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Quaternary Environments Dating Methods I

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Accuracy Versus Precision Precision means that the samples have low amount of error associated with the dating 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 Accuracy means that the samples are dated to the true age of the sample We strive for both accuracy and precision in dating techniques We strive for both accuracy and precision in dating techniques

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Accuracy Versus Precision

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Relative Versus Absolute Dating Relative Dating Relative Dating Principle of Superposition Principle of Superposition Absolute Dating Absolute Dating Provides solid chronological dates (within error bars) that are related to a calendar year scale Provides solid chronological dates (within error bars) that are related to a calendar year scale

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Methods Radioisotopic Methods Radioisotopic Methods Based on rate of atomic disintegration Based on rate of atomic disintegration Paleomagnetic Methods Paleomagnetic Methods Relies on past reversals of the Earths magnetic field Relies on past reversals of the Earths magnetic field Organic and Inorganic Chemical Methods Organic and Inorganic Chemical Methods Based on time-dependent chemical changes in a sample Based on time-dependent chemical changes in a sample Biological Methods Biological Methods Based on the growth of an organism Based on the growth of an organism

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Using Radioactivity in Dating Radiometric dating Radiometric dating Useful radioactive isotopes for providing ages Useful radioactive isotopes for providing ages 87 Rb/ 87 Sr – 47.0 billion years 87 Rb/ 87 Sr – 47.0 billion years 232 Th/ 208 Pb – 12.1 billion years 232 Th/ 208 Pb – 12.1 billion years 238 U/ 206 Pb – 4.5 billion years 238 U/ 206 Pb – 4.5 billion years 40 K/ 40 Ar – 1.3 billion years 40 K/ 40 Ar – 1.3 billion years 235 U/ 207 Pb – 713 million years 235 U/ 207 Pb – 713 million years 14 C/ 14 N– 5,730 years (5,570 Libby years) 14 C/ 14 N– 5,730 years (5,570 Libby years)

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Sources of Error A closed system is required A closed system is required To avoid potential problems, only fresh, unweathered samples should be used To avoid potential problems, only fresh, unweathered samples should be used

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Using Radioactivity in Dating Reviewing basic atomic structure Reviewing basic atomic structure Nucleus Nucleus Protons – positively charged particles with mass Protons – positively charged particles with mass Neutrons – neutral particles with mass Neutrons – neutral particles with mass Electrons – negatively charged particles that orbit the nucleus Electrons – negatively charged particles that orbit the nucleus

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Using Radioactivity in Dating Reviewing basic atomic structure Reviewing basic atomic structure Atomic number Atomic number An elements identifying number An elements identifying number Equal to the number of protons in the atoms nucleus Equal to the number of protons in the atoms nucleus Mass number Mass number Sum of the number of protons and neutrons in an atoms nucleus Sum of the number of protons and neutrons in an atoms nucleus Identifies an isotope Identifies an isotope

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Using Radioactivity in Dating Reviewing basic atomic structure Reviewing basic atomic structure Isotope Isotope Variant of the same parent atom Variant of the same parent atom Differs in the number of neutrons Differs in the number of neutrons Results in a different mass number than the parent atom Results in a different mass number than the parent atom

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Using Radioactivity in Dating Radioactivity Radioactivity Spontaneous changes (decay) in the structure of atomic nuclei Spontaneous changes (decay) in the structure of atomic nuclei Types of radioactive decay Types of radioactive decay Alpha emission Alpha emission Emission of 2 protons and 2 neutrons (an alpha particle) Emission of 2 protons and 2 neutrons (an alpha particle) Mass number is reduced by 4 and the atomic number is lowered by 2 Mass number is reduced by 4 and the atomic number is lowered by 2

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Using Radioactivity in Dating Types of radioactive decay Types of radioactive decay Beta emission Beta emission An electron (beta particle) is ejected from the nucleus An electron (beta particle) is ejected from the nucleus Mass number remains unchanged and the atomic number increases by 1 Mass number remains unchanged and the atomic number increases by 1

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Using Radioactivity in Dating Types of radioactive decay Types of radioactive decay Electron capture Electron capture An electron is captured by the nucleus An electron is captured by the nucleus The electron combines with a proton to form a neutron The electron combines with a proton to form a neutron Mass number remains unchanged and the atomic number decreases by 1 Mass number remains unchanged and the atomic number decreases by 1

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Common Types of Radioactive Decay

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Using Radioactivity in Dating Using Radioactivity in Dating Parent – an unstable radioactive isotope Parent – an unstable radioactive isotope Daughter product – the isotopes resulting from the decay of a parent 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 Half-life – the time required for one-half of the radioactive nuclei in a sample to decay

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Using Radioactivity in Dating Radiometric dating Radiometric dating Principle of radioactive dating Principle of radioactive dating The percentage of radioactive atoms that decay during one half-life is always the same (50 percent) 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 However, the actual number of atoms that decay continually decreases Comparing the ratio of parent to daughter yields the age of the sample Comparing the ratio of parent to daughter yields the age of the sample

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Radioactive decay curve

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Radiocarbon Dating Dating with 14 C Dating with 14 C Half-life of 5730 years Half-life of 5730 years Used to date very recent events Used to date very recent events 14 C is produced in the upper atmosphere 14 C is produced in the upper atmosphere Useful tool for anthropologists, archeologists, and geologists who study very recent Earth history Useful tool for anthropologists, archeologists, and geologists who study very recent Earth history

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Sources of Error in 14C Dating Problems of Sample Selection and Contamination Problems of Sample Selection and Contamination Young carbon effects Young carbon effects Old carbon effects Old carbon effects Variation in 14C content in the ocean reservoir Variation in 14C content in the ocean reservoir Fractionation Effects Fractionation Effects

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14 C Age of Sea Water

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Radiocarbon Variation Over the Last 2000 years

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Calibration Curve Showing Departure

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Calibrated Curve

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Radiocarbon Plateaus

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14 C Bomb Spike

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Potassium Argon Dating ( 40 K/ 40 Ar) Instrumental in dating sea-floor basalts and providing the timing of magnetic reversals Instrumental in dating sea-floor basalts and providing the timing of magnetic reversals Used in dating lava flows Used in dating lava flows Also for dating metamorphic events Also for dating metamorphic events

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Potassium Argon Dating ( 40 K/ 40 Ar) 39 K and 41 K, Stable 39 K and 41 K, Stable 40 K unstable and 0.012% of all potassium 40 K unstable and 0.012% of all potassium 40 K decays to 40 Ca and 40 Ar 40 K decays to 40 Ca and 40 Ar Ca is common in rocks and is therefore not useful in dating 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 Measure the amount of 40 Ar in the lab and the amount of 40 K is also measured from the sample

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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 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 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 Dating is done on sanidine, plagioclase, biotite, hornblende, and olivine in volcanic rocks and glauconite, feldspar, and sylvite in sedimentary rocks

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Problems of 40 K/ 40 Ar Assumptions Assumptions No Ar was left in the rock at formation No Ar was left in the rock at formation System has remained closed since formation System has remained closed since formation Checks 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 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 Problem Loss of Ar due to diffusion, recrystallization, solution, and chemical reactions Loss of Ar due to diffusion, recrystallization, solution, and chemical reactions

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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 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 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 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

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Uranium Series Dating 238 U and 235 U have a decay process that cascades through a series of elements 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 Each decay stage can be used as a dating tool Thermal Ionization Mass Spectrometry (TIMS) allows very accurate estimates from small samples Thermal Ionization Mass Spectrometry (TIMS) allows very accurate estimates from small samples U series are useful in dating corals and speleothems 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 Mollusks seem to be an open system in relation to U and are not generally conducive to U series dating

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Problems of U Series Dating Assumes the initial 230 Th/ 234 U, 234 U/ 238 U, and 231 Pa/ 235 U ratios 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 Likely in ocean sediments but more in flux in the atmosphere Assumes a closed system Assumes a closed system

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Luminescence Dating Principles and Applications Light emitted from a mineral crystal (usually quartz or feldspars) when exposed to heat or light 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 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 The clock is set to zero by heating or optical bleaching Therefore Loess and fluvial sediments make good candidates for luminescence dating Therefore Loess and fluvial sediments make good candidates for luminescence dating

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Luminescence Thermoluminescence (TL Dating) Thermoluminescence (TL Dating) When the light is emitted as a result of thermal hearting When the light is emitted as a result of thermal hearting First published Wintle and Huntley 1979 First published Wintle and Huntley 1979 Optical and Infrared Stimulated Luminescence (OSL and IRSL Dating) Optical and Infrared Stimulated Luminescence (OSL and IRSL Dating) Visible or infrared energy emitted in response to radiation Visible or infrared energy emitted in response to radiation

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Problems in TL Dating Assumes that the relationship between the radiation dose and the resulting luminescence is a linear relationship; not always the case 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 <5,000 yrs the rate of electron accumulation is slow, possibly needing to exceed a threshold Some minerals may reach saturation >300,000 yrs Some minerals may reach saturation >300,000 yrs Anomalous Fading – Minerals do not hold the electrons beyond a few weeks Anomalous Fading – Minerals do not hold the electrons beyond a few weeks Variations in environmental dose; related to moisture content for one example Variations in environmental dose; related to moisture content for one example

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Optical and Infrared Stimulated Luminescence (OSL and IRSL Dating) Zero in the modern sediments Zero in the modern sediments Sensitive to light bleaching setting the system to zero 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 Multiple measurements are possible because short stimulation to the light source does not deplete the potential luminescence

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Fission Track Dating Uranium will decay through fission, splitting the nucleus and shooting the two halves into the mineral Uranium will decay through fission, splitting the nucleus and shooting the two halves into the mineral The results are fission tracks from 10-20μm in length The results are fission tracks from 10-20μm in length Some glassy minerals will loose their fission tracks through heating, setting the clock to zero Some glassy minerals will loose their fission tracks through heating, setting the clock to zero Different minerals have different annealing temperatures Different minerals have different annealing temperatures

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Fission Track Dating The samples are polished and etched with a chemical that brings out the tracks 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 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 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 The known fission rate of 238 U is used to calculate the age of the sample

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Fission Track Dating Can be used in apatite, micas, sphene, and zircons 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 Can also be used in rocks such as volcanic ash, obsidian, basalts, granites, tuffs, and carbonatites Ranges from 10 3 to 10 8 years Ranges from 10 3 to 10 8 years The error associated with this technique is hard to determine and is seldom reported 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 Repeat measures are the best, but are seldom undertaken because of time constraints

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Dendrochronology Temperate trees produce annual rings. Temperate trees produce annual rings. The trees are recording all of the environmental variables that affect tree growth. 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. Can be used to date objects with annual resolution back 10,000 years in the best circumstances.

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Neutron capture (A) and Beta emission (B) Neutron capture (A) and Beta emission (B)

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