GEOLOGICAL SETTINGS

The Bornholm Basin with water depth of about 100
m is located east of the island of Bornholm, in the
south–western Baltic Sea. It has its boundaries at
50030’–55045’N and 14030’–16030’E. The basin is
assumed to be bordered by 50 m isobaths (Fig.1) thus
covering the area of 14 000 sq. km. It is surrounded
by shallow bottom areas with depth ranging between
25 and 30 m. Saline and dense waters occasionally
penetrating to the Baltic from the North Sea, find
their way from the Danish passages first to the Arkona
Basin with depth of about 46 metres, then entering the
Bornholm Basin through the Bornholm Gat channel.
The Słupsk Trench is another considerably deep basin
to the east of the Bornholm Basin, separated from the
latter by a moraine ridge, thus forming Słupsk Sill with
its ridge coming at the depth of 56 m.

The Bornholm Basin has smooth slopes what only
go steeper in the west. Here on a small plateau Christiansö
Island is found. This island is separated from the
Bornholm Island by a valley with depth ranging between
60 and 70 m. At the flat bottom of the Bornholm
Basin main types of sediments are terrigenous aleuro–
pelitic and pelitic mud containing respectively 50-70
and 70-79% of the < 0.01 mm fraction (Emelyanov et
al. 1995a). Redox potential Ept is normally negative
in the mud and reaches –135 mV (Eh=+65 mV).
During the last 40 years no less than five times
near-bottom waters in the Bornholm deep were stagnated:
1969, 1971–1972, 1976, 1982 and 1989 (Shpaer,
Larionov 1982; Stryuk et al. 1995). According to
Emelyanov et al. (1995b) and Emelyanov (1996) the
interface of O2-H2S in the water strata lay at the depth
about 80 m.

Recent sediments (0-5 cm) found in the Bornholm
Basin are terrigenous, including also pelitic mud. Mud
contains elevated (3-6%) Corg (up to 7.84% max.). According
to our previously accepted classification this
is either sapropel (5-10% Corg) or sapropel–like (3-5%
Corg) mud.
Among clastic minerals in sediments quartz is prevailing,
normally accounting for 80-90% of the 0.1-0.05
mm light sub–fraction. The amount of feldspar is up
to 2%, mica up to 5-–6%; in some samples glauconite
can also occur. Clastic part of the heavy sub–fraction
of 0.1-0.05 mm (specific weight >2.9) is comprised
of mica, hornblende, epidote–clinozoisite, ilmenite–
hematite and garnet. Terrigenous minerals (quartz in
light sub–fraction, ilmenite and magnetite in the heavy
one) are more frequently found in the marginal parts
of the basin, whereas mica and glauconite are more
common in the central area of the basin (Trimonis et
al. 1995). Illite is the prevailing clay mineral, followed
by kaolinite, montmorillonite and chlorite.

Plant remains are plentiful in the upper sediment
layer. These are darnel of cereals, grains of pollen and
spores, and diatom skeletons. Also sediments reveal
some whole shells of Macoma baltica mollusc and
their fragments.

Authigenous minerals found in the mud are mainly
iron sulphides, phosphates, including some phosphates
that build on fish bones. Though rather unexpected,
limonite and hydrogoethite are also frequently found in
the reduced sediments. Also some brown orbicular and
black irregular–shaped iron sulphide micro–nodules
occur in bottom sediments.