Radiocarbon Based Paleoseismic Dating Gordon Seitz San Diego State University

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

Carbon Has Three Naturally Occurring Isotopes Isotopes have the same chemical properties but different atomic weights Carbon-12 98.9% (stable – 6 neutrons) Carbon-13 1.1% (stable – 7 neutrons) Carbon-14 0.000000000001% (radioactive – 8 neutrons) If 14C atoms were people, there would be at most ONE 14C atom on the whole earth!

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

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

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

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

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.

∆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

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

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

The FN Tandem Accelerator

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 Hog Lake Exceptional section completeness Even this stratigraphy required physical event horizon correlations to avoid errors!

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

Rare In Situ Carbon Samples at Salt Creek

When possible date different sample materials

Detrital Charcoal

In Situ Macro Samples: Seeds or fragile charcoal

In Situ Plant Matter

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

A bit more detail.

Chronological Model- Event Age Calculation

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.

Terrestrial cosmogenic nuclide surface exposure dating 65 ka 50 ka

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