Bråviken

This about 40 km long bay of the Baltic Sea is situated outside the town of Norrköping along the south-east coast of Sweden. On 1 March 1982 five sediment cores (nos. 736-740) were sampled through the ice in the inner, north-western part of this bay. Two years later (23 February 1984) five more cores (nos. 980-984) were sampled through the ice in the same area. As exemplified below, it was possible to correlate seasonal layers between those cores which had been sampled at water depths exceeding 23 m.

Radiographic comparison between the upper part of cores 740, 981, and 982 from the inner , north-western part of Bråviken (northwest of Esterön).

As shown above, two new varves were formed between 1982 and 1984. This suggests that the varves below also represent annual signals. Coring sites 740 and 981 were situated rather close to each other at water depths of 29 and 26 m respectively. The distance between coring sites 981 and 982 amounted to about 200 m, but the difference in water depth was only 0.2 m. As shown, core-to-core correlations, based on X-ray radiographs, improve the possibilities of calculating spatial and temporal variations in sedimentation rate.

Variation in content of solids and in annual sedimentation rates in the upper part of core 982 from Bråviken.

The part of the varves which had the lowest density was formed in winter and was in this case taken as the boundary between the given sedimentological years. This means that the period for formation of the given annual varves may be somewhat longer or shorter than 365 days.

On 28 May 1979 sediment cores 521 and 522 were sampled at depths during coring of 2 m and 1 m respectively in Svensksundsviken, a small, shallow bay in the southern part of the larger bay Bråviken. As exemplified below erosion dominated over deposition at these coring sites.

Radiograph of core 522 from Svensksundsviken, a small and very shallow bay in the southern part of Bråviken.

At this coring site erosion dominated over deposition, and as a result consolidated, varved clay has been exposed. Modern bioturbation structures now characterize the uppermost part of these old deposits. The film-density curve also illustrates the vertical variation in bulk density.

File in Swedish.

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