HR: 1340h
AN: B43D-1581    [Abstracts]
TI: ?14C of Atmospheric CO2 over the Subtropical and Equatorial Pacific and at Point Barrow, Alaska
AU: * Xu, X
EM: xxu@uci.edu
AF: University of California, Irvine, Earth System Science Department, Irvine, CA 92697-3100,
AU: Trumbore, S
EM: setrumbo@uci.edu
AF: University of California, Irvine, Earth System Science Department, Irvine, CA 92697-3100,
AU: Ajie, H
EM: hoajie@uci.edu
AF: University of California, Irvine, Earth System Science Department, Irvine, CA 92697-3100,
AU: Tyler, S
EM: styler@uci.edu
AF: University of California, Irvine, Earth System Science Department, Irvine, CA 92697-3100,
AB: Δ¹⁴C is a unique tracer for studying the carbon cycle, especially for discriminating between fossil and biosphere carbon emissions. However observations of Δ¹⁴C variation in atmospheric CO₂ are available for only a few locations. We have been measuring atmospheric ¹⁴CO₂ in boundary layer air over the subtropical and equatorial Pacific and at stations in the US since 2002 to expand the ¹⁴CO₂ database with high precision data that sample either at high temporal resolution (Point Barrow, Alaska) or high spatial resolution (cross-equatorial mid-Pacific). These data provide observational constraints for the roles of ¹⁴C isotope disequilibirum in the tropical terrestrial biosphere, the Southern ocean, and fossil fuel burning and enhance our understanding of the patterns of atmospheric ¹⁴CO₂ distribution and its seasonal variation. Five transects of atmospheric ¹⁴CO₂ were collected on shipboard over the Pacific Ocean between Los Angeles (34°N, 118°W) and Auckland, New Zealand (34°N, 177°W) from fall 2002 to summer 2005. Abundances of CO and CH₄ in addition to CO₂, and their stable isotopes were also measured for these samples. The high precision of our Δ¹⁴C analysis (~2‰ based on duplicate measurements) allows us to observe relatively small variations over the latitude span investigated. All five transects show that Δ¹⁴C in atmospheric CO₂ were relatively uniform from the equatorial region to 30°S latitude. They also indicate a consistent decreasing trend in Δ¹⁴C (~7‰) northward of ~6°N to 30°N latitude, consistent with an increase in fossil fuel input in the northern hemisphere. From fall 2002 to summer 2005, Δ¹⁴C decreased by an average rate of 6‰/year, with a slightly higher rate of decrease over the southern ocean. Correlation between CO mixing ratio and Δ¹⁴C indicates short-term atmospheric circulation may significantly affect the ¹⁴CO₂ distribution pattern and its latitudinal gradient. In addition, signals from seasonal variation of ¹⁴CO₂ could be superimposed on the transects' latitudinal variation. We have also been measuring Δ¹⁴C in two air samples biweekly from the Point Barrow Observatory, Alaska (71°N, 157°W) since July 2003. In this period, Δ¹⁴C decreased by 5- 6‰/year, to ~52‰ in Feb-2007. We find a distinct seasonal cycles for ¹⁴C, with a broad minimum around April and a maximum in September with an amplitude of 5-7‰. This seasonal pattern is highly variable from year to year. Increasing ¹⁴C values may reflect injection of stratospheric air in April and May, and higher soil respiration with enriched ¹⁴CO₂ between May to August; rapid declines may be due to reduction in soil respiration and changes in the poleward advection of fossil fuel burned in the winter months.
DE: 1030 Geochemical cycles (0330)
DE: 1610 Atmosphere (0315, 0325)
DE: 4251 Marine pollution (0345, 0478)
DE: 4806 Carbon cycling (0428)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting