Abstract: This paper presents the high-resolution data on sulfur compounds distribution and their isotopic composition for a gravity core from the western part of the Black Sea. Pyritazation processes in the uppper part of lake beds is considered in detail and potential sources of hydrogen sulfide are discussed. The pyrite sulfur isotope composition is used as an indicator of paleoconditions in the basin during Late-Quaternary time.Download introduction and summary of my diploma thesis (PDF)
The Black Sea is the largest anoxic marine system.
There are two sources for supply of water to the waterbody of the Black
Sea. On the one hand seawater incurses into the Black Sea from Mediterranean
via the street of Bosporus with an inflow of 200km3 per year.
On the other hand there is an inflow of freshwater from the surrounding
areas, especially central and middle-eastern europe with an brutto inflow
of 320km3 per year. The most important river is the danube.
Because of the different salinities and weights, the freshwater and seawater
mixing is limited to the uppermost 100-150m. In this depth an pycnocline
detaches the surface water from the bottom waters because the mixing between
surface waters and bottom water is strongly restricted. That´s why
the whole bottom water is exchanged only one time in 1000 years.
There is a transport of dead organism and plant
material through the water column to the seafloor. Microbial degradation
of this material needs oxygen to produce energy by oxidation of reduced
organic material to form carbondioxide. This leads to a completely consumption
of free oxygen in the watercolumn below the pycnocline. At anoxic conditions
some microorganisms are able to use sulphate (SO42-)
for oxidation of organic material. In the consequence the toxic hydrogensulphide
is produced and no macrocospic live is possible in the waterbody and at
the seafloor-water interface below the pycnocline. Large amounts of organic
material reach the bottom of the sea. Organic material akkumulate in the
sediments and can be concentrated up to 20%. These kinds of sediments are
called sapropel.
The occurence of hydrogensuphide in the watercolumn and within the sediments makes the precipitation of different sulphides possible. The most important ones are iron-sulphides like pyrite, greigite and iron-monosulphide. During the last glaciation the Black Sea wasn´t connected with the Mediterranean. At that time the stratification in the water column does not exist because the basin had been filled only with freshwater. The picnoclyne couldn´t develop because there was only one source of water. Freshwater is poor with sulphate. So sulphate-reduction shouldn´t take place in the sediments deposited under limnic conditions. Nevertheless you can find ironsulphides in the limnic sediments. A downward flow of sulphate in the porefluid through the the sediments supplies sulphate to the limnic sediments. This sulphate serves as the sulphur source for iron-sulphides.
Very often sapropels contain large concentrations of some minor elements like Re, Mo, Cd, U or V. This enables the opportunity to identify the redoxconditions within the watercolumn during sedimentation. On the illustrations below you can see enrichment factors of some minor elements in Black Sea sapropels compared with average values of mediterranean sapropels and middle values for Centurion/Turon boundary event Black Shales. These sediments are furter examples for sediments which had been deposited under anoxic conditions. The value for a sediment which has been depositet under oxic conditions (average shale, Wedepohl 1971) is one.
This work happens in cooperation with Dr. Michael Böttcher (Max Planck Institute for Marine Microbiology, Bremen).
Further
informations about Black Sea sediments and other type anoxic basins
look here.
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