Lecture 9: Crisis after crisis: mid-Palaeozoic extinctions. Flashcards
The early Palaeozoic world: atmosphere
lower O2atm levels
Sun only 95% as luminous as today
but compensated for as CO2atm ~8-16x higher than present atmospheric levels (PAL)
greenhouse conditions
Why was there sluggish circulation in the early Palaeozoic oceans?
Sluggish circulation driven by brines sinking in equatorial areas
Cambrian ‘explosion’ saw the appearance of what
but Ordovician saw…
the ‘appearance’ of many high level taxa (e.g., phyla)
- massive increase in biomass
- biodiversity and ecological complexity but mostly at level of family / genera / species
but Ordovician saw the most dramatic increase in lower level taxa in the entire Phanerozoic
What was the Great Ordovician Biodiversification Event?
massive radiation in sessile benthic suspension feeders
first extensive non-microbial reef systems
plankton and nekton diversified
e.g., acritarchs, graptolites and fish
The end of the Ordovician (Hirnantian stage), 445-444 Ma
witnessed…
what was affected?
A two-step extinction event associated with significant stable isotope excursions
nearly everything…
benthos, plankton, nekton
85% of (marine) species, 40% genera, 25% of families died
What is a “mass extinction event”?
when the rate of extinction increases above the rate of speciation
The losers in a “mass extinction event”?
most groups decimated in two waves, but not all died out synchronously
complete turnover in graptolite community
What happened in the mass extinction event”? The geological evidence: 1
massive oxygen isotope excursion: +4‰ δ18O (heavier)
δ18O changes with temperature and/or ice volume
+4‰ δ18O would imply 10°C drop in tropics - unreasonable
there must also have been ice
Evidence points to ice…
glacially striated rock pavements
diamictites / dropstones
erosion (‘incision’ due to sea level fall)
shallow water sediments: oncolites
Ice down south
map the glacial deposits on palaeogeographic reconstruction
ice sheet over supercontinent of Gondwana, centred on south pole
at same time as ice, marine faunas are very restricted: cold-adapted brachiopod Hirnantia worldwide
Why the freeze? The geological evidence: 2
large +ve excursion in δ13C in organic matter and carbonates (CaCO3)
Caledonian mountain building increased weathering on land
weathering produced HCO3-
washed into ocean, locked up as carbonates on seafloor
reduced CO2atm cooling glaciation
The geological evidence: 3: change in sedimentation
change in sedimentation
black shales before and after glaciation, pale, bioturbated muds during
Explaining the two-step extinction event
Step 1…
cooling, water locked up in ice sheets sea level fall of up to 100m
epicontinental seas shrank or disappeared, killed shallow water faunas: 1st extinction
glaciation produced vigorous ocean circulation bottom waters cooled and oxygen levels rose
surviving faunas adapted to cold, oxygen-rich waters (e.g., Hirnantia)
Explaining the two-step extinction event
Step 2…
Gondwana began to migrate away from the south pole
ice sheets began to melt
ocean circulation slowed
world ocean returned to stratified, low-oxygen state
high-oxygen survivors of the first extinction were decimated: 2nd extinction
the few survivors would go on to radiate and diversify in the Silurian period
The survivors went on the thrive
in the Silurian and the Devonian…
Devonian extinctions
still many unanswered questions
‘pulsed’ extinctions:
Givetian/Frasnian (less studied) Frasnian (‘F’) ~374.5 Ma Frasnian/Fammenian (‘F/F’), ~372 Ma Devonian/Carboniferous focus here on F and F/F
The devastated Devonian seas
Extensive reef complexes had been lost
some Palaeozoic reef-builders never recovered
the armoured fish ‘finale’
brachiopods, which had dominated early Palaeozoic benthos, lost 75% of species, never recovered
The devastated Devonian land
~ 50% of species became extinct
the first vertebrates to make land-fall were nearly all lost
hardly any arthropod fossils (e.g., insects) known from the latest Devonian
Did meteorite impacts drive the extinctions?
Alamo impact crater, SE Nevada (inside Area 51!), impact breccia shocked quartz crystals Slijan impact crater, Sweden, 52 km dia. microtektites (melted rock = glass)
Impacts can drive global cooling
impact creates dust cloud
cuts out sunlight, inhibits photosynthesis
The Devonian world: similar but different
oceans still generally low oxygen, warm, saline deep waters and black shales
on land, plants were taking over: first forests
deeper root systems extensive biological weathering creates soils
releases Ca2+, Mg2+, P, HCO3-
Carbon cycle changes
extensive deep weathering of soils releases more and more Ca2+, Mg2+, P, HCO3-
rivers transport these breakdown products to the ocean
precipitate as carbonates (limestones)
locks up CO2atm as rock
Plants drive change
second carbon cycle - on land
O2atm keeps building up (higher than today)
land plants have unbalanced the system
plants begin to suck up CO2atm
Wood and charcoal: carbon cycle changes further
Archaeopteris forests produce secondary wood
locks up more CO2atm as Corg
high O2atm levels encourage fire
wildfires burn Corg produce charcoal
very resistant, serves as a new sink for carbon
locks up more CO2atm
CO2atm plummets
