Geoscience Reference
In-Depth Information
Fig. 17.3
Repeated oxic - anoxic conditions can clearly be seen in vertical sediment profiles, here
core MGGN-2010-28 from the central Gulf of Finland (Photo H. Vallius/GTK)
feature is partly an indication of occasional inflows of saline water from the North
Sea. Normally the hypoxic conditions between the inflows are present for some
years or perhaps a decade at bottoms below the halocline. Anoxia starts after a
period of stagnation, when no new saltwater inflows from the Danish Sounds have
occurred for a while. The anoxia finally ends when new oxygen-rich saltwater
inflows ventilate the bottom waters of the Baltic Sea, which later are pushed into the
Gulf of Finland. This kind of cyclic changes can be considered normal in the Baltic
Sea and are normally seen as alternating laminated and homogeneous sequences in
into two different kinds of anoxia, the more or less permanent anoxia below the
halocline and the seasonal, short-term, anoxia/hypoxia which occurs more or less
yearly. Both these will be explained later.
17.3 Materials and Methods
This study is based on the data of a multitude of cruises of Finnish and Russian
research vessels in the Gulf of Finland arranged by the Geological Survey of Finland
(GTK) and the A.P. Karpinsky Russian Research Geological Institute (VSEGEI)
during the last decades. The seafloor has been surveyed using different techniques,
such as echo sounding, side scan sonar, shallow seismics, and still photography, as
well as observation with ROV cameras, not to mention surface sampling with differ-
ent kinds of samplers. The amount of survey line kilometres, which exceeds many
thousands, has been complemented with thousands of surface samples. During each
survey, the seafloor has first been mapped by echo sounding after which it has been
inspected by different ground truthing methods, including surface sampling. Thus,
there is at present a rather good picture of the Holocene sedimentary environment
of the Gulf of Finland.
17.4 Present Situation
The Gulf of Finland is unfortunately very sensitive to changes, as the water vol-
ume is small because of the shallow water depth of the gulf. Because of the rather
strong freshwater inflow from east and inflow of saltier surface water from west, the
water column is usually strongly stratified, which connives seafloor anoxia. Rather
