A multi-box model study of the role of the biospheric metabolism in the recent decline of delta18O in atmospheric CO2

Abstract

From around 1993 to 1997, the NOAA‐CU δ18O measurements at Pt. Barrow, Mauna Loa, Cape Kumukahi, Cape Grim and the South Pole show a decrease in atmospheric CO2δ18O of about 0.5‰. Recently, Gillon and Yakir (2001) have attributed this decrease to a conversion of C3 forests to C4 grasslands through anthropogenic land‐use change. However, their explanation can account for only about 0.02‰ yr−1 decrease rate. In this paper we offer a viable alternative explanation. We have used a multi‐box model of the global carbon cycle and its δ18O to show that an increase in biospheric respiration (CO2 flux from plant with lower‐than‐atmosphere δ18O), combined with a decrease in the amount of CO2 (with higher‐than‐atmosphere δ18O) diffusing back from plant leaves before being assimilated as part of the gross primary production (GPP), could produce the observed decline in the atmospheric CO2δ18O. This decrease in the CO2 back diffusion out of leaves could be interpreted as an overall increase in both biospheric activities of photosynthesis and respiration. Change in the metabolic activities of the biosphere as a possible cause for the observed decrease in δ18O is a reasonable hypothesis, since isotopic fractionations that occur during CO2 exchange processes (photosynthesis and respiration) between the atmosphere and the biosphere contribute significantly to the observed variations in atmospheric CO2δ18O, while contribution from the net air‐sea CO2 exchange is negligible.

Long‐term record of atmospheric CO2 and stable isotopic ratios at Waliguan Observatory: Seasonally averaged 1991–2002 source/sink signals, and a comparison of 1998–2002 record to the 11 selected sites in the Northern Hemisphere

“…558% (55 data sets, R 2 = 0.18, P < 0.01). The estimated d s for 18 O (À7.662 ± 2.113)% at WLG is in agreement with a mean source or sink where the 18 O of CO 2 is in equilibrium with soil water (varying from À9 to À2%)[Ishizawa et al, 2002]. Thus, though subject to a large degree of uncertainty due to relatively poor linear relationship (R 2 < 0.2) compared to that of the d 13 C and CO 2 relationship, the observed d 18 O seasonal cycle is most likely a result of the exchange of atmospheric CO 2 with soil water throughout the year.…”

“…[17] As cited [Flanagan et al, 1997;Friedli et al, 1987;Ishizawa et al, 2002;Keeling, 1961;Mook et al, 1983;Nakazawa et al, 1997b;Trolier et al, 1996] and discussed above, d 18 O in atmospheric CO 2 is determined by exchange with vegetation, leafwater, soils and the ocean. It is the exchange of CO 2 with the terrestrial biosphere (through ecosystem photosynthesis and respiration) that drives much of the seasonal variation in the d 18 O of atmospheric CO 2 , especially over the continental NH.…”

Seasonal variation of the oxygen isotopic ratio of atmospheric carbon dioxide in a temperate forest, Japan

“…[33] D A affects significantly the seasonal variation of d 18 O of atmospheric CO 2 inside the canopy, because its seasonal changes are larger than those of D R , A and R. To estimate D A accurately, it is important to know the ratio (r) of the assimilated CO 2 flux to the back diffusion CO 2 flux for photosynthesis and its seasonality, which are uncertain at present. Recently, Ishizawa et al [2002] also suggested that the secular declines of about 0.5% in d 18 O of atmospheric CO 2 measured at Pt. Barrow, Mauna Loa, Cape Kumukahi, Cape Grim and the South Pole between 1993 and 1997 were caused by the decrease in CO 2 diffusing back to the atmosphere from plant leaves.…”

Abundance of mass 47 CO2 in urban air, car exhaust, and human breath

“…In global models of the isotopic budget of CO 2 , its d 18 O value is dominated by exchange with plant and soil water, and the anthropogenic component is generally neglected (Ishizawa et al, 2002). In urban air and in some continental stations, however, the anthropogenic component is likely to be significant.…”