1. Radiocarbon Based Paleoseismic Dating
Gordon Seitz
San Diego State University

2. False color image of Earth’s magnetic field
14C - 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

3. Carbon Has Three Naturally Occurring Isotopes
Isotopes have the same chemical properties but different atomic weights
Carbon % (stable – 6 neutrons)
Carbon % (stable – 7 neutrons)
Carbon % (radioactive – 8 neutrons)
If 14C atoms were people, there would be at most ONE 14C atom on the whole earth!

4. Radiocarbon in the Environment
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
C reservoirs adapted from WS Reeburgh

5. INTCAL 04 14C-Calendar Calibration
cf. Reimer et al., 2004
Hughen et al., 2004

6. Dendrochronological 14C-Calendar Calibration
INTCAL 04
Tree ring data are the “gold standard”
True record of atmospheric 14CO2
Replication of overlapping sequences
cf. Reimer et al., 2004
Stuiver et al., 1998

7. Dendrochronological 14C-Calendar Calibration
cf. Reimer et al., 2004
Stuiver et al., 1998
Flat spots, steep bits, and small age reversals are common.

8. The amount of 14C in the environment increased at the beginning of the Atomic Age
Above-ground nuclear testing from the late 1950’s through early 1960’s produced an excess amount of 14C in the atmosphere.

9. ∆14C is the concentration of 14C relative to pre-industrial atmosphere
The 14C “Atmospheric Bomb Curve” 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

10. Chemical Reactions Change Plant or Animal Material to Graphite
Dissolve carbonate or combust organic matter under vacuum
Transfer CO2 gas
Chemical reaction turns CO2 into graphite and H2O

11. Make Target for AMS Measurement
Pound into target holder
Load into sample wheel

12. The FN Tandem Accelerator

13. 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.

14. Complete 17-event record San Jacinto fault at Hog Lake
Exceptional section completeness
Even this stratigraphy required physical event horizon correlations to avoid errors!

15. Somewhat typical Non-in situ or detrital carbon site.
Difficulties: non-continuous layers and section incomplete

16. Rare In Situ Carbon Samples at Salt Creek

17. When possible date different sample materials

18. Detrital Charcoal

19. In Situ Macro Samples: Seeds or fragile charcoal

20. In Situ Plant Matter

21. C-14 Date Scatter Plot: first step in data evaluation

22. A bit more detail.

23. Chronological Model- Event Age Calculation

24. 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.

25. Terrestrial cosmogenic nuclide surface exposure dating
65 ka
50 ka

26. Luminescence dating
water
content
(after Olley et al., 1998)