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

Tree-ring dating calibration 0-12k yr BP (Reimer et. al. 2004)

Reimer et. al. 2004

Discusses calibration data for Northern Hemisphere C14 using tree-ring measurements 0-12k BP

www.radiocarbon.org

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

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!

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.

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

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

Accuracy and Precision

Tested by:a.) measuring samples that overlap

TRCb.) 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

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

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!

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%!

AAAAGGGGHHH!!!

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

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

Reimer

Tree Ring Data Sets Random Walk Model (RWM) –

account of uncertainty in calendar and 14C age to calculate underlying CC

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

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

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

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

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

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)