Are the Siberian traps
one of Earths biggest killers?
The end Permian or
Permian-Triassic mass extinction around 252mya, is marked by distinct losses of
biodiversity in both terrestrial and marine realms. An estimated 52% of all families went
extinct, with 96% of all marine species and 70% of all terrestrial species such
as insects; plants and vertebrates vanished in a geological instant. The marine
realm had significant loss of suspension feeders and carnivores (bryozoan,
crinoids, brachiopods and foraminifera) and almost all the reef dwellers. It
remains the largest mass extinction event recorded in Earths history. Much like
the Cambrian explosion the Permian-Triassic (P-T) mass extinction is still
being debated today as there is still not one universally excepted hypothesis.
There is evidence
that a negative stable carbon isotope excursion of 3‰ - 6‰ was
roughly synchronous with the mass extinction suggesting a major shift in the
global carbon cycle. Proposed mechanisms
of this excursion include the reduction of primary productivity, oxidation of
sedimentary organic matter, volcanic degassing, burning of forests, outbursts
of methane from methane hydrates or a combination of all these processes.
Carbon dioxide is a greenhouse gas and has a long average lifetime in the
atmosphere, it has the ability to accumulate over periods of time and increase
the average global temperature as it absorbs long wave radiation. Nonetheless,
this carbon isotope shift is indicative of either a global drop in photosynthesis
or global warming, or even both. A drop in photosynthesis expresses a lack of
carbon fixation by plants and a disruption to the biosphere. This is exhibited
by the lack of coal beds for around 6mya afterward. Ultimately less carbon
dioxide was extracted from the atmosphere causing further increases in the
global temperature. But what could cause a drop in photosynthesis?
Carbon excursion on the P-Tr boundary
All organisms have
a tolerance to certain environmental conditions they are able to live in.
Abiotic factors such as temperature, salinity, nutrient input, soil,
precipitation and in particular sunlight availability for photosynthetic
organisms, affect whether an organism will be able to survive in an environment.
A drop in photosynthesis across the P-T boundary demonstrates that the abiotic
environment has undergone changes, which have resulted in the significant loss of
photosynthesizing organisms. It could have been changes that the organisms were
intolerant to e.g. high temperatures or less light may have been penetrating to
the surface because of volcanic emissions. This drop in primary productivity in
the biosphere means animals higher up the food chain become susceptible to
changes in the food supply as the number of phytoplankton and land plants
decrease.
In conjunction
with the increase in CO2 and drop in photosynthesis
there is also evidence for abnormally high ocean and air temperatures, ocean
acidification and widespread ocean anoxia. These changes can come in
association with the rapid addition of greenhouse gases. So what could cause a
rapid addition of greenhouse gases to the atmosphere over a short geological
time period?
Links between the atmosphere, hydrosphere, and biosphere showing ocean
acidification
It is a known fact
that there is a strong correlation between continental flood basalts and mass
extinctions. Three of the largest mass extinctions recorded in Earth’s history
strangely coincide with large outpourings of basaltic magma in a continental
flood basalt regime (CFB). The Permian-Triassic extinction was synchronous with
the Siberian Traps, the Triassic-Jurassic extinction with the Central Atlantic
Magmatic Province and the Cretaceous-Tertiary extinction with the Deccan Traps.
Is this pure chance?
Paleogeography of the end-Permian
The eruption of
the Siberian Traps is associated with or possibly promoted by rifting
(east-west extension) of the West Siberian Basin during the Permian-Early
Triassic. The magma is thought to be associated with a hot mantle plume and the
decompression melting that occurs as you decrease the lithostatic load over an
area during a rifting event. Eruptions of the Siberian Traps began roughly
300,000 years before the P-T boundary and occurred during and after the mass
extinction. Enough magma was erupted to cover an area the size of the United
States with magma kilometres thick. Approximately two-thirds of this magma
erupted during and prior to the P-T boundary with the remainder erupting during
the following 500,000 years. Radiometric dating suggests the eruptions
continued for around 1mya, which fits the hypothesis of rapid addition of
greenhouse gases to our atmosphere. Because as magma erupts it brings to the
surface gases that are incompatible with silicate minerals e.g. CO2and H2O, so they get released into the atmosphere. Extensive volcanism over
a short period could therefore explain the CO2 excursions. The rising of magma could also have induced the release
of methane from methane hydrates or the evoking of polar gas hydrate release in
permafrost regions due to earth warming.
The eruption of
the Siberian Traps and release of a high volume of greenhouse gases is thought
to have had runoff effects in the atmosphere, hydrosphere and the biosphere. The
uptake of atmospheric greenhouse gases CO2 causes ocean acidification and the amount of CO2 has particular controls on whether calcite will be precipitated or
dissolved.
Equilibrium equation for
the precipitation and dissolution of calcite or aragonite
The addition of CH4 and CO2 also produces acid rain when it reacts with water. The warmer temperatures accelerate the
hydrological cycle, which causes a greater amount of corrosion on land and
nutrient runoff into surrounding shallow waters. Sufficient nutrient input
causes primary production in the photic zone to increase. As the primary
producers die and fall to the bottom of the ocean, bacteria begin to deplete
available oxygen as they decompose organic material. Over time this leads to
the formation of an oxygen minimum zone and anoxic deeper waters. Its likely
that ocean warming weakened the thermal gradient within oceans leading to a
more stratified ocean which further accentuated the absence of oxygen in deeper
waters due to a lack of overturning. Ocean
warming makes it easier to achieve anoxic states because warm water has a lower
capacity to hold dissolved oxygen. During an anoxic event, if certain bacteria
are consuming organic matter in the absence of oxygen they produce a toxic gas H2S (hydrogen sulphide). In
certain concentrations most organisms can tolerate small quantities of H2S but if concentrations pass a
certain threshold then it becomes toxic. It is suggested by some that the
widespread anoxia caused immense volumes of H2S gas to bubble out of the
ocean and contribute to the Permian-Triassic extinction.
Current
understandings of ecosystems proves that a lot of organisms can’t adapt quickly
enough to severe shifts in environmental conditions. And if they do organisms
can only withstand a certain threshold when eventually conditions become
unliveable or toxic. The simultaneous episodes of mass extinction and global
warming during the end-Permian give strong evidence to the collapse of both
marine and terrestrial ecosystems being somewhat associated with the carbon
excursion. The producer of greenhouse gases in the end-Permian must have had
the ability to produce copious volumes of CH4 and in particular CO2. The Siberian Traps most likely had the capacity to do that. Other
hypothesis swaying from the Siberian Traps include the assemblage of the
supercontinent Pangaea. It’s thought that a multitude of shallow marine basins,
which was the dominant habitat for most marine invertebrates, were destroyed
when the continents moved together and as this was happening ocean currents
were channeled and deflected. Thus changing the ocean circulation and therefore
regional climate systems. This altered the net primary production occurring on
earth and generated a carbon excursion. Another hypothesis dismisses the
volcanogenic gas release and identifies the trigger as the end-Permian mantle
plume. Suggesting that as it rose due to rifting and decompression melting, it
heated sediments and methane hydrates under Siberia, which invoked the release
of methane gas.
Both these
alternative hypothesis however still support the idea tectonics could be an
instigator in the Permian-Triassic mass extinction. The eruption of the
Siberian traps gives evidence of the complex relationships between Earth’s
spheres (in this case: atmosphere, hydrosphere and geosphere). In this case the
geosphere (eruptions) warmed the atmosphere, this affected the hydrosphere and
the biosphere by warming, increased acidic rain and ocean warming leading to
anoxia. This then linked back to the atmosphere as the lack of photosynthesises
meant lower amounts of carbon extracted from the atmosphere and further
warming. It proves tectonic process’ may have bigger implications on life than
we previously thought, because potentially in this case they have not created
life but destroyed it.
The only question
left on our minds now is, if without the tectonics rifting apart the West Siberian
Basin, would there have ever been an event or a process that had the capability
of producing enough greenhouse gases to promote the cascading collapse of
entire ecosystems. Would the end-Permian mass extinction ever have occurred if
tectonics didn’t instigate volcanic activity?
Josephine Turnbull
REFERENCES:
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likely culprit for end-Permian extinction. Retrieved from
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Britannica. (2014). Permian extinction.
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