2.2.4. Measurements of Stable Isotopes of Atmospheric CO₂

Since 1989 the Stable Isotope Laboratory at INSTAAR has been measuring the stable isotopic composition of CO₂ from weekly flask samples of air obtained from the CMDL network of sites.  Begun with a selection of only six sites and two ships in 1990, the measurement effort has grown to include approximately 50 stationary sites in the CMDL program as well as all of the shipboard samples.  During 1997 over 6400 flasks were analyzed for d¹³C and d¹⁸O, complementing the CO₂ mole-fraction measurements made by CMDL.  A description of the measurements and calibration procedures is presented by Trolier et al. [1996]. 

Uptake of CO₂ by plants or respiratory loss from plants or herbivores changes the isotopic composition of atmospheric CO₂.  In the majority of plants the process of photosynthesis discriminates against the uptake of ¹³C relative to ¹²C, thus producing isotopically labeled organic matter.  In contrast, the exchange of CO₂ between the atmosphere and the oceans produces only a very small isotopic signature.  In principle this distinction allows us to discriminate between terrestrial and oceanic sources and sinks of atmospheric CO₂.  There is a group of plants, however, that employ a different photosynthetic pathway that does not produce much isotopic fractionation, making its isotopic signature look more like the oceans, and we have to account for that. 

The ¹⁸O signature is governed by the interaction of the carbon cycle with the hydrological cycle.  Oxygen isotopic exchange takes place in liquid water and occurs during the hydration reaction CO₂ + H₂O <---> H₂CO₃.  Respiratory CO₂ emanating from soils equilibrates with the oxygen in soil moisture, and CO₂ diffusing in and out of photosynthesising leaves equilibrates with leaf water.  Leaf water is often enriched in ¹⁸O relative to soil water so that the study of ¹⁸O in atmospheric CO₂ has the potential to quantitatively separate respiration from photosynthesis in carbon cycle studies.  With CO₂ and ¹³C/¹²C alone one could only study the net exchanges between the atmosphere and the terrestrial biosphere and oceans.

The degree to which isotopic measurements made on atmospheric samples are useful is seriously constrained by the precision of the mass spectrometer used.  For example, a change of just 0.024‰ d¹³C measured globally translates to an equivalent switch between a terrestrial or an oceanic source to the atmosphere of 1 GtC (10¹⁵ g C).  Hence, a very high precision instrument is desirable.  In 1990 we began making measurements with a VG SIRA Series 2 mass spectrometer.  This instrument was capable of measurement precisions of 0.03‰ for d¹³C and 0.05 for d¹⁸O (one sigma).  A Micromass Optima mass spectrometer was purchased and tested in 1996 and brought on line in late 1996 exclusively for making flask measurements.  The overall reproducibility for the Optima system in 1997 (one sigma standard deviation for replicate analyses) is 0.012‰ for d¹³C and 0.031‰ for d¹⁸O.  This new system incorporates an automated 40-port custom manifold and a sample extraction system using all stainless steel parts and is based on the methods proven on the SIRA instrument.  Air samples are extracted at a rate of 40 scc min^-1 on both systems.  However, the more efficient trapping of CO₂ and greater sensitivity of the new Optima system require 25% less sample, which shortens analysis time by a proportional amount.  This also allows more air to be left in the flask for any subsequent analyses, such as isotopes of carbon in methane, or a repeat analysis should it be required.  Tests have shown that with the typical amount of sample left in the flask when it arrives for isotopic analysis, only one extraction was possible with the SIRA instrument, whereas with the Optima instrument, two to three extractions are now possible with acceptable reproducibility.  With further extractions the flask pressure drops below a threshold where significant isotopic fractionation occurs, possibly as a result of CO₂ coming off the flask walls with a very different d¹⁸O and d¹³C. 

The increased capacity attained with the 40-port manifold allowed us to make over 11,560 separate analyses on flasks and reference gases in 1997.  The combined effect of improved precision and increased capacity is a significant increase in the utility of the isotopic measurements towards characterizing the present day carbon cycle.

As one check on data quality, we began in late 1996 to measure three aliquots of air from a standard cylinder in the middle of each flask run on the mass spectrometer.  This additional cylinder, called the "trap tank" provides for a continuous check on the performance of the entire system: the extraction of CO₂ from air, the isotopic ratio analysis, and the final corrections for N₂O and ¹⁷O.  Figure 2.6 shows the standard deviation for d¹³C values measured on three aliquots of the trap tank plotted as a 10-point running mean.  The average standard deviation is slightly less than 0.01‰ and is nearly always less than 0.015‰.  The same information is shown in Figure 2.7 for d¹⁸O of the trap tank. The average standard deviation is 0.03‰, and values are always less than 0.05‰.  Thus most of the improvement in the isotopic measurements in moving analyses from the SIRA to the OPTIMA has been in the d¹³C values where standard deviations have improved by about a factor of three; d¹⁸O has improved by less than a factor of two.  The difference lies, we believe, in the extraction process, which is sensitive to the presence of water in the air samples.  We note that when dry air from several cylinders is intercompared, such as when standard cylinders are analyzed, the d¹⁸O standard deviations are significantly less, generally 0.015‰ or less; the d¹³C standard deviations are also lower, usually about 0.005‰.

Fig. 2.6.   Ten-point running mean of the standard deviation of d¹³C the trap tank as a function of run number in per mille.  The data covers the period from June 1997 to January 1998.  

  Fig. 2.7.   Ten-point running mean of the standard deviation of d¹⁸O of the trap tank as a function of run number in per mille.  The data covers the period from June 1997 to January 1998.

A sample of the isotope data is given in Figure 2.8 which shows the time series of CO₂ mole fraction, d¹³C, and d¹⁸O from Niwot Ridge, Colorado.  The striking anti-correlation between the seasonal cycles of CO₂ and d¹³C is a reflection of terrestrial photosynthesis and respiration.  The CO₂ absorbed by plants is depleted in ¹³C leaving behind an atmosphere slightly enriched in ¹³C.  The d¹⁸O seasonal cycle is quite different from that of d¹³C.  For example, when the net flux from the terrestrial biosphere is already directed into the atmosphere at the end of the growing season, the respired CO₂ from soils at higher latitudes is still pulling down atmospheric d¹⁸O.  Soil moisture at high latitudes is very depleted in ¹⁸O, a signature that is picked up by respired CO₂ [Ciais et al., 1997].  Annually averaged d¹⁸O is not expected to display a decadal trend, but can change dramatically from year to year, most likely because it is subject to the strong opposing forces of respiration (depleted in ¹⁸O) and photosynthesis (leaf water enriched in ¹⁸O).

Fig. 2.8.   CO₂, d¹³C and d¹⁸O at Niwot Ridge, Colorado.

Field testing of a new air sampling apparatus began at the Samoa Observatory, American Samoa (SMO) in September 1994 and at Cape Kumukahi, Hawaii, in May 1995.  The new Airkit differs from the older Martin and Kitzis sampler (MAKS) version in two important ways.  It has a thermoelectrically cooled condenser to remove most of the water vapor from the air stream, and a microprocessor to control the sampling process in order to minimize the chance for operator error.  The effect of drying the air samples is most dramatic for the measurement of ¹⁸O/¹⁶O in CO₂.  Without drying, d¹⁸O in the samples collected in humid tropical locations is highly variable and consistently more depleted in ¹⁸O due to isotopic exchange between water and CO₂, which presumably occurs on the wall of the flask [Gemery et al., 1996].  Figure 2.9 shows the improvements in the samples from SMO.  For ¹⁸O, the majority of the "wet" samples (MAKS) was rejected because of poor pair agreement.  The fortuitously retained pairs tend to be isotopically "light."  Since the samples have been dried with the Airkit the true d¹⁸O signature of CO₂ in the central equatorial Pacific is emerging.  There appears to have been a small decrease, about 0.08‰ yr^-1, in the last 3 years.  There is also an improvement in the d¹³C values measured in the Airkit samples.  The latter samples have a defined, but small, seasonal cycle that was only vaguely hinted at in the earlier MAKS samples. 

Fig. 2.9.   Oxygen-18 in flask samples from Cape Matatula, Samoa.

[BACK]    [CONTENTS]    [NEXT]