Atmospheric%2014CO2%20over%20the%20mid%20Pacific%20Ocean%20and%20at%20Point%20Barrow,%20Alaska,%20USA%20from%202002%20to%202004%20Xiaomei%20XU%20([e-mail]),%20Susan%20TRUMBORE,%20Henry%20AJIE,%20Stanley%20TYLER,%20and%20Jim%20RANDERSON%20Earth%20System%20Science%20Department,%20University%20of%20California,%20Irvine,%20CA

1
Atmospheric 14CO2 over the mid Pacific Ocean and
at Point Barrow, Alaska, USA from 2002 to
2004Xiaomei XU (xxu_at_uci.edu), Susan TRUMBORE,
Henry AJIE, Stanley TYLER, and Jim
RANDERSONEarth System Science Department,
University of California, Irvine, CA
92697/USANir KRAKAUER Division of Geological and
Planetary Sciences, MC 100-23, California
Institute of Technology, Pasadena, CA 91125/USA
B23A-0948
INTRODUCTION
Figure 2. Pacific transect D14C
Figure 5. Comparison of NH air D14C data3
14CO2 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 14CO2 is
essential for many of these applications. We
have recently begun measuring atmospheric 14C in
the mid-Pacific and at stations in the U.S. to
enhance our understanding of the patterns of
atmospheric 14C distribution and its seasonal
variation.
D14C ()
D14C ()
SAMPLE COLLECTION AND MEASUREMENT
Atmospheric CO2 samples over the Pacific Ocean
were collected in two shipboard transects. The
first one is between Manzanillo, Mexico (16N,
109W) and Auckland, New Zealand (34N, 177W)
from Sept. 23 to Oct. 4, 2002. The second
transect is between Los Angeles, US (34N, 118W)
and Auckland from July 28 to Aug. 10, 2003.
Figure 3. Point Barrow D14C
Samples from Niwot Ridge and Montaña de Oro are
plotted in Figure 5 with 14CO2 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.
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(ClO4)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
1.
Factors controlling the present short-term
atmospheric D14CO2 Sources of high D14C
Stratosphere /troposphere mixing Exchange with
terrestrial biosphere Sources of low D14C
Exchange with ocean Southern Ocean lowers
D14C Fossil fuel burning Other influences
Lateral atmospheric mixing
D14C ()
CONCLUSIONS
Figure 4. Point Barrow d13C and pCO2
We have also been monitoring 14CO2 from three
fixed surface sites in the US a coastal site at
Point Barrow, Alaska (71N, 157W) a
mid-continental site at Niwot Ridge, CO (41N,
105W) and a coastal northern hemispheric site
at Montaña de Oro State Park, CA (35N, 121W).
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 6L canister which
was then pressurized to 2 atm by a oil free
pump.

1. Our 14CO2 results from North America are
   consistent with the European data.
2. Pacific Ocean transects indicate a larger
   decrease of 14CO2 in southern latitudes from 2002
   to 2003 which may be due to exchange with lower
   14C Southern Ocean surface water.
3. The time series from Point Barrow indicates
   seasonal variation of 14CO2. This seasonality may
   show the influence of 14C-enriched CO2 released
   to the atmosphere by respiration from terrestrial
   ecosystems.
4. Overall, our results confirm large-scale patterns
   in atmospheric 14C predicted using carbon cycle
   models coupled with models of atmospheric
   transport 6.

?13C ()
pCO2 (ppm)
Upon returning to the lab, CO2 is cryogenically
purified on a vacuum line, sub sampled for d13C
analysis, and then reduced to graphite using
titanium hydride, zinc, and a cobalt catalyst
2. The graphite is analyzed for ?14C at the
W.M. Keck AMS facility at UC Irvine. These air
samples were also measured for C-trace gas
abundance (CO, CH4 in addition to CO2, and their
stable isotopes).
We plan to continue measuring radiocarbon in CO2
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 D14C.
RESULTS AND DISCUSSION
The time series of atmospheric 14CO2 at Point
Barrow, Alaska from July 12, 2003 to Oct. 25,
2004 shows a general decreasing trend with time
(Figure 3). The average D14C 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 D14C in spring/summer are
likely associated with (1) stronger stratosphere
injection in April and May, and (2) increased
soil respiration with enriched 14CO2 from May
through August. Decreased soil respiration and
greater fossil fuel burning in the winter cause
the 14CO2 decreases. This seasonality was also
observed at Jungfraujoch by Levin et al 5
(Figure 5). Low D14C values in Pt. Barrow air
correlate with wind direction, indicating that
part of the temporal variation may be caused by
the advection of low 14C air from lower latitudes.
REFERENCES
The 2002 transect (Figure 2) showed that D14C in
atmospheric CO2 in this latitude range was
relatively uniform spatially during the
collection period. The average D14C value of all
24 samples was 79.92.3 (1s). The spread of the
data was comparable to our analytical error
estimated by repeated standard measurements.
There was a slight decreasing trend in D14C of
air CO2 northward of 6N, 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 D14C
value of 75.42.2 (1s), 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
14CO2 in surface waters of the Southern Ocean or
a natural seasonal cycle of air-sea gas exchange.
The distribution of D14C is not correlated with
either d13C nor CO2 mixing ratio.

• Tyler, S C, H O Ajie, A L Rice, A M McMillan, R J
  Cicerone, D C Lowe, (1999) Measurements of CH4
  Mixing Ratio and ?13C in Air Along a Transect
  over the Pacific Ocean between Auckland, New
  Zealand and Los Angeles, California, EOS
  Transactions AGU, B51B-07.
• Vogel, J.S., (1992) A rapid method for
  preparation of biomedical targets for AMS,
  Radiocarbon, 34, 344-350.
• Julia Gaudinski, Data of Sweden, Falls Creeks, OR
  and Blodgett, OR in Figure 5 are from her
  unpublished work (personal communication).
• Levin, I. and V. Hesshainer (2000) Radiocarbon
  A unique tracer of global carbon cycle dynamics,
  Radiocarbon, 42(1), 69-80.
• Levin, I. and Kromer, B. (2004) The Tropospheric
  14CO2 level in Mid-Latitudes of he northern
  hemisphere (1959-2003), Radiocarbon, 46(3), 1-12.
• Randerson, J.T., I.G. Enting, E.A.G. Schuur, K.
  Caldeira, and I.Y. Fung (2002) Seasonal and
  latitudinal variability of troposphere ?14CO2
  Post bomb contributions from fossil fuels,
  oceans, the stratosphere, and the terrestrial
  biosphere. Global Biogeochem. Cycle, 16(4),
  1112-1130.