Oceanic anoxia during the Permian–Triassic transition and links to volcanism - Volcanism and Global Environmental Change - page 285

Ocean circulation rate (Sv)

0 10 20 30

3

Oxygenated

2.5 Intermediate-water anoxia

Deep-water anoxia
Intermediate-water sulfidia
Deep-water sulfidia

2

1.5

Present-day representative

1

0.5

Figure 18.3 Ocean redox conditions based on changes in ocean circulation rate
(in sverdrups, Sv) and marine phosphate input

in a one-dimensional ocean

biogeochemical model (modi ed after Ozaki et al ., 2011 ). Circles represent Earth

system modelling results of end-Permian euxinia from Meyer et al .( 2008 ). Note
that the decrease in ocean circulation alone is insuf cient to cause ocean anoxia.

biogeochemical representations of the marine C, N, P, S and other cycles, future
work should test the hypothesis that Siberian Traps eruption was large enough
to cause the observed deoxygenation.
Geochronology supports the link between volcanism and the pattern of anoxia at
the Permian - Triassic transition. The intensi cation of anoxia - euxinia at the PTB

suggested by both local and global palaeoredox proxies is coeval with the initiation
of Siberian Traps eruption, especially the early explosive phase with phreatomag-
matic pipes (Mundil et al ., 2004 ; Svensen et al ., 2009 ; Burgess et al ., 2014 ).
It is not clear whether records of local-scale anoxia in the late Permian represent
the baseline redox state of late Palaeozoic oceans or enhanced deoxygenation.
Another open question is how well the later Siberian Traps eruptive history
matches the pulses of anoxia in the Early Triassic. Radiometric ages for late
Siberian Traps magmatism extend into the Spathian (Kamo et al ., 2003 ), so anoxic
pulses could be driven by later eruptive phases. However, the timing of potential
Early Triassic peaks in volcanic activity remains unknown, preventing correlation
with perturbations in marine geochemistry. The emerging view of Early Triassic
anoxia as punctuated rather than persistent

fits better with the proposed volcanic
driver because the residence time of CO 2 in the ocean - atmosphere system is on the
scale of 100 - 200 kyr (Archer et al ., 1997 ), making it dif cult to maintain elevated

rates of nutrient input for millions of years. If flood basalt volcanism was the main

trigger of Permian - Triassic anoxia, one would expect a contraction of anoxic
conditions soon after cessation of volcanism, as weathering draws down CO 2
and the climate cools. Siberian Traps volcanism can explain the widespread anoxia
and euxinia at the PTB, but its role as a potential driver for Early Triassic anoxia
deserves more attention.