The Phanerozoic
Several intervals of deep ocean anoxia occurred throughout the Phanerozoic (see review by Meyer and Kump,
2008). Their most prominent geological expressions
include organic-rich deposits (“black shales”) from the
Cambrian of China, the Ordovician of North America,
the Silurian of North Africa, and the Devonian-Lower
Carboniferous of the USA and Europe. Most of these
Paleozoic deposits represent shallow marine environments, and synchronous records from the deep oceans
are largely missing. In contrast, there is evidence for widespread deepwater anoxia around the Permian-Triassic
boundary, coinciding with the most severe extinction
event in Earth history (Meyer and Kump, 2008). In the
Mesozoic, a number of rather short-termed but severe
and widely recognized intervals of ocean anoxia occurred,
the so-called oceanic anoxic events (OAEs; Schlanger and
Jenkyns, 1976). Although their full global expansion is
debated (e.g., Trabucho Alexandre et al., 2010), there is
evidence that at least the North Atlantic and Tethys were
periodically affected by intense deepwater anoxia to
euxinia, whereas comparable records from the Pacific
and Indian Oceans are scarce. The Toarcian Oceanic
Anoxic Event (T-OAE) and the Cenomanian-Turonian
Boundary Event (CTBE or OAE2) are considered to be
the most widespread occurrences of Mesozoic marine
anoxia (Jenkyns, 2010). The Cenozoic sediment record
is comparably poor in black shale-type deposits.
Organic-rich sediments related to the Paleocene-Eocene
Thermal Maximum (PETM) are mostly limited to shallow
marine environments (Cohen et al., 2007). However, the
deep Arctic Ocean record shows clear signs of Paleocene
and Eocene deepwater anoxia, most likely related to its
restricted circulation and stable water column stratification. The most recent witnesses of short-termed deepwater
anoxia are Pliocene-Pleistocene sapropels of the Eastern
Mediterranean (Emeis et al., 2000; De Lange et al., 2008).
“Modern analogues”
In the modern ocean, there are no true analogues for open
marine black shale formation. However, a number of
“near-analogue” environments can serve as natural laboratories to study some of the physical, chemical, and biological processes that interacted in past anoxic oceans
(Demaison and Moore, 1980). The most prominent example is the Black Sea, Earth’s largest permanently euxinic
marine basin (Degens and Ross, 1974). It is characterized
by moderate primary productivity but strong salinity stratification and a very restricted connection to the Mediterranean. A similarly well-studied but less restricted and less
euxinic marine environment is the Cariaco Basin, and also
a number of fjords and bays around the world (e.g.,
Framvaren Fjord, Saanich Inlet, Kau Bay) exhibit multiannual deepwater anoxia to euxinia (Richards, 1965;
Tissot and Welte, 1984). Also coastal upwelling areas
(e.g., off Namibia, Chile, Peru, Arabian Sea) share common features with past anoxic oceans as they display high
seasonal primary productivity, leading to deepwater oxygen consumption and occasional development of sulfidic
water masses (Copenhagen, 1953). Over the past decades,
the study of geochemically similar but spatially restricted
analogues has advanced our understanding of past anoxic
oceans. A wide range of geochemical proxies is now at
hand to unravel the paleoenvironmental setting from sedimentary archives.
Environmental controls: organic matter export,
water column stratification, plate tectonics, and
astronomical forcing
Two fundamental environmental parameters are traditionally suggested as drivers of oceanic anoxia: high primary
productivity inducing enhanced organic matter export to
the seafloor and enhanced organic matter preservation by
limited deepwater ventilation, i.e., low oxygen
(Demaison and Moore, 1980). Modern end-members of
this “productivity versus preservation” debate are the
Black Sea, on the one hand (moderate productivity, severe
basin restriction, salinity stratification, absence of molecular oxygen below redoxcline), and upwelling areas, on the
other hand (full connection to the open ocean, seasonally
very high productivity). Recent studies suggest that most
black shale deposits do not allow a strict differentiation
between organic matter productivity and preservation as
dominant cause for organic carbon accumulation and
anoxia. Past examples of restricted anoxic basins include
the mid-Cretaceous proto-North Atlantic and the Paleogene Arctic Ocean. Past high-productivity regions with
anoxic water masses developed in the mid-Cretaceous
equatorial Atlantic as nutrients were “trapped” due to specific circulation patterns (Trabucho Alexandre et al.,
2010). To create and maintain marine anoxia, a range of
important biogeochemical feedback mechanisms needs
to be considered. One well-known “anoxia-productivity
feedback” is the efficient recycling of phosphate from
marine sediments if bottom-water oxygen is strongly
depleted (Ingall et al., 1993; Slomp et al., 2004; März
et al., 2008). Recycled phosphate can then be reintroduced
into the photic zone and sustain high-productivity and
organic matter export.
Although the onsets and terminations of widespread
ocean anoxia are still debated, they appear to be related
to plate tectonics and volcanism (Meyer and Kump,
2008). Astronomical forcing can regulate the intensity of
anoxia once favorable conditions have been created
(De Lange et al., 2008). Plate tectonic constellations are
responsible for the restriction of ocean basins (e.g., the
mid-Cretaceous North and South Atlantic), reducing the
renewal and oxygenation of deepwater masses (Erbacher
et al., 2001). Volcanic activity introduces large amounts
of the greenhouse gas CO 2 into the atmosphere, leading
to both global warming and enhanced chemical
weathering (Turgeon and Creaser, 2008). Warming
reduces the oxygen solubility in the oceans and the vertical mixing of its water masses, and chemical weathering
ANOXIC OCEANS
21
Several intervals of deep ocean anoxia occurred throughout the Phanerozoic (see review by Meyer and Kump,
2008). Their most prominent geological expressions
include organic-rich deposits (“black shales”) from the
Cambrian of China, the Ordovician of North America,
the Silurian of North Africa, and the Devonian-Lower
Carboniferous of the USA and Europe. Most of these
Paleozoic deposits represent shallow marine environments, and synchronous records from the deep oceans
are largely missing. In contrast, there is evidence for widespread deepwater anoxia around the Permian-Triassic
boundary, coinciding with the most severe extinction
event in Earth history (Meyer and Kump, 2008). In the
Mesozoic, a number of rather short-termed but severe
and widely recognized intervals of ocean anoxia occurred,
the so-called oceanic anoxic events (OAEs; Schlanger and
Jenkyns, 1976). Although their full global expansion is
debated (e.g., Trabucho Alexandre et al., 2010), there is
evidence that at least the North Atlantic and Tethys were
periodically affected by intense deepwater anoxia to
euxinia, whereas comparable records from the Pacific
and Indian Oceans are scarce. The Toarcian Oceanic
Anoxic Event (T-OAE) and the Cenomanian-Turonian
Boundary Event (CTBE or OAE2) are considered to be
the most widespread occurrences of Mesozoic marine
anoxia (Jenkyns, 2010). The Cenozoic sediment record
is comparably poor in black shale-type deposits.
Organic-rich sediments related to the Paleocene-Eocene
Thermal Maximum (PETM) are mostly limited to shallow
marine environments (Cohen et al., 2007). However, the
deep Arctic Ocean record shows clear signs of Paleocene
and Eocene deepwater anoxia, most likely related to its
restricted circulation and stable water column stratification. The most recent witnesses of short-termed deepwater
anoxia are Pliocene-Pleistocene sapropels of the Eastern
Mediterranean (Emeis et al., 2000; De Lange et al., 2008).
“Modern analogues”
In the modern ocean, there are no true analogues for open
marine black shale formation. However, a number of
“near-analogue” environments can serve as natural laboratories to study some of the physical, chemical, and biological processes that interacted in past anoxic oceans
(Demaison and Moore, 1980). The most prominent example is the Black Sea, Earth’s largest permanently euxinic
marine basin (Degens and Ross, 1974). It is characterized
by moderate primary productivity but strong salinity stratification and a very restricted connection to the Mediterranean. A similarly well-studied but less restricted and less
euxinic marine environment is the Cariaco Basin, and also
a number of fjords and bays around the world (e.g.,
Framvaren Fjord, Saanich Inlet, Kau Bay) exhibit multiannual deepwater anoxia to euxinia (Richards, 1965;
Tissot and Welte, 1984). Also coastal upwelling areas
(e.g., off Namibia, Chile, Peru, Arabian Sea) share common features with past anoxic oceans as they display high
seasonal primary productivity, leading to deepwater oxygen consumption and occasional development of sulfidic
water masses (Copenhagen, 1953). Over the past decades,
the study of geochemically similar but spatially restricted
analogues has advanced our understanding of past anoxic
oceans. A wide range of geochemical proxies is now at
hand to unravel the paleoenvironmental setting from sedimentary archives.
Environmental controls: organic matter export,
water column stratification, plate tectonics, and
astronomical forcing
Two fundamental environmental parameters are traditionally suggested as drivers of oceanic anoxia: high primary
productivity inducing enhanced organic matter export to
the seafloor and enhanced organic matter preservation by
limited deepwater ventilation, i.e., low oxygen
(Demaison and Moore, 1980). Modern end-members of
this “productivity versus preservation” debate are the
Black Sea, on the one hand (moderate productivity, severe
basin restriction, salinity stratification, absence of molecular oxygen below redoxcline), and upwelling areas, on the
other hand (full connection to the open ocean, seasonally
very high productivity). Recent studies suggest that most
black shale deposits do not allow a strict differentiation
between organic matter productivity and preservation as
dominant cause for organic carbon accumulation and
anoxia. Past examples of restricted anoxic basins include
the mid-Cretaceous proto-North Atlantic and the Paleogene Arctic Ocean. Past high-productivity regions with
anoxic water masses developed in the mid-Cretaceous
equatorial Atlantic as nutrients were “trapped” due to specific circulation patterns (Trabucho Alexandre et al.,
2010). To create and maintain marine anoxia, a range of
important biogeochemical feedback mechanisms needs
to be considered. One well-known “anoxia-productivity
feedback” is the efficient recycling of phosphate from
marine sediments if bottom-water oxygen is strongly
depleted (Ingall et al., 1993; Slomp et al., 2004; März
et al., 2008). Recycled phosphate can then be reintroduced
into the photic zone and sustain high-productivity and
organic matter export.
Although the onsets and terminations of widespread
ocean anoxia are still debated, they appear to be related
to plate tectonics and volcanism (Meyer and Kump,
2008). Astronomical forcing can regulate the intensity of
anoxia once favorable conditions have been created
(De Lange et al., 2008). Plate tectonic constellations are
responsible for the restriction of ocean basins (e.g., the
mid-Cretaceous North and South Atlantic), reducing the
renewal and oxygenation of deepwater masses (Erbacher
et al., 2001). Volcanic activity introduces large amounts
of the greenhouse gas CO 2 into the atmosphere, leading
to both global warming and enhanced chemical
weathering (Turgeon and Creaser, 2008). Warming
reduces the oxygen solubility in the oceans and the vertical mixing of its water masses, and chemical weathering
ANOXIC OCEANS
21