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

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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: 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 Basic theory Sample collection and processing Calibration

Radiocarbon dating Tsurue Sato Gayatri Marliyani October 31, 2012

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

Once an organism dies, it ceases to obtain more 14 C 14 C decays reducing the concentration within organism after death 14 C 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 14 C dating 14 C ---> 14 N + ß + neutrino

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

Applicable range Within the last 50,000 to 60,000 years A: Present amount of 14 C A 0 : Original amount of 14 C t: Time it takes to reduce the original amount to the present amount k: Half-life of 14 C (5,370 years) (* Libby half-life is 5,568 years) (Trumbore, 2000)

How the 12 C / 14 C Ratio Works

Two ways to measure 14 C (1) Beta-decay counting ( 14 C 14 N + - ): Measure radioactivity (decay constant x no. of 14 C atoms). (2)Accelerator mass spectrometry (AMS) Count individual 14 C atoms to get 14 C/ 12 C 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 CO 2 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 14 C from the packaging, the packaging should also be airtight to avoid contact with atmospheric 14 C 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-treatments4: CO 2 production 5: Graphitization6: Pressing7: Sample analysis b. ABA washc. Combustion Sample preparation and analysis a. Physical separationa. Into quartz tubeb. Sealing a. Adding CO 2 b. Adding H 2 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 14 C 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 14 C Industrial revolution to present, increase in values of CO 2 in atmosphere which decreases the ratio of 14 C to 12 C 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 14 C 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

Limitation 14 C can only be used to date organic material Samples cant be too old or too young, from ~300 - ~50,000 years, limited due to half life (approximately 9 half lives) 14 C dating accuracy is dependent upon a consistent ratio between 12 C and 14 C (equilibrium) –The assumption of equilibrium is FALSE –There are factors that can affect 14 C in the atmosphere Ancient fossils as well as coal contain 14 C residue

Solutions measure all three C isotopes ( 12 C, 13 C, 14 C) concentrate 14 C and extend counting time measure individual atoms (AMS) precisely identify exchange reservoirs calibrate conventional dates to calendar years (tree rings, corals) combine 14 C 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 14 C 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 lecture Basic theory Sample collection and processing Calibration

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