Interannual variability in the oxygen isotopes of atmospheric CO2 driven by El Niño.

The stable isotope ratios of atmospheric CO(2) ((18)O/(16)O and (13)C/(12)C) have been monitored since 1977 to improve our understanding of the global carbon cycle, because biosphere-atmosphere exchange fluxes affect the different atomic masses in a measurable way. Interpreting the (18)O/(16)O variability has proved difficult, however, because oxygen isotopes in CO(2) are influenced by both the carbon cycle and the water cycle. Previous attention focused on the decreasing (18)O/(16)O ratio in the 1990s, observed by the global Cooperative Air Sampling Network of the US National Oceanic and Atmospheric Administration Earth System Research Laboratory. This decrease was attributed variously to a number of processes including an increase in Northern Hemisphere soil respiration; a global increase in C(4) crops at the expense of C(3) forests; and environmental conditions, such as atmospheric turbulence and solar radiation, that affect CO(2) exchange between leaves and the atmosphere. Here we present 30 years' worth of data on (18)O/(16)O in CO(2) from the Scripps Institution of Oceanography global flask network and show that the interannual variability is strongly related to the El Niño/Southern Oscillation. We suggest that the redistribution of moisture and rainfall in the tropics during an El Niño increases the (18)O/(16)O ratio of precipitation and plant water, and that this signal is then passed on to atmospheric CO(2) by biosphere-atmosphere gas exchange. We show how the decay time of the El Niño anomaly in this data set can be useful in constraining global gross primary production. Our analysis shows a rapid recovery from El Niño events, implying a shorter cycling time of CO(2) with respect to the terrestrial biosphere and oceans than previously estimated. Our analysis suggests that current estimates of global gross primary production, of 120 petagrams of carbon per year, may be too low, and that a best guess of 150-175 petagrams of carbon per year better reflects the observed rapid cycling of CO(2). Although still tentative, such a revision would present a new benchmark by which to evaluate global biospheric carbon cycling models.

Isotopic metrology of carbon dioxide. I. Interlaboratory comparison and empirical modeling of inlet equilibration time, inlet pressure, and ion source conductance.

We report a pilot study of high-precision differential isotope ratio measurements made on replicate samples of pure carbon dioxide using three instruments of identical manufacture. Measurement protocols were designed to explore the effects of sample size, ion source conductance, and inlet changeover equilibration time on the raw measurements. Our goal was better understanding of factors that influence these measurements in order to establish procedures for highly reproducible and accurate determinations of Reference Material (RM) isotopic compositions. Evaluation and modeling of reported data illuminated effects consistent with two instrumental memory sources--one short-lived (t((1/2)) approximately 10 s) and the other long-lived (t((1/2)) approximately 6-10 min), uncompensated by normal background measurements--that can significantly influence measurements made by the dual inlet method. These biases, proportional to the difference in isotopic compositions between the measured sample and reference gases, decrease in magnitude with increasing sample size, source conductance, and equilibration time. We observed biases as high as 0.1 per thousand per 10 per thousand difference between sample and reference gases. These memory sources may be responsible for measured delta(13)C values of RMs generally being highly reproducible within any single laboratory but less reproducible among independent laboratories. The magnitude of the bias is consistent with the ranges of delta(13)C values reported in prior laboratory intercomparisons. Uncertainties are most likely due to high and variable long-lived memory among the instruments tested.