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Modern Records of Carbon and Oxygen Isotopes in Atmospheric Carbon Dioxide and Carbon-13 in Methane

Introduction

This page provides an introduction and links to records of carbon-13 (13C), carbon-14 (14C), and oxygen-18 (18O) in atmospheric carbon dioxide (CO2), and also to 13C in methane (CH4) in recent decades. We emphasize large data bases each representing many currently active stations. Records have been obtained from samples of ambient air at remote stations, which represent changing global atmospheric concentrations rather than influences of local sources. Fossil carbon is relatively low in 13C and contains no 14C, so these isotopes are useful in identifying and quantifying fossil carbon in the atmosphere. Although the 14C record is obfuscated by releases of large amounts during tests of nuclear weapons, this isotope is nonetheless useful in tracking carbon through the carbon cycle and has limited use in quantifying fossil carbon in the atmosphere. Oxygen-18 amounts are determined by the hydrological cycle as well as biospheric influences, so they are often harder to interpret but are nonetheless useful in hydrological studies. Oxygen-18 and deuterium (2H) in polar ice cores provide information about past temperature long before the beginning of instrumental records. A gateway page to chronologies of isotopes in ice cores is here.

The World Data Center for Greenhouse Gases (WDCGG) provides isotope data for stations around the world. In addition to the remote stations that reflect global background conditions, many stations are located in areas influenced by large urban or regional sources. Isotopic signatures may be useful for investigating influences of specific sources on CH4 or CO2 concentrations. While this site provides the greatest geographical coverage, data may not always be as current as from the individual sources listed above, and the period of record as given below therefore ends with "recent" rather than "current"” which applies to the latest data available directly from the individual contributors.

These data have graciously been made freely available for access and distribution; the original investigators made the effort to obtain the data and assure their quality. To assure proper credit is given, please follow the instructions in the headers of the data files, in readme files, and/or at the end of this page when using any of this material. If data accessed from this site are to be used in a publication we strongly recommend some contact with the principal investigators at an early stage of the work to be sure the data are being interpreted and used correctly (Some organizations insist on this; see instructions on the home pages or at the top of the header files). Neither the principal investigators nor CDIAC is responsible for misuse of these data.

Contributors

The following organizations have current data from multiple sites.

  1. The Commonwealth Scientific and Industrial Research Organization (CSIRO) of Australia
  2. The Global Monitoring Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration (NOAA)
  3. The World Data Center for Paleoclimatology at the National Oceanic and Atmospheric Administration (NOAA), which archives the ice-core data
  4. The Scripps Institution of Oceanography (Scripps CO2 program)
  5. The world data center for greenhouse gases (WDCGG) archives data from these contributors, as well as data from other sources around the world
  6. CDIAC maintains records for additional individual locations covering limited time periods.

Period of Record

Isotope Scripps* NOAA* CSIRO* WDCGG*
13C in CO2 1977-current 1990#-current 1992-currrent 1990#-recent
18O in CO2 1977-current 1990#-current   1990#-recent
14C in CO2 1992-current 2003-2007   1954§-recent
13C in CH4   1998-current   1991§-recent

 *Date is for the earliest record, most stations have shorter records. The Scripps graphics gallery shows the varying record lengths.
#From December 1989 at ships of opportunity near specific latitude-longitude intersections in the western Pacific Ocean.
§One station, Baring Head, New Zealand, has 14CO2 records back to 1954 and 13CH4 back to 1991, Other 13CH4 records start in 1998 or later.

Station Locations

  • CSIRO: The last item in the FTP directory is a link to the current map of stations.
  • SCRIPPS: 13CO2, 14CO2 and C18O2 isotopic data are presented along with the CO2 data for each station.
  • NOAA Click on a location to see which isotopes are measured there. One can also get graphics from this page.
  • WDCGG: Select an isotopic species from the parameter list.

graphics Graphics

Graphics for organizations with current data from multiple sites can be found at:

  • CSIRO: 13C at Cape Grim Tasmania is in the bottom right.
  • NOAA: This link gets to the visualization page for all gases and the isotopes 13C, 14C and 18O in CO2 and 13C in CH4. After selecting a location, select Carbon Cycle Gases from the list at the left. Then select a plot type and parameter of interest (e.g. Carbon-13/Carbon-12 in Carbon Dioxide) from the last three items on the "parameter" list, and click "Submit". Move mouse over any point on the map to get its 3-letter station code (e.g. SPO is the South Pole).
  • SCRIPPS: Graphics for 13C, 14C and 18O in CO2

image Data

Find information about 3-letter station codes here.

Methods

Measurement

Stable isotopes are measured using Mass spectrometry (MS): http://www.esrl.noaa.gov/gmd/outreach/isotopes/mass_spec.html in which molecules are bombarded with electrons to create ions with particular mass to charge ratios. Accelerated ionic molecules of normal carbon dioxide, CO2, with 44 atomic mass units (a.m.u.), 13CO2 (45 a.m.u.) and carbon dioxide containing an 18O atom (46 a.m.u.) will follow different paths when exposed to an electromagnetic field. The number of ions passing along a particular path is measured by an electronic signal. A nontechnical introduction to mass spectrometry may be found here: http://old.post-gazette.com/downloads/20030324Mass_spectrometer.pdf.

A CO2 molecule containing a 14C atom "weighs in" at 46 a.m.u. and the number of such atoms in any sample only is about one per trillion carbon atoms. It also decays at a rate that is proportional to the amount in a sample. These complications make measurement difficult. In the past, the decay rate was measured by counting the beta particles (the decay product) emitted over some time period, which was very time consuming if an accurate estimate was to be obtained.

More recently, accelerator mass spectrometry (AMS) at: http://www.esrl.noaa.gov/gmd/outreach/isotopes/accelerator.html has been used for 14C analysis. The accelerator produces 14C, 13C and 12C ions which can then be separated by conventional mass spectrometry, except that the signal produced by the small number of 14C ions has to be multiplied via a gas-filled ionization chamber or a solid state device to obtain an accurate reading. Carbon-13 atoms follow a separate path to a more conventional detector (Faraday cup) where their signal is measured for use in normalizing the sample for isotopic fractionation. See: http://www.physics.utoronto.ca/~isotrace/whatis.html for more details. AMS is much faster than the "beta counting" method; smaller samples can be analyzed and sensitivity is increased to provide greater accuracy.

Reporting Data

The ratio of the amount of a less common stable isotope to that of the most common isotope of that element (the isotope ratio) in a sample is compared to the isotope ratio in some standard reference material. The example below provides the general formula, using carbon-13 as an example.

Value (per mil) = 1000 × [(13C/12C)sample – (13C/12C)reference ]/(13C/12C)reference
= 1000 × {[(13C/12C)sample/(13C/12C)reference] -1}

The multiplier of 1000 is convenient because isotope ratios and their differences are typically small; the results are expressed as per mil. The reference standard for 18O and for 13C (in CO2 or CH4) is derived from calcite material known as NBS 19, available from the National Institute of Standards and Technology. This standard is usually reported as "Vienna-PDB," or V-PDB for historical reasons.

Values of the unstable isotope 14C are reported in terms of the ratio of decay activity in a sample to that in a standard, which was originally oxalic acid derived from sugar beets (OxA-I). This standard is linked to oxalic acid II (OxA-II) available from the National Institute of Standards and Technology. Activities in the sample and in the standard are first normalized, using 13C amounts present, to account for isotopic fractionation, which that does not occur in atmospheric CO2 but may occur during sample preparation. Again, the reference standard for 13C is V-PDB. The normalized 14C activities of the sample and the standard are in Becquerels per gram of carbon. Subtraction of 1 from the ratio of normalized activity of the sample to that of the standard gives a calculation analogous to that for the stable isotope ratios shown above, except that normalized activities are used in place of isotope ratios. The result is multiplied by 1000 for convenience, so the values are also given as "per mil."

Trends

Carbon-13 in CO2 is decreasing, as the fraction of atmospheric CO2 that is realized from combustion of fossil carbon is increasing. Ratios of 13C/12C in CO2 tend to be lower in the Northern Hemisphere, suggesting a fossil-fuel source that resides mainly in the Northern Hemisphere.

Carbon-13 in CH4 has decreased since 2008, but the short record (only back to 1998) combined with multiple sources precludes any simple explanation at this point. Ratios of 13C/12C in CH4 tend to be lower in the Northern Hemisphere.

Carbon-14 in CO2 is decreasing, and 14C/12C ratios are lower in the Northern Hemisphere than in the Southern Hemisphere, suggesting a northern hemisphere source of 14C-depleted carbon (e.g., fossil fuels). However, things are not quite that simple; although 14C from bomb testing has largely been removed from the atmosphere by the biosphere, the biosphere is now giving some back, precluding any simple interpretation of the rate of 14C decline. For more on this topic, see Levin et al. (2010).

Variations in 18O in CO2 reflect not only the carbon cycle, but the water cycle as well. Oxygen-18 evaporated from the oceans will eventually fall out as precipitation and make its way into CO2 respired from the biosphere. Therefore these variations reflect complex processes and are not always easily interpreted, although 18O is useful in hydrological studies. Oxygen-18 in CO2 has an annual cycle but has otherwise tended to stay constant in recent decades. Like the other isotopes discussed above, ratios of 18O/16O tend to be lower in the Northern Hemisphere.

References

  • Allison, C.E., and R.J. Francey. 2007. Verifying Southern Hemisphere trends in atmospheric carbon dioxide stable isotopes, J. Geophys. Res. 112 D21304, doi: 10.1 029/2006J0007345
  • Francey, R.J., L.P. Steele, R.L. Langenfelds, et al. 1998. Atmospheric carbon dioxide and its isotopes, methane, carbon monoxide, nitrous oxide and hydrogen from Shetland. Atmos. Environ. 32 3331-3338.
  • Graven, H.D., T. P. Guilderson and R. F. Keeling, 2012. Observations of radiocarbon in CO2 at seven global sampling sites in the Scripps flask network: Analysis of spatial gradients and seasonal cycles. J. Geophys. Res. 117 D02302, doi:10.1029/2011JD016533
  • Keeling, C.D., S. C. Piper, R. B. Bacastow, M. Wahlen, T. P. Whorf, M. Heimann, and H. A. Meijer. 2001. Exchanges of atmospheric CO2 and 13CO2 with the terrestrial biosphere and oceans from 1978 to 2000. I. Global aspects, SIO Reference Series, No. 01-06, Scripps Institution of Oceanography, San Diego, 88 pages.
  • Keeling, C.D., Piper, R. B. Bacastow, M. Wahlen, T. P. Whorf, M. Heimann, and H. A. Meijer. 2005. Atmospheric CO2 and 13CO2 exchange with the terrestrial biosphere and oceans from 1978 to 2000: observations and carbon cycle implications, pages 83-113, 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.
  • Levin, I, et al. 2010. Observations and modelling of the global distribution and long-term trend of atmospheric 14CO2, Tellus 62B, 26–46. doi:10.1111/j.1600-0889.2009.00446.x
  • Lehman, S.J., J.B. Miller, R.J. Sparks, et al. 2007. A new high precision 14CO2 time series for North American continental air. J. Geophys. Res. 112, D11.
  • Turnbull, J.C. 2007, P. Rayner, J. Miller, et al. 2009. On the use of 14CO2 as a tracer for fossil fuel CO2: Quantifying uncertainties using an atmospheric transport model J. Geophys. Res. 114 D22.

Citing This Material

Modern Isotope Data

  • CSIRO: CSIRO requests that use of these data in any paper or presentation be accompanied by acknowledgement of the source of the data (CSIRO Marine and Atmospheric Research GASLAB) and that the version of the data (as specified by release date) be explicitly stated.
  • NOAA: Citations to NOAA personnel are given in the "readme" files for each species.
  • For C-14 data from NOAA cite Turnbull et al. (reference given above)
  • SCRIPPS: Scripps requests citing Keeling et al. 2001 or, if it is necessary to cite a peer-reviewed article, cite Keeling et al 2005. (References are given above and at the bottom of this page.) We also suggest the reference given at the top of each data page, for example:
    R. F. Keeling, S. C. Piper, A. F. Bollenbacher and S. J. Walker, Scripps CO2 Program, Scripps Institution of Oceanography, University of California, La Jolla, California USA 92093-0244, Data accessed September 15, 2012.
  • For 14C data from Scripps cite Graven et al. 2012 (reference given above).
  • WDCGG: Citation instructions are given in red on the WDCGG home page. By use of these data “you accept that an offer of co-authorship will be made through personal contact with the data providers or owners whenever substantial use is made of their data. In all cases, an acknowledgement must be made to the data providers or owners and the data centre when these data are used within a publication."
  • CDIAC:
    • If accessing the data from this site: Please also cite: Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy.
    • If citing material from this page only, cite as: Modern Records of Carbon and Oxygen Isotopes in Atmospheric Carbon Dioxide and Carbon-13 in Methane, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy. Path: http://cdiac.ornl.gov/trends/co2/modern_isotopes.html.