Monthly Atmospheric ¹³C/¹²C Isotopic Ratios for 10 SIO Stations

Graphics    Digital Data

Investigators

C.D. Keeling, A.F. Bollenbacher, and T.P. Whorf
Carbon Dioxide Research Group,
Scripps Institution of Oceanography,
University of California,
La Jolla, California 92093-0444, U.S.A.

Period of Record

1977-2002

Methods

Stable isotopic measurements for atmospheric ¹³C/¹²C and ¹⁸O/¹⁶O at global sampling sites have been carried out by Dr. C.D. Keeling and co-workers at Scripps Institution of Oceanography (SIO) since 1977. These isotopic measurements complement the continuing global atmospheric and oceanic CO₂ measurements initiated by Keeling in 1957.

The isotopic program began in 1977 as a joint project between Dr. Keeling and Dr. Willem Mook, director of the Centrum voor Isotopen Onderzeok (CIO) at the University of Groningen, The Netherlands, and continued until 1992. During this time period, cryogenically extracted CO₂ samples were shipped to the Univeristy of Groningen, and analyzed using either of two Sira mass spectrometers manufactured by VG Instument Inc. Final results were calculated and reported back to SIO, leaving details of mass spectrometer performance and calibration methods up to Dr. Mook and an assistant, Hans Roeloffzen. Dr. Mook left the Centrum voor Isotopen Onderzoek in 1989, and the first 15 years of this data set has been re-evaluated by Dr. Harro Meijer, his replacement as director.

Beginning in 1992, the Scripps isotopic program became a joint project with Dr. Martin Wahlen, also of SIO. Until September of 2000, the analyses were performed at SIO using Dr. Wahlen's VG Prism II dual-inlet stable isotope mass spectrometer, and SIO personnel were directly involved in the calibration process [Bollenbacher et al., 2002, Guenther et al. 2002]. In September of 2000, SIO began using its own Micro Mass Optima dual-inlet mass spectrometer. They have continued to use the same daily working standards as were used on Dr. Wahlen's Prism mass spectrometer, enabling them to combine both the Prism and the more recent Optima data without changing calibration procedures.

The CIO and SIO data sets have both been referenced to international isotopic standards [Bollenbacher et al., 2002, Guenther et al., 2002]. Comparison of these two data sets were made in 1990 and 1991 using secondary standards and samples from the La Jolla sampling station analyzed by both laboratories. These comparisons indicated an offset between the calibrations of the two laboratories in which the ¹³C/¹²C ratio of the SIO version of the compared data was found to be, on average, 0.112 per mil more positive than for the CIO data. Published results of SIO which include ¹³C/¹²C data [Gruber et al., 1999, Keeling et al., 1989, Keeling et al., 1995, and Keeling et al. 2004] are based on adjusting the SIO data to agree with the calibration of the CIO data by adding -0.112 per mil. The monthly data presented here also include that adjustment. The offset may arise largely because of differing procedures regarding international calibration as to whether two or three point calibrations are preferred.

These monthly averages have been adjusted in a manner similar to that used for SIO monthly CO₂ data. Annual averages of these monthly data, as well as of the annual fitted function (4-harmonics with gain factor and spline), are included to provide a more comprehensive set of annual data when missing months of data do not allow an annual average to be calculated. Also, the annual average data have been augmented (data in parentheses) when only 1 or 2 months are missing for that year, by substituting the monthly fit value(s) for missing data and then averaging the 12 values. The month(s) without data are shown as 99.999 while this recomputed annual average is shown in parentheses. Data are reported in terms of parts per mil difference from the Peedee belemnite (PDB) standard.

Trends

The ¹³C isotope is stable and heavier than the normal form of carbon (¹²C), and plants tend to selectively assimilate the lighter isotopes during the photosynthetic process. This results in the following features of the ¹³C/¹²C ratio in the atmosphere: (1) a seasonal cycle occurs with the heavier isotope at relatively high concentrations during the summer, as plants selectively remove the lighter isotope from the atmosphere, and (2) a general decrease with time, as more fossil carbon (which originally was plant material, and consequently biased toward the lighter isotope) is injected into the atmosphere from the combustion of fossil fuels. Additionally, about 95% of fossil-fuel carbon emissions are from the Northern Hemisphere, and there is a 6-12 month time lag before this material is transported by the atmosphere to the various stations in the Southern Hemisphere. The seasonal cycle reverses and its amplitude decreases in the Southern Hemisphere, where the seasons are opposite those in the Northern Hemisphere and there is much less land area to support a terrestrial biosphere.

References

• Bollenbacher, A.F., P.R. Guenther, C.D. Keeling, E.F. Stewart, M.Wahlen, and T.P. Whorf, 2002. Calibration methodology for the Scripps ¹³C/¹²C and ¹⁸O/¹⁶O stable isotope program, 1992-1996, 143 p.
• Gruber, N., C.D. Keeling, R.B. Bacastow, P.R. Guenther, T.J. Leuker, M. Whalen, H.A.J. Meijer, W.G. Mook, T.F. Stocker, 1999. Spatiotemporal patterns of carbon-13 in the global surface oceans and the oceanic Suess effect, in Global Biogeochemical Cycles, 13:307-335.
• Guenther, P. R., A. F. Bollenbacher, C. D. Keeling, E. F. Stewart, and M. Wahlen, 2002. Calibration methodology for the Scripps ¹³C/¹²C and ¹⁸O/¹⁶O stable isotope program, 1996-2000, 118 p.
• Keeling, CD., P.R. Guenther, and D.J. Moss, 1986. Scripps Reference Gas Calibration System for Carbon Dioxide-in-Air Standards: Revision of 1985. World Meteorological Organization, Geneva, Switzerland, Technical Document No. 25 and No. 25A, 76p.
• Keeling, C.D., R.B. Bacastow, A.F. Carter, S.C. Piper, T.P. Whorf, M. Heimann, W.G. Mook, and Hans Roeloffzen. 1989. Aspects of Climate Variability in the Pacific and the Western Americas, Pages 165-236 in: Peterson, D. H., ed., Geophysical Monograph 55, American Geophysical Union, Washington DC.
• Keeling, C.D., T.P. Whorf, M. Wahlen & J. van der Plicht, 1995. Interannual extremes in the rate of rise of atmospheric carbon dioxide since 1980, Nature 375:666-670.
• Keeling, C.D., S.C. Piper, R.B. Bacastow, M. Wahlen, T.P. Whorf, M. Heimann, and H.A. Meijer, 2004.
  Atmospheric CO₂ and ¹³CO₂ exchange with the terrestrial biosphere and oceans from 1978 to 2000: Observations and carbon cycle implications, in A History of Atmospheric CO₂ and its Effects on Plants, Animals, and Ecosystems", editors, Ehleringer, J.R., T.E. Cerling, M.D. Dearing, Springer Verlag, New York, pp. 83-113.

3/2005