Geosciences 308 10/31/02
Direct-dating of fossils
Next few lectures consider how fossils
are used in estimating the age of fossils and rocks, and in determining that
two rocks were deposited at the same time (even if you don't know the precise
age in millions of years).
Reminder: absolute
age (age in years)
vs relative age (older, younger, same)
Some very young fossils can be
directly-dated to assign an absolute age (in years)
1.
Growth rings: in wood
(dendrochronology) or coral (sclerochronology) or mollusks (sclerochronology).
Building
chronologies with distinctive banding patterns
Ok
back to a few thousand years
2.
Radiocarbon
There
are only a few circumstances that allow you to geochemically analyze a fossil
to directly determine its age. This is
because organic matter and hard parts, don't incorporate many of the naturally
radioactive isotopes used in dating.
With one major and very useful exception. Radiocarbon.
Radiocarbon, or carbon-14,
commonly written 14C, is a naturally occurring, continuously
produced, radio-isotope of carbon (most of which is in the stable forms of
Carbon 12 and Carbon 13). Carbon 14 is
said to be an unstable isotope in that it undergoes radioactive decay through
time to become a stable isotope, 14 Nitrogen.
That rate of decay is known,
and like the decay of other radioactive elements, it follows the exponential
decay curve: that is a constant percentage
decrease in the number of radioactive atoms per unit time.
With Carbon 14, half the
carbon 14 atoms are gone after 5,730 years.
This 5,730 years is said, therefore, to be radiocarbon's half-life. Half of what is remaining is gone in
another 5,730 years.
So, after two half-lives,
only 1/4 of the original radiocarbon content is left; after three half-lives,
only 1/8 remains, and so on.
Compared to other isotopes used in
dating, Carbon-14 has a very short half life.
It's difficult to detect any radiocarbon in samples older than 40,000
years. Thus, radiocarbon can be used
only in very young fossils: those
younger than 40,000 years.
Unlike other radioisotopes, Carbon 14 is
continuously produced in the upper atmosphere.
Here's how the system works:
Carbon 14 is generated in the upper
atmosphere by cosmic rays bombarding Nitrogen atoms. Nitrogen gives up a proton and carbon-14 is produced. This production occurs at a nearly constant
rate.
The radiocarbon produced in the upper
atmosphere is oxidized to carbon dioxide, which is then incorporated directly
into plant matter through photosynthesis, or dissolved in the ocean.
Thus, plant material like wood, takes up
carbon 14 throughout life; animals that eat plant material then also take up
radiocarbon through their lives, animals that secrete shells out of CaCO3
also take up carbon 14 through their lives.
Upon death, no new radiocarbon is taken
in and the radiocarbon already there simply decays.
Thus, the difference between the amount
in the organically produced material (wood, shell, bone) and the original
amount (assumed a constant - pretty close – see below for caliobration) is a
function of the time-since-death of the organism.
Note:
works only with organic carbon (wood, plants, material from plants,
shell, bone, organic material. Only
good to 40,000 years.
Examining/correcting
for two key assumptions:
a) constant production and b) rapid mixing between atmosphere and oceans:
a)
Constant
production rate of radiocarbon. Evaluation with tree rings
radiocarbon date the wood from individual
tree-rings of known age:
deviation of line from perfect 1:1 slope
means that radiocarbon age slightly
underestimates calendar date. Note some higher frequency wiggles too.
Corrections within historic time needed
with very young material because
--burning of fossil fuels puts more
“dead” carbon in atmosphere, thus making
living
material have a smaller proportion of radiocarbon than would be expected
--atmospheric
testing of atomic weapons put an excess of radiocarbon in the
atmosphere
(and biosphere), making post-1950 specimens appear younger than they actually
are. Radiocarbon ages reported as years
before present, with “present” defined as 1950.
b)
Rapid mixing of atmosphere and oceans. Although the radiocarbon in the
upper
atmosphere is mixed throughout the atmosphere and shallow oceans, deep ocean
waters are not well-mixed with the atmosphere
--Dates
on historically collected live specimens from the ocean: example
Gulf of California shells collected in ~1936 are 1,000 years old,
according to radiocarbon dating: Source
of carbon is from old dissolved CO2 upwelled from deep water. The “reservoir effect”. Varies from place to place, oceanwide
average is about 400 yrs.
--This so-called “reservoir effect” means
that a correction must be applied to
marine samples.
Two applications:
Samples
of fresh water peat recovered from cores on the continental shelf. The age of the youngest fresh water peat in
the core (these peats occur just above marine sediments) marks the time that
sea level rose above mean high water at that location.
Relative to present-day sea
level, the older the highest peat in the section, the deeper below present sea
level it's found.
A
sea level curve can be drawn that shows the rate of rise in sea level since
18,000 years ago – the time of the last glaciation – when sea level was at its
lowest. Sketch on board.
B. Shroud of Turin
Burial
shroud with image of man -Burial shroud
of
Jesus? But radiocarbon age of 1260-1390 AD
3.
Amino acid "dating"
Living
organic matter is composed of proteins, which are composed of amino acids. Even after death, some organic matter
persists even after the rest of the soft parts have decayed away. This organic matter is trapped inside the
shell or bones of the organism.
After
death, some of these amino acids behave in a very predictable way: two amino acids are mirror images of each
other:
L-isoleucine (left-handed)
D-alloisoleucine
(right-handed)
In
living organisms the left-handed form is present. After death, some of that L-isoleucine degrades (or racemizes) to
D-alloisoleucine.
Thus,
the ratio of alloisoluecine to isoluecine, or the A/I ratio, is a measure of
the time-since death of the organism.
Low A/I young
High
A/I older
But,
the rate at which this change takes place depends on temperature. The warmer the temperature, the faster the
reaction. So, as a dating technique, it
depends on shells or bones having the same temperature histories. Means you can't compare from one climatic
regime to another.
Can
be calibrated with radiocarbon to give absolute ages.
Has
been applied to shells as old as 1.20 million years (but poorly calibrated).
Example:
Colorado delta shells. Why the
decrease in abundance with increasing age?
-
Destruction
through time?
-
Decreased
production in the past?