Olaf Dellwig

Changing sedimentation in tidal flat sediments of the southern North Sea from the Holocene to the present: a geochemical approach

Abstract 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.

Trace metals in Holocene coastal peats and their relation to pyrite formation (NW Germany)

Abstract 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.

Lead in sediments and suspended particulate matter of the German Bight: natural versus anthropogenic origin

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 (<5 mg l−1). Besides pollution, RTF contain elevated amounts of natural Pb-rich materials (K-feldspars and heavy minerals) due to a man-made high-energy environment (dike building) in comparison to HTF. 206Pb/207Pb ratios of RTF (1.192±0.019) are similar to the local geogenic background, determined from HTF (1.207±0.008). In contrast, Pb isotope ratios of nearshore SPM (1.172±0.007) and offshore SPM (1.166±0.012) show a distinct shift towards the anthropogenic/atmospheric signal of 1.11–1.14. This difference between RTF and SPM supports the assumption of low deposition rates of fine material in the intertidal systems. As the 206Pb/207Pb ratios of SPM do not reach the pure anthropogenic signal, the adsorbed Pb fraction was examined (leaching). However, the leachates also contained large amounts of geogenic Pb (SPM ≈40%, recent sediments ≈60%). The authors assume that the uptake of natural Pb occurs in nearshore waters, presumably in the turbid intertidal systems. Possible sources for dissolved Pb are mobilisation during weathering (geogenic signal) and dissolution of oxihydroxide coatings with subsequent release from porewaters, and unspecific riverine input. Comparatively small parts of SPM leave the coastal water mass and reach the open North Sea. This process therefore leads to a decontamination of the tidal flat sediments. Due to more pronounced atmospheric input, the offshore SPM becomes enriched in anthropogenic Pb as indicated by decreasing 206Pb/207Pb ratios with increasing distance from the coast.

Tube‐dwelling invertebrates: tiny ecosystem engineers have large effects in lake ecosystems

There is ample evidence that tube-dwelling invertebrates such as chironomids significantly alter multiple important ecosystem functions, particularly in shallow lakes. Chironomids pump large water volumes, and associated suspended and dissolved substances, through the sediment and thereby compete with pelagic filter feeders for particulate organic matter. This can exert a high grazing pressure on phytoplankton, microorganisms, and perhaps small zooplankton and thus strengthen benthic-pelagic coupling. Furthermore, intermittent pumping by tube-dwelling invertebrates oxygenates sediments and creates a dynamic, three-dimensional mosaic of redox conditions. This shapes microbial community composition and spatial distribution, and alters microbe-mediated biogeochemical functions, which often depend on redox potential. As a result, extended hotspots of element cycling occur at the oxic-anoxic interfaces, controlling the fate of organic matter and nutrients as well as fluxes of nutrients between sediments and water. Surprisingly, the mechanisms and magnitude of interactions mediated by these organisms are still poorly understood. To provide a synthesis of the importance of tube-dwelling invertebrates, we review existing research and integrate previously disregarded functional traits into an ecosystem model. Based on existing research and our models, we conclude that tube-dwelling invertebrates play a central role in controlling water column nutrient pools, and hence water quality and trophic state. Furthermore, these tiny ecosystem engineers can influence the thresholds that determine shifts between alternate clear and turbid states of shallow lakes. The large effects stand in contrast to the conventional limnological paradigm emphasizing predominantly pelagic food webs. Given the vast number of shallow lakes worldwide, benthic invertebrates are likely to be relevant drivers of biogeochemical processes at regional and global scales, thereby mediating feedback mechanisms linked to climate change.

The indicative meaning of diatoms, pollen and botanical macro fossils for the reconstruction of palaeoenvironments and sea-level fluctuations along the coast of Lower Saxony; Germany

Abstract 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.

Regional Differences of Hydrographical and Sedimentological Properties in the Beibu Gulf, South China Sea

ABSTRACT Bauer, A.; Radziejewska, T.; Liang, K.; Kowalski, N.; Dellwig, O.; Bosselmann, K.; Stark, A.; Xia, Z.; Harff, J.; Böttcher, 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 Seas Northern Shelf, Journal of Coastal Research, Special Issue, No. 66, pp. 49–71. Coconut Creek (Florida), ISSN 0749-0208. Analyzing the Beibu Gulfs hydrography and sediment properties is crucial for the understanding of naturally and anthropogenically induced matter and energy fluxes in the South China Seas 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) investigations. On the basis of hydrographical profiles (temperature, salinity and σt) taken at 25 stations, four ecological zones are identified in the study area for the sampling period in September/October 2009. These zones are mainly influenced by riverine input and tidal mixing, water mass transport through the Qiongzhou Strait which also affects the gulfs circulation, and South China Sea waters in the southern Beibu Gulf. The zonation extends from the coastal areas in the northern Beibu Gulf and west of Hainan Island across the central regions to the southern part of the gulf. The study demonstrates that the hydrographical peculiarities of the different zones influence not only the biogeochemical features (chlorophyll, nutrients, suspended particulate matter) of the water column but also the deposition of sediments and their biological (plant pigment contents and foraminifera) and geochemical (pore water properties) characteristics. Both, the near-shore area and the zone in the vicinity of the Qiongzhou Strait show relatively high chlorophyll concentrations and therefore give evidence of enhanced primary production in the entire water column. Whereas the grain size and the foraminifera in the surface sediments follow the hydrodynamically controlled sedimentation conditions, plant pigment contents in the surface sediments additionally follow the productivity pattern in the water column. Depending on the depositional environments with their respective sedimentology and organic matter contents, the geochemical sediment properties reflect the primary production within the water column as well.

Bacterial communities potentially involved in iron-cycling in Baltic Sea and North Sea sediments revealed by pyrosequencing

To gain insight into the bacterial communities involved in iron-(Fe) cycling under marine conditions, we analysed sediments with Fe-contents (0.5-1.5 wt %) from the suboxic zone at a marine site in the Skagerrak (SK) and a brackish site in the Bothnian Bay (BB) using 16S rRNA gene pyrosequencing. Several bacterial families, including Desulfobulbaceae, Desulfuromonadaceae and Pelobacteraceae and genera, includingDesulfobacterandGeobacter, known to reduce Fe were detected and showed highest abundance near the Fe(III)/Fe(II) redox boundary. Additional genera with microorganisms capable of coupling fermentation to Fe-reduction, includingClostridiumandBacillus, were observed. Also, the Fe-oxidizing families Mariprofundaceae and Gallionellaceae occurred at the SK and BB sites, respectively, supporting Fe-cycling. In contrast, the sulphate (SO4 (2-)) reducing bacteriaDesulfococcusandDesulfobacteriumwere more abundant at greater depths concurring with a decrease in Fe-reducing activity. The communities revealed by pyrosequencing, thus, match the redox stratification indicated by the geochemistry, with the known Fe-reducers coinciding with the zone of Fe-reduction. Not the intensely studied model organisms, such asGeobacterspp., but rather versatile microorganisms, including sulphate reducers and possibly unknown groups appear to be important for Fe-reduction in these marine suboxic sediments.

Black Sea temperature response to glacial millennial-scale climate variability

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.

Effects of De-Embankment on Pore Water Geochemistry of Salt Marsh Sediments

Abstract 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 carbon [TOC], total sulphur [TS]) and selected major elements (Si, Al, Fe, Mn). Additionally, reactive iron and manganese were analysed. Sediments along the study transect are characterised by quartz dominance and very low TC (TOC and TIC) and TS contents. The iron content is comparable to other salt marsh sediments. The high percentage of reactive iron (up to 40%) indicates that salt marsh sediments form an important iron source for pore waters, as confirmed by high pore water concentrations of dissolved iron (up to 583 μM). Dissolved iron in pore waters most likely results from reduction and dissolution of oxidised iron minerals by organic ligands or Fe(II) organic complexes. Iron complexation by humic substances and siderophores in combination with circum-neutral pH values keep iron in solution. Therefore, the studied salt marshes presumably form an important iron reservoir, which may account for elevated pyrite contents frequently observed in Holocene coastal peats. Flooding of the salt marsh during a storm surge resulted in a considerable increase in pore water iron (8-fold), manganese (21-fold), phosphate (5-fold), and ammonia (7-fold) concentrations. These results show that seawater restoration (de-embankment) should be handled very carefully, especially with regard to nutrient release and subsequent changes in pore water quality.

Benthic Bacterial Community Composition in the Oligohaline-Marine Transition of Surface Sediments in the Baltic Sea Based on rRNA Analysis

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.