Fraction Modern Research Articles

Homogeneity evaluation of Chinese sugar carbon (CSC) standard for AMS 14C measurement

The Chinese national radiocarbon standard Chinese Sugar Carbon (CSC), or often called the “Chinese Sucrose Charcoal,” was investigated for its homogeneity and its suitability as a radiocarbon standard for Accelerator Mass Spectrometer (AMS). We randomly chose ten new CSC bottles and two opened bottles and measured their carbon isotope compositions. δ13C results obtained by Isotope Ratio Mass Spectrometer (IRMS) showed relatively large variation: from −15.9 to −22.2‰ with an average of −19.0±1.2‰ (n=49), indicating CSC grains in the mg range are not isotopically homogeneous. However, 14C results of CSC, expressed as Fraction Modern (FM), are well constrained to 2.4‰ after fractionation correction to −25‰. Measurements from all 12 bottles gave an average FM value of 1.3533±0.0034 (n=55). This value is about 7‰ higher that the FM value of 1.346±0.002, converted from the originally reported R value of 1.362±0.002 (n=14, with δ13C of −19.32, n=9) [1,2]. The R value of 1.362 is the ratio of 14C activity of CSC to 95% of OXI, not an FM value. The online simultaneous AMS 13C/12C measurement that allows for correction of all isotopic fractionation, including manufacture, graphitization and machine induced fractionation, is the key for obtaining consistent 14C measurements on this standard. Without further homogenization, CSC may not be a good choice of standard for an AMS that does not have the capability of measuring online AMS δ13C. Further study is needed to see if mixing/grinding can reduce the inhomogeneity. Re-measurement is suggested with more laboratories involved in order to get a better defined consensus value.

Carbon export and cycling by the Yukon, Tanana, and Porcupine rivers, Alaska, 2001–2005

Loads and yields of dissolved and particulate organic and inorganic carbon (DOC, POC, DIC, PIC) were measured and modeled at three locations on the Yukon River (YR) and on the Tanana and Porcupine rivers (TR, PR) in Alaska during 2001–2005. Total YR carbon export averaged 7.8 Tg C yr−1, 30% as OC and 70% as IC. Total C yields (0.39–1.03 mol C m−2 yr−1) were proportional to water yields (139–356 mm yr−1; r2 = 0.84) at all locations. Summer DOC had an aged component (fraction modern (FM) = 0.94–0.97), except in the permafrost wetland‐dominated PR, where DOC was modern. POC had FM = 0.63–0.70. DOC had high concentration, high aromaticity, and high hydrophobic content in spring and low concentration, low aromaticity, and high hydrophilic content in winter. About half of annual DOC export occurred during spring. DIC concentration and isotopic composition were strongly affected by dissolution of suspended carbonates in glacial meltwater during summer.

High-Precision AMS 14C Results on TIRI/FIRI Turbidite

Unleached aliquots of TIRI/FIRI turbidite were analyzed by accelerator mass spectronomy (AMS) over a timespan of 18 months. Individual analyses ranged from 18,090–18,245 yr BP with reported errors between 30–50 yr. The weighted average fraction modern (FM) of these 28 measurements is 0.10378 ± 0.00008 (which equates to 18,199 ± 8 yr BP) and the measurements show a 1 standard deviation scatter of 0.00044 (±35 yr). The fractional error of these results indicates reproducibility of individual measurements at the 4 (1σ) level, which is consistent with the quoted counting-statistics-based errors. Laboratories engaged in the determination of 14C results at reasonably high precision should consider taking advantage of the TIRI and FIRI sample materials in the role of process standards. Additional suites of high-precision data are necessary to refine the accuracy of these sample materials.