enhances terrestrial nutrient input into the oceans (Adams
et al., 2010). Global warming leads to eustatic sea-level
rise, concomitant widespread flooding, and an expansion
of shelf sea areas. Besides the increased potential for shelf
anoxia, these shallow marine areas also trap a large fraction of siliciclastic detritus before it reaches the deep
basins. Therefore, organic-rich deepwater deposits are
often characterized by reduced terrestrial dilution
(Arthur et al., 1988).
Recognizing anoxia in sedimentary archives
As most multicellular organisms (such as benthic foraminifera) require oxygen to survive, anoxic environments
are usually strongly deprived in macroscopic life. The sedimentological expressions of this lack of burrowing
macrobenthos are finely laminated deposits without any
bioturbation.
Both organic and inorganic geochemistry provides a
number of proxies to detect anoxia/euxinia in different
parts of a water column. For instance, a biomarker to
detect euxinic conditions in the photic zone is
isorenieratene, an organic pigment exclusively produced
by phototrophic green and purple sulfur bacteria
(Sinninghe Damsté et al., 1993).
A first-order geochemical approach to reconstruct
marine anoxia was the organic carbon to total sulfur
(C/S) ratio of Black Sea deposits (Berner, 1984). While
normal, oxic marine sediments exhibited C/S ratios
around 3, the production of H 2 S by organic matter degradation in the Black Sea, and its subsequent precipitation as
metal sulfides (mostly pyrite, FeS 2 ), resulted in C/S ratios
<3. A better representation of the C-S-Fe interrelationship
is a ternary diagram with the end-members organic C,
total S, and reactive Fe, as it allows a distinction between
anoxic systems where pyrite formation was limited by
H 2 S or reactive Fe (Dean and Arthur, 1989).
Under anoxic ferruginous conditions, the sulfate depletion of the water column prevents any enrichment of sedimentary S and hence the recognition of water column
anoxia using bulk sedimentary C-S-Fe proxies (Poulton
and Canfield, 2011). To distinguish non-sulfidic from
sulfidic conditions, a sequential Fe extraction scheme distinguishes between the relative amounts of the Fe pool that
can potentially react, or already have reacted, with H 2 S
(Poulton and Canfield, 2005). In addition to C-S-Fe systematic, the selective removal of sedimentary P relative
to organic C under anoxic conditions results in increasing
sedimentary C/P ratios under increasingly oxygendepleted bottom-water conditions (Algeo and Ingall,
2007).
Among the most powerful redox proxies are trace metal
enrichments/depletions relative to their lithogenic background contents (Brumsack, 2006). Redox-sensitive
metals change their redox states under oxic, suboxic, and
anoxic conditions (e.g., Mn, U, V); sulfide-forming metals
precipitate sulfide phases/coprecipitate with pyrite if H 2 S
is available (e.g., Mo, Cd, Zn). While questions remain
in our understanding of natural trace metal systematics,
several metals have emerged over the years as reliable
redox indicators. While Mn tends to be depleted from sediments already under suboxic conditions, Re becomes
enriched within sulfidic microenvironments in the sediment. Uranium is enriched if full anoxia is reached in the
sediments, while Cd and Zn precipitate as sulfides under
weakly sulfidic conditions. Molybdenum requires stronger euxinia in the water column to be transferred to
particle-reactive thiomolybdates. The interpretation of
trace metal records is complicated by the fact that some
of these elements serve as micronutrients and are
pre-concentrated via plankton (Böning et al., 2004) before
burial. They may as well respond to specific redox conditions in the surface sediment, while others respond to
redox changes in the water column. Early diagenetic processes pose another challenge, as trace metals may be
redistributed within the sediment, biasing the primary
redox record.
Beyond element contents and speciation in marine sediments, the stable isotopic signatures of metals like Mo
and Fe are increasingly used as redox indicators (Anbar
and Rouxel, 2007). The specific value of Mo isotopes is
based on the quantitative removal of Mo from a water column under fully euxinic conditions without any isotope
fractionation. Through isotope and mass balance calculations, this ultimately gives the percentage of the global
oceanic water mass that was fully euxinic at a given point
in time. The stable Fe isotope composition of marine sediments, in conjunction with total Fe enrichment patterns, is
controlled by the input of isotopically light Fe from
suboxic shelf sediments and the shuttling and distribution
of this excess Fe within a suboxic redoxcline. Therefore,
the depth of the redoxcline in a water column and the
extent of underlying euxinic water masses can be
reconstructed (Eckert et al., 2013). However, the full
application of these isotope systems as paleo-redox proxies will require a better understanding of these trace metals
and their isotopes in the marine environment under a variety of boundary conditions.
Conclusions
Our understanding of anoxic conditions in the oceans has
greatly advanced over the past decades. The classic “productivity versus preservation” discussion sparked many
of these advances and has motivated scientists to take a
closer look at the underlying mechanisms causing, sustaining, and terminating ocean anoxia. This was achieved
by combining studies of modern and ancient anoxic
marine environments and by working across scientific disciplines. Nowadays, geoscientists can take advantage of a
large set of analytical tools and paleoenvironmental proxies that led to a better understanding of, e.g., the causes for
the development of anoxic conditions, their effects on
organisms, global element cycles and the climate system,
duration and spatial extent of anoxia and euxinia, the
degree of anoxia or euxinia, and the reasons for their
22
ANOXIC OCEANS
et al., 2010). Global warming leads to eustatic sea-level
rise, concomitant widespread flooding, and an expansion
of shelf sea areas. Besides the increased potential for shelf
anoxia, these shallow marine areas also trap a large fraction of siliciclastic detritus before it reaches the deep
basins. Therefore, organic-rich deepwater deposits are
often characterized by reduced terrestrial dilution
(Arthur et al., 1988).
Recognizing anoxia in sedimentary archives
As most multicellular organisms (such as benthic foraminifera) require oxygen to survive, anoxic environments
are usually strongly deprived in macroscopic life. The sedimentological expressions of this lack of burrowing
macrobenthos are finely laminated deposits without any
bioturbation.
Both organic and inorganic geochemistry provides a
number of proxies to detect anoxia/euxinia in different
parts of a water column. For instance, a biomarker to
detect euxinic conditions in the photic zone is
isorenieratene, an organic pigment exclusively produced
by phototrophic green and purple sulfur bacteria
(Sinninghe Damsté et al., 1993).
A first-order geochemical approach to reconstruct
marine anoxia was the organic carbon to total sulfur
(C/S) ratio of Black Sea deposits (Berner, 1984). While
normal, oxic marine sediments exhibited C/S ratios
around 3, the production of H 2 S by organic matter degradation in the Black Sea, and its subsequent precipitation as
metal sulfides (mostly pyrite, FeS 2 ), resulted in C/S ratios
<3. A better representation of the C-S-Fe interrelationship
is a ternary diagram with the end-members organic C,
total S, and reactive Fe, as it allows a distinction between
anoxic systems where pyrite formation was limited by
H 2 S or reactive Fe (Dean and Arthur, 1989).
Under anoxic ferruginous conditions, the sulfate depletion of the water column prevents any enrichment of sedimentary S and hence the recognition of water column
anoxia using bulk sedimentary C-S-Fe proxies (Poulton
and Canfield, 2011). To distinguish non-sulfidic from
sulfidic conditions, a sequential Fe extraction scheme distinguishes between the relative amounts of the Fe pool that
can potentially react, or already have reacted, with H 2 S
(Poulton and Canfield, 2005). In addition to C-S-Fe systematic, the selective removal of sedimentary P relative
to organic C under anoxic conditions results in increasing
sedimentary C/P ratios under increasingly oxygendepleted bottom-water conditions (Algeo and Ingall,
2007).
Among the most powerful redox proxies are trace metal
enrichments/depletions relative to their lithogenic background contents (Brumsack, 2006). Redox-sensitive
metals change their redox states under oxic, suboxic, and
anoxic conditions (e.g., Mn, U, V); sulfide-forming metals
precipitate sulfide phases/coprecipitate with pyrite if H 2 S
is available (e.g., Mo, Cd, Zn). While questions remain
in our understanding of natural trace metal systematics,
several metals have emerged over the years as reliable
redox indicators. While Mn tends to be depleted from sediments already under suboxic conditions, Re becomes
enriched within sulfidic microenvironments in the sediment. Uranium is enriched if full anoxia is reached in the
sediments, while Cd and Zn precipitate as sulfides under
weakly sulfidic conditions. Molybdenum requires stronger euxinia in the water column to be transferred to
particle-reactive thiomolybdates. The interpretation of
trace metal records is complicated by the fact that some
of these elements serve as micronutrients and are
pre-concentrated via plankton (Böning et al., 2004) before
burial. They may as well respond to specific redox conditions in the surface sediment, while others respond to
redox changes in the water column. Early diagenetic processes pose another challenge, as trace metals may be
redistributed within the sediment, biasing the primary
redox record.
Beyond element contents and speciation in marine sediments, the stable isotopic signatures of metals like Mo
and Fe are increasingly used as redox indicators (Anbar
and Rouxel, 2007). The specific value of Mo isotopes is
based on the quantitative removal of Mo from a water column under fully euxinic conditions without any isotope
fractionation. Through isotope and mass balance calculations, this ultimately gives the percentage of the global
oceanic water mass that was fully euxinic at a given point
in time. The stable Fe isotope composition of marine sediments, in conjunction with total Fe enrichment patterns, is
controlled by the input of isotopically light Fe from
suboxic shelf sediments and the shuttling and distribution
of this excess Fe within a suboxic redoxcline. Therefore,
the depth of the redoxcline in a water column and the
extent of underlying euxinic water masses can be
reconstructed (Eckert et al., 2013). However, the full
application of these isotope systems as paleo-redox proxies will require a better understanding of these trace metals
and their isotopes in the marine environment under a variety of boundary conditions.
Conclusions
Our understanding of anoxic conditions in the oceans has
greatly advanced over the past decades. The classic “productivity versus preservation” discussion sparked many
of these advances and has motivated scientists to take a
closer look at the underlying mechanisms causing, sustaining, and terminating ocean anoxia. This was achieved
by combining studies of modern and ancient anoxic
marine environments and by working across scientific disciplines. Nowadays, geoscientists can take advantage of a
large set of analytical tools and paleoenvironmental proxies that led to a better understanding of, e.g., the causes for
the development of anoxic conditions, their effects on
organisms, global element cycles and the climate system,
duration and spatial extent of anoxia and euxinia, the
degree of anoxia or euxinia, and the reasons for their
22
ANOXIC OCEANS