Download Quaternary Environments Introductory Lecture

Quaternary Environments
Dating Methods II
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)
Earth’s 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
Magnetic Field
Magnetostratigraphy

Use of magnetic reversals as a
chronometer
 Dipole
changes synchronous around the
world
 Not dependent upon fossil associations or
similar rocks
Magnetization of Rocks and
Sediments

Thermoremnant Magnetization (TRM)

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Detrital Remnant Magnetization (DRM)

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Magnetic particles become aligned with the ambient
magnetic field as they settle through the water column
Post-Depositional DRM


Currie Point – Below which the igneous rock’s magnetic
record is fixed
Effective on lava flows and baked clays at archaeological
sites
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
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

Paleomagnetic Timescale

Major Polarity Epochs (chrons)

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Polarity Events (subchrons)

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Persist from 10,000 to 100,000 years
Geomagnetic Excursions


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
Persist around 1,000,000 years
Short-term geomagnetic fluctuations (Cryptochrons)
Persist for a few thousand years
Due to the non-dipole variation
δ18O in marine sediments and their relationship
with astronomical forcing has been used to
refine the timing of magnetic reversals
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

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

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

Amino-Acid Dating
First studies in 1968 (Hare and Mitterer
1968)
 Can be conducted on small samples <2mg
in mollusks or foraminifera
 Can also be conducted of wood,
speleothems, and corals

Problems With Amino-Acid Dating
Must be calibrated to provide absolute
dates
 Very sensitive to temperature history

 An
uncertainty of +/- 2°C is equivalent to an
age uncertainty of +/-50%
Can also be affected by contamination and
leaching
 Rates vary from one Genus to another

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

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

Problems With Obsidian Hydration
Must be regionally calibrated to provide
absolute dates
 Dependent upon temperature
 Varies with sample composition
 Not 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 Hydration
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

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/monticchio_tephrochronology/content_en.html
Thephrochronology
http://www.grancampo.de/english/tephra/tephra3.htm
Lichenometry
Lichen are a symbiotic relationship
between algae and fungi
 The algae provide carbohydrates through
photsynthesis
 The fungi provide a protective environment

Lichen – Bush-like form
 Crustose – Flat disc-like forms
 Foliose
Rhizocarpon geographicum from
Norway
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
Growth Curves of Lichen
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
Biological Problems With
Lichenometry
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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
Environmental Problems With
Lichenometry

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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
Sampling Problems With
Lichenometry

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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