Environmental Engineering Reference
In-Depth Information
an early stage as the Black Sea and Mediterranean Sea were reconnected. In
sediments on the slope, deposition of this sapropel started significantly later.
The rise of the O
2
-H
2
S-interface from the bottom at 2200 m depth to a water
depth of 500 m has been calculated from the rate of sedimentation of organic
matter and on the chemical characteristics of contemporaneous sediment layers
at the two depths, and probably lasted between 2300 to 3000 years [15]. The
bottom part of Unit II (Unit IIb) has a higher organic carbon content and was
deposited during a period of higher productivity, whereas the layers above (Unit
IIa) contain a higher fraction of terrigenous material deposited under a period of
lower primary productivity and stronger terrestrial inputs due to an intensified
erosion of the Eurasian continent [26, 27]. During the deposition of Unit IIa,
however, the water column was probably largely oxic [65].
Increasing salinity allowed the final invasion of the marine coccolithophorid
Emiliania huxleyi
between 3500 and 1600 yr BP (depending on the dating
method). This event initiated the deposition of finely laminated coccolith ooze,
defined as Unit I. These microlamina are thought to represent varves [63] of
annual events like spring blooms [28]. The sediments contain 3-5% organic
carbon, with the white laminae consisting mainly of coccoliths (over 90 weight
%) deposited during spring and fall blooms. The intervening dark laminae are
enriched in terrigenous material. These laminated layers are separated from an
earlier invasion (at the very beginning of Unit I; [65]) by a several cm-thick
transition sapropel which consists largely of terrigenous material and contains
9% TOC. The transition sapropel was deposited over a time interval of 400-1000
years.
Independent evidence for the vertical extent of the anoxic zone may come
from fossil biomarkers of green sulfur bacteria. Diaromatic carotenoids and
their sulfur-linked derivatives have been used as indicators of the presence
of green sulfur bacteria in various past depositional environments, even those
which are now located on land like the Messinian Vena del Gesso basin (North-
ern Apennines, Italy) [36]. Similarly, isorenieratene [59] and sulfurized isore-
nieratane [65, 77] have been detected in the analyses of organic sulfur fractions
from Black Sea sediments. In Unit IIb, free isorenieratene was found [17, 65, 72]
as well as farnesane withδ
13
C values typical for green sulfur bacteria which use
the reverse TCA cycle for CO
2
fixation. Isorenieratene reaches ranges between
2-72 µg/gdw in Unit I and up to 1.4 µg/gdw in Unit IIb [59]. Compared to free
isorenieratene, sulfurized isorenieratane reached lower concentrations in Unit
I (8.3 - 13.0 µg·(g dry weight)
−
1
) but higher concentrations in Unit IIb (50.0
µg·(g dry weight)
−
1
) [65, 77]. Taken together, these data indicate that photic
zone anoxia, hence a large anoxic water body, developed during the deposition
of Unit IIb and Unit I.
Recently, the 16S rRNA gene sequences of
Chlorobium
sp. BS-1 have been
detected in sediment layers of the Black Sea by PCR employing specific primers
