Helmar Kunzendorf

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.

Holocene history of the Baltic Sea as a background for assessing records of human impact in the sediments of the Gotland Basin

Sediment cores from the Gotland Basin were studied for their siliceous microfossil assemblages and organic carbon content to compare recent environmental changes in the Baltic Sea with its natural long-term history. Age models were constructed using 210Pb, 137Cs and corrected and calibrated 14C dates. The transgression that marks the onset of the Ancylus Lake stage is recorded in the sediments as a small increase in organic carbon coinciding with a peak in diatom abundance and increased diatom diversity. A minor occurrence of brackish-freshwater diatoms is recorded in the Ancylus Lake c. 9950–9750 cal. yr BP (c. 8900–8800 14C yr BP), correlating with the onset of the Initial Litorina Sea in the Bornholm Basin. A high-productivity event is recorded in the end of the Post-Litorina Sea and corresponds to the Mediaeval warm event. An alteration in the diatom assemblage contemporaneous with a decrease in organic carbon, interpreted as representing a deterioration in the climate, correlates with the start of the ‘Little Ice Age’ about 850–700 cal. yr BP. A change dated to ad 1950–1960 is probably an effect of increased nutrient availability in the open Baltic Sea. This effect of eutrophication was probably caused by increased discharge of nutrients deriving from fertilizers, as the responding diatom species partly indicate a cold climate rather than a warm one, as would have been expected if this had been only a response to the warmer climate documented during the last 100 years or so.

Long term variations in backbarrier salt marsh deposition on the Skallingen peninsula - the Danish Wadden Sea

Abstract Accretion on a natural backbarrier salt marsh was determined and modeled as a function of high tide level, initial salt marsh level and distance from the salt marsh edge. Accretion measurements were based on up to 67-year-old marker horizons, supplemented by 210Pb/137Cs datings. The salt marsh is situated on the backbarrier of the Skallingen peninsula in the northern part of the Danish Wadden Sea. The tidal range (mean 1.5 m) is strongly affected by wind tide which occasionally adds up to about 3 m to the astronomical high tide level. Accretion is restricted to a narrow vertical band from about 0.1 m below to about 0.7 m above the mean high water level. In the outer part of the backbarrier (close to the tidal flat) mean accretion is about 4 mm yr−1 and in the inner part it is about 2 mm yr−1. The decrease takes place in a similar manner as the decrease across a flood plain. The major part of salt marsh accretion is associated with high water levels corresponding to weather conditions characterized by gales. The long term variation of salt marsh accretion correlates with variations in the North Atlantic Oscillation winter index. The number of over-marsh high tides decreases exponentially with high tide level. The function constants controlling this distribution vary in direct ratio to the mean sea level. Plausible near future scenarios of tidal development were obtained by extrapolating this relation. Three sea level scenarios were tested: (i) sea level rise continues at a constant (long term) rate of 2.3 mm yr−1, (ii) sea level rise continues at a constant (short term) rate of 4.2 mm yr−1 and (iii) sea level rise accelerates to a constant rate of 6.4 mm yr−1. In the first case accretion on the salt marsh will keep pace with the high water level rise. In the second case deposition in the inner part of the salt marsh will lag behind while that of the outer part of the salt marsh will keep pace with the rising high water level. In the third case, corresponding to a ‘worst case’ scenario for the 21th century, the salt marsh will gradually drown.

Stable isotopes in recent sediments of Lake Arendsee, NE Germany: Response to eutrophication and remediation measures

Abstract Recent sediments of Lake Arendsee, a small eutrophic lake in NE Germany, were examined in high temporal resolution in order to investigate the applicability of stable isotopes as proxies to trace paleoenvironmental conditions. The age model of the sediment core is based on varve counting and 137Cs/210Pb dating and reveals that the sedimentary record covers the last 70 years, during which the lake has undergone major changes in productivity and eutrophication in response to nutrient loading. Since 1965, TOC, TIC, TN and TP accumulation rates in general reflect the change from oligo-/mesotrophic to eutrophic level. The isotopic compositions of autochthonous organic matter and authigenic calcite (whitings) demonstrate that carbon isotope fractionation takes place in a CO2-limited pool. In accordance with the historical limnological observations, enhanced eutrophication is reflected by δ13CTOC increases of over 3‰ from 1934 to 1985 as a result of less 12C/13C fractionation by photosynthesis during intense primary productivity. Decreasing δ13CTIC values indicate that biogenically induced calcite formation occurred during enhanced primary production accompanied by CO2 limitation. The gradual trend to more light nitrogen isotopic signatures towards the sediment surface reflects the dominance of air nitrogen fixation of cyanobacteria blooms during the last decades. The δ18OTIC distribution suggests that plankton blooms between 1934 and 1978 had shifted towards the early season with lower temperatures as a result of increasing nutrient level, which subsequently favor plankton blooms even in colder waters. Since the mid 1980s, decreasing δ13CTOC and increasing δ13CTIC values since 1985 indicate a more efficient 12C/13C fractionation during photosynthesis with no CO2 limitation. This improvement in water quality is in accordance with several remediation measures carried out since the late 1970s. Linear isotopic trends are interrupted by high TIC, TOC, TN and TP accumulation rates in the early 1970s, which can be clearly attributed to the artificial drainage of a nearby wetland carried out during that time. This event can be considered an important external source for nutrient-loaded water and particulate matter and is displayed by significantly heavier δ13CTIC and δ18OTIC values in the sedimentary record.

Sedmentation in the Central Baltic Sea as Viewed by Non-Destructive Pb-210-dating

Abstract Geografisk Tidsskrift, Danish Journal of Geography 98:1–9. A non-destructive gamma-spectrometric method was used to assay the sedimentation in the central Baltic Sea. Three cores from the Gotland Deep with water depths exceeding 230 m showed relatively high Pb-210 and Cs-137 but relatively little variation between cores in the curves for unsupported Pb-210. A constant rate of supply (CRS) modelling of the data revealed that there is a tendency of increasing linear sedimentation rates, from 0.5 to 2 mm a−1, since 1950. With the measured bulk density profiles this is accompanied by increasing sediment accumulation rates (from about 200 to 600 g m−2 a−1) There are generally moderately increased Cs-137 values found in the deep and these may mainly be ascribed to the Chernobyl accident. A core from the central Gdansk Basin taken at a water depth of 117 m shows relatively lower Pb-210 values and a more structured unsupported Pb-210 distribution with depth. A very high activity (about 4000 Bq kg−1) sedi...

Effects of morphological changes on metal accumulation in a salt marsh sediment of the Skallingen peninsula, Denmark

In 1931 a red-colored, sandy marker horizon was placed on the emergingSkallingen salt marsh. Sedimentation on top of the marker horizon sincethen shows two opposing tendencies. Coincident with salt marshdevelopment the sediments display up to 1964 a fining upward sequencewith an increasing content of organic matter. Since 1964 a nearby creekhas meandered towards the sampling plot. Consequently, the sedimentsbecome coarser with a decreasing organic matter content. Themorphological induced changes in sedimentary conditions strongly influencemetal content in the sediments and thereby hide anthropogenic inducedconcentration variations. Thus, an apparently diminishing Zn content (perkg dry weight) since 1964 could indicate lesser load to the area. However,corrected for grain size effects there is an increasing content of Zn. Othermetal concentrations (e.g. Cu) show a diminishing trend when corrected forgrain size effects and therefore indicate a reduced anthropogenic inducedload of these metals to the salt marsh.

Pollen deposition rates in peat and lake sediments from the Pinus sylvestris L. forest-line ecotone of northern Norway

A comparative investigation of pollen deposition in peat and limnic sediments from the Pinus sylvestris-Alnus incana forest line in Dividalen, central Troms, northern Norway was performed by pollen analyses of sub-modern sediments. The possibilities of using 210Pb and 137Cs-datings in calculations of pollen influx in peat and limnic sediments are discussed. Correlation between the lead–cesium peat chronology and a marker-based limnic chronology is achieved at AD 1945–1946 by comparing the percentual pollen-curves. The upper 14 cm of the peat and 12 cm of the limnic sequence are hence supposed to represent the period ca. 1945–ca. 1996. The mean percentual representation of Pinus pollen is nearly twice as high in the peat sediments as in the limnic sediments, probably due to a slight over-representation from pine trees at the edge of the mire. The mean percentual pollen values for Alnus and Betula pubescens-type is fairly equal in peat and lake sediments, while the pollen influx rates are higher for all tree taxa in the lake sediments. This may be caused by the lake being exposed to a larger regional component than the smaller and topographically more sheltered mire. To achieve reliable pollen influx data at a near annual resolution, one needs a good control of the sedimentation processes, close intervals of isotope measurements and additional chronostratigraphically fixed marker horizons.