Seasonal Carbon Isotope Cycles

The carbon isotope composition of CO2 in the atmosphere fluctuates in annual cycles, much like CO2 in the atmosphere itself.  Carbon 12 is the most common isotope, representing about 99% of atmospheric carbon. Carbon 13 represents most of the remaining percent.  Carbon 14 is an extremely small component (about one part per trillion), mostly generated by nuclear tests during the 1960s.  The amount of C14 in the air has been rapidly declining since the elimination of above-ground nuclear testing. 

The ratio of C13 to C12 is expressed as a standard measure: dC13/C12  (usually pronounced "del-thirteen").  The measure represents the ratio of C13 to C12, as compared to a standard ratio.  The equation for dC13/C12 is:

dC13/C12 =  ((C13/C12 ratio of sample / C13/C12 ratio of standard) -1) * 1000. 

The equation simply expresses the difference between the sample and the standard, expressed in tenths of percent.

The light isotope, C12, is more easily taken up in plants during photosynthesis.  Thus, plants, and plant-derived carbon (including you and me, and other things which eat plants) have negative dC13/C12 ratios.  Coal, which is derived from wood, and oil, which is derived from algae,  also have negative dC13/C12 ratios (in the range of -20 to -35).  Natural gas, which can be formed by several mechanisms, may have a very negative dC13/C12 ratio (from -20 to -50).

So when dC13/C12 is rising in the atmosphere, as happens in the Northern Hemisphere summer,  it is because plants are taking up the light C12, and the ratio of C13 remaining in the atmosphere is increasing.  When dC13/C12 is falling, as happens in the fall through spring, it is because animals and bacteria in the biosphere are respirating, giving back the C12 taken up by plants.  In addition, coal, oil and gas are being burned, giving back C12 taken up by plants long ago, and causing the dC13/C12 ratio to fall.

Let's look at the data.  

As seen in previous posts, atmospheric monitoring stations have collected data on CO2 concentrations and isotope rations across a wide range of latitudes.

Isotope trends show a similar seasonal cyclicity and amplitude dependence on latitude as seen in global CO2 concentrations.

This is a chart of d C13/C12 observations in carbon dioxide, at latitudes ranging from the arctic circle to the south pole.  There is an annual, asymmetric cyclicity to the measurements, and a gradual downward trend, indicating progressively lighter isotopic composition in the atmosphere.

Let's take a more detailed look at the cycles.   This chart shows the d C13/C12 readings from 2003 to 2006.  Observation stations are color-coded by latitude, with warm colors indicating the southern hemisphere, and cool colors showing the northern hemisphere.  The asymmetrical Northern Hemisphere cycles are exactly the inverse of what we saw in the previous CO2 charts.

Let's take a close look at the seasonal nature of the d C13/C12 cycles.   The isotope signature rises sharply in the Northern Hemisphere summer, and falls through the remainder of the year, sharply at first, and then more gradually.  Southern hemisphere observations show a very weak opposite polarity to the northern hemisphere.  If we take the southern hemisphere readings as a baseline for global d C13/C12, it is clear that there are forces producing both positive and negative seasonal deflections from that baseline.

We can remove the long-term trend from the chart, to see the cyclicity better.   I subtracted a one-year moving average of measurements at each station from the data, to produce the chart of relative fluctuation.  Differences between northern and southern hemispheres are apparent, as is the asymmetry of the cycles.

Here is a closer look at the cycles with the long-term trend removed.  Observations from the northern hemisphere have very high amplitude cycles.  Southern hemisphere cycles are relatively flat, exactly as we saw previously in the previous CO2 composition charts.

We can make a plot of peak-to-trough amplitude by latitude, as we did for the CO2 cycles.  Annual cycles grow larger at higher latitudes in the Northern Hemisphere, falling slightly near the North Pole, exactly as seen in the CO2 chart.

And the amplitude of d C13/C12 cycles by latitude can be compared to the distribution of population, as we did with cycles of atmospheric CO2.

Conclusions:

• Northern Hemisphere d C13/C12 observations show a seasonal cyclicity, rising in the summer and falling in the winter.  The cycles are consistent with light C12 being absorbed by plants during the growing season, and with light C12 being released to the atmosphere by plant oxidation and more fossil-fuel use during the winter.  
• The cycles show both positive and negative deflections in the northern hemisphere, relative to the southern hemisphere baseline.  The absorption of C12 by plants in the growing season is the strongest and sharpest part of the cycle.  
• The long term trend is toward more negative d C13/C12, consistent with the accumulation of atmospheric CO2 from fossil fuel use.

I will leave a few loose ends and questions to address, but will place this on the blog today.   I will try to tie up some of the loose ends as soon as I can.

Loose Ends:  

>  The observed amplitudes of the d C13/C12 isotope cycles should be compared to the estimated isotope changes produced by fossil fuel emissions.   These can be modeled, using known annual consumption volumes  and d C13/C12 ratios.  Are cycles produced by annual fossil-fuel consumption of the same size as the observed cycles?

>  The magnitude of the long-term d C13/C12 trend should be compared to estimates from fossil fuel use, to establish a reasonable origin for increasingly negative isotope signature in the atmosphere over time.

Questions:

>  As with the CO2 cycles, the sharpest annual changes in the d C13/C12 cycle occur in the summer months, when light isotopes are being taken up by plant growth.  We can easily see that agriculture is concentrated in the Northern Hemisphere.  What is the estimated impact of agriculture on CO2 and isotope cycles?

>  What is the reason for the sharply negative rebound in d C13/C12, following the summer growing season? >  We should be able to explain the full shape of the CO2 and isotope annual cycles.  The steepest part of the fall occurs before the main heating season for fossil fuel consumption.  The answer might be a rapid oxidation of plant matter by bacterial action, burning of agricultural waste, or some interaction of CO2 sources and sinks.  Demonstration of the actual mechanism, supported by data, would be helpful.

> The amplitude of d C13/C12 cycles, like the amplitude of CO2 cycles, increases northward to a point beyond the Arctic Circle, before slightly diminishing at the highest latitudes observed.   It is not clear why this is so.  Some factors to consider are: 1)  the volume of air available for dispersion decreases per degree of latitude  northward, simply due to the curvature of the earth.  2)  Mixing with southern latitude air diminishes at higher northern latitudes.  3)  Winter fossil fuel usage per capita probably increases at highest northern latitudes.  For example, the gas utility serving Anchorage, Alaska, has about a 12-fold swing in fuel delivery in winter, as compared to summer months.

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This article is the third of a series of articles about global CO2.  The final article consolidates and summarizes the previous posts.

1)  The Keeling Curve
      http://dougrobbins.blogspot.com/2011/05/keeling-curve.html

2)  The Keeling Curve and Seasonal Carbon Cycles

      http://dougrobbins.blogspot.com/2012/03/keeling-curve-and-seasonal-carbon.html

3)   Seasonal Carbon Isotope Cycles

      http://dougrobbins.blogspot.com/2012/03/seasonal-carbon-isotope-cycles.html

4)   Long-Term Trends in Atmospheric CO2

      http://dougrobbins.blogspot.com/2012/03/long-term-trends-in-atmospheric-co2.html

5)   Modeling Global CO2 Cycles

      http://dougrobbins.blogspot.com/2012/04/modeling-global-co2-cycles.html

6)   The Keeling Curve Summary:  Seasonal CO2 cycles and Global CO2 Distribution
       http://dougrobbins.blogspot.com/2013/05/the-keeling-curve-seasonal-co2-cycles.html

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References:

References:
Atmospheric CO2 Carbon Isotope Data:

Keeling, R.F. S.C. Piper, A.F. Bollenbacher, and S.J. Walker. 2010. Monthly atmospheric ¹³C/¹²C isotopic ratios for 11 SIO stations. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.

http://cdiac.ornl.gov/trends/co2/iso-sio/iso-sio.html

Global Emissions average isotope data

Boden, T.A., G. Marland, and R.J. Andres. 2013. Global, Regional, and National Fossil-Fuel CO₂ Emissions. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. doi 10.3334/CDIAC/00001_V2013

http://cdiac.ornl.gov/ndps/db1013_v2012.html

ftp://cdiac.ornl.gov/pub/db1013_v2012/db1013.global.dat

All CO2 data in this article is credited to C. Keeling and other at the Scripps Institute of Oceanography, also Gaudry et al, Ciattaglia et al, Columbo and Santaguida, and Manning et al.  The data can be found on the Carbon Dioxide Information Analysis Center; http://cdiac.ornl.gov/trends/co2/

World Background Map for charts courtesy ESRI.

The population chart was prepared by "Radical Cartographer" Bill Rankin.  http://www.radicalcartography.net/

I used the version of the map posted here: http://www.geekosystem.com/world-population-latitude-longitude/