Gravity corer with rectangular, transparent coring tubes

The used gravity corer ready for sampling, and equipped with a core catcher of type Niemistö.

Most of the X-rayed cores were sampled by the gravity corer shown above, and in about 0.7 or 1.0 m long coring tubes. Normally, this gravity corer is equipped with rectangular coring tubes made of transparent acrylic glass, with an inside section of 30 mm x 60 mm, and a wall thickness of 4 mm. The brass cutter at the lower end of the tubes is sharpened on the outside. The lower end of the tubes may be closed by incerting a rectangular plate through a slit in the brass cutter. The tubes are surmounted by a hollow, circular disk of acrylic glass, which fits into the coring head. High compacting pressures may arise within the tubes of gravity corers if the flow-through of water is hindered during the coring. Therefore, the corer head is fitted with a valve system, that allows an unrestricted flow of water through the coring tube both during descent and during penetration into the sediment. On lowering, when the lowering wire is taut, the valve is held in open position one by means of two inner hooks attached to the flange of the valve haft, and by the weight of the sampler. Upon contact with the bottom, when the wire slackens, the position of the valve changes to open position two. It remains in this position as long as the wire is slack and the corer penetrates the sediment, by means of two outer hooks attached to the top disc on the valve haft, and by the weight of the valve. The moment the wire tightens and the withdrawal of the corer starts, the hooks are automatically loosened, whereby the valve opening is closed.

The closing mechanism in position 1, 2, and 3. From Axelsson and Håkanson 1978 (J. Sed. Petrol. 48).

Two or four smaller or larger lead weights, weighing 2.5 or 5 kg, may be added to a movable brass frame mounted externally on the coring tube before sampling. Thus the total weight of the sampler can be increased in 5 kg steps. A core catcher of type Niemistö may also be added to the brass frame before sampling. When sampling in shallow water the coring tubes may be pressed down by hand with the help of rods and an adapter fastened to the corer head.

After sampling the coring tubes with bottom water on top of the sediment cores are transported, stored, and X-rayed in an upright position. If possible they should be stored in a cold-storage. Sometimes colour photos are taken before the cores are extruded from the coring tubes.

Part of a transparent coring tube with an inside section of 30 mm x 60 mm and showing the result of sampling.

It is important not to disturb the sediment surface during sampling, and the sediment cores should be taken with attention being paid to minimizing any compaction.

The uppermost, bioturbated sediment layer with adjoining bottom water in the 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.

Part of a coring tube with bottom water on top of core 1289 from a depth of 39 m in Byfjorden.

The gray layers in this core are due to dredging operations. The thickest of these layers was formed in 1974. Compare with the radiographs of core 1290 from the same area.


 

X-ray photo to the left and colour photo to the right of the uppermost part of core 717 from a depth of 19 m in Lake Stor-Laisan.

The hard, rust-coloured layers at a core depth of 3-4 cm also contained some ash and coarse organic detritus. These layers were probably formed in the early 1960´s, when construction works took place further upstream for hydropower development. Date of coring: 1981-08-19.

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