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B 23 A-0948 Atmospheric 14 CO over the mid Pacific Ocean and at Point Barrow, Alaska, USA from 2002 to 2004 2 Xiaomei XU ([email protected] edu), Susan TRUMBORE, Henry AJIE, Stanley TYLER, and Jim RANDERSON Earth System Science Department, University of California, Irvine, CA 92697/USA Nir KRAKAUER Division of Geological and Planetary Sciences, MC 100 -23, California Institute of Technology, Pasadena, CA 91125/USA INTRODUCTION is a useful tracer for studying the carbon cycle, in terms of determining residence times and fluxes between different carbon reservoirs, and understanding the various underlying processes. Knowledge of the regional and global distribution of atmospheric 14 CO 2 is essential for many of these applications. We have recently begun measuring atmospheric 14 C in the mid-Pacific and at stations in the U. S. to enhance our understanding of the patterns of atmospheric 14 C distribution and its seasonal variation. University of California, Irvine Figure 5. Comparison of NH air 14 C data 3 2 14 C (‰) 14 CO Figure 2. Pacific transect 14 C UCI SAMPLE COLLECTION AND MEASUREMENT Atmospheric CO 2 samples over the Pacific Ocean were collected in two shipboard transects. The first one is between Manzanillo, Mexico (16°N, 109°W) and Auckland, New Zealand (34°N, 177°W) from Sept. 23 to Oct. 4, 2002. The second transect is between Los Angeles, US (34°N, 118°W) and Auckland from July 28 to Aug. 10, 2003. Upon returning to the lab, CO 2 is cryogenically purified on a vacuum line, sub sampled for 13 C analysis, and then reduced to graphite using titanium hydride, zinc, and a cobalt catalyst . The graphite is analyzed for 14 C at the W. M. Keck AMS facility at UC Irvine. These air samples were also measured for C-trace gas abundance (CO, CH 4 in addition to CO 2, and their stable isotopes). RESULTS AND DISCUSSION The 2002 transect (Figure 2) showed that 14 C in atmospheric CO 2 in this latitude range was relatively uniform spatially during the collection period. The average 14 C value of all 24 samples was 79. 9± 2. 3‰ (1 s). The spread of the data was comparable to our analytical error estimated by repeated standard measurements. There was a slight decreasing trend in 14 C of air CO 2 northward of 6°N, consistent with an increase in fossil fuel inputs to air in the northern hemisphere. The 2003 transect was similar to that of 2002 transect, in terms of the latitudinal distribution. It gave an average 14 C value of 75. 4± 2. 2‰ (1 s), indicating a decrease of approximately 4. 5‰ per year. The decrease is more profound in the southern part of the transect, which may be result from exchange with lower 14 CO 2 in surface waters of the Southern Ocean or a natural seasonal cycle of air-sea gas exchange. The distribution of 14 C is not correlated with either 13 C nor CO 2 mixing ratio. 14 C (‰) Factors controlling the present short-term atmospheric 14 CO 2: Sources of high 14 C • Stratosphere /troposphere mixing • Exchange with terrestrial biosphere Sources of low 14 C • Exchange with ocean– Southern Ocean lowers 14 C • Fossil fuel burning Other influences • Lateral atmospheric mixing Figure 4. Point Barrow 13 C and p. CO 2 CONCLUSIONS p. CO 2 (ppm) We have also been monitoring 14 CO 2 from three fixed surface sites in the US: a coastal site at Point Barrow, Alaska (71°N, 157°W); a mid-continental site at Niwot Ridge, CO (41°N, 105°W); and a coastal northern hemispheric site at Montaña de Oro State Park, CA (35°N, 121°W). The bi-weekly sampling at Point Barrow was incorporated into the ongoing CMDL sampling network starting July 12, 2003. Air samples were collected into a pre-evacuated 6 L canister which was then pressurized to ~ 2 atm by a oil free pump. Samples from Niwot Ridge and Montaña de Oro are plotted in Figure 5 with 14 CO 2 data from three European sites from 1995 to 2003 [4, 5]. Figure 5 shows that all North America data are consistent with the European data except for a couple high points. 13 C (‰) Figure 1. A high purity compressor was used to collect air samples. Air is filtered and dried by a series of stainless steel traps filled with Mg(Cl. O 4)2 before passing through the compressor which has been specially cleaned to pressurize air samples without contamination. The samples are collected into aluminum cylinders which have been treated internally to minimize wall effects . Figure 3. Point Barrow 14 C 1. Our 14 CO 2 results from North America are consistent with the European data. 2. Pacific Ocean transects indicate a larger decrease of 14 CO 2 in southern latitudes from 2002 to 2003 which may be due to exchange with lower 14 C Southern Ocean surface water. 3. The time series from Point Barrow indicates seasonal variation of 14 CO 2. This seasonality may show the influence of 14 C-enriched CO 2 released to the atmosphere by respiration from terrestrial ecosystems. 4. Overall, our results confirm large-scale patterns in atmospheric 14 C predicted using carbon cycle models coupled with models of atmospheric transport . We plan to continue measuring radiocarbon in CO 2 in the mid-Pacific and at the surface US stations in different seasons for the next several years to obtain a more complete picture of seasonal and latitudinal variation in atmospheric 14 C. The time series of atmospheric 14 CO 2 at Point Barrow, Alaska from July 12, 2003 to Oct. 25, 2004 shows a general decreasing trend with time (Figure 3). The average 14 C of this time series was 66. 6‰ with a range of about 13‰. The distribution shows a strong hint of a seasonal cycle with amplitudes of ~10‰. Minimum values were observed in May and maximum values in September. Increases in 14 C in spring/summer are likely associated with (1) stronger stratosphere injection in April and May, and (2) increased soil respiration with enriched 14 CO 2 from May through August. Decreased soil respiration and greater fossil fuel burning in the winter cause the 14 CO 2 decreases. This seasonality was also observed at Jungfraujoch by Levin et al  (Figure 5). Low 14 C values in Pt. Barrow air correlate with wind direction, indicating that part of the temporal variation may be caused by the advection of low 14 C air from lower latitudes. REFERENCES 1. Tyler, S C, H O Ajie, A L Rice, A M Mc. Millan, R J Cicerone, D C Lowe, (1999) Measurements of CH 4 Mixing Ratio and 13 C in Air Along a Transect over the Pacific Ocean between Auckland, New Zealand Los Angeles, California, EOS Transactions AGU, B 51 B-07. 2. Vogel, J. S. , (1992) A rapid method for preparation of biomedical targets for AMS, Radiocarbon, 344 -350. 3. Julia Gaudinski, Data of Sweden, Falls Creeks, OR and Blodgett, OR in Figure 5 are from her unpublished work (personal communication). 4. Levin, I. and V. Hesshainer (2000) Radiocarbon – A unique tracer of global carbon cycle dynamics, Radiocarbon, 42(1), 69 -80. 5. Levin, I. and Kromer, B. (2004) The Tropospheric 14 CO 2 level in Mid-Latitudes of he northern hemisphere (19592003), Radiocarbon, 46(3), 1 -12. 6. Randerson, J. T. , I. G. Enting, E. A. G. Schuur, K. Caldeira, and I. Y. Fung (2002) Seasonal and latitudinal variability of troposphere 14 CO 2: Post bomb contributions from fossil fuels, oceans, the stratosphere, and the terrestrial biosphere. Global Biogeochem. Cycle, 16(4), 1112 -1130.