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The Influence of Air and Sea Exchange on the Carbon Isotope Distribution

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The Influence of Air and Sea Exchange on the Carbon Isotope Distribution GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 6, NO. 3, PAGES 315-320, SEPTEMBER 1992 THE INFLUENCE OF AIR AND SEA EXCHANGE ON TI-IE CARBON ISOTOPE DISTRIBUTION IN THE SEA Wallace S. Broecker Lamont-DohertyGeological Observatory of Columbia University,Palisades, New York Ernst Maier-Reimer Max-PlanckInstitut f'tir Meterologie Hamburg,Germany Abstract.We explorehere the influence of thetemperature However,for carbonisotope ratios, this tie is not perfect dependenceof isotope fractionation between atmospheric CO2 [Keir, 1991]. The reasonis that while trace metals and andocean •CO2 on thedistribution of carbonisotopes in the nutrientscirculate only throughthe sea,carbon circulates ocean.This is accomplishedby ananalysis of departuresfrom throughthe atmosphere as well. The temperaturedependence theexpected Redfield tie between PO4 and 8•3C. We findthat of thecarbon isotope fractionation between atmospheric CO2 for thesurface ocean, the temperature influence largely andsurface ocean •CO2 tendsto disruptthe correlation compensatesfor thebiologic influence. In thedeep ocean, the between •3C/•2C ratios and the concentrationof the nutrients temperatureinfluence imprinted at thesites of deepwater andtrace metals. The extentof thisdisruption depends on the formationreduces somewhat the biologically induced difference relativestrength of mixingwithin the seaand CO2 exchange betweenthe carbon isotope ratios for deep waters produced in betweenthe ocean and atmosphere. Because of thecomplex the northern Atlantic and in the Antarctic. These same features natureof mixingwithin the sea,it provesdifficult to estimate arereproduced in theHamburg ocean model. In orderto themagnitude of thedisruption. Ocean general circulation assessthe impact of changesin theratio of oceanmixing rate models(GCMs) offer a meansto assessthe sensitivityof this to windspeed, we havemade a modelrun in whichCO2 disruptionto alteredclimatic conditions. exchahgerates between air and sea were everywhere doubled. Asexpected, the influence of thethermodynamic effect on the OPPOSING TENDENCIES: BIOLOGIC VERSUS oceaniccarbon isotope distribution is magnified. TffERMODYNAMIC The carbonisotope-nutrient constituent separation about INTRODUCTION whichwe are concernedhas its origin in the isotope fractionationbetween atmospheric CO2 and surface ocean Ourknowledge of thepaleocirculation patterns in thedeep ECO2. Were thereno air-seafractionation, then the oceancomes largely from measurements of stable isotope distributionof 8aK?values within the seashould be tightly ratios(carbon and oxygen) and of tracemetal concentrations correlatedwith phosphateconcentration. The relationship (cadmiumand barium) in theshells of planktonicand benthic would be as follows: foraminiferafound in deep-seasediments. The distributionsin theocean of •3C72Cand Cd/Ca are generally viewed to be 8•3C.8•3CM.O. _ Aphoto C (1) highlycorrelated one to theother and to thoseof PO4and NO3. =EC02 •'ø'F)org (PO,-PO, M-ø') whereM.O. standsfor meanocean, Aphoto is the carbon isotopeseparation during marine photosynthesis andC]P)org Copyright1992 is thecarbon to phosphorusratio in marineorganic matter. by the AmericanGeophysical Union. TakingAphoto to be -19%o, C/Porg to be 128 and ZCO2U'ø' to be 2200 grnol/kg,the equationbecomes Papernumber 92GB01672. 0886-6236/92/92G B-01672510.00 8•aC - 8•aCM'ø' = 1.1 (PO,M'ø' - PO4) (2) 316 Broeckerand Maier-Reimer: Influence of CO,.Exchange on The 8•3C for mean ocean carbon is about 0.5%o and the mean DeepWater (NADW) liesbelow this trend and that for new oceanPO,• concentration is about2.2 I.unol/kg. Hence AntarcticBottom Water (AABW) lies abovethis trend (see Figure4). Thesewaters mix in theproportion one part 8•C = 0.5 + 1.1 (2.2 - PO,0 = 2.9 - 1.1 PO,• (3) NADW t• twoparts AABW to formthe water entering the deepIndian and Pacific oceans. This equationis of coursean approximationfor Aphoto The 8a3C-PO•coordinates for watersfrom the warm regions changeswith theCO,_ content of surfacewater; ZCO2 increases of thesurface ocean fall belowthe value expected if the•5x3C - with increasingalkalinity and phosphate concentration, and phosphaterelationship were exactly biologic. This departure C/Porglikely varies somewhat from place to place in thesea. is notentirely thermodynamic; the values for surfacewater However,the deviations produced in thisway aresmall havebeen influenced by thedrop in atmospherica3C72C ratio comparedto thosestemming from air-seaisotope causedby fossilfuel burning.Between 1800 and the time of fractionation.Since the highestphosphate content surface theGEOSECS surveys (1972 - 1978),the 8•3C for watersare foundin thepolar oceans, one would expect them to atmosphericCO,_ dropped by about1.2%o [Friedli et al., 1986]. havethe lowestsurface water 8•3C values. For example,as Modelresults suggest that this change caused surface ocean the ambientPO4 value in Antarcticsurface waters is about 1.6 Bmol/kgand that in tropicaland temperate surface waters averages0.2 lamol/kg,the 8x3C for Antarcticsurface waters should be about 1.5%o lower than in Antarctic surface waters. Workingin the oppositedirection is the thermodynamic effectrelated to air-seaCO,. exchange which tends to makethe 8aK2values for coldsurface waters higher than those for warm surfacewaters. At equilibriumthe carbonisotope separation betweenatmospheric CO,_ and surface ocean ECO2 increases by about1%o per for each10øC decrease in watertemperature. Becauseof thisdependence, were thermodynamic equilibrium achieved,it wouldproduce a strongcarbon isotope difference in theopposite sense; Antarctic surface waters would have 2 to 3%ohigher 8aK2 values than tropical and temperate surface waters. Where between the two extremes the actual situation lies o io 20 3o dependson therelative strength of air-seaCO,_ exchange on SURFACE WATER TEMP (*C) onehand and mixing between the sea's water masses on the other. The formerworks to establishthermodynamic equilibriumand the latterto establisha perfectcorrelation with phosphate.It will alsodepend on the distributionof (•••01 •1 I I I I _ phosphorusin high-latitudesurface waters. 2 ß BIOLOGICI.1%o//.z. mol/kgCOEFFICIENT - o OBSERVATIONS o Shownin Figure 1 is a plot of carbonisotope ratio against temperaturefor surfacewater samples obtained during the GEOSECSexpeditions to theAtlantic and Indian oceans + INDIAN [Bacastowand Maier-Reimer, 1990]. As can be seen,no trend 0 N ATLANTIC with temperatureis discernable.In thisfigure, the same ß S ATLANTIC i I i i I, I I carbonisotope results are alsoplotted against phosphate 0.0 0.5 1.0 1.5 content.Although a smalldecrease in b•sCwith phosphate PHOSPHATE(/.z. rnol/kg) concentrationis seen,it is not nearlyas largeas would be expectedif the distributionwere exactlybiologic. Roughly Fig. 1. (Upperpanel) The relationshipbetween the •JxsC value speaking,in today'ssurface ocean the thebiologic tendency is for dissolvedY-,CO2 and the temperatureof surfacemixed layer nearlybalanced by the thermodynamictendency. A similar samplescollected in theAtlantic and Indian Oceans as part of situationexists in the PacificOcean [Kroopnick et al., 1977]. theGEOSECS program. The line showsthe trendexpected if As reproducedin Figure2, a traverseof surfacewaters collected thermodynamicequilibrium were achieved with atmospheric along150øW during the Hudson1970 expedition does show a CO,_.(Lower panel) The relationshipbetween the b•sC value decreasefor polarsamples in the senseexpected from for dissolved•CO2 andthe phosphate content of surfacemixed fractionationduring photosynthesis, but it is only about0.7%o layersamples from the Atlanticand Indian oceans collected as (insteadof 1.5%o).Unfortunately, no PO4determinations were partof theGEOSECS program. The line is the trendexpected madeon thesesamples. were no exchangewith atmosphericCO,_ to occur. The carbon The carbonisotope-phosphate trend in Pacificand Indian isotopemeasurements were made in theUniversity of Hawaii Oceandeep waters follows expectation (see Figure 3). The laboratoryof PeterKroopnick and in the ScrippsInstitution 8•sCvalue drops by about1.1%o per additional I.tmol/kg of for Oceanographylaboratory of HarmonCraig [Kroopnick, PO•. However,the 8xsC-PO•coordinate of new NorthAtlantic 1980, 1985; Ostlund et al., 1987]. Broeckerand Maier-Reimer: Influence of CO,_Exchange on •3C 317 2.5 I I I I I If theRedfield slope of 1.1%ooper Ixmol/kg is assumedto be uniformthroughout the sea.then a measureof the influenceof thermodynamicequilibration on theõ•3C value for anygiven 2.3- watersample is givenby therelationship 2.1- Aõ•3C= õ•C + 1.1 PO4- 2.7 (4) '" ,.' .',.:\. where2.7%0 is an arbitraryconstant introduced to bring the Aõ•aCvalues for waterslying on thetrend for deepPacific and Indian Ocean waters close to zero. New NADW has a Aõ•C 1.7- value of about -0.5%0 and new AABW a AõlaC value of about +0.2%0.Phosphate deficient surface waters in thewarm parts of the ocean have values of about -1.0%oobefore correction for 1.5- the fossilfuel CO2induced change and -0.3%0after this I correctionhas been made (see Figure 4). i 1.3 A mapshowing the Aõ•K2 values for surfacewater samples S 610ø 4'0ø o 60ø N obtainedas part of the GEOSECSprogram is shownin Figure LATITUDE 5. Note that the ZX•J•C values for surface waters in the Fig. 2. Carbonisotope measurements on theT•CO,_ from Antarcticare positivewhile thosefor the restof surfaceocean surfacewater samples collected during the Hudson 1970 are negative. expeditionalong 150øW in thePacific Ocean [Kroopnick et The differencein carbonisotope ratio between deep Atlantic al.,
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