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SCIen IFIC REPORTS | 7: 10129 | DOI:10.1038/s41598-017-10974-y
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O ganic ca bon budge o he
eas e n bounda y o he No h
A lan ic sub opical gy e: majo
ole o DOC in mesopelagic
espi a ion
Ye ay San ana-Falcón
1, Xosé An ón Ál a ez-Salgado2, Ma ía Dolo es Pé ez-He nández
1,3,
Alonso He nández-Gue a1, E an Mason
4 & Ja ie A ís egui1
T anspo s o suspended pa icula e (POCsusp) and dissol ed (DOC) o ganic ca bon a e in e ed om
a box-model co e ing he eas e n bounda y o he No h A lan ic sub opical gy e. Co esponding
ne espi a ion a es (R) a e ob ained om a ne o ganic ca bon budge ha is based on he anspo
es ima es, and includes bo h e ical and la e al luxes. The o e all R in he mesopelagic laye
(100–1500 m) is 1.6 ± 0.4 mmol C m−2 d−1. DOC accoun s o up o 53% o R as a esul o d awdown
o o ganic ca bon wi hin Eas e n No h A lan ic Cen al Wa e (ENACW) ha is en ained in o sinking
Medi e anean O e low Wa e (MOW) ha leads o o ma ion o Medi e anean wa e (MW) a
in e media e dep hs (~900 m). DOC ep esen s 90% o he espi ed non-sinking o ganic ca bon. When
con e ed in o oxygen uni s, he compu ed ne espi a ion a e ep esen s less han hal he oxygen
u iliza ion a es (OUR) epo ed o he mesopelagic wa e s o he sub opical No h A lan ic. Mesoscale
p ocesses in he a ea, no quan i ied wi h ou app oach, could accoun in pa o he OUR di e ences
obse ed be ween ou ca bon budge and o he published s udies om he No h A lan ic, al hough
seasonal o in e annual a iabili y could also be esponsible o he di e ence in he es ima es.
Oceanic espi a ion occu s o e he en i e wa e column1, 2, as opposed o pho osyn hesis, which is es ic ed o
he epipelagic laye ( oughly abo e 150 m). The non- espi ed ac ion o he biogenic o ganic ca bon p oduced
in he epipelagic laye is suscep ible o downwa d expo in he o m o dissol ed (DOC) and pa icula e (POC)
o ganic ca bon, suppo ing espi a ion in he da k ocean2, 3. A he global scale, he ela i e con ibu ion o DOC
o oxygen consump ion in he da k ocean has been es ima ed o be 10–20%4, 5, bu i may inc ease signi ican ly in
egions o deep-wa e con ec ion5–7. Pa adoxically, he sum o DOC and sinking POC collec ed wi h sedimen
aps does no equen ly accoun o he es ima ed da k ocean espi a ion a es8–10. This con lic ing imbalance
could be a consequence o me hodological unce ain ies in he es ima ion o espi a ion a es and/o ca bon
luxes10–12. Fo ins ance, he bulk o pa icula e ma e ial in he wa e column is composed o slow-se ling o sus-
pended pa icles13–15 ha can escape om sedimen aps due o hei buoyancy16. Se e al s udies ha e sugges ed
ha suspended pa icles a e an impo an sou ce o o ganic ma e o p oka yo ic o ganisms in he mesopelagic
zone17, 18. The e o e, suspended pa icles would ep esen an unquan i ied sou ce o “missing” o ganic ca bon,
especially in a eas whe e la e al ad ec ion is in ense, such as in Eas e n Bounda y Cu en sys ems19. Indeed,
i has been al eady shown ha , using a box-model app oach, suspended pa icula e o ganic ca bon (POCsusp)
could suppo up o 59% o he o al mesopelagic espi a ion in he sou hwes e n sec o o he Cana y Cu en 20,
a egion s ongly a ec ed by he coas al-ocean expo o pa icula e ma e ial om he NW A ican coas al
upwelling sys em21, 22.
1Ins i u o de Oceanog a ía y Cambio Global, IOCAG, Uni e sidad de Las Palmas de G an Cana ia, ULPGC, 35017, Las
Palmas de G an Cana ia, Spain. 2CSIC, Ins i u o de In es igaciones Ma inas, Edua do Cabello, 6, 36208, Vigo, Spain.
3Depa men o Physical Oceanog aphy, Woods Hole Oceanog aphic Ins i u ion, Woods Hole, Massachuse s, USA.
4Ins i u o Medi e áneo de Es udios A anzados, CSIC-UIB. C. Miquel Ma quès, 21, 07190, Espo les, Illes Balea s,
Spain. Co espondence and eques s o ma e ials should be add essed o Y.S.-F. (email: ye ay[email p o ec ed])
Recei ed: 26 Ap il 2017
Accep ed: 14 Augus 2017
Published: xx xx xxxx
OPEN
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SCIen IFIC REPORTS | 7: 10129 | DOI:10.1038/s41598-017-10974-y
In he p esen s udy, ollowing he app oach used by Alonso-González and cowo ke s in he sou he n Cana y
Cu en 20, we es ima e he con ibu ion o DOC and POCsusp luxes o ne espi a ion in he eas e n bounda y o
he No h A lan ic sub opical gy e (Fig.1). Ou o e a ching hypo hesis is ha , con a y o he sou he n Cana y
Cu en whe e POCsusp is an impo an componen o he ca bon budge 20, 23, he DOC con ibu ion o o al
espi a ion in he s udy a ea is la ge han ha o POCsusp. The wo a eas a e oceanog aphically dis inc . On he
one hand, he coas line o ien a ion in ou egion is un a o able o coas al upwelling, which is absen o limi ed
jus o he summe mon hs24, 25. On he o he hand, ou egion is cha ac e ized by he densi y-d i en exchange
o wa e masses be ween he Medi e anean Sea and he eas e n No h A lan ic26. As a consequence, Eas e n
No h A lan ic Cen al Wa e (ENACW) is en ained in o he dense as - lowing Medi e anean O e low Wa e
(MOW) ha exi s h ough he S ai o Gib al a 27–29 and is subduc ed along he slope in o he Gul o Cádiz (ca.
33–37°N)30–33 o below 1000 m dep h, whe e i sp eads in o he A lan ic as Medi e anean Wa e (MW)27–29, 34–37.
This en ainmen o ENACW in o MOW may p omo e he downwa d lux o DOC o in e media e laye s in his
pa icula egion, as has al eady been shown o an h opogenic ca bon dioxide38.
In a p e ious s udy39, based on a ough es ima ion o o ganic ca bon luxes in a box egion ha included he
S ai o Gib al a , i has been sugges ed ha DOC emine aliza ion could explain abou 90% o he ino ganic
ca bon p oduced in he wa e column. Ou s udy es s his hypo hesis, p o iding di ec measu emen s o POCsusp
and DOC luxes o in eg a ion wi hin a box-model analysis. We aim o con ibu e o he unde s anding o he
o ganic ca bon dynamics in his complex egion, which is cha ac e ized by he con luence o wa e masses o
di e en o igin, wa e mass en ainmen and subduc ion, and in e mi en upwelling24, 40–43.
Resul s
O ganic ca bon dis ibu ions. The DOC and POCsusp da a used in his wo k we e collec ed du ing a
esea ch c uise in au umn 2009. The c uise consis ed o a se o hyd og aphic s a ions dis ibu ed egula ly along
h ee ansec s (no he n, wes e n and sou he n) ha delimi he box p esen ed in Fig.1 (see Ma e ials and
Me hods o u he de ails on sampling and analysis). POCsusp and DOC dis ibu ions om 0 o 4000 m (Fig.2)
show ha su ace POCsusp concen a ions along he no he n ansec dec ease o sho e om 4–6 µmol C L−1 in
he coas al a ea o abou 2 µmol C L−1 in he open ocean. Below 500 m POCsusp concen a ions a e 1 µmol C L−1
o less, excep a ound a seamoun a ~14° W. Wa e s wi h ela i ely high POCsusp deepen a s a ion 30. Su ace
DOC concen a ions a e highe han 60 µmol C L−1 o he en i e ansec . A deepening o DOC occu s a s a-
ions 30–37.
Su ace POCsusp concen a ions along he wes e n ansec a e highe han 1–2 µmol C L−1. A deepening o
POCsusp- ich wa e s a s a ions 56–58 coincides wi h he shoaling o isoneu al su aces be ween wo mesoscale
Figu e 1. Loca ion o he CTD s a ions. Biogeochemical s a ions a e numbe ed. AC, ACC, CanC, and PC e e
o Azo es Cu en , Azo es Coun e -Cu en , Cana y Cu en , and Po ugal Cu en . AI and MO indica e he
su ace A lan ic in low and bo om Medi e anean o e low. Blue/ ed a ows indica e anspo in o/ou o he
s udy egion. S a ion 75 is a he ESTOC si e. The map was p oduced using M_Map oolbox (h ps://www.eoas.
ubc.ca/~ ich/map.h ml) on MATLAB R2013a, h ps://www.ma hwo ks.com/p oduc s/ma lab/. Global ETOPO
is used o ba hyme ic da a (h ps://www.ngdc.noaa.go /mgg/global/).
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eddies44. POCsusp concen a ions emain a ound 1 µmol C L−1 along he es o he sec ion wi h li le a ia ion.
DOC is abou 60–70 µmol C L−1 nea he su ace, and dec eases o less han 50 µmol C L−1 wi h dep h. Cen al
wa e s wi h high DOC concen a ions descend along sloping isoneu als a s a ions 53–56. The deepening o
DOC no h o 35°N coincides wi h he loca ion o he Azo es Cu en sys em44.
The sou he n ansec shows su ace POCsusp concen a ions o 2–3 µmol C L−1. A wes wa d dec ease is
obse ed below 250 m. Su ace DOC concen a ions show alues abo e 70–80 µmol C L−1. A pa ch o ela i ely
high POCsusp below 1000 m dep h coincides wi h cool and esh wa e s44. DOC shows alues abo e 70–80 µmol
C L−1 a he su ace, wi h maximum concen a ions abo e 100 µmol C L−1 a subsu ace dep hs (~200 m) in
he eas e n s a ions close o he Lanza o e Passage. In e media e wa e s (below he 27.38 isoneu al) show low
DOC concen a ions, excep a s a ions 64–65 whe e a on be ween a cyclonic eddy and he sou hwa d Cana y
Cu en is pe iodically obse ed44.
Mass anspo s. Mass anspo s ac oss he no he n, wes e n, and sou he n ansec s a e aken om a p e-
ious s udy by Pé ez-He nandez and co-wo ke s who de eloped an in e se box model wi h da a om he same
c uise44 (see Ma e ial and Me hods o u he de ails abou he in e se model and i s eliabili y). A summa y o
he in eg a ed la e al mass anspo a each ansec is p esen ed in Fig.3. No e ha luxes in o/ou o he box a e
indica ed by posi i e/nega i e signs, espec i ely.
A p edominan inwa d (sou hwa d) anspo is obse ed ac oss he no he n ansec (~37°N; blue line),
in e up ed only by he ou wa d (no hwa d) anspo o in e media e wa e s (isoneu als 27.38 o 27.92). The
mass anspo ac oss he wes e n ansec (~24°W; ed line) is s ongly inwa ds (eas wa d) in he su ace and
Figu e 2. Ve ical sec ion (0–4000 m) o POCsusp (µmol C L−1; uppe panels) and DOC (µmol C L−1; lowe
panels) o he no he n (le ), wes e n (middle), and sou he n ( igh ) ansec s. Biogeochemical s a ions a e
indica ed a he op o he uppe panels. G ay do ed lines indica e neu al densi y.
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cen al wa e s, and p edominan ly ou wa ds (wes wa d) below. The mass anspo ac oss he sou he n ansec
(~29°N; g een line) is ou wa ds (sou hwa d) in he uppe 700 me e s o he wa e column, while he low o in e -
media e and deep wa e s a ies by less han ±2 S .
POCsusp and DOC anspo s. O ganic ca bon anspo s along isoneu als a e calcula ed by mul iplying
he co esponding POCsusp and DOC concen a ions (Fig.2) by he mass anspo s (Fig.3) (see Ma e ials and
Me hods o mo e de ails on he es ima ion and obus ness o he ca bon anspo s). Fo he case o DOC,
anspo s o he non- e ac o y pool DOC (e-DOC) a e calcula ed. The e-DOC pool is ob ained by sub ac ing
a backg ound e ac o y DOC concen a ion (~45 mmol C m−³ in he s udy a ea) om he measu ed DOC. We
only conside he e-DOC ac ion because he e ac o y ac ion enewal is housands o yea s45, 46, a pe iod
much longe han he decadal enewal ime o he s udy box.
POCsusp and e-DOC anspo s in he a ea (Fig.4) may be di ided in o a sou hwa d lux o su ace and cen al
wa e s, in e up ed by a weak no hwa d anspo o he uppe in e media e wa e s, ha is obse ed bo h a he
no he n and sou he n ansec s. Sou hwa d anspo p e ails again in he deepes wa e s. The zonal ci cula ion
is p edominan ly eas wa d a he su ace and cen al wa e s. The end e e ses in he in e media e and deep lay-
e s whe e a wes wa d anspo is obse ed, especially a he bo om o he in e media e laye .
POCsusp and e-DOC luxes in he su ace, cen al, in e media e, and deep wa e s a each ansec a e summa-
ised in Table1. The inpu s (posi i e alues) o o ganic ca bon h ough he no he n and wes e n ansec s exceed
he ou pu s (nega i e alues) o he sou he n ansec a bo h su ace and cen al laye s. Weak ho izon al ou pu
luxes a e ound bo h in he in e media e and he deepes laye s.
Discussion
To e alua e he o ganic ca bon sou ces wi hin he box we build a mass balance (Fig.5a) and an associa ed o ganic
ca bon budge (Fig.5b). Fo he su ace wa e s (0–100 m), 1.2 ± 0.1 S en e he box h ough he no he n, sou h-
e n and wes e n ansec s, and −0.78 ± 0.1 S lea e he box as he A lan ic in low in o he Medi e anean Sea (see
Ma e ials and Me hods). As a esul , he mass balance o inpu s minus ou pu s o he box would be 0.4 ± 0.2 S .
We p opose ha his olume is anspo ed om he su ace o he cen al wa e s o pa ially balance he loss o
ENACW o in e media e wa e s when en ained in o he MOW (see below he discussion abou he mass balance
o he cen al wa e s). The balance o o ganic ca bon, i.e. POCsusp and e-DOC, anspo ed ac oss he no he n,
wes e n and sou he n ansec s indica es ha he su ace wa e s o he box ecei e an ex e nal inpu om he
Figu e 3. In eg a ed mass anspo s o he no he n (blue diamonds o e a dashed line), wes e n ( ed
c osses o e a do ed line) and sou he n (g een ci cle dashed line) ansec s. The sign o he ne anspo is
nega i e/posi i e o di e ging/con e ging low ou o /in o he box. No h A lan ic Cen al Wa e (NACW),
Medi e anean Wa e (MW), sub-A ic In e media e Wa e (SAIW), An a c ic In e media e Wa e (AAIW),
Lab ado Sea Wa e (LSW), and No h A lan ic Deep Wa e (NADW) wa e masses a e indica ed bo h by colo
and ac onym. Y-axis is neu al densi y.
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A lan ic Ocean o 30.0 ± 2.0 × 108 mol C d−¹. A he same ime, acco ding o ou es ima es (see Ma e ials and
Me hods), −11.4 ± 2.9 × 108 mol C d−¹ is expo ed wi hin su ace wa e s o he Medi e anean Sea. This numbe
lies wi hin he ange o he A lan ic in low o TOC p e iously measu ed in Ap il 1998 (7.6 × 108 mol C d−¹)47, and
in Sep embe 1997 (27.1 × 108 mol C d−¹)48, a e sub ac ion o e ac o y DOC.
Fu he mo e, he non- espi ed o ganic ca bon in su ace wa e s may be anspo ed o he da k ocean by u bu-
len di usion, passi e sinking, and ac i e anspo 49, 50. We es ima ed i s he e ical u bulen di usion a 100 m
dep h as F = −ε × R/(N2 × (1 − R)) × (DOCz2 − DOCz1)/(z2 − z1)51. We conside ed (1) he a e aged DOC in he
uppe 100 m (DOCz1 cen e ed a z1 = 50 m) and om 100 o 200 m dep h (DOCz2, cen e ed a z2 = 150 m) o he
calcula ion o he e ical DOC g adien , (2) he squa e o he B un -Väisälä equency (N2) be ween 50 and 150 m,
and (3) cons an alues o he dissipa ion a e (ε = 10−8 m3 s−2) and he Richa dson numbe (R = 0.2). We ob ained
an a e aged alue o 0.4 ± 0.3 mmol C m−² d−¹, (a e age ± SD o 47 s ns) ha would gi e a o al downwa d DOC lux
o abou 5.7 ± 3.7 × 108 mol C d−¹, when mul iplied by he su ace a ea o he box (1.4 × 106 km2).
Figu e 4. POCsusp (uppe panels) and DOC (lowe panels) luxes (108 mol C d−¹) a each ansec . Posi i e/
nega i e alues indica e inpu s/ou pu s o/ om he box. Wa e masses a e indica ed bo h by colo and ac onym:
No h A lan ic Cen al Wa e (NACW), Medi e anean Wa e (MW), sub-A c ic In e media e Wa e (SAIW),
An a c ic In e media e Wa e (AAIW), Lab ado Sea Wa e (LSW), and No h A lan ic Deep Wa e (NADW).
Y-axes is neu al densi y.
No he n T ansec Wes e n T ansec Sou he n T ansec Global
POCsusp e-DOC POCsusp e-DOC POCsusp e-DOC POCsusp e-DOC
Su ace 1.79 ± 0.02 15.29 ± 0.04 7.27 ± 0.2 85.20 ± 2.8 −7.45 ± 0.5 −70.46 ± 3.1 1.48 ± 0.2 28.51 ± 2.0
Cen al 2.52 ± 0.3 17.83 ± 2.3 0.87 ± 0.03 9.97 ± 0.7 −1.34 ± 0.1 −17.46 ± 2.9 1.93 ± 0.2 −0.12 ± 0.06
In e media e −0.12 ± 0.06 −0.39 ± 0.2 −1.03 ± 0.09 −7.73 ± 0.9 0.22 ± 0.01 0.88 ± 0.02 −0.96 ± 0.1 −7.53 ± 0.5
Deep 4.04 ± 0.4 16.04 ± 1.5 −2.41 ± 0.3 −13.24 ± 0.9 −1.68 ± 0.1 −5.95 ± 0.5 −0.24 ± 0.3 −3.79 ± 1.0
Table 1. POCsuspand e-DOC luxes (108 mol C d−1) o su ace, cen al, in e media e, and deep wa e s o he
no he n, wes e n, and sou he n ansec s. E o ba s a e gi en. Posi i e/nega i e alues indica e inpu /ou pu s
o/ om he box.

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Sinking POC luxes ob ained wi h su ace e he ed aps a 200 m dep h and ex apola ed o 150 m wi h he
Ma in equa ion52 we e app oxima ely 0.55 mmol C m−² d−¹ a he Eu opean S a ion o Time-se ies in he Ocean
(ESTOC)53. In he no heas A lan ic (39–43°N, 17–19°W), he minimum e ical POC luxes we e es ima ed
o be 0.36 ± 0.09 mmol C m−² d−¹ om d i ing aps a 200 m dep h du ing la e summe 54. Al hough he use o
d i ing aps has been equen ly c i icized o unde es ima ing expo luxes55–57, compa ison wi h POC luxes a
he Be muda Time Se ies S a ion (BATS) de i ed om bo h su ace- e he ed aps and Tho ium-23456 shows ea-
sonable ag eemen du ing low p oduc i i y pe iods. A he ESTOC si e, he lowes p oduc i i y alues ha e been
ob ained du ing au umn (~10 mmol C m−² d−¹)57; conside ing an e- a io (expo /p oduc ion a io) o 5–10%55,
he e ical POC lux du ing he less p oduc i e season would be 0.5–1.0 mmol C m−² d−¹. The e o e, an a e age
alue o he expo a e om d i ing aps o 0.36 o 1.0 mmol C m−² d−¹ is ep esen a i e o he sinking lux
om su ace wa e s wi hin he box a ha ime o he yea . Agg ega ion and disagg ega ion o sinking pa icles by
bio ic and abio ic mechanisms could modi y he sinking a es and magni ude o POC luxes wi h dep h ac oss he
da k ocean58–62, al hough ou box model app oach does no allow iden i ica ion o hese p ocesses.
In addi ion, he es ima ed a e aged e ical lux o POC media ed by mig an zooplank on (ac i e lux) was
0.22 mmol C m−² d−¹ sou h o he island o G an Cana ia63, accoun ing o 25% o he sinking POC lux (passi e
lux) in he egion. Mo e ecen ly, a mean ac i e lux o 0.28 mmol C m−² d−¹ (29% o he passi e lux) has been
es ima ed no h o G an Cana ia64; hese es ima es ag ee wi h hose obse ed a BATS (0.17 mmol C m−² d−¹)65
and he Hawaiian Ocean Time Se ies (HOT; 0.26 mmol C m−² d−¹)66. Howe e , he ac i e lux accoun ed o only
8% and 11–44% o he passi e lux a hei espec i e egions65, 66. I we assume ha he ac i e lux ep esen s a
bes 20% o he passi e lux in ou egion, he passi e + mig a o y POC lux om he su ace o he cen al wa e s
would be a ound 0.82 mmol C m−² d−¹ which, mul iplied by he a ea o ou box, yields a o al POC lux om he
su ace o he cen al wa e s o 11.4 × 108 mol C d−¹.
The cen al wa e s (100–700 m) wi hin he box ecei e an o e all la e al inpu o POCsusp and e-DOC
o 10.7 ± 1.9 × 108 mol C d−¹ om he su ounding A lan ic Ocean and 2.4 ± 1.5 × 108 mol C d−¹ om he
Medi e anean ou low h ough he S ai o Gib al a . In addi ion, i is well known ha MOW is subduc ed wi hin
he s udy box o mo e han 1000 m and ha he cen al wa e s a e en ained in o he MOW as i sinks29, 38. In ac ,
he mass balance o cen al wa e s wi hin he box (Fig.5a), conside ing he ne wa e lows ac oss he no he n,
sou he n, and wes e n ansec s (2.1 ± 0.1 S ), he Medi e anean ou low in o he A lan ic (0.67 ± 0.1 S ; see
Ma e ials and Me hods), and he inpu o 0.4 ± 0.2 S om he su ace wa e s, is 3.1 ± 0.4 S . The e o e, in o de
o main ain conse a ion o olume in he cen al wa e s, 3.1 ± 0.4 S should en e he in e media e laye by
Figu e 5. Schema ic diag am showing (a) he mass anspo and (b) he o ganic ca bon budge s in he box
egion. The wa e column is di ided in o su ace (0–100 m), cen al (100–700 m), in e media e (700–1500 m),
and deep (below 1500 m) laye s. A ows indica e ne anspo (S ) o o ganic ca bon (108 mol C d−¹) ac oss he
open bounda ies. Ligh blue indica es la e al inpu s (posi i e alues), ligh ed indica es la e al ou pu s (nega i e
alues), ligh iole indica es e ical en ainmen , ligh g een indica es e ical luxes om he laye abo e. Ne
espi a ion a es o each compa men a e indica ed.
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en ainmen o he cen al wa e s in o he subduc ing Medi e anean ou low. Ou olume calcula ion is in ag ee-
men wi h ea lie es ima es (2–3 S 67; 2.6 S 68; 1.3 S 28; 2.3 S 38). To ob ain he o ganic ca bon anspo ed by his
olume, we mul iply he a e age o ganic ca bon concen a ion a he bo om o he cen al laye by 3.1 ± 0.4 S
o ob ain a downwa d POC + e-DOC lux o 14.8 ± 3.7 × 108 mol C d−¹, 90% o which is DOC. Simila ly, we also
calcula e he o ganic ca bon anspo associa ed wi h he 0.4 ± 0.2 S o su ace wa e s ha en e he cen al
wa e s yielding 6.6 ± 2.2 × 108 mol C d−¹, 10% as POCsusp and 90% as DOC (Fig.5b).
Fo he in e media e wa e s (700–1500 m), aside om he o ganic ca bon ha en e s his laye by en ain-
men o ENACW in o he subduc ing MOW (14.8 ± 3.7 × 108 mol C d−¹), he o he o ganic ca bon anspo s
associa ed wi h he wa e mass luxes lea e his laye . The ne balance o POCsusp and e-DOC anspo ac oss he
no he n, wes e n and sou he n ansec s is −8.5 ± 0.6 × 108 mol C d−¹. This indica es an expo om he box o
he adjacen A lan ic Ocean, and is especially in ense ac oss he wes e n ansec (see Table1), con a y o he
su ace and cen al wa e s whe e o ganic ca bon was impo ed om he A lan ic Ocean. Fu he mo e, he mass
balance o he in e media e wa e s (Fig.5a) indica es ha he e should be an en ainmen o 1.3 ± 0.5 S in o he
deep wa e s o olume conse a ion, ha ansla es in o a d awdown o o ganic ca bon o 3.3 ± 1.4 × 108 mol C
d−¹, 76% o which is DOC (Fig.5b). In addi ion, an a e age pa icle lux below 700 m o 0.46 mmol C m−² d−¹ has
been ob ained om h ee si es loca ed wi hin he Cana y Cu en egion69. Assuming his alue o ou egion o
s udy, we ob ain a e ical POC supply o in e media e wa e s o 6.4 × 108 mol C d−¹.
Finally, he mass anspo balance o he deep wa e s (below 1500 m), 0.4 ± 0.4 S , is compa ible wi h mass
conse a ion wi hin he unce ain y o he es ima es. POCsusp + e-DOC anspo ed om he in e media e wa e s
by en ainmen (3.3 ± 1.4 × 108 mol C d−1), sinking POC (2.4 × 108 mol C d−1), o ganic ca bon bu ial in o he sed-
imen s (0.06 × 108 mol C d−¹)70, and he ca bon balance a he open bounda ies o he box (−4.0 ± 1.3 × 108 mol C
d−¹) yield a negligible ca bon excess o 1.6 ± 1.9 × 108 mol C d−¹. The sinking POC lux a 1500 m was de i ed
om he lux a 700 m using he Ma in equa ion52.
O e all, he o ganic ca bon balance o he su ace wa e s associa ed wi h he la e al luxes ac oss he
no he n, wes e n and sou he n ansec s (30.0 ± 2.2 × 108 mol C d−¹), losses om he S ai o Gib al a
(−11.4 ± 2.9 × 108 mol C d−¹), en ainmen (−6.6 ± 2.0 × 108 mol C d−¹), ac i e + passi e anspo luxes
(−11.4 × 108 mol C d−¹), and u bulen di usion (−5.7 ± 3.7 × 108 mol C d−¹), yields −5.1 ± 4.1 × 108 mol C d−¹.
Assuming s eady s a e condi ions, his balance esul s in a ne he e o ophy (R > 0) o abou −0.4 ± 0.3 mmol C
m−² d−¹ a he ime o his s udy.
The o e all balance o o ganic ca bon o he cen al wa e s includes he POCsusp + DOC la e al luxes
ac oss he A lan ic bounda ies o he box (10.7 ± 2.1 × 108 mol C d−¹; 82% as DOC) and he S ai o Gib al a
(2.4 ± 1.6 × 108 mol C d−¹; 62% DOC), passi e + ac i e luxes o POC om su ace wa e s (11.4 × 108 mol C
d−¹), and o in e media e wa e s (−6.4 × 108 mol C d−¹), downwa d DOC luxes by u bulen di usion
(5.7 ± 3.7 × 108 mol C d−¹), and en ainmen om he su ace (6.6 ± 2.2 × 108 mol C d−¹), and o he in e media e
wa e s (−14.8 ± 3.7 × 108 mol C d−¹). All hese o ganic ca bon luxes esul in a ne R o 15.6 ± 6.3 × 108 mol C
d−¹ (1.1 ± 0.4 mmol C m−² d−¹).
Las ly, o in e media e wa e s he POCsusp + e-DOC includes he balance ac oss he no he n, wes e n and
sou he n bounda ies (−8.5 ± 0.6 × 108 mol C d−¹), en ainmen om cen al wa e s (14.8 ± 3.7 × 108 mol C d−¹)
and in o deepe wa e s (−3.3 ± 1.4 × 108 mol C d−¹), and e ical downwa d POC luxes om cen al wa e s
(6.40 × 108 mol C d−¹) and o deepe laye s (−2.4 × 108 mol C d−¹), esul ing in a ne R o 7.0 ± 4.0 × 108 mol C
d−¹ (0.5 ± 0.3 mmol C m−² d−¹) assuming s eady-s a e condi ions.
The ne R wi hin he mesopelagic laye , assumed o be equi alen o he sum o he cen al and he in e medi-
a e wa e s, is 22.7 ± 5.2 × 108 mol C d−¹ (1.6 ± 0.4 mmol C m−² d−¹). This alue is lowe han hose ob ained wi h
a simila box model wes o he Cana y Islands20: 2.4–5.1 mmol C m−² d−¹. Acco ding o hese au ho s, POCsusp
would suppo 28–59% o he o al mesopelagic R in ha egion. Howe e , hei es ima es we e based solely on
POCsusp luxes, assuming wo scena ios in which he con ibu ion o DOC was ixed a 15 and 30%. In he p esen
s udy, we ha e measu ed DOC and calcula ed ha i con ibu es abou 53% o o al mesopelagic R wi hin he
box. I has been hypo hesized ha DOC ep esen s abou 90% o he espi a ion o non-sinking o ganic ca bon
in he mesopelagic laye om an empi ical ca bon mass balance wi hin a box bounded by he S ai o Gib al a ,
24–41° N and 22° W39. Ou esul s indica e ha DOC ep esen s ha ac ion when only he non-sinking o ganic
ca bon is conside ed, in ag eemen wi h he high DOC consump ion a es ob ained in a eas unde going wa e
mass ans o ma ion6, 71.
The es ima ed mesopelagic R is much lowe han he oxygen u iliza ion a es (OUR) calcula ed o he No h
A lan ic sub opical gy e72–74 (9–11 mmol C m−² d−¹ as con e ed using a -O2/C mola a io o 1.475). In addi ion,
a ca bon budge no h o ou box egion du ing summe 200976 es ima ed a o al inpu o o ganic ca bon o he
wiligh zone (ca. 50–1000 m) o ~7.7 mmol C m−² d−¹, due mainly o sinking POC. OUR alues anging om
4.7 o 16.6 mmol C m−² d−¹ be ween 100 o 750 m dep h we e ob ained using i ium and 228Ra me hodologies
in he No h A lan ic sub opical gy e77. Simila in eg a ed alues (4.9 o 13.0 mmol C m−² d−¹) o ca bon espi-
a ion o he 100–1000 dep h zone we e ob ained bo h by 7Be and POC lux a enua ion me hodologies in he
sub opical No h A lan ic78. A compila ion o da a om he Paci ic ocean79 shows OUR alues anging om
0.04 o 18.1 mmol C m−² d−¹. In he No h Paci ic, OUR alues in eg a ed o e he i s 600 m dep h80 a e -
aged 4.6 mmol C m−² d−¹, while highe OUR alues o 9.8 o 11.7 mmol C m−² d−¹ ha e been ecen ly epo ed
o a mon hly ime-se ies a HOT12. Values ob ained du ing he p esen s udy migh , howe e , unde es ima e
he annual mean since ou s udy ook place du ing he unp oduc i e au umn when POCsusp concen a ions a e
pa icula ly low56. Addi ionally, he egion wi hin he box is s ongly in luenced by mesoscale ea u es such as
ilamen s and eddies, whose e ec s on ca bon seques a ion emain o be quan i ied. The enhancemen o ca bon
luxes o he ocean in e io d i en by mesoscale eddies has been epo ed in a numbe o s udies81–88. Indeed,
injec ion o o ganic ca bon om su ace wa e s was especially in ense in se e al a eas, such as in he an icyclonic
eddy a he no hwes e n co ne o he box (s a ions 33–35), o he Azo es F on (s a ions 52–5544). Mo eo e ,
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upwelling ilamen s may play a key ole in he coas al-ocean anspo o o ganic ma e 89, 90. The Cape Ghi ila-
men lies wi hin he egion o s udy, and may expo o he open ocean 29 o 63% o he annual a e aged p ima y
p oduc ion associa ed wi h he coas al upwelling91, 92. All o hese ea u es occu episodically wi hin he box, and
could enhance he supply o o ganic ca bon o mesopelagic wa e s.
Nea he ESTOC si e, ne R a es a he mesopelagic zone (ca. 150–700 m) o 2.0–3.1 mol C m−² y−¹ (5.5–
8.5 mmol C m−² d−¹) we e ob ained by using a ace conse a ion model applied o clima ological da a93. They
also ob ain consis en R a es based on ETS (elec on anspo sys em) espi a o y ac i i y o he same egion a
he end o Ma ch. Di e ences be ween hese es ima es and ou alues could be due o highe la e al ad ec ion o
DOC and POCsusp om he coas al upwelling egion a ha si e56, and highe p oduc i i y a he end o he la e
win e bloom.
In summa y, his s udy p o ides an es ima e o ne espi a ion in he mesopelagic wa e s wi hin a box enclos-
ing he eas e n bounda y o he No h A lan ic sub opical gy e. This es ima e is based on he assump ion ha he
box is in s eady s a e, i.e., he e is nei he accumula ion o o ganic ca bon en e ing he box no consump ion o
p e iously accumula ed o ganic ca bon. This assump ion is accep able o he budge o he cen al, in e media e,
and deep wa e s as hei decadal enewal imes p eclude seasonali y. Howe e , seasonal a iabili y is p onounced
in he su ace laye and, hence, he s eady s a e assump ion could lead o biased alues o he annual ne luxes.
Fu he mo e, al hough he con ibu ion o mesoscale phenomena needs o be add essed in u u e s udies, ou
esul s show ha a majo ac ion o he mesopelagic o ganic ca bon demand in his a ea is uelled by DOC.
Indeed, he o ma ion o MW has been ound o be he main mechanism o he expo o an h opogenic ca bon
in his egion38. Despi e he inaccu acy o he s eady-s a e assump ion he ca bon budge p esen ed he e shows
ha he e ical en ainmen o ENACW in o subduc ing MOW con ibu es o he expo o DOC o mesopelagic
wa e s, whe e i may cons i u e he main subs a e suppo ing da k ocean espi a ion. Likewise, due o he wes -
wa d ad ec ion o in e media e wa e s, his p ocess may also ep esen a signi ican supply o o ganic ca bon o
he eas e n No h A lan ic ou side he box.
Ma e ials and Me hods
Hyd og aphy and seawa e sampling. Be ween Oc obe 15 and No embe 11, 2009 he RV Hespé ides
ca ied ou an in ensi e hyd og aphic su ey in he eas e n bounda y o he No h A lan ic sub opical gy e a
he con luence egion be ween he Azo es Cu en Sys em, he Po ugal Cu en and he Cana y Cu en . A g id
o 81 conduc i i y- empe a u e-dep h (CTD) s a ions was dis ibu ed along a box egion de ined by 28.7–37.0°N
and 24.5°W. A 47 o hese s a ions seawa e samples we e aken o analysis o dissol ed (DOC) and pa icula e
o ganic ca bon (POC; Fig.1, numbe ed do s).
Reade s in e es ed in a de ailed desc ip ion o he hyd og aphy and wa e mass cha ac e iza ion a e e e ed
o Pé ez-He nández and cowo ke s44.
O ganic ca bon. Disc e e samples o POC we e ob ained a selec ed dep hs om he su ace up o he bo -
om (5, 25, 50, 100 200, 400, 600, 800, 1000, 1500, 2000, 3000, 4000 and 5000 m) by means o a ose e sample
equipped wi h wen y- ou 10 L Niskin bo les. Al hough we a e awa e ha pa icles sink a di e en a es, we
assume ha all he POC collec ed in oceanog aphic bo les co esponds o he slowly-se ling o suspended pool
(POCsusp)20. Thus, his pool ep esen s an uppe h eshold o he pa icula e o ganic ma e ial ha is suscep ible
o la e al anspo . Wa e samples (4 L) o POCsusp we e collec ed in polyp opylene bo les and il e ed h ough
p ecombus ed (450 °C, 12 hou s) 25 mm Wha man GF/F- il e s (po e size 0.7 µm). The il e s we e w apped in
p ecombus ed aluminium oil and ozen a −20 °C. In he labo a o y, he il e s we e hawed and d ied o e -
nigh a 55 °C, hen placed o e nigh in a desicca o sa u a ed wi h HCl umes o emo e ino ganic ca bon, and
d ied again in a second desicca o wi h silica gel o a leas 24 hou s. Finally, he il e s we e packed in in slee es
be o e being analysed wi h a Pe kin-Elme 2400 CHN elemen al analyse ollowing s anda d p o ocols94. Unused
p e-combus ed GF/F- il e s we e ea ed in he same way and used as handling blanks. Blanks anged om 0.06
o 0.7 µmol C L−1. DOC adso p ion on o he il e s was es ima ed a se e al andom s a ions, a h ee di e en
dep h le els, as he amoun o ca bon e ained on a backing il e placed unde nea h he main il e . The alues
anged om 0.17 o 1.73 µmol C pe 25 mm-diame e GF/F- il e , which a e simila o p e ious es ima es20, 95, 96.
A he same s a ions and dep h le els, samples o he analyses o DOC we e collec ed in 250 mL acid-cleaned
all-glass lasks. Samples om he uppe 100 m we e immedia ely il e ed h ough p ecombus ed (450 °C, 4 hou s)
47 mm Wha man GF/F il e s in an acid-cleaned all-glass il a ion sys em. Fil e ed su ace and un il e ed deepe
wa e samples we e collec ed in 10 mL p ecombus ed glass ampules (450 °C, 12 hou s). A e acidi ica ion wi h
H3PO4, he ampoules we e hea -sealed and p ese ed in he da k a 4 °C un il analysis in he labo a o y wi h a
Shimadzu TOC-V o ganic ca bon analyse by high empe a u e ca aly ic oxida ion (HTCO). The sys em was
calib a ed daily wi h po assium hyd ogen ph hala e (99.95–100.05%, p.a., Me ck). The p ecision o he DOC
calib a ion was ± 1 μmol L−1. The pe o mance o he ins umen was es ed wi h he ca bon e e ence ma e i-
als (CRM) p o ided by D. A. Hansell (Uni e si y o Miami, USA). Measu ed concen a ions o he CRM we e
45.5 ± 1.7 µmol C (n = 10); he ce i ied alue is 44–46 µmol C L−1 (lo #09-06 om he Flo ida S ai a 700 m).
O ganic ca bon budge . The wa e column was di ided in o disc e e neu al densi y laye s97. The uppe
ou laye s coincide oughly wi h he main he mocline wa e s (below 27.38 neu al densi y laye s; 0–700 m),
he ollowing h ee laye s wi h in e media e wa e s (27.38–27.92; 700–1500 m), and he lowes se en laye s
wi h deep wa e s (27.92–28.10; below 1500 m). Ou es ima es we e based on he in e se box model used by
Pé ez-He nández and cowo ke s44 o ob ain mass anspo s (M) o each laye along he no he n, wes e n, and
sou he n ansec s. The model uses he he mal wind equa ion a e calcula ing he e e ence le el eloci ies and
hei unce ain ies. These eloci ies a e calcula ed assuming geos ophy, and mass and p ope y conse a ion. The
in e se box model used has been ex ended om a p e ious one98 o include he app oxima e conse a ion o mass
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and anomalies o salini y and hea , and o allow ans e be ween laye s. This model also conside s adjus men o
esh wa e luxes and Ekman anspo s in each sec ion. The in e se p oblem consis s o 37 equa ions and 130
unknowns: 99 o e e ence eloci ies, 26 o e ical eloci ies and e ical di usion, 4 o Ekman anspo s and
1 o he eshwa e lux. To sol e he in e se p oblem, he Gauss-Ma ko me hod, which p oduces a minimum
e o a iance solu ion om ini ial es ima es o he unknowns, was used. M a each laye is mul iplied by he a e -
aged suspended and dissol ed o ganic ca bon concen a ions (C) a he same laye o ob ain he co esponding
la e al ca bon luxes (M × C). To epo hese ca bon luxes, he laye s we e g ouped in o su ace, cen al, in e -
media e, and deep wa e s. The la e al ca bon luxes we e summed o ob ain he co esponding ca bon budge s
o each laye .
The obus ness o he ca bon luxes and budge s calcula ed in his s udy was es ed by means o he ollowing
pe u ba ion es (see supplemen a y ma e ial): ca bon luxes we e calcula ed as (M ± e M) × (C ± e C), whe e
e M and e C ep esen he unce ain y o he es ima ion o M and C, espec i ely. The e o e, he alues o M and
C a e pe u bed wi hin he limi s o he unce ain y o hei espec i e es ima ions. A o al o 100 pe u ba ions
we e pe o med o each ca bon lux and budge . The a e age o hese 100 alues is conside ed he op imum solu-
ion and he co esponding s anda d de ia ion (SD) an es ima e o he unce ain y o he luxes and budge s99.
See Pé ez-He nández and cowo ke s44 o a ho ough analysis o he alues o M and e M used in he p esen
calcula ions. Conce ning he unce ain y o POCsusp and DOC measu emen s, hese we e ob ained by calcula ing
he SD o all suspended and dissol ed o ganic ca bon measu ed in each laye a each ansec .
To ob ain he o ganic ca bon luxes in o/ou o he Medi e anean Sea, mass anspo s ac oss he S ai o
Gib al a we e se a −0.78 ± 0.1 S o he su ace ou low o he Medi e anean and 0.68 ± 0.1 S o he bo om
in low o he eas e n No h A lan ic100 (1 S = 109 kg s−¹). These mass anspo s, which a e wi hin he ange o
o he es ima es in he li e a u e101–107, we e mul iplied by he a e age concen a ions o POCsusp (3.4 mmol m−³ in
he ou low and 1.6 mmol m−³ in he in low) and e-DOC (13.7 mmol m−³ in he ou low and 2.7 mmol m−³ in he
in low) measu ed in he same egion by A ís egui and co-wo ke s (unpublished da a om May 2014).
Ne espi a ion (R) o POCsusp and e-DOC wi hin each laye o he box is ob ained as ollows:
=− −
TOC
d
d
IOR
(1)
Assuming s eady s a e condi ions (dTOC/d = 0), he balance o inpu s (I) minus ou pu s (O) o o al o ganic
ca bon (TOC = POCsusp + e-DOC) ac oss he open bounda ies mus equal he o al ne espi a ion wi hin he
box;, i.e., R = I − O.
Da a A ailabili y. The da ase s gene a ed du ing and/o analysed du ing he cu en s udy a e a ailable om
he co esponding au ho on easonable eques .
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