Environmental Engineering Reference
In-Depth Information
oxidized atmosphere and surface water) and Metazoan life during the terminal
Neoproterozoic (oxidized atmosphere and surface water).
In more general terms and applicable throughout earth's history, the depo-
sition of black shales, organic carbon and pyrite-rich siliciclastic sediments, is
attributed to anoxic conditions in (larger) parts of the water column [89]. Such
conditions of bottom water anoxia are further supported through the absence
of bioturbation as indicated by the lack of trace fossils.
In addition, the size distribution of sedimentary pyrite is increasingly being
utilized as a proxy signal for water column anoxia [91]. Microframboids formed
in a necessarily anoxic water column are distinctly different in size and shape
compared to diagenetic (even more so late diagenetic) pyrite formed in the
sediment.
3.2 Geochemical Proxy Signals: C-S-Fe
The biogeochemical cycling of redox-sensitive elements in sedimentary en-
vironments results in characteristic changes in their abundances and ratios.
One of the key processes in marine sediments is the anaerobic mineralization
of sedimentary organic matter through fermenting and subsequently sulphate
reducing bacteria. Sulphate is reduced at the expense of organic matter which
is oxidized, thereby buffering the atmospheric oxygen abundance [9]. This is
expressed in a positive correlation between the abundances of organic carbon
and pyrite sulphur, a traditional proxy signal for normal marine, i.e. oxic bottom
water [11, 43, 61, 62] vs. semi-euxinic and euxinic bottom water conditions
[61]. Subsequently, it was realized that the availability of reactive iron was an
additional important parameter in euxinic environments [63, 64]. No positive
correlation between organic carbon and pyrite sulphur can be observed in such
sediments.
Apart from C-S-Fe systematics described above, substantial effort has been
devoted to a detailed quantification of different sedimentary iron species [60,
64]. Depending upon analytical data, two parameters were defined:
the degree of pyritization (DOP)
=
+
DOP
Fe
PYR
/
Fe
PYR
Fe
HCl
the degree of anoxicity (DOA)
DOA
=
(
Fe
PYR
+
Fe
MAG
+
Fe
OX
+
Fe
CARB
)/
Fe
T
.
Both allow anoxic depositional conditions to be distinguished from oxic
conditions. A DOP value less than 0.45 reflects deposition under normal marine,
well-oxygenated bottom water conditions, whereas a DOP
>
0.75 suggests
anoxic bottom water conditions. Intermediate values are thought to reflect
