Stable Isotopes of Oxygen 31 Overview

The three stable oxygen isotopes, "'O, ''O, and lsO in both, C02 and 02, constitute important tracers of the global carbon and oxygen cycles. Besides revealing crucial information in local process studies (see Lloyd, this volume), they may also be observed and modeled on the global scale. Unlike carbon or oxygen, however, these isotopes are not conserved in the carbon or oxygen cycle, but are constantly exchanged at a few critical connection points with the hydrological cycle (which contains much more oxygen than is present in C02 and 02). Figure 1 shows the two cycles schematically in the lower two panels and indicates the locations where exchanges with water determine the isotopic composition of the flows of C02 and 02 (blue arrows). On land these are:

1. The leaf water, which determines the isotopic composition of the 02 generated by photosynthesis and the C02 generated by autotrophic respiration.

2. The soil/groundwater determining the isotopic composition of C02 generated by heterotrophic respiration.

In the oceans, the isotopic signature of seawater determines the 02 produced by photosynthesis and the oxygen isotopic composition of DIC generated during remineralization of organic material and carbonate.

In addition, fractionation processes during phase transitions between the major carbon and oxygen reservoirs modify the isotopic composition of atmospheric C02 and 02. These fractionation steps are indicated in Figure 6 with the red whisker symbols on the exchange flow arrows. In the stratosphere, there exists an exchange link between the oxygen isotope cycles of C02 and 02 generated by photochemical processes (Bender et al., 1994). While this link is of minor importance for the atmospheric budgets of the oxygen isotopes, it is essential for the isotopic l70 anomaly (see Section 3.4 below).

Both, (a) changes in the isotopic composition of the hydrological cycle due to, e.g., climate variations and (b) changes in the carbon and oxygen fluxes between the reservoirs therefore can induce changes in the oxygen isotopic composition of atmospheric C02 and 02. Dynamically, after a perturbation a new atmospheric steady state of the oxygen isotope ratios establishes within a few years in the case of 180/l60 in C02 and within about 1200 years in

Seasonal cycle O.

Zotino, 60 N 90 E

Seasonal cycle O.

Zotino, 60 N 90 E

FIGURE 4 Relationship between the seasonal cycles of CO, and 02 in the interior of the Eurasian continent (at Zotino, 60°N, 90°E) within the planetary boundary layer. The left diagram shows the seasonal signal components in 02 induced from the terrestrial and oceanic seasonal sources; the right-hand panel shows the modeled relation between the seasonal cycles of O, and C02 (monthly averages).

FIGURE 4 Relationship between the seasonal cycles of CO, and 02 in the interior of the Eurasian continent (at Zotino, 60°N, 90°E) within the planetary boundary layer. The left diagram shows the seasonal signal components in 02 induced from the terrestrial and oceanic seasonal sources; the right-hand panel shows the modeled relation between the seasonal cycles of O, and C02 (monthly averages).

the case of l80/l60 in 02 (Bender et al., 1994). This difference arises from the different atmospheric turnover times of CO, and 02. Observations of spatiotenrporal isotopic variations in the atmosphere may be related to either one or a combination of these two principal driving factors. Clearly, there exists a remarkable correspondence between the cycles of the oxygen isotopes and the global cycles of carbon and oxygen, which becomes evident on comparing the upper and lower panels in Figure 1.

-180 -150 -120 -90 -60 -30 0 30 60 90 120 150

FIGURE 5. Slope of the modeled relationship between the seasonal cycles of O, and CO, in the planetary boundary layer. The color scale has been selected such that values in the Northern Hemisphere are highlighted, where the relationship between the seasonal signals of the two tracers is essentially linear. The black dot indicates the location of the Zotino (60°N, 90°E) station displayed in Figure 4.

3.2 lsO in C02

The 180/l60 ratio of C02 is primarily controlled by exchanges with the terrestrial carbon systems. Exchanges with the surface ocean are controlled by temperature-dependent fractionation during gas exchange and the isotopic signature of surface waters, which is highly correlated with the salinity (Craig and Gordon, 1965). Marine biological processes do not significantly affect this signature.

Of interest are the atmospheric seasonal cycles, the annual mean gradients, and interannual variations as recorded at the global monitoring networks. If the 180/l60 signature of surface waters is known (i.e., ground water and the evaporatively enriched leaf water), then both, the observed seasonal cycle and the meridional gradient of l80/l60 in C02 provide a powerful tool to constrain on regional and global scales the gross photosynthesis (GPP) of the terrestrial biosphere. A forward modeling study to demonstrate this has been performed by Ciais et al. (1997a, b). The atmospheric observations may also be used in an inverse modeling approach to constrain GPP and its driving factors on a regional basis (Peylin et al., 1999). Interannual variations of l80/l60 in C02 have been documented in observations, but are relatively difficult to interpret, as these are controlled to a large extent by changes in the driving hydrological cycle over land.

3.3 The Dole Effect

Terrestrial and oceanic biospheric processes drive with similar relative weight the l80/l60 ratio of atmospheric 02. The most important effect is the fractionation occurring during consumption of O, by heterotrophic respiration. This fractionation leads to a global atmospheric l80/l60 isotope ratio enriched by about 23.5%

relative to ocean water (Dole, 1935; Morita, 1935), known as the Dole effect. From measurements on ice cores it has been possible to establish the history of the Dole effect over the last glacial cycles. Since the magnitude of the Dole effect hinges on a multitude of factors, it is puzzling that it remained relatively constant over the glacial climate cycles (Bender et al, 1994; Severinghaus et al, 1998; Petit et al., 1999). This has often been interpreted to mean that, the terrestrial and oceanic biospheric production co-varied in a fixed ratio. More detailed modeling studies (Hoffmann et al, 1998) demonstrate, however, that other factors, such as changes in the hydrological cycle and/or changes in the geographical location of the major terrestrial vegetation activity during the ice-age cycles, complicate the interpretation of the observed constancy of the Dole effect.

Because of their expected small size, atmospheric variations of the l80/l60 in O, have not been observed up to now. Seasonal variations and mean annual gradients are expected to be on the order of a few permeg (Seibt, 1997). Just as the combination of CO, and 02 provides a very powerful constraint on the carbon cycle, a combination with measurements of the oxygen isotope ratios in both CO, and 02 would constitute two further powerful constraints.

3.4 170

Oxygen consists of three stable isotopes: l60, ''O and lsO. For most applications only the ratio between the more abundant isotopes, lsO and "'O, is measured. If all fractionation processes in the environmental system were purely mass-dependent, measurements of ''0/l60, would be redundant, as they could be predicted from the l80/l60 ratio. However, it has recently been observed that photochemical exchange between O,, Oi; and C02 in the stratosphere involves mass-independent fractionation among the oxygen isotopes (1990; Thiemens et al, 1995a, b Thiemens, 1999). Thereby O, becomes anomalously depleted, while C02 becomes anomalously enriched. Because of this, measurements of ''0/l60 in atmospheric 02 and/or C02 provide an independent piece of information. Conveniently, one may define an ''O anomaly tracer (Al70) (Thiemens et al., 1995b)

where the symbol 8 denotes an isotope ratio expressed as a deviation from a standard in units of %o. The scaling factor in the definition of this anomaly tracer has been chosen so that it captures the mass-dependent fractionation. Thus, wherever at a phase transition a mass-dependent oxygen isotope fractionation occurred, A' O would remain constant. Thus, A''0 constitutes a tracer with pathways identical to l80/l60 in the cycles of CO, and O, except that it is not fractionated. It is constantly being generated in the stratosphere and "destroyed" at the exchanges with the hydrological cycle in the land biosphere and the ocean. Because of these properties, measurements of this tracer are easier to interpret as compared to the 180/160 ratio alone. Two first examples of the application of this tracer to biogeochemistry have been presented by Luz et al, (1999).

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