Radiographic analysis of sediment cores from Gamlebyviken

The core-to-core correlation in this bay is based on radiographs of nine sediment cores (cores 1133-1141). The coring sites were situated at water depths between 11 and 59 m Two of the (35) radiographs of these cores are shown below.

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Radiographic comparison between the upper, gas-rich part of cores 1133 and 1135, sampled at water depths of 49 and 46 m respectively, on the same day, 1985-07-20, and at a distance of about 500 m from each other in Gamlebyviken. The dark, more or less round or elliptical spots mark the presence of gas in bubble-phase.

In core 1135 the border between the upper, mainly annually laminated and the lower, mainly bioturbated sediments was situated at a core depth of 29 cm, probably corresponding to the year 1953. For the period 1953 - 1985, the calculated annual mean mass sedimentation rate (salt-free) amounted to 1.5 kg/m² and the annual mean linear sedimentation rate to 9 mm at coring site 1135. However, it must be stressed that given linear sedimentation rates for modern deposits normally will decrease with time and especially with increasing overburden pressure due to gravitational compaction. Some of the couplets are poorly developed. Therefore, the given chronology and the given sedimentation rates are somewhat uncertain.

Most of the sampled sediment cores expanded after sampling due to the formation and expansion of gas bubbles, which resulted from the reduced hydrostatic pressure and temperature rise. Core 1134 (sampled at a depth of 59 m) expanded 3.8 cm (increase in core length from 66.5 to 70.3 cm = 6 %).

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