radiocarbon based paleoseismic dating gordon seitz san diego state university
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Radiocarbon Based Paleoseismic DatingRadiocarbon Based Paleoseismic Dating
Gordon Seitz
San Diego State University
1414C - A quick primerC - A quick primer
Formed primarily in the stratosphere (1° slow or thermal neutron capture by 14N) Galactic rays 1-10 GeV protons & alpha particles interact with other atoms to produce (2°)
neutrons.
Production of 14C is influenced by: Bkg galactic flux of cosmic rays Geomagnetic shielding Solar wind
Rapidly oxidized to 14CO > 14CO2
False color image of Earth’s magnetic field
Carbon Has Three Naturally Occurring IsotopesCarbon Has Three Naturally Occurring Isotopes
Carbon-12 98.9% (stable – 6 neutrons)Carbon-13 1.1% (stable – 7 neutrons)
Carbon-14 0.000000000001% (radioactive – 8 neutrons)
Isotopes have the same chemical properties but different atomic weights
If 14C atoms were people, there would be at most ONE 14C atom on the whole earth!
Radiocarbon in the Environment Radiocarbon in the Environment
Carbon 14 Strat-trop exchange processes Rapidly oxidized to 14CO > 14CO2
Interhemispheric mixing (~2yrs) Residence time in troposphere <10yrs Natural abundance (14C/12C) ~ 1x10-12
Boon and bane: Carbon cycle Terrestrial Biosphere Ocean Carbon Cycle
Cosmic Rays14N + n ➔ 14C + p14C + O2 ➔ 14CO2 750 Gt
1020 Gt38100 Gt5000 Gt1580 Gt
66-100 E6 Gt3
14C ➔ 14N + β−τ1/2 = 5730 yrsCO2CO2CO2
C reservoirs adapted from WS Reeburgh
INTCAL 04 14C-Calendar Calibration INTCAL 04 14C-Calendar Calibration
cf. Reimer et al., 2004 Hughen et al., 2004
05000100001500020000250000500010000150002000025000
CALENDAR YEARS BP
INTCAL 04
CoralTree RingCariaco
Dendrochronological 14C-Calendar Calibration Dendrochronological 14C-Calendar Calibration
0300060009000120000200040006000800010000
CALENDAR YEARS BPcf. Reimer et al., 2004 Stuiver et al., 1998
INTCAL 04
Tree ring data are the “gold standard” True record of atmospheric 14CO2 Replication of overlapping sequences
Dendrochronological 14C-Calendar Calibration Dendrochronological 14C-Calendar Calibration
cf. Reimer et al., 2004 Stuiver et al., 1998
05001000150020000500100015002000
CALENDAR YEARS BP
INTCAL 04
Flat spots, steep bits, and small age reversals are common.
Above-ground
nuclear testing
from the late
1950’s through
early 1960’s
produced an
excess amount of 14C in the
atmosphere.
The amount of 14C in the environment increased at the beginning of the Atomic Age
The The 1414C “Atmospheric Bomb Curve”C “Atmospheric Bomb Curve”Can be used for dating recent events (e.g. forensics)Can be used for dating recent events (e.g. forensics)
Units:
•X-axis
Calendar year
•Y-axis
∆14C [‰]
∆14C is the concentration of 14C relative to pre-industrial atmosphere
Chemical Reactions Change Plant or Animal Chemical Reactions Change Plant or Animal Material to GraphiteMaterial to Graphite
Dissolve carbonate or combust organic matter under vacuum
Transfer CO2 gas
Chemical reaction turns CO2 into graphite and H2O
Make Target for AMS MeasurementMake Target for AMS Measurement
Pound into target holder Load into sample wheel
C-14 Dating Uncertainty versus Context Uncertainty
- There are two endmember paleoseismic sites in terms of context uncertainty:
- In situ Carbon sedimentary sections: clear association of the sample age to layer age
-Non in situ or detrital charcoal sections: unknown age offset, i.e. the dating samples are older by an unknown amount. These dates provide one-sided constraints
-Important considerations: section completeness and event horizon correlations.
Complete 17-event record San Jacinto fault at Complete 17-event record San Jacinto fault at Hog LakeHog Lake
Exceptional section completeness
Even this stratigraphy required physical event horizon correlations to avoid errors!
Somewhat typical Non-in situ or detrital carbon site.Somewhat typical Non-in situ or detrital carbon site.
Difficulties: non-continuous layers and section incomplete
C-14 Date Scatter Plot: first step in data evaluationC-14 Date Scatter Plot: first step in data evaluation
Hard versus Soft Data
- more data does not always improve the chronologic model.
- key data i.e reliable dates should be used to pin the chronology
-Important considerations: due to stratigraphic incompleteness and event horizon correlation ambiguities. Dating methods have their limitations and cannot replace careful fieldwork.