HR: 09:30h
AN: B11E-07    [Abstracts]
TI: Seasonal isotopic fractionation of atmospheric CO₂ reveals a mismatch between observations and model simulations
AU: * Ballantyne, A
EM: apballantyne@gmail.com
AF: NOAA Earth System Research Laboratory, 325 Broadway, Boulder, CO 80305,
AU: Miller, J
EM: john.b.miller@noaa.gov
AF: NOAA Earth System Research Laboratory, 325 Broadway, Boulder, CO 80305,
AU: Tans, P
EM: pieter.tans@noaa.gov
AF: NOAA Earth System Research Laboratory, 325 Broadway, Boulder, CO 80305,
AB: Here we present a novel approach in using the carbon isotopic composition of the atmosphere and the biosphere to identify inconsistencies in model predictions of water and carbon exchange between the biosphere and atmosphere. We evaluated different methods for estimating the isotopic signature of source CO₂ (δ_s) on seasonal timescales. We determined that the most effective method for retrieving information about isotopic discrimination by the terrestrial biosphere was by analyzing the residual differences between CO₂ and δ¹³CO₂ observations from the planetary boundary layer and the free troposphere. Seasonal patterns of δ_s derived from atmospheric sampling sites in the Northern Hemisphere showed a coherent increase in δ_s during the Boreal summer. Intra-annual measurements of δ¹³C in the cellulose of Pinus taeda growing in a pine forest of the Southeastern, US also indicated consistent increases in δ_s during the growth season. However, an identical analysis of isotopic tracers in a global biosphere-atmosphere transport model indicated a decline in δ_s during the growth season. The mismatch between observations and model predictions was greatly improved when stomatal conductance was allowed to vary as a function of vapor pressure deficit instead of relative humidity. These results suggest that model predictions of conductance may be inconsistent, especially in terrestrial regions where vapor pressure deficit and relative humidity are temporally covariant.
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
DE: 0428 Carbon cycling (4806)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting