Direct-dating of fossils

Transcription

1 Geosciences /2/04 Direct-dating of fossils Absolute (calendar) age vs relative age Direct dates vs associated dates Methods for direct-dating 1. Growth rings. Dendrochronology and sclerochronology Individual fossils and cross-dating 2. Radiocarbon dating Radiocarbon incorporated into living material Exponential decay and half-lives Two key assumptions: (1) near-constant production rate and (2) rapid mixing Correcting for the assumptions: (1) tree-ring calibration and (2) reservoir ages Two applications: constructing a sea level curve and dating the Shroud of Turin 3. (if time available) Amino-acid dating Calibrating amino acid ratios Colorado delta shells Next two lectures consider how fossils are used in estimating the age of fossils and rocks, and in determining that two rocks were deposited at the same time (even if you don't know the precise age in millions of years). Reminder: absolute age (age in years) vs relative age (older than, younger than, same as) Some young fossils can be directly-dated to assign an absolute age (in years) 1. Growth rings: in wood (=dendrochronology) or accretionary skeleltons (coral, or mollusks) (=sclerochronology). Trees in seasonal climates add wood during the summer and slow or stop growth in the winter. Result a series of rings, each representing one year.

2 In good growing years (warm, wet, for example) the ring is wide, in bad years (cold, dry) the ring is thin. Provides information about environmental conditions from variations in ring width. Matching up distinctive bands from one tree to another allows construction of a master chronology that extends beyond the age of a single tree. Master chronologies back to about 12,000 years before present. Tree-ring illustrations from 2. Radiocarbon There are only a few circumstances in which one can geochemically analyze a fossil to directly determine its age. This is because organic matter and hard parts, don't incorporate many of the naturally radioactive isotopes used in dating. With one major and very useful exception. Radiocarbon. Radiocarbon, or carbon-14, commonly written 14 C, is a naturally occurring, continuously produced, radio-isotope of carbon (most of which is in the stable forms of Carbon 12 and Carbon 13). Carbon-14 is said to be an unstable isotope in that it undergoes radioactive decay through time to become a stable isotope, 14 N, or Nitrogen-14 - one that doesn t decay any farther..

3 That rate of decay is known, and like the decay of other radioactive elements, it follows the exponential decay curve: that is a constant percentage decrease in the number of radioactive atoms per unit time. With 14C, half of the 14 C atoms are gone after 5,730 years. This 5,730 years is said, therefore, to be radiocarbon's half-life. Half of what is remaining is gone in another 5,730 years. So, after two half-lives, only 1/4 of the original radiocarbon content is left; after three half-lives, only 1/8 remains, and so on. Radiocarbon ages reported as years before present, with present defined as Compared to other isotopes used in dating, Carbon-14 has a very short half-life. It's difficult to detect any radiocarbon in samples older than 50,000 years. Thus, radiocarbon can be used only in very young fossils: those younger than 50,000 years.

4 How does radiocarbon get incorporated into potential fossils? Unlike other radioisotopes, 14 C is continuously produced in the upper atmosphere. Here's how the system works: 14 C is generated in the upper atmosphere by cosmic rays bombarding Nitrogen atoms. Nitrogen gives up a proton and 14 C is produced. This production occurs at a nearly constant rate. The radiocarbon produced in the upper atmosphere is oxidized to carbon dioxide, which is then incorporated directly into plant matter through photosynthesis, or dissolved in the ocean. Thus, plant material like wood, takes up 14 C throughout life; animals that eat plant material then also take up radiocarbon through their lives, animals that secrete shells out of CaCO 3 also take up 14 C through their lives. Upon death, no new radiocarbon is taken in and the radiocarbon already there simply decays. Thus, the difference between the amount in the organically produced material (wood, shell, bone) and the original amount (assumed a constant - pretty close see below for calibration) is a function of the time-since-death of the organism. Note: works only with organic carbon (wood, plants, material from plants, shell, bone, organic material. Only good to ~50,000 years. Examining/correcting for two key assumptions: 1. Constant production rate of radiocarbon. Is the rate of production in the upper atmosphere constant or does it vary? If it varies, how much does it vary? Evaluation with tree rings Radiocarbon-date the wood from individual tree-rings of known age:

5 The deviation of the line from perfect 1:1 slope means that radiocarbon age slightly underestimates calendar date. Note some higher frequency wiggles too. 2. Rapid mixing of atmosphere and oceans. Is the radiocarbon produced in the upper atmosphere rapidly mixed through the atmosphere and ocean? Although the radiocarbon in the upper atmosphere is mixed throughout the atmosphere and shallow oceans, deep ocean waters are not well-mixed with the atmosphere. How do we know this? Dates on historically collected live specimens from the ocean: example Gulf of California shells collected in ~1936 are 1,000 years old, according to radiocarbon dating: Source of carbon is from old dissolved CO 2 upwelled from deep water. This is the reservoir effect. Varies from place to place, ocean-wide average is about 400 yrs, greater in some places (like the G of C), less in others The reservoir effect means that a correction must be applied to marine samples. Typically about 400 years is subtracted from the raw radiocarbon date. Some other corrections Corrections within historic time also needed with very young material because --burning of fossil fuels puts more dead carbon in atmosphere, thus making living material have a smaller proportion of radiocarbon than would be expected --atmospheric testing of atomic weapons put an excess of radiocarbon in the atmosphere (and biosphere), making post-1950 specimens appear younger than they actually are. Two applications: A. sea level curves Samples of fresh water peat recovered from cores on the continental shelf. The age of the youngest fresh water peat in the core (these peats occur just below marine sediments) marks the time that sea level rose above mean high water at that location. Relative to present-day sea level, the older the highest peat in the section, the deeper below present sea level it's found. A sea level curve can be drawn that shows the rate of rise in sea level since 18,000 years ago the time of the last glaciation when sea level was at its lowest. Sketch on board. B. Shroud of Turin Burial shroud with image of man -Burial shroud of Jesus, many thought?

6 Small unlabelled samples of the shroud (along with samples of cloth of known ages) were sent to three radiocarbon labs (including the one here at UA). But radiocarbon ages of AD were reported. More than a millennium too young to be the burial shround of Jesus. 3. Amino acid "dating" Living organic matter is composed of proteins, which are composed of amino acids. Even after death, some organic matter persists even after the rest of the soft parts have decayed away. This organic matter is trapped inside the shell or bones of the organism. After death, some of these amino acids behave in a very predictable way: two amino acids are mirror images of each other: L-isoleucine (left-handed) D-alloisoleucine (right-handed) In living organisms the left-handed form is present. After death, some of that L- isoleucine degrades (or racemizes) to D-alloisoleucine. Thus, the ratio of alloisoluecine to isoluecine, or the A/I ratio, is a measure of the timesince death of the organism. Low A/I young High A/I older

7 But, the rate at which this change takes place depends on temperature. The warmer the temperature, the faster the reaction. So, as a dating technique, it depends on shells or bones having the same temperature histories. Means you can't compare from one climatic regime to another. Why bother with amino acid dates? Cost and speed. A/I ratios can be calibrated with radiocarbon to give absolute ages. Example: Colorado delta shells. Why the decrease in abundance with increasing age? - Destruction through time? - Decreased production in the past? Amino acid dating has been applied to shells as old as 1.20 million years in cool climates (but the dates are poorly calibrated).