Quaternary Environments Dating Methods I Accuracy Versus Precision Precision means that the samples have low amount of error associated with the dating Accuracy means.
Download ReportTranscript Quaternary Environments Dating Methods I Accuracy Versus Precision Precision means that the samples have low amount of error associated with the dating Accuracy means.
Quaternary Environments Dating Methods I
Accuracy Versus Precision
Precision
means that the samples have low amount of error associated with the dating
Accuracy
means that the samples are dated to the true age of the sample We strive for both accuracy and precision in dating techniques
Accuracy Versus Precision
Relative Versus Absolute Dating
Relative Dating Principle of Superposition Absolute Dating Provides solid chronological dates (within error bars) that are related to a calendar year scale
Methods
Radioisotopic Methods
Based on rate of atomic disintegration
Paleomagnetic Methods
Relies on past reversals of the Earth’s magnetic field
Organic and Inorganic Chemical Methods
Based on time-dependent chemical changes in a sample
Biological Methods
Based on the growth of an organism
Using Radioactivity in Dating
Radiometric dating
Useful radioactive isotopes for providing ages
87 Rb/ 87 Sr
232 Th/ 208 Pb
238 U/ 206 Pb
40 K/ 40 Ar
235 U/ 207 Pb
14 C/ 14 N – 47.0 billion years – 12.1 billion years – 4.5 billion years – 1.3 billion years – 713 million years – 5,730 years (5,570 Libby years)
Sources of Error
A closed system is required
To avoid potential problems, only fresh, unweathered samples should be used
Using Radioactivity in Dating
Reviewing basic atomic structure
Nucleus
Protons – positively charged particles with mass
Neutrons – neutral particles with mass
Electrons – negatively charged particles that orbit the nucleus
Using Radioactivity in Dating
Reviewing basic atomic structure
Atomic number
An element’s identifying number
Equal to the number of protons in the atom’s nucleus
Mass number
Sum of the number of protons and neutrons in an atom’s nucleus
Identifies an isotope
Using Radioactivity in Dating
Reviewing basic atomic structure
Isotope
Variant of the same parent atom
Differs in the number of neutrons
Results in a different mass number than the parent atom
Using Radioactivity in Dating
Radioactivity
Spontaneous changes (decay) in the structure of atomic nuclei
Types of radioactive decay
Alpha emission
Emission of 2 protons and 2 neutrons (an alpha particle)
Mass number is reduced by 4 and the atomic number is lowered by 2
Using Radioactivity in Dating
Types of radioactive decay
Beta emission
An electron (beta particle) is ejected from the nucleus
Mass number remains unchanged and the atomic number increases by 1
Using Radioactivity in Dating
Types of radioactive decay
Electron capture
An electron is captured by the nucleus
The electron combines with a proton to form a neutron
Mass number remains unchanged and the atomic number decreases by 1
Common Types of Radioactive Decay
Using Radioactivity in Dating
Parent
– an unstable radioactive isotope
Daughter product
– the isotopes resulting from the decay of a parent
Half-life
– the time required for one-half of the radioactive nuclei in a sample to decay
Using Radioactivity in Dating
Radiometric dating
Principle of radioactive dating
The percentage of radioactive atoms that decay during one half-life is always the same (50 percent)
However, the actual number of atoms that decay continually decreases
Comparing the ratio of parent to daughter yields the age of the sample
Radioactive decay curve
Radiocarbon Dating
Dating with 14 C
Half-life of 5730 years
Used to date very recent events
14 C is produced in the upper atmosphere
Useful tool for anthropologists, archeologists, and geologists who study very recent Earth history
Sources of Error in 14C Dating
Problems of Sample Selection and Contamination Young carbon effects Old carbon effects Variation in 14C content in the ocean reservoir Fractionation Effects
14
C Age of Sea Water
Radiocarbon Variation Over the Last 2000 years
Calibration Curve Showing Departure
Calibrated Curve
Radiocarbon Plateaus
14
C Bomb Spike
Potassium Argon Dating (
40
K/
40
Ar)
Instrumental in dating sea-floor basalts and providing the timing of magnetic reversals Used in dating lava flows Also for dating metamorphic events
Potassium Argon Dating (
40
K/
40
Ar)
39 K and 41 K, Stable 40 K unstable and 0.012% of all potassium 40 K decays to 40 Ca and 40 Ar Ca is common in rocks and is therefore not useful in dating Measure the amount of 40 Ar in the lab and the amount of 40 K is also measured from the sample
Potassium Argon Dating (
40
K/
40
Ar)
Long half-life makes this useful over long time scales but not really usable at less than 100,000 years Optimal time range is around 30 ma and up to 1 ba rocks can be dated Dating is done on sanidine, plagioclase, biotite, hornblende, and olivine in volcanic rocks and glauconite, feldspar, and sylvite in sedimentary rocks
Problems of
40
K/
40
Ar
Assumptions No Ar was left in the rock at formation System has remained closed since formation Checks The ratio of 36 Ar to 40 Ar is known in the atmosphere and can be measured in the rock to determine atmospheric contamination Problem Loss of Ar due to diffusion, recrystallization, solution, and chemical reactions
40
Ar/
39
Ar Dating
A problem with 40 K/ 40 Ar dating is that K and Ar are measured at different places in the rock This can be solved by irradiating the samples and converting 39 K to 39 Ar With the known ratio of 40 K to 39 K, the amount of 40 K can be calculated from the same lattice structure as the 39 Ar
Uranium Series Dating
238 U and 235 U have a decay process that cascades through a series of elements Each decay stage can be used as a dating tool Thermal Ionization Mass Spectrometry (TIMS) allows very accurate estimates from small samples U series are useful in dating corals and speleothems Mollusks seem to be an open system in relation to U and are not generally conducive to U series dating
Problems of U Series Dating
Assumes the initial 230 Th/ 234 U, 234 U/ 238 U, and 231 Pa/ 235 U ratios Likely in ocean sediments but more in flux in the atmosphere Assumes a closed system
Luminescence Dating Principles and Applications
Light emitted from a mineral crystal (usually quartz or feldspars) when exposed to heat or light The light emitted is related to the amount of ionizing radiation that the sample has been exposed to from sediment The clock is set to zero by heating or optical bleaching Therefore Loess and fluvial sediments make good candidates for luminescence dating
Luminescence
Thermoluminescence (TL Dating) When the light is emitted as a result of thermal hearting First published Wintle and Huntley 1979 Optical and Infrared Stimulated Luminescence (OSL and IRSL Dating) Visible or infrared energy emitted in response to radiation
Problems in TL Dating
Assumes that the relationship between the radiation dose and the resulting luminescence is a linear relationship; not always the case <5,000 yrs the rate of electron accumulation is slow, possibly needing to exceed a threshold Some minerals may reach saturation >300,000 yrs Anomalous Fading – Minerals do not hold the electrons beyond a few weeks Variations in environmental dose; related to moisture content for one example
Optical and Infrared Stimulated Luminescence (OSL and IRSL Dating) Zero in the modern sediments Sensitive to light bleaching setting the system to zero Multiple measurements are possible because short stimulation to the light source does not deplete the potential luminescence
Fission Track Dating
Uranium will decay through fission, splitting the nucleus and shooting the two halves into the mineral The results are fission tracks from 10-20 μm in length Some glassy minerals will loose their fission tracks through heating, setting the clock to zero Different minerals have different annealing temperatures
Fission Track Dating
The samples are polished and etched with a chemical that brings out the tracks The tracks are counted, then the sample is heated, annealing the tracks. Then the sample is irradiated with a slow neutron beam and the tracks from the fission of 235 U are counted The number of induced tracks is proportional to the amount of 238 U in the sample The known fission rate of 238 U is used to calculate the age of the sample
Fission Track Dating
Can be used in apatite, micas, sphene, and zircons Can also be used in rocks such as volcanic ash, obsidian, basalts, granites, tuffs, and carbonatites Ranges from 10 3 to 10 8 years The error associated with this technique is hard to determine and is seldom reported Repeat measures are the best, but are seldom undertaken because of time constraints
Dendrochronology
Temperate trees produce annual rings.
The trees are recording all of the environmental variables that affect tree growth.
Can be used to date objects with annual resolution back 10,000 years in the best circumstances.
Neutron capture (A) and Beta emission (B)