SEDIMENTARY ENVIRONMENTS/Anoxic Environments 499 Figure 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 dense water that drives much of the present-day oceanic circulation Oceanic deep waters are therefore envisaged to have been replaced by dense, warm, saline waters generated in evaporitic tropical-shelf seas Warm water holds considerably less dissolved oxygen than colder water, and this factor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 stimulate 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 deposition was widespread, without implying that all oceans, beneath the surface waters, were simultaneously 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 superanoxia coincides with an extreme greenhouse climate and with the eruption of a giant volcanic province In fact, the marine anoxia–global warming–massive volcanism 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 extinction event) The Early Jurassic provides another classic example, with volcanism in the Karoo region of South Africa being correlated with a significant marine extinction 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 (Germany) 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 environments indicates that there are two routes to producing 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