history of our planet week 4.1

extinction. triggers of snowball earths and mass extinction events

organisation of time

hadean at start, quaternary currently

time segments

eon, era, period, epoch, age

causes of mass extinctions - external triggers

extraterrestrial impact, massive volcanism

causes of mass extinctions - internal feedbacks

global climate change, ocean anoxia and acidification

mass extinctions defined for marine animal genera

genera = class of species

can seen marine better due to fossils

big 5 extinction

End Ordovician (O-S)

Late Devonian

End Permian (P-Tr)

End Triassic (Tr-J)

End cretaceous (K-P)

  + present day

late permian model

end permian mass extinction

Extinction of families:

63% land (over 75% species)

Only things living for a number of years were ferns (tall – pleuromeia fossils)

49% marine (over 90% species lost)

61% total

end permian mass extinction on land

The lystrosaurus mammal- like shovel lizard – nearly the only 4 legged creature living through this period

o   Mutagenesis of plants

o   Forests lost for 5 Myr

§  Replaced by the lycopsids including ferns

§  Extensive erosion

end permian mass extinction Volcanic event:

End Permian saw massive volcanic eruptions of the Siberian traps – a large igneous province

Spewed our 2million km^3 of lava covering 1.6 million^2 to a depth of us to 3000m

volume was erupted in less than 1 million years

Recent study says 60,000 yr (a geological instant)

Believe lava was being pushed into vast beds of coal, setting coal on fire – releasing more co2 as the coal beds are being ignited and making its way to the atmosphere

end permian mass extinction- Carbon cycle perturbation:

Volcanic eruptions release co2 and lava intruded into coal beds igniting them and releasing more co2

Carbon isotope record shows the release of large amounts of isotopically light (^13C-depleted) material

Either organic carbon or methane

Evidence for extreme global warming of ~10*C

end permian mass extinction- Ocean acidification:

When large amounts of CO2 are added to the atmosphere and dissolve in the ocean this acidifies it

This inhibits the formation of calcium carbonate shells (calcium carbonate shells required for calcifying organisms which are important for photosynthesis)

Key for carbon cycle and marine food chains

The second phase pf extinction was particularly bad for calcifying organisms

end permian mass extinction- Ocean anoxia:

Extinctions suggest that mush of the pecan became anoxic (dissolved oxygen)

E.g. loss of shelf bottom dwelling communities

Anoxic and euxinic – toxic H(2)S

Euxinic - Lots of hydrogen sulphide

What generate anoxia?

Increase in oxygen demand due to increases phosphorus input from land

Decrease in oxygen supply due to warming reducing oxygen solubility

In the sediments there are lots of different elements locked up in mud. One of these in sulphur (can be stable), but with very low oxygen conditions at the bottom of the sea, that water sediment interface has a chemical reaction and sulphur starts to leak out. Means there was a reltively narrow range of depth at which any animal could have lived bc too high = too hot and too low = no oxygen or too much hydrogen sulphide

end cretaceous world

Lump of rock size of Manhattan ~10km radius impacted earth

end cretaceous mass extinction

60% of all genera became extinct

Extinctions occurred very rapidly

Unusual in its selectivity:

Large land animals suffered extinction, most famously the (non-avian) dinosaurs

But those who could shelter in water/soil/burrows or who had larvae/eggs underground (e.g. crocodiles) survived

Evergreen vegetation suffered more than higher-latitude deciduous vegetation

end cretaceous world - impact event

Hypothesised 1980 by walter alvarez based on global iridium (Ir) anomaly (only could have happened from a large meteor)

1991- alan hildebrand published evidence of a 180km wide impact crater at Chicxulub dated at 65Million years ago

Estimated to be caused by a 10km diameter asteroid

end cretaceous world - volcanic event

End cretaceous saw massive volcanic eruptions of the deccan traps

A large igneous province

800 Kyr basalt outpouring emplaced 365,000km^3 of lava

Eruptions were underway before the impact event, warming the climate – putting the earth in a more vulnerable state

end cretaceous world -The ‘Strangelove’ ocean

Ocean photosynthesis (primary production) was wiped out

Calcareous nanoplankton (marine phytoplankton) suffered 90% species loss only 12 species survived

Globally synchronous extinction

Supports ‘impact winter’ hypothesis of dark, cold conditions with long periods without sunlight

Photosynthetic activity largely stops working

end cretaceous world -Effects on the carbon cycle:

The biological pump takes up CO2 from the atmosphere and transfers it to the deep

Shutting this down caused a 2-3x increase in atmospheric CO2

This added to CO2 from deccan traps eruptions causing global warming

end cretaceous world - Oxygen catastrophe:

Oxygen was toxic to most microbial life at the time

Anaerobic habitats on land and in the surface ocean largely disappeared

But the formation if the ozone layer offered new protection from the UV radiation

Methane was removed from the atmosphere contributing to the glaciations including the first ‘snowball earth’ 2.2 Ga

how cold during snowball earth?

end cretaceous world - runaway feedback

Cooling leads to ice advance

More ice reflects more sunlight

Less sunlight absorbed

More cooling…more ice advance…

Once ice crosses a critical latitude (~30 degrees)

Runaway - snowball earth

end cretaceous world? snowball earth- when did it happen

~2220 Ma (makganyene)

Long gap then:

~710 Ma (sturtian)

~ 640 Ma (marinoan)

end cretaceous world? exit from snowball earth

Trigger: co2 build up

Amplifier: reverse ice-albedo feedback

Should produce a very hot, high CO2 world in the aftermath

end cretaceous world? snowball earth- Cap carbonates:

These occur right after the glaciations (i.e above them in the geological strata)

They are explained in terms of mixing a high CO2 atmosphere with an ocean full of alkaline ions (e.g. Ca^2+) to form carbonates

they are enormous, consistent with a huge amount of CO2 being removed

end cretaceous world? snowball earth- What triggered the events:

Geologists link snowballs to the break up of supercontinents (e.g. redinia)

Basalt outpouring produced a highly weatherable surface

Increased runoff promoted high weathering rates and co2 drawdown

is this how Gaia works?