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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