Environmental Engineering Reference
In-Depth Information
Globally averaged
palaeoredox proxies
Local palaeoredox proxies
Black shales
& poorly
bioturbated
Evaporite
d
Framboidal
pyrite
Isoren-
ieratane
Ce/Ce
*
d
98/95
Mo
U ppm
d
238
U
34
S
244
246
A./B/
248
250
Smith.
Dien.
Gries.
252
PTB
254
256
258
260
262
Figure 18.2 Summary of the prevalence of anoxia through time (by proxy).
Blue
oxic oceans, grey
some evidence for anoxia
suboxia, black
strong
¼
¼
-
¼
34
S evaporite record is
evidence for widespread anoxia, white
no data. The
¼
δ
interpreted to re
ect changes in the proportion of sulfur buried as pyrite under
euxinic conditions. Note the abrupt change from more oxic to more anoxic oceans
at the PTB, especially in globally averaged palaeoredox proxies.
See also colour
con
rmed the existence of euxinic conditions in the photic zone (Grice
et al
.,
2005
;
Hays
et al
.,
2007
). Uranium and molybdenum proxies are opening opportunities
to quantify the extent of anoxia globally from data in local sections (Brennecka
et al
.,
2011
; Proemse
et al
.,
2013
).
Taken together, redox proxy data provide evidence of a clear trend toward rapid
deoxygenation across the PTB (
Figure 18.2
). New proxies that represent globally
averaged redox conditions tend to contradict the idea that widespread end-Permian
anoxia began early in the late Permian (Isozaki,
1997
). Uranium and molybdenum
records indicate generally oxic ocean conditions right up to the main extinction
horizon and a rapid expansion of anoxia at that time. Pyrite framboids, bio-
markers and cerium-anomaly records require some localized deoxygenation in
the Changhsingian, especially in deeper-water settings in Panthalassa, the Boreal
Ocean and the Perth Basin (
Figure 18.1
). In the Tethys region, typically fossilifer-
ous Changhsingian strata suggest that any dysoxic conditions were not extreme
enough to restrict animal life. By the PTB, however, the only remaining palaeo-
redox records indicating oxic conditions come from shallow-marine sections,
