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

The FN Tandem AcceleratorThe 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 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

Rare In Situ Carbon Samples at Salt CreekRare In Situ Carbon Samples at Salt Creek

When possible date different sample materialsWhen possible date different sample materials

Detrital CharcoalDetrital Charcoal

In Situ Macro Samples: Seeds or fragile charcoalIn Situ Macro Samples: Seeds or fragile charcoal

In Situ Plant MatterIn Situ Plant Matter

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

A bit more detail.A bit more detail.

Chronological Model- Event Age CalculationChronological 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.

50 ka

65 ka

Terrestrial cosmogenic nuclide surface exposure dating

Luminescence dating

watercontent

(after Olley et al., 1998)