RSS

Scientific Dating Systems 2

20 Jun

We continue our look at the different scientific/archaeological dating systems used by researchers in their attempt to place dates upon the unknowable or those parts of the past we can date within reasonable time frames.

One note here, all those who advocate a long history or age to our earth and universe ignore one vital detail, possibly two. First, they ignore and deny the existence of evil, its deceiving nature and its lack of truth and two that all secular systems are fallible and built by those who are blinded by sin and unbelief.

1. Jerusalem Fell in 587 Not 586 BC by C. Ermal Allen

Biblical scholarship will be forever indebted to Edwin Thiele for his discovery that finally made sense of the confusing dating systems for kings mentioned in the OT. Two keys unlocked the secrets that had mystified scholars for centuries: (1) the year in some time-lines began with the month Nisan (in the spring), and in others it began with Tishri (in the fall). Furthermore, (2) the first year of the king’s reign was sometimes counted from the year he actually began and at other times from his first full year, counting the initial months prior to the new year as his accession year.

When these two keys are combined, the result is four different ways of dating any particular year in the reign of a king. See the chart below, based on Thiele’s work, showing the possibilities for dating the last days of Jerusalem, when it was besieged and destroyed by the Babylonians.

(2005). Bible and Spade (2005), 18(1), 25.

2. Origins: Fixing the Millennium– Just how did we get to the year 2000 anyway?By Leonora Neville

Most of us are already familiar with the Common Era (C.E.) as a secular version of the Anno Domini (A.D.) chronological system, which dates events according to “the Year of Our Lord,” or the birth of Jesus. But when exactly did people start dating things from the time of Christ? Obviously when Jesus was born no one had a calendar saying it was year 0. Herod had no way of knowing he came to power in the year 37 B.C. In fact, it was not until hundreds of years after the time of Jesus that anyone tried to reckon the years that had elapsed since his birth.

Most of the earliest Christians were converted Jews, who relied on the Jewish lunar calendar; but as Christianity spread to other groups, most people continued to use the Roman calendar introduced by Julius Caesar in 46 B.C.E. (Before the Common Era). Created with the help of the great Alexandrian astronomer Sosigenes, Caesar’s “Julian” calendar formally established a solar year measuring 12 months or 365 1/4 days. (The quarter was made up in an extra day every fourth or “leap” year.)

While the Julian Calendar effectively standardized the length of the year throughout the Roman Empire—and made it easy to refer to a particular date within a given year—a variety of options were still available for distinguishing one year from another. The Romans frequently referred to a particular year by the names of the consuls who had ruled at the time. Roman historians sometimes also numbered years from “the founding of the city of Rome” (ab urba condita) in what we would call 753 B.C.E. A third way of numbering years was by fixing them in relation to the Indiction, or 15-year tax cycle.

In the fourth century, many Christians began situating themselves within the “Era of the Martyrs,” which started in 284 C.E., with the Roman emperor Diocletian’s persecution of Christians. Citizens of Antioch in Syria pegged “year 1” to 49 B.C.E. in commemoration of Julius Caesar’s dictatorship. In the fifth century many Greek-speaking Christians started to number years from the creation of the world (Anno Mundi), which they believed occurred in either 5493 or 5509 B.C.E. By the tenth century C.E., Anno Mundi dating—with the world’s creation fixed at 5509 B.C.E.—became standard in the Byzantine Empire and hence in the Orthodox countries of Eastern Europe…

While the A.D. system spread throughout Europe during the Middle Ages, the basic calendar in use was still Julius Caesar’s. But the Julian Calendar’s reckoning of 365 1/4; days in one solar year was too long by 11 minutes and 14 seconds each year. This meant that over time astronomical phenomena fell out of sync with their fixed dates in the calendar. By the 16th century, the spring equinox occurred ten days before March 21!

In the 1570s a group of church astronomers, led by the Jesuit scholar Christopher Clavius (1537–1612), recalculated the length of the solar year and arrived at a more precise estimate of 365.2422 days. (Their estimate was remarkably accurate; modern scientists calculate the year as 365.242199 days.) Since the difference between the Jesuit solar year (365.242 days) and the Julian year (365.25 days) added up to a total of 3.12 days every 400 years, the Jesuits proposed that three out of every four centennial years should no longer be leap years. (For example, the centennial years 1700, 1800 and 1900 were not leap years, but the year 2000 will be one.) This modification to the Julian system saves three days every four centuries and ensures that the modern calendar loses only .0003 days each year.

In 1582, Pope Gregory XIII issued a papal bull officially adopting the Jesuit calendar reforms. To eliminate the already existing time lag between astronomical phenomena and their fixed dates, he also cut ten days out of October that year. By papal decree, October 4 was followed by October 15, 1582.

Unfortunately, Gregory’s reforms were ill-timed. At the moment of his decree, much of Europe was embroiled in the Wars of Religion (1562–1598) and a good many people in Europe (to say nothing of the rest of the world) were unwilling to change their calendars just because the Pope said so. While the new Gregorian calendar was adopted fairly quickly by the Catholic countries of Europe, the German Protestant nations refused to accept Gregory’s reforms until 1699. England clung to the Julian calendar until 1752!

Shanks, H. (Ed.). (2004). Archaeology Odyssey 03:01.

3. Radiometric Dating: A Christian Perspective byDr. Roger C. Wiens  — http://asa3.org/ASA/resources/Wiens.html

The next few pages cover a broad overview of radiometric dating techniques, show a few examples, and discuss the degree to which the various dating systems agree with each other. The goal is to promote greater understanding on this issue, particularly for the Christian community.

Many people have been led to be skeptical of dating without knowing much about it. For example, most people don’t realize that carbon dating is only rarely used on rocks. God has called us to be “wise as serpents” (Matt. 10:16) even in this scientific age. In spite of this, differences still occur within the church.

A disagreement over the age of the Earth is relatively minor in the whole scope of Christianity; it is more important to agree on the Rock of Ages than on the age of rocks. But because God has also called us to wisdom, this issue is worthy of study.

Rocks are made up of many individual crystals, and each crystal is usually made up of at least several different chemical elements such as iron, magnesium, silicon, etc. Most of the elements in nature are stable and do not change. However, some elements are not completely stable in their natural state. Some of the atoms eventually change from one element to another by a process called radioactive decay. If there are a lot of atoms of the original element, called the parent element, the atoms decay to another element, called the daughter element, at a predictable rate. The passage of time can be charted by the reduction in the number of parent atoms, and the increase in the number of daughter atoms.

Radiometric dating can be compared to an hourglass. When the glass is turned over, sand runs from the top to the bottom. Radioactive atoms are like individual grains of sand–radioactive decays are like the falling of grains from the top to the bottom of the glass. You cannot predict exactly when any one particular grain will get to the bottom, but you can predict from one time to the next how long the whole pile of sand takes to fall.

Once all of the sand has fallen out of the top, the hourglass will no longer keep time unless it is turned over again. Similarly, when all the atoms of the radioactive element are gone, the rock will no longer keep time

Unlike the hourglass, where the amount of sand falling is constant right up until the end, the number of decays from a fixed number of radioactive atoms decreases as there are fewer atoms left to decay (see Figure 1). If it takes a certain length of time for half of the atoms to decay, it will take the same amount of time for half of the remaining atoms, or a fourth of the original total, to decay. In the next interval, with only a fourth remaining, only one eighth of the original total will decay.

By the time ten of these intervals, or half-lives, has passed, less than one thousandth of the original number of radioactive atoms is left. The equation for the fraction of parent atoms left is very simple. The type of equation is exponential, and is related to equations describing other well-known phenomena such as population growth. No deviations have yet been found from this equation for radioactive decay.

Table 1. Some Naturally Occurring Radioactive Isotopes and their half-lives

Radioactive Isotope

(Parent)

Product

(Daughter)

Half-Life

(Years)

Samarium-147 Neodymium-143 106 billion
Rubidium-87 Strontium-87 48.8 billion
Rhenium-187 Osmium-187 42 billion
Lutetium-176 Hafnium-176 38 billion
Thorium-232 Lead-208 14 billion
Uranium-238 Lead-206 4.5 billion
Potassium-40 Argon-40 1.26 billion
Uranium-235 Lead-207 0.7 billion
Beryllium-10 Boron-10 1.52 million
Chlorine-36 Argon-36 300,000
Carbon-14 Nitrogen-14 5715
Uranium-234 Thorium-230 248,000
Thorium-230 Radium-226 75,400

Potassium-Argon. Potassium is an abundant element in the Earth’s crust. One isotope, potassium-40, is radioactive and decays to two different daughter products, calcium-40 and argon-40, by two different decay methods. This is not a problem because the production ratio of these two daughter products is precisely known, and is always constant: 11.2% becomes argon-40 and 88.8% becomes calcium-40. It is possible to date some rocks by the potassium-calcium method, but this is not often done because it is hard to determine how much calcium was initially present.

Argon, on the other hand, is a gas. Whenever rock is melted to become magma or lava, the argon tends to escape. Once the molten material hardens, it begins to trap the new argon produced since the hardening took place. In this way the potassium-argon clock is clearly reset when an igneous rock is formed…

Although potassium-argon is one of the simplest dating methods, there are still some cases where it does not agree with other methods. When this does happen, it is usually because the gas within bubbles in the rock is from deep underground rather than from the air. This gas can have a higher concentration of argon-40 escaping from the melting of older rocks.

This is called parentless argon-40 because its parent potassium is not in the rock being dated, and is also not from the air. In these slightly unusual cases, the date given by the normal potassium-argon method is too old. However, scientists in the mid-1960s came up with a way around this problem, the argon-argon method, discussed in the next section.

Argon-Argon. Even though it has been around for nearly half a century, the argon-argon method is seldom discussed by groups critical of dating methods. This method uses exactly the same parent and daughter isotopes as the potassium-argon method. In effect, it is a different way of telling time from the same clock. Instead of simply comparing the total potassium with the non-air argon in the rock, this method has a way of telling exactly what and how much argon is directly related to the potassium in the rock.

In the argon-argon method the rock is placed near the center of a nuclear reactor for a period of hours. A nuclear reactor emits a very large number of neutrons, which are capable of changing a small amount of the potassium-39 into argon-39. Argon-39 is not found in nature because it has only a 269-year half-life. (This half-life doesn’t affect the argon-argon dating method as long as the measurements are made within about five years of the neutron dose).

The rock is then heated in a furnace to release both the argon-40 and the argon-39 (representing the potassium) for analysis. The heating is done at incrementally higher temperatures and at each step the ratio of argon-40 to argon-39 is measured. If the argon-40 is from decay of potassium within the rock, it will come out at the same temperatures as the potassium-derived argon-39 and in a constant proportion. On the other hand, if there is some excess argon-40 in the rock it will cause a different ratio of argon-40 to argon-39 for some or many of the heating steps, so the different heating steps will not agree with each other

Rubidium-Strontium. In nearly all of the dating methods, except potassium-argon and the associated argon-argon method, there is always some amount of the daughter product already in the rock when it cools. Using these methods is a little like trying to tell time from an hourglass that was turned over before all of the sand had fallen to the bottom.

One can think of ways to correct for this in an hourglass: One could make a mark on the outside of the glass where the sand level started from and then repeat the interval with a stopwatch in the other hand to calibrate it. Or if one is clever she or he could examine the hourglass’ shape and determine what fraction of all the sand was at the top to start with.

By knowing how long it takes all of the sand to fall, one could determine how long the time interval was. Similarly, there are good ways to tell quite precisely how much of the daughter product was already in the rock when it cooled and hardened.

In the rubidium-strontium method, rubidium-87 decays with a half-life of 48.8 billion years to strontium-87. Strontium has several other isotopes that are stable and do not decay. The ratio of strontium-87 to one of the other stable isotopes, say strontium-86, increases over time as more rubidium-87 turns to strontium-87.

But when the rock first cools, all parts of the rock have the same strontium-87/strontium-86 ratio because the isotopes were mixed in the magma. At the same time, some of the minerals in the rock have a higher rubidium/strontium ratio than others. Rubidium has a larger atomic diameter than strontium, so rubidium does not fit into the crystal structure of some minerals as well as others…

The Samarium-Neodymium, Lutetium-Hafnium, and Rhenium-Osmium Methods. All of these methods work very similarly to the rubidium-strontium method. They all use three-isotope diagrams similar to Figure 4 to determine the age. The samarium-neodymium method is the most-often used of these three. It uses the decay of samarium-147 to neodymium-143, which has a half-life of 105 billion years. The ratio of the daughter isotope, neodymium-143, to another neodymium isotope, neodymium-144, is plotted against the ratio of the parent, samarium-147, to neodymium-144.

If different minerals from the same rock plot along a line, the slope is determined, and the age is given by the same equation as above. The samarium-neodymium method may be preferred for rocks that have very little potassium and rubidium, for which the potassium-argon, argon-argon, and rubidium-strontium methods might be difficult.

The samarium-neodymium method has also been shown to be more resistant to being disturbed or re-set by metamorphic heating events, so for some metamorphosed rocks the samarium-neodymium method is preferred. For a rock of the same age, the slope on the neodymium-samarium plots will be less than on a rubidium-strontium plot because the half-life is longer.

However, these isotope ratios are usually measured to extreme accuracy–several parts in ten thousand–so accurate dates can be obtained even for ages less than one fiftieth of a half-life, and with correspondingly small slopes.

The lutetium-hafnium method uses the 38 billion year half-life of lutetium-176 decaying to hafnium-176. This dating system is similar in many ways to samarium-neodymium, as the elements tend to be concentrated in the same types of minerals. Since samarium-neodymium dating is somewhat easier, the lutetium-hafnium method is used less often….

Uranium-Lead and related techniques. The uranium-lead method is the longest-used dating method. It was first used in 1907, about a century ago. The uranium-lead system is more complicated than other parent-daughter systems; it is actually several dating methods put together. Natural uranium consists primarily of two isotopes, U-235 and U-238, and these isotopes decay with different half-lives to produce lead-207 and lead-206, respectively.

In addition, lead-208 is produced by thorium-232. Only one isotope of lead, lead-204, is not radiogenic. The uranium-lead system has an interesting complication: none of the lead isotopes is produced directly from the uranium and thorium. Each decays through a series of relatively short-lived radioactive elements that each decay to a lighter element, finally ending up at lead. Since these half-lives are so short compared to U-238, U-235, and thorium-232, they generally do not affect the overall dating scheme.

The result is that one can obtain three independent estimates of the age of a rock by measuring the lead isotopes and their parent isotopes. Long-term dating based on the U-238, U-235, and thorium-232 will be discussed briefly here; dating based on some of the shorter-lived intermediate isotopes is discussed later.

The uranium-lead system in its simpler forms, using U-238, U-235, and thorium-232, has proved to be less reliable than many of the other dating systems. This is because both uranium and lead are less easily retained in many of the minerals in which they are found. Yet the fact that there are three dating systems all in one allows scientists to easily determine whether the system has been disturbed or not.

Using slightly more complicated mathematics, different combinations of the lead isotopes and parent isotopes can be plotted in such a way as to minimize the effects of lead loss. One of these techniques is called the lead-lead technique because it determines the ages from the lead isotopes alone. Some of these techniques allow scientists to chart at what points in time metamorphic heating events have occurred, which is also of significant interest to geologists…

ree rings do not provide continuous chronologies beyond 11,800 years ago because a rather abrupt change in climate took place at that time, which was the end of the last ice age. During the ice age, long-lived trees grew in different areas than they do now. There are many indicators, some to be mentioned below, that show exactly how the climate changed at the end of the last ice age. It is difficult to find continuous tree ring records through this period of rapid climate change. Dendrochronology will probably eventually find reliable tree records that bridge this time period, but in the meantime, the carbon-14 ages have been calibrated farther back in time by other means.

Calibration of carbon-14 back to almost 50,000 years ago has been done in several ways. One way is to find yearly layers that are produced over longer periods of time than tree rings. In some lakes or bays where underwater sedimentation occurs at a relatively rapid rate, the sediments have seasonal patterns, so each year produces a distinct layer.

Such sediment layers are called “varves”, and are described in more detail below. Varve layers can be counted just like tree rings. If layers contain dead plant material, they can be used to calibrate the carbon-14 ages.

Another way to calibrate carbon-14 farther back in time is to find recently-formed carbonate deposits and cross-calibrate the carbon-14 in them with another short-lived radioactive isotope. Where do we find recently-formed carbonate deposits?

If you have ever taken a tour of a cave and seen water dripping from stalactites on the ceiling to stalagmites on the floor of the cave, you have seen carbonate deposits being formed. Since most cave formations have formed relatively recently, formations such as stalactites and stalagmites have been quite useful in cross-calibrating the carbon-14 record.

What does one find in the calibration of carbon-14 against actual ages? If one predicts a carbon-14 age assuming that the ratio of carbon-14 to carbon-12 in the air has stayed constant, there is a slight error because this ratio has changed slightly. Figure 9 shows that the carbon-14 fraction in the air has decreased over the last 40,000 years by about a factor of two.

This is attributed to a strengthening of the Earth’s magnetic field during this time. A stronger magnetic field shields the upper atmosphere better from charged cosmic rays, resulting in less carbon-14 production now than in the past. (Changes in the Earth’s magnetic field are well documented. Complete reversals of the north and south magnetic poles have occurred many times over geologic history.)

A small amount of data beyond 40,000 years (not shown in Fig. 9) suggests that this trend reversed between 40,000 and 50,000 years, with lower carbon-14 to carbon-12 ratios farther back in time, but these data need to be confirmed.

…The offset is generally less than 1500 years over the last 10,000 years, but grows to about 6,000 years at 40,000 years before present. Uncalibrated radiocarbon ages underestimate the actual ages. Note that a factor of two difference in the atmospheric carbon-14 ratio, as shown in the top panel of Figure 9, does not translate to a factor of two offset in the age. Rather, the offset is equal to one half-life, or 5,700 years for carbon-14.

This is only about 15% of the age of samples at 40,000 years. The initial portion of the calibration curve in Figure 9 has been widely available and well accepted for some time, so reported radiocarbon dates for ages up to 11,800 years generally give the calibrated ages unless otherwise stated. The calibration curve over the portions extending to 40,000 years is relatively recent, but should become widely adopted as well.

Ice Cores. One of the best ways to measure farther back in time than tree rings is by using the seasonal variations in polar ice from Greenland and Antarctica. There are a number of differences between snow layers made in winter and those made in spring, summer, and fall. These seasonal layers can be counted just like tree rings.

The seasonal differences consist of a) visual differences caused by increased bubbles and larger crystal size from summer ice compared to winter ice, b) dust layers deposited each summer, c) nitric acid concentrations, measured by electrical conductivity of the ice, d) chemistry of contaminants in the ice, and e) seasonal variations in the relative amounts of heavy hydrogen (deuterium) and heavy oxygen (oxygen-18) in the ice.

These isotope ratios are sensitive to the temperature at the time they fell as snow from the clouds. The heavy isotope is lower in abundance during the colder winter snows than it is in snow falling in spring and summer. So the yearly layers of ice can be tracked by each of these five different indicators, similar to growth rings on trees.

Ice cores are obtained by drilling very deep holes in the ice caps on Greenland and Antarctica with specialized drilling rigs. As the rigs drill down, the drill bits cut around a portion of the ice, capturing a long undisturbed “core” in the process. These cores are carefully brought back to the surface in sections, where they are catalogued, and taken to research laboratories under refrigeration. A very large amount of work has been done on several deep ice cores up to 9,000 feet in depth. Several hundred thousand measurements are sometimes made for a single technique on a single ice core.

A continuous count of layers exists back as far as 160,000 years. In addition to yearly layering, individual strong events (such as large-scale volcanic eruptions) can be observed and correlated between ice cores. A number of historical eruptions as far back as Vesuvius nearly 2,000 years ago serve as benchmarks with which to determine the accuracy of the yearly layers as far down as around 500 meters.

As one goes further down in the ice core, the ice becomes more compacted than near the surface, and individual yearly layers are slightly more difficult to observe. For this reason, there is some uncertainty as one goes back towards 100,000 years. Ages of 40,000 years or less are estimated to be off by 2% at most.

Ages of 60,000 years may be off by up to 10%, and the uncertainty rises to 20% for ages of 110,000 years based on direct counting of layers (D. Meese et al., J. Geophys. Res. 102, 26,411, 1997). Recently, absolute ages have been determined to 75,000 years for at least one location using cosmogenic radionuclides chlorine-36 and beryllium-10 (G. Wagner et al., Earth Planet. Sci. Lett. 193, 515, 2001).

These agree with the ice flow models and the yearly layer counts. Note that there is no indication anywhere that these ice caps were ever covered by a large body of water, as some people with young-Earth views would expect…

Thermoluminescence. There is a way of dating minerals and pottery that does not rely directly on half-lives. Thermoluminescence dating, or TL dating, uses the fact that radioactive decays cause some electrons in a material to end up stuck in higher-energy orbits.

The number of electrons in higher-energy orbits accumulates as a material experiences more natural radioactivity over time. If the material is heated, these electrons can fall back to their original orbits, emitting a very tiny amount of light. If the heating occurs in a laboratory furnace equipped with a very sensitive light detector, this light can be recorded. (The term comes from putting together thermo, meaning heat, and luminescence, meaning to emit light).

By comparison of the amount of light emitted with the natural radioactivity rate the sample experienced, the age of the sample can be determined. TL dating can generally be used on samples less than half a million years old. Related techniques include optically stimulated luminescence (OSL), and infrared stimulated luminescence (IRSL).

TL dating and its related techniques have been cross calibrated with samples of known historical age and with radiocarbon and thorium dating. While TL dating does not usually pinpoint the age with as great an accuracy as these other conventional radiometric dating, it is most useful for applications such as pottery or fine-grained volcanic dust, where other dating methods do not work as well…

Well, the situation is very similar for the dating of rocks, only we have rock records rather than historical records. Consider the following:

  • There are well over forty different radiometric dating methods, and scores of other methods such as tree rings and ice cores.
  • All of the different dating methods agree–they agree a great majority of the time over millions of years of time. Some Christians make it sound like there is a lot of disagreement, but this is not the case. The disagreement in values needed to support the position of young-Earth proponents would require differences in age measured by orders of magnitude (e.g., factors of 10,000, 100,000, a million, or more). The differences actually found in the scientific literature are usually close to the margin of error, usually a few percent, not orders of magnitude!
  • Vast amounts of data overwhelmingly favor an old Earth. Several hundred laboratories around the world are active in radiometric dating. Their results consistently agree with an old Earth. Over a thousand papers on radiometric dating were published in scientifically recognized journals in the last year, and hundreds of thousands of dates have been published in the last 50 years. Essentially all of these strongly favor an old Earth.
  • Radioactive decay rates have been measured for over sixty years now for many of the decay clocks without any observed changes. And it has been close to a hundred years since the uranium-238 decay rate was first determined.
  • Both long-range and short-range dating methods have been successfully verified by dating lavas of historically known ages over a range of several thousand years.
  • The mathematics for determining the ages from the observations is relatively simple.

4. Dating Methods in Science: Strata, Fossils and Age of the Earth — http://darwiniana.org/datingmethods.htm

Because of the distortions and lies spread by fundamentalists about scientific dating there is a need for a centralized source of information on the topic. A few examples of such lies are presented at the very bottom of this page.

For each dating or chronological method there is a link in the box at right to take you to that section of this page. There, you will find a brief description of the method, plus links to take you to other webpages with more extensive information.

Dating is not necessary to demonstrate that evolution is a fact. Chronological sequence is all that is really required. However, human beings love to see factual precision, and we want to know how old something is.

Please remember that all dating methods, even those termed “absolute,” are subject to margins of error. We say the Earth is 4.56 ± 0.02 billion years old. That is a very small amount of possible error range. There are 20 methods shown here. Modern studies almost always use two or more methods to confirm dating work and to build confidence in the results obtained.

Steno’s Law – The Law of Superposition : A bit of history about Nicolas Steno, who formulated the Law of Superposition.

Geologic Time: Relative Time Scale : James Hutton and William Smith advanced the concept of geologic time and strengthened the belief in an ancient world. Hutton, a Scottish geologist, first proposed formally the fundamental principle used to classify rocks according to their relative ages. He concluded, after studying rocks at many outcrops, that each layer represented a specific interval of geologic time.

Further, he proposed that wherever uncontorted layers were exposed, the bottom layer was deposited first and was, therefore, the oldest layer exposed; each succeeding layer, up to the topmost one, was progressively younger. The Major Divisions of Geologic Time are shown here, arranged in chronological order with the oldest division at the bottom, the youngest at the top.

Relative Time, Superposition and Cross-cutting Relationships : Geologic intrusions, faults and unconformities are explained and pictured.

Stratigraphy and Cross-Dating/Biostratigraphy : Stratigraphy is the study of strata, or layers. Specifically, stratigraphy refers to the application of the Law of Superposition to soil and geological strata containing archaeological materials in order to determine the relative ages of layers. Cross-dating is a technique used to take advantage of consistencies in stratigraphy between parts of a site or different sites, and objects or strata with a known relative chronology. A specialized form of cross-dating, using animal and plant fossils, is known as biostratigraphy.

Correlation by Fossils : Correlation means matching the order of geologic events in one place with the order of geologic events in another place. By far, the most widespread method of correlation uses fossils

Geologic Time: Index Fossils : Keyed to the relative time scale are examples of index fossils, the forms of life which existed during limited periods of geologic time and thus are used as guides to the age of the rocks in which they are preserved.

William “Strata” Smith, a civil engineer and surveyor, was well acquainted with areas in southern England where “limestone and shales are layered like slices of bread and butter.” His hobby of collecting and cataloging fossil shells from these rocks led to the discovery that certain layers contained fossils unlike those in other layers. Using these key or index fossils as markers, Smith could identify a particular layer of rock wherever it was exposed. Because fossils actually record the slow but progressive development of life, scientists use them to identify rocks of the same age throughout the world.

Cosmic-ray Exposure Dating : This dating method relies on measuring certain isotopes produced by cosmic ray impacts on exposed rock surfaces. Because cosmic rays constantly bombard meteorites flying through space, this method has long been used to date the “flight time” of meteorites–that is the time from when they were chipped off a larger body (like an asteroid) to the time they land on Earth.

Coral Slide Set from NOAA : Corals exhibit seasonal growth bands very much like those in trees. Sometimes these bands are visible to the naked eye; usually, however, they are more visible in an x-ray like the one shown at right. When paleoclimatologists drill a coral core, they can count the growth bands and date samples exactly. Long cores can cover several hundred years; this portion of a core from Urvina Bay in the Galápagos Islands covers the period from 1716 to 1735 A. D.

Tidal Slowdown, Coral Growth, and the Age of the Earth : In certain modern corals we find growth-bands that indicate yearly, monthly, and even daily growth. There are about thirty daily bands per month and about 365 daily bands per year for modern corals and shellfish. But careful analysis of the growth-bands of fossil corals and shellfish from the Devonian and Pennsylvanian has confirmed that years in these periods contained more days than years do now (about 400).

Amino Acid Geochronology : This is a relative, and sometimes absolute, dating method that relates the diagenesis of fossil protein preserved in carbonate materials with time (geologic age of the sample) and temperature (long term chemical temperature of the enclosing sediment). Stratigraphic applications of the method have been demonstrated from both marine and non-marine sequences all over the world using a variety of carbonate fossil materials including mollusks, foraminifera, bone, ostrich egg shells, ostracodes, and tooth enamel. A brief explanation is given at Bear Lake Methods: Amino Acid Dating.

Amino Acid Racemization : Provides a frank discussion of possible problems encountered when using this method, and the need for cross-checking results against other methods.

Advertisements
 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

 
%d bloggers like this: