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Author Topic:   Introduction To Geology
Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


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Message 232 of 294 (684867)
12-19-2012 3:05 AM


Radiocarbon Dating
Radiocarbon dating
Introduction
In this article we shall discuss how radiocarbon dating works, the conditions under which it can be applied, and the limitations of the method.
The isotopes
There are three important isotopes underlying the process of radiocarbon dating.
14N (nitrogen-14) is converted to 14C (carbon-14) in the upper atmosphere as a result of bombardment by neutrons in so-called cosmic rays: high-energy particles bombarding the Earth's atmosphere from outer space. Such an isotope is said to be cosmogenic. On formation, the newly-born carbon atom quickly oxidizes to form a molecule of carbon dioxide (CO2).
14C is an unstable isotope of carbon, and so decays back to 14N via beta decay with a half-life of about 5730 years. Because the quantity of 14C being produced annually is more or less constant, whereas the quantity being destroyed is proportional to the quantity that exists, it can be shown that the quantity in the atmosphere at any given time will be more or less constant: the processes of production and decay of 14C produces an equilibrium.
Also of importance is the stable carbon isotope 12C; this makes up 98.89% of atmospheric carbon, as opposed to only 0.0000000001% 14C. The balance is made up by the stable isotope 13C, which need not concern us in this article.
The terrestrial carbon cycle
The terrestrial carbon cycle is fairly simple: plants get their carbon from the atmosphere via the process of photosynthesis; herbivores get their carbon from plants, and carnivores from the herbivores. After the death of the organism, processes of decay will return its carbon to the atmosphere, unless it is sequestered --- for example in the form of coal.
This means that when an organism is alive, its ratio of 14C/12C will be the same as the ratio in the atmosphere. But of course when the organism dies it is cut off from the source of atmospheric carbon, the 14C will start to decay to 14N, and the ratio will begin to change.
The method
This immeditately suggests a method of dating organic remains. If we measure the amount of 12C and the amount of 14C in an organic sample, then since we know the atmospheric ratio and the amount of 12C present, we can deduce how much 14C was present originally. And then since we know how much was present originally, since we can measure how much is present now, and since we know the decay rate of 14C, it is trivial to compute the age of the sample.
This method is variously known as radiocarbon dating, carbon dating, 14C dating, or C-C dating.
One of the nice things about this method is that we don't have to worry about carbon being lost from the sample. Because we are measuring the abundance of two isotopes of carbon, and because isotopes of the same element will be chemically identical, no ordinary process can preferentially remove 12C or 14C, and so any process of carbon removal will leave the 12C/14C ratio the same, and the method will still work.
Limitations of the method
The method has various limitations. First of all, the quantity of 14C is going to be small enough to begin with, being only 0.0000000001% of atmospheric carbon, and then as the decay process progresses, it's going to get smaller and smaller. After about 60,000 years, the quantity will be too small for our instruments to measure accurately, and the best we'll be able to say is that the sample is about 60,000 years old or more. For this reason radiocarbon dating is of more interest to archaeologists than to geologists.
Two effects also interfere with the dating of very recent samples. The testing of thermonuclear weapons produced an increase in atmospheric 14C, peaking in the mid-1960s; and the burning of fossil fuels has been causing an increase in atmospheric 12C; this has not been accompanied by a corresponding increase in 14C because as the carbon in coal and oil is old, the amount of 14C they contain is infinitesimal. Fortunately it is rarely necessary to use radiocarbon methods to date very recent samples.
Thirdly, it is in the nature of the method that it can only be applied to organic remains: it makes no sense to apply it to rocks or to mineralized fossils.
Fourthly, the carbon in the organic remains does have to originate with the terrestrial carbon cycle and with plants performing photosynthesis. If this is not the case, it is sometimes possible to correct for the fact; in other cases it makes dating impossible.
For example, marine carbon behaves quite differently from carbon in the terrestrial cycle. The residence time of carbon in the ocean can be measured in hundreds of thousands of years (where the residence time of carbon is defined as the average time an atom of carbon will stay in the ocean). This increases the apparent age of the sample by about 400 years, depending on where in the ocean the organism lived and died. Given a latitude and longitude, an appropriate correction to the date is supplied by the Marine Reservoir Database.
Since humans eat seafood, this can also affect the carbon dating of humans, and what is worse it does so in an inconsistent manner, since human consumption of seafood varies with location and culture. However, the marine component of diet can be estimated by measuring the ratio of the stable isotopes 15N/13C in the sample: this will be higher the more seafood the individual consumed. This allows archaeologists to estimate the magnitude of this effect and correct for it.
Another source of carbon we have to take into account is the weathering of limestone. The result of this is to supply streams, rivers, and lakes with a source of dissolved calcium carbonate; if freshwater shellfish (for example) use this to construct their shells, then they are using a source of carbon which is millions of years old. Clearly applying radiometric dating in such a case is pointless. Another source of old carbon is the outgassing from volcanoes: in locations where this is a significant source of CO2, plants growing in the area will appear older than they actually are.
Even participation in the terrestrial carbon cycle does not quite guarantee the date: we could, for example, imagine termites eating their way through the wood of a 200 year old house; these termites would date to 200 years old or more (depending on the age of the tree). By and large, however, organisms tend to consume fresh vegetation or fresh meat, so this problem is unlikely to arise in practice.
Comparison with known dates
One way we can check the efficacy of radiocarbon dating is to compare the dates it produces with dates known on historical grounds, to ensure that it does indeed give us the right answer. The graph below shows the results obtained by the pioneer of the method, W.F. Libby, showing the measurements of 14C made on artifacts of known date compared with what would be expected on the basis of their dates as known on historical and archaeological grounds.
The fit is quite good, as you can see, even though the measurements were made in the infancy of the science when measuring techniques were inferior to those in use today and the half-life of 14C was not known so accurately as is now the case.
We can also compare radiocarbon dates with dates known on other grounds. For example, we have discussed the use of varves for dating; now since varves incorporate organic material as they are formed, we can check that when we radiocarbon date a varve, we get the same date for it as we obtain by counting the varves.
Again, it is obviously possible to carbon-date one of the rings of a tree, and to compare the date produced by radiocarbon dating with the date produced by dendrochronology. Such dates typically agree to within 1 or 2 per cent.
Calibrated dating
Although the radiocarbon dates agree closely with dendrochronology, they do not agree exactly. It is generally agreed that the dendrochronological dates should be considered the more accurate. The proportion of 14C in the atmosphere is not absolutely constant; for example, it can be reduced by volcanic activity, since the carbon dioxide emitted by volcanoes is richer in 12C than atmospheric carbon dioxide. By comparison the behavior of the genera of trees used in dendrochronology is more reliable and consistent.
It is therefore standard procedure to tweak the raw radiocarbon dates to bring them in line with dendrochronology, producing what are known as calibrated radiocarbon dates. This allows us to combine the greater accuracy of dendrochronology with the wider applicability of radiocarbon dating.
Edited by Dr Adequate, : No reason given.
Edited by Dr Adequate, : No reason given.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


(1)
Message 237 of 294 (685022)
12-20-2012 2:18 AM


Radiocarbon Etc
Lot of interesting suggestions here. Thanks. I shall expand the article on radiocarbon.
I shall be mentioning U-Th in a later article --- I felt it belonged with Th-Pa and Ra-Pb rather than with U-Pb and Pb-Pb.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


(2)
Message 239 of 294 (685161)
12-20-2012 5:34 PM


Cosmogenic Surface Dating
Cosmogenic surface dating
Introduction
In the article we shall discuss the techniques and applications of cosmogenic surface dating.
Applications
Unlike other dating methods, which tell us how long it is since a rock was formed, cosmogenic surface dating tells us how long a rock has been exposed on the surface.
In some cases, as when the rock is a lava flow, this amounts to the same thing. But there are other ways in which a rock can become exposed, as for example when a glacier erodes the topsoil covering bedrock: when the glacier melts, the bedrock will be exposed.
Cosmogenic isotopes
In the article on radiocarbon dating we have already introduced one cosmogenic isotope, 14C, which is produced by cosmic rays from 14N.
For cosmogenic surface dating, the two most commonly used isotopes are the cosmogenic isotopes 10Be, which is produced from 16O and which has a half-life of 1.39 million years; and 26Al, which is produced from 26Si and which has a half-life of 717,000 years.
The method
Because the isotopes we're using have a short half-life, it follows that if a rock has been buried for a few million years the quantities of these isotopes will be negligible. But when the rock becomes exposed on the surface, and so exposed to cosmic rays, these cosmogenic isotopes will begin to accumulate in the rock.
The rate at which they do so will depend on a number of factors, including:
* The exposure of the rock. A nearby obstacle such as a mountain will shield the rock from cosmic rays coming from that direction, reducing the creation of cosmogenic isotopes.
* The elevation of the rock. If the rock is on top of a mountain, then the cosmic rays have less atmosphere to travel through to get to the rock, and so more of them will make the journey all the way to the rock without being absorbed in the atmosphere on the way.
* The depth from which we take the sample. Cosmic rays can penetrate a few meters through rock or soil, but the further they travel the more likely they are to be absorbed, so a rock sample will get more exposure to cosmic rays if it is taken from the surface than if it is taken from a meter down.
If we take all the relevant factors into account, and calculate, estimate, or simply measure the amount of cosmic rays a given rock is exposed to per year, and if we measure the quantities of the cosmogenic isotopes in a sample of the rock, then we can figure out how long the rock has been exposed.
Limitations of the method
The quantity of the relevant isotopes in the rock will not simply grow without limit with longer and longer exposure to cosmic rays; rather they will tend towards a maximum. In practice, we are not going to be able to tell the difference between a rock which has reached 99.9% of this maximum and one which has reached 99.99%. Consequently, the practical limit for the use of cosmogenic surface dating seems to be about 10 million years; after that, one old rock looks much like another. The lower limit for application of the method seems to be about ten years, because of practical limits on the accuracy with which we can measure the quantities of the relevant isotopes.
Edited by Dr Adequate, : No reason given.
Edited by Dr Adequate, : No reason given.

  
Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


(2)
Message 240 of 294 (687597)
01-14-2013 6:49 AM


U-Th, U-Pa, And Ra-Pb
U-Th, U-Pa, and Ra-Pb
Introduction
In this article we shall discuss three similar methods that can be used to date marine and lacustrine sediments: the U-Th, U-Pa, and Ra-Pb methods.
The isotopes
The methods discussed in this article each require two isotopes: a parent isotope which is soluble (or the commonly occurring compounds of which are soluble) and a radioactive daughter isotope which is not soluble.
The table below shows three such systems together with the half-life of the daughter isotope.
MethodParentDaughterHalf-life
U-Th234U230Th75,000 yrs
U-Pa235U231Pa32800 yrs
Ra-Pb226Ra210Pb22 yrs
The method
The parent isotope will be present dissolved in the ocean or in lakes, but when decay takes place the insoluble daughter isotope will precipitate out as sediment and will form part of the upper layer of marine or lacustrine sediment. It will subsequently be buried in its turn by further sediment, and being radioactive will undergo decay.
Now, if there was absolutely none of the parent isotope present in the sediment, then the calculation would be very simple: when we have dug down through the sediment up to the point where the daughter isotope is only half as abundant as it is on the surface, then we would have dug back through one half-life's worth of time; and in general we could write:
t = h log2(N/Ns)
where
* t is the age of the sediment;
* h is the half-life of the daughter isotope;
* Ns is the quantity of the daughter isotope on the surface layer of sediment;
* N is the quantity of the daughter isotope at the depth we're trying to date.
That would be the simple case: however it will not necessarily be true that there will be none of the parent isotope in the sediment. There may well be some, but this is not a problem, since we can measure the quantity of the parent isotope present in the upper layers of sediment and take this into account in our calculations. The crucial point is that there will be more of the daughter isotope than could be accounted for by the decay of the parent within the sediment.
Note on the use of Ra-Pb
All the methods described here are somewhat limited in their usefulness by the short half-lives of the daughter isotopes. This is particularly true of 210Pb; since it has a half-life of only 22 years, this makes it useless for most geological purposes. However, it can be used to gauge the rates of deposition of marine sediment as an alternative to the use of sediment traps.
This method has a couple of advantages over sediment traps. First, it is quicker: it doesn't take long to obtain a sediment core sample, whereas a sediment trap has to be left in place for at least a year to produce useful results.
Second, use of Ra-Pb allows us to measure the sedimentation that has taken place over the course of a century or so and average it, reducing the effect of annual fluctuations on the figures we obtain.
Alternate use of U-Th
We can make an alternative use of the fact that 234U is soluble and 230Th is not.
First of all, this means that 234U will be incorporated into the structure of marine organisms such as corals. Secondly, it means that 234U will be incorporated into speleothems and 230Th will not, just as with the U-Pb method discussed in the article on U-Pb and related methods.
There is, however, a difference between U-Pb and U-Th: 230Th is radioactive. Whereas this was essential to its use in dating marine sediments, it is actually an inconvenience when dating organic remains or speleothems, since it means that the 230Th will not only be produced by decay, but also destroyed by it.
As a consequence, what happens is that the quantity of 230Th in the sample will tend towards secular equilibrium: the point at which the thorium is being produced at the same rate as it is being destroyed. This fact, combined with the practical difficulty of measuring whether the level of 230Th has reached 99.9%, 99.99%, or 99.999% of secular equilibrium, limits the useful range of the method to about 500,000 years.
Because this method can be applied to organic materials, it can be correlated with the radiocarbon method, and the dates produced by both methods can be shown to be concordant.
Edited by Dr Adequate, : No reason given.
Edited by Dr Adequate, : No reason given.
Edited by Dr Adequate, : No reason given.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


(1)
Message 242 of 294 (687726)
01-15-2013 9:37 PM


Erratum
Foreveryoung has pointed out that most cosmic rays do not in fact come from the Sun, and I have amended the relevant articles accordingly.
I will now write out a hundred times "I have never actually studied astronomy, and should not presume that I know anything about it."

  
Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


(1)
Message 243 of 294 (687734)
01-16-2013 6:04 AM


Paleomagnetic Dating
Paleomagnetic dating
Introduction
In this article we shall discuss how we can use the paleomagnetism in rocks to attach dates to them (paleomagnetic dating). The reader may find it useful to go back and read the main article on paleomagnetism before continuing.
Polar wander and dating
One we have dated a sufficient number of rocks and measured the orientation of the magnetism they contain, we can build up a picture of how the position or apparent position of the poles over time.
So if we are then faced with a rock the date of which we do not know, then we do know (of course) the latitude and longitude at which we found it, and we can measure the orientation of its magnetism, and so we can look at the global picture we've built up of continental drift, and so figure out when the rock must have formed in order to have its magnetism oriented in just that direction.
Magnetic reversals and dating
Once we have dated a sufficient number of rocks and found out whether they have normal or reverse polarity, we can likewise build up a timeline for the occurrence of the reversals.
As noted in a previous article, magnetic reversals come at irregular intervals. This means that the pattern of normal and reverse polarity in an assemblage of rocks can be distinctive in the same way (though for a completely different reason) that growth rings in a tree can be distinctive. We might, for example, see a long period of reverse polarity, followed by six very quick switches of polarity, followed by a long period of normal polarity; and this might be the only time that such a thing occurs in our timeline.
So if we are presented with an undated rock, and we find a really distinctive pattern of paleomagnetic reversals within it, we may be able to identify the one time at which such a sequence of magnetic reversals took place.
Strengths and weaknesses of the method
The reader will observe that it is necessary to be able to date some rocks, in fact a lot of rocks, before paleomagnetic dating can be brought into play. You may therefore be wondering why, if we have perfectly good dating methods already, we don't just use them.
However, the advantage of paleomagnetic dating is that we can use it on different rocks from those susceptible to our ordinary methods of absolute dating: while most radiometric methods usually require igneous rocks, paleomagnetism can be measured in sedimentary rocks.
One problem which may arise is that the direction of the poles from a given location, or the pattern of magnetic reversals, may repeat over a long enough period of time, so that the paleomagnetic data we get when we measure these factors is not unique to a single time in the history of the Earth.
It is possible to get round this problem if we can find an approximate date of the rocks by other means. For example, if by considering their stratigraphic relationship to a datable igneous rock, we can establish that they are (for example) less than 20 million years old, then it may turn out that the paleomagnetic data, though not unique over the whole history of the Earth, are unique over the course of the last 20 million years, and then we can go ahead and use paleomagnetic dating.
Edited by Dr Adequate, : No reason given.

  
Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 246 of 294 (687961)
01-18-2013 2:41 AM
Reply to: Message 245 by petrophysics1
01-17-2013 5:02 PM


Re: Try Looking
Those are good reasons, thank you.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


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Message 247 of 294 (687962)
01-18-2013 2:45 AM


Sclerochronology
Sclerochronology
Introduction
In this article we shall discuss the basis of sclerochronology, a method of dating shells and corals by analysis of their growth patterns.
Growth patterns in shells and corals
When shelly organisms grow, many types lay down bands of new growth in a way that regularly reflects the passage of time: for example, laying down one growth band per day (as many corals do), or one growth band every low tide, as mussels do.
Some corals lay down distinct bands of skeletal calcium carbonate on a daily basis and also display seasonal patterns, so that they keep count both of days and of years. In the same way, mussels deposit their growth bands every low tide, but also show variations according to the phase of the moon, so that they keep count both of low tides and of lunar months. The photograph below shows the clam Arctica islandica, a popular species with sclerochronologists.
It is possible to use these growth patterns to date recent sediments in a manner analogous to dendrochronology. However, there is a more interesting way of using this data, which we shall discuss in the remainder of this article.
Tidal braking
the friction of tides that the Moon causes on the Earth slows down the Earth's rotation: this is known as tidal braking. The effect, though small, is measurable by the high-precision clocks used by astronomers, and so can be established directly as well as on theoretical grounds: at present, the effect amounts to a day getting shorter by 2.3 milliseconds over the course of a century (see here for more details). This may not sound like much, but it adds up: over the course of 100 million years, that would add up to a change in the length of a day of 38 minutes.
This means that in the past days must have been shorter. As the length of a year is constant, this means that in the past there must have been more days per year: using the present rate of slowing as a basis, there would have been about ten more days per year a 100 million years ago.
We should note that in fact scientists do not simply extrapolate the present rate of slowing in a linear manner to calculate past rates of rotation, but rather calculate this from the physics of the Earth-Moon system. For the purposes of this article, it is not necessary to go into the details of the calculation.
Tidal braking and sclerochronology
This immediately suggests a way of dating corals and shellfish. Take corals as an example. As I have said, they lay down daily bands, and the way in which they do so displays seasonal fluctuations. This means that by counting the number of daily bands per year, we can find out how many days there were per year at the time when they were formed. In the same way, by looking at mussels we can find out how many low tides there were per lunar month when they grew.
So by calculating how tidal braking has changed the number of days in a year or a lunar month, we can put a date on the organisms: for example, a coral showing 375 daily growth bands per year must have grown around 100 million years ago.
Weaknesses of the method
The number of days per year or per lunar month changes so slowly over time that we cannot expect sclerochronology to be as precise as radiometric methods such as U-Pb. If a change of one day per year corresponds to the passage of 10 million years, then this limits the precision with which we can resolve the age of a shell or coral.
What is more, the change in day length is not as predictable as the decay of radioactive isotopes. The graph below shows changes in day length from 1860 to 1980.
As you can see, although there is a general tendency for the Earth to slow down, occasionally it has sped up.
Over the longer term, the magnitude of tidal braking will depend on the exact interaction of the Earth and Moon. Such things as the position of the continents and of mid-ocean ridges will affect tidal patterns, and these change over time as we have seen in our discussion of plate tectonics. Then again, the formation of polar ice-caps, and the concomitant fall of sea-levels would speed up the Earth's rotation as a consequence of the law of conservation of angular momentum.
Because of these considerations, geologists prefer to use radiometric methods rather than sclerochronology where it is possible to do so, even though radiometric dating is rather more expensive. However this sclerochronological technique, even if it is rarely used in practice, has a distinct theoretical significance: it acts as an check on the validity of radiometric methods. When we find approximately 400 days per year in the Devonian period and about 390 in the Carboniferous (see J. Wells (1963) Coral Growth and Geochronometry, Nature 197(4871), 948-950) then this is in line with the dates put on these periods by radiometric methods. Since the mechanisms of coral and shell growth are completely unrelated to the process of radioactive decay, this provides a completely independent check on radiometric dating. The agreement between sclerochronology and radiometric dating is therefore a good reason to have confidence in both.
Edited by Dr Adequate, : No reason given.
Edited by Dr Adequate, : No reason given.
Edited by Dr Adequate, : No reason given.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


(1)
Message 249 of 294 (688053)
01-18-2013 6:08 PM


Tidal rhythmites and dating
Tidal rhythmites and dating
Introduction
In this article we shall explain what tidal rhythmites are, how they are formed, and their implications for dating.
As this article re-uses ideas introduced in the previous article on sclerochronology, the reader will need to have read that article first.
Rhythms and rhythmites
Rhythmites are sedimentary rocks which display a repetitive vertical succession of types of sediment. We have already discussed varves, which are a kind of rhythmite. In this article we shall be interested in rhythmites produced by the action of the tide.
Tidal cycles include:
* A semidiurnal cycle. In most locations the moon produces two high tides and two low tides a day.
* A diurnal cycle. Some locations, such as the Gulf of Mexico, get only one high tide and one low tide per day.
* A mixed cycle. This can be seen in many locations on the west coast of America. In a mixed cycle, there are two high tides and two low tides per day, but one high tide is higher than the other, and one low tide is lower than the other.
* A fortnightly cycle. The highest high tides and lowest low tides (spring tides) occur at new moons and full moons, when the moon is either in line with or opposite the sun.
* Tidal cycles such as the apsidal cycle (presently lasting 8.85 years) and the nodal cycle (18.6 years) can also be distinguished; for the purposes of this article we may overlook them.
In additional to these tidal cycles, the rhythm of the seasons can also have their effect on sedimentary deposition. Varves are a special case of this, though typically varves are so thin that cycles of shorter duration are not discernible. Some sediments, however, will display a full range of cycles from semidiurnal to annual.
Rhythmites and dating
Any or all of the cycles mentioned above can be recorded in nearshore sediments. So it is possible to look at nearshore sedimentary rocks and, depending on which rhythms are recorded in the rock, to find out how many days there were in a month, or days in a year, or months in a year, or all of these facts, at the time when the rhythmite was deposited.
This allows us to subject these rhythmites to the same analysis as is used in sclerochronology, except that we can look at days in a month and months in a year as well as days in a year. The fact that there is close agreement between the number of days in a year as calculated on the basis of rhythmites and by the use of sclerochronology is a reason to have confidence in both methods, since it is hard to see how both could be wrong and yet coincidentally in agreement.
The same caveats apply to the use of this type of rhythmite for dating as apply to sclerochronology, and for just the same reasons. Also as with sclerochronology, the agreement of data from rhythmites with dates produced by radiometric dating is a reason to have confidence in radiometric methods.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


(2)
Message 251 of 294 (688233)
01-20-2013 11:42 PM


Fossils And Absolute Dating
Fossils and absolute dating
Introduction
In this article we shall discuss how fossils can be used for the purposes of absolute dating.
Fossils and dating
We have already discussed the construction of the geological column. If our stratigraphic methods show that fossil A was always deposited below fossil B whenever we are in a position to compare their dates of deposition, then we can conclude that species A is older than species B. We can apply the same sort of reasoning to the stratigraphic relationships of fossils and datable rocks.
For example, suppose that using stratigraphic methods, we can show that a particular fossil is always older than rocks which are 14 million years old or less, and always younger than rocks which are 16 million years old or more, whenever we are in a position to make a comparison.
Now, it is a fundamental principle of science --- arguably, the only fundamental principle of science --- that a rule that works every time we can test it must be taken as true unless and until we find a counterexample. So in this case we would have to conclude that this fossil species is between 14 and 16 million years old wherever we find it, even in those cases where there are no datable rocks that we can compare it to.
But this means that we can now use the fossil species to date the sedimentary rocks in which it is found; and we can say that those fossils found in the same strata as this species must be the same age; those species which stratigraphy tells us are older than it is must be more than 16 million years old; and those species which stratigraphy tells us are younger than it is must be less than 14 million years old.
Hence we can use datable rocks to put dates on fossil species; and then we can use the fossil species to put dates on other rocks which would otherwise be difficult to date.
Those fossils we have described as "index fossils" are particularly suitable for this purpose, since they have a wide geographical distribution but only inhabit a thin slice of time.
Advantages of the method
There are three main advantages of using fossils for dating in this manner.
First of all, we may want to date a stratum which is a long way up or down from any rocks we can date using radiometric methods. In this case, the use of fossils will be absolutely the best method available.
Second, it is much faster than any more technical method. Why send a rock to a laboratory and wait for a reply when you can just glance at the fossils it contains and say: "Ah yes, Early Ordovician"?
Third, by the same token, it's much cheaper. Radiometric dating requires specialized equipment: lasers, spectrometers, or in the case of Ar-Ar dating a small nuclear reactor. Even the humblest items of equipment come at a price: laboratories that carry out U-Pb dating wash the bottles they use for two years continuously to eliminate contamination. Rather than employ the services of such a laboratory, it is so much cheaper for the geologist to recognize a well-known species of ammonite, trilobite, foraminiferan, or whatever, the age of which is already known.
Edited by Dr Adequate, : No reason given.
Edited by Dr Adequate, : No reason given.

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 Message 252 by RAZD, posted 01-21-2013 5:26 PM Dr Adequate has replied

  
Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


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Message 253 of 294 (688299)
01-21-2013 6:45 PM
Reply to: Message 252 by RAZD
01-21-2013 5:26 PM


Re: Fossils And Absolute Dating
The typical creationist complaint is that this is circular reasoning: rocks to date the fossils that then date the rocks ...
... but it is not circular: the first rocks are dated by radiometric methods the second rocks are usually of a type that cannot be dated by radiometric methods, sedimentary rocks, which is where the fossils are found.
Well, as I said:
Hence we can use datable rocks to put dates on fossil species; and then we can use the fossil species to put dates on other rocks which would otherwise be difficult to date.
This "circular reasoning" thing has always struck me as one of the odder creationist fantasies. They are surely not actually imagining some geologist saying: "I know this rock is ten million years old because I know the fossils in it are ten million years old; and I know the fossils are ten million years old because I know the rock they're in is ten million years old." I think as usual they aren't imagining anything at all, they're merely reciting words; if they tried to attach meaning to them they'd realize that what they're saying can't really be true.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


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Message 254 of 294 (689381)
01-30-2013 6:31 AM


Absolute Dating: An Overview
Absolute dating: an overview
Introduction
In this article, we shall take a look back at the methods of absolute dating, and see how we know that they can be relied on.
Basis of the methods
One argument in favor of the absolute dating methods presented in the preceding articles is that they should work in principle. If they don't, then it's not just a question of geologists being wrong about geology, but of physicists being wrong about physics and chemists being wrong about chemistry; if the geologists are wrong, entire laws of nature will have to be rewritten. Science, since it concerns just one universe with one set of laws, constitutes a seamless whole; we cannot unpick the single thread of absolute dating without the whole thing beginning to unravel.
Still, it has happened in the past that scientists have thought they'd got hold of a law of nature and then found out it was false. There is no particular reason to suspect that this will turn out to be the case when it comes to the laws underlying absolute dating; nonetheless, an argument from principle alone can never be entirely convincing. Let us therefore turn to the evidence.
Sea-floor spreading
You will recall from our discussion of sea-floor spreading that the sea-floor spreads out from mid-ocean rifts, and so ought to be younger nearer the rifts and progressively older further away from them.
What is more, we can measure the rate of spreading directly by GPS, SLR, and VLBI. This means that if we didn't have any other way of doing absolute dating, we would as a first approximation take the age of basalt on a spreading sea floor to be the distance from the rift divided by the rate of spreading.
Now if we estimate the age of the sea-floor like that, then we get a good agreement with the dates produced by radiometric methods. It is hard to think that this is a coincidence; it is also hard to think of any mechanism that could produce this agreement other than that the rocks are as old as radiometric methods tell us.
Marine sediment
We began our discussion of absolute dating by saying that sedimentation rates could not be relied on for absolute dating. If there is one possible exception to this, it would be the deposition of marine sediment, since it is not subject to erosion, and since we would expect the rates of deposition of the various sediments to be, if not actually constant, then not subject to such a degree of variation as (for example) glacial till. Based on the known rates of deposition, we may therefore at least say that the depths of marine sediment found on the sea floor are consistent with the ages of the igneous rocks beneath them as produced by radiometric dating.
Radiometric dating and paleomagnetism
The polarity of the Earth's magnetic field is a global phenomenon: at any given time it will either be normal everywhere or reversed anywhere. So if our methods of radiometric dating are correct, then we would predict that rocks dated to the same age would have the same polarity, which they do.
If this does not completely prove that radiometric dating is correct, it does at least show that (barring a wildly improbable coincidence) there is at least a one-to-one relationship between the dates produced by radiometric methods and the true dates, and so it must be taken as an argument in favor of these methods.
Comparison with historical dates
It is possible to test radiocarbon dating by using it to put a date on historical artifacts of known date, and to show that it is usually very accurate.
It has also been possible to test Ar-Ar dating against the historical record, since it is sufficiently sensitive to date rocks formed since the inception of the historical record. For example, Ar-Ar dating has been used to give an accurate date for the eruption of Vesuvius in 79 A.D, as recorded by Roman historians at the time. (See Lanphere et al., 40Ar/39Ar ages of the AD 79 eruption of Vesuvius, Italy, Bulletin of Volcanology, 69, 259—263.)
Radiocarbon dating, varves, and dendrochronology
Because varves contain organic material, it is possible to compare the dates from varves with the dates produced by radiocarbon dating, and see that they are in good agreement. We also see close agreement between dendrochronology and uncalibrated radiocarbon dates.
Now, each of these three methods relies on a different underlying physical process: radioactive decay, outwash from glaciers, the growth of trees. We can hardly suppose that there is some single mechanism which would interfere with all three of these very different processes in such a way as to leave the dates derived from them still concordant.
But it is equally far-fetched to imagine that three different mechanisms interfered with the three processes in such a way as to leave the dates concordant; that would require either a preposterous coincidence, or for natural processes to be actually conspiring to deceive us: an idea which is, if anything, even more preposterous.
Now, preposterous things do happen occasionally. But in this case there is a perfectly reasonable and straightforward explanation for why the dates are concordant, namely that they are correct.
Radiometric dating, sclerochronology and rhythmites
Similar remarks may be made about the agreement between radiometric dating of rocks, sclerochronology, and dating by rhythmites.
Are we to believe that one single mechanism interfered with the decay of radioactive isotopes, the secretion of calcium carbonate by molluscs, and the action of the tide? Absurd. But are we instead to believe that three separate mechanisms interfered with these processes in such a way as to leave all the dates concordant? That would be equally absurd. The straightforward explanation for the concordance of the dates is that they are in fact correct.
Consider the following analogy: a clockmaker sells us an electric clock, a pendulum clock, and a spring-driven clock, and guarantees that they are shockproof. Skeptical of the clockmaker's claim, we subject the clocks to shock: we shake them, drop them, hit them with hammers and shoot them out of a cannon. Throughout this process, they all go on showing exactly the same time. Is it plausible that we have damaged their very different internal mechanisms in such a way that they are all running fast or slow but still in perfect synchrony? Or is it more likely that they are synchronized because nothing that's happened to them has affected their working?
Agreement with relative dating
Relative dating by definition does not produce actual dates, but it does allow us to put an order on the rocks, and so if absolute dating is to be trusted, it should agree with this order, telling us, for example, that Ordovician rocks are older than Triassic rocks; and it does.
It is hard to see this as a coincidence; it is equally hard to think of some alternate explanation of why we can correlate isotope ratios or sclerochronological data with the relative order of rocks as deduced from stratigraphic methods --- other than the straightforward explanation that absolute dating is producing the right dates.
Internal consistency of radiometric dates
In our discussion of radiometric dating, we have seen that many, indeed most, radiometric methods are self-checking.
So in the U-Pb method, we check that the two uranium isotopes produce concordant dates. In the Ar-Ar method, we check that step heating yields the same date at every step. In Rb-Sr, Sm-Nd, Lu-Hf, Re-Os, La-Be, La-Ce and K-Ca dating, we check that the data we plot on the isochron diagram lies on a straight line.
These precautions allow us to throw out most data that have been produced by confounding factors such as atmospheric contamination, weathering, hydrothermal events, metamorphism, metasomatism, etc.
It is, as we have explained, possible for the occasional incorrect date to slip through this filter, since it is possible for some of these confounding factors to accidentally change the isotope ratios in such a way as to produce something that looks like a good date: apparently concordant dates for Ar-Ar or U-Pb, or a false isochron for the various isochron methods.
It would indeed be remarkable if this never happened, since one-in-a-thousand chances do in fact occur one time in a thousand. But by the same token, the other 999 times they don't, and so although any particular date produced by these methods might be called into question, it must be the case that the vast majority of dates that pass through these filters must be good; for we can hardly suppose that the confounding factors are actively conspiring to deceive us, and so these long-shot events must be as rare as statistical considerations would lead us to expect.
Mutual consistency of radiometric dates
You might perhaps suggest that if some unknown factor, contrary to our present understanding of physics existed that sped up or slowed down radioactive decay in the past, then we would expect the radiometric dates to be concordant whether they were right or wrong.
This is, as I say, contrary to our present understanding of physics, and so is mere unfounded speculation. What is more, the reader should recollect that "radioactive decay" is not the name of one process; it is the name of any process that rearranges the nucleus. So to leave dates produced by different radiometric methods still concordant, nature would somehow have to conspire to fool us by changing the rate of alpha decay, of beta decay, of electron capture, in such a way that the different dating methods based on these different modes of decay come up with the same dates.
Another point to bear in mind is that a change in the rate of radioactive decay, even if it was carefully coordinated in this way, would still not change every radiometric date in the same direction: if, for example, radioactive decay sped up at some time in the past then this would make U-Pb or Ar-Ar dates older than they should be, but it would make the dates produced by cosmogenic surface dating younger than they should be.
Summary
It is possible to doubt any particular date obtained by absolute dating methods. But it would be bizarre to doubt the general picture they paint. For what we see is a massive agreement between the different radiometric methods, varves, dendrochronology, sclerochronology, rhythmites, paleomagnetic data, sedimentation rates, sea-floor spreading, and relative dating methods.
For the dates obtained by absolute dating to be wrong in general and yet wrong in such a way as to be in agreement with one another and with other observations, we would have to suppose either that we are looking at an inconceivably massive coincidence, or that the whole Earth is a fraud designed to deceive us.
Ideas to the latter effect have actually been proposed from time to time; most notably by the nineteenth century religious zealot Philip Gosse, whose eccentric work Omphalos proposed that the Earth was a mere few thousand years old, but that God had created it to look much older. To this the Reverend Charles Kingsley memorably answered: "I cannot believe that God has written on the rocks one enormous and superfluous lie for all mankind". That of course would be a theological rather than a geological question, and so is outside the scope of this textbook. What can be said is that geology is a science, and that in science it is necessary to proceed on the basis that the universe is not a lie; because if we believed that, we could believe that anything at all was the case and disregard all evidence to the contrary. The scientific method compels us, then, to disregard the possibility of divine malice; and mere natural processes, being mindless, cannot be actually malevolent.
What, then, of coincidence? Well, there are limits to the degree of coincidence we can believe in, otherwise again we could believe nearly anything. The scientific method requires us to discard such remote possibilities unless there is at least a hint of a shred of evidence for them.
We are left with the conclusion that the great majority of the dates produced by absolute dating methods must be reasonably accurate.
Edited by Dr Adequate, : No reason given.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


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Message 256 of 294 (689779)
02-04-2013 2:38 PM


Onwards!
OK, I've updated the glossary again. That makes 542 glossary items, and ~88,000 words, or, to put it another way, about the same length as Persuasion but a little shorter than Nineteen Eighty-Four. Both of which I would have rather written, but that's how the cookie crumbles. If I was the bastard lovechild of George Orwell and Jane Austen, this would probably have been a much more interesting thread. My anecdotes about my family life alone would have made it unmissable reading.
I shall now move on to paleoclimatology. This was not in fact part of my original plan, I intended originally to stop at the previous article, but as I went on I realized that I'd have to deal with paleoclimatology, and since I had to, I'd have to deal with the methods it employs just as I had to talk about the methods of absolute dating: according to the principles by which this textbook is written, I can't just say: "Here are the results geologists say are true, kindly oblige me by taking them as gospel". So here goes.

  
Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


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Message 257 of 294 (690034)
02-08-2013 3:39 AM


Paleoclimatology: Introduction
Paleoclimatology: introduction
Introduction
In this article we shall take a brief overview of what paleoclimatology is before getting down to specifics.
What is paleoclimatology?
As the name suggests, paleoclimatology is the study of ancient climates. Climate can be defined as broad trends in the weather: the fact that it rained in Seattle yesterday is merely weather; the fact that it often rains in Seattle is climate.
By definition, paleoclimatology must include the whole of climatology, plus some techniques for finding out about past conditions. Now, climatology is a complicated science, because the climate is composed of many parts which interact with one another in complex ways. A full introduction to the subject would have to deal with the influence and relations of such factors as the salinity of the ocean, the biological productivity of the land, the composition of the atmosphere, the effects of chemical weathering on climate, the effects of climate on chemical weathering, the deposition of coal, the short and long term effects of volcanoes, variations in the Earth's orbit, and dozens of other considerations. To explore all these factors in the detail they really deserve would so unbalance this textbook as to make it require a new title along the lines of "Introduction To Geology Plus Everything You Ever Wanted To Know About The Climate".
Instead, we shall focus more on methods for finding out what the climate was like in the past than on understanding the underlying principles that drive the climate.
Methods: measurement
The gold standard of measurement would be instrumental measurement, such as is performed with a thermometer, a barometer, a rain gauge, etc. However, such measurements have only been made in the recent past, and obviously geologists have to look elsewhere for paleoclimatic data: they have to look at the sedimentary and fossil records.
In paleoclimatology, the relationship between the geological record and the facts is not so straightforward as in other areas of geology: there is nothing that stands in the same simple relationship to (for example) ancient temperatures as aeolian sand does to ancient deserts, or fossils do to ancient organisms. Instead, geologists have found it necessary to develop proxies. A proxy might be defined as something we can measure which is not the thing we'd actually like to measure, but which bears a known relationship to it. A proxy can be justified in various ways: by testing that it presently indicates what we think it indicates, by observation of the natural world or of laboratory experiments; on theoretical grounds that argue that the proxy ought to indicate what we think it indicates; by comparison to proxies in which we already feel confident; or any combination of these considerations. We shall look at a selection of these proxies in subsequent articles.
Methods: models
As we have noted above, the climate is complicated, climatic models are complicated, and to keep from doubling the length of this book we shall have to treat climate models more or less as black boxes containing the accumulated knowledge of climatologists.
That said, the construction of climate models is of interest to us: we can use models to figure out what the climate should have been like in the past, and then compare this with the evidence we have for what the climate actually was like. If the two match up, then this increases our confidence in both our proxies and our models.
This also gives us a check on our reconstruction of past geological events. For example, if geologists tell us that there used to be a mountain range along the coast of a continent, and if climate models tell us that the mountain range should have created a rain-shadow desert on its leeward side, then if we find evidence of a desert in the right place with the right date, this increases our confidence in the models and in the geologists' claim that there used to be a mountain range where they say it was. In this way paleoclimatology can not only draw evidence from the other fields in historical geology, but can also contribute evidence to them.
Edited by Dr Adequate, : No reason given.
Edited by Dr Adequate, : No reason given.

  
Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


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Message 258 of 294 (690096)
02-08-2013 10:08 PM


Sediments And Paleoclimates
Sediments and paleoclimates
Introduction
In this article we shall look at how various kinds of sediment are characteristic of the climates in which they are formed, so that the lithified counterparts of these sediments can be used as an indicator of climatic conditions in the past.
The reader may find it useful to look back at the main articles on deserts, glaciers, paleosols, ooids, and coal to put the remarks below into their proper perspective.
Sediments and paleoclimates
There are a number of types of sediment which are symptomatic of the climatic conditions under which they formed.
* Glacial sedimentation and erosion. As we have discussed in the article on glaciers, we can detect their former presence by such clues as striations, glacial till, and dropstones. Obviously the presence of glaciers indicates a cold climate.
* Similarly the presence of aeolian sand, with its distinctive large-scale cross-bedding, pinstripe laminae, etc, indicates an arid and usually a hot climate. The same may be said of features such as playa lakes, indicative of low rainfall and a high rate of evaporation.
* Redbeds are sedimentary rocks cemented together chiefly by iron oxides; these are characteristic of a dry climate.
* Ooids are formed only in warm, shallow, agitated water, and so are indicators of a warm climate.
* Coal requires a peat swamp, and therefore cannot form in an arid climate.
* The soil types known as laterites are produced in tropical conditions with seasonal alternation between a monsoon season and a dry season, and so the corresponding paleosols indicate such conditions where they are found in the geological record.
How do we know?
For information about how we know that glacial till is deposited by glaciers, or that coal is lithified peat, and so forth, the reader should refer to the main articles on those topics.
Given that knowledge, the inference from the sedimentary rocks to the climate in which they were formed is a fairly obvious one. It is difficult, after all, to suppose that in times gone past glaciers flowed in conditions of sweltering heat, and it is a downright contradiction in terms to think of a dry swamp. Similar remarks apply to the other sediment types listed above; unless the laws of physics or chemistry were significantly different in the past than they are in the present, it is hard to see how sediments in the past could be characteristic of different climates than the corresponding sediments in the present.
Edited by Dr Adequate, : No reason given.

  
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