LECTURE 6

Question 1

cretaceous - palogene extinction (65 mya)

K/Pg

Answer 1

the dinosaur killer

• at least 50% of all species lost
• nothing over 25kg survived on land
• larger body = needed more to survive
• one exception = crocodile/alligator = generalists - not fussy eaters and can survive without food for a while
• 80-90% of marine species
• includes marine reptiles, ammonites, mosasaurs, plesiosaurs

Question 2

K/Pg boundary

Answer 2

• mass extinctions changes in the fossil record
• rock layers deposited 65mya record the changes
• looks like a thick black band in the rock
• palogene = sandstone sediments
• cretaceous = eg mudstone

Question 3

causes of the K/Pg extinction

alvarez hypothesis

Answer 3

• thought to be impact of large asteroid
• found high iridium levels at the K/Pg contact
• iridium is rare on earth
• huge spike to 3.7ppb (20x normal concentration)
• typically 0.2ppb
• recorded globally
• iridium known to exist in higher concentrations in extra-terrestrial objects
• so thinks the object must have been at least 10km in diameter

further evidence:

• soot layers associated with iridium layers = massive global fires
• fern spores = first to colonise a fire-impacted area
• tektites = natural glass produced by melting rocks during impact = shows large impact
• shocked quartz = planar deformation/lamellae
• tsunami deposits
• the crater = drilling off coast of Yucatan Peninsula, Mexico reveals odd rocks = suevite
• suevite = rock of brecciated material (glass and rock) glass = partial melting and rapid cooling

Question 4

chicxulub impact crater

Answer 4

• geophysical surveys
• shows earths gravitational field
• gravity = proxy to rock density
• see concentric rings (like a crater structure)

Yucatan Peninsula, Mexico

• seismic refraction tomography shows density of ocean bottom layer
• shows that sediment had filled in the crater
• crater size = 180-240km diameter
• meteor size = about 10km in diameter
• large complex crater:
  • concentric ringed structure
  • central peak or peak ring

Question 5

impact

Answer 5

• meteor was ~10km diameter
• shallow angle of entry (20-30degrees)
• energy of impact = 6.2x107 tonnes of TNT
• 100km3 rock vaporised
• air blast >1500km radius
• ejecta deposited north west of the impact site
• winds in excess of 1000km per hour near the impact site possible
• ejecta would have been several hundred meters thick near impact
• along campeche bank, 350-600km from chicxulub, impact deposits of ~50 to ~300 meters have been logged in boreholes