BILD 3 Lecture Notes - Lecture 13: Geologic Time Scale, Radiometric Dating, Extinction Event
13 Jun 2018
School
Department
Course
Professor
BILD 3 Lecture 13
5/2/2018
• Date old rocks via radiometric dating
o Some elements have unstable isotopes that has its own characteristic half-life
o The aout of the stale daughter isotope that’s preset is how you alulate
the age from the parent isotope
o When half is parent and half is daughter, that is one half-life
o Inverse exponential curve: one half-life is ½, then decays to a quarter of the
total, then an eighth of the total
o C14 is use for more recent objects
o Living organisms have a stable ratio of C14/C12 until they die, at which point C14
decays exponentially
• Changes in the fossil record
o Geological time scale:
▪ Idea developed before Darwin
▪ Based on distinctive fossil taxa
▪ Originally based on relative ages of fossils
▪ Some boundaries are mass extinctions
▪ High concentrations of a species=high population/predominates, helps to
set benchmarks for the relative evolution of other taxa in the strata
• Background extinction
o Is the extinction that occur at a normal rate
o From the fossil record: 10-100 species/year
o Due to random events that occur
o Not typically greater than the rate of new species that are forming
• Mass extinction
o Is greater than 75% of species that go extinct within a short period of time due to
some large scale event
o Most commonly known: end of the Cretaceous that ended the age of the
Dinosaurs
o Can happen over than instantaneous timescales (not just a meteorite hitting the
Earth)
o We are in the midst of what may become the 6th mass extinction
o 5 mass extinctions: Ordovician, Devonian, Permian, Triassic, and Cretaceous
• For the first 4 billion years of the planet, not a lot was happening
• Precambrian= 4.6BYA to 542MYA
• Current estimate of earliest life on Earth is ~3.8BYA
• Three evolutionary milestone during the Precambrian
o Gradual oxygen increase in the environment
▪ First environments of Earth had low O2 concentration; evidence of
oxidized iron in oldest rocks prior to saturation
▪ Therefore, it is thought that first life was anaerobic
▪ These would give rise to photosynthetic cyanobacteria
find more resources at oneclass.com
find more resources at oneclass.com
