Transatlantic equatorial distribution of nitrous oxide and methane

Deep-Sea Research I 49 (2002) 1175–1193

Transatlantic equatorial distribution of nitrous oxideand methane

Claude Oudota, Philippe Jean-Baptisteb,*, Elise Fourr!eb, Claire Mormichea,Michael Guevela, Jean-Fran-cois Ternon

c, Pierre Le Corred

a Institut de Recherches pour le D!eveloppement, Centre de Brest, 29280 Plouzan!e, FrancebCommissariat "a l’Energie Atomique, Laboratoire des Sciences du Climat et de l’Environnement, CEA-CNRS, CEA Saclay,

Batiment 709, 91191 Gif/Yvette, cedex, Francec Institut de Recherches pour le D!eveloppement, Cayenne, Guyane Fran-caise, France

dLaboratoire de Chimie Marine, Observatoire Oc!eanologique, Universit!e de Bretagne Occidentale, 29682 Roscoff cedex, France

Received 14 August 2001; accepted 19 March 2002

Abstract

During January–March 1993, the vertical distributions of dissolved nitrous oxide (N2O) and methane (CH4), and

their atmospheric mixing ratios were measured in the equatorial Atlantic on the WOCE lines A6 (71300N) and A7

(41300S). Measured mean N2O and CH4 atmospheric mixing ratios were 0.316 and 1.786 ppm respectively, with an

analytical precision of 1%. Surface waters were everywhere supersaturated with both gases (mean excess DN2O=8%

and DCH4=32%), with significantly higher values close to the continents. The N2O vertical distributions on both

sections display a broad maximum centred near 400m depth, at the level of the oxygen minimum. The CH4 vertical

distributions also show a sharp subsurface maximum at the base of the mixed layer, associated with a chlorophyll a

maximum. Computed N2O and CH4 sea–air fluxes are in the range 1.1–1.8 mmolm�2 d�1 and 1.2–2.0 mmolm�2 d�1

respectively. For N2O, we calculate that the upward transport through the thermocline matches the horizontal

transport in the mixed layer and the emission to the atmosphere. Hence, the N2O budget is balanced within the

experimental uncertainties. This is not the case for CH4, whose vertical transport from the maximum concentration

layer accounts for o10% of the sea–air flux. It follows that the CH4 budget requires a sustained production within the

mixed layer itself to maintain CH4 supersaturation and balance its escape to the atmosphere. r 2002 Elsevier Science

Ltd. All rights reserved.

Keywords: Nitrous oxide; Methane; Atlantic ocean; Supersaturation; Air–sea exchange

1. Introduction

Nitrous oxide (N2O) and methane (CH4) areimportant atmospheric trace gases which influence

directly and indirectly the earth environment. Inthe troposphere, N2O acts as a greenhouse gas,with a radiative impact about 200 times that ofCO2 on a molecular basis (IPCC, 1996). In thestratosphere, it is a major source of the NOradicals involved in the destruction of the ozonelayer (Crutzen and Schmailzl, 1983). Methane

*Corresponding author. Fax: +33-1-6908-7716.

E-mail address: [email protected] (P. Jean-Baptiste).

0967-0637/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 7 - 0 6 3 7 ( 0 2 ) 0 0 0 1 9 - 5

is an another greenhouse molecule, with asubstantial radiative impact, 20 times that ofCO2 (IPCC, 1996). Natural sources account for50–60% of the total N2O emissions, 25–30% ofwhich are due to marine nitrification and deni-trification processes (IPCC, 2001). Oceanicmethane emission due to in situ biologicalproduction is much smaller, representing about2% of the cumulative natural and anthropogenicsources (Cicerone and Oremland, 1988; Lambertand Schmidt, 1993). According to Bange et al.(1994) 75% of the total oceanic flux of CH4 arisesfrom coastal areas, which account for only 7% ofthe total surface of the oceans. However thesefigures have large uncertainties due to the lack ofdata.In the following, we report oceanic and atmo-

spheric N2O and CH4 measurements acquired inthe equatorial Atlantic ocean (CITHER-1 cruise,1993) as part of the World Ocean CirculationExperiment (WOCE). This new data set offers adetailed picture of their transatlantic distributionalong the equator. We discuss its main character-istics with respect to water masses and biologicalprocesses. We also study the upper ocean budget ofthese trace gases and their flux to the atmosphere.

2. Methods

About 1000 N2O and 500 CH4 oceanic sampleswere analyzed during the CITHER-1 cruise of theN.O. L’ATALANTE from January 2 to March 19,1993. The cruise included two transatlantic zonalsections at 71300N and 41300S (WOCE Hydro-graphic Programme lines A6 and A7) and twosmall meridian transects at 31500W and 351W(Fig. 1). Air samples were also analyzed at 29 sitesalong the track of the vessel. Hydrographic andgeochemical tracer data, including oxygen, nutri-ents and CFCs, are available from Arhan et al.(1998); Oudot et al. (1998) and Andri!e et al.(1998), respectively.The water samples were taken in 100ml glass

bulbs immediately following CFC and O2 sam-pling. Microbial activity was suppressed by theaddition of 100 ml of 3% HgCl2. The waters werestored in the dark at ambient temperature prior to

their analysis, which was carried out within 12hours after sampling.The bulbs were connected via Cajont fittings to

the gas extraction line of a DELSIt DI 200 gaschromatograph equipped with an electron capturedetector (ECD) for N2O and a flame ionisationdetector (FID) for CH4. Dissolved gases weretransferred under a helium flow (140mlmin�1)onto specific traps (Molecular Sieve 13X at 01C forN2O and activated charcoal at �801C for CH4).These traps were subsequently heated (1801C forN2O and 601C for CH4) and the gases wereinjected into chromatographic columns (3.5m longPorapak Q column for N2O and 2m longMolecular Sieve 5A column for CH4).Acquisition and processing of the chromato-

grams were performed with a DATA-JET integra-tor linked to a SPECTRA-PHYSICSt computerworkstation equipped with the WINNER software.The method for the simultaneous analysis of CH4

and N2O was based on the work of Butler et al.(1987). The analytical protocol for N2O wasadapted from earlier work of Oudot et al. (1990).The measurements were calibrated with a

standard gas mixture (0.37 ppm N2O and1.8 ppm CH4) supplied by the industrial gascompany BOC (England). This BOC standardwas checked against a primary standard (0.311470.0011 ppm N2O and 1.70570.008 ppm CH4)provided by R. Weiss (Scripps Institution ofOceanography). The calibration curves were con-structed by injecting the standard mixture withthree loops of different volumes (3.36270.003,9.72870.004, 20.08670.005 cm3), calibrated byweighing with distilled water. The volume of eachloop was corrected for the dead volume of theinjection valve (0.3670.10 cm3). The analyticalerror (2 sigma), deduced from the reproducibilityon a series of replicates, is 70.15 nmol kg�1 forN2O, and 70.02 nmol kg�1 for CH4.The air samples were collected according to the

same protocol as that applied for atmospheric CO2

(Oudot et al., 1995). Air was dried on a silica geltrap before injection in the sampling loop of theGC system. Analytical precision on atmosphericN2O and CH4 measurements, established frommultiple analyzes of the BOC standard, is morethan 1% for both gases.

C. Oudot et al. / Deep-Sea Research I 49 (2002) 1175–11931176

Fig. 1. Cruise track and position of the hydrographic stations during CITHER 1 (January 2–March 19, 1993, N.O. L’ATALANTE). The stations sampled for N2O and

CH4 are indicated by squares. Stars indicate air sampling locations.

C.

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eaR

esearch

I4

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N2O and CH4 supersaturations with respect totheir atmospheric concentration were calculatedfrom the solubility equations of Weiss and Price(1980) and Wiesenburg and Guinasso (1979)respectively.Seawater was also collected in dark glass bottles

to measure the phytoplankton pigment (chloro-phyll a) concentration. Each sample (volume=295 cm3) was filtered on a glass fiber filter (Herb-land et al., 1985), which was stored in a freezer(�601C) until on-shore analysis. The measure-ments were carried out by processing the filtersaccording to the standard fluorometric methoddescribed by Herbland et al. (1985), with pigmentextraction done with methanol instead of ethanol.The uncertainty in the chlorophyll concentration isabout 5%.

3. Results

3.1. Atmospheric N2O and CH4 measurements

The mean atmospheric mixing ratios of N2Oand CH4 during the cruise (January–March 1993)were 0.316 and 1.786 ppm respectively. Our N2Odata are in good agreement with previous esti-mates (Oudot et al., 1990; Weiss et al., 1992; Bangeet al., 1996a; De Wilde and Helder, 1997; Reeset al., 1997) after the 0.001 ppm/yr secular trend ofKhalil and Rasmussen (1992) and Machida et al.(1995) is taken into account. Based on the CH4

secular trend and interhemispheric gradient (Bateset al., 1996), the CH4 atmospheric concentration inthe Atlantic equatorial belt during the CITHER-1(1993) cruise was 5% above that interpolated fromthe Ascension Island (81S) and Barbados (131N)estimates reported by Dlugokencky et al. (1998).Although this discrepancy could be due to someundetected analytical bias, we speculate that itmay also be linked to the exceptionally large CH4

supersaturation of surface waters observed duringthis cruise (see below).

3.2. Oceanic surface N2O and CH4 supersaturation

Measured N2O surface concentrations wereeverywhere in excess of solubility equilibrium with

the atmosphere (Fig. 2). Away from the coastlines,the mean excess was DN2O=(873)%. Super-saturation increased significantly toward the con-tinental margins, especially close to Africa, withvalues up to 15%. Surface layer N2O super-saturations recorded during the CITHER-1 cruiseare in the same range as those observed in theAtlantic equatorial zone during the TTO-TAS andSAVE cruises (2%oDN2Oo18%) by Weiss et al.(1992).Ocean surface CH4 concentrations showed large

excesses relative to solubility equilibrium with theatmosphere (Fig. 3). Measured DCH4 values var-ied from 16% to 22% in the western and centralpart of the basin to more than 50–100% on theeastern side. The highest values were found onthe 41300S transect, extending as far as 1500 km offthe coast of Africa. Measured supersaturationin the eastern and central equatorial Atlanticare substantially higher than those published inthe literature for comparable areas: 5–20% in thecentral Atlantic (Conrad and Seiler, 1988) and8.573.5% in the central Pacific between 51N and51S (Bates et al., 1996). The high CH4 excessesrecorded in the eastern Atlantic basin are com-parable to the values reported by Owens et al.(1991) and Patra et al. (1998) in the centralArabian Sea and along the coast of India (40%oDCH4o100%), and by Upstill-Goddard et al.(1999) off the coast of Oman (115%oDCH4o130%).

3.3. Water column N2O and CH4 distributions

N2O was supersaturated throughout the watercolumn (Fig. 4). The vertical distribution of N2Oshows a concentration maximum around 400mdepth which is more pronounced in the easternbasin (Fig. 5). This maximum (up to 60 nmol/kg)coincides with the well documented oxygen mini-mum (Reid, 1989), which also is attenuated fromeast to west (Arhan et al., 1998). This pattern istypical of the inverse relationship between N2Oand O2 extensively described in the literature(Yoshinari, 1976; Cline et al., 1987; De Wildeand Helder, 1997). Between 2000 and 4000m, N2Oconcentrations decrease below 20 nmol/kg. Thiswater layer corresponds to the well ventilated


Fig. 2. Distribution of surface N2O supersaturations (%) along (a) 71300N, (b) 351W, (c) 31500W, and (d) 41300S.

C. Oudot et al. / Deep-Sea Research I 49 (2002) 1175–1193 1179

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Mean ± SD = 20 ± 4 %

Coastline of Africa

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Fig. 3. Distribution of surface CH4 supersaturations (%) along (a) 71300N, (b) 351W, (c) 31500W, and (d) 41300S.


North Atlantic Deep Water (NADW) flowingfrom the Subarctic along the continental slope ofAmerica (Reid, 1989; Arhan et al., 1998) andcharacterised by an oxygen maximum. In the coreof the NADW arriving at 71300N along the SouthAmerican coast (501W), N2O concentrationsdecrease to o16 nmol/kg. In the less oxygenatedAntarctic Bottom Waters (AABW) N2O concen-trations increase again (Arhan et al., 1998).Between 241W and 301W, in the deepest part ofthe Brazil Basin, N2O peaks at 23 nmol/kg. Thestrong topographic control on the AABW flowexplains why this N2O increase is not detected eastof the Mid-Atlantic Ridge.The vertical distribution of CH4 displays a

subsurface maximum at the base of the mixedlayer in the range 3–5 nmol/kg (Fig. 6). A max-

imum concentration of 5.6 nmol/kg (correspond-ing to a 170% supersaturation level) was measuredat a 33m depth at station 196 (71300N, 151W) closeto the African coast. Conrad and Seiler (1988) alsoreported CH4 concentrations between 2 and4 nmol kg�1 in the upper 250m of the watercolumn of the tropical Atlantic, and Tillbrookand Karl (1995) measured concentrations above5 nmol/kg off the coast of California. Below thismaximum, CH4 concentrations decrease monoto-nically and fall below the solubility equilibriumaround 200–300m depth, thus leaving deeperwaters undersaturated with respect to the atmo-sphere (Fig. 7).

4. Discussion

4.1. Surface layer N2O–CH4 distributions

and ocean-atmosphere fluxes

Observed N2O and CH4 excesses in the equator-ial Atlantic surface waters represent the net effectof biological production in the water column(either local or advected by enriched waters) andgas exchange with the atmosphere.The large N2O supersaturation recorded off the

African coast is the consequence of the uplift ofN2O rich waters by the coastal upwelling (Pierottiand Rasmussen, 1980; Cline et al., 1987; De Wildeand Helder, 1997). N2O supersaturation alsoincreases at the western end of the 71300N section,off the coast of French Guyana. This area isinfluenced by the discharge of the Amazon Riverwhose waters, characterised by a high load ofsediments and organic matter, are enriched in N2O(Butler et al., 1987; Bange et al., 1996b; Kroezeand Seitzinger, 1998). Measurements carried out inthe same area during the TTO-TAS Leg 1 (Weisset al., 1992) also indicate high N2O supersatura-tions (Fig. 8) at the mouth of the Amazon (DN2Oup to 13–14%) and in the Amazon plume(8%oDN2Oo9%) which is spread northwest-wards by the North Brazil Current (Ternon et al.,1999).Unlike N2O, no CH4 increase was detected in

the western Atlantic near the Amazon influence.One possibility is that CH4 is rapidly oxidised by

0 2 400 6

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PTH

(m)

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7

0 200 400 600

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0

7˚30N (East)

7˚30N (West)

4˚30S (East)

4˚30S (West)

N2O excess (%)

−2000

−3000

−4000

−5000

Fig. 4. N2O supersaturation (%) vertical profiles at 71300N

(plus) and 41300S (crosses). Red and black colors are for the

western and eastern basins respectively.


Fig. 5. Vertical N2O distribution (nmol kg�1) along (a) 71300N, (b) 41300S, (c) 351W, and (d) 31500W.


intense microbial activity (Jones and Amador,1993). The large DCH4 values in the easterntropical Atlantic are likely to be due to the highbiological activity and methane production in thisregion under the influence of a strong coastalupwelling (De Angelis and Lee, 1994).The flux F (in mmolm�2 d�1) of N2O or CH4 to

the atmosphere is given by Eq. (1):

F ¼ kðC � C�Þ; ð1Þ

where k is the air–sea exchange coefficient of thegas species, and C and C* are the concentrationsof N2O or CH4 at the ocean surface and atsolubility equilibrium with the atmosphere, respec-tively. The gas transfer coefficients were computedfrom the shipboard wind speed at 10m using therelationships of Liss and Merlivat (1986) (hereafterL&M86) and Wanninkhof (1992) (hereafter W92),which provide a lower and upper limit of the air–sea fluxes (Jean-Baptiste and Poisson, 2000).

Fig. 5 (continued).


Fig. 6. Vertical CH4 distribution (nmol kg�1) along (a) 71300N, (b) 351W, (c) 31500W, and (d) 41300S.


Schmidt numbers of both gases were taken fromWanninkhof (1992).Although the N2O and CH4 surface super-

saturations are higher in the eastern basin, theocean-atmosphere gas fluxes are not significantlydifferent owing to higher wind speeds (and hencegas transfer coefficient) along the western bound-ary (Table 1). Our average sea–air flux estimatesfor the equatorial zone (1.1–1.8 mmolm�2 d�1 forN2O and 1.2–2.0 mmolm�2 d�1 for CH4) areconsistent with the compilations of Bange et al.(1994, 1996b).

4.2. Subsurface N2O maximum

The inverse relationship between N2O produc-tion and oxygen consumption is a signature of themicrobial activity in the water column. Yoshinari(1976), who first showed that the oceanic N2Oexcess (DN2O) was, as a general rule, proportionalto the apparent oxygen utilisation (AOU), sug-gested that nitrification was the main source ofN2O. This interpretation was largely adopted inlater studies dealing with DN2O and AOUdistributions in various oceanic areas (Naqviet al., 1994; De Wilde and Helder, 1997; Rees

A

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Amazon R .

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Lat

itu

de

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-2 0 2 4 6 8 10 12 14 16

Fig. 8. Distribution of the surface N2O (%) supersaturation on TTO-TAS Leg1-December 1982 (Weiss et al., 1992), showing the

impact of the Amazon River.

−100 50 0 50 100 150 200

CH excess (%)

−1000

−800

−600

−400

−200

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DE

PT

H (

m)

4−

Fig. 7. CH4 supersaturation (%) vertical profiles along 71300N

(plus) and 41300S (crosses).


et al., 1997; Patra et al., 1999). Isotopic studies(15N/14N and 18O/16O), however, have suggesteda possible coupling between nitrification anddenitrification as a N2O production mechanism,even in aerobic zones (Yoshida et al., 1989; Naqviet al., 1998).Fig. 9 displays the relationship between DN2O

and the AOU in the aphotic layer betweenthe upper thermocline and 1000m depth for the71300N section (complete results are given inTable 2). The slope of the regression line (DN2O/AOU) is about 25% higher in the eastern TropicalAtlantic Ocean than in the western sector. This

Table 1

Sea–air N2O and CH4 net flux (mmolm�2 d�1) in the eastern and western equatorial Atlantic, calculated according to Liss and Merlivat

(1986) and Wanninkhof (1992). Wind speeds are shipboard wind velocities at 10m height (in m/s)

Location Wind speed N2O flux CH4 flux

L&M86 W92 L&M86 W92

71300N–West (411W–511W) 8.771.0 1.570.7 2.471.2 1.470.3 2.270.6

71300N–East (151W–251W) 6.270.4 1.470.5 2.270.8 1.870.6 2.870.9

351W (41160S–51N) 8.371.1 1.770.3 2.770.5 1.570.4 2.470.7

31500 (41300S–51N) 5.171.6 0.670.6 1.170.8 0.970.9 1.871.2

41300S–West (281W–351W) 7.770.6 1.070.3 1.670.4 1.070.5 1.770.9

41300S–East (101W–41E) 5.471.0 0.870.4 1.470.6 1.671.6 2.772.5

Mean CITHER 1 7.071.5 1.170.6 1.870.9 1.270.8 2.071.3

0

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AOU (µmol kg-1)

∆N2O

(nm

ol k

g-1

)

7˚30'N-East(15˚W - 25˚W)

7˚30'N-West(41˚W - 51˚W)

∆ N2O/AOU = 0.211 ± 0.015 nmol µmol -1

∆ N2O/AOU = 0.264 ± 0.020 nmol µmol -1

Fig. 9. Relationship between the N2O excess (DN2O) and the apparent oxygen utilisation (AOU) along 71300N.

Table 2

Regression of DN2O (nmol kg�1) vs. AOU (mmol kg�1) betweenthe upper thermocline and 1000m depth in the eastern and

western equatorial Atlantic

Location DN2O/AOU

(nmolmmol�1)R2 N

71300N–West (411W–511W) 0.21170.015 0.843 41

71300N–East (151W–251W) 0.26470.020 0.770 53

351W (41160S–51N) 0.19070.013 0.881 31

3150’W (41300S–51N) 0.23470.015 0.884 36

41300S–West (281W–351W) 0.22170.017 0.923 17

41300S–East (101W–41E) 0.27270.019 0.851 36

R2 is the correlation coefficient and N is the number of

observations for each location.


increase, observed on both sides of the equator(71300N and 41300S), probably results from theenhanced biological activity and remineralisationof organic matter in coastal upwelling ecosystems.Nitrate regeneration during nitrification is accom-panied by the release of large quantities of N2O(Codispoti and Christensen, 1985). The DN2O/AOUratios observed in the eastern Tropical Atlanticduring CITHER 1 (0.234–0.272nmolmmol�1) areamong the highest listed in the literature. With theexception of the high value of 0.36nmolmmol�1

reported by Elkins et al. (1978) for active areasof the Central Pacific, most of the previouslyreported values are in the range 0.076–0.219nmolmmol�1 (De Wilde and Helder, 1997).

4.2.1. N2O upward transport and the N2O budget

The upward N2O flux at the base of the mixedlayer was calculated from Eq. (2):

FN2O ¼ KzðdN2O=dzÞ; ð2Þ

where FN2O is the N2O flux entering the mixedlayer (expressed in mmolm�2 d�1), Kz is thecoefficient of vertical turbulent diffusion, anddN2O/dz is the vertical N2O concentration gradi-ent (expressed in mmolm�4). The best estimate forKz; deduced from the CITHER 1 hydrographicdata with an inverse model (Lux et al., 2001), is4.3m2 d�1. In a recent study of the nitrate budgetof the central Atlantic mixed layer, Planas et al.(1999) estimated Kz in the range 1.45–5.84m2 d�1

between 31S and 61300N at 301W, a valueconsistent with the above estimate. Fluxes to the

mixed layer are summarised in Table 3 along withsea–air fluxes to allow for comparison. In thewestern equatorial Atlantic, both fluxes agree wellwithin the uncertainties, showing that there is aglobal balance between the N2O flux upwardthrough the mixed layer and its escape rate tothe atmosphere. This suggests that the lateraltransport contribution to the N2O budget is ofminor importance in this part of equatorialAtlantic, owing to the negligible horizontal con-centration gradient. On the other hand, in theeastern basin, the N2O isolines slope upward,revealing a significant East–West N2O con-centration gradient. This gradient, coupled withthe eastward mean zonal velocity in the mixedlayer (CITHER-1 Cruise Report, 1994), mayexplain why the N2O upward flux entering themixed layer is 2–3 times larger than the sea–airflux. The horizontal N2O transport Fh in themixed layer can be estimated from the advectionequation (3):

Fh ¼ UhrCh; ð3Þ

where rCh is the horizontal N2O concentrationgradient, h is the mixed layer thickness (hE50m)and U is the mean zonal velocity.With an average horizontal N2O concentration

gradient rCh in the mixed layer of the order of0.75� 10�5 nmol kg�1m�1, the 2–3mmolm�2 d�1

difference between the vertical transport at the baseof the mixed layer and the sea–air flux requires amean zonal velocity in the range 6.2–9.3 cm/s. Thisrange agrees well with the CITHER-1 ADCP data

Table 3

CH4 and N2O upward transport at the base of the mixed layer in the eastern and western equatorial Atlantic. The ocean-atmosphere

fluxes of Table 1 are also recalled for comparison

Location CH4 vertical

transport

(mmolm�2 d�1)

CH4 sea–air flux

(mmolm�2 d�1)

N2O vertical

transport

(mmolm�2 d�1)

N2O sea–air flux

(mmolm�2 d�1)

71300N–West (411W–511W) 0.0870.04 1.470.3/2.270.6 1.070.4 1.570.7/2.471.2

41300S–West (281W–351W) 0.0270.02 1.070.5/1.770.9 0.970.3 1.070.3/1.670.4

351W (41160S–51N) 0.0270.02 1.570.4/2.470.7 1.470.3 1.770.3/2.770.5

71300N–East (151W–251W) 0.1770.10 1.870.6/2.870.9 4.271.1 1.470.5/2.270.8

41300S–East (101W–41E) 0.0870.08 1.671.6/2.772.5 3.071.2 0.870.4/1.470.6

31500W (41300S–51N) 0.1270.05 0.970.9/1.871.2 1.370.2 0.670.6/1.170.8


(CITHER-1 Cruise Report, 1994), suggesting N2Obudget has been correctly assessed.

4.3. Subsurface CH4 maximum

The subsurface CH4 peak is closely associatedwith the chlorophyll a maximum situated at the

base of the mixed layer (Fig. 10). This chlorophylla peak, which reflects the abundance of phyto-plankton, is the result of a trade-off betweenphotosynthetic processes on the one hand, whichsupports the growth of the phytoplankton, andzooplankton grazing and degradation of theorganic matter on the other (Herbland, 1983).

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T (˚C)

CH4 (34˚50'W)

CH4 (1˚30'E)

T (34˚50'W)

T (1˚30'E)

(e)

0

100

200

300

400

500

600

Dep

th (

m)

Chl.a (34˚50'W)

Chl.a (1˚30'E)

0 0.5 1.0 1.5 2.0

(f)

Se ction 7˚30'N

Se ctions 35˚W a nd 3˚50'W

Se ction 4˚30'S

T (˚C)

CH4 (nmol kg-1) Chlorophyll-a (µg l-1)

Dep

th (

m)

Dep

th (

m)

Fig. 10. Selected temperature, methane and chlorophyll a vertical profiles.


Recent studies suggest that, in the well oxygenatedoceanic layers close to the surface, methane isproduced by methanogenic bacteria present inreducing microhabitats associated with suspendedparticles, fecal pellets, or in zooplankton guts(Owens et al., 1991; Karl and Tilbrook, 1994).Unfortunately, no data is available on thedistributions of zooplankton and particulate mat-ter, thus preventing us from further specifying theconditions of CH4 production.

4.3.1. Thermocline CH4 residence time

The undersaturation of CH4 below 200–300msuggests that there is a balance between thedownward diffusive transport of CH4 from thesubsurface maximum and its loss by oxidation. Byanalogy with the apparent oxygen utilisation, weuse the concept of apparent methane utilisation(AMU). Fig. 11 shows the variation of AMU as afunction of the 3H–3He age (Jean-Baptiste et al,1992; Jean-Baptiste et al., 1998) for the whole dataset between the CH4 maximum and the 1000mdepth horizon. The slope of the linear regressionleads to a methane utilisation rate of the order of0.03 nmol kg�1 yr�1. If translated into a downwardCH4 flux from the maximum concentration layerdown to 1000m, this consumption rate requires adownward flux of 0.08 mmolm�2 d�1.The corresponding residence time of CH4 at

1000m, where the average concentration is0.970.2 nmol kg�1, is 3077 yr. Few papers report

estimates of the rate of methane in situ consump-tion. A majority of studies aimed at estimatingCH4 oxidation rates rely on incubation experi-ments carried out in specific ecosystems, such asthe Southern California Bight (Ward and Kilpa-trick, 1993) or the Sargasso Sea (Jones, 1991), andproduce much higher rates (up to three orders ofmagnitude). Scranton and Brewer (1978) estimateda minimum CH4 consumption rate within theNorth Atlantic Deep Water (NADW) of 0.15nmol kg�1 yr�1, a value five times higher than ours.Recently however, Rehder et al. (1999) pointedout that the residence time of CH4 (14 yr) deducedfrom the Scranton and Brewer (1978) estimateappeared too low to be compatible with the rate ofadvection-diffusion deduced from the CFC-11distribution in the NADW. The new residencetime of CH4 below 600m deduced from theirmodel is close to 50 yr. CH4 production andconsumption rates depend greatly on local condi-tions. This large spatial variability may explain theconsiderable amount of uncertainty that charac-terises the residence time of CH4 in the variousoceanic environments.

4.3.2. CH4 upward transport and the CH4 budget

The vertical transport of CH4 from the subsur-face maximum can be estimated from the verticaldiffusive flux given by Eq. (4):

FCH4¼ KzðdCH4=dzÞ; ð4Þ

where FCH4is the CH4 vertical flux (expressed in

mmolm�2 d�1), Kz is the coefficient of verticaldiffusion (Kz ¼ 4:3m2 d�1) and dCH4/dz the ver-tical concentration gradient (expressed inmmolm�4). The calculated CH4 vertical fluxesfrom the layer of maximum CH4 concentrationare reported in Table 3. As expected from therelative intensity of the CH4 maximum along theequator, these fluxes are on average 3 times largerin the eastern basin (0.1270.04 mmolm�2 d�1)than in the western part of the equatorial belt(0.0470.03 mmolm�2 d�1).The downward average CH4 flux of

0.08 mmolm�2 d�1 calculated in Section 4.3.1 fromthe CH4 consumption rate falls in the samerange as that of the vertical diffusive flux(0.04–0.12 mmolm�2 d�1). Hence, within the

0 20 40 80 100

Tritium-helium age (yr)

0.0

1.0

2.0

3.0

App

aren

t Met

hane

Util

izat

ion

(nm

ol/k

g)

60

Fig. 11. Plot of the apparent methane utilisation (AMU) vs.3H–3He age.


uncertainties, CH4 supply from the maximumconcentration layer is able to match CH4 con-sumption in the thermocline below. On the otherhand, the upward CH4 transport into the mixedlayer is lower by one to two orders of magnitudethan the CH4 flux escaping at the ocean surface,regardless of which air–sea exchange coefficient(L&M86 or W92) is used. In the eastern part of the71300N section, where the methane vertical gradi-ent is the highest, the CH4 upward transportaccounts for only 6–9% of the sea–air flux. Animbalance of the same order of magnitudewas previously observed in the northwesternsubtropical Atlantic (10–201N) by Scranton andBrewer (1977). They found a diffusive flux fromthe subsurface maximum (0.12 mmolm�2 d�1)much smaller than the flux at the air–sea interface(2.30 mmolm�2 d�1), thus implying a substantialCH4 production in the surface layer itself.More recently, Ward (1992) compared the air–sea flux of CH4 (3.2 mmolm�2 d�1) and thediffusive flux (0.285 mmolm�2 d�1) for the South-ern Californian Bight and equally concluded thatan additional CH4 source was necessary toaccount for the sea–air flux. Tilbrook and Karl(1995) reported a similar situation in the oligo-trophic gyre of the northern central Pacific Oceanclose to Hawaii with an average air–sea flux ofCH4 of 2.5 mmolm�2 d�1 and a diffusive flux of0.2 mmolm�2 d�1. These converging results pointto the fact that the CH4 supersaturation in surfacewaters is maintained mainly by in situ production.Of lesser importance is the upward flux from thesubsurface maximum, which accounts for o10%of the total CH4 escape rate.

5. Conclusions

The vertical distributions of nitrous oxide andmethane along the WOCE A6-A7 transects acrossthe equatorial Atlantic (January–March 1993)display characteristic features linked to the pro-duction and removal processes of both gases andto the characteristics of the various water masses.The main conclusions of the present study are thefollowing:

* The N2O and CH4 atmospheric mixing ratiosmeasured during the cruise were 0.316 and1.786 ppm respectively, with a precision about1%. Whereas N2O results agree well withavailable data, CH4 measurements are 5%above literature estimates. The reason for thisdiscrepancy remains unclear.

* CH4 surface supersaturation was 32% onaverage, with values exceeding 100% off theAfrican coast. N2O surface supersaturations didnot exceed 20%, and remained close to theiraverage value of (873)% at most of the openocean stations. For both gases, the largestexcesses were observed on the African side ofthe equatorial belt.

* The vertical distributions of N2O and CH4 bothdisplayed subsurface maxima. The N2O max-imum, more pronounced in the eastern basin,was centred around 400m depth, and coincidedwith the oxygen minimum. The observed(DN2O/AOU) ratios were among the highestever listed in the literature. The CH4 maximum,located at the base of the mixed layer (approxi-mately at 40m depth in the east and 100mdepth in the west) was closely associated with achlorophyll a maximum, suggesting a couplingmechanism between phytoplankton dynamicsand the CH4 production.

* The estimated rate of methane consumptionin the thermocline and below (down to 1000m)in the Atlantic equatorial zone is 0.03 nmolkg�1 yr�1, corresponding to a residence time of3077 yr at 1000m.

* The N2O upward transport from the layer ofmaximum concentration into the mixed layermatches the horizontal advection and thesea–air flux, thus showing that the surfacemixed layer N2O budget is balanced withinthe experimental uncertainties. On the contrary,CH4 upward transport accounts foro10% of the sea–air flux and points to an addi-tional CH4 in situ production in the mixed layeritself.

Acknowledgements

The CITHER-1 cruise of the WOCE programwas supported by the Institut de Recherches pour


le D!eveloppement (IRD, ex-ORSTOM), the In-stitut Fran-cais de Recherche pour l’Exploitationde la Mer (IFREMER) and the Institut Nationaldes Sciences de l’Univers (INSU) through theProgramme National d’Etude de la Dynamique duClimat (PNEDC). The Universit!e de BretagneOccidentale (UBO) and the Laboratoire desSciences du Climat et de l’Environnement (LSCE)also took part in this project. The authors wish tothank Captain Gourmelon, Captain Derouet andthe crew of the N.O. L’ATALANTE for theircontribution to the success of the cruise. We alsothank C. Colin and A. Morli"ere (IRD), ChiefScientists, M. Arhan and H. Mercier of theLaboratoire de Physique des Oc!eans (CNRS-IFREMER-UBO), and B. Bourl"es and Y. Gour-iou (IRD), for providing the hydrographic(CTD-O2) data. We are particularly grateful toR. Weiss (SIO) for providing us with a refer-ence for the calibration of our N2O and CH4

measurements.

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Transatlantic equatorial distribution of nitrous oxide and methane