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| Titel |
Biominerals of nanocrystalline greigite (Fe3S4) in sediments of the Black Sea and the sill at the Bosporus |
| VerfasserIn |
Anton Preisinger, Selma Aslanian, Roman Beigelbeck, Namik Cagatay, Wolf-Dieter Heinitz, Johann Wernisch |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056125
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| Zusammenfassung |
| The biomineralization of iron sulfide is a biologically controlled mineralization where single
spinel crystals of greigite (Fe3S4) are formed intracellularly by magnetosomes in sulfate
reducing bacteria (SRB). Each single crystal is surrounded by a protein-containing lipid
bilayer membrane shaping a cuboctahedral 3D-nanocrystalline greigite. We analyzed
sediment cores taken from the Black Sea and the sill of the Bosporus regarding their greigite
content. Two different types of clusters of twinned ferrimagnetic nanocrystalline greigites
(magnetotactic and framboidal greigites) were found. Their amount within a reference
volume was quantitatively analyzed by counting clusters on ore microscopy photographs and
scanning electron microscopy pictures. Ferrimagnetic greigite clusters are only
stable in anoxic environments and change under oxic conditions to non-magnetic
pyrite (FeS2)+FeOOH. We utilized these properties in order to answer hitherto
unsolved questions about the climate and the paleohydrology of the Black Sea, the
Marmara Sea, and the sill of the Bosporus. In particular, detailed scientific knowledge
regarding water level, sediment in- and outflow, and oxic/anoxic-state of the Black
Sea during the last 20.000 years can be obtained. The formation, occurrence, and
distribution of ferrimagnetic greigites in SRB in anoxic sea water allow inferences about
cold/warm-periods of this region as well as the paleohydrological history of the Black
Sea.
The entrance of water to the Black Sea from the Danube (NW-site) and from the Marmara
Sea over the Bosporus (SW-site) is subjected to a periodicity: During glacier melt in the Alps,
higher water content enters the Black Sea resulting in a lower concentration of SO42- ions.
This process is reversed during glacier growth. Such a rhythmic behavior can be observed and
reproduced for the last 8000 years directly by analyzing framboidal greigite clusters taken
from gravity cores of the Black Sea. The last change from a cold to a warm period took
place about 1833 yearsBP, which can be seen in our analysis because in a warmer
period, the occurrence of the greigite clusters is statistically lower than in a colder
one.
Between 16 and 12 kyearsBP, water and sediments coming from the Danube and the
Strait of Kerch entered the Black Sea. Water and sediment particles were transported by
counter-clockwise acting Coriolis- and centrifugal forces. These sediments were deposited in
the shelf zone parallel to the Bulgarian coast. At the end of the Younger Dryas (11.7
kyearsBP), the sea level of the Black Sea increased and a sublacustrine river valley
parallel to the Bulgarian coast in direction towards to the Bosporus was formed.
We analyzed five representative cross sections of this river valley regarding the
determination of the cold/warm-periods by utilizing our greigite-based method as well as a
Sediment-Echo-Sounder (SES 2000).
Nowadays, the water exchange between the Black and the Marmara Sea takes place
through the Bosporus strait with sill depths of 32 - 34 m (southern end) and 60 m (northern
end). We analyzed the sedimentation at the sill of the Bosporus and the Golden Horn by
means of an SES 2000. The results provide detailed information about the historical evolution
of the water flow between both seas during the last 10.000 years. The sill of the Bosporus is
separated into two channels where water enters on the Asian side from the Marmara Sea and
on the European side from the Black Sea. Measurements of clusters of magnetotactic
3D-nanocrystalline greigites in gravity cores of the Black Sea indicate the change of the
velocity of the water entrance from the Marmara in the Black Sea between 8 - 10 kyearsBP.
The entrance starts sporadic at 10 kyearsBP with a maximum between 9600-8800
yearsBP.
Concluding, all our results contradict the “catastrophic flood”-hypothesis of the Bosporus
by Ryan and Pitman. |
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