Environmental Engineering Reference
In-Depth Information
4.2 The Early Paleozoic
Following the break-up of Rodinia, the early Paleozoic world was char-
acterized by a large continental area (Gondwana) above the South Pole and
dispersed continents (such as Laurentia, Baltica, Siberia and others) in low and
mid-latitudes [78]. An overall high sea level resulted in the flooding of large
continental areas and the development of distinct facies variations including
the deposition of extensive laminated non-bioturbated sediments (black shale
deposits). Abundant occurrences, such as Cambrian varved black shales [94],
the Ordovician and Silurian graptolite shales [12] or Devonian and Carbonif-
erous black shales deposits [25] on many continents suggest deposition from
an apparently oxygen deficient deeper part of the water column during much
of early and middle Paleozoic times (Fig. 1). Combined sedimentological and
paleontological evidence was utilized to propose a model for different facies
developments and animal habitats in the early Paleozoic [13]. This shows the
spatial distribution of proximal oxic, bioturbated sediments, termed shelly fa-
cies, followed by distal non-bioturbated, anoxic, pyritic black shales of the so
called graptolite facies. Analogous to modern anoxic settings, the water column
would have contained a chemocline and a distinct vertical sequence of primary
productivity in the photic zone, followed downward by aerobic respiration, den-
itrification and sulphate reduction. The lateral extent of this facies distribution
was dependent on the prevailing climatic conditions which were responsible
for changes in sea level but more importantly for an initially sluggish oceanic
circulation. Progressive ventilation of the early and mid-Paleozoic oceans with
oxygen is viewed as a consequence of growing ice sheets in the high latitudes
and a subsequent ocean circulation comparable to the modern world.
Additional evidence stems from temporal variations in the carbonate carbon
[87] and sulphate sulphur [38] isotopic composition. Isotope mass balance
calculations based on these records suggest enhanced anaerobic respiration of
sedimentary organic matter via bacterial sulphate reduction, consistent with a
view of oxygen-deficient bottom waters.
Apart from an overall appearance of (possibly) prolonged oxygen deficiency
in the early and mid-Paleozoic deep ocean, evidence has been provided for a
link between anoxia and extinction events during this time interval [35, 40, 90].
4.3 The Permian-Triassic Transition
Numerous causes have been proposed for the greatest mass extinction during
Earth history at the Permian Triassic boundary (e.g. [6, 41, 42, 66, 71, 93]).
Among them, widespread end-Permian oceanic anoxia (Fig. 1) is indicated by
respective laminated organic and pyrite rich black shales from shallow and deep
water environments [92].
