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Pelatihan : Techniques in Active Tectonic Study Juni 20-Juli 2, 2013 Instruktur: Prof. J Ramon Arrowsmith (JRA) Dari Arizona State University (ASU) - US. Tempat Pelaksanaan:

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slide1

Pelatihan :

Techniques in Active Tectonic Study

Juni 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 geochronology outline of this lecture
Carbon-14 geochronologyOutline of this lecture
  • Basic theory
  • Sample collection and processing
  • Calibration
slide3

Radiocarbon dating

Tsurue Sato

Gayatri Marliyani

October 31, 2012

slide4

Theory

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

slide5

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

14C ---> 14N + ß + neutrino

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

determining the starting amount
Determining the Starting Amount
  • 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)

Two types of carbon used in the dating process: 12C and 14C

Different ratio

Same ratio

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

Two Things Need to Know to determine how many half-lives have expired
  • How fast it decays (measured in half-lives). This is known (5,730 years --> Cambridge half life).
  • The starting amount of C-14 in the fossil.

A Critical Detail

slide8

Applicable range

Within the last 50,000 to 60,000 years

A: Present amount of 14C

A0: Original amount of 14C

t: Time it takes to reduce the original amount to the present amount

k: Half-life of 14C (5,370 years)

(*Libby half-life is 5,568 years)

(Trumbore, 2000)

two ways to measure 14 c
Two ways to measure 14C
  • (1) Beta-decay counting (14C → 14N + b-): Measure radioactivity (decay constant x no. of 14C atoms).
  • 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
Example of Material
  • Corals and foraminifera.
  • Textiles and fabrics.
  • Water.
  • etc
  • Charcoal, wood, twigs and seeds
  • Bone.
  • Marine, estuarine and riverine shell.
  • Peat
  • Lake muds and sediments.
  • Soil.
  • Pollen.
  • requirements:
  • Carbon originally fixed from atmospheric CO2
  • Not contaminated
  • Found in situ
  • Well-preserved
sampling error precaution
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
slide14

Sample preparation and analysis

3: Pre-treatments

4: CO2 production

a. Physical separation

b. ABA wash

a. Into quartz tube

b. Sealing

c. Combustion

5: Graphitization

6: Pressing

7: Sample analysis

a. Adding CO2

b. Adding H2

c. Graphitization

AMS

slide15

Lawarence Livermore National Laboratory AMS

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

calibration
Calibration
  • 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.
  • 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
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
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
limitation
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
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
slide25

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 geochronology outline of this lecture1
Carbon-14 geochronologyOutline of this lecture
  • Basic theory
  • Sample collection and processing
  • Calibration