Although a subject of controversy, anoxic condi-tions are thought by many to favour the preservation of organic matter, and many anoxic sediments are organic-rich.. On the face of it, fo
Trang 1This 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
condi-tions 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
char-acteristically 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:
Ammon-ites), 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:
Brachio-pods) (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
‘dys-aerobic’ was first coined for modern forms found
living on the seafloor within the OMZ of the
Califor-nian Borderland Low-oxygen bottom waters provide
harsh conditions for animals with the result that only a low-diversity, but often abundant, community of crea-tures 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 dis-play a shell morphology of broad, flat, often circular valves, sometimes with dense, fine radial ribs These are loosely termed ‘paper pectens’, and this charac-teristic 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 depos-ition, although levels need not have been very high However, the geochemistry of anoxic sediments im-plies 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 geochem-ical and palaeontologgeochem-ical 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 pro-posed for dysaerobic faunas, which envisaged per-sistently low seafloor oxygen levels, and as a result the alternative names ‘episodically dysaerobic’ and
‘poikiloaerobic’ have been proposed However, nei-ther 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 descrip-tive 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 Oxygen-restricted biofacies 3 and 4 are very common, and they typically combine attributes of anoxic depos-ition (lamination and trace-metal enrichment) and dysaerobic deposition (presence of low-diversity ben-thic 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 ther-mohaline circulation regime It therefore came as
Figure 2 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.
SEDIMENTARY ENVIRONMENTS/Anoxic Environments 497