Geology Reference
In-Depth Information
1994), developing largely from photosynthetic oxygen
production. The SOM forms on two different time
scales, episodic (by physical events in which nutricline
water is transported into the mixed layer and then ven-
tilated to the atmosphere, producing a phytoplankton
bloom) and gradual (developing from an input of 'new'
nutrients transported from the depths or/and by sink-
ing particles).
grained, dark-colored and compacted sediment, usu-
ally with distinct lamination and commonly more than
0.5% organic matter. The dark gray color is caused by
finely dispersed iron sulfides and organic compounds.
Argillaceous black shales were common in the Prot-
erozoic, the Early Paleozoic, and during the Jurassic
and Cretaceous (Kukal 1990).
Controls on the mode of life and the biofacies by
oxygen are described by the terms
Oxygen-minimum zone: Decomposition of organic
matter produced in the surface waters causes a decrease
in oxygen in many intermediate waters. The depth of
that layer ranges in the Pacific Ocean and in the Indian
Ocean from 100 to 1500 m, but oxygen values less than
0.5 ml/l water occur only between 500 and 1000 m.
Benthic environments are only affected where the mini-
mum layer intersects with shelf and slope environments.
In ancient times, the oxygen minimum layer was of
variable thickness and intensity. Reduced or expanded
oxygen minimum zones are caused by variations in pri-
mary production and density stratification within the
water body, which in turn control the advection and
diffusion of oxygen. Expanded oxygen minimum zones
have been reported from the Cretaceous.
aerobic: normal benthic fauna, presence of free oxy-
gen, no oxygen restriction,
dysaerobic: impoverished benthic fauna stressed by
low bottom-water oxygen values
anaerobic: no benthic fauna, due to the lack of free
oxygen.
These zones fit depth-related environments in which
oxygen values remain rather constant. The term poikilo-
aerobic (Oschmann 1991) refers to annual and seasonal
fluctuations of low-oxygen values in shallow shelf and
epeiric environments that are reflected by low diver-
sity and an opportunistic benthic fauna. The term
exaerobic (Savrda and Bottjer 1987) refers to a biofa-
cies near the anaerobic-dysaerobic boundary charac-
terized by specialized benthic shelly organisms harbor-
ing chemosymbiotic bacteria.
12.1.5.1 Terminology and Classification
Declining oxygen gradients cause a decrease in benthic
faunal diversity until the oxygen-free anaerobic zone
is reached (Rhoads and Morse 1971; Byers 1977;
Savrda and Bottjer 1987). Ranking by diversity reveals
paleo-oxygenation gradients (Wignall 1990).
Oxygen-restricted biofacies (ORB) and inferred
paleo-oxygenation levels can be derived from the num-
ber of species and the sediment fabric (Wignall and
Hallam 1991). Examples were summarized by Allison
et al. (1995). ORB 1 and ORB 2 are equivalent to the
anaerobic biofacies and characterized by the total ab-
sence of benthic organisms and a fissile sediment fab-
ric. ORB 3 and ORB 4 contain only a few benthic spe-
cies which are very rare in ORB 3 or abundant on some
bedding planes (ORB 4). Sediment fabric is planar lami-
nated but bioturbation may occur. ORB 4 corresponds
to the poikiloaerobic and episodically disaerobic facies.
ORB 5 and ORB 6 are characterized by bioturbation.
ORB 3 to ORB 6 correspond to the dysaerobic biofa-
cies.
Levels of dissolved free oxygen are classified by the
content of free oxygen in the water expressed as ml/l,
and indicated by the terms
• oxic: >1ml/l O 2
• dysoxic: 1.0 to 0.2 ml/l O 2
• suboxic: 0 to 0.2 ml/l O 2
• anoxic: <0.2 ml/l O 2 , commonly no free O 2 .
The term euxinic refers to anoxic conditions but with
free H 2 S in the water column.
Anoxic sediments and anoxic bottom waters are pro-
duced where there is a deficiency in oxygen due to very
high organic productivity and a lack of oxygen replen-
ishment to the water or the sediment, e.g. in the case of
water stratification or stagnation (Southam et al. 1982).
Extended periods of anoxia exclude virtually all higher
invertebrates from the benthic environment.
Most anoxic sediments are siliciclastic sediments
represented by black shales . This term denotes a fine-
12.1.5.2 Recognizing PaleoOxygenation
Generally, low organic content, light rock colors,
existence of diverse and highly-diverse benthic faunas
and trace fossils, and evidence of hydraulic activity (e.g.
cross-stratification and high-energy textures (Sect.
12.1.1) are taken as proxies of oxic environments. High
organic content, dark rock colors, bituminous smell,
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