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. .
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]