Geochronology The delta 18 O record (from ice cores) shows a very dramatic increase in delta 18O that starts a period of abnormally high d18O. This period has been termed the Younger Dryas Cold Event. However, when looked for in other proxy methods, it was found that the start of this event (the dramatic increase) is typically found to be almost two thousand years older than shown in the ice record (13kyrBP instead of 11kyrBP). These extreme events that are distinguishable in many proxy records are important because they allow us to match chronologies so we can take out as much uncertainty as possible. Tree rings (non-bound proxy?) Tree rings are the classic and most basic example of a proxy for chronology. A tree grows best in Spring, in warmish, moist conditions with plenty of sun, and then develops bark (by poor growth) in the late Summer, when it’s dry and too warm, preparing itself for fall and winter. Tree rings are a product of this. Each ring represents a year of growth, with the light area being fast growth (spring), and the dark area being slow growth (late summer). Hence we can count the number of summers the tree has lived in by counting the dark rings. We can also learn something about the conditions of each spring by the width of the lighter bands. This method isn’t infallible, as the seasons don’t always live up to their expectations, and hence the tree may react to an odd summer in a different way, causing confusion between what is and isn’t a tree ring (for example, if there is an extremely dry spring, very little growth may mean two rings and very close and indistinguishable from each other). However, remember that counting tree rings only works if you know when it was cut down, as this is the outermost ring. Without that information, all you have is the length of it’s life and the condition of those years, but you cannot pin those conditions on any given years. This is a non-bound proxy. Another useful non-bound proxy are stalagmites. Stalagmites also form “layers” in a similar way to tree rings. Each layer represents a year. Ice cores and lake sediments also form layers similarly. However, in order to use the layers, you need to have a definite age of one of them, in order to extrapolate the age of it’s neighbours. Radiometric dating This need to define the need of a ring was reduced/satisfied when radiometric dating arose. Radiocarbon dating (specifically) uses the amount of C14 isotope in a dead sample to estimate when it “died” (cut off it’s equilibrium from the environment). This calculation requires 3 pieces of information. The initial amount of C14 in the environment, the amount of C14 in the sample now, and the half life of C14. Then, using C(now) = C(initial)exp(lambda.*t), we can solve for t. This works with C14 because C14 is a radioactive isotope of carbon. However, one difficult thing to work out is what C(initial) was. Unfortunately for this calculation, the amount of C14 in the atmosphere is not a constant over time (unsurprisingly). C14 is produced by the bombardment of nitrogen atoms (N14) in the atmosphere by cosmic-ray-derived neutrons. Neutron + N14 = C14 + proton. Hence the amount of cosmic-ray-derived neutrons that enter the atmosphere determine the amount of C14 in the atmosphere and therefore determines the amount of C14 that enters into carbon stores like trees or stalagmites. The cosmic ray flux from the universe onto earth is constant/stable, however, cosmic rays are deflected by solar wind, hence the more active the sun is, the less cosmic rays enter our atmosphere. In conclusion, radiocarbon production is affected by solar wind and solar activity, such that the more solar wind, the less C14 is produced in the atmosphere. This variable amount of C14 means we need to come up with a calibration curve for C14. This is done by taking a proxy method that incorporates both C14 and another proxy method like tree rings. Trees can be used for this. We can use the tree rings as a definite age, and then for each year measure the C14. By relating these two, we come up with a calibration curve that takes the C14 “age” and then shows you what it’s actual age is. See figure below. This is great, however, there are obvious problems. If the C14 age correlates to two or more calendar ages, then you don’t know which one to take and you must look at the context to decide which age is more likely. We have only been measuring solar activity for about 400 years, so it is difficult to use that as context for old samples, and from our records we have yet to understand it’s pattern, so cannot extrapolate back. Furthermore, collected data seems to suggest that C14 production rate is also controlled by slow changes in the magnetic field, namely the slow weakening of the geomagnetic field leading to a gradual increase in 14C production. Another problem we find with radiocarbon dating is the assumption that our system is in equilibrium with the atmosphere. Some systems, like corals or stalagmites not only consist of organic carbon (equilibrium with atmosphere), but also of carbon contributions from other carbon forms nearby, for example limestone bedrock (stalagmites) or bicarbonate ions in seawater (corals). This adds ‘dead’ carbon that is older than the system we are interested in, making the data appear older than it really is. For example, stalagmite growth occurs by the addition of minerals to the stalagmite from water that is passing by. Typically stalagmites occur in caves, underground. This means that the water will likely have passed through some other bedrocks in the process of getting there from the surface (where it would have been in equilibrium with the atmosphere). If the rock that it has passed through has been carbon rich, then it may pick up lots of this carbon (note this carbon will be very old with very little C14 as it would have been deposited a long time ago – relative to the lifetime of C14). This will dilute the concentration of C14 in the water, and hence reduce the concentration of C14 in the stalagmites it goes to help form, making the stalagmite seem older by C14 dating. The only way around this problem is the use of another dating method to try and correct for the ‘reservoir effect’ (described above). U-Th dating is one method that can be useful in these situations. Once the ‘reservoir effect’ has been calculated for one area of the stalagmite it can be applied to the whole thing (given there’s no reason to think it may have changed). The last problem with radiocarbon dating is referred to as the bomb spike. The period after the second world war saw a serious increase in the testing of nuclear bombs, which caused an artificial increase in the amount of radiocarbon in the atmosphere. The PTBT agreement on 10 October 1963 stopped this, but by that time the radiocarbon was already well and truly away from it’s natural level. It is currently returning slowly to it’s natural level, however this deviation means that radiocarbon dating doesn’t work for things younger than 1955. Hence all reported data take their datum as 1950, despite many of them saying ‘present’. U-Series dating Above is the U238 decay chain. It shows the series of decays that U238 undergoes until it reaches a stable daughter isotope. Activity = concentration x decay constant (lambda), where lambda = 0.693/half-life (don’t know where they got that from). Secular equilibrium is when the activity ratios of any isotopes in a decay chain are equal to 1. This is only reached in very ancient systems. Note, we are interested in activity here, not concentration. See the example in the slide below to understand. This series allows us to do daughter-deficiency dating. This is a method that works in situations where there is initially no daughter atoms present, but the parent atom is there. For example, we can use this method in stalagmites, because Th is insoluble, but U is soluble, hence stalagmites form with U but no Th due to their method of formation. This means that any Th found in the stalagmite must be from decay of a U atom. Normally, we would just use C=Czero.*exp(-lambda.*t) of the parent isotope. But in the U238 series, daughter elements are also radioactive, so instead of measuring using concentrations, activity ratios are used. The idea is that the youngest samples will be furthest from secular equilibrium (i.e. will have ratios of less than 1) and ancient samples will tend to 1. The ratio equation here is more complicated than ones we’ve seen before. A ratio of 1 correlates to an age of 500 ka or older (not possible to determine exact age after 500ka. U series dating doesn’t need a calibration curve, because we don’t need to know initial concentration, we can just use the secular equilibrium. Only major issue with this is that the sample may be contaminated by detrital Th (will mainly be Th232, which is not produced by the decay of uranium, but might have some Th230?). If the ratio of Th230/Th232 is less than 20, then the date might not be useable. You can do a detrital Th correction but this is mainly guesswork. Globally synchronous cooling during Younger Dryas Event? A 1996 paper suggested that the YD event was characterised by cold in Australia as well. They used U-Th dating, however very few terrestrial Southern Hemisphere records of YD exist, so difficult to find samples to test. The dating methods used then were very low resolution by modern standards. A 2013 paper later dated the same stalagmite and showed that the original dates were completely off. Moral of story : Although U-Th dating is theoretically sound, it can still be suspect, so treat all dating (even if U-Th), with caution. Cosmogenic nuclide surface exposure dating This method utilises the fact that rocks are ‘weathered’ by cosmogenic radiation. Atoms on the surface of the rock can be made radioactive by the neutrons of the radiation, and we can use this to measure how long that face of the rock has been exposed to the atmosphere. This gives us a method of dating when glaciers retreated, or when the rock was moved, or broken (exposing a fresh face). There are a couple problems, namely that different surfaces could have been bombarded by cosmic rays at different rates (due to orientation), and that the surface may have shifted (changing that rate). In practice you measure all surfaces and use the distribution of that to form a most likely age. Summary Different materials require different dating techniques, and you must understand the formation of the material and the context of the material in order to judge the results (stalagmites for example). Generally, chronologies become more uncertain as the material gets older U-series dating of carbonates is becoming more and more relevant and important. Currently at the forefront of dating (this, however, does not mean that it is infallible!).
