Himmerfjärden Bay

Radiographic analysis of sediment cores.

The coastal bay Himmerfjärden is situated in the southern part of the waterway connecting the Lake Mälaren region with the Baltic Sea. This bay has received effluent water since 1974 from a sewage treatment plant located at the northern shore of the bay. The core-to-core correlation in this bay is based on radiographs of 10 sediment cores, cores 634 - 636 sampled on 6 August 1980, cores 1111 - 1114 sampled on 26 June 1985, and cores 1205 - 1207 sampled on 15 November 1986. The coring sites were situated at water depths between 25 and 49 m. For location see the map further down. All of these cores were annually laminated in the uppermost part, as exemplified by the radiographs below.

Radiographic comparison between the upper part of sediment cores 636, 1205, and 1206 from a depth of 26, 26, and 30 m respectively in the northern part of Himmerfjärden. (Location of sediment surface during X-raying).

Cores 1205 and 1206 were sampled six years later than core 636. The oxygen deficit resulted here in a change from bioturbated to annually laminated sediment structures in 1978 at coring places 636 and 1205 and one year earlier (in 1977) at the somewhat deeper coring place 1206.

Downcore variation in dry bulk density in the upper part of sediment core 1206, calculated from the recorded film density along the centerline of the radiograph.

Annual mass sedimentation rates at coring site 1206. Mean annual value for this 10-year period = 286 mg/cm². This value of the annual mean mass sedimentation rate (salt-free) corresponded to an annual mean linear sedimentation rate of 17 mm at the prevailing overburden pressure in 1986.

Core 1206 was gasrich, as shown by the radiograph above, and expanded about 2 cm in length after sampling due to the formation and expansion of gas bubbles, which resulted from the reduced hydrostatic pressure and temperature rise. In this core the mean values of the wet and dry bulk density and the loss on ignition amounted to 1.09, 0.14 (salt-free) and 9.6 % respectively for the upper 0.1 m of sediment. The salinity, the density , and the pH of the bottom water in the coring tube was 6 ‰, 1.005 and 7.3 respectively.

Radiograph of the upper part of sediment core 1114 from a depth of 30 m in the northern part of Himmerfjärden. The curve, showing the downcore variation in dry bulk density, is based on film-density values, recorded along the centerline of the radiograph. As shown above the difference in density between the bottom water in the transparent coring tube and the uppermost sediment layer was very insignificant

The uppermost sediment layer was extremely loose in the 4 (in the upper part annually laminated) sediment cores, sampled on 26 June 1985. The thickness of the uppermost fluid layer amounted to about 9 cm in core 1111 (sampled from a depth of 49 m), to about 7.5 cm in core 1112 (from a depth of 47 m), to about 4 cm in core 1113 (from a depth of 31 m), and to about 5 cm in core 1114 (from a depth of 30 m). In these fluid sediment layers the mean value of the water content amounted to about 99 % by weight, the loss on ignition to about 20 % by weight, and the silt content (salt-free) to about 8 g/l. This means that the mean value of solids in these loose, fluid parts of future, not yet compacted annual layers amounted to only about 3 per mil of the total sediment volume.

Downcore variation in water percentage by volume (inclusive gas) in the upper part of sediment core 1114.

As shown above the amount of solids was very low also in parts of the fully developed annual varves in the upper part of core 1114. In general the hardest parts of the annual varves was formed in connection with the autumn storms.

Coring sites in Himmerfjärden.

File in Swedish.

Back

Back to my front page.