Oceanic anoxia during the Permian–Triassic transition and links to volcanism - Volcanism and Global Environmental Change - page 283

where the surface waters may have been oxygenated by wind mixing despite the
expansion of oxygen minimum zones onto the continental shelves.
After the PTB, palaeoredox constraints indicate intermittent oxic and anoxic
conditions throughout the Early Triassic moving toward more oxic conditions
in the Middle Triassic. Lithologic observations, cerium anomalies and uranium
concentrations together point toward recurring intervals of widespread anoxia in
the Early Triassic: the most widely recognized at the PTB through the Griesbachian,
another at the Smithian/Spathian boundary event, and another in the Spathian.
Apparent disagreements among proxies about the redox state of Permian -
Triassic oceans likely result from differences in sensitivity across proxies as well
as spatial heterogeneity in marine redox conditions ( Figure 18.2 ). For example,
even though globally averaged proxies support a well-ventilated late-Permian
ocean, the Peace River Basin (British Columbia) could have experienced
upwelling, photic-zone euxinia and preservation of isorenieratane due to local
controls on circulation patterns. In addition, variation in redox conditions on
geologically short timescales could create disagreements among proxies. For
example, seasonal or other short-term anoxic pulses might leave biomarker
evidence of euxinia within beds that also contain animal fossils. Most local proxies
agree with widespread anoxia at the PTB, if not the speci c timing of anoxic pulses

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in the Early Triassic. Although the S evaporite record seems to indicate

persistent anoxia throughout the Early Triassic ( Figure 18.2 ), this is likely a
consequence of low temporal resolution of this proxy, rather than a contradiction
of other, more punctuated records. Con

δ

icts among the proxies recording a
globally averaged palaeoredox signal can be resolved due to differing sensitivities
to the degree of deoxygenation; Mo is more sensitive to euxinia while U is
sensitive to anoxia, explaining the widespread episodes of Early Triassic anoxia
evident in U records but not in Mo records.
Palaeoredox proxies that record a globally averaged signal point toward a
major expansion of anoxic waters at the end-Permian extinction horizon, but these
records have not yet been developed much beyond the PTB interval ( Figure 18.2 ).
With primarily records of local geochemical conditions below the boundary,
the baseline redox state of Permian oceans remains poorly constrained. Likewise,
while there is evidence for reoccurring widespread anoxia later in the Early
Triassic, we do not yet know if these episodes were as extensive or persistent
as that at the PTB.

18.5 Volcanism and anoxia
The potential for massive volcanism to drive ocean anoxia is well established
(reviewed in Meyer and Kump, 2008 ). Because oxygen solubility is