Tree Rings, Carbon Dating and the Age of
the Universe
by
Kenny A. Chaffin
All Rights Reserved © 2013 Kenny A. Chaffin
How do we measure time? With a
clock of course, but what is that? It’s really just a defined amount of movement
which repeats at regular intervals like a pendulum or a slowly rotating pointer.
The first time piece we used may have been our own beating hearts. Certainly
the Sun and Moon in their regular cycles, solstices and equinoxes have forever
in our history, eyes and brains been used to measure time. The seasons of
course were used as well to predict animal migrations and for agriculture. But
how do we know the age of the Universe, the age of the Earth, fossils or other
things on it when we were not there with our stopwatches in hand? We have only
an incomplete recorded history for the last few hundred years, very sketchy
records and artifacts before that. But by using the natural time cycles,
processes and events in nature we can obtain ages for most things in the
universe, including the universe itself.
Each year trees grow in summer and
go dormant in winter. This growth adds a distinctive and easily seen layer of
cells around the trunk of the tree and when seen in cross-section appears as
concentric rings. Certainly this was noticed well before writing and one of our
smarter ancestors realized these rings were due to annual growth and that each
ring not only represented a year but the growing conditions during that year.
By counting the rings we can tell how many years the tree has grown. We can
also tell if it was a good year or bad year climate-wise. In good years the
growth layer will be thicker due to the favorable conditions. In lean years the
growth layer will be thinner.
Tree
Rings from a one meter wide trunk.
Trees of course do not live forever
so how far back can we go with this type of dating? The oldest tree we know of is
a bristlecone pine that is 5063 years old. From this tree alone we can tell
something about the climate 2000 years ago during the time of Buddha, or the
conditions during the Battle of Thermopylae. In addition by matching up the
thick/thin pattern of the rings in samples from different trees either living
or dead we can extend our tree ring dating 11,000 years into the past. Beyond
that, samples of wood living, dead or petrified that includes rings capable of aligning
are few and far between. Still this could be extended further into the past if
we find well preserved samples, fossils, etc. Oh and in case you’re wondering
if someone cut down that 5000 year old bristlecone pine to count its rings in
its trunk, the answer is -- thank goodness, no. We can take a ‘core sample’ from
the tree without any serious damage by using a hollow drill and removing just a
small cross-section from the trunk of the tree without harming it. This is the
same method we use to take ocean and arctic ice core samples.
Another method of determining age
of organic samples is through carbon dating. And that doesn’t mean carbon-based
life-forms hooking up on Facebook. It works only with samples from dead plants
or animals all of which contain carbon and it relies on measuring the relative
ratios of carbon isotopes in the sample. Willard Libby received the Nobel Prize
for chemistry in 1949 for discovering this technique. It can measure as far
back as 60,000 years. The reason it relies on dead organic matter is that a
living organism is constantly metabolizing (flushing out old carbon in waste
and replacing it with new carbon from the environment) and will have an equal
ratio of carbon-12 (C12) to carbon-14 (C14). When that organism
dies the carbon is not replaced by bodily functions. The C14 is
radioactive with a half-life of 5,730 ± 40 years and will gradually decay to Nitrogen
over time changing the ratio of C12 to C14. By measuring the ratio of the carbon isotopes we
can get a reasonably accurate date of death. This method works great for
determining the age of bones and plant materials. Despite a few ‘gotchas’ in
using this method such as the randomness of decay and the sampling accuracy,
when used correctly is a highly reliable measurement of age in organic samples.
By comparing carbon dates against tree
ring dates we can validate the accuracy of both against one-another. Carbon dating
extends our reach into the past (at least for organic items) to 60,000 years
ago. There are other methods as well such as ice and ocean floor core samples. As
snow falls in the Arctic and Antarctic where it never melts it forms layers (much
like tree rings) that build up year after year, season after season. By taking
ice core samples using a hollow drill (like with the tree rings) we can obtain
samples as old as 800,000 years ago – almost a million years. By matching the
layers against tree-ring data we can synchronize the dates thus providing a
glimpse deeper into the past. Like tree rings the width of the layer will tell
us the amount of snowfall that year and from that we can get an indication of
the climate over time. Ice cores also provide data on the atmosphere of the
planet at the time the snow fell. This is because the snow traps bubbles of air
and other particulates as it falls and is compressed by additional layers above
it. The air bubbles can be analyzed for various elements such as CO2,
Nitrogen, Oxygen, Ammonia, and other gasses that were in the atmosphere when
the snow fell. The particulates can give information on volcanic eruptions,
dust storms, pollen and a multitude of other information on climate, weather
and environmental status.
19 cm long section of an ice core showing 11 annual layers
with summer layers (arrowed) sandwiched between darker winter layers
A similar accumulation takes place on
the ocean floor as microscopic creatures die and fall to the bottom along with
any particles, dust, pollen, etc. These layers too can be synced up with the
ice cores and our other measurement methods. These sea floor sediment cores extend
our view to 145 million years ago. We can use the same type of analysis to
determine the climate by measuring the thickness of the layers made up of these
microscopic creatures. This will tell us something about the conditions, whether
they were thriving or starving. The pollen and dust can give us information
such as prevailing winds determined by the source of the pollen as well as
amount, type and other details. The composition of the chalky shells (calcium,
carbon, etc.) can tell us about the ocean, the atmosphere, water temperature and
more.
Of course normal dry land does a
similar layering process as dust, dirt and other particles are carried into and
settle from the atmosphere. There is a gradual ‘sinking’ of the past into the
Earth which archeologists and others know well. Significant climate changes and
events can be quite evident in those layers. The deeper we dig the farther into
the past we delve. We can identify layers by comparing cross sections from
different geological areas or even countries. Certain world shattering events
such as the Chicxulub asteroid impact that killed the dinosaurs can be
identified in strata by its tell-tale iridium rich K-T boundary layer (now
called Cretaceous–Paleogene (K–Pg) boundary) which is found in sediments
world-wide.
Geological layers
There are also other radioactive
substances with much larger half-lives than carbon including potassium-argon and
uranium-lead dating with half-lives of over a million years. These radioactive
isotopes are used to date dinosaur bones and rocks back to the formation of the
Earth. And of course we can sync this clock up with the C14 , tree
ring and other data to provide accurate dating back to the formation of the
Earth. These methods along with geological stratigraphic methods have allowed use
to establish a full geological timescale. We continue to add events to our ‘all
time’ calendar as well as synchronizing it with new information as it becomes
available.
The age of the Earth has been
measured by this radiometric dating of rocks and meteor samples and is currently
thought to be 4.54 ± 0.05 billion years old. The oldest minerals analyzed to
date are zircon crystals from Australia which give a date of at least 4.404
billion years old. The oldest meteors we have sampled are 4.567 billion years
old which would be the upper limit for the age of the Earth. These meteors
would have been formed and been left over from the formation of the solar
system 4.6 billion years ago which initially formed the Earth. Also judging by
the age of the Sun (see next section) this validates the 4.54 billion year age
of the Earth.
Determining
the age of stars such as our sun relies on stellar evolution models, mass,
membership in a group of stars formed together, luminosity, type of star, and
other factors. As stars age their luminosity decreases – they fade out. For
main sequence stars knowing the mass of the star and its luminosity you can
determine its age. If a star is a member of a cluster it can be assumed that
all stars in the cluster are approximately the same age. Using these factors we
can estimate the age of our sun as 4.6 billion years. Other techniques which
have to do with protoplanetary disc and gravitational collapse of molecular
clouds leading to star formation can play into determining a star’s age. If the
molecular cloud or protoplanetary disc is still highly visible around the star it
would of course be younger than one with a fully collapsed/formed planetary
system. By comparing fully dispersed protoplanetary clouds to fully collapsed
clouds and know the typical time frame required for formation of a solar system
an estimate of a star’s age can be made. There is always potential error in
determining these ages but by putting all the puzzle pieces together – stellar
evolution, brightness, location, etc. we believe we can determine the age of
individual stars quite well.
UGC 12158 is an excellent example of a barred spiral galaxy taken by
the Hubble space telescope.
So what of the age of galaxies and
the Universe itself? The age of galaxies is really just the age of the stars in
them, but some implications could be made based on the distribution of the
stars. In particular in a spiral galaxy, because it is ‘spinning’ it is thought
that the stars in the arms will gradually spread apart. The more dispersed the stars
and arms are, the older the galaxy would be. But this should also be evident
from determining the age of the stars in those spiral arms. And again like
matching up our tree rings across samples and to radiometric dating we can
match up the spreading of the spiral arms based on our knowledge of galactic
evolution with the age of the stars in those arms and confirm that they are
equal in order to validate them. Now on to the thing we and everything we know of
are part of -- the Universe itself.
Our estimate of the age of the
universe is determined by its date of origin. Wait, what? If we know the origin
of the universe we know its age, right? No, not exactly. Until Edwin Hubble
determined the distances to and between various stars and learned that those distances
were increasing with time we had no idea about the age of the Universe. Once we
knew the universe was expanding and knew the expansion rate then it is a simple
matter to extrapolate backwards to the time everything in the universe was at a
single point – the Big Bang! That would be its beginning and that would be its
birthdate. You could be forgiven if you thought Hubble was the one to realize
and calculate it, but actually it was Allan Sandage in 1958 almost 30 years
after Hubble’s initial work. He found the age of the Universe to be 13.7
billion years old – amazingly accurate even today. Our most recent estimate and
measurement was done using the space telescopes from the WMAP study and the Planck
space probe to measure the cosmic background radiation. This has given us our
latest and best estimate of the age of the universe at 13.798 billion years
old. Perhaps not amazingly this number fits well in our overall understanding
of the universe, our third generation sun, and our observations of galactic and
stellar formation. There are of course still many puzzles such as dark energy,
string theory and quantum gravity to be solved but I’d say we’ve done quite
well on the measuring time frontier and in understand the age of many and sundry
things. Certainly there will be some adjustments as time goes on as there
always is in science, but for now we seem to have a reasonable handle on what
time it really is. All the puzzle pieces align and the stars are brightly
shining.
References/Resources/Links
Tree Ring Dating - Dendrochronology:
Tree Ring Image:
Oldest Trees:
Carbon Dating:
Ice Core Dating:
Ice Core Example Image:
Radiometric Dating:
Dating Dinosaur Bones:
Sea Floor Cores:
Ocean Sediments:
Geological Layers Image:
Geological Time Scale:
Age of the Earth:
Age of Stars:
Our Sun:
Age of the Universe:
Spiral Galaxy Image:
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