Oceanic deep waters are there-fore envisaged to have been replaced by dense, warm, saline waters generated in evaporitic tropical-shelf seas.. Warm water holds considerably less dissolve
Trang 1dense water that drives much of the present-day
oceanic circulation Oceanic deep waters are
there-fore envisaged to have been replaced by dense, warm,
saline waters generated in evaporitic tropical-shelf
seas Warm water holds considerably less dissolved
oxygen than colder water, and this factor alone will
tend to encourage deep-sea anoxia Some modelling
experiments suggest that an ocean with warm saline
deep waters will circulate more rapidly than one with
colder waters, with the result that upwelling may
have been more vigorous This in turn would
stimu-late more plankton productivity, thereby intensifying
the mid-water OMZ Intense oceanic volcanism,
which is also correlated with the OAEs, may also
have supplied increased levels of nutrients to the
oceans and fostered further productivity
The Cretaceous OAEs were originally intended to
denote intervals when oxygen-poor marine
depos-ition was widespread, without implying that all
oceans, beneath the surface waters, were
simultan-eously anoxic In essence they refer to intervals
when anoxic environments were very widespread
However, there is evidence to suggest that true global
oceanic anoxia may have in fact happened, with the
most notable example occurring at the transition
from the Permian to the Triassic Appropriately
enough, this has been termed a superanoxic event,
and it is probably no coincidence that this interval is
also marked by the greatest marine mass extinction
of all time (see Palaeozoic: End Permian Extinctions)
Like the Bonarelli Event, the Permo-Triassic
super-anoxia coincides with an extreme greenhouse climate
and with the eruption of a giant volcanic province In
fact, the marine anoxia–global warming–massive vol-canism triumvirate is seen several times in Phanerozoic history, and invariably coincides with extinction events, although of considerably different magnitudes (the Bonarelli Event is marked by only a minor extinc-tion event) The Early Jurassic provides another classic example, with volcanism in the Karoo region of South Africa being correlated with a significant marine ex-tinction event and widespread deposition of black shales, particularly in north-west Europe, where they are variously known as the Jet Rock (England), Schistes Cartons (France), and the Posidonienschiefer (Ger-many) Linking these various phenomena into a cause-and-effect scenario is a key goal of much current geological research
Productivity versus Preservation
All black shales appear to have formed in anoxic environments, and black shales are the source all the world’s oil and much of its natural gas; therefore understanding and predicting the occurrence of anoxic environments is a key goal in hydrocarbon exploration However, it is remarkably difficult to constrain the key attributes of anoxic-environment development As noted above, study of modern envir-onments indicates that there are two routes to produ-cing organic-rich sediments, and they are distinctly different In high-productivity settings, supplied with abundant nutrients, the seafloor is overwhelmed by the flux of organic matter, and there is insufficient oxygen to decay it all As a consequence oxygen-poor
or anoxic conditions develop, but this is merely due to
Figure 4 Oxygen restricted biofacies based on fossil content and sediment properties The fossils have been divided into three categories First, nekton are free swimming animals and pseudoplankton are forms that attach to floating objects, such as driftwood, in the surface waters Such species are not affected by bottom water oxygen levels and so are found in a broad range of sediments Second, nektobenthos are swimming forms, such as ammonites, that probably lived near the seafloor and so were absent from environments developing bottom water anoxia Third, benthic species live on the seafloor and cannot tolerate bottom water anoxia Reproduced from Wignall PB (1994) Black Shales Oxford Monographs in Geology and Geophysics Oxford: Oxford University Press.
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