May 15, 2000 - ages of DOC in the deep N. Atlantic and N. Pacific Oceans are. 4,000 [Bauer et ... signatures [Williams and Gordon, 1970] indicate that oceanic.
GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 10, PAGES 1495-1498, MAY 15, 2000
Radiocarbon distributions in Southern Ocean dissolved and
particulate organic matter Ellen R. M. Druffel
Department of EarthSystemScience,Universityof Californiaat Irvine,Irvine,CA James E. Bauer Schoolof Marine Sciences,Collegeof William & Mary, GloucesterPt, VA
Abstract. Dissolved organic carbon (DOC)is the largest actively exchangingpool of organic carbon in the ocean, yet
defined, but include bacterial utilization [Carlson and Ducklow, 1996; Cherrier et al., 1996; Williams and Carlucci,
its sources andsinksarenotwellconstrained. The average•4C 1976], photochemicaldegradation[Moppet et al., 1991] and agesof DOC in the deepN. Atlantic and N. Pacific Oceansare 4,000 [Bauer et al., 1992; Druffel et al., 1992] and 6,000 years [Williams and Druffel, 1987], respectively, and representthe beginning and end of the deep ocean conveyor [Broecker, 1991]. Here we reportthat the deep SouthernOceanDOC has a
sorption to particles [Druffel and Williams, 1990; Santachi, 1991].
The2000-year•4Cagedifference between deepN. Pacific anddeepN. AtlanticDOCis largerthanthe 1500 year•4Cage differencefor DIC, possibly owing to excess 'young' IX)C
14Cage(5,600 y) muchcloserto that of the deepN. Pacific, from higher river inputs to the Atlantic [Opsahl and Bennet, but its concentrationin seawater(41_+2taM) is nearly equalto that of the deep N. Atlantic. The radiocarbon and concentrationdata indicate that most, but not all, deep DOC is transported conservatively with the ocean's conveyor. A younger (post-bomb) sourceof DOC to the N. Atlantic is the most likely explanation for the large age difference we observe between deep DOC in the Atlantic and Southern Oceans. Other possibilities are a source of older IX)C or a smaller microbial sink in the S. Ocean, or perhapsa possible slowdownof S. Ocean deep water formation during the past century [Broecker et al., 1999].
1997] or higherfluxesof 'young' (post-bomb)organicmatter from the euphotic zone in the Atlantic [Deuser et al., 1988;
Druffel et al., 1992]. It hasbeen hypothesizedthat most deep DOC is transportedconservativelybetweenthe N. Atlantic and the N. Pacific via the deepconveyorbelt [Druffel et al., 1992].
Measurements of the•4Cageandconcentration of deepIX• in the S. Ocean, located between these two end members, are
essential to test this hypothesis. Methods
Radiocarbonin DOC, DIC and suspendedparticulateorganic
carbon(POCsusp > 1 tamdiameter)wasmeasured in samples
Introduction
collected from 22 - 24 depths between 3 and 5441 meters in
The•4Cagesof DOCaresignificantlyolder(by 3300-3800 the S. Ocean in December 1995 -January 1996. This area is •4Cy)than thoseof dissolved inorganiccarbon(DIC) in the dominatedby westwardflowing surfacewatersthat have a weak samewatersof the deep (> 1500 m depth) N. Atlantic and N. PacificOceans.This is dueto recyclingof DOC on century-tomillennial timescales [Druffel et al., 1992; Williams and Druffel, 1987]. Most deepDOC is believed to be utilized at
thermocline, even in summer. Deep waters include Pacific Deep Water (1000-2500 m, low oxygen, high silica), the core of the N. Atlantic Deep Water (2500-4500 m, high salinity,
extremely low rates by free-living bacteria [Barber, 1968], whichlikely accountsfor the decreasein DOC in the deepsea from about 48 tam in the GreenlandSea to 34 tam in the N. Pacific (49øN) [Druffel et al., 1992; Hansell and Carlson, 1998]. Global riverine inputs alone could account for observed •4C residence times of DOC in the world oceans
hundredmetersof the bottom (cold and dense) [Reid, 1986]. Sampleswere treated and oxidized accordingto previously describedtechniques(seeFigure 1a,b caption)and the resultant
[Hedgeset al., 1997], however, lignin content [MeyersSchulte and Hedges, 1986; Opsahl and Bennet, 1997],
1987] on cobaltcatalystfor •4C analysisusing accelerator
low silica) and Antarctic Bottom Water found within a few
CO2extracted andpurified.TheCO2fromthesesamples was split~ 1'10,withthe smallfractionfor •5•3Canalysisandthe
large fraction converted to a graphite target [Vogel et al.,
mass spectrometry (AMS) at the Center for AMS Res. at the Statistical molecularcomposition[Hedgeset al., 1992] and •5•3C Lawrence Livermore National Laboratory. signatures[Williams and Gordon, 1970] indicate that oceanic uncertainties(1 o) for individual AMS A•4C measurements IX• contains only small amounts of identifiable terrestrial ranged from + 5 to + 7%0; the total uncertainty for DIC A•4C valueswas_+6%0,andfor DOC andPOC A•4Cwas_+13%o. organic matter. Instead, autochthonousmarine production in surfacewaters is believed to be the primary source of the standing IX)C pool in the oceans [Williams and Druffel, Results 1987]. The ultimate sinks of IX• are even more poorly
TheDOCA14Cvaluesfromthe S. Oceansamples (Fig. l a) Copyright2000 by the AmericanGeophysicalUnion.
rangedfrom-366%0 at 3 m depthto -517%oat 2876 m depth.
TheseA•4Cvaluesare the lowestyet observedfor surface
Papernumber1999GL002398.
waters. The weak thermocline at high latitudes [Gille, 1999]
0094-8276/00/1999GL002398505.00
allows•4C-poor, deepDOCto mix verticallyalongsurfaces of 1495
1496
DRUFFEL AND BAUER: RADIOCARBON IN SOUTHERN OCEAN ORGANIC MATTER
4c (%ø) -600
-400
-200
4c (%,) o
200
-2oo
0
-lOO
o
lOO
200
r •' 'r 1 'r 1
lOOO
1000-
--•
_~
_
_
2000-
2000 Depth (m)
"i
DIC
..J
Depth (m)
_
,
3000-
3000 ,
_
..i
4000
_
4000
_
_
_
,
_
DOC
5000
_
_
5000 _,
.... _
_.• -i
_
*-i
'*i _
6000
_
6000
Figure1. a) A14C of Dec andDIC in samples collected fromStationFLUSH(54øS,176øW)in the S. Ocean fromDecember 1995 -January1996 (filled andopencircles,respectively),fromthe N. CentralPacificsite (31øN,159øW)fromJune- July 1987(openandfilledtriangles, respectively)[Druffel et al., 1992]andfrom the Sargasso Seasite(33ø50'N63ø30'W)in May - June1989(openandfilleddiamonds, respectively)[Bauer et al.,
1992;Druffelet al., 1992]. Thedashed line showsthe A•4Cvaluesfor seawater DIC collectedduringthe GEOSECS expedition,Stn 290 (nearthe FLUSHsite) at 58øS,174øWon 25 February1974 [Ostlundand Stuiver,1980]. The Dec samples(0.65 liters seawater) werefrom seawater that wasacidifiedto removeDIC andoxidizedusinghighenergy,UV-oxidation[Baueret al., 1992;WilliamsandDruffel, 1987]. The recovery of Dec usinghigh-energy UV oxidationhasbeencompared to othermethods(e.g., high-temperature catalytic oxidation,HTCO), andthe two methodshavebeenfoundto yield identicalDec concentrations for deepPacific waters[PeltzerandHayward,1996;Sharpet al., 1995]. In addition,recentresultsfroma globalsurveyof deep oceanDOC usingHTCe [HansellandCarlson,1998] showthat concentrations obtainedby this methodare within lgM of our valuesobtainedby UV oxidation for latitudesmatchingthose of our three sites. The analyticalprecisionof the UV methodis _+1gM Dec, andindependent replicatesampleanalysestypically agreeto within 2 }aM [Baueret al., 1998; Baueret al., 1992]. The DIC samples(0.5 liters seawater)were acidifiedandstrippedof CO2 by recirculated nitrogengas[Masielloet al., 1998;McNicholet al., 1994].
b) Alncof POCsusp in samples collected fromtheS. Ocean (filledcircles), theN. CentralPacificsite(open triangles)[Druffelet al., 1992] andthe Sargasso Sea (open triangles)[Baueret al., 1992; Druffel et al., 1992]. Suspended POC wascollectedusingin situ pumpswhichpumpedseawaterfor 2-8 hoursthrough4 quartzfiber
filters(0.8 gmporediameter)mounted in PVC holders[Williamset al., 1980]. The filters with POCsusp were acidified,driedandcombusted in doublequartztubesat 850øC [Druffelet al., 1992]. The concentrationsof POC (gg/1) at each depth were determinedfrom the manometricmeasurementof CO2 gas obtained after combustion of each filter and the liters of water filtered. Radiocarbon measurementsfor all samples are
madeby C. E. Franks on the VG Micromass 602E isotope ratio mass spectrometerin the laboratory of L. Keigwin at WHOI.
constant density into the surfacewaters. This range in DOC
1950s and early 1960s, has penetratedboth dissolvedcarbon
A•4Cvaluesfromsurface to deepin the S. Ocean(151%o)is poolsto a depthof > 1000 m (pre-bomb,DIC A•4Cvaluesin similarto thatobserved for the DIC A14Cvalues(201%o)(Fig. surfaceand deep waterswas - -160%0[Linick, 1980]). la), thoughthe DOC A•4Cvaluesare 340-390%0lower.The In contrast, POCsusp A•4Cvalues (Fig.lb) ranged from32%0 similaritiesin the shapesof the DOCandDIC A•4C profiles at 16 m (= surfaceDIC A•4Cvalues)to -111%oat 5391 m (50 m showthatbomb•4C,produced in the stratosphere in the late above bottom), indicatingthat even the deepestPeCsusp
DRUFFEL
AND BAUER: RADIOCARBON
IN SOUTHERN
OCEAN ORGANIC
MATTER
1497
samplescontainedsomepost-bombcarbon. Concentrations
of PeCs.sp in surface (3-85 m) waters(2.6 - 10 pM) werethe
o
highest we have observedin the open ocean and were nontrivial comparedto the surfaceD(• pool (51 pM). Deep values,however,averaged0.08pM which are similar to other openoceansites[Druffel et al., 1992].
o o
-400
ø ø
ø
o -450 0
Analysis
Thedecrease in A14C of POCsusp withdepthmaybe dueto incorporation of 'old' DOC onto POCsusp by heterotrophic uptakeor sorption[DruffelandWilliams,1990] (Figure1b). This scenariorequires-14% of POCsusp in the mid-water column(2800m, A•4C= -38%0)to consistof old DOC (with a
-500
. * .*. * ø %øø ß
,,.•'
'55032 34 36 38 40 42 44 46 [DOC]
pM
A•4C= -500%0). Othercombinations of phasesand A values canproduce theobserved reduction in A•4Cof PeCsusp.Figure2. A14Cof deep(>1500m)Dec versusconcentration The quantitativeimportanceof DOC sorptiononto POCsusp.of DOC in the Sargasso Sea (open diamonds), N. Central while unknown, has been speculatedto have a significant
Pacific (open triangles) [Druffel et al., 1992] and the S. Ocean
impacton the A•4Cof the POCpool [Druffelet al., 1996'
(filled circles, this work).
A•4C value-_ surfacePOC -_ surfaceDIC = +20%0), then the
than 100% yield.
The curve is the calculated
relationship betweenDOCconcentration andA•4C,assuming Druffel and Williams, 1990]. that DOC •4Cagesat a constantrate from the averageN. Vertical mixing between the local surface and deep Atlanticvalue(-390%0,4000 •4C years)to the averageN. reservoirsadequately describesmassand isotope balancesof Pacificvalue(-525%0,6000 •4Cyears). The line is slightly DOC in the three oceans. For example, if surfaceDOC in the curved dueto thenaturallogrelationship between AIacandinc S. Oceanis comprisedof a mixtureof deepDOC (41 pM with age. The low DOC concentration for the S. Ocean point average A14C of-500%0> 1500 m depth)andmodem,surface- obtained from the water sample at 1700m shows a value of less produced DOC (51 I.tM minus41 pM = 10 pM, with an average 35.5 glVIanda A•4C= -483%0,andprobablyrepresents
totalsurface DOC(51 pM) wouldhavea A14Cvalueof -398%0 (41/51.(-500) + 10/51.(20)). This agreeswith the averageof An important clue is the small but significant lowering of threeobservedDOC A•4C valuesin the upper30 m of the mixedlayer, -372 _+5 (sd)%0. Similar calculationsreported themeandeepDOCA•4Cbetweenthe S. OceanandN. Pacific
in previously for theN. AtlanticandN. Pacificrevealed Alac (25%0or 4-0014Cyrs), whichis closeto the shiftobserved values for surface DOC of-214%o and-206%0, compared to the DIC 14Cage for S. Ocean-Pacific deepwatertransit Thus, constant measuredsurfacevalues of -210%o and -179%o, respectively [Fiadeiro, 1982; Stuiver et al., 1983]. [Druffel et al., 1992] (Fig. 1a). remineralization or sorptivelossesin this sector,with aging
of •4Cduringremineralization, wouldbe WhendeepDOC A•4Cvaluesareplottedas a functionof andnon-fractionation DOC concentrations for our three sites(Fig. 2), it appearsthat either the S. Oceanvaluesdepartfrom the curve(shown) that
consistent with our observations.
linksthedeepN. AtlanticandN. Pacific values,or that the N. Atlanticvaluesdepartfrom a curve(not shown)that links the deep N. Pacific and the deep S. Ocean. Either DOC concentrationsin the S. Ocean are high, or those in the N.
comparedto starting plankton organic matter (- 18 to -
20%o)(unlike theN. Pacificor S. Ocean)remainenigmatic,but may reflecta relatively largercontributionof terrigenousC3 plant material(8•3C=-28 to -30%o)to the Atlantic. This
Atlantic
materialcarrieswithit a significantlyelevatedA•4Csignature
are low.
Thelower8•3Cvaluesin N. AtlanticDOC (-21 to -22%0)
On one hand, if remineralization and aging alone of deep [Hedgeset al., 1986;Raymondand Bauer, 1998]. Alternatively, the AI4cof S. OceanDOCcouldbe too low, DOC had occurredduring its transport via the deepconveyor, we wouldhaveexpectedthe DOC concentrations in the deepS. andexplainedeitherby a 3 pM (-8 %)addition of 'dead'DOC
Oceanto havebeenlower(andperhaps theA•4Cvaluesto have (A14C=-1000%o) to S. Ocean deepwater(originally-460%o, been higher) than those observed, i.e., closer to the curve shownin Fig. 2. Bacterialconsumptionlags phytoplankton productionof organic matter in the S. Oceanwater column [Karl, 1993], which arguesfor DOC concentrationsthat are too high in the S. Ocean.
39 pM) or a >8-17% additionof a very 'old' fraction of the Dec (>-1000to -700%o)(Fig. 2). Sourcesof low A•4Ccarbon to the S. Oceancould include black (elemental) carbon whose
AI4cvalues(-500to -900%o) aresignificantly lowerthanthe
bulk organic carbon (-200 to -700%o) in the same sediment Ontheotherhand,deepN. AtlanticDOCAlaCvaluesmay horizons (0-50 cm) from our S. Ocean site [Masiello and be highbecauseof riverineinputsof DOC to the Atlantic that Druffel, 1998]. Someblack carbon is submicronin size, thus are4- and6-fold greaterper unit volumethan to the Pacific or it could contributeto the low Dec A14Cvalues in the S. Ocean S. Oceans[Sverdrupet al., 1942], respectively.Aging of DOC [Masiello and Druffel, 1998]. Inputs from the Antarctic
duringits transitfrom the N. Atlanticto the S. Ocean(1600 continentalmarginmay also significantlyinfluencethe A14C
•4Cyrs) appears too largecompared to the watertransittime of Dec andPec in the S. Ocean,becauselow A14Cvalues were determined fromDIC •4Cagedifferences (700 14Cyrs) [Stuiver found in organic carbon from surface sediments of the et al., 1983]. Selectivereplacementof youngerDec, removed by remineralization, photo-oxidation or some other process(es), with older DOC couldaccountfor the large age decrease, but wouldrequirea nearly 1'1 exchangeof DOC in order to maintain
similar concentrations.
continentalslope in the Ross Sea (-500 to -700%o)[DeMaster andRagueneau,1996]. RecentA•4Cmeasurements andmass balance calculations off the east and west coasts of the U.S.
suggest that'old'Dec andPOCsu,inputsfromoceanmargins to the open ocean interior may be greaterthan inputs of
recently produced organic carbon derived from the surface ocean[Bauerand Druffel, 1998]. Another possibility may be relatedto the recent report involving the possible slowdown of S. Ocean deep water formation during the 20th-century suggestingthat ventilation of this deep water mass is episodic rather than in steady state [Broecker et al., 1999]. That DOC in this region could be agedlonger at depth over the past few hundredyears may be the reason for the old DOC (Druffel and Bauer, in prep).
Furtherstudies,includingcompound specific•4Canalysis [Eglinton et al., 1997], will help to determine the relative importance of selective replacement of young DOC by old DOC betweenthe N. Atlantic and S. Ocean, inputs of old DOC in the S. Ocean and net remineralization
losses between
the S.
Oceanand N. Pacific in the deepocean. Acknowledgments. We thank Sheila Griffin and Dave Wolgast for field, analytical and laboratory support. Mark Schrope, Cartie Masiello, Ai Ning Loh, Bob Wilson, Michael Wolgast, Michelle Taranto and the officers and the crew of the R/V Melville helped with sampling, Mousamy Khan and Eben Franks with analytical support and Carrie Masiello, Cindy Lee, Rick Jahnke and two anonymousreviewers provided helpful commentson earlier versions of the manuscript. This work was supported by the Chemical Oceanographyprogram of the U.S. National ScienceFoundation.
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(Received September 16, 1999; revisedFebruary24, 2000;; acceptedMarch 24, 2000.)
