Earth Sciences 2240F/G Lecture Notes - Lecture 7: Luis Walter Alvarez, Seafloor Spreading, Mesozoic
11 Jun 2018
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End Cretaceous Mass Extinction 6/12/2018 12:49:00 AM
1.0 The Cretaceous Period of the Mesozoic Era
• Mesozoic dominated by tectonic effects of the breakup of Pangaea
• Rapid sea floor spreading due to opening of Atlantic in cretaceous
produced high mid ocean ridges
• Displaced water from basins and produced huge continental
seaways
• Throughout cretaceous, sea levels high and there was an
abundance of shallow warm seas again
• First half of cretaceous, global temp was at least 8C of that today,
the greatest greenhouse period with highest atmospheric CO2 since
Precambrian
• Most prolific period earth ever experienced
• End of cretaceous, 65-75% al all species (marine and terrestrial)
died
o What survived? Mainly mammals, insects, birds, flowering
plants, corals and some fish
• Gradualists say took place over period as great as 1 my,
catastrophists say critical time between 1 and 1000 years
• Speculation not helped by:
o Incomplete fossil records
o Incomplete preservation of fossils
o Reworking of sediments by burrowers
2.0 The KT Boundary
• second worst mass extinction since Precambrian times
• 65.5 mya at KT boundary
• most prominent was that of nearly all dinosaurs and that of large
reptilian swimmers in oceans, most devastating was the dramatic
decline in small plants both terrestrial and marine at the base of the
food chain, without them organisms all along the food chain
disappeared
• 1980, Luis Alvarez and son found unbroken rock sequence from
Cretaceous to Tertiary
o found extraordinarily high values for the element iridium (Ir)
only in that thin layer of clay marking the exact time
boundary (which is accepted to be a sensitive tracer of
extraterrestrial matter become of its strong depletion in
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End Cretaceous Mass Extinction 6/12/2018 12:49:00 AM
earth’s crust (0.05ppb) as compared to chondritic meteorites
(500 ppb), this clay layer contained 30 ppb
o first evidence that extinction had anything to do with
sometime out there
• everywhere in world that KT boundary layer has been sampled has
found this Ir anomaly
• calculated that found values could only have come from an asteroid
measuring at least 10 km in diameter
• concluded that such an asteroid impacting earth would have
produced enough dust in atmosphere to shut off global
photosynthesis for several years, with resulting collapse of food
chain
• Two holes in theory:
o No impact site of sufficient size was known
o There apparently was a perfect correlation with an enormous
eruption of lava in India (called the Deccan Traps) which
many scientists believed could have supplied the atmospheric
dust, CO2 and SO2 and even Ir (if magma came from core
mantle boundary)
• Mixed with clay at many localities were
o Tiny glass beads (tektites)
o Fragments of minerals showing extensive shock features
o Lots of carbon (apparently representing soot form burned
vegetation)
• Dr. Alan Hildebrand adjunst prof of UWO did Hard science to define
character of KT boundary and to define crater itself
o Discovered fireball layer, the top layer of the KT boundary
layer, 3 mm thick and represents 1500 cu km of carbon rish
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