Hallsfjärden Bay

This bay of the Swedish Baltic Coast, being situated outside the town of Södertälje, is also part of an important internal waterway connecting the Lake Mälaren region with the Baltic Sea. A great number of sediment cores (87) were sampled in this bay during the period 1978 - 1989 and X-rayed, the uppermost part in stereo. The majority of the cores (78) were sampled in the northern part of the bay (north of Fläsklösa). A few of the radiographs are shown below. The given chronology has been checked against marker layers, formed in connection with the dumping of dredged material. The oldest of the identified marker layers was mainly formed in 1923.

Upper part of core 510 from Hallsfjärden. The film density is recorded along the centerline of the radiograph. The arrows mark the position of the sediment surface in October 1978 and in May 1979, that is just before and after a period of dredging and dumping operations.

Transmission densitometers are used for film-density measurements along the centerline of the radiation images, as examplified above. The film-density values are then used to calculate the downcore variations in sediment density and in other, density-related values. Results of the X-ray densitometric calculations are also examplified by the curves below, showing the variations in dry bulk density in the upper parts of sediment cores 512 and 979.

Radiograph of the upper part of core 512 from a depth of 17 m in Hallsfjärden.

The hard (light) layer at a sediment depth of 11 cm reflects dredging operations in 1965. The vertical variation in dry bulk density is calculated from the recorded film density along the centerline of the radiograph.

Radiograph of the upper part of core 979 from a depth of 30 m in Hallsfjärden and radiographically calculated variations in dry bulk density and in content of solids in the annual varves. Mean rate of sedimentation 1966 - 1984 = 1.68 kg of solids/m².

In this core the loosest part of the varves (in this bay generally formed in summer) is chosen as the border between the sedimentological years (the annual varves). In the deeper part of this bay, as in most bays along the coast of Sweden, the rate of sediment accumulation is generally highest during and just after the autumn storms. Erosion and resuspension, especially of newly deposited sediments, may be rather considerable in connection with heavy storms. This reworking of the sediment results in an accumulation of greater amounts of sediment in the deeper parts of the bays. Sediment focusing, as a result of erosion and resuspension, is therefore of great importance for the spatial variation in sedimentation rate.

Radiographic comparison between the uppermost part of sediment cores 510, 514, and 513. The arrows mark the positions of the sediment surface on 18 October 1978 and 15 May 1979, i.e. just before and after a period of dredging and dumping operations. From Axelsson, V., 2002: Monitoring sedimentation by radiographic core-to-core correlation, Geo-Marine Letters 21/4.

Parts of a marker layer formed during the winter of 1978 - 1979, when dredged material was transported to and dumped in the deepest part of Hallsfjärden, are shown by the radiographs above. A considerable part of the dredged material was suspended during the dumping and later redeposited from a turbid underflow which spread out from the disposal area and covered the deeper parts of the bay. The thickness of the redeposited dredged material decreased with decreasing water depth and with distance from the disposal area. Coring sites 510, 514, and 513 were situated at depths of 20, 23, and 22 m, and about 150 m west of, 800 north of, and 400 m south of the disposal area respectively.

A larger file in Swedish.

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