Geoscience Reference
In-Depth Information
(Tamsalu and Myhrberg 1995 ) . Salinity stratification is normally strong in the gulf,
with a halocline at about 60 m.
The bottom of the Gulf of Finland is made up of different kinds of sediments. The
basins of modern accumulation differ in shape and size depending on which area is
observed. They are separated from each other by submarine shallows made up of
coarser material, such as gravel, sand, and especially glacial till which can be seen
as different moraine formations. Sometimes, the bedrock penetrates the sediment as
outcrops on different water depths.
It has been assumed by Kankaanpää et al. ( 1997 ) that sedimentation basins
with active sedimentation would cover about 1/4 of the total area of the Gulf of
Finland. Experience from echo soundings in the Gulf of Finland has revealed that
the assumption is probably slightly overestimated, but it is the best guess in the
absence of full coverage data. Thus, we can assume that the areal coverage of soft
Holocene mud, which today is actively incorporated in binding/release of matter
to/from the water phase, is about 25%. That is the area of all sediments which react
very fast to changes in physico-chemical conditions, some 7,000 km 2 according to
the estimate by Kankaanpää et al. ( 1997 ) .
Especially in the eastern Gulf of Finland, ferromanganese concretions are rather
common (Fig. 17.2 ) . They occur on moderate depths where there is a gentle slope
of clay usually ending in a basin of modern gyttja-clay accumulation. Usually the
abundant concretion areas are located on the edge of the accumulation basin so that
the concretions are partly covered with gyttja-clay. The conditions during growth
of the concretions have to be permanently oxic. As soon as conditions change to
anoxic either because of changes in the surrounding area or because of burial of the
concretions into the sediment, they start to dissolve (Zhamoida et al. 2007 ) .
One characteristic feature of the Baltic Sea as well as the Gulf of Finland is
that through its entire marine postglacial history it has been occasionally anoxic
(hypoxic) (Kotilainen et al. 2000 , Winterhalter 2001 , Zillén et al. 2008 ) . This cyclic
Fig. 17.2 Spheroidal
concretions from the eastern
Gulf of Finland. Diameter of
sieve is 25 cm (photo
H. Vallius)
 
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