Pelatihan :

Techniques in Active Tectonic StudyJuni 20-Juli 2, 2013

Instruktur: Prof. J Ramon Arrowsmith (JRA)Dari Arizona State University (ASU) - US

Tempat Pelaksanaan: Ruang Pangea, Laboratorium Gempabumi (LabEarth) –

Puslit Geoteknologi LIPI dan Kuliah lapangan akan dilakukan disekitar Sesar Lembang, Jawa Barat.

* Lebih jelas baca TOR/KAK dan daftar acara

Carbon-14 geochronologyOutline of this lecture

• Basic theory• Sample collection and processing• Calibration

Radiocarbon dating

Tsurue SatoGayatri Marliyani

October 31, 2012

http://science.howstuffworks.com/environmental/earth/geology/carbon-141.htm

Theory

• Once an organism dies, it ceases to obtain more 14C• 14C decays reducing the concentration within

organism after death• 14C decays by beta emission, emission of an electron

and a neutron changing into a proton, thus reverting back into nitrogen

The emitted beta particles (ß) are what is counted in Libby's "gas proportional“ method of 14C dating

14C ---> 14N + ß + neutrino

Determining the Starting Amount

Two types of carbon used in the dating process: 12C and 14C• 12C is a stable isotope (it does not decay)• When an organism is alive it has the same ratio (12C to

14C) that is found in the atmosphere (1-trillion to 1)

Same ratio Different ratio

1. How fast it decays (measured in half-lives). This is known (5,730 years --> Cambridge half life).

2. The starting amount of C-14 in the fossil.

A Critical Detail

Two Things Need to Know to determine how many half-lives have expired

The C-14 dating method relies on measuring the amount of 14C in the material

Applicable range

Within the last 50,000 to 60,000 years

A: Present amount of 14CA0: Original amount of 14Ct: Time it takes to reduce the original amount to the present amountk: Half-life of 14C (5,370 years) (*Libby half-life is 5,568 years)

(Trumbore, 2000)

How the 12C / 14C Ratio Works

Two ways to measure 14C(1) Beta-decay counting (14C → 14N + b-): Measure

radioactivity (decay constant x no. of 14C atoms).

(2)Accelerator mass spectrometry (AMS) Count individual 14C atoms to get 14C/12C ratio

One gram of "modern" carbon produces about 14 beta-decay events per minute. To measure the age of a 1g sample to a precision of +/- 20 years one needs 160,000 counts, or about 8 days of beta-counting.

AMS allows you to do the same measurement on a 1 milligram sample in a few minutes.

Example of Material• Charcoal, wood, twigs and seeds• Bone.• Marine, estuarine and riverine

shell.• Peat• Lake muds and sediments.• Soil.• Pollen.

• Corals and foraminifera.

• Textiles and fabrics.• Water. • etc

requirements:• Carbon originally fixed from

atmospheric CO2

• Not contaminated• Found in situ• Well-preserved

Application: paleoseismic study, bracketing the age of earthquakes

Sampling procedure

Sampling error precaution

• samples should be packaged in chemically neutral materials to avoid picking up new 14C from the packaging, the packaging should also be airtight to avoid contact with atmospheric 14C

• the stratigraphy should be carefully examined to determine that a carbon sample location was not contaminated by carbon from a later or an earlier period

3: Pre-treatments 4: CO2 production

5: Graphitization 6: Pressing 7: Sample analysis

b. ABA wash c. Combustion

Sample preparation and analysis

a. Physical separation a. Into quartz tube b. Sealing

a. Adding CO2 b. Adding H2 c. Graphitization AMS

https://www-pls.llnl.gov/data/assets/images/about_pls/centers_and_institutes/bioams/ams2.jpg

Lawarence Livermore National Laboratory AMS

• Results of 14C dating are reported in radiocarbon years, and calibration is needed to convert radiocarbon years into calendar years

• Un-calibrated radiocarbon measurements are usually reported in years BP where 0 (zero) BP is defined as AD 1950

• The most popular and often used method for calibration is by dendrochronology.

Calibration

the age of a certain carbonaceous sample can be easily determined by comparing its radiocarbon content to that of a tree ring with a known calendar age.

If a sample has the same proportion of radiocarbon as that of the tree ring, it is safe to conclude that they are of the same age.

Factors affecting the amount of carbon in the atmosphere

• Atomic bomb testing in the 1950s elevated atmospheric 14C

• Industrial revolution to present, increase in values of CO2 in atmosphere which decreases the ratio of 14C to 12C

• Cosmic ray flux rate changes, e.g. supernova• Magnetic field changes can modify intensity of

cosmic ray flux• Short term- sun activity (solar flares) major factor,

paired with low 14C production rates

INTCAL09 Radiocarbon Age Calibration Curve for 0-50.000 years cal BP

(Reimer et al, 2009)

• Curve developed rom archive of tree rings, marine (corals and planktonic foraminifera) and highly resolved speleothems

• Calibration software: OxCal

Burbank and Anderson, 2011, Tectonic Geomorphology, Chapter 3

Calibration by OxCal

Calibration by OxCal

Limitation

• 14C can only be used to date organic material• Samples can’t be too old or too young, from ~300

- ~50,000 years, limited due to half life (approximately 9 half lives)

• 14C dating accuracy is dependent upon a consistent ratio between 12C and 14C (equilibrium)– The assumption of equilibrium is FALSE– There are factors that can affect 14C in the atmosphere

• Ancient fossils as well as coal contain 14C residue

Solutions

• measure all three C isotopes (12C, 13C, 14C)• concentrate 14C and extend counting time• measure individual atoms (AMS)• precisely identify exchange reservoirs• calibrate conventional dates to calendar years

(tree rings, corals)• combine 14C with other dating methods• understand stratigraphic context

References

Hua, Q., and Barbetti, M., 2004, Review of tropospheric bomb 14C data for carbon cycle modeling and age calibration purposes, Radiocarbon, vol. 46, no. 3, p. 1273-1298.

Libby, W. F., 1960, Radiocarbon dating: Nobel Lecture, December 12, 1960. 23 September 2012, http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1960/libby-lecture.pdf.

Lienkaember, J. J., and Ramsey, C. B., 2009, OxCal: Versatile tool for developing paleoearthquake chronologies – A primer: Seismological Research Letters, vol. 80, no. 3, p. 431 – 434.

Trumbore, S. E., 2000, Radiocarbon geochronology, in Noller, J. Sl., Sowers, J. M., and Lettis, W. R., eds., Quaternary geochronology: AGU Ref. Shelf, vol. 4: Washington, D. C., p..41-60.

UCI AMS Facility, 2011, Combustion protocol, Dec. 26, 2011, http://www.ess.uci.edu/ams/Text%20bodies/Combustion%20protocol.pdf.

UCI KCCAMS Facility, 2011, Acid/Base/Acid (ABA) Sample pre-treatment, Dec. 26, 2011, http://www.ess.uci.edu/ams/Text%20bodies/ABA_protocol.pdf.

--, 2009, AMS settings to 14C measurements, January 22, 2009, http://www.ess.uci.edu/ams/Text%20bodies/UCI%20KCCAMS%20-%20AMS%20settings%20to%2014C%20measurements.pdf.

--, 2011, Graphitization protocol: hydrogen reduction method (organic samples), December 26, 2011, http://www.ess.uci.edu/ams/Text%20bodies/Organic%20graphitization%20protocol.pdf.

--, 2011, Swipe protocol, April 28, 2011, http://www.ess.uci.edu/ams/Text%20bodies/Swipe%20protocol%20complete.pdf.

Carbon-14 geochronologyOutline of this lecture

• Basic theory• Sample collection and processing• Calibration