GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 8, NO. 3, PAGES 363-376, SEPTEMBER 1994 The Dole effect and its variations during the last 130,000 years as measured in the Vostok ice core Michael Bender • and Todd Sowers 2 GraduateSchool of Oceanography,University of RhodeIsland, Kingston Laurent Labeyrie Centredes Faibles Radioactivites, Centre National de la RechercheScientifique, Commissariat a L'EnergieAtomique, Gif-sur- Yvette, France Abstract. We review the currentunderstanding of the Dole effect (the observeddifference between the •5180of atmospheric0 2 andthat of seawater)and its causes, extend the record of variationsin theDole effectback to 130kyr before present using data on the •5180 of 0 2obtained from studying the Vostok ice core(Sowers et al., 1993), anddiscuss the significanceof temporalvariations. The Dole effectreflects oxygenisotope fractionation during photosynthesis, respiration, and hydrologic processes (evaporation, precipitation,and evapotranspiration). Our bestprediction of the present-dayDole effect,+20.8 %0,is considerablylower thanthe observedvalue, + 23.5 %0,and we discusspossible causes of this discrepancy. During the past 130 kyr, the Dole effecthas been 0.05 %0lower thanthe present value, on average.The standarddeviation of the Dole effect from the meanhas been only _+0.2 %0,and the Dole effect is nearly unchangedbetween glacial maxima and interglacial periods. The smallvariability in theDole effect suggeststhat relative rates of primaryproduction in theland and marine realms have been relatively constant.Most periodicvariability in the Dole effectis in the precessionband, suggesting that changes in thisglobal biogeochemical term reflects variations in low-latitudeland hydrology and productivity or possiblyvariability in low-latitudeoceanic productivity. Introduction they can potentially be made to do so. Ice core reconstructionsof the concentrationsof bioactivegases in air Our understanding of the response of the biosphere to provide an integratedglobal signal which complementsthe Pleistoceneclimate change is actually quite limited. For the proxy studies cited above. Variations in the concentrationsof terrestrial realm, our most detailed knowledge comes from themost abundant bioactive gases (CO 2, CHn, N20 ) reflect,in extensive studiesof pollen in sedimentswhich record climate part, global scale changes in the marine and terrestrial during and after the last glacial termination [e.g., COHMAP, biospheres.Atmospheric concentrations of CHn havebeen 1988]. Pollen data for earlier times are extremely informative found to vary with a period correspondingto that of the but geographicallyrestricted. In the marine realm, the most precession of the equinoxes [Chappellaz et al., 1990]. ambitious attempts to understand the response of the Concentrationsare higher when temperaturesare warmerand biosphere to climate have focused on studies of primary precipitationis greater. Glacial-interglacialvariations in the productivity in the past inferred from two proxy indicators: concentrationof CO2 havebeen attributed to changesin the organic carbon accumulation rates [e.g., Sarnthein et al., fertility and carbonexport of the upperwater ecosystem [e.g., 1988, 1992] and foraminiferal taxonomy [Mix, 1989a, b]. Mix, 1989a; Knox and McElroy, 1984], althoughvariations Paleoproductivity estimates made from many other proxies in oceancirculation and otherfactors must also have played an have enlivened the discussionwith respectto specific areasof important role (e.g., Boyle, 1988; Broecker, 1989). On a the ocean. shortertimescale, the transientminimum in the CO2 Proxy studiesof the responseof the terrestrial and marine concentrationof air at the end of the last glacial termination biosphere are limited in that they do not generally provide reflects rapid growth of the land biosphere[Neftel et al., information on a global scale, although with enough effort 1988]. Theb•80 of O2 is an additionalvariable which reflects the 1Alsoat Centredes Faibles Radioactivites, CNRS-CEA, Gif-sur- globalresponses of the land and marinebiospheres to climate Yvette, France. change,albeit in a complexmanner. The Dole effectis defined 2Presentlyat Lamont-Doherty Earth Observatory of Columbia asthe difference between the b•80 of atmospheric02 in airand University,Palisades, NY. theb•80 of contemporaneousseawater. The magnitudeof the Dole effect, which today is about +23.5 %o [Kroopnick and Copyright1994 by the AmericanGeophysical Union. Craig, 1972],mainly reflects the isotopic composition of 02 producedby marine and terrestrialphotosynthesis, as well as Paper number94GB00724. the extentto which the heavy isotopeis discriminatedagainst 0886-6236/94/94GB-00724510.00 during respiration. As previously discussed, /5180 of 364 BENDER ET AL.: DOLE EFFECT IN THE VOSTOK ICE CORE atmospheric02 variedover glacial/interglacialtimescales in The5180 of 02 producedby photosynthesisis similar to responseto changesin the •5180of seawater[Horibe et al., thatof the sourcewater [e.g., Guy et al., 1993]. The •5180of 1985; Bender et al., 1985, Sowers et al., 1993]. Other factors, 0 2 producedby marineplants today is thus0 %o.The •5180 of whichare of secondaryimportance in controllingthe •5•80of 0 2 producedon the continentshas been estimatedto lie atmospheric0 2, are (1) changesin terrestrialand marine between+ 4 and + 8 %o[Dongmann, 1974; Farquhar et al., fertility, (2) varying isotope fractionation associatedwith the 1993]. These elevated •5180values are the result of elevated hydrologic cycle, and (3) changes in respiratory isotope leaf water•5180 values resulting from evapotranspiration. effects on either a speciesor communitylevel. The•5180 of 02 consumedbyrespiration is ~ 20 %oless than The responsetime of •5•80of 02 is comparableto the thatof the source02 [Guyet al., 1993;Kiddon et al., 1993]. turnovertime of 02 in air with respectto photosynthesisand At isotopicsteady state, the •5•80of 02 moleculesactually respiration,about 1.2 kyr (Table 1). In general,increases in consumedby respirationmust be thesame as the •5•80 of 02 the ratio of terrestrialto marineproduction will causethe •5•80 producedby photosynthesis[Lane and Dole, 1956]. Today of atmospheric02 to rise. Withinthe terrestrial realm, large- thisconstraint is satisfied with •5•80 of atmospheric02 = + scale changesin ecology, precipitation,and humidity can all 23.5 %o. In the eventof deviations,the steadystate value of causevariability in the Dole effect. the Dole effect would be restoredwith an e-foldingtime equal In this paper we present a record of variations in the Dole to theturnover time of 02 in air, ~ 1.2kyr. effect over the last 130 kyr, based on the analysis of air We now recognizethat severaladditional processes affect occludedin the GISP2 and Vostok ice cores. We then explore the•5180 of atmospheric02 [e.g.,Berry, 1992]. Themost the implicationsof this record for changesin the fertility of importantis evapotranspiration,which can causevery large the land and marine biospheresand changesin hydrologic enrichmentsin the •5•80of leaf waterof landplants because of fractionationof oxygen isotopesduring the Late Pleistocene. the preferrentialevaporation of the light isotope[Dongrnann et al., 1974; Farris and Strain, 1978; Forstel, 1978]. Due largely to the meticulouswork of R. D. Guy and colleagues Dole Effect Reconsidered [Guy et al., 1989, 1992], our estimates of isotope fractionationduring photosynthesisand respirationare based The •5•80of surfaceseawater today is, on average,close to on much better data than was available to Lane and Dole zero on the SMOW scale, while that of 02 is constant [1956]. In addition, we now have better estimates of the throughoutthe atmosphereat a value of about+ 23.5 %o. Over relative rates of land and oceanproduction. Finally, we can the time period of interest here, photosynthesis and now begin to assessthe impactof severaladditional processes respiration are the most important reactions producing and on the Dole effect,including isotopic exchange between CO 2 consuming02 . The isotopiccomposition of 02 in air must and02 drivenby photochemicalprocesses in the stratosphere therefore be understood in terms of isotope fractionation (first suggestedby Dole et al. [1954], p. 66), mixing in the associatedwith these reactions [Lane and Dole, 1956; Berry, aphoticzone of theocean and its influence on the •5•80 of 02 1992; and referencestherein]. consumedby respirationin that environment,and equilibrium Table la. Terrestrial Mass Balance of 02 and •SlsOof 02 Production Production Reference Grossproduction excluding photorespired 0 2 (GPP) 14.1 Farquhar et al. [ 1993] Gross production including photorespiration(14.1/0.69) 20.4 Farquhar et al. [ 1980] Process Fraction of respiratory Isotope effect Reference 0 2 consumption •5180of terrestrialphotosynthetic 02 w. r. t. SMOW 4.4 %o Farquhar et al. [ 1993] Discriminationagainst O 18 during respiration Dark respiration 59% 18.0 Guy et al. [ 1992, 1993] 10% Mehler reaction 15.1 Guy et al. [ 1992] Photorespiration 31% 21.2%o Guy et al. [ 1992] Flux weighted terrestrial respiratoryisotope effect, 18.7%o excluding dark respiration Equilibriumenrichment in •5180of leafwater w. r. t. air +0.7%o Bensonand Krause [ 1984] Terrestrialrespiratory isotope effect (= 18.7%o-0.7%o) 18.0%o Terrestrial Dole effect 22.4 BENDER ET AL.: DOLE EFFECT IN THE VOSTOK ICE CORE 365 Table lb. Marine Mass Balance of O2 and •180 of O2 Production term Production Reference (x1015moles/yr) Marinegross production (=4 x seasonalnet production) 12 Keelingand Shertz[