Quaternary EnvironmentsQuaternary EnvironmentsDating Methods IIDating Methods II

PaleomagnetismPaleomagnetism

Major ReversalsMajor Reversals Aperiodic global-scale geomagnetic reversalsAperiodic global-scale geomagnetic reversals Dipole changesDipole changes

Secular VariationsSecular Variations Smaller amplitude quasi-periodic variationsSmaller amplitude quasi-periodic variations Non-dipole fieldNon-dipole field Regional in scale (1000 – 3000km)Regional in scale (1000 – 3000km)

Earth’s Magnetic FieldEarth’s Magnetic Field

Produced by electrical currents in the coreProduced by electrical currents in the core Still not fully understoodStill not fully understood Acts like bar magnet inclined at 11Acts like bar magnet inclined at 11° from the ° from the

axis of rotationaxis of rotation InclinationInclination

Variation on the horizontal planeVariation on the horizontal plane DeclinationDeclination

Variation from true geographical northVariation from true geographical north

Magnetic FieldMagnetic Field

MagnetostratigraphyMagnetostratigraphy

Use of magnetic reversals as a Use of magnetic reversals as a chronometerchronometer Dipole changes synchronous around the Dipole changes synchronous around the

worldworld Not dependent upon fossil associations or Not dependent upon fossil associations or

similar rockssimilar rocks

Magnetization of Rocks and Magnetization of Rocks and SedimentsSediments

Thermoremnant Magnetization (TRM)Thermoremnant Magnetization (TRM) Currie Point – Below which the igneous rock’s magnetic Currie Point – Below which the igneous rock’s magnetic

record is fixedrecord is fixed Effective on lava flows and baked clays at archaeological Effective on lava flows and baked clays at archaeological

sitessites Detrital Remnant Magnetization (DRM)Detrital Remnant Magnetization (DRM)

Magnetic particles become aligned with the ambient Magnetic particles become aligned with the ambient magnetic field as they settle through the water columnmagnetic field as they settle through the water column

Post-Depositional DRMPost-Depositional DRM Based on the water content for some sediments, they Based on the water content for some sediments, they

may take on their magnetic characteristic after depositionmay take on their magnetic characteristic after deposition Chemical Remnant Magnetization (CRM)Chemical Remnant Magnetization (CRM)

Post-Depositional magnetization due to chemical Post-Depositional magnetization due to chemical changes in magnetic mineralschanges in magnetic minerals

Problems With PaleomagentismProblems With Paleomagentism

DRM is not instantaneous DRM is not instantaneous Sediments are subject to bioturbation Sediments are subject to bioturbation

(especially effecting post-depositional (especially effecting post-depositional DRM)DRM)

Overturned sediment may give false Overturned sediment may give false excursionsexcursions

Post-Depositional magnetic changes due Post-Depositional magnetic changes due to chemical recrystallizationto chemical recrystallization

Paleomagnetic TimescalePaleomagnetic Timescale

Major Polarity Epochs (chrons)Major Polarity Epochs (chrons) Persist around 1,000,000 yearsPersist around 1,000,000 years

Polarity Events (subchrons)Polarity Events (subchrons) Persist from 10,000 to 100,000 yearsPersist from 10,000 to 100,000 years

Geomagnetic ExcursionsGeomagnetic Excursions Short-term geomagnetic fluctuations (Cryptochrons)Short-term geomagnetic fluctuations (Cryptochrons) Persist for a few thousand yearsPersist for a few thousand years Due to the non-dipole variationDue to the non-dipole variation

δδ18O in marine sediments and their relationship 18O in marine sediments and their relationship with astronomical forcing has been used to with astronomical forcing has been used to refine the timing of magnetic reversalsrefine the timing of magnetic reversals

Paleomagnetic Master ChronologyPaleomagnetic Master Chronology

Small secular changes in the magnetic fieldSmall secular changes in the magnetic field Can create a chronostratigraphic templateCan create a chronostratigraphic template Based on an a well-dated magnetostratigraphic Based on an a well-dated magnetostratigraphic

record from a type sectionrecord from a type section Undated sediments should have close to the Undated sediments should have close to the

same sedimentation rate and not have been same sedimentation rate and not have been disturbeddisturbed

Dating checked through other lines of evidence Dating checked through other lines of evidence such as tephrochronology such as tephrochronology

Dating Methods Involving Chemical Dating Methods Involving Chemical ChangesChanges

Amino acid analysis of organic samplesAmino acid analysis of organic samples Amount of weathering that an inorganic Amount of weathering that an inorganic

sample has experienced sample has experienced Chemical fingerprinting fo volcanic ashesChemical fingerprinting fo volcanic ashes

Amino-Acid DatingAmino-Acid Dating

All living organisms contain amino acidsAll living organisms contain amino acids Living organisms have levo (left rotating) Living organisms have levo (left rotating)

formationformation Amino acid formation is dextro (right Amino acid formation is dextro (right

rotating) after an organism diesrotating) after an organism dies D/L ratios can give the age of a sampleD/L ratios can give the age of a sample Can date samples from a few thousand Can date samples from a few thousand

years old to a few million years oldyears old to a few million years old

Amino-Acid DatingAmino-Acid Dating

First studies in 1968 (Hare and Mitterer First studies in 1968 (Hare and Mitterer 1968)1968)

Can be conducted on small samples <2mg Can be conducted on small samples <2mg in mollusks or foraminiferain mollusks or foraminifera

Can also be conducted of wood, Can also be conducted of wood, speleothems, and coralsspeleothems, and corals

Problems With Amino-Acid DatingProblems With Amino-Acid Dating

Must be calibrated to provide absolute Must be calibrated to provide absolute datesdates

Very sensitive to temperature historyVery sensitive to temperature history An uncertainty of +/- 2An uncertainty of +/- 2°C is equivalent to an °C is equivalent to an

age uncertainty of +/-50%age uncertainty of +/-50% Can also be affected by contamination and Can also be affected by contamination and

leachingleaching Rates vary from one Genus to anotherRates vary from one Genus to another

Temperature Records FromTemperature Records From Amino-Acid Dating Amino-Acid Dating

If the age of the sample is known, the If the age of the sample is known, the temperature history can be determinedtemperature history can be determined

Temperature is the main thing that Temperature is the main thing that controls racemization rates so solving for controls racemization rates so solving for temperature resolves much of the errortemperature resolves much of the error

Relative dating with Amino Acid Relative dating with Amino Acid Racemization can produce an Racemization can produce an aminostratigraphyaminostratigraphy

Obsidian HydrationObsidian Hydration

Fresh surfaces of obsidian will react with Fresh surfaces of obsidian will react with water from the atmosphere or soil to water from the atmosphere or soil to create a hydration rindcreate a hydration rind

The thickness of the hydration rind can be The thickness of the hydration rind can be measured and used to tell the age of the measured and used to tell the age of the samplesample

Mainly used in archaeology can also date Mainly used in archaeology can also date glacial or volcanic events glacial or volcanic events

Problems With Obsidian HydrationProblems With Obsidian Hydration

Must be regionally calibrated to provide Must be regionally calibrated to provide absolute datesabsolute dates

Dependent upon temperatureDependent upon temperature Varies with sample compositionVaries with sample composition Not very preciseNot very precise

Obsidian hydration profiles from Crooks Canyon in Northern California. The large number of readings between 0.8 and 1.5 microns indicate occupation at the very

end of the Terminal Prehistoric Period, as well as during the Historic Period.

http://www.farwestern.com/crookscanyon/pagei.htmMcGuire and Waechter 2004

Obsidian HydrationObsidian Hydration

ThephrochronologyThephrochronology

Airborne pyroclastic material ejected Airborne pyroclastic material ejected during a volcanic eruptionduring a volcanic eruption

Form isochronous stratigraphic markersForm isochronous stratigraphic markers Must be dated by Must be dated by 4040K/K/4040Ar or fission-track Ar or fission-track

datingdating Can be used in for bounding datesCan be used in for bounding dates

ThephrochronologyThephrochronology Petrographic and chemical studies can identify unique tephra Petrographic and chemical studies can identify unique tephra

signatures which can then be used in a tephrochronologysignatures which can then be used in a tephrochronology

http://www.gfz-potsdam.de/pb3/pb33/projects/monticchio_tephrochronology/content_en.html

ThephrochronologyThephrochronology

http://www.grancampo.de/english/tephra/tephra3.htm

LichenometryLichenometry

Lichen are a symbiotic relationship Lichen are a symbiotic relationship between algae and fungibetween algae and fungi

The algae provide carbohydrates through The algae provide carbohydrates through photsynthesisphotsynthesis

The fungi provide a protective environmentThe fungi provide a protective environment Foliose Lichen – Bush-like formFoliose Lichen – Bush-like form Crustose – Flat disc-like formsCrustose – Flat disc-like forms

Rhizocarpon geographicumRhizocarpon geographicum from from NorwayNorway

LichenometryLichenometry

Most used to date glacial deposits in tundra Most used to date glacial deposits in tundra environmentsenvironments

Also used to date lake-level, sea-level, glacial Also used to date lake-level, sea-level, glacial outwash, trim-lines, rockfalls, talus stabilization, outwash, trim-lines, rockfalls, talus stabilization, former extent of permanent snow coverformer extent of permanent snow cover

Assumes constant growth rate of lichen so that Assumes constant growth rate of lichen so that the largest diameter lichen will be the oldestthe largest diameter lichen will be the oldest

Growth Curves of LichenGrowth Curves of Lichen

Lichen DatesLichen Dates

SpeciesSpecies DiameterDiameter AgeAge LocationLocation

Alectoria minusculaAlectoria minuscula 160mm160mm 500-600 yrs500-600 yrs Baffin IslandBaffin Island

Rhizocarpon geographicumRhizocarpon geographicum 280mm280mm 9,500 +/-1500 yrs9,500 +/-1500 yrs Baffin IslandBaffin Island

Rhizocarpon alpicolaRhizocarpon alpicola 480mm480mm 9,000 yrs9,000 yrs Swedish LaplandSwedish Lapland

Biological Problems With Biological Problems With LichenometryLichenometry

Growth rate differs by generaGrowth rate differs by genera Variable growth rate (fastest when the lichen is Variable growth rate (fastest when the lichen is

young)young) Lag time in origination Lag time in origination Hard to identify to the species levelHard to identify to the species level Competition (some allelopathic) between Competition (some allelopathic) between

individuals at high densityindividuals at high density

Environmental Problems With Environmental Problems With LichenometryLichenometry

Growth dependent on substrate type (surface Growth dependent on substrate type (surface texture and chemical composition)texture and chemical composition)

Dependent upon climatic factorsDependent upon climatic factors Slower growth rates occur with low Slower growth rates occur with low

temperatures, short growing seasons, and low temperatures, short growing seasons, and low precipitation precipitation

Snow cover may inhibit lichen growthSnow cover may inhibit lichen growth

Sampling Problems With Sampling Problems With LichenometryLichenometry

Must be calibrated regionallyMust be calibrated regionally Growth curves may not be linearGrowth curves may not be linear Must locate the largest lichen on the surfaceMust locate the largest lichen on the surface Irregular growth of older lichenIrregular growth of older lichen Some colluvium may have older lichenSome colluvium may have older lichen Error bars should be 15-20% and larger with Error bars should be 15-20% and larger with

extrapolated datesextrapolated dates