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from clastic sections are more complicated due to the local effects of bottom-water
redox changes superimposed on the global shift in seawater uranium concentra-
tions (Algeo
et al
.,
2012
; Shen
et al
.,
2012
). The sole
238
U record for the
PTB demonstrates a rapid negative isotopic shift coincident with the extinction
horizon, also consistent with a global increase in seafloor anoxia (Brennecka
et al
.,
2011
).
Studies that extend beyond the extinction horizon indicate that anoxia
continued, at least episodically, through much of Early Triassic time. Uranium
concentrations in carbonates suggest that anoxic conditions were prevalent
throughout the Griesbachian into the Dienerian (Ehrenberg
et al
.,
2008
). Con-
odont Th/U data from south China point toward multiple peaks in anoxia, in the
Griesbachian, Smithian
-
earliest Spathian, and mid Spathian, after which
the ocean became oxygenated through the Middle Triassic (Song
et al
.,
2012
).
δ
18.3.5 Molybdenum
The Mo isotope fractionations between seawater and modern marine sediments
deposited under different redox conditions allows the use of ancient sediments to
track the evolution of the Mo isotope composition of ancient oceans; to quantify
the partitioning of Mo isotopes by oxic, suboxic
-
anoxic and euxinic sinks in
ancient oceans; and, consequently, to quantify oceanic oxygen levels (Siebert
et al
.,
2003
; Neubert
et al
.,
2008
; Dahl
et al
.,
2010
).
For most of the late Palaeozoic, seawater
δ
98/95
Mo values remained at around
þ
2.0
‰
, which is slightly lower than those of modern seawater and implies a
similar oxygen content (Dahl
et al
.,
2010
). At the end-Permian mass extinction
horizon, seawater
δ
98/95
Mo values decreased to
(Voegelin
et al
.,
2009
; Silva-
Tamayo
et al
.,
2013
; Zhou
et al
.,
2012
; Proemse
et al
.,
2013
). This decrease, which
is recorded by successions located along the Tethys and the Panthalassic oceans,
has been interpreted as the global expansion of oceanic euxinia during the main
extinction event (
Figure 18.1
). Low
δ
~
0
‰
98/95
Mo values (
þ
1.5
) characterize the
Early Triassic oceans and suggest the return to anoxic conditions (Silva-Tamayo
et al
.,
2013
).
‰
18.4 Pattern of anoxia
-
a summary
High-resolution records and novel palaeoredox proxies have sharpened and altered
our picture of oceanic anoxia across the Permian
-
Triassic transition. Rather
than being prolonged and persistent, anoxia appears to have waxed and waned in
several discrete episodes during Early Triassic time (Wignall and Twitchett,
2002
;
Song
et al
.,
2012
). Biomarkers for green sulfur bacteria in PTB sediments have
