Lecture 9: Crisis after crisis: mid-Palaeozoic extinctions.

Question 1

The early Palaeozoic world: atmosphere

Answer 1

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

Question 2

Why was there sluggish circulation in the early Palaeozoic oceans?

Answer 2

Sluggish circulation driven by brines sinking in equatorial areas

Question 3

Cambrian ‘explosion’ saw the appearance of what

but Ordovician saw…

Answer 3

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

Question 4

What was the Great Ordovician Biodiversification Event?

Answer 4

massive radiation in sessile benthic suspension feeders
first extensive non-microbial reef systems
plankton and nekton diversified
e.g., acritarchs, graptolites and fish

Question 5

The end of the Ordovician (Hirnantian stage), 445-444 Ma
witnessed…

what was affected?

Answer 5

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

Question 6

What is a “mass extinction event”?

Answer 6

when the rate of extinction increases above the rate of speciation

Question 7

The losers in a “mass extinction event”?

Answer 7

most groups decimated in two waves, but not all died out synchronously
complete turnover in graptolite community

Question 8

What happened in the mass extinction event”? The geological evidence: 1

Answer 8

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

Question 9

Evidence points to ice…

Answer 9

glacially striated rock pavements
diamictites / dropstones
erosion (‘incision’ due to sea level fall)
shallow water sediments: oncolites

Question 10

Ice down south

Answer 10

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

Question 11

Why the freeze? The geological evidence: 2

Answer 11

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

Question 12

The geological evidence: 3: change in sedimentation

Answer 12

change in sedimentation

black shales before and after glaciation, pale, bioturbated muds during

Question 13

Explaining the two-step extinction event

Step 1…

Answer 13

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)

Question 14

Explaining the two-step extinction event

Step 2…

Answer 14

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

Question 15

The survivors went on the thrive

Answer 15

in the Silurian and the Devonian…

Question 16

Devonian extinctions
still many unanswered questions
‘pulsed’ extinctions:

Answer 16

Givetian/Frasnian (less studied)
Frasnian (‘F’) ~374.5 Ma
Frasnian/Fammenian (‘F/F’), ~372 Ma 
Devonian/Carboniferous
focus here on F and F/F

Question 17

The devastated Devonian seas

Answer 17

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

Question 18

The devastated Devonian land

Answer 18

~ 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

Question 19

Did meteorite impacts drive the extinctions?

Answer 19

Alamo impact crater, SE Nevada (inside Area 51!), 
impact breccia
shocked quartz crystals
Slijan impact crater, Sweden, 52 km dia.
microtektites (melted rock = glass)

Question 20

Impacts can drive global cooling

Answer 20

impact creates dust cloud

cuts out sunlight, inhibits photosynthesis

Question 21

The Devonian world: similar but different

Answer 21

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-

Question 22

Carbon cycle changes

Answer 22

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

Question 23

Plants drive change

Answer 23

second carbon cycle - on land
O2atm keeps building up (higher than today)
land plants have unbalanced the system
plants begin to suck up CO2atm

Question 24

Wood and charcoal: carbon cycle changes further

Answer 24

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

Question 25

Colder and colder…

Answer 25

climate cools and dries
latitudinal faunal zones contract, narrow
further cooling drives extinction of faunas adapted to warm conditions…
descent into the icehouse…
(probably a very complex series of successive events, which may have included massive eruptions and mountain building too)

Question 26

What did greenhouse conditions mean for circulation of early Palaeozoic oceans?

Answer 26

greenhouse conditons meant ocean circulation was very different oceans: no “global conveyor belt”

Question 27

What were the early Palaeozoic oceans like?

Answer 27

nutrient-rich, oxygen-poor, warm saline deep waters

high sea-level, vast shallow seas, little land (equator and S. pole)

Question 28

How many major mass extinction events in the Phanerozoic?

Answer 28

5 major mass extinction events in the Phanerozoic
end-Ordovician the second most severe in last 541 My
biggest extinction: end-Permian (“The Great Dying”, 252 Ma)