JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 106, NO. C5, PAGES 9377-9383, MAY 15, 2001
Bubbles: An estimate of their role in the global oceanic flux of carbon Edward
C. Monahan
and Hans
G. Dam
Department of Marine Sciences,Universityof Connecticutat Avery Point, Groton, Connecticut
Abstract. The amountof particulateorganiccarbon(POC) formed annuallyvia the scavenging of dissolvedorganiccarbon(DOC) by bubblesis calculatedto be 1.9 Gt C
yr-•. In arriving at thisestimate usewasmadeof (1) published descriptions of thebubble spectrumassociated with the alphaplumesbeneathspillingwavecrests,(2) the published relationshipof stageA whitecapcoverageto wind speed,(3) data on the scaledepth of alphaplumesand on the risevelocityof bubblescoatedwith organicfilm, (4) published bubblescavenging efficiencies,and (5) recordedconcentrations of DOC in oceanicsurface
waters. Thisrateof DOC scavenging bybubbles represents a 1.7%yr-• turnoverof the globaloceanicDOC pool, and eachyear producesan amountof POC equalto 17% of the POC pool in the uppermost300 m of the ocean.Bubble scavengingturns over the DOC presentin the shallowlayer of bubble influenceonce every 67 days.Moreover, within the depth of influenceof large bubbles(uppermost1.4 m of the water column), bubblescavenging representsan annual threefold turnoverof the DOC pool and a POC productionequivalentto thirtyfold of the POC pool. It is noted that the amount of DOC transformedinto POC eachyear via bubble scavengingis equal to the amount of carbon presentin the carbondioxideestimatedto be absorbedby the oceaneachyear from the atmosphere. 1.
into solution
Introduction
The role playedby the bubblesinjectedinto the surfacelayer of the ocean by breakingwavesin scavengingsurfactantorganic material and thus generatingorganicallyrich particles has long been a subjectfor speculationand experiment[e.g., Baylor and Sutcliffe, 1963; Sutcliffeet al., 1963; Baylor et al., 1977;Blanchardet al., 1981].Sincethe dissolvedorganiccarbon (DOC) pool in the world oceanexceedsthe particulate organiccarbon(POC) pool by at least 1 order of magnitude [Murray,1994;Kepkay,1994],anyprocessthat is evenmodestly effective at convertingDOC to POC will have a significant impact on the oceanicPOC budget.Aggregationof dissolved and colloidalmatter by risingbubblesat the air-seainterfaceis known as surfacecoagulationand is recognizedas a mechanismby whichdissolvedorganicmaterial(DOM) is converted into particulateorganicmaterial(POM) [Johnson et al., 1986; Kepkay,1994;Mari, 1999;Passow,2000].Consequently, surface coagulationmay representa significantsinkof the DOC pool. The goal of this paper is to assessthe global scavengingof
before
it has a chance
to reach the sea surface
[Johnson and Cooke,1980, 1981].In thesepapers,Johnsonand Cookeprovidewhat is arguablythe mostconvincing and elegant demonstrationof the formation of organic particlesby bubbles. However, even if a bubble survives, reaches the sea surface, and bursts, as is the case with most of the bubbles,
particularly the larger ones, produced by a breaking wave, there is a high probabilitythat an organicallyrich particlewill be produced[e.g.,Blanchard,1989].That is becausewhen the film cap of a bubble resting on the sea surfacebursts, the consequentpressurerelease causesthe remaining dimple or cavityto reboundand form an upwardmovingjet or spikeof water. Most suchjets formed by bubbleswith radii 2 months (67 days). This is of the same order of the DOC material is likely to be constitutedby surfactantssuch magnitudeasthe reportedturnovertime of DOC in modelsof as lipids [Skop et al., 1991], there is experimentalevidence oceanicsurfacewaters previouslypublishedby, e.g.,Archer et [•;uti•andTomai&1988]in support of thenotionthatthese al. [1997] andAndersonand Williams[1999].Hence, within the surfactantsare likely to determinethe adsorptionpropertiesof area of influence of bubbles the sink of DOM due to bubble the bulk DOC. These authors documented for model subscavenging appearsto be asimportantasotherwell-recognized stancesand natural seawaterthat adsorptionof small organic DOC lossmechanisms,such as hydrolysis,mixing, and phomolecules(aminoacids)with functionalgroupscharacteristic tooxidation.
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Sincebubble scavengingof DOC representsa sink for the Andreas, E. L., J. B. Edson, E. C. Monahan, M.P. Rouault, and S. D. Smith, The spraycontributionto net evaporationfrom the sea:A DOC pool and a sourcefor the POC pool, it wouldbe instrucreviewof recentprogress,BoundaryLayerMeteorol.,72, 3-52, 1995. tive to estimatewhat fraction of thesetwo poolsis turned over Archer, D., E. T. Peltzer, and D. L. Kirchman, A timescale for disannuallyby this mechanism.Most of the changesof the DOC solvedorganiccarbonproductionin equatorialPacificsurfacewaand POC occur in near-surface waters. For instance, DOC
ters, GlobalBiogeochem.Cycles,11, 435-452, 1997.
decreases with depthin the uppermost300 m of the oceanand Baylor, E. R., and W. H. Sutcliffe,Dissolvedorganicmatter in seawater as a sourceof particulate food, Limnol. Oceanogr.,8, 369-371, is relativelyconstantbelow this depth [Peltzerand Hayward, 1963. 1996;Andersonand Williams, 1999, and referencestherein]. Baylor, E. R., M. B. Baylor, D.C. Blanchard,L. D. Syzdek,and C. The DOC pool in this layer is estimatedto be 110 Gt C, and Appel, Virus transferfrom surfto wind,Science,198, 575-580, 1977. the POC pool is estimatedto be 11 Gt C [Murray,1994].Thus Blanchard,D.C., The electrificationof the atmosphereby particles from bubblesin the sea,Prog. Oceanogr.,1, 71-202, 1963. bubblescavenging of DOC representsa turnoverrate of 1.7%
yr-• of the DOC pooland17%yr-• of thePOC poolin this
Blanchard,D.C., Bubble scavengingand the water-to-air transfer of organicmaterial in the sea,Adv. Chem.Set.,145, 360-385, 1975.
layer stretchingfrom the surfacedown to 300 m. However,if Blanchard,D.C., Bacteriaand other materialsin dropsfrom bursting we were to consideronly the DOC pool within the depthscale bubbles,in Climateand Health Implicationsof Bubble-Mediated SeaAir Exchange,edited by E. C. Monahan and M. A. Van Patten,pp. for large bubble influence(D = 1.4 m), we would obtain a 1-16, Conn. Sea Grant Coll. Program, Groton, Conn., 1989. very different picture. Since neither the DOC nor the POC concentrationis constantwith depth, we need to have repre- Blanchard,D.C., and L. D. Syzdek,Bubbletube: Apparatusfor determiningrate of collectionof bacteria by an air bubble rising in sentativevaluesfor thesetwo poolsin the uppermost1.4 m of water, Limnol. Oceanogr.,19, 133-138, 1974. the ocean. We assume that DOC concentrations are, on aver-
Blanchard,D.C., L. D. Syzdek,and M. E. Weber, Bubble scavenging
of bacteriain freshwaterquicklyproducesbacterial enrichmentin age,100/xM (summarizedbyPeltzerandHayward[1996])and airborne drops,Limnol. Oceanogr.,26, 961-964, 1981. that POC represents10% of the DOC pool (10 /xM) in the Carey, W. M., J. W. Fitzgerald,E. C. Monahan, and Q. Wang, Mealayer stretchingfrom 0 to 1.4 m. (These concentrationsare surementof the soundproducedby a tippingtroughwith freshand equivalentto 0.6 and 0.06 Gt of DOC and POC, respectively, salt water, J. Acoust. Soc. Am., 93, 3178-3192, 1993. for the globalocean.)Hence the rate of DOC bubblescaveng- Carlson, C. A., H. W. Ducklow, and A. F. Michaels, Annual flux of
ing(1.9Gt yr-•) wouldresultin an annualthreefoldturnover of the DOC pool and thirtyfold turnoverof the POC pool in the depth of influenceof large bubbles(D). Thus it would appearthat the role of bubblesshouldnot be ignoredin future investigations and modelsof DOC and POC dynamicsin nearsurface waters.
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Hayami, S., and Y. Toba, Drop productionby burstingof air bubbles of POM at 100 m is estimatedto be of the order of 8 Gt yr on the sea surface, 1, Experiments at still sea water surface,J. [Yamanakaand Tajika, 1997].A future challengeto oceanogOceanogr.Soc.Jpn., 14, 1-6, 1958. rapherswill be to elucidatewhat fraction of the POC formed Houghton,J. T., G. J. Jenkins,and J. J. Ephraums,ClimateChange: by bubble scavengingof DOC becomespart of the sinking TheIPCC ScientificAssessment, CambridgeUniv. Press,New York, 1990. POC flux and what fraction is recycledin near-surfacewaters.
Acknowledgments. H.G.D. wishesto acknowledgethe supportfor his researchprovidedby the National ScienceFoundationvia grant OCE-9725976.
E.C.M.
wishes to thank P. van Patten and the Connect-
icut Sea Grant Office for assistance in the preparationof this manuscriptand the accompanying figure.The authorsthank S. Rambergof ONR for callingto their attentionthe work of R. Skopand R. Skopfor providingvaluable reprints of his papers. We thank N. Ivosevicfor guidanceon the adhesionpropertiesof surfactantsand for pointing
outtheworkbyV. •;uti• andcolleagues. E.C.M.thanks G. Deanefor providingclarificationsregardinghis publishedbubblespectra.
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