Thomas Leipe

Changes in the C, N, P burial rates in some Baltic Sea sediments over the last 150 years - relevance to P regeneration rates and the phosphorus cycle

Three major depositional basins (the Gdansk, Bornholm and Eastern Gotland basins) of the Baltic Proper, which together account for .50% of the depositional areas in the entire Baltic Sea, have accumulated increased amounts of sediment and organic carbon during the last 50 years, as is shown in 210 Pb-dated sediment cores. The shallow Arkona Basin has had constant accumulation rates and rate increases of bulk sediment, organic carbon and aluminium in the Bornholm Basin are parallel and are interpreted to reflect increased material input from land or from erosion of shallow-water areas. In the Gdansk and Eastern Gotland basins, the accumulation rates of mineral matter have risen at lower rates than those of organic carbon and point to preferential enrichment of organic matter. At increasing rates of burial, the material in the Gdansk and Eastern Gotland basins appears to have been depleted in phosphorus relative to nitrogen and organic carbon. Using C:N:P ratios from a sediment trap mooring in the Gotland Basin and from fluffy layer material in the Pomeranian Bight as characteristic ratios for sedimenting material, we find evidence for significant increases in the ratios of C to P in sediments younger than 15‐25 years in cores from the Eastern Gotland basin, coincident with H2S build-up in deep waters. Paired oxygen and phosphorus concentration data from the sub-halocline water column of the Gotland Basin covering the time since 1970 suggest that the phosphate reflux is fed from two sources: At the transition from oxic to anoxic conditions, iron-bound phosphate is suddenly liberated at the sediment‐water interface and results in a concentration jump of approximately 2 mmol dm 23 phosphate in deep water. During anoxic periods with H2S in bottom waters, phosphate diffuses out of the sediment and adds to the dissolved phosphate pool. Our data imply that the sediment contributed approximately 14,000 t a 21 of phosphorus to the water column of the deep Gotland Basin since the early 1970s. Compared to total P input to the Baltic Sea (1993: 39,000 t), the reflux of P from anoxic sediments in the Gotland Basin thus is a major source of P for the Baltic Sea. On time scales of years and decades, the reflux of phosphorus from the sediments may be the reason for discrepancies between the ratios of nitrogen and phosphorus in the Baltic Sea and for a lack of effects of declining phosphate discharge into this large ecosystem. q 2000 Elsevier Science B.V. All rights reserved.

Manganese cycling in the Gotland Deep, Baltic Sea

Abstract Manganese plays an important role as both an electron donor and acceptor in redox processes of stratified marine environments. Here we present results on Mn cycling in the water column of the Gotland Deep, Baltic Sea—a basin with periodically observed anoxic conditions in bottom waters. In the period 1999–2001, the deeper part of the Gotland Basin was permanently anoxic, and no inflows of significant amounts of oxygenated North Sea water were recorded below the halocline. The upward Mn(II) flux at the chemocline varied between 28 and 77 μmol m−2 day−1 (avg. 53 μmol m−2 day−1) and it was balanced by the downward flux of oxidized Mn over the entire year. The vertical flux of Mn(II) in the water column was mostly regulated by the flux of settling Mn oxides with a minor contribution of Mn(II) diffusing from bottom sediments (7.1–8.2 μmol m−2 day−1). The potential Mn(II) oxidation contributed no more than 2% to the total transferable electron flux (potential chemosynthesis) from the anoxic into the oxic zone, whereas the flux of settling Mn oxides presumably accounted for 3–30% of sulfide oxidation. Two different morphotypes of Mn-rich particles, formed during Mn(II) oxidation in the Gotland Basin chemocline, were identified using SEM-EDX: amorphous particles (92%) and Metallogenium-like particles. About 68% of all Mn-rich particles were associated with large aggregates containing an organic matrix. Manganese represented more than 60% of the total elemental composition of the particles; calcium, iron and silica were also detected.

Material transport from the nearshore to the basinal environment in the southern Baltic Sea I. Processes and mass estimates

Abstract Processes involved in erosion, transport and deposition of cohesive materials are studied in a transect from shallow (16 m) to deep (47 m) water of the SW Baltic Sea. The wave- and current-induced energy input to the seabed in shallow water is high with strong variability and suspended matter concentrations may double within a few hours. Primary settling fluxes (from sedimentation traps) are less than 10 g m −2 day −1 , whereas resuspension fluxes (evaluated from sedimentation flux gradients) are 15–20 times higher and the residence time for suspended matter in the water column is 1–2 days. Settling velocities of aggregates are on average six times higher than for individual particles resulting in an enhanced downward transport of organic matter. Wave-induced resuspension (four to six times per month) takes place with higher shear stresses on the bottom than current-induced resuspension (three to five times per month). The short residence time in the water column and the frequent resuspension events provide a fast operating benthic–pelagic coupling. Due to the high-energy input, the shallow water areas are nondepositional on time scales longer than 1–2 weeks. The sediment is sand partly covered by a thin fluff layer during low-energy periods. The presence of the fluff layer keeps the resuspension threshold very low ( −2 ) throughout the year. Evaluated from 3-D sediment transport modeling, transport from shallow to deep water is episodic. The net main directions are towards the Arkona Basin (5.5×10 5 t per year) and the Bornholm Basin (3.7×10 5 t per year). Energy input to the bottom in deep water is low and takes place much less frequently. Wave-induced resuspension occurs on average once per month. Residence time of particles (based on radioactive isotopes) in the water column is half a year and the sediment accumulation rate is 2.2 mm year −1 in the Arkona Basin.

Material transport from the near shore to the basinal environment in the southern Baltic Sea II: Synthesis of data on origin and properties of material

The Pomeranian Bight (southern Baltic Sea) is a mixing zone between waters of the Baltic Proper and the river Oder, which drains a densely populated and highly industrialised catchment of central Europe. The bight is a nondepositional area, and all material produced in its water column, from erosion of strata at the seafloor and cliffs, and delivered by rivers, is transported near the seafloor to the depositional areas of the Arkona, Bornholm and Gdansk basins. In this contribution, we assess the origin, transformation and mass fluxes of material through the bight based on an integrated field study conducted in the period 1996–1998. The transport mechanism is by wave- and current-induced resuspension and settling cycles, which effectively enrich organic-rich material and associated substances (organic pollutants, heavy metals) in deeper water; the estimated transport time is less than 6 months. The phases in which the material is transported are suspended matter in the water column, a particle- and aggregate-rich benthic boundary layer of 20 m water depth), the fluffy layer is not readily distinguished from the underlying soft, organic-rich sediment and the change in physical and chemical properties is gradual. The organic matter passing through the coastal zone in the southern Baltic is unaffected by biological or chemical modifications in composition. We find no evidence for a preferential removal of nitrogen or phosphorus, even if the speciation of phosphorus changes from biological compounds to minerals. The compositional changes which we see, i.e., in the nitrogen isotopic composition and in trace metal concentrations, are mainly caused by dilution of the river signal. In the case of

Regional geochemical baselines for sedimentary metals of the tropical São Francisco estuary, NE-Brazil

River catchments of the humid tropics deliver a large amount of the world’s particulate suspended matter and associated elements to the coastal zone (Hay, 1998; Meybeck, 1993). The estuaries and mangrove habitats set along this transport pathway transform, retain and accumulate a fraction of the river-borne particulates and metals in their sediments (Lee, 1995; Jennerjahn and Ittekkot, 2002; Machado and Lacerda, 2004). The metals are preferentially buried with fine-grained sediments and also affected by the mineralogy and content of organic matter (Hornberger et al., 1999). Many metal accumulation studies in coastal systems focused on the establishment of geochemical baselines for the evaluation of the degree of natural and/or anthropogenic metal loadings in the system. The tropical and sub-tropical coastal zone of Brazil has been subject to manifold metal contamination studies of its embayments, estuaries, coastal lagoons and mangrove habitats embedded in these systems affected by a varying degree of multiple human impacts, such as urbanization, industrialization, deforestation, land erosion and agricultural practices (Patchineelam et al., 1988; Lacerda et al., 1992, 1999, 2006; Marins et al., 2004). However, consistent geochemical baselines are still relatively scant and in practice may also only be established on a regional basis, as the grain size, mineralogical and chemical composition of suspended matter of Brazils tropical rivers varies greatly in accor-

Late Quaternary Climate Variations Reflected in Baltic Sea Sediments

Late Pleistocene to Holocene climate change of the Atlantic and the northern European realm is reflected by the facies of sediments in the Baltic Sea. The sedimentary sequence have been subdivided into zones reflecting the main postglacial stages of the Baltic Sea basin development according to sediment echosounder profiling and investigating sediment cores from the central Baltic. The changes in the environment of Baltic Sea bottom water is displayed by sediment physical, geochemical, and microfossil proxies. These proxies mark the main shift in the sedimentary facies of the Baltic Basin at 8.14 cal. years BP, from a freshwater to a brackish/marine environment due to the Littorina transgression of marine water masses from the North Sea. The downhole physical facies variation from the Eastern Gotland can be correlated basinwide. Thickness maps of the freshwater and the brackish sediments ascribe the general change in the hydrographic circulation from a coast-to-basin to a basin-to-basin system along with the Littorina transgression. Variations in the salinity of the brackish Littorina Baltic Basin are attributed to changes in the North Atlantic Oscillation (NAO) ascribing the wind forces driving the inflow of marine water into the Baltic Basin. Time series analysis of facies variation reveals distinct periodicities of 900 and 1,500 years. These periods can be compared with data from North Atlantic marine sediments and Greenland ice cores identifying global climate change effects in Baltic Basin sediments.

226Raexcess/Ba growth rates and U-Th-Ra-Ba systematic of Baltic Mn/Fe crusts

Abstract We analyzed 238U, 234U, 232Th, 230Th, and 226Ra by thermal ionization mass spectrometry (TIMS) and Ba by inductively coupled plasma optical emission spectrometry (ICP-OES) on eight Mn/Fe crusts from the Mecklenburg Bay (SW Baltic) and on one from the Bothnian Bay (N Baltic) to test the 226Raex/Ba ratio as potential geochronometer. 226Raex/Ba ratios decrease as a function of depth within the concretions in all analyzed profiles. Calculated diffusion coefficients are relatively low (∼9 · 10−7 cm2/yr for Ra and 5 · 10−7 cm2/yr for Ba) and suggest that diffusion is negligible for the Ra and Ba record. In addition, 226Raex/Ba ages are consistent and independent from the growth rate and growth direction within a crust. Thus, the decline in 226Raex/Ba ratio is most likely due to radioactive decay of 226Raex, although the influence of varying oxic conditions has still to be evaluated. 226Raex/Ba growth rates range from 0.021 to 0.0017 mm/yr and tend to be lower than those calculated and based on stratigraphic methods (1 to 0.013 mm/yr). 226Raex/Ba ages of concretions from shallow water environment (20 m depth, Mecklenburg Bay/SW Baltic) cover a time interval from 990 ± 140 yr to 4310 ± 310 yr BP corresponding to the stabilization of the sea level close to the present position about 5500 to 4500 yr ago. One sample from greater depth (70 m, Bothnian Bay-/N Baltic) showed a higher 226Raex/Ba age of 6460 ± 520 yr BP.

Linkage between acoustic parameters and seabed sediment properties in the south-western Baltic Sea

Acoustic profiling methods are widely used to provide a rapid view into geological structures. For the interpretation of acoustic profiling results (single- and multi-beam), reliable geo-acoustic models are needed. Suitable geo-acoustic models covering a wide range of sediment types do not exist to date for the Baltic Sea. Based on surface sediment datasets, geo-acoustic models have been set up for the prediction of acoustical parameters derived from sedimentological data for south-western Baltic Sea surface sediments. Empirical relationships were created to predict key in situ parameters (p-wave velocity, wet bulk density) from sedimentological core data, notably grain density and water content. The Gassmann-Hamilton equations were used to set up a more generic physically based model. For the first time semi-empirical equations for the calculation of the elastic frame modulus and the solid sediment particle modulus were established by an iterative Gassmann-Hamilton fitting procedure. The resulting models have a remarkably good performance with, for example, a calculated sound velocity accuracy of about 17–32 m s–1 depending on model input data. The acoustic impedance of seafloor sediments can be estimated from single-beam echosounding if the contribution of seafloor reflectivity is extracted from the total acoustic signal. The data reveal a strong linkage between acoustic impedance and selected sediment properties (e.g. grain size, water content). This underlines the potential for effective mapping of seafloor sediment properties (e.g. habitat mapping). Furthermore, these geo-acoustic models can be used by marine geologists for a precise linkage between sediment facies identified in longer cores and corresponding acoustic facies recorded by high-resolution seismic profiling in future work.

BIOGEOCHEMICAL AND PHYSICAL CONTROL ON SHELF ANOXIA AND WATER COLUMN HYDROGEN SULPHIDE IN THE BENGUEL A COASTAL UPWELLING SYSTEM OFF NAMIBIA

Shelf anoxia and recurring sulphidic water column conditions are characteristic features of the coastal upwelling system off Namibia. The development of oxygen-depleted water column conditions is linked to the relative dominance of South Atlantic Central Water, which flows southward from the Angolan Dome over Eastern South Atlantic Central Water. Interand intra-annual variations in the strength of upwelling influence the thickness and stability of the relatively stagnant boundary layer. Hydrogen sulphide accumulation in this boundary layer is mainly driven by the diffusive flux of hydrogen sulphide from the sediment. The hydrogen sulphide derives from the rapid degradation of organic material by bacterial sulphate reduction in the topmost 20 cm of sediment. Low reactive iron contents in the diatomaceous mud belt limit iron sulphide precipitation and sulphide oxidation by oxidized iron. In the absence of oxygen, iron, and manganese as important electron acceptors, sulphide oxidation proceeds largely by the reduction of nitrate by the large sulphur bacteria Beggiatoa and Thiomargarita, which cover large areas of the shelf. Regional differences in the distribution of these bacteria affect the development of sulphidic bottom waters. While hydrogen sulphide is quantitatively oxidized in sediments covered by Beggiatoa mats, only a fraction of the sulphide is removed by Thiomargarita. Areal estimations of aerobic water column respiration, diffusive fluxes of hydrogen sulphide from the sediment, and rates of bacterial sulphate reduction indicate that oxidation of sulphide at the sediment-water interface and oxidation of water column sulphide may comprise up to 25 % of the total oxygen consumption in the coastal

Storm Disturbance of Sediment Contaminants at a Hot-Spot in the Baltic Sea Assessed by 234Th Radionuclide Tracer Profiles

Fly ash sludges from an abandoned metal smelter were dumped into the shallow inner part of the Mecklenburg Bay until 1971, representing the most severe heavy metal contamination hot-spot along the German coast. Half of the dumped Zn (455 t) and Pb (173 t) inventory was found to be spread from the originally 0.5 km2 hot-spot site to a now 360 km2 affected adjacent area. Wave-driven resuspension during gale events produced large pulses of contaminated sediments from this hot-spot due to the only 23 m water depth. Instantaneous sediment mixing down to 10 cm occurs during such a wave event as evidenced by activity profiles of the short-lived radionuclide 234Th in sediment cores. According to these estimated sediment exchange fluxes in the transport bottom area, each wave event may have mobilized Zn and Pb pulses on the order of several hundreds of kilograms from the dump site, sufficient to build up a plume in sediments of the outer bay area. With each centimeter (approximately 5 yr) of additional natural sediment capping, however, the amount of metal remobilization would decrease by about 50%.