Quaternary Environments Dating Methods II

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Quaternary EnvironmentsDating Methods II
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Paleomagnetism

• Major Reversals
• Aperiodic global-scale geomagnetic reversals
• Dipole changes
• Secular Variations
• Smaller amplitude quasi-periodic variations
• Non-dipole field
• Regional in scale (1000 3000km)

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Earths Magnetic Field

• Produced by electrical currents in the core
• Still not fully understood
• Acts like bar magnet inclined at 11 from the
  axis of rotation
• Inclination
• Variation on the horizontal plane
• Declination
• Variation from true geographical north

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Magnetic Field
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Magnetostratigraphy

• Use of magnetic reversals as a chronometer
• Dipole changes synchronous around the world
• Not dependent upon fossil associations or similar
  rocks

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Magnetization of Rocks and Sediments

• Thermoremnant Magnetization (TRM)
• Currie Point Below which the igneous rocks
  magnetic record is fixed
• Effective on lava flows and baked clays at
  archaeological sites
• Detrital Remnant Magnetization (DRM)
• Magnetic particles become aligned with the
  ambient magnetic field as they settle through the
  water column
• Post-Depositional DRM
• Based on the water content for some sediments,
  they may take on their magnetic characteristic
  after deposition
• Chemical Remnant Magnetization (CRM)
• Post-Depositional magnetization due to chemical
  changes in magnetic minerals

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Problems With Paleomagentism

• DRM is not instantaneous
• Sediments are subject to bioturbation (especially
  effecting post-depositional DRM)
• Overturned sediment may give false excursions
• Post-Depositional magnetic changes due to
  chemical recrystallization

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Paleomagnetic Timescale

• Major Polarity Epochs (chrons)
• Persist around 1,000,000 years
• Polarity Events (subchrons)
• Persist from 10,000 to 100,000 years
• Geomagnetic Excursions
• Short-term geomagnetic fluctuations
  (Cryptochrons)
• Persist for a few thousand years
• Due to the non-dipole variation
• d18O in marine sediments and their relationship
  with astronomical forcing has been used to refine
  the timing of magnetic reversals

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Paleomagnetic Master Chronology

• Small secular changes in the magnetic field
• Can create a chronostratigraphic template
• Based on an a well-dated magnetostratigraphic
  record from a type section
• Undated sediments should have close to the same
  sedimentation rate and not have been disturbed
• Dating checked through other lines of evidence
  such as tephrochronology

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Dating Methods Involving Chemical Changes

• Amino acid analysis of organic samples
• Amount of weathering that an inorganic sample has
  experienced
• Chemical fingerprinting fo volcanic ashes

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Amino-Acid Dating

• All living organisms contain amino acids
• Living organisms have levo (left rotating)
  formation
• Amino acid formation is dextro (right rotating)
  after an organism dies
• D/L ratios can give the age of a sample
• Can date samples from a few thousand years old to
  a few million years old

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Amino-Acid Dating

• First studies in 1968 (Hare and Mitterer 1968)
• Can be conducted on small samples lt2mg in
  mollusks or foraminifera
• Can also be conducted of wood, speleothems, and
  corals

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Problems With Amino-Acid Dating

• Must be calibrated to provide absolute dates
• Very sensitive to temperature history
• An uncertainty of /- 2C is equivalent to an age
  uncertainty of /-50
• Can also be affected by contamination and
  leaching
• Rates vary from one Genus to another

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Temperature Records From Amino-Acid Dating

• If the age of the sample is known, the
  temperature history can be determined
• Temperature is the main thing that controls
  racemization rates so solving for temperature
  resolves much of the error
• Relative dating with Amino Acid Racemization can
  produce an aminostratigraphy

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Obsidian Hydration

• Fresh surfaces of obsidian will react with water
  from the atmosphere or soil to create a hydration
  rind
• The thickness of the hydration rind can be
  measured and used to tell the age of the sample
• Mainly used in archaeology can also date glacial
  or volcanic events

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Problems With Obsidian Hydration

• Must be regionally calibrated to provide absolute
  dates
• Dependent upon temperature
• Varies with sample composition
• Not very precise

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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.htmMc
Guire and Waechter 2004
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Obsidian Hydration
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Thephrochronology

• Airborne pyroclastic material ejected during a
  volcanic eruption
• Form isochronous stratigraphic markers
• Must be dated by 40K/40Ar or fission-track dating
• Can be used in for bounding dates

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Thephrochronology

• Petrographic and chemical studies can identify
  unique tephra signatures which can then be used
  in a tephrochronology

http//www.gfz-potsdam.de/pb3/pb33/projects/montic
chio_tephrochronology/content_en.html
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Thephrochronology
http//www.grancampo.de/english/tephra/tephra3.htm
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Lichenometry

• Lichen are a symbiotic relationship between algae
  and fungi
• The algae provide carbohydrates through
  photsynthesis
• The fungi provide a protective environment
• Foliose Lichen Bush-like form
• Crustose Flat disc-like forms

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Rhizocarpon geographicum from Norway
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Lichenometry

• Most used to date glacial deposits in tundra
  environments
• Also used to date lake-level, sea-level, glacial
  outwash, trim-lines, rockfalls, talus
  stabilization, former extent of permanent snow
  cover
• Assumes constant growth rate of lichen so that
  the largest diameter lichen will be the oldest

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Growth Curves of Lichen
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Lichen Dates
Species Diameter Age Location
Alectoria minuscula 160mm 500-600 yrs Baffin Island
Rhizocarpon geographicum 280mm 9,500 /-1500 yrs Baffin Island
Rhizocarpon alpicola 480mm 9,000 yrs Swedish Lapland
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Biological Problems With Lichenometry

• Growth rate differs by genera
• Variable growth rate (fastest when the lichen is
  young)
• Lag time in origination
• Hard to identify to the species level
• Competition (some allelopathic) between
  individuals at high density

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Environmental Problems With Lichenometry

• Growth dependent on substrate type (surface
  texture and chemical composition)
• Dependent upon climatic factors
• Slower growth rates occur with low temperatures,
  short growing seasons, and low precipitation
• Snow cover may inhibit lichen growth

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Sampling Problems With Lichenometry

• Must be calibrated regionally
• Growth curves may not be linear
• Must locate the largest lichen on the surface
• Irregular growth of older lichen
• Some colluvium may have older lichen
• Error bars should be 15-20 and larger with
  extrapolated dates

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