Transcript Chapter 4
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) Detrital Remnant Magnetization (DRM) 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) Polarity Events (subchrons) Persist from 10,000 to 100,000 years Geomagnetic Excursions 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 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 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 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