Quaternary Environments Dating Methods I. Accuracy Versus Precision Precision means that the samples have low amount of error associated with the dating.

Quaternary EnvironmentsQuaternary EnvironmentsDating Methods IDating Methods I

Accuracy Versus PrecisionAccuracy Versus Precision

PrecisionPrecision means that the samples have means that the samples have low amount of error associated with the low amount of error associated with the datingdating

AccuracyAccuracy means that the samples are means that the samples are dated to the true age of the sampledated to the true age of the sample

We strive for both accuracy and precision We strive for both accuracy and precision in dating techniquesin dating techniques

Accuracy Versus PrecisionAccuracy Versus Precision

Relative Versus Absolute DatingRelative Versus Absolute Dating

Relative DatingRelative Dating Principle of SuperpositionPrinciple of Superposition

Absolute DatingAbsolute Dating Provides solid chronological dates (within Provides solid chronological dates (within

error bars) that are related to a calendar year error bars) that are related to a calendar year scalescale

MethodsMethods

Radioisotopic MethodsRadioisotopic Methods Based on rate of atomic disintegrationBased on rate of atomic disintegration

Paleomagnetic MethodsPaleomagnetic Methods Relies on past reversals of the Earth’s magnetic fieldRelies on past reversals of the Earth’s magnetic field

Organic and Inorganic Chemical MethodsOrganic and Inorganic Chemical Methods Based on time-dependent chemical changes in a Based on time-dependent chemical changes in a

samplesample

Biological MethodsBiological Methods Based on the growth of an organismBased on the growth of an organism

Using Radioactivity in DatingUsing Radioactivity in Dating

Radiometric datingRadiometric datingUseful radioactive isotopes for providing agesUseful radioactive isotopes for providing ages

8787Rb/Rb/8787Sr Sr – 47.0 billion years– 47.0 billion years 232232Th/Th/208208Pb Pb – 12.1 billion years– 12.1 billion years 238238U/U/206206Pb Pb – 4.5 billion years– 4.5 billion years 4040K/K/4040Ar Ar – 1.3 billion years– 1.3 billion years 235235U/U/207207Pb Pb – 713 million years– 713 million years 1414C/C/1414NN – 5,730 years (5,570 Libby – 5,730 years (5,570 Libby

years)years)

Sources of ErrorSources of Error

A closed system is requiredA closed system is required To avoid potential problems, only fresh, To avoid potential problems, only fresh,

unweathered samples should be usedunweathered samples should be used


Reviewing basic atomic structureReviewing basic atomic structureNucleusNucleus

ProtonsProtons – positively charged particles with mass – positively charged particles with mass NeutronsNeutrons – neutral particles with mass – neutral particles with mass Electrons Electrons – negatively charged particles that – negatively charged particles that

orbit the nucleusorbit the nucleus


Reviewing basic atomic structureReviewing basic atomic structureAtomic numberAtomic number

An element’s identifying numberAn element’s identifying number Equal to the number of protons in the atom’s Equal to the number of protons in the atom’s

nucleusnucleus

Mass numberMass number Sum of the number of protons and neutrons in an Sum of the number of protons and neutrons in an

atom’s nucleusatom’s nucleus Identifies an isotopeIdentifies an isotope


Reviewing basic atomic structureReviewing basic atomic structureIsotopeIsotope

Variant of the same parent atomVariant of the same parent atom Differs in the number of neutronsDiffers in the number of neutrons Results in a different mass number than the Results in a different mass number than the

parent atomparent atom


RadioactivityRadioactivitySpontaneous changes (decay) in the structure Spontaneous changes (decay) in the structure

of atomic nucleiof atomic nuclei Types of radioactive decayTypes of radioactive decay

Alpha emissionAlpha emission Emission of 2 protons and 2 neutrons (an alpha Emission of 2 protons and 2 neutrons (an alpha

particle)particle) Mass number is reduced by 4 and the atomic Mass number is reduced by 4 and the atomic

number is lowered by 2number is lowered by 2


Types of radioactive decayTypes of radioactive decayBeta emissionBeta emission

An electron (beta particle) is ejected from the An electron (beta particle) is ejected from the nucleusnucleus

Mass number remains unchanged and the atomic Mass number remains unchanged and the atomic number increases by 1number increases by 1


Types of radioactive decayTypes of radioactive decayElectron captureElectron capture

An electron is captured by the nucleusAn electron is captured by the nucleus The electron combines with a proton to form a The electron combines with a proton to form a

neutronneutron Mass number remains unchanged and the atomic Mass number remains unchanged and the atomic

number decreases by 1number decreases by 1

Common Types of Radioactive Common Types of Radioactive DecayDecay


ParentParent – an unstable radioactive isotope – an unstable radioactive isotope Daughter productDaughter product – the isotopes resulting – the isotopes resulting

from the decay of a parentfrom the decay of a parent Half-lifeHalf-life – the time required for one-half of – the time required for one-half of

the radioactive nuclei in a sample to decay the radioactive nuclei in a sample to decay


Radiometric datingRadiometric datingPrinciple of radioactive datingPrinciple of radioactive dating

The percentage of radioactive atoms that decay The percentage of radioactive atoms that decay during one half-life is always the same (50 during one half-life is always the same (50 percent)percent)

However, the actual number of atoms that decay However, the actual number of atoms that decay continually decreasescontinually decreases

Comparing the ratio of parent to daughter yields Comparing the ratio of parent to daughter yields the age of the samplethe age of the sample

Radioactive decay curveRadioactive decay curve

Radiocarbon DatingRadiocarbon Dating Dating with Dating with 1414C C

Half-life of 5730 yearsHalf-life of 5730 yearsUsed to date very recent eventsUsed to date very recent events1414C is produced in the upper atmosphereC is produced in the upper atmosphereUseful tool for anthropologists, archeologists, Useful tool for anthropologists, archeologists,

and geologists who study very recent Earth and geologists who study very recent Earth historyhistory

Sources of Error in 14C DatingSources of Error in 14C Dating

Problems of Sample Selection and Problems of Sample Selection and ContaminationContamination Young carbon effectsYoung carbon effects Old carbon effectsOld carbon effects

Variation in 14C content in the ocean Variation in 14C content in the ocean reservoir reservoir

Fractionation EffectsFractionation Effects

1414C Age of Sea WaterC Age of Sea Water

Radiocarbon Variation Over the Radiocarbon Variation Over the Last 2000 yearsLast 2000 years

Calibration Curve Showing Calibration Curve Showing DepartureDeparture

Calibrated CurveCalibrated Curve

Radiocarbon PlateausRadiocarbon Plateaus

1414C Bomb SpikeC Bomb Spike

Potassium Argon Dating (Potassium Argon Dating (4040K/K/4040Ar)Ar)

Instrumental in dating sea-floor basalts Instrumental in dating sea-floor basalts and providing the timing of magnetic and providing the timing of magnetic reversalsreversals

Used in dating lava flowsUsed in dating lava flows Also for dating metamorphic eventsAlso for dating metamorphic events


3939K and K and 4141K, StableK, Stable 4040K unstable and 0.012% of all potassiumK unstable and 0.012% of all potassium 4040K decays to K decays to 4040Ca and Ca and 4040ArAr Ca is common in rocks and is therefore Ca is common in rocks and is therefore

not useful in datingnot useful in dating Measure the amount of Measure the amount of 4040Ar in the lab and Ar in the lab and

the amount of the amount of 4040K is also measured from K is also measured from the samplethe sample


Long half-life makes this useful over long time scales but not Long half-life makes this useful over long time scales but not really usable at less than 100,000 yearsreally usable at less than 100,000 years

Optimal time range is around 30 ma and up to 1 ba rocks Optimal time range is around 30 ma and up to 1 ba rocks can be datedcan be dated

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

Problems of Problems of 4040K/K/4040ArAr

AssumptionsAssumptions No Ar was left in the rock at formationNo Ar was left in the rock at formation System has remained closed since formationSystem has remained closed since formation

ChecksChecks The ratio of The ratio of 3636Ar to Ar to 4040Ar is known in the atmosphere Ar is known in the atmosphere

and can be measured in the rock to determine and can be measured in the rock to determine atmospheric contaminationatmospheric contamination

ProblemProblem Loss of Ar due to diffusion, recrystallization, solution, Loss of Ar due to diffusion, recrystallization, solution,

and chemical reactionsand chemical reactions

4040Ar/Ar/3939Ar DatingAr Dating

A problem with A problem with 4040K/K/4040Ar dating is that K and Ar are Ar dating is that K and Ar are measured at different places in the rockmeasured at different places in the rock

This can be solved by irradiating the samples and This can be solved by irradiating the samples and converting converting 3939K to K to 3939ArAr

With the known ratio of With the known ratio of 4040K to K to 3939K, the amount of K, the amount of 4040K can K can

be calculated from the same lattice structure as the be calculated from the same lattice structure as the 3939ArAr

Uranium Series DatingUranium Series Dating

238238U and U and 235235U have a decay process that cascades U have a decay process that cascades through a series of elementsthrough a series of elements

Each decay stage can be used as a dating toolEach decay stage can be used as a dating tool Thermal Ionization Mass Spectrometry (TIMS) allows Thermal Ionization Mass Spectrometry (TIMS) allows

very accurate estimates from small samplesvery accurate estimates from small samples U series are useful in dating corals and speleothemsU series are useful in dating corals and speleothems Mollusks seem to be an open system in relation to U and Mollusks seem to be an open system in relation to U and

are not generally conducive to U series datingare not generally conducive to U series dating

Problems of U Series DatingProblems of U Series Dating

Assumes the initial Assumes the initial 230230Th/Th/234234U, U, 234234U/U/238238U, U, and and 231231Pa/Pa/235235U ratiosU ratios Likely in ocean sediments but more in flux in Likely in ocean sediments but more in flux in

the atmospherethe atmosphere Assumes a closed systemAssumes a closed system

Luminescence Dating Principles Luminescence Dating Principles and Applicationsand Applications

Light emitted from a mineral crystal (usually Light emitted from a mineral crystal (usually quartz or feldspars) when exposed to heat or quartz or feldspars) when exposed to heat or lightlight

The light emitted is related to the amount of The light emitted is related to the amount of ionizing radiation that the sample has been ionizing radiation that the sample has been exposed to from sedimentexposed to from sediment

The clock is set to zero by heating or optical The clock is set to zero by heating or optical bleachingbleaching Therefore Loess and fluvial sediments make good Therefore Loess and fluvial sediments make good

candidates for luminescence datingcandidates for luminescence dating

LuminescenceLuminescence

Thermoluminescence (TL Dating) Thermoluminescence (TL Dating) When the light is emitted as a result of When the light is emitted as a result of

thermal heartingthermal hearting First published Wintle and Huntley 1979First published Wintle and Huntley 1979

Optical and Infrared Stimulated Luminescence Optical and Infrared Stimulated Luminescence (OSL and IRSL Dating)(OSL and IRSL Dating) Visible or infrared energy emitted in response Visible or infrared energy emitted in response

to radiationto radiation

Problems in TL DatingProblems in TL Dating

Assumes that the relationship between the radiation Assumes that the relationship between the radiation dose and the resulting luminescence is a linear dose and the resulting luminescence is a linear relationship; not always the caserelationship; not always the case <5,000 yrs the rate of electron accumulation is slow, <5,000 yrs the rate of electron accumulation is slow,

possibly needing to exceed a thresholdpossibly needing to exceed a threshold Some minerals may reach saturation >300,000 yrsSome minerals may reach saturation >300,000 yrs

Anomalous Fading – Minerals do not hold the electrons Anomalous Fading – Minerals do not hold the electrons beyond a few weeksbeyond a few weeks

Variations in environmental dose; related to moisture Variations in environmental dose; related to moisture content for one examplecontent for one example

Optical and Infrared Stimulated Optical and Infrared Stimulated Luminescence (OSL and IRSL Dating)Luminescence (OSL and IRSL Dating)

Zero in the modern sedimentsZero in the modern sediments Sensitive to light bleaching setting the system to zeroSensitive to light bleaching setting the system to zero Multiple measurements are possible because short Multiple measurements are possible because short

stimulation to the light source does not deplete the stimulation to the light source does not deplete the potential luminescencepotential luminescence

Fission Track Dating Fission Track Dating

Uranium will decay through fission, splitting the Uranium will decay through fission, splitting the nucleus and shooting the two halves into the nucleus and shooting the two halves into the mineralmineral

The results are fission tracks from 10-20The results are fission tracks from 10-20μμm in m in lengthlength

Some glassy minerals will loose their fission Some glassy minerals will loose their fission tracks through heating, setting the clock to zerotracks through heating, setting the clock to zero

Different minerals have different annealing Different minerals have different annealing temperaturestemperatures

Fission Track DatingFission Track Dating

The samples are polished and etched with a The samples are polished and etched with a chemical that brings out the trackschemical that brings out the tracks

The tracks are counted, then the sample is The tracks are counted, then the sample is heated, annealing the tracks. Then the sample heated, annealing the tracks. Then the sample is irradiated with a slow neutron beam and the is irradiated with a slow neutron beam and the tracks from the fission of tracks from the fission of 235235U are countedU are counted

The number of induced tracks is proportional to The number of induced tracks is proportional to the amount of the amount of 238238U in the sampleU in the sample

The known fission rate of The known fission rate of 238238U is used to U is used to calculate the age of the samplecalculate the age of the sample

Fission Track DatingFission Track Dating

Can be used in apatite, micas, sphene, and Can be used in apatite, micas, sphene, and zirconszircons

Can also be used in rocks such as volcanic ash, Can also be used in rocks such as volcanic ash, obsidian, basalts, granites, tuffs, and obsidian, basalts, granites, tuffs, and carbonatitescarbonatites

Ranges from 10Ranges from 1033 to 10 to 1088 years years The error associated with this technique is hard The error associated with this technique is hard

to determine and is seldom reportedto determine and is seldom reported Repeat measures are the best, but are seldom Repeat measures are the best, but are seldom

undertaken because of time constraintsundertaken because of time constraints

DendrochronologyDendrochronology

Temperate trees produce annual rings.Temperate trees produce annual rings. The trees are recording all of the The trees are recording all of the

environmental variables that affect tree environmental variables that affect tree growth.growth.

Can be used to date objects with annual Can be used to date objects with annual resolution back 10,000 years in the best resolution back 10,000 years in the best circumstances.circumstances.

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

Quaternary Environments Dating Methods I. Accuracy Versus Precision Precision means that the samples have low amount of error associated with the dating.

Documents

number of protons

different mass number

neutron mass number

number equal

nucleus slide

dating parent

dating precision

dating accuracy