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Radiocarbon Calibration

Radiocarbon Calibration. By Jonathan Chacon ESS 433 14November2007. Fairbanks et. al. 2005. Paired radiocarbon and 230Th/ 234U/ 238U ages determined on coral samples help extend radiocarbon calibration curve (RCCC) from 12k-50k yr BP

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Radiocarbon Calibration

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  1. Radiocarbon Calibration By Jonathan Chacon ESS 433 14November2007

  2. Fairbanks et. al. 2005 • Paired radiocarbon and 230Th/ 234U/ 238U ages determined on coral samples help extend radiocarbon calibration curve (RCCC) from 12k-50k yr BP • Tree-ring dating calibration 0-12k yr BP (Reimer et. al. 2004)

  3. Reimer et. al. 2004 • Discusses calibration data for Northern Hemisphere C14 using tree-ring measurements 0-12k BP • www.radiocarbon.org

  4. Fairbanks • Extended TRC by using coral reef cores from Barbados (Atlantic), Kiritimati Atoll (CenPac), and Araki Is. (WesPac) • Accurate calibration essential for measuring time and rates of change for numerous scientific fields

  5. Fairbanks cont. • (CO2)14 produced by cosmic rays in atmosphere, mixes in troposphere and exchanges w/ reactive C reservoirs of ocean and biosphere, where it decays • THUS the use of coral core samples!

  6. Fairbanks cont. • Existing RCC data sets infer calendar ages based on interpolations and correlations of local climate proxies in deep sea cores to chronologies of ice core proxies, or assumptions about sedimentation rates.

  7. Fairbanks Methodology • Require that each data point has measured calendar age (230Th/ 234U/ 238U) and RC age w/ known errors independent of each other • Other studies not included b/c they use samples w/ 1-5% calcite, which corrupts RC data • Fairbanks -0.2% calcite samples

  8. RC Age Calibration • All RC ages are readily subject to contamination by modern C during sample handling and processing • 230Th/ 234U/ 238U and 14C ages have independent errors measured, computed, and accounted for

  9. Accuracy and Precision • Tested by: • a.) measuring samples that overlap TRC • b.) making paired 230Th/ 234U/ 238U and 231Pa/235Uages on select samples • c.) through 230Th/ 234U/ 238U, 231 Pa/235U, and 14C measurements in laboratory calibration samples

  10. 230Th/ 234U/ 238U • Abundance determined by Thermal Ionization Mass Spectrometry (TIMS) opened wide range of dating applications • New half-life estimates for 230Th and 234U • Improvements made to sample pretreatment quality control

  11. Advantages to Fairbanks • Quality of coral samples combined w/ high precision 230Th/ 234U/ 238U and 14C measurements • Goal= 3 yrs make 400 new 230Th/ 234U/ 238U and 14C to fill in gaps of RCC 0-50k BP w/ samples of 100 yr resolution or better • Rigorous error estimation!

  12. Quality Control • <0.2% calcite = most important screening criteria, uses X-ray Diffraction • 230Th/ 234U/ 238U and 231Pa/235U ages determined by Multi-Collector Magnetic Sector double focusing Inductively Coupled Mass Spectrometer (MC-MS-ICPMS) w/ age uncertainty of ~0.5%!

  13. AAAAGGGGHHH!!!

  14. More Methodology • No benefit to amalgamating subsets of calibration pts. from other data sets b/c dramatic increase in scatter • 10-29k BP = all pristine coral samples exclusively from marine environments • Consistency of screening, handling, analysis, and precision of measurements justify “stand-alone” CC

  15. Fairbanks to Reimer • Fairbanks CC overlaps w/ existing TRCC and extend calibration to 50k BP • Paired 230Th/ 234U/ 238U and 14C age determinations fr. pristine corals provide, high precision/accuracy RCCC for use beyond superior TRCC

  16. Reimer • Tree Ring Data Sets • Random Walk Model (RWM) – account of uncertainty in calendar and 14C age to calculate underlying CC

  17. IntCal04 • Group that established criteria for acceptance of data into calibration set • Tree ring measurements used 0-12.4k BP, marine data 12.4-26k BP • www.radiocarbon.org

  18. Tree Ring Data Sets (0-12.4k BP) • Dendrochronological dating and cross-checking tree rings is required • Rigorous testing by internal replication of many overlapping sections: • Heidelberg Data Set • Belfast Data Set • Waikato Data Set • Seattle Data Set • Pretoria and Groningen Data Sets • Arizona Data Set

  19. Regional Offsets • Trees assumed to have sampled well-mixed atmosphere • -lat-dependent diffs in ocean area, surface water ages, and wind speeds cause 14C offsets of ~1‰ (8 14C yr) for most of the Northern Hemisphere with the exception of the high Arctic • - growing season diffs, altitudinal effects, melting permafrost, proximity to areas of intense ocean upwelling, and volcanic emissions -> significant in certain localities and time periods

  20. Sources of Uncertainty • Counting statistics -> standard deviation in 14C age difference of replicate samples (mathematical process) • Others may be the result of differences in sampling, sample pretreatment, or laboratory operation, which are difficult to quantify for individual samples

  21. Data Collection • For comparison purposes, calculated a weighted avg of all updated tree-ring C14 measurements w/in a 10 year window • RWM assumes equal rise/fall in atmospheric 14C levels

  22. Reimer Conclusions • -b/c RWM accounts for scatter around “true calibration curve”, IntCal04 curve is smoother than the tree-ring-derived portion of IntCal98 (0-11440k BP)

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