Environmental Engineering Reference
In-Depth Information
Table 18.1 Sensitivity of various proxies to paleoredox conditions. The categories for
degree of deoxygenation are: suboxic (0.3
1.5 mL/L O
2
), dysoxic (0.1
0.3 mL/L O
2
),
-
-
anoxic (0
0.1 mL/L O
2
) and euxinic (anoxic and sul
dic) (Kaiho,
1994
).
-
Degree of
deoxygenation
Water depth of
deoxygenation
Spatial extent of
deoxygenation
Proxy
Poorly bioturbated to
laminated shales
Dysoxic to
anoxic
Bottom water
Local
Framboidal pyrite
Euxinic
Water column
Local
Isorenieratane and
aryl isoprenoids
Euxinic
Photic zone
Local
Sulfur isotopes
Euxinic
Unclear
Unclear
Cerium anomaly
(Ce/Ce
Suboxic to
anoxic
Bottom water
Local
*
)
Uranium concentration
Anoxic
Bottom water
Local/global
*
Uranium isotopes
Anoxic
Bottom water
Global
Molybdenum isotopes
Anoxic to
euxinic
Bottom water
Global
*
Interpreting the spatial extent of deoxygenation from uranium concentrations depends
on context. Anoxic facies can record local changes in anoxia and uranium preservation,
but shallow-marine carbonates may approximate global ocean changes in uranium
concentration and the extent of oceanic anoxia.
known from PTB sections, especially in the Tethys region (e.g. Logan and Hills,
1973
), and they have been used to implicate ocean anoxia as a cause of the
end-Permian mass extinction (Hallam,
1989
). Numerous PTB sections from
the Tethys and Panthalassic Oceans exhibit lithological characteristics typically of
dysoxic to anoxic settings, such as reduced intensity of bioturbation and sizes
of burrows, increased abundance of framboidal pyrite and increased prevalence of
black shales (reviewed in Wignall and Twitchett,
2002
). In shallow-water sections,
the onset of these indicators is typically coincident with the main extinction interval
in the fossil record (e.g. Wignall and Hallam,
1992
; Wignall and Twitchett,
1996
).
However, several lines of evidence suggest that the variation in ocean redox chemis-
try was temporally and spatially complex. Changes in the lithology and chemistry of
deep-marine shales from Japan and British Columbia suggest that the onset of anoxia
was more gradual, beginning near the middle
-
late Permian transition, peaking in
intensity near the PTB, and waning gradually through Early Triassic time (Isozaki,
1997
). Variation in ichnofauna and ichnofabric across a palaeo-depth gradient in
northwest Canada indicates shallow-water environments were typically better oxy-
genated than deeper-water environments during the earliest Triassic (Griesbachian)
time (Beatty
et al
.,
2008
). Precipitation of carbonate crystal fans in outer shelf
deposits of the uppermost Lower Triassic (Spathian) Virgin Limestone in the western
USA points toward local persistence or recurrence of anoxic conditions even several
million years after the main extinction event (Woods
et al
.,
1999
).
