Radiographic analysis of sediment cores from Örserumsviken
This shallow and only about 1.3 km long bay is located just southeast of the town of Västervik in southeastern Sweden. In 1999 eleven sediment cores were sampled in and nearby this bay and X-rayed, the uppermost part in stereo, in order to document the sedimentary sequence in the upper part of the bottom deposits. The qualitative information obtained from the radiographic images was quantified by the use of X-ray densitometric methods.
The uppermost, bioturbated sediment layer with adjoining bottom water in the 6 cm wide coring tube with the sediment core O1 from the coastal bay Örserumsviken.
In the larger part of this shallow bay the bottom was in 1999 overgrown by a knee-high underwater-meadow of Characeae.
Locations of the eleven X-rayed sediment cores (O1 -O11), sampled in and nearby the bay Örserumsviken.
Radiograph of an area with a substantial unconformity in sediment core O1 from the innermost part of Örserumsviken. Water depth during coring: 1.52 m. Date of coring: 1999-08-24.
This unconformity (see also the curve below), situated at a core depth of about 18 cm, was probably formed during the dredging operations in 1978/79. The accumulated amount of solids above this unconformity amounted in August 1999 to 1.0 g/cm2. This means that the mean annual rate of sedimentation at this coring station probably amounted to 0.5 kg of solids/m2 during the 20-year period 1979-1999, corresponding to a mean linear rate of sedimentation of 9 mm/year. A similar type of unconformity (time gap) in the sedimentary sequence was also found in the upper part of the other cores from the inner part of the bay. In core O2 the unconformity was situated at a core depth of about 20 cm.
Downcore variation in content of solids in sediment core O1.
As shown by this diagram the content of solids was very low above the unconformity in the sedimentary sequence. This was also the case in the other cores from the inner part of the bay. In the four innermost cores (O1 - O4) the mean value of the content of solids amounted to only about 40 g/litre of sediment for the core depth 0 - 10 cm.
Radiographs of the upper, middle and lower part of sediment core O4. The lead letters X and B are used for marking corresponding levels on the partly overlapping radiographs. Water depth during coring = 1.8 m.
In this core the coarser organic matter consisted of among others bark flakes and decayed Characeae.
Stereoradiographs of the uppermost part of sediment core O9 with adjoining bottom water from the outer part of Örserumsviken. Water depth during coring = 4.5 m. These stereoradiographs are shown in a reduced scale. This makes it possible to study the sedimentary structures in three dimensions without the help of a stereoscope. The light dots mark the presence of coarser minerogenic particles (sand and gravel) as well as erosion products in the form of irregular aggregates.
When studying the stereoradiographs, You should start with your nose close to the monitor, then pull your face slowly away from it, and look through the image.
Click here to get a better resolution of the stereoradiographs.
Hardness index (the accumulated amount of solids down to a sediment depth of 10 cm) of sediment cores O1 - O11. For location, see the map above.
The harness index of the four sediment cores O1 - O4 from the inner part of Örserumsviken is very low: 0.460, 0.393, 0.374, and 0.418 for cores 01, 02, 03, and 04 respectively. These values correspond to an average dry and wet bulk density as low as about 0.04 and 1.04 respectively, and to an average water content and porosity as high as about 96 % and 98 % respectively.
The modern bottom deposits in this bay, as in many other bays along the coast of Sweden, are polluted. However, due to the very low content of solids in the uppermost, polluted layers, as illustrated by the hardness-index diagram above, the content of pollutants expressed per volume of sediment in these layers is comparatively low in relation to the content expressed as per weight of solids.
One of the most important ways, both from environmental and economical points of view, to improve the bottom conditions in polluted bays might (besides the cleaning of waste water) often be to create a new active layer on top of the polluted deposits. This is especially the case for several lakes and coastal bays in Sweden which are covered by ice in winter and where there is plenty of clean glaciofluvial sand in the neighbourhood that can be transported to and deposited as a thin layer on the ice above the polluted bottom areas. During break-up of the ice this layer will settle, partly cover, and partly compact the polluted layers below. This procedure should be repeeted until the new active layer is thick enough. With active layer is here meant the part of the top layer that takes part in the exchange processes between sediment, bottom water, and biota. Except for natural recovery, this type of capping by natural, granular material, such as sand, is the least expensive way of remediation.
A larger file in Swedish.
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