea region in response to the Northern Hemisphere climate oscillations. During colder and drier stadials, the aeolian input increased relative to the riverine discharge, potentially due to southward shifted and/or stronger westerly winds and due to changes in the vegetation cover. An evaporation exceeding freshwater supply by rainfall and rivers possibly caused higher salinity and a lower lake level. The environmental status during MIS 4 and 2 is very much comparable with the stadial conditions during MIS 3. During warmer and more humid interstadials, lower salinity and presumably positive lake level changes most likely resulted from increased precipitation and river discharge. This likely increased primary productivity through an augmented nutrient supply. Lowest average salinities are suggested for the middle part of MIS 3 in response to enhanced meltwater from the disintegrating Fennoscandian Ice Sheet and/or by generally more humid conditions.

In autumn 2002 a time-series station was installed in the tidal inlet between the Islands of Langeoog and Spiekeroog (Southern North Sea, NW Germany) to continuously measure physical, chemical, and meteorological parameters, even during extreme weather conditions (gale-force storms, drifting ice). Inside the pole of the station sensor tubes are installed in direction of the prevailing tidal currents. The tubes are equipped with hydrographic sensors (pressure, temperature, conductivity) and allow retrieval of water for nutrient analysis by automated instruments located inside the pole. Dissolved methane and the nutrients ammonia, nitrite, nitrate, phosphate, and silicate are measured at the station. Nutrient patterns obtained during spring and summer 2006 reflect the development of nutrient concentrations during the annual spring phytoplankton bloom. Methane measurements showed a tidal dependence with maxima at low tide, which emphasizes the tidal flat sediments as the dominating source for methane in the open coastal water column.

In this study, we present a multilevel in situ pore water sampler that allows pore water sampling down to 5 m sediment depth. The sampler forms a crucial tool to study biogeochemical processes on different time scales in advective pore water systems. After insertion into the sediment, the sampler stays on site, allowing repetitive sampling at identical locations and depth intervals. The sampler has been successfully tested for 1 year in sandy sediments in the backbarrier tidal flats of Spiekeroog Island at the German North Sea coast. Depth profiles of redox-sensitive elements show a high depth resolution and are not affected by oxidation artifacts during extraction. Seasonal variations because of advection and changing microbial activity are apparent for some element species even at sediment depths of 5 m.

Dissolved and particulate Mn concentrations were investigated on a seasonal scale in surface waters of the NW German Wadden Sea (Spiekeroog Island) in 2002 and 2003. As the Wadden Sea forms the transition zone between the terrestrial and marine realms, Mn was analysed in coastal freshwater tributaries and in the adjoining German Bight as well. Additionally, sediments and porewaters of the tidal flat sediments were analysed for Mn partitioning and microbial activity. Dissolved Mn concentrations show strong tidal and seasonal variation with elevated concentrations during summer at low tide. Summer values in the Wadden Sea (av. 0.7 μM) are distinctly higher than in the central areas of the German Bight (av. 0.02 μM), suggesting a possible impact of the Wadden Sea environment on the Mn budget of the North Sea. Seasonality is also observed for particulate Mn in the Wadden Sea (winter av. 800 mg kg − 1 ; summer av. 1360 mg kg − 1 ). Although particles are relatively Mn-poor during winter, the high SPM load during this season causes elevated excess concentrations of particulate Mn, which in part exceed those of the dissolved phase. Therefore, winter values cannot be ignored in balance calculations for the Wadden Sea system. Porewater Mn concentrations differ depending on sediment type and season. Maximum concentrations are found in surface sediments at a mixed flat site (190 μM) during summer, while winter values are distinctly lower. This indicates that enhanced microbial activity owing to higher temperature during summer leads to increased reduction of Mn-oxides in surface sediments and enhances the corresponding diffusive and advective Mn flux across the sediment-water interface. Draining of Mn-rich porewaters from sediments is also documented by analyses of tidal creek waters, which are highly enriched in Mn during summer. Furthermore, an important Mn source is freshwater discharged into the Wadden Sea via a flood-gate. The concentration of dissolved Mn in freshwater was highly variable during the sampling campaigns in 2002 and 2003, averaging 4 μM. In contrast, particulate Mn displayed a seasonal behaviour with increasing contents during summer. On the basis of salinity variations in the Wadden Sea, the total amount of Mn contributed to the Wadden Sea via freshwater was estimated. This balance shows the importance of the freshwater environment for the Mn inventory of the Wadden Sea. During winter the total Mn inventory of the Wadden Sea water column may be explained almost completely by freshwater discharge, whereas in summer the porewater system forms the dominating source.

In comparison to similar low-sulfate coastal environments with anoxic-sulfidic sediments, the Achterwasser lagoon, which is part of the Oder estuary in the SW Baltic Sea, reveals unexpectedly high pyrite concentrations of up to 7.5 wt%. Pyrite occurs mainly as framboidal grains variable in size with diameters between 1 and 20 μm. Pyritization is not uniform down to the investigated sediment depth of 50 cm. The consumption of reactive-Fe is most efficient in the upper 20 cm of the sediment column, leading to degrees of pyritization (DOP) as high as 80 to 95%. Sediment accumulation in the Achterwasser takes place in high productivity waters. The content of organic carbon reaches values of up to 10 wt%, indicating that pyrite formation is not limited by the availability of organic matter. Although dissolved sulfate concentration is relatively low (

Interdisciplinary studies carried out on marine and terrestrial deposits, such as tidal flats, lagoonal deposits, and salt marshes have provided the basis for interpreting Holocene sea-level fluctuations along the southern North Sea coast. Radiocarbon dating provides the timeframe for the studies. The employed palaeoecological techniques include analysis of pollen, diatoms, and botanical macro remains. Benefits and constraints of each technique are discussed using examples of sediment cores from around the southern North Sea. Sea-level index points have been defined using the results of these interdisciplinary studies and then employed to reconstruct the sea-level oscillations of the last two millennia.

We report first results from a study on water and element exchange across the land-ocean boundary at the southern Baltic Sea. The focus is set on ferruginous fresh ground waters escaping at the shore line, flowing in air contact before entering a subterranean mixing zone with brackish Baltic Sea water. The present study combines the results from multiple sampling campaigns that investigated the composition of several springs as well as the surface and subsurface development of fresh waters on the way to the Baltic Sea. This is achieved by a combination of hydroand solid-phase geochemical and stable isotope measurements with ground water dating. Results are compared to the composition of groundwaters recovered from wells in the catchment area and the local isotope meteoric water line developed for Warnemünde. The spring water is shown to be impacted by the dissolution of biogenic carbon dioxide and marine carbonate as well as the oxidation of pyrite in glacial sediments. Dating yields a surprisingly high diversity between some closely associated springs with average ages of about 25 to 32 years, but different mixing proportions with older tritium-free ground-water.

Sediments (568) and suspended particulate matter (SPM, 302 samples) of the southern German Bight and the adjacent tidal flat areas were analysed for selected major elements (Al, Fe, K), trace metals (Mn, Pb), and 206Pb/207Pb ratios using XRF, ICP–OES, ICP–MS. For selected samples a leaching procedure with 1 M HCl was used to estimate the Pb fraction associated with labile phases (e.g. Mn/Fe-oxihydroxide coatings) in contrast to the resistant mineral matrix. Enrichment factors versus average shale (EFS) reveal elevated Pb contents for all investigated sediments and SPM in the following order: Holocene tidal flat sediments (HTF, human-unaffected) 5 mg l−1) < offhore SPM (

Three drill cores from the marshlands of NW Germany, which cover the entire Holocene, were analyzed at high-resolution for bulk composition, Al, Fe, selected trace metals, and stable sulfur isotopes. The drill cores contain two lithological types of peat: (i) basal peats overlying Pleistocene sands and (ii) intercalated peats situated between clastic sediments of predominantly marine origin. The peat layers are characterized by distinct enrichments in pyrite due to microbial sulfate reduction under almost open system conditions with respect to seawater sulfate as shown by sulfur isotope partitioning. The main Fe source seems to be the freshwater environment. The determination of dissolved and particulate Fe of channels and small rivers close to the study area revealed a 50-fold higher Fe content of the freshwater environment when compared with North Sea water. Pyrite enrichments are explained by two scenarios: (i) pyrite formation coincides with fen reed peat growth (basal and intercalated) under the influence of a brackish water zone (salinity app. 5–15) and (ii) pyrite was formed after peat growth in the lowest limnic basal peat intervals. Maximum pyrite accumulation (TS 28%) occurs in latter peats that contain thin clastic layers as a result of tidal channel activities after peat formation. The occurrence of clastic layers may have favoured the inflow of saline groundwater. The peat layers are also characterized by enrichments in redox-sensitive trace metals (As, Mo, Re, U) and Cd, whereas Co, Cr, Cu, Mn, Ni, Pb, Tl, and Zn reflect the geogenic background. Leaching experiments have shown that As, Co, Cu, Mo, Re, and Tl are predominantly fixed as sulfides and/or incorporated into pyrite. The remaining trace metals show no distinct trends, only Cr reveals a strong relation to the lithogenic detritus. Seawater is the dominating source for As, Cd, Mo, Re, and U. The remaining trace elements seem to have a freshwater source similar to Fe. In contrast to the distribution of pyrite, highest amounts of redox-sensitive trace metals are seen in fen reed peats (basal and intercalated) that were formed under a direct influence of seawater and brackish water, respectively. Therefore, we suggest that saline groundwater entering the basal peats was probably depleted in redox-sensitive trace metals, e.g. owing to microbially induced reduction of trace metals and subsequent precipitation as sulfides or fixation by organic matter.

The last glacial–interglacial transition or Termination I (T I) is well documented in the Black Sea, whereas little is known about climate and environmental dynamics during the penultimate Termination (T II). Here we present a multi-proxy study based on a sediment core from the SE Black Sea covering the penultimate glacial and almost the entire Eemian interglacial ( ( 133.5 ± 0.7 ) – ( 122.5 ± 1.7 ) ka BP ). Proxies comprise ice-rafted debris (IRD), O and Sr isotopes as well as Sr/Ca, Mg/Ca, and U/Ca ratios of benthic ostracods, organic and inorganic sediment geochemistry, as well as TEX 86 and U K′ 37 derived water temperatures. The ending penultimate glacial (MIS 6, 133.5 to 129.9 ± 0.7 ka BP ) is characterised by mean annual lake surface temperatures of about 9 °C as estimated from the TEX 86 palaeothermometer. This period is impacted by two Black Sea melt water pulses (BSWP-II-1 and 2) as indicated by very low Sr/Ca ostracods but high sedimentary K/Al values. Anomalously high radiogenic 87 Sr/ 86 Sr ostracod values (max. 0.70945) during BSWP-II-2 suggest a potential Himalayan source communicated via the Caspian Sea. The T II warming started at 129.9 ± 0.7 ka BP , witnessed by abrupt disappearance of IRD, increasing δ 18 O ostracod values, and a first TEX 86 derived temperature rise of about 2.5 °C. A second, abrupt warming step to ca. 15.5 °C as the prelude of the Eemian warm period is documented at 128.3 ka BP. The Mediterranean–Black Sea reconnection most likely occurred at 128.1 ± 0.7 ka BP as demonstrated by increasing Sr/Ca ostracods and U/Ca ostracods values. The disappearance of ostracods and TOC contents > 2 % document the onset of Eemian sapropel formation at 127.6 ka BP. During sapropel formation, TEX 86 temperatures dropped and stabilised at around 9 °C, while U K′ 37 temperatures remain on average 17 °C. This difference is possibly caused by a habitat shift of Thaumarchaeota communities from surface towards nutrient-rich deeper and colder waters located above the gradually establishing halo- and redoxcline.

A drill core from the marshlands of NW Germany covering the entire Holocene was analysed at high-resolution by geochemical (XRF, Cirm-MS) and microfacies methods (diatoms, thin sections, SEM) in order to provide information about the palaeoenvironmental development. In addition, microbial experiments were carried out with sulphate-reducing bacteria (SRB) and cellulose fermenting fungi in seawater using peat as substrate. The core contains an intercalated reed peat layer (2.73–3.02 m) and a basal peat (5.78–6.42 m). The basal peat consists in the lowest part of fen woodland and raised bog peat, while the upper part is formed by a reed peat bed. The peat layers are characterised by a distinct enrichment of pyrite with negative bulk sediment δ 34 S values (av. −11.5‰, range −2.6 to −26.7‰) reflecting microbial sulphate reduction in some peat intervals under almost open system conditions with respect to the input of seawater sulphate. Pyrite occurs exclusively with framboidal texture in close vicinity to the organic matter which indicates localised pyrite formation at the decay site in microenvironments with high concentrations of dissolved Fe. Microbial experiments reveal that peat does not represent an appropriate substrate for SRB and that SRB have to rely on the preceding decomposition of peat by other microorganisms in microenvironments. The total sulphur content of the peat layers averages to 7.8 wt% with a maximum value of 28.2 wt% unusually found within the basal peat at the transition of fen woodland and raised bog peat. The pyrite enrichments can be explained by two different scenarios which are related to the intensity of the sea-level rise. The first scenario applies to a moderate sea-level rise and is decisive for the reed peats where pyrite formation coincides with peat growth. A brackish zone is assumed for the peat-forming environment which is influenced by iron-rich freshwater and sulphate-rich seawater. As the freshwater forms the major Fe source the amounts of pyrite increase to a certain degree with decreasing salinity in this environment. The palaeosalinity is estimated by the investigation of littoral and pelagic diatoms. The second scenario applies to the extensive pyrite formation within the basal peat at the transition of fen and bog peat. This pyrite was likely formed after peat growth favoured by a steep sea-level rise. Thin sections of this peat interval reveal clastic layers (1–3 mm) with completely pyritised interfaces. As these clastic layers contain marine pelagic diatoms (e.g. Paralia sulcata ) we propose that they are caused by tidal channel activities during a phase of a steep sea-level rise. The marine incursion led to a partial buoyancy of the basal peat at weak transition zones, i.e. the transition from fen to bog peat. As a result of the bouyancy, marine suspended particulate matter can be introduced laterally in-between the peat sections. When the peat settles down a clastic layer remains whose interface with the peat may have favoured the inflow of waters of higher salinity. Therefore, an enhanced formation of pyrite was possible due to the combination of sulphate-rich ground water and iron-rich peatland waters.

This study presents geochemical evidence for a change in depositional energy conditions of tidal flat sediments (southern North Sea) from the Holocene, i.e. human unaffected, to present-day conditions. We investigated Holocene and present tidal flat sediments and suspended particulate matter (SPM) from the NW German coastal area (Spiekeroog Island back-barrier area and Jade Bay), as well as sediments from the Helgoland Island mud hole area. Samples were analysed for bulk parameters (TC, TIC), major (Al, Ca, Fe, Mg, K, P, Si, Ti), and trace elements (Ba, Pb, Rb, Sr, V, Zn, Zr). Enrichment factors versus average shale reveal four groups of elements for the investigated Holocene and present sediments. Fe, Mg, K, Ba, Rb, and V show a shale-like behaviour and enrichments of Ca and Sr reflect the occurrence of carbonate, whereas higher levels of P, Pb, and Zn in the present samples are due to pollution. The fourth group consists of Si, Ti, and Zr, which may be used as indicators of depositional energy because these elements are concentrated by particle sorting effects. The most pronounced geochemical difference between the Holocene and present tidal flat sediments is an enrichment of Zr in the present samples. As Zr is commonly associated with heavy minerals, this enrichment indicates a higher depositional energy environment in the present sediments, which can be traced to modern dike building. The same effect, i.e. increasing current velocities, is responsible for a general depletion of fine-grained, Al-rich, material in the present sediments. The examination of SPM shows that large amounts of this fine-grained material are present in the water column and may be transported from the intertidal system into the open North Sea. The comparison of a calculated Holocene clay accumulation rate with modern estimates of SPM deposition in the German Bight reveals about a two-fold higher deposition of fine material in the Holocene tidal flats. As the sediments from the Helgoland mud hole show a geochemical composition similar to Holocene tidal flat sediments, we assume that the Helgoland mud hole may serve as a proximal depocentre in the southern North Sea for the SPM exported from the back-barrier systems.

Holocene deposits of the NW German coastal plain consist of many different lithological facies, for example tidal flats, brackish water sediments and peat beds. The effects of the Holocene sea-level rise on palaeoenvironmental conditions of this coastal sequence were studied by inorganic geochemical methods in conjunction with diatom analyses. Three hundred samples from two parallel sediment cores which cover the entire NW German Holocene were taken at high resolution and were examined for major and minor elements and bulk parameters. Selected samples were analysed for redox-sensitive trace elements and REE distribution, reactive iron and bulk sediment 34 S/ 32 S-ratios. Chemical parameters, for example Si/Al and Ca/Sr-ratios and total organic carbon (TOC) contents correlate with depositional factors such as wave-energy and lithofacies changes, which allow a detailed reconstruction of the palaeoenvironment. Diatom analyses reveal information about changes between marine, brackish and limnic conditions and serve to reconstruct palaeosalinity. Early diagenetic effects are evident in the TOC-rich intervals. Most peat layers are affected by sulphate reduction and resulting pyrite formation as well as by enrichments in redox-sensitive trace elements. The highest enrichments are seen for As, Mo, Re and U, indicating a distinct seawater influence. S-isotope ratios of peat samples are compatible with pyrite formation under both open- and closed-system conditions, depending on exposition to seawater. The inorganic geochemical and diatom data suggest limnic conditions at the base of the sequence and repeated changes towards marine conditions within the overlying clastic units. On the other hand, data obtained from the clastic units yield evidence of a recurrent succession from open to restricted marine, brackish-lagoonal, and finally fen environments. Clastic sediments overlying peat layers, correlate with the increase of marine-derived geochemical signatures and pelagic diatoms attest transgressive overlaps. The analyses suggest that major controls on the palaeoenvironments were (i) climate-related oscillations of the coast line and (ii) the morphology of the coastal region allowing marine incursions even into distal semi-terrestrial lowlands.

Holocene depositional sequences in the coastal zone of the North Sea reflect climate-driven sea-level changes that led to lateral interfingering of marine, brackish, lagoonal and limnic sediments and peat layers. This succession is reflected in an 18 m sediment core drilled in the marsh lands of the river Weser. The core spanning the entire Holocene was analyzed at high stratigraphic resolution for major and minor elements, bulk and molecular constituents and microfacies characteristics. Abundant triterpenoids in sediments of the basal organic unit indicate nutrient-rich limnic paleoconditions not yet influenced by the rising sea level. Lithological description of several sedimentary units overlying the basal sequence initially suggested a mainly lagoonal setting with occasional influence by tidal channel activities; geochemical and microfacies analyses confirmed repeated marine ingression. Marine conditions are indicated by pyrite enrichment, the presence of sterathiols formed by the reaction of sterols with reduced sulfur species, and the occurrence of marine pelagic diatoms. Towards the top of the sedimentary sequence, estuarine conditions become more and more pronounced as the river Weser established its modern course whereas it had been quite variable previously. Estuarine conditions are reflected by sterol and amino acid distributions and by limnic diatom species that are mixed with marine pelagic forms. Although the majority of the clastic units consist of silty to clayey sediments, some intercalated quartz sand layers reflect the increase of hydrodynamic energy characteristic of lower tidal flats and channels. Lumps of reworked peat within fine clastic deposits also reflect high energy events. Even a dark layer within the upper clastic unit, formerly considered a fossil soil horizon, is not the direct result of regression-induced soil development, but originates mainly from eroded fossil organic material from the basal unit. This interpretation was established by biomarker analysis and radiocarbon dating.

In the southern North Sea, an observatory is permanently installed in the tidal flat area of Spiekeroog Island. The observatory permits continuous nutrient (silica, phosphate, nitrate, nitrite) measurements in the open water column. Nutrients vary on tidal and seasonal time scales depending on phytoplankton dynamics and nutrient recycling in tidal flat sediments. Model results indicate that dissolved nutrients are exported from the tidal flat system to coastal waters of the North Sea. Thus, nutrient cycling in tidal flat areas may influence nutrient and phytoplankton dynamics in coastal waters of the southern North Sea.