Tree Rings, Carbon Dating and the Age of the Universe

(From How Do we know? A few things we've learned from science)

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.