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Quaternary Environments Dating Methods II. Paleomagnetism  Major Reversals  Aperiodic global-scale geomagnetic reversals  Dipole changes  Secular.

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Presentation on theme: "Quaternary Environments Dating Methods II. Paleomagnetism  Major Reversals  Aperiodic global-scale geomagnetic reversals  Dipole changes  Secular."— Presentation transcript:

1 Quaternary Environments Dating Methods II

2 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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4 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

5 Magnetic Field

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

7 Magnetization of Rocks and Sediments  Thermoremnant Magnetization (TRM)  Currie Point – Below which the igneous rock’s 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

8 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

9 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  δ18O in marine sediments and their relationship with astronomical forcing has been used to refine the timing of magnetic reversals

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11 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

12 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

13 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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16 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

17 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

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19 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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21 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

22 Problems With Obsidian Hydration  Must be regionally calibrated to provide absolute dates  Dependent upon temperature  Varies with sample composition  Not very precise

23 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. and Waechter 2004

24 Obsidian Hydration

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

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

27 Thephrochronology

28 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

29 Rhizocarpon geographicum from Norway

30 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

31 Growth Curves of Lichen

32 Lichen Dates SpeciesDiameterAgeLocation Alectoria minuscula 160mm yrs Baffin Island Rhizocarpon geographicum 280mm 9,500 +/-1500 yrs Baffin Island Rhizocarpon alpicola 480mm 9,000 yrs Swedish Lapland

33 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

34 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

35 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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