Oceanic anoxia during the Permian–Triassic transition and links to volcanism - Volcanism and Global Environmental Change - page 284

temperature-dependent, the release of volcanic CO 2 and resulting greenhouse
warming decrease oxygen supply to ocean water. In addition, global warming
enhances the hydrologic cycle, continental weathering and nutrient delivery
to the oceans (e.g. phosphate), which, in turn, stimulates biological production
and increases oxygen demand. The development of anoxic conditions creates a
positive feedback, liberating more phosphate from sediments and increasing
nutrient fluxes (Van Cappellen and Ingall, 1994 ).

Numerical Earth system modelling has been used to investigate the factors that
led to widespread ocean anoxia during the Permian and Triassic. While ocean
stagnation has long been linked to anoxia in the palaeoceanographic literature,
most modelling studies have shown that end-Permian stagnation was physically
unlikely. In an early review of modelling studies, Winguth and Maier-Reimer
( 2005 ) concluded that the end-Permian oceans were likely well-ventilated and that
anoxia was more likely a result of changes in carbon cycling than the physical
mixing of the oceans. A fully coupled, high-resolution global climate simulation
of end-Permian conditions also showed reduced, but not absent, overturning
circulation (a 25% increase in the ideal age of water at 3000 m), and that marine

biotic changes were likely in

~

uenced by rapid increases in atmospheric p CO 2

(Kiehl and Shields, 2005 ).
Many numerical models require elevated nutrient contents of the ocean to
generate anoxic conditions. In multiple studies, anoxia and euxinia are more
widespread in the Palaeo-Tethys Ocean than the Panthalassic Ocean due to the
nutrient trapping circulation of the Tethys Ocean (Meyer et al ., 2008 ; Winguth and
Winguth, 2012 ). Simulations using Earth system models of intermediate complex-
ity that also included a marine sulfur cycle required at least a tripling of phosphate
(the limiting nutrient over long timescales; Tyrrell, 1999 ) to generate PZE in the
Palaeo-Tethys Ocean and in areas of upwelling ( Figure 18.3 ; Meyer et al ., 2008 ).
However, the extent of oxygen depletion and hydrogen sul

de build-up varies
widely between model parameterizations. At least one model generates oxygen
depletion but no extreme anoxia - euxinia even under 10 modern phosphate
concentrations (Winguth and Winguth, 2012 ). However, primary production in
this model was limited by Fe supply, and euxinia would have surely resulted from
a combination of high Fe and high phosphate conditions.
Siberian Traps volcanism makes an attractive trigger for PTB anoxia, because
of the potential for rapid, massive CO 2 release (Kamo et al ., 2003 )and
weathering - nutrient feedbacks. There have been multiple efforts to estimate
total CO 2 release from the volume of erupted basalt and degassing country rocks
(see Chapters 10 - 12 ) . However, we have yet to quantify the corresponding
amount and duration of greenhouse warming, or the expected increase in
nutrient delivery to the oceans. Using Earth system models that incorporate