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30 - 2 ANGERT ET AL.: BIOLOGICAL EFFECTS ON THREE O 2 ISOTOPESThe substrate water in leaves is enriched in 17 O and 18 Othrough evapotranspiration [Dongmann, 1974]. As a result,terrestrial photosynthesis produces O 2 that, on average, isenriched in these heavy isotopes. In both marine andterrestrial systems, the major cause of the heavy isotopeenrichment is preferential removal of 16 O (over 17 O and18 O) by biological uptake mechanisms [Lane and Dole,1956]. This preferential removal is mass-dependent and therelative increase in 17 O/ 16 O is about half (0.52) of therelative increase in 18 O/ 16 O (Figure 1). In addition toevapotranspiration and biological uptake, atmospheric O 2is also affected by photochemical reactions in the stratosphere[Luz et al., 1999]. These reactions preferentiallytransfer 17 O and 18 O from O 2 to CO 2 in a mass-independentway. Hence, in these reactions the ratio of the increase in17 O/ 16 O over 18 O/ 16 O in CO 2 is equal to 1 or larger[Lämmerzahl et al., 2002; Thiemens, 1999]. As a result,O 2 becomes mass-independently depleted in the heavyisotopes [Bender et al., 1994]. At the surface of the Earth,the 17 O depletion of atmospheric O 2 is removed byexchange through photosynthesis and uptake. Therefore,the net result is depletion in 17 O of atmospheric O 2 incomparison to O 2 affected by biological uptake alone(Figure 1). The magnitude of this depletion depends onthe ratio of production of 17 O depleted O 2 in the stratosphere,and its destruction by biological cycling. Thus, pastvariations in the 17 O depletion can be used to infer changesin global biological productivity.[4] In order to apply this approach, it is necessary that therelationship between 17 O/ 16 O and 18 O/ 16 O to be known withgreat accuracy. However, the triple isotope relationships indifferent physical-chemical mass-dependent processes,varies slightly about 0.52 [Luz and Barkan, 2000; Matsuhisaet al., 1978; Young et al., 2002]. If this is also the casefor O 2 consumption by different biological mechanisms,then a change in the ratio of uptake by the different pathwayswill result in variations in the magnitude of the 17 Odepletion. As a consequence, past variations in this parameterwill be misinterpreted.[5] In interpretations of variations in the Dole Effect it isusually assumed that the major driving force is changes inthe terrestrial to marine production rate (terrestrial producedO 2 is heavier than marine produced, because of the enrichmentof leaf water in evapotranspiration). An impliedassumption is that the enrichment due to biological uptakeremains constant. In the present paper, we explore thepossibility that time variations in Dole Effect were alsoaffected by changes in the relative rates of O 2 uptake bydifferent biological mechanisms, that have different fractionationsassociated with them. Consumption (or uptake) ofO 2 in plants occurs via four mechanisms: ordinary respirationthrough the cytochrome oxidase pathway (COX), respirationby the alternative oxidase pathway (AOX), Mehlerreaction, and photorespiration. The first two processes takeplace in light as well as in dark conditions, while the lattertwo occur only under illumination. In addition, O 2 diffusionin air precedes biological uptake in some cases. The discriminationagainst 18 O associated with the AOX (28%)isstronger than the discrimination associated with other mechanisms(COX 18%, Mehler 15%, photorespiration Figure 1. Schematic plot (not to scale) of 17 O and 18 Orelative to 16 O variations. Oxygen is produced by photosynthesisand it is fractionated by biological uptake in amass-dependent way, and by stratospheric photochemistryin a mass-independent way. The balance between these twotypes of fractionation control the triple isotopic compositionof atmospheric O 2 .22%). Hence, an increase in the proportion of O 2 uptake byAOX will significantly intensify the Dole Effect.[6] To improve the interpretation of past variations in thetriple O 2 isotopes and the Dole Effect, we have investigatedthe effects of biological mechanisms in controlled laboratoryexperiments. In one set of experiments, we studiedtriple isotope variations due to uptake alone by COX andAOX, and due to removal by diffusion. In a second set ofexperiments, conducted in an airtight terrarium, we analyzedthe combined effects of production and consumptionby both dark and light reactions in steady state betweenphotosynthesis and uptake.2. Triple Isotope Systematics2.1. Terminology[7] Atmospheric O 2 is depleted in 17 O due to stratosphericreactions. Hence, O 2 that is produced by photosynthesiswill have 17 O excess with respect to atmospheric O 2 .This 17 O excess was formulated in previous papers [Luz andBarkan, 2000; Luz et al., 1999] as D 17 O=d 17 O 0.521 d 18 O. However, this definition is somewhat problematicsince it contains a linear approximation to the mass-dependentfractionation, and because the parameter 0.521 (thatrepresents mass-dependent processes) depends on thechoice of reference material. Here, we follow the suggestionof Miller [2002] for a new definition of the 17 O excess thatis free of these problems:17 R18 R17 D ln C ln : ð1Þ17R18 ref R ref
ANGERT ET AL.: BIOLOGICAL EFFECTS ON THREE O 2 ISOTOPES 30 - 3[8] The new definition of the 17 O excess will be noted as17 D to distinguish it from the old definition (D 17 O). 17 Rstands for the isotope ratio 17 O 16 O/ 16 O 2 , 18 R stands for theisotope ratio 18 O 16 O/ 16 O 2 , and the subscript ‘‘ref’’ stands forthe reference. It should be noted that 17 D is a calculatedvalue that depends both on the standard used and on thevalue of the parameter C. This parameter represents thetriple isotope relationships in mass-dependent processes.However, as was mentioned in the introduction, theserelationships vary slightly about 0.52 for different processes.As a result, The chosen C value can only correspond to therelationship associated with one of the processes, preferablythe main one. In this study we choose to use a value of C =0.516, which corresponds to ordinary dark respiration(COX), the most wide spread global O 2 uptake mechanism.[9] In the old definition of the 17 Oexcess(D 17 O) thevariations in the isotopic composition were described on ad 17 O versus d 18 O plot. To describe the variation in the isotopiccomposition according to the new definition, two new termsthat will be used for the axes of such plot are suggested,Ln 17 O lnLn 18 O ln17 R17R ref18 R18R ref¼ ln d 17 O þ 1Þ¼ ln d 18 O þ 1Þ:[10] Another advantage of 17 D,Ln 17 O and Ln 18 O is thatthey are dimensionless. Hence, the value of a sample versusthe reference exactly equals to the value of the referenceversus the sample with the opposite sign (this is not true forD 17 O, d 17 O and d 18 O). For convenience, Ln 17 O and Ln 18 Oare expressed as permil (%) and 17 D as per meg deviationsfrom the standard.[11] The slope of a line passing between two points (Aand B) on a Ln 17 O versus Ln 18 O plot, will be defined as l,as in previous work [e.g., Miller, 2002], ln 17 R A = 17 R Bl : ð4Þln 18 R A = 18 R B[12] Following Mook [2000], the relationship between thediscrimination against17 O and 18 O (relative to 16 O) ispresented asq ln ð17 aÞlnð 18 aÞ ; ð5Þwhere 17 a and 18 a are the fractionation factors x R p / x R s . Thesubscripts ‘‘p’’ and ‘‘s’’ stand for ‘‘product’’ and ‘‘substrate,’’respectively (x can be 17 or 18).[13] Blunier et al. [2002] used an additional relationshipbetween the discriminations,g ð2Þð3Þ17 a 118a 1 ¼ 17 e18e ; ð6Þwhere x e is x a 1.[14] The terms q and g describe the inherent relationshipbetween the discriminations, but cannot be measureddirectly. What can be measured is the slope (l) that iscontrolled by these relationships and the processes thattake place in the system. In the special case of a systemat steady state, in which production equals uptake, theslope (l) is equal to q (see section 2.2). In the specialcase of a system where only uptake takes place, the slope(l) is equal to g (see section 2.3). Hence, by conductingexperiments in such systems, q or g could be calculatedfrom the measured l. In other systems (for example, inones where mixing takes place) the slope can be differentfrom both q and g. The difference between the value of qand g is small (0.003), but since the 17 O excessvariations in O 2 are minute such a difference cannot beneglected.[15] This paper deals with the effect of the biosphere onthe triple isotopic composition of the atmosphere. Since thebiosphere-atmosphere system is close to production-uptakesteady state, we chose to use q to report the relationshipbetween discriminations.2.2. Production-Uptake Steady State[16] In this study, we used a terrarium similar to that ofLuz et al. [1999] to estimate the q value associated withphotorespiration. To understand how the q value of aprocess can be evaluated from such experiment, we willdiscuss the triple isotope systematics of a closed system inbiological production-uptake steady state. Such a systemcan also be used as a model for the Earth atmosphere andbiosphere without the effects of stratospheric photochemistryand the hydrological cycle.[17] The starting point for such a system is oxygen that isproduced by photosynthesis in photosystem 2 from water(noted as ‘‘W’’) without isotopic fractionation. Uptake byrespiration causes enrichment in the heavy isotopes. Sincethe fractionation in biological uptake is mass dependent, theenrichment in 17 O will be about half of the enrichment in18 O. In steady state, depending on the value of q, the systemwill reach either the point that is marked as ‘‘BSS1’’(Biological Steady State 1) or ‘‘BSS2’’ in Figure 2. TheLn 18 O value of the O 2 in steady state relative to thesubstrate water is illustrated by the horizontal distancebetween ‘‘BSS1’’ (or ‘‘BSS2’’) and ‘‘W,’’ and is thesystem’s equivalent of the ‘‘Dole Effect.’’[18] A system in production-uptake steady state can beanalyzed graphically as in Figure 2, or, more rigorously, bya model that deals with the fluxes of the three isotopicspecies. Such a one-box model of production-uptake steadystate can be formulated as follows:y P ¼ y U;where y P is production rate of y O 16 O (y can be 16, 17 or18), and y U is uptake rate of y O 16 O.[19] Equation (7) can be rewritten as follows:ð7Þ16 P x R W ¼ 16 U x R BSS x a; ð8Þwhere x can be 17 or 18, the subscript ‘‘W’’ stands foroxygen produced by photosynthesis, and ‘‘BSS’’ stands forthe system air in biological steady state.
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