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6.10 Biological Fluxes in the Ocean and Atmospheric Pco2

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6.10 Biological Fluxes in the Ocean and Atmospheric Pco2 6.10 BiologicalFluxesinthe Ocean andAtmospheric p CO2 D.Archer University ofChicago,IL, USA 6.10.1 INTRODUCTION 275 6.10.2 CHEMICAL REARRANGEMENT OF THE WATERS OF THE OCEAN 276 6.10.2.1 The Organic Carbon BiologicalPump 276 6.10.2.1.1 Nutrient limitation andnewproduction 276 6.10.2.1.2 Iron asalimitingmicronutrient 276 6.10.2.1.3 Measuringcarbon export 277 6.10.2.2 CaCO3 Production andExport 277 6.10.2.2.1 Calciteandaragonite 277 2 2 6.10.2.2.2 Temperatureand[CO3 ]regulation ofCaCO3 production 278 6.10.2.2.3 Latitudinaldistribution discrepancy 278 6.10.2.2.4Organic C/CaCO 3 production ratio 278 6.10.2.3 SiO2 Production andExport 278 6.10.2.4GeochemicalSignatureofthe BiologicalPump 279 6.10.2.4.1 Organic carbon redissolution depth 279 6.10.2.4.2 CaCO3 watercolumn redissolution 279 6.10.2.4.3 SiO2 sinkingandredissolution 280 6.10.2.4.4The ballast modelfor sinkingparticles 281 6.10.2.5DirectAtmospheric p CO2 Signatureofthe BiologicalPump 281 6.10.2.5.1 Organic carbon pump 281 6.10.3 CONTROLS OF MEAN OCEAN CHEMISTRY 282 6.10.3.1.1 Controls on organic matterburial 283 6.10.3.1.2 CaCO3 285 6.10.3.1.3 SiO2 286 6.10.3.1.4Indirectatmospheric p CO2 signatureofthe biologicalpump 286 6.10.4SUMMARY 287 REFERENCES 287 6.10.1 INTRODUCTION asnitrateandiron,the physicsofsinkingorganic matter,andthe production andredissolution of The basic outlinesfor the carbon cycleinthe phytoplankton companion minerals CaCO3 and ocean,asitisrepresented inoceancarbon cycle SiO2 ,inthe watercolumn andinthe sediment. models,weresummarized byBroeckerandPeng The carbon cycleistypically brokendown into (1982).Since then,ongoingfieldresearch has two components,bothofwhich will be summar- revised thatpicture, unearthingnewdegreesof ized here.The firstisthe biologicalpump, freedom andsensitivitiesinthe oceancarbon effectingthe redistributingofbiologically active cycle, which mayprovide cluestochangesinthe elements like carbon,nitrogen,andsilicon within behavior ofthe carbon cycleoverthe glacial the circulatingwaters ofthe ocean. The secondis cyclesanddeeperinto geologicaltime.These the ultimateremovaloftheseelements byburialin sensitivitiesinclude newideasabout the chem- sediments. Thesetwo componentsofthe carbon istry andcyclingofphytoplankton nutrientssuch cycletogethercontrol the meanconcentrations of 275 276 BiologicalFluxesinthe OceanandAtmospheric p CO2 many chemicals inthe ocean,includingoceanpH such asdeposition from the atmosphere(asinthe andthe p CO2 ofthe atmosphere. caseofiron),or localproduction (asinnitrogen Thischapteroverlaps considerably withthe fixation).Traditionally,oceanographers have only two others from thisTreatiseIhavehad the distinguished “new” versus “recycled”primary pleasureto read:Chapters 6.04and6.19.My production usingthe different forms ofbiologi- chapterisdistinguished, Isuppose, bythe callyavailablenitrogen(EppleyandPeterson, perspectiveofageochemicaloceanmodeler, 1979). Whenphytoplankton degrade, theirnitro- attemptingto integratenewfieldobservations genisreleased inthe reducedform,asammonia, into the context ofthe oceancontrol ofthe p CO2 of which correspondstonitrogen’s chemicaloxi- the atmosphere. dation stateinproteins andDNA.Ammonia released withinthe euphotic zoneisquickly assimilated bythe next generation ofphytoplank- ton. Ammonia released below the euphotic zone 6.10.2 CHEMICAL REARRANGEMENT escapesthisfate, becausethe darkness precludes OF THE WATERS OF THE OCEAN photosynthesis. Instead, thisammonia gets oxi- 6.10.2.1 The Organic Carbon BiologicalPump dized bybacteria to nitrate.Therefore, most of the biologically availablenitrogenindeepwaters The cyclingofdissolved carbon andassociated ofthe oxic oceanisinthe form ofnitrate.Using chemicals (such asoxygen,nitrogen,andphos- thisdichotomy,oceanographers candistinguish phorus)inthe oceanisdrivenbythe livesand betweennewproduction,supported byupwelling deathsofphytoplankton.Sunlightisrequired asan from below,asthe uptake ofisotopicallylabeled energysource for photosynthesis,andisavailable nitrate, versus recycled production asthe uptake insufficient intensity only into the top 100 morso ofisotopicallylabeled ammonia.The ratioof ofthe ocean; the rest ofthe oceanisdark, or at newproduction to production hasbeencalled the least too darkto makealivingharvestingenergy f -ratiobybiologicaloceanographers (Dugdale fromlight. Thenetgeochemicaleffectof andGoering, 1967). Valuesofthe f -ratiorange photosynthesisinthe oceanistoconvert dissolved from 10% to 20% inwarm,oligotrophic, low- constituentsinto particles,piecesofsolid material nutrient open-oceanconditions,to 50% inblooms thatcantake leaveoftheirparent fluid bysinking. andnear-shoreconditions (EppleyandPeterson, Astheydo,theystripthe surface waters of 1979). However,the apparent correlation whichevercomponent locally limits the growth between f -ratioandprimary production maybe ofphytoplankton (typically nitrogenoriron), anartifactofdifferencesinsea-surface andcarrythe entiresuiteofbiologicallyuseful temperature(Laws etal .,2000). elementstodepth. Asthe particlessink, theydegrade inthe water column,or theyreach the seafloor,to degrade withinthe sediments or to exitthe oceanbyburial. 6.10.2.1.2 Iron asalimitingmicronutrient Thiscombination ofremovalatthe sea surface andaddition atdepthmaintains higherconcen- MartinandFitzwater(1988)originally proposed trations ofbioactiveelements andcompoundsat thatthe availability ofthe micronutrient iron may depthinthe oceanthanatthe surface.Theshape be limitingphytoplankton production inremote ofadepthprofileofdissolved nitrogeninthe partsofthe worldocean. Thisproposalwas form ofnitrate, alimiting“fertilizer” or nutrient to convincingly provenbyaseriesofopen-ocean phytoplankton,resemblesthoseofotherbioactive fertilization experiments,inthe equatorialPacific (Coale etal .,1996; Martin etal .,1994)andinthe compounds,includingC,PO 4 ,Si, Fe, andCd.This process,fundamentaltothe oceancarbon cycle, Southern Ocean(Boyd etal .,2000). Although hasbeencalled the biologicalpump (Broeckerand photosynthesisratesandchlorophyllconcen- Peng, 1982; see Chapter6.04). trations increased significantly inall cases,the ironfertilization wasless effectiveatstimulating carbon sinkingto depth, especially inthe Southern 6.10.2.1.1 Nutrient limitation andnew Ocean,wherethe added ironrecycled inthe production euphotic zoneuntilitdispersed bylateralmixing somemonthslater(Abraham etal .,2000). Several Phytoplankton insurface waters,upon com- attempts havebeenmadeto modelthe iron cyclein pletion oftheirearthly sojourns,sufferoneoftwo the ocean(ArcherandJohnson,2000; Fung etal ., fates. Eithertheyaredegraded andconsumed 2000;LefevreandWatson,1999).Although withinthe surface waters ofthe ocean,or they dust deposition delivers moreiron to the surface maysinkto the deepsea.Onaverage, the ofthe ocean(Fung etal .,2000),uncertainty removalofdissolved nutrients from the surface remains about the availability ofthatirontothe zoneto depthmustbe balanced bymixingup of phytoplankton. Ithasbeenargued thatdust dissolved nutrients from below,or othersources deposition mayplayonly aminor roleinregulating ChemicalRearrangement ofthe Waters ofthe Ocean 277 phytoplankton abundance (ArcherandJohnson, waters. Verticalconvection thus drivesexportof 2000; LefevreandWatson,1999). TheNorth sea-surface DOC to depth.Upwellingbringslow- PacificOcean,inparticular,appears to be a DOC watertothe sea surface,wherephotosyn- problemarea, withhigh deposition ofdust (Duce thesisbegins the production ofDOC.ThisDOC andTindale, 1991)failingto drivedepletion ofsea- canbe exported laterallyorback to the ocean surface nutrients. interior.Onaglobalmean,DOC export accounts for less thanhalfofthe exportoforganic matter from the surface ocean,andonly atiny fraction to 6.10.2.1.3 Measuringcarbon export the deepsea (Yamanaka andTujika, 1997). Inthe steadystate(thatistosay,on along Nutrient supply. Inspiteofthe developments of enough timeaverage),the uptake ofnewnutrients, sediment trapcalibration byradiogenic isotopes nitratefor example, must be balanced bythe andexport oforganic matterinthe suspended or removalofnutrients inexported organic matter. dissolved states,problems remaininbalancing Oceanographers havedefined aquotient called the carbon andnutrient exportagainst supplyby e -ratio,describingthe exportefficiencyofeupho- physicalfluid transport. Thisproblemismost tic zoneorganic matter,exportproduction/gross acuteinthe subtropicalgyres,intensely sampled production. Onlongenough timescales,the e -ratio intwo time-seriesJGOFS stations:onenear mustequalthe f -ratio. Inthisway,the biological Bermuda (BATS)andonenearHawaii (HOT). pump acts to limitthe productivity ofthe ocean, Netverticalmotionsinthe upperoceanare pacingitaccordingto the overturningtimescaleof determined bythe divergence ofsurface currents, the oceancirculation. drivenbyfriction withthe wind.The direction Sediment traps andsinkingparticles. The andintensity ofthe resultingverticalmotion is measurement oforganic carbon exportfrom the determined bythe curl ofthe windstress,which surface oceanhasremained problematic over leadstonetupwellinginthe subpolargyres, the decades. Theworkhorseofthe oceanographic carryingnutrients verticallyinto the euphotic community for determiningsinkingparticulate zone, but downwardinthe subtropicalgyres, fluxesisthe moored sediment trap. Sediment traps supplyingno nutrients for photosynthesisatall areplagued bybiasesfrom hydrodynamicsand (Peixoto andOort,1992). Onasmallerspatial from “swimmers,”which actively enterthe open scale, eddiesandfronts leadto verticalmotion and collection
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