HR: 1340h
AN: B23A-0947    [Abstracts]
TI: Sensitivity analysis and quantification of uncertainty for isotopic mixing relationships in carbon cycle research
AU: * Zobitz, J M
EM: zobitz@math.utah.edu
AF: Department of Mathematics, University of Utah, 155 S 1400 E, Salt Lake City, UT 84112 United States
AU: Keener, J P
EM: keener@math.utah.edu
AF: Department of Mathematics, University of Utah, 155 S 1400 E, Salt Lake City, UT 84112 United States
AU: Bowling, D R
EM: bowling@biology.utah.edu
AF: Department of Biology, University of Utah, 257 S 1400 E, Salt Lake City, UT 84112 United States
AB: Quantifying and understanding the uncertainty in isotopic mixing relationships is critical to isotopic applications in carbon cycle studies at all spatial and temporal scales. Studies associated with the North American Carbon Program will depend on stable isotope approaches and quantification of isotopic uncertainty. An important application of isotopic mixing relationships is determination of the isotopic content of large-scale respiration ($\delta^{13}$C$_{R}$) via an inverse relationship (a Keeling plot) between atmospheric CO$_{2}$ concentrations ([CO$_{2}$]) and carbon isotope ratios of CO$_{2}$ ($\delta^{13}$C). Alternatively, a linear relationship between [CO$_{2}$] and the product of [CO$_{2}$] and $\delta^{13}$C (a Miller/Tans plot) can also be applied. We used an extensive dataset from the Niwot Ridge Ameriflux Site of [CO$_{2}$] and $\delta^{13}$C in forest air to examine contrasting approaches to determine $\delta^{13}$C$_{R}$ and its uncertainty. These included Keeling plots, Miller/Tans plots, Model I, and Model II regressions Our analysis confirms previous observations that increasing the range of measurements ([CO$_{2}$] range) reduces the uncertainty associated with $\delta^{13}$C$_{R}$. For carbon isotope studies, uncertainty in the isotopic measurements has a greater effect on the uncertainty of $\delta^{13}$C$_{R}$ than the uncertainty in [CO$_{2}$]. Reducing the uncertainty of isotopic measurements reduces the uncertainty of $\delta^{13}$C$_{R}$ even when the [CO$_{2}$] range of samples is small ($<$ 20 ppm). As a result, improvement in isotope (rather than CO$_{2}$) measuring capability is needed to substantially reduce uncertainty in $\delta^{13}$C$_{R}$. We also find for carbon isotope studies no inherent advantage to using either a Keeling or a Miller/Tans approach to determine $\delta^{13}$C$_{R}$.
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting