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The Joy of High Tech

by

Rodford Edmiston

Being the occasionally interesting ramblings of a major-league technophile.

Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. And if you find a mistake, please let me know about it.

Getting a Date

The first objective methods of measuring the age of objects older than recent history (say, within the past thousand years) often gave results which surprised many archeologists, paleontologists and geologists. So much so that the dates were often rejected, or explained away, often in derisive tones. In the late Nineteenth Century the consensus was that the Earth was young and humanity not much younger. So early estimates made by physicists about the age of the Earth and other bodies in the Solar system were greeted with great skepticism. ("The Sun is nine thousand years old?! Nonsense!") Then the various disciplines started getting actual, hard data, and some human artifacts turned out to be older than the previously accepted age of the Earth, and some rocks were given ages vastly older than even that!

Science is an interdisciplinary institution, with many checks and balances. And more and more disciplines were providing strong evidence that time was much deeper than most educated people had previously thought.

One of the criticisms of evolution in the period leading up to the first, objectively obtained data on the actual age of the Earth was that not enough time had passed for it to produce the variety of life seen in the world. Supporters of evolution who stated that the diversity shown in both the currently living animals and plants and that shown in fossils was proof that the Earth was much older than thought were accused - justifiably - of arguing in circles. Hard data that there actually had been enough time vindicated them. But such is the conservatism of the human mind that many scientists insisted the ages being determined were proof that the science behind the dating methods must be wrong.

Stratigraphy - identifying layers of soil and/or rock by their characteristics, such as included fossils - can provide relative dates, using the reasonable assumption that new is deposited on top of old, as long as the layers remain undisturbed. The problem is that many natural processes can overturn soil or strip away some layers so that new layers are deposited directly onto very old ones. Even rocks will flow like thick putty when deep underground. (Something else the conservatives rejected, since that required pressure and time outside their world view.) If layers are tilted, folded or otherwise distorted the stratigraphic sequence will become difficult - or even impossible - to read. Even when the stratigraphic record is clear it only gives relative ages, not absolute ones. Still, once accepted this was the first reliable, reproducible, scientific method for dating fossil beds or layers of human-produced debris with respect to each other.

The Latin names given to various geological periods probably seem quaint and archaic to modern students. Those periods were given names instead of dates because before reliable dating methods their ages could only be assigned relatively, through stratigraphy. That is, distinct bands with typical fossils or types of rocks could be found in many rock formations, and their relative positions - and therefore relative ages - could be determined through diligent study. Cenozoic rocks are on top of Mesozoic rocks which are on top of Paleozoic rocks, but how old are any of them?

The temporal boundaries of these eras - and the multiple sub-eras in each - were marked by such things as significant changes in rock composition, or the types of fossils found. Several different distinct layers were found widely around the world. Some were claimed to represent the Noachian flood, because they contained fossils of sea creatures. (It is now known that most of these are seabed sediments turned to stone and raised by vast, slow, powerful geological processes of which scientists at the time were unaware.) Slowly, scientists began to realize just how much material there actually was in these rocks. In some places fine-grained rocks held thousands of layers of sedimentary material, each layer presumably marking a season, or perhaps an entire year. These layers alone represented long stretches in human terms - many of them recording spans of time longer than what had previously been thought of as the total age of the Earth - and they were in the middle of multiple thick layers. People were starting to stretch their minds, at least in respect to the concept of how old things actually were. But this was still only supposition, a matter of relative positioning in the timeline.

Then came radioactivity.

The old alchemists' dream, of changing one substance into another, turned out not only to be true, but to be happening all the time in nature. Moreover, these changes proceeded in a specific order at a very steady rate. If you could measure that rate, find a piece of, say, undisturbed radium ore, and accurately measure the ratio of decay products radium atoms produce to the remaining amount of radium, you could get the age of the actual piece of ore. Theoretically.

Of course, ore is ore because it isn't the pure substance. So, after getting the decay rate (which was not easy, since some materials have half-lives longer than the current age of the universe; even a tiny error in measuring these rates would cause huge errors in calculated ages) they next had to develop methods for getting the ratios.

In 1905, the British physicist Lord Rutherford made the first specific suggestion to use radioactivity as a tool for measuring geologic time directly. It turns out that there are only a few radioactive materials which are actually useful for dating purposes. And while there is some overlap in the date ranges for which they work, there are also some large gaps. Still, there were a few early successes. In 1907, Professor B. B. Boltwood, a radiochemist at Yale University, published a list of geologic ages based on radioactivity. Although Boltwood's ages have since been revised, they did show correctly that the duration of geologic time would be measured in terms of hundreds of millions to billions of years. The ages were astounding, and could only be obtained for certain, specific layers in the geologic record. However, the dates he obtained put the layers in the same order determined by stratigraphy. Now they were getting somewhere!

More methods of dating through the detection of radioactive isotopes or their decay products were developed. Some turned out to be useful for things besides rocks.

The Potassium/Argon method is good for a range of about 10,000 years BCE (Before Common Era) to over 4,000,000 years BCE. The Rubidium/Strontium method is useful for 25 to 90 million years. The Uranium/lead methodologies are more flexible, due to uranium having both several useable radioactive isotopes with widely differing half-lives, and the fact that there are several decay paths whose byproducts can provide ages for a wide range of periods. U/L can even be used for some limited dating of (once) living things. Calcium-rich items (such as bones or shells) buried in wet ground which contains enough uranium will preferentially absorb uranium from their surroundings while not absorbing the thorium and protactinium daughter products of uranium, which are not water soluble. Therefore, any daughter products found in the materials are the result of uranium decay within them, and the amount and type can be used to date the object.

All these methods are for dating the formation of rocks. For living things the most common method of radiological dating uses the decay of Carbon 14. Cosmic rays striking the atmosphere produce a radioactive isotope of carbon, C-14, as a result of the bombardment of nitrogen by neutrons from the cosmic rays. This unstable isotope is incorporated into living things through respiration and consumption of other living things. Once this stops - that is, the organism dies - there is no longer any carbon intake. So by measuring the ratio of C-14 to other carbon isotopes in a dead bit of tissue you can know when it died. For example, if carbon from a sample of wood is found to contain only half as much carbon-14 as that from a living tree, the estimated age of the old wood would be 5,730 years, one half-life of C-14. For many years Carbon-14 analysis was the standard for dating once-living materials. Then astronomers discovered that the cosmic ray flux wasn't constant. Today, the C-14 calendar has been calibrated by other methods and is considered quite reliable, when used within its limitations.

C-14 can be used to acquire dates from a few years in the past to nearly 100,000 years back, but the method is usually restricted to a range of 200 to 40,000 years. Note that the global burning of fossil fuels tends to skew the carbon isotope balance of living things over much of the Earth if they lived in the late Twentieth and early Twenty-First centuries. The carbon in these materials is so old that nearly all the C-14 they originally had is long decayed.

Besides these isotopic methods, there are many other reliable dating methods which do not depend on radioactive decay, some of which depend on knowledge of a culture's artifacts and which can be used for historical and recent prehistoric dating. Specific styles in pottery, jewelry, tomb decoration and so forth can often give a date to within a century or two, and occasionally much closer. One recent example of a situation where dating by style has been used is the tomb cask or ossuary box which is suspected of belonging to James, brother of Jesus. The exact time when the style of decoration used on the box came into use is known to within a few years, and this decoration was only used for a pretty limited time (a century or two). The time when James died is therefore within the period when the box was created.

One very useful dating method is dendrochronology, dating by tree rings. This has actually been used to calibrate Carbon-14 dates. Pretty impressive, when you consider that it was first dismissed as fringe science. However, it depends on calibration for each region where it is used, and that can be very time-consuming and effortful. Also, weather patterns repeat through time, so for long-dead wood some other method may be needed to pick which set of reference tree rings it actually matches. Combined with carbon dating to give a general period, tree-ring dating may date the death of the tree a piece of wood came from to a specific year.

In still bodies of water and some other situations the layers of silt formed can be dated, as mentioned above, both by distinguishing between annual layers (much as with tree rings) and by looking for specific materials in the inclusions. (Some types of pollen means Spring, certain types of insects mean Summer, etc.) This method can be used to obtain dates from both lake-bottom mud and fossilized sediments. A similar method can be used with the ice in glaciers and polar caps.

Related to this is a group of disciplines which use bits of vegetable matter found with items. Presumably, a berry found on a plate with an abandoned meal was put there during the time of year when such berries were on the vine or on the ground. Pollen grains are very distinct, and an expert can distinguish between pollen from different varieties of the same plant. Finding a tomb with a fine dusting of pollen on the floor from plants which only bloom during late Spring probably means the tomb was last open in late Spring.

Another non-isotopic method of dating is paleomagnetism. Some materials pick up the direction of the Earth's magnetic field when they cool after being heated; since this direction changed globally in a known pattern this can sometimes be used to date when something cooled.

Then there is thermoluminescence. Many materials, when heated, will release energy captured when high-energy photons - such as gamma rays - passed through them during the period since they were last heated; reheating the object and measuring the amount of specific types of photons released can give a somewhat reliable date, depending on circumstances. Thermoluminescence can be used on materials such as fired pottery, stones caught in fires and volcanic tephras.

Which of these methods are the most accurate? That depends on the situation. For instance, if we know that a certain log was cut just before use, and can tie it into the local dendrochronology calendar, we might be able to get the exact year a structure was built or a piece of furniture was made. If an undisturbed site has a bunch of coins with dates which cut off at a certain point, we can be pretty sure they weren't buried (or whatever) much after that, and could not have been buried before the latest year on any of them. However, careful judgement, based on extensive knowledge, must be used with such dating methods. In ancient Egypt (most of which had no forests) wood was important and valuable enough to be reused over and over. A toothpick found in a tomb may have started as a huge log centuries earlier. Also, many coins remain remained in use for centuries, because until modern times making coins was difficult and there weren't many of them.

Chemical dating methods are generally not accurate except in certain circumstances. The fluoride dating method depends on the fact that buried bones absorb the compound from the soil. The technique can be very useful in certain circumstances. Getting an absolute date from a patina (corrosion or other surface effect cause by environmental exposure) is pretty much an educated guess. There are too many factors involved in the rate of effect, including not knowing whether as-found conditions are typical of previous conditions. However, differences in patina on the same surface can provide relative dating. The ossuary box mentioned above did turn out to be legitimate; however, the inscription giving the name of the occupant was probably forged, and recently. The patina on the part of the inscription containing the name James had been disturbed. The dealer claimed he did this when cleaning the box, but the difference in patina led to a closer examination, and strong evidence emerged that the original name had been altered.

If Carbon-14 and dendrochronology are applicable, most other dating methods are pretty much just used to confirm the dates they give.