Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 2340

SEDIMENTARY ENVIRONMENTS/Anoxic Environments 497 Figure Scanning electron microscope image of a pyrite fram boid, from the annulata Shale (Upper Devonian) of northern Ger many Such spherical accumulations of tiny pyrite microcrystals (microcrysts) form in prolific numbers in anoxic marine environ ments This is particularly the case for manganese, which is often concentrated as a carbonate on the margins of anoxic areas Although a subject of controversy, anoxic conditions are thought by many to favour the preservation of organic matter, and many anoxic sediments are organic-rich This is manifest in the dark colour of the sediments, and the epithet ‘black shale’ is often used to describe anoxic sediments Such sediments often also display very fine lamination, and they characteristically weather into thin sheets to produce a rock known as a paper shale The preservation of fine laminae is testimony to the fact that animals cannot live in anoxic conditions Thus, the worms and other creatures that normally disrupt delicate laminae by their burrowing activities are absent On the face of it, fossils should also be absent from anoxic sediments because the anoxia inhibits all but microbial life However, on the contrary, many black shales contain abundant and well-preserved fossil remains Many of these belong to creatures that swam, such as fishes (see Fossil Vertebrates: Fish) and ammonites (see Fossil Invertebrates: Ammonites), and so their presence is readily explained: they presumably lived in the oxygenated surface waters and sank into the anoxic deeper waters only after death (Figure 3A) However, many fossils in anoxic sediments belong to ostensibly bottom-living forms, notably bivalves (see Fossil Invertebrates: Bivalves) and brachiopods (see Fossil Invertebrates: Brachiopods) (Figure 3B) For a long time such occurrences have been a subject of debate, but most workers now agree that they are dysaerobic fossils The term ‘dysaerobic’ was first coined for modern forms found living on the seafloor within the OMZ of the Californian Borderland Low-oxygen bottom waters provide harsh conditions for animals with the result that only a low-diversity, but often abundant, community of creatures can survive Low diversity–high abundance is a common attribute of black-shale fossil assemblages too, and they are generally regarded as representing a community tolerant of low oxygen Ancient dysaerobic assemblages are often dominated by bivalves that display a shell morphology of broad, flat, often circular valves, sometimes with dense, fine radial ribs These are loosely termed ‘paper pectens’, and this characteristic morphology has evolved again and again in black-shale depositional environments (Figure 3B) By implication, the presence of fossils in a black shale suggests the presence of oxygen during deposition, although levels need not have been very high However, the geochemistry of anoxic sediments implies that there was no oxygen at all during deposition For example, uranium is concentrated in sediments only in the complete absence of oxygen; otherwise it occurs as a highly soluble ion that is not precipitated Thus, there is often a discrepancy between geochemical and palaeontological evidence for oxygen levels This can be resolved if it is appreciated that black shales record a range of depositional environments in which the average depositional conditions were anoxic but seafloor oxygen was present during some, probably brief, intervals These oxygenation events would allow transient colonization by rapidly dispersing species such as paper pectens This is a distinctly different concept from that originally proposed for dysaerobic faunas, which envisaged persistently low seafloor oxygen levels, and as a result the alternative names ‘episodically dysaerobic’ and ‘poikiloaerobic’ have been proposed However, neither has really caught on in the literature, and the term dysaerobic continues to be used In order to get away from these generic terms, a simple descriptive oxygen-restricted biofacies scheme has been provided based on fossil and sediment attributes (Figure 4) Based on British Jurassic examples, the scheme recognizes a gradient of features thought to record improving seafloor oxygen levels Oxygenrestricted biofacies and are very common, and they typically combine attributes of anoxic deposition (lamination and trace-metal enrichment) and dysaerobic deposition (presence of low-diversity benthic fossils); they undoubtedly formed in the variably oxygenated conditions described above Oceanic Anoxic Events The modern oceans are extremely well ventilated, and deep waters are everywhere supplied with high levels of oxygen, thanks primarily to a vigorous thermohaline circulation regime It therefore came as