Environmental Engineering Reference
In-Depth Information
Following Berner's original classification of sediments [8], oxic sediments
are deposited under an O
2
-concentration greater than 30µM, suboxic sediments
are characterized by O
2
-concentrations between 30 and 1 µMO
2
. Anoxic
sediments with less than 1 µMO
2
were differentiated into non-sulphidic (H
2
S
<
1 µM) and sulphidic (H
2
S
>
1 µM) deposits.
Any earth science text book contains ample descriptions of anoxia, even
global anoxia. For example, we view the distant past, the early part of earth's
history (the Archaean) as a world, in which the entire ocean-atmosphere sys-
tem was anoxic, followed by a Great Oxygenation Event and subsequent
oxygenation of atmospheric, surface and deep water environments [29, 30].
However, alternative views are also considered [51]. Prolonged deep ocean
anoxia has also been proposed for the early Paleozoic [12]. Multiple levels of
black shale deposition during the Cretaceous are described as oceanic anoxic
events (OAEs) with global implications [3]. Large scale, possibly global anoxic
conditions have been considered as cause for mass extinctions [88]. Finally, nu-
merous modern oceanic settings show anoxic/euxinic water column conditions
[21].
On a smaller scale, any sedimentary environment contains a transition from
oxic to anoxic conditions as reflected by decreasing oxygen abundance with
depth. Pertinent lithological (e.g., grain size) and biological (e.g., bioturbation,
microbial activity) characteristics determine how steep this gradient is and how
quickly the pore waters become anoxic.
Considering these examples, it is obvious that any discussion of anoxia, in
particular with respect to the geobiological consequences, will strongly depend
on the scale of observation. This contribution will focus on large scale anoxia,
affecting the entire or at least large parts of the atmosphere-ocean-system.
Following a previous comprehensive account of modern and ancient anoxia
[5], it is the aim of this review to update and extend the temporal record of
global anoxia.
2. MODERN ANOXIA
A long history of continuous research in modern anoxic environments has
significantly improved our understanding of causes and consequences of water
column anoxia. Based on differences in the oceanographic framework, three
different types of marine anoxic environments have been suggested [21]: (a)
silled basins, (b) upwelling zones and (c) stable oxygen minimum zones in the
open ocean. Prominent examples include (a) the Black Sea, the Baltic Sea, the
Cariaco Basin, the Framvaren Fjord, or the Saanich Inlet; (b) the South-West
African shelf off Namibia, or the Peruvian/Chilean shelf, and (c) the Arabian
Sea and northern Indian Ocean, or the Gulf of California, respectively. Results
