Saturday, October 24, 2015

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.

References/Resources/Links

Tree Ring Dating - Dendrochronology:

http://en.wikipedia.org/wiki/Dendrochronology

Tree Ring Image:

http://en.wikipedia.org/wiki/File:Tree.ring.arp.jpg

Oldest Trees:

http://en.wikipedia.org/wiki/List_of_oldest_trees

Carbon Dating:

http://en.wikipedia.org/wiki/Radiocarbon_dating

Ice Core Dating:

http://en.wikipedia.org/wiki/Ice_core

Ice Core Example Image:

http://en.wikipedia.org/wiki/File:GISP2_1855m_ice_core_layers.png

Radiometric Dating:

http://en.wikipedia.org/wiki/Radiometric_dating

Dating Dinosaur Bones:

http://science.howstuffworks.com/environmental/earth/geology/dinosaur-bone-age1.htm

Sea Floor Cores:

http://en.wikipedia.org/wiki/Core_sample

Ocean Sediments:

http://highered.mcgraw-hill.com/sites/dl/free/0073016543/398471/chamberlin1e_samplechapter.pdf

Geological Layers Image:

http://en.wikipedia.org/wiki/File:OrdOutcropTN.JPG

Geological Time Scale:

http://en.wikipedia.org/wiki/Geological_time_scale

Age of the Earth:

http://en.wikipedia.org/wiki/Age_of_the_Earth

Age of Stars:

http://en.wikipedia.org/wiki/Stellar_age_estimation

Our Sun:

http://en.wikipedia.org/wiki/Sun

Age of the Universe:

http://en.wikipedia.org/wiki/Age_of_the_universe

Spiral Galaxy Image:

http://en.wikipedia.org/wiki/File:UGC_12158.jpg

Friday, June 19, 2015

Extinction!

Since a sixth mass extinction appears to be under way...

Extinction!

From my book: How do we know?

Kenny A. Chaffin

Over 98% of known species are dead – gone, kaput, vamoosed, disappeared, extinct! We will be too someday by those odds, but for now let us enjoy our time under the sun. That 98% doesn’t count the unknown species; those that we could have no idea about, like the ones just getting going when Theia struck the nascent Earth and turned it once more into a molten sea of lava. In the last half billion years there have been at least five planet-spanning extinction events in which half of all animal species on Earth have been destroyed. Could it happen again and if so is there anything we can do about it?

The answer of course is, certainly it could happen again and very likely will and odds are there is very little we could do about it. The best we might be able to do would be to get out of the way. The earliest extinction may have been from the formation of our Moon when Theia, a Mars-sized protoplanet slammed into the Earth some half-a-billion years after its initial formation. At that time the early Earth would have likely had surface water, atmosphere and have been cool enough to support primitive life or its origin whether from panspermia or abiogenesis. Life literally could have been gaining a foothold on Earth when the massive impact by Theia would have rebooted life to begin again.

Once life returned it was still no easy pickings. Early simple single celled organisms and their predecessors would live in a kind of stasis for billions of years as best we can tell. The geological and fossil records are virtually non-existent. The earliest indicators of life we have are geological layers and stromatolites produced by cyanobacteria. It would be another 2.5 – 3 billion years before these single cells began cooperating and forming multicellular aggregates and another billion years – approximately 4 billion years since the Earth’s formation before the first simple animals appeared. There would be what has been called the Cambrian explosion 540 million years ago right around Earth’s four billionth birthday when there was a major expansion in the diversity of life. It is this abundance of life and the traces it left in the geological record which allows us to gain insight into extinctions. By excavating and analyzing layer upon layer of sediment left on the sea floor and subsequently pushed to the surface or through direct core sampling we can count, test, and examine layers in relation to one another and determine reasonably precisely when life was abundant, when certain species died out or when all life seems to struggle or disappear.

Our first glimpse of the great Cambrian explosion came from the Burgess shale a 505 million year old fossil bed in Canada which contains some of the best preserved fossils of all time. This plethora of Cambrian life has been validated in other sites around the world as well. There may have been a greater diversity of life at that time than at any other in Earth’s history. The Ordovician-Silurian extinction which followed would definitely put a damper of that explosion.

The O-S event was actually two closely spaced extinction events in the 450-440 mya timeframe. It killed off 70% of all species and is ranked second only to The Great Dying of 251 mya. At 450 mya the cephalopods may have been the dominant life-form, but many others existed. There were sea urchins and jawed eels and jawless vertebrate fishes. This event killed off two-thirds of all those species.

Of course what most people want to know is what caused it? Was it an asteroid like killed the dinosaurs, was it an ice age or global warming? The fact is that we cannot always associate a specific climatic change or other cause with an extinction event.

See: Extinction Intensity Graph

There have been five ‘major’ extinctions and dozens of smaller extinctions which we see in the fossil record. There is even a hypothesis that extinctions occur every 26 million years and are somehow linked to a dark companion star (named Nemesis of course) that is orbiting around the sun and perturbs the Oort cloud and/or asteroid belt. There is of course no evidence supporting this nor does it match precisely with the timing of extinction events. Other associated and/or more relevant possibilities are flood basalt flows of magma from the Earth’s interior. This would be events such as the Deccan and Siberian trap events which released massive flows of lava as well as volcanic gasses into the atmosphere. Sea-level falls appear to be associated with all of the major extinction events, but it is unclear if this is a cause or a result of other events.

And of course there is the one we all fear – an impact event such as the Chicxulub impact which killed the dinosaurs 65 million years ago. An ocean impact can have an even more devastating effect than a land impact. The energy – particularly heat generated by an ocean impact can release massive amounts of CO₂ gas which would kill any air-breathing organisms. Since ocean impacts do not always leave an obvious crater it is difficult to attribute as the cause of a mass extinction. There is always the possibility for climate to shift out of control producing either an ice age or global warming that can be disastrous to all life. Again like the sea-level falls these conditions are thought to be more a result of others than the cause themselves. There are other less obvious but possible events which either contribute to or trigger extinction events such as near-space supernova explosions, anoxia conditions in the ocean, super volcano events such as the Toba eruption, unusual plate tectonics movement and others. (See the links in the resource section if you really want to dig deeper)

It is interesting to note that immediately following the Ordovician-Silurian extinction (450 mya) we see the first plant life move onto land from the seas. This may be an example of how an extinction event can ‘accelerate’ evolution. Not that evolution can be actually accelerated, but by clearing out the environment or opening up niches for new life to flow into there can be a greater rate of expression of mutations that take advantage of those previously unexploitable environmental niches. We see this happen following the extinction of the dinosaurs with the rise of mammals to take advantage of the vacant environmental areas. Following the O-S extinction sharks became the dominant predator in the ocean. Crawling insects arose on land and would soon be flying. The first tetrapods wriggled and climbed their way onto land to become the ancestors of all four-limbed vertebrate land animals including humans. At this time the most recent supercontinent Pangaea was being formed from the convergence of Euramerica and Gondwana.

While those tetrapod descendants were likely affected by the next mass extinction it would have its predominant effect on marine species. Overall some 70% of species on the planet were destroyed by the Late Devonian Extinction which lasted an incredible 15-20 million years from 375-360 mya. There is some evidence for multiple ‘pulses’ of extinction during this period. Even given the long duration there is no definitive evidence as to the cause. We know ocean anoxia existed but that was likely a result rather than the cause.

Every extinction event we’ve seen required 5-10 million years for species to re-adapt, re-evolve, and re-enter the environmental niches opened by the extinction. In the case of extreme extinction events such as the Permian-Triassic Extinction it was more like 30 million years. This was known as The Great Dying and over 90% of all species were destroyed, life itself was close to being wiped from the planet. This took place 251 million years ago. Whereas the Devonian extinction affected primarily the oceans, the P-T extinction was devastating to everything. It ended the dominance of mammal-like reptiles on land. It destroyed 96% of all ocean species and 70% of all land species down to the insects which is unique to this particular extinction. There are numerous speculations as to the cause. The Siberian Trap event is the prime candidate which lasted for a million year period from 251-250 mya. A secondary possibility is an asteroid impact, perhaps in the ocean. Beyond those, the fossil evidence shows a sea level change, anoxia, drought-like conditions and apparent shifts in ocean circulation. Given the near total extinction of life we see significant evidence of this event in most any and all areas of the environment – in the geological layers, ocean core samples as well as Arctic ice cores in all regions of the world.

Life would recover from this the worst devastation ever during the next 50 million years until the Triassic-Jurassic extinction event 201 mya just before Pangaea began to break up. Compared to others it was short-lived at only 10,000 years yet it still destroyed 70% of all living species at the time. Many of the land animals - archosaurs, therapsids, and large amphibians were killed off which left open an environment the dinosaurs would rule for the next 140 million years. There was an evident and gradual climate change associated with this extinction but that alone would not have induced it. A proposed meteor impact could have, but no evidence has been found. There was a large basalt flood, of the Central Atlantic Magmatic Province (CAMP) in the area of what would become the mid-Atlantic ridge as Pangaea was beginning to break up. Many think this is the most likely source for the Triassic-Jurassic extinction which made way for the thunder lizards.

See: CAMP Magmatism in Pangea

And now the one you’ve been waiting for, the Cretaceous-Paleogene Extinction (K-T) of 66 mya which killed those very same dinosaurs and made way for us via a mammal domination of the Earth which continues to this day. Of the five major extinctions this is the one that is tied to an asteroid impact. The theory is supported by a geological layer of iridium found world-wide and whose thickness varies by location. Iridium is associated with asteroids and the distribution of this layer – its thickness and position matches with the location of the Chicxulub crater in the Gulf of Mexico off of the Yucatán peninsula. The layer is thicker closer to the impact point and in predominant wind directions and thinner in remote regions – e.g. the opposite side of the planet. The fact that this layer is world-wide tells us that it sent massive amounts of dust and debris into the atmosphere. The impact brought on a global and extended winter due to dust blanketing the atmosphere. The dust and debris in the atmosphere was so thick that it blocked most sunlight preventing photosynthesis and many plants died leaving a massive hole at the bottom of the food chain which propagated upward and resulted in the death of the dinosaurs at the top of that food chain. Also implicated in this extinction is the Deccan Trap Event -- a basaltic flow in northern India which spanned the same time period. Some 75% of all species were destroyed, but the environment was opened to the small surviving mammals that would thrive, expand and diversify into new creatures such as horses, whales, bats, primates and more leading eventually to ourselves.

What of future extinctions? There is certainly the possibility of an asteroid strike as was seen dramatically over Russia recently and with the recent close-passes of largish asteroids. It is thought that the Yellowstone caldera is well past due for a massive explosion or basaltic flow. It could all happen again and there would be very little we could do. We are in no position to evacuate the planet in large or in small. We would simply die…or depending on the extent of the event perhaps survive in some small and or primitive manner to potentially rebuild civilization again.