Transcript Eg, fluorine dating

Fossils
• Remains of organisms (often partially or wholly replaced by
“rock”)
• Bones, teeth, shells,
• hard exoskeletons
• Usually in sedimentary rocks (water carries and drops small
particles that later formed layered rock) or in volcanic ash
sediments
Fossil primates rare
Habitat
Dating fossils
Relative dating
• Stratigraphic Correlation – matching strata
from different locations based on chemical
similarities between layers
• E.g., fluorine dating – vertebrate bones/teeth
in fluorine containing soils accumulate
fluorine over time. More fluorine = older.
• Law of Superposition
• Fossils in a lower stratum (layer) of rock are
older than those above it.
• RELATIVE AGE - older   younger
Relative Dating
• Cosmic collisions
• Evidence of a significant impact, reported in early
1980s
• Chemical evidence: iridium
Cretaceous / Tertiary boundary
95 localities
Iridium: rare on earth
common in asteroids
& meteorites
Geological evidence: crater
Asteroid 10-15 km dia
65 mya
Shift from Cretaceous to Tertiary
Epochs marked by dinosaur extinction
Significance to us?
Absolute dating fossils
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Radiometric dating
based on rates of radioactive decay
Natural conversion of an element into a different form of
that element (isotope)
1. Rates constant and independent of environmental
factors
2. Rates of decay are known
3. Amount of decay from a parent element into a
daughter isotope = a geologic clock.
Radiometric Dating
1. Determine ratio of parent
isotope to daughter isotope.
2. Convert ratio to number of halflives elapsed.
3. Multiply number of elapsed halflives X number of years it takes for
a half-life to elapse
4. This is the age estimate of that
rock.
Potassium/Argon clock
• Decay of 40K (potassium) produces 40Ar (argon
gas)
• Potassium bearing rocks.
• Igneous rock: heat drives off previously
accumulated Argon gas
• Sets the “clock” to zero
• As rock cools and solidifies, 40K continues to
decay to 40Ar which is trapped inside the rock.
• To date the rock, it is reheated and the amount of
40Ar given off is measured.
• The ratio of 40K to 40Ar permits dating the rock.
• 40K has a half-life of 1.3 billion years.
• In 1.3 billion years, 1/2 of the original 40K will
have been converted to 40Ar
• In 2.6 billion years, 1/4 of the original 40K will
remain.
• Radiocarbon Dating: 14C  14N
• 14C has a half life of 5,730 years
• Clock set to zero by death of the individual.
• Examining carbon-14 to nitrogen-14 ratio in organic
remains.
• Only if organism-derived carbon is present and only
back to 50,000 - 75,000 yBP)
Fossils found in sedimentary rocks, so fossils usually have relative dates
Absolute dating
Dendrochronology – matching tree ring width variation
Only where trees were present
Seasonal climate
Dates back to 8,000 - 12,000 yBP)
Dating estimates from genetic data
Assumption: average mutation rate is constant.
pod dinosaurs
A bird and some dinosaur relatives