Time series of precisely measured atmospheric constituents are a critical component to understanding the fate and transport of fossil fuel carbon in the contemporary carbon cycle. Following the intrusion of anthropogenic 14C culminating in the early 1960s, the 14C/12C ratio (D14C), has decreased. Atmospheric 14C reflects variations in fossil fuel emissions (14C-free), ocean-atmosphere exchange, stratosphere-troposphere mixing, and terrestrial ecosystem fluxes. As a consequence of the redistribution of anthropogenic 14C into and with the terrestrial and ocean reservoirs, the spatio-temporal variability of 14CO2 has decreased.
Present day measurements at clean-air stations exhibit seasonal cycles with amplitudes of 10 and differences in annual-mean values as a function of latitude of 3 to 5. Data by Levin and Hessheimer provide evidence for D14C latitudinal variability. In the early 1990s the mean annual D14C latitudinal profile exhibited a maximum in the tropics (~+2.5 relative to global mean) and minimums in the mid-latitude NH (~ -3) and the far southern latitudes (~ -3). The simplest interpretation of the decrease in the NH is influence of burning of 14C-free fossil fuels whereas the low values over the S. Ocean reflect exchange with recently upwelled low-14C water. The main limitation to 14C/12C measurements is technological: most of the atmospheric signals of interest are at or near the detection limit of some methods.
We have begun to determine the 14CO2 content to 1-2 by accelerator mass spectrometry (AMS) from atmospheric flask samples collected by the SIO CO2 and O2/N2 network. In order to ascertain sample handling and extraction we have analyzed individual replicate CO2 samples collected at La Jolla. The CO2 was reduced to graphite using iron catalyst and a methodology similar to that described by Vogel et al. Targets were analyzed at 100K to 50K (14C atoms) per individual acquisition cycle and for each acquisition cycle, normalized to the 6 bracketing primary (OX1) standards. Groups were cycled to approach 1E6 14C atoms analyzed. Replicate flasks analyzed on the same wheel have a fractional error of 0.9 (n=18 pairs). These results indicate that flask handling and extraction is very reproducible. Since those early experimental measurements, we have begun to analyze archived ampules of CO2. The average error in all flask measurements to date is 1.2. Measurement of 46 pairs of replicate flask samples from La Jolla analyzed over the last ~18 months exhibit a pooled standard deviation of 1.75 and generally show agreement within one-sigma error.
See more of Poster Session II
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)