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dysaerobic or dysoxic conditions with low O 2 , but no H 2 S [63]. Alternatively, a
DOA > 0.38 has been shown to reflect deposition beneath anoxic bottom water
conditions [59].
3.3 Trace Element Abundances and Ratios
Abundances and ratios of redox-sensitive trace elements are frequently uti-
lized to assess the redox conditions of modern and ancient sedimentary systems
[37, 49, 76]. Classical examples include V, Cr, Ni, Co, Mo or U, all reacting
differently under varying oxygen content, thus representing valuable proxy sig-
nals particularly in laminated sediments. A positive correlation with organic
carbon abundances is further consistent with the aspect of enhanced preserva-
tion of organic matter and enrichment of numerous trace elements under anoxic
conditions (for a recent review see [37]).
In addition to trace element abundances, the isotopic composition of selected
redox-sensitive elements like molybdenum represents an equally powerful in-
dicator for anoxia [1].
3.4 Biomarker Evidence
Biomarkers are molecular fossils which can be found in sediments/sedi-
mentary organic matter and which reflect the former presence and metabolic
activity of living organisms [24, 58]. Organic geochemical methods provide
the means for identifying the precursor organisms but also for characterizing
the depositional environment and/or bacterial and inorganic degradation during
diagenesis. For the present discussion, the prevalence and extent of anoxic con-
ditions in the water column of a sedimentary environment is the prime question.
Undoubtedly, the anaerobic microbial reworking of sedimentary organic mat-
ter, either in the sediment or in anoxic bottom waters, is an important process
in sedimentary environments. However, the ability to reconstruct photic-zone
anoxia would indicate a possible contribution of anaerobic organisms to pri-
mary productivity via anoxygenic photosynthesis.
Unequivocal evidence for anoxic water conditions within the photic zone is
provided by the presence of bacterial pigments (such as isorenieratene) which
are uniquely biosynthesized by strictly anaerobic bacteria. The presence of
isorenieratene and derivatives, together with their characteristic carbon isotopic
composition, point to the (former) activities of photosynthetic green sulphur
bacteria (Chlorobiaceae) which live under low light intensities and sulphide
dissolved in the water column. Clear indications for anoxygenic photosynthe-
sis exist in recent environments, such as the Black Sea (e.g. [67]), but evidence
is growing also from ancient sedimentary environments, like the Mediterranean
sapropels of Pliocene age (e.g. [14, 53]), Cretaceous sediments from the North
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