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Remineralization ratios of carbon, nutrients, and oxygen in the

North Atlantic Ocean: A field databased assessment

Helmuth ThomasDepartment of Marine Chemistry and Geology, Royal Netherlands Institute of Sea Research (NIOZ), Texel, Netherlands

Received 22 June 2001; revised 7 December 2001; accepted 5 April 2002; published 26 September 2002.

[1] Remineralization ratios of carbon, nutrients, and oxygen have been assessed in theNorth Atlantic Ocean along the WOCE 1A/E section. The study is based on an extensivefield data set comprising dissolved inorganic carbon (DIC), nitrate and nitrite (NO3/2),phosphate (PO4), and oxygen (O2) data as well as hydrographic data. A procedure hasbeen introduced which normalizes DIC data to constant salinity and temperature andcorrects for the contamination from anthropogenic CO2. The remaining variability on thenormalized DIC values (DICbio) can be attributed to the remineralization of organic matter.DICbio can thus be seen as carbon-analogy to the apparent oxygen utilization (AOU).The consecutive evaluation of the remineralization ratios obtains two different regimesseparated at the density level r = 1027.7 kg m�3. In the shallower level the ratios(C/N = 4.5; C/P = 67, AOU/C = 2.0; N:P = 15; AOU:P = 134; AOU/N = 9.0) are shiftedtoward relatively higher nutrient release and higher oxygen consumption with respectto the Redfield ratios of particulate organic matter (POM). In contrast, in the deeper levelsthe ratios are shifted toward relatively higher carbon release and lower oxygen demand(C/N = 11; C/P = 152, AOU/C = 0.86; N:P = 13.9; AOU:P = 130; AOU/N = 9.4). Thedepth integrated inventories of the remineralization products (DIC, NO3/2, PO4, andAOU) provide water column averaged ratios for the investigation area (C/N = 8.8;C/P = 124, AOU/C = 1.1; N:P = 14.2; AOU:P = 131; AOU/N = 9.3) which imply ahigher efficiency of the biological carbon pump in the North Atlantic Ocean thanpredicted with respect to the elemental composition of POM. INDEX TERMS: 4805

Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4806 Oceanography: Biological

and Chemical: Carbon cycling; 4835 Oceanography: Biological and Chemical: Inorganic marine chemistry;

4845 Oceanography: Biological and Chemical: Nutrients and nutrient cycling; KEYWORDS: North Atlantic

Ocean, dissolved inorganic carbon, carbon-nutrient relationships, Redfield ratios, biological CO2 pump,

organic matter remineralization

Citation: Thomas, H., Remineralization ratios of carbon, nutrients, and oxygen in the North Atlantic Ocean: A field databased

assessment, Global Biogeochem. Cycles, 16(3), 1051, doi:10.1029/2001GB001452, 2002.

1. Introduction

[2] The uptake of atmospheric CO2 by the oceans via the‘‘biological carbon pump’’ is driven by primary productionin the euphotic zone. Dissolved inorganic carbon (DIC) andnutrients are converted to particulate organic matter (POM)during photosynthesis. Only a minor part of the POMescapes remineralization in the surface layer by settlingdown into the deeper water column [Eppley and Peterson,1979]. The export production in turn is either remineralizedin the deeper water column causing DIC and nutrient release,or it is eventually buried in the sediments over geologicaltimescales. The DIC thus exported from the surface layer isreplenished by CO2 from the atmosphere and thereforerepresents the oceanic uptake of atmospheric CO2 via the‘‘biological carbon pump.’’ The DIC and nutrients which are

released in the deeper water column are then transported bythe thermohaline circulation to the upwelling areas, wherefinally CO2 is given back to the atmosphere.[3] The concept of Redfield et al. [1963] has been applied

in order to assess the DIC released by remineralization ofPOM in the deeper water column. This biogeochemical keyconcept is based on the assumption that the elemental ratiosof carbon and nutrients in freshly produced POM are equalto the corresponding release ratios of DIC and nutrientsduring remineralization of POM in the deeper water col-umn. Redfield’s classical ratios of carbon (C), nitrogen (N),phosphorus (P), and oxygen (O2) were given to C:N:P: �O2 = 106:16:1:138 with the oxygen ratio being inferredfrom stoichiometric considerations. Accordingly, changes innutrient concentrations in the deeper water column can beconverted to the associated carbon units with reference tocarbon/nutrient relationships observed in freshly producedPOM. Alternatively, Anderson and Sarmiento [1994] sug-

GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 16, NO. 3, 1051, doi:10.1029/2001GB001452, 2002

Copyright 2002 by the American Geophysical Union.0886-6236/02/2001GB001452$12.00

24 - 1

gested that the remineralized DIC can be derived using theoxygen/carbon ratio observed during photosynthesis [Laws,1991]. Several methods have followed one of these routes inorder to assess the biological carbon pump.[4] The Redfield concept with its assumption of fixed

conversion factors has been a very useful tool for firstestimates of the biological carbon pump. However, for moredetailed analysis, for example assessing the penetration ofanthropogenic CO2 using separation concepts introduced byBrewer [1978] and Chen and Millero [1979], this assump-tion has to be carefully reconsidered, since the separationconcepts strongly depend on the Redfield ratios [e.g.,Wanninkhof et al., 1999]. It has become evident that eitherduring primary production or during remineralization ofPOM this assumption does not necessarily hold true. Forexample, a wide systematic change of carbon/nutrient ratioshas been observed during photosynthesis [e.g., Thomas etal., 1999, and references therein; Copin-Montegut, 2000;Osterroht and Thomas, 2000; Kortzinger et al., 2001]. Theelemental composition of particles trapped in the deeperwater column may also show a systematic change in theelemental composition with depth [e.g., Olesen and Lunds-gaard, 1995; Knauer et al., 1979; Wakeham et al., 1984;Honjo and Manganini, 1993]. Moreover, several studieshave analyzed the related concentration changes of DIC,nitrate (NO3/2), phosphate (PO4), and oxygen in the deeperwater column arguing in favor of changes of the releaseratios [e.g., Minster and Boulahdid, 1987; Shaffer, 1996;Hupe and Karstensen, 2000; Hupe et al., 2001].[5] The North Atlantic Ocean as a high latitude regime

plays a key role in taking up atmospheric CO2 via thesolubility pump [e.g., Thomas et al., 2001]. Moreover, it ischaracterized by high primary productivity which causeshigh CO2 uptake via the biological pump [e.g., Falkowski

et al., 1998]. Despite this relevance, there is still only sparseinformation available about the remineralization ratios ofPOM in the deeper water column of theNorthAtlantic Ocean.Due to the lack of DIC datamost earlier studies were confinedto nutrient/oxygen ratios [Takahashi et al., 1985;Minster andBoulahdid, 1987;Fanning, 1992]. Other studies excluded thehigh latitude regions of the Atlantic Ocean, notably the NorthAtlantic [e.g., Anderson and Sarmiento, 1994] or focused onglobal rather than on regional scales [Shaffer, 1996].[6] Here, a new method is proposed to assess the remi-

neralization ratios of POM in the deeper water column ofthe North Atlantic Ocean referring to highly accurate DICdata determined according to Johnson et al. [1993] andWOCE nutrient data. The key idea is to develop a procedurewhich corrects the observed DIC data for the variability ofthe background and the anthropogenic CO2 signal. Theremaining signal is then only affected by the remineraliza-tion of organic matter which thus enables the assessment ofthe remineralization ratios including carbon-related ratios.The corrected DIC value would be analogous to theapparent oxygen utilization (AOU).

2. Data

[7] The data were collected during the German WOCEcruise Meteor 30/3 from 24 November 1994 through 15December 1994 covering the WOCE A1/E section(Figure 1). Dissolved oxygen (O2) and the nutrients nitrate(including nitrite, NO3/2) and phosphate (PO4) were deter-mined in 920 samples from 43 stations according to theWOCE operations manual [World Ocean CirculationExperiment, 1994; Koltermann et al., 1996]. The overallerrors were lower than 0.2% for O2, 1.38% for NO3/2, and1.27% for PO4. The 360 samples from 42 stations were

Figure 1. Cruise track and stations occupied along the WOCE A1 line during Meteor cruise 30/3.Stations east of Greenland (WOCE line A1/E) are subjects of investigation. Figure 1 is reprinted fromThomas and Ittekot [2001] with permission from Elsevier Science.

24 - 2 THOMAS: ORGANIC MATTER REMINERALIZATION IN THE NORTH ATLANTIC

analyzed for dissolved inorganic carbon (DIC) immediatelyafter sampling using the coulometric SOMMA systemdescribed by Johnson et al. [1993]. DIC reference samples(CRM, produced by A. Dickson, Scripps Institution ofOceanography) were used to calibrate the measurementsfor each cell individually. The overall error of the systemwas determined to ±1.5 mmol kg�1 (<0.1%) during onboardoperation. The calculations of the equilibrium reactions ofthe carbonate system were performed using the programCO2SYS, version 1.04 [Lewis and Wallace, 1998].

3. Observations

[8] Detailed discussion of the O2 and DIC distributions aswell as of hydrographic topics relevant for the WOCE A1/Esection (Figure 1) are given by Thomas and Ittekkot [2001,and references therein] or, for example, by Stoll et al.[1996]. The discussion here is thus confined to a briefdescription of the distributions of DIC, O2, nutrients, anddensity. An overview of the concentrations observed in therelevant water masses is given in Table 1.

[9] The surface concentrations of PO4 (Figure 2a) increaseslightly in a westerly direction from approximately 0.5 to 0.8mmol kg�1. The Irminger Sea, characterized by youngerLabrador Seawater (LSW) [e.g., Sy et al., 1997], reveals arather homogenous level of �1.1 mmol kg�1 PO4, whichdecreases slightly in the Denmark Strait Overflow Water(DSOW) above the bottom (1.0 mmol kg�1). East of the Mid-Atlantic Ridge (MAR) the Subpolar Mode Water (SPMW),separating the surface waters from the Intermediate Water(IW), shows PO4 concentrations between 0.8 and 1.1 mmolkg�1. The IW can be identified by the intrusion of nutrient-rich waters from the south (1.2 mmol kg�1 PO4), while theolder LSW below the IW shows again lower concentrationsof approximately 1.1 mmol kg�1 PO4. The deeper watermasses, North Atlantic Deep Water (NADW) and AntarcticBottom Water (AABW), show increasing PO4 levels of1.2–1.4 and >1.5 mmol kg�1, respectively. In general,NO3/2 shows the same features as PO4 and the values aredepicted in Table 1 (Figure 2b). The pattern of the oxygenconcentrations (Figure 2c) can be seen as mirror image of thenutrient concentrations, although this image is disturbed bystrong solubility effects. Accordingly, highest values can beobserved in the surface waters. In the subsurface waters of theIrminger Sea (younger LSW), O2 concentrations of approx-imately 280–300 mmol kg�1 are found increasing in thedeeper DSOW because of its very low temperatures

(�310 mmol kg�1). Within the eastern basin, SPMW andLSW show lower oxygen concentrations (230–260 mmolkg�1) which increase again in the LSW (280 mmol kg�1). Asexpected, the DIC concentrations (Figure 2d) follow thenutrient pattern with lowest concentrations in the surfacewaters. The LSW in the western basin is characterized byconcentrations of about 2148 mmol kg�1 which decrease inthe DSOW to approximately 2110 mmol kg�1 (not visible inFigure 2d because of the lower spatial resolution of the DICsamples). In the eastern basin the SPMW reveals concen-trations between 2120 and 2150 mmol kg�1 increasing toapproximately 2170 mmol kg�1 in the IW. A relative DICminimum is observed in the LSW of 2150 mmol kg�1, whilein the deeper NADWand AABW the concentrations increaseto 2190 and 2200 mmol kg�1, respectively. The density levelof r = 1027.7 kg m�3 (Figure 2e) ascends in westerlydirection from �1600 m depth to less than 200 m at around39�E. At the Greenland shelf this level is observed at a depthof approximately 800 m. As discussed later, this density levelindicates the separation line of the upper and lower watercolumn regarding the remineralization ratios of POM.

4. DIC Changes in the Deeper Water ColumnDue to Remineralization of Organic Matter

[10] In order to assess carbon export from the surface tothe deeper waters via the biological carbon pump, theremineralization ratios of POM are usually referred to.Briefly, these ratios can be obtained employing ‘‘preformedconditions‘‘ of a water mass and assigning the differencesbetween preformed conditions and observations to theremineralization of organic matter. The preformed condi-tions describe the state of a water mass at the sea surfacebefore a water mass is subducted. The relevant parametersto describe the preformed conditions regarding DIC aretemperature (T ), salinity (S ), Alkalinity (AT) and the partialpressure of CO2 (pCO2). In order to define the preformedconditions of DIC an absolute value of AT is thus required.This might be obtained exploiting linear relationshipsbetween salinity and alkalinity, which, however, revealregional variability [e.g., Millero et al., 1998]. The descrip-tion of the preformed condition of an individual sample thusrequires reliable knowledge of both the origin of the watermasses constituting this sample and their mixing history.[11] In order to avoid the above indicated shortcomings of

describing preformed conditions a two-step procedure isemployed to extract the biological variability from the

Table 1. Major Water Masses and Corresponding DIC, Nutrient, and Oxygen Concentrationsa

Water Masses ApproximateDepth,m

ApproximateDIC,

mmol kg�1

ApproximateO2,

mmol kg�1

ApproximateAOU,

mmol kg�1

ApproximateNO3/2,

mmol kg�1

ApproximatePO4,

mmol kg�1

Surface waters 2080–2130 260–300 0–20 8–13 0.5–0.8SPMW Subpolar Mode Water 250–900 2120–2150 230–250 20–70 11–17 0.8–1.1IW Intermediate Water 900–1200 2170 230–260 85 19–20 1.2LSW, Irminger Sea Labrador Seawater >500 2148 280–300 30 17 1.1DSOW Denmark Strait Overflow Water 2500–3000 2110 310 30 15 1.0LSW, Iceland Basin Labrador Seawater 1800 2150 280 45 19 1.2NADW North Atlantic Deep Water 2500–3000 2190 260–280 80 19–22 1.2–1.4AABW Antarctic Bottom Water >4000 2200 <250 90 >23 >1.5

aReprinted from Thomas and Ittekot [2001] with permission from Elsevier Science.

THOMAS: ORGANIC MATTER REMINERALIZATION IN THE NORTH ATLANTIC 24 - 3

observed DIC concentrations. This procedure (1) normal-izes observed DIC concentrations to standard T, S, and ATconditions and (2) corrects for the anthropogenic CO2

contamination. Since calcification is of minor occurrencein the North Atlantic Ocean [e.g., Kortzinger et al., 2001], itcan be neglected here. The parameter describing biologicalvariability in DIC (DICbio) is thus computed as

DICbio ¼ DICobs � DDICsol � DDICant; ð1Þ

considering the contributions of solubility effects comparedto standard conditions (DDICsol) and anthropogenic CO2

(DDICant) to the observed DIC concentrations (DICobs).

4.1. Normalization of DIC Concentrations forSolubility Effects

[12] The solubility of CO2 in seawater is determined by itsT, S, and AT conditions, which cause part of the observedvariability in the DIC concentrations. The suggested nor-malization procedure to standard T, S, and AT conditionsgenerates a homogenous DIC background; in front of thatsolely remineralization processes and DDICant cause thevariability in DIC. Since the procedure accounts for differ-ences between observed and standard conditions, the influ-ence of the absolute value of AT on those differences isnegligible and the generalized description of AT according to

Figure 2. Distributions of (a) PO4, (b) NO3/2, (c) O2, (d) DIC, and (e) density observed along theWOCE A1/E section. The density level r = 1027.7 kg m�3 (dash-dotted line) in Figure 2e represents theseparation line of the water column into a shallower and deeper part. See text for details. Figure 2d isreprinted from Thomas and Ittekot [2001] with permission from Elsevier Science.


Millero et al. [1998] [AT = 520.1 + 51.24S] is referred to. Atemperature of T = 6�C, a salinity of S = 35 and acorresponding AT have been chosen as standard conditions,since T = 6�C and S = 35 represent approximately the meanvalues observed during the cruise. Keeping in mind theabove AT/S relationship a set of standard DIC values(DICstd) is defined for Tn = 6�C and Sn = 35 and givenpCO2 values between 280 and 370 matm. In order todescribe the solubility effect (DDICsol), i.e., the impact ofdifferent T, S, and AT conditions on the DIC concentrations,the difference between DICstd and the DIC calculated fordifferent temperatures between 0� and 20�C and differentsalinities between 34 and 37 at different pCO2 conditionsDIC[34–37, 0–20�C, pCO2]:

DDICsol ¼ DICstd � DIC 34�37;0�20�C;pCO2½ �: ð2Þ

The multiparameter analysis of the results of this exerciseprovides a linear equation which describes DDICsol just as afunction of S, T [in �C], and pCO2 [in ppm]:

DDICsol ¼ �1312:8þ S 38:6� T 8:8þ pCO2 0:04: ð3Þ

[13] Note that this equation does not yet account for thecorrection for anthropogenic CO2 contamination; it onlydescribes the normalization of the DIC at different pCO2

conditions. Figure 3 shows the correction for a given pCO2

of 350 matm. For the present study the pCO2 of theindividual samples has been estimated following Thomasand Ittekkot [2001]. Accordingly, the pCO2 can be esti-mated with regard to the ventilation ages of water massesand the time history of the atmospheric CO2. The ventila-tion ages of the key water masses observed on the WOCEA1/E section are adopted and interpolated for all samplesbetween the key water masses. Referring to the time historyof the atmospheric CO2, the corresponding pCO2 is thenobtained. Because of the low pCO2 coefficient, any pCO2

disequilibrium can be neglected here. An alternative to

determining the pCO2 for each sample individually is toassume a fixed (mean) pCO2 value instead of a specific one.

4.2. Correction for Anthropogenic CO2 Contamination

[14] Most of the methods assessing the penetration ofanthropogenic CO2 in the oceans follow the separationconcept introduced by Brewer [1978]. This concept, whichhas been modified and improved by several authors [e.g.,Chen and Millero, 1979; Gruber et al., 1996; Kortzinger etal., 1998], refers to remineralization ratios of organic matterin order to separate out the biological component of DIC.Applying this concept to the present study would thereforenot allow an independent determination of the reminerali-zation ratios of POM, since these then would be part of theinitial conditions. Recently, an alternative approach ofassessing anthropogenic CO2 in the oceans has been sug-gested which refers to ventilation ages of water masses andCO2 equilibrium chemistry of seawater and is independentfrom remineralization ratios of POM. This approach hasbeen applied on a global scale [Thomas et al., 2001] andalso on the current data set of North Atlantic Ocean[Thomas and Ittekkot, 2001]. The latter data have beenused to quantify DDICant in the present study (equation (1)).

4.3. Biological Variability of DIC (DICbio)

[15] This two-step procedure has been applied to theobserved DIC concentrations and the associated T and Svalues. The cumulative profiles (Figures 4a and 4b) indicatethat the values are decreased by the normalization in thedeeper waters mainly as a consequence of the low temper-atures whereas in the upper layers the values will beincreased compared to the observations. Since the gradientsin salinity are rather small in the investigation area, thenormalization of the S (and AT) is of minor relevancecompared to the temperature correction. As expected, thecorrection for anthropogenic CO2 contamination decreaseswith depth. The variability of DICbio provides a measure of

Figure 3. Plot of the function (equation (3)) to normalize observed DIC values to a constant temperature(6�C) and salinity (35) shown for a pCO2 of 350 matm. The cross indicates the zero point with respect toTn = 6�C and Sn = 35.


the carbon which has been released by remineralization oforganic matter from a homogeneous background. DICbio

thus enables the assessment of the remineralization ratioswith respect to oxygen and nutrients and can been seen as thecarbon analogy to the apparent oxygen utilization (AOU).An absolute value of remineralized DIC as provided by theAOU for oxygen is not given at this stage, since the absolutevalue of DICbio depends on the standard conditions. There-fore, the obtained relationships of AOU to DIC, NO3/2, andPO4 will be exploited later to quantify water column inven-tories of remineralized DIC, NO3/2, and PO4.

5. Release Ratios of DIC and Nutrients Due to theRemineralization of POM

[16] The remineralization ratios of POM will be discussedfor DIC and the nutrients NO3/2, and PO4 and AOU.Already, the qualitative view of the property/property plotsindicates two different biogeochemical regimes as exempli-fied by the DICbio/AOU plot (Figure 5). Apart from the

surface layer samples (triangles), two different groups ofsamples can be identified, one characterized by a less steepslope (black dots) and one by a steeper slope (diamonds).The detailed analysis of this feature leads to the separationof the water column along the density horizon r = 1027.7 kgm�3 into an upper and deeper part (see also Figure 2e). Thesurface waters, the SPMW, and the IW constitute upperdensity range (r < 1027.7 kg m�3), and the layers below(r > 1027.7 kg m�3) are mainly characterized by both typesof LSW and the deeper NADW and AABW.[17] Surface layer samples (associated with AOU < 20

mmol kg�1) have not been considered for further discussion,since both AOU and DIC are affected by air-sea exchangeof O2 and CO2, respectively. Geometric mean functionalregression analysis [Sprent and Dolby, 1980] has beenperformed on the property/property relationships. As indi-cated by Figure 5, the water column can be separated intotwo regimes with different biogeochemical characteristics.Accordingly, the remineralization ratios of carbon, nutrientsand oxygen (Table 2) are shown for the water column above

Figure 4. (a) The impact of the normalization procedure is shown for the cumulated DIC profiles. Thediamonds indicate the observed values, whereas the dots represent the T- and S-normalized values and thecrosses represent the values which have been additionally corrected for the contamination ofanthropogenic CO2 (DDICant). (b) The corrections are shown for the individual samples. The trianglesshow the correction for DDICant, the dots show the normalization for T and S, and the crosses show thetotal correction. Since the observed variability in S is rather low (Smin � 34 and Smax � 35.5), the changesof the T/S normalization (dots) from negative to positive values can mainly be attributed to decreasingtemperatures.


and below r = 1027.7 kg m�3 in Figures 6 and 7,respectively.

5.1. C:N Ratios

[18] The present study obtains for the upper layers of theNorth Atlantic Ocean above r = 1027.7 kg m�3 a carbon tonitrogen remineralization ratio of C:N = 4.5 ± 0.3 (Figure 6a)whereas for the deeper layer a C:N ratio of 11.0±0.5 isobtained (Figure 7a). The shift of the ratios implies relativelyhigh nitrogen release by particle remineralization comparedto carbon, whereas in the deeper layers the remaining carbonenriched tissue is remineralized. This general trend has beenfound in several studies which include carbon-related ratios.Good agreement is found with the gradients reported byShaffer [1996] which increase from C:N = 5.0 in the upperlayers to 9.3 in the deeper layers. Shaffer [1996] applied amodeling technique, and the corresponding C:N ratios havebeen converted from the given C:P and N:P ratios. Theresults of the present study are also in agreement with datafrom particle trap studies in the North Atlantic Ocean byHonjo and Manganini [1993]. The authors found the same

gradient of increasing C:N ratios with depth which werefound to be somewhat higher (6–8) for in the shallowerlayers but similar for the deeper layers (8–10), respectively.The low C:N ratio in the shallower layers presented here isalso in reasonable agreement with data reported by Takaha-shi et al. [1985] for the isopycnal surfaces r = 1027.0 kg m�3

and r = 1027.2 kg m�3.

5.2. C:P Ratios

[19] For the shallower layer above r = 1027.7 kg m�3, acarbon to phosphorus release ratio of C:P = 67 ± 5 has beenobtained, whereas it increases in the deeper layers to 152 ± 6(Figures 6b and 7b). Both the shallower and the deeper C:Pratios are significantly different from the Redfield C:P ratioof 106:1. As already observed regarding the C:N ratios, thelow C:P ratio in the shallower layers might be a hint to apreferential release of nutrients, whereas the carbon tissue isexported to the deeper layers causing high C:N and C:Pratios [e.g., Thomas et al., 1999]. The deviation from thestrictly Redfieldian behavior is confirmed by the sedimenttrap data [Honjo and Manganini, 1993] which show alsosimilar C:P ratios in both the shallower and deeper layers.Moreover, the C:P ratios by Takahashi et al. [1985] for theisopycnal surfaces r = 1027.0 kg m�3 and r = 1027.2 kg m�3

as well as the modeled C:P ratios and their gradient byShaffer [1996] can be confirmed in general. The C:N andC:P ratios for the North Atlantic Ocean obtained in this studycannot support results of depth-invariant ratios as reportedby earlier studies for different oceanic regimes.

5.3. AOU:C Ratios

[20] The AOU:C ratio changes from 2.0 ± 0.1 in theshallower layer to 0.86 ± 0.04 (Figures 6c and 7c) in thedeeper layer as already indicated by Figure 5. The resultsclearly imply two regimes with different biogeochemicalcharacteristics. Similar gradients of decreasing AOU:C havebeen reported, for example, by Hupe and Karstensen [2000]for the Arabian Sea (AOU:C = 1.56 in 500 m and AOU:C =1.25 in the deepest layers) as well as by Shaffer [1996](AOU:C = 1.5 in 100 m and AOU:C = 1.28 in 3000 m),although it has to be noted that the differences in the extremevalues of the latter studies are smaller than in the presentstudy. Taking into account stoichiometric considerations, theapparently high AOU:C ratio of the shallower layers is inagreement with the above low C:N release ratio (4.5:1),which might be justified by a high oxygen demand duringcombustion of organic matter as indicated by the idealizedequation for amino acid combustion with short carbonchains (equations (4a) and (4b) with 4 and 5 carbon atoms):

C:N ¼ 4 CH3 � CH2 � CHðNH2Þ � COOHþ 6:5O2

) 4CO2 þ 1HNO3 þ 4H2O

¼> AOU:C ¼ 1:6; ð4aÞ

Figure 5. Plot of AOU versus normalized DIC values(DICbio). The separation of the water column is denoted bydifferent symbols. The triangles mark the surface waterscorresponding to AOU < 20 mmol kg�1 which have beenexcluded from further discussion. The diamonds mark theupper water column characterized by r < 1027.7 kg m�3, andthe dots mark the deeper part below r = 1027.7 kg m�3. SeeFigure 2e for location of the density level r = 1027.7 kg m�3.

Table 2. Remineralization Ratios Obtained for the WOCE A1/E Section in the North Atlantic Ocean

Ratio C:N C:P AOU:C N:P AOU:P AOU:N

r < 1027.7 kg m�3 4.5 ± 0.3 67 ± 5 2.0 ± 0.1 15 ± 0.3 134 ± 9 9.0 ± 0.6r > 1027.7 kg m�3 11.0 ± 0.5 152 ± 6 0.86 ± 0.04 13.9 ± 0.3 130 ± 6 9.4 ± 0.5


C:N¼ 5 CH3 � ðCH2Þ2 � CHðNH2Þ � COOHþ 10:5O2

) 5CO2 þ 1HNO3 þ 10H2O

¼> AOU:C ¼ 2:1; ð4bÞ

Carbohydrates

HCOHð Þ6 þ 6O2 ) 6CO2 þ 6H2O ¼> AOU:C ¼ 1: ð4cÞ

Furthermore, high AOU:C ratios have been reported for theNorth Atlantic Ocean by Takahashi et al. [1985] (AOU:C =1.7 and AOU:C = 1.96 at r = 1027.0 kg m�3 and r = 1027.2kg m�3, respectively) and Kortzinger [1995] (1.97 and 1.98,respectively). The low AOU:C ratio found for the deeperlayers might be interpreted as hint for the combustion ofalready high oxygenated organic matter. For example, thecombustion of carbohydrates would equal an AOU:C ratioof 1:1 (equation (4c)).

5.4. N:P Ratios

[21] The N:P-remineralization ratio obtained to be 15 ±0.3:1 for the shallower layer (Figure 6d and Table 2), whichis slightly lower than the Redfieldian value of POM (C:P =16:1). The ratio decreases with depth to N:P = 13.9 ± 0.3:1(Figure 7d and Table 2). The general trend of these results isin good agreement with data reported by Minster andBoulahdid [1987], although their data show higher values

in the shallower layers (16.73 to 15.5:1 at r = 1027.0 to1027.4 kg m�3). The N:P ratios of POM reported by Honjoand Manganini [1993] reveal some variability and thus showonly a weak agreement with the data presented here. Theratios of this study might not confirm with depth constantN:P ratios; however, since the difference between the shal-lower and deeper N:P ratios is rather small, a note of cautionshould be taken.

5.5. AOU:P Ratios

[22] The AOU:P remineralization ratios presented here ofAOU:P = 134 ± 9:1 for the shallower and 130 ± 6:1 for thedeeper layers (Figures 6e and 7e and Table 2) appear to berather constant, taking into account the uncertainties. Thevalues confirm the range of Redfield et al.’s [1963] AOU:Pratio which was inferred from stoichiometric considerations.The AOU:P ratios reported by Minster and Boulahdid[1987] for layers at r = 1027.4 and 1027.8 kg m�3 (142:1and 127:1, respectively) are in agreement with the findingsof this study. It is worth noting here that during theremineralization of organic matter, the relationship betweenoxygen and phosphorus is different from that between forexample nitrogen and oxygen. The released phosphatewater column can been seen as product of the hydrolysisof organic phosphoric acid esters, a process which does notconsume oxygen. In contrast the release of NO3/2 to the

Figure 6. Remineralization ratios of DIC nutrients and oxygen obtained for the upper water columnabove r = 1027.7 kg m�3. The plots are shown with the same scale as Figure 7 to enable better comparison.


water column points to the oxidation of amino acids or otherreduced nitrogen compounds (see also equations (4a) and(4b)). Thus, the AOU:N ratio (to be discussed below)describes a joint process of oxygen and nitrogen, whereasthe AOU:P ratio rather describes parallel processes.

5.6. AOU:N Ratios

[23] The AOU:N remineralization ratios have beenobtained to AOU:N = 9.0 ± 0.6:1 for the shallower andAOU:N = 9.4 ± 0.5:1 for the deeper layers, respectively(Figures 6f and 7f and Table 2). In the given range ofuncertainty, both ratios appear to be similar. The value ofthe AOU:N ratios is in agreement with the results byMinster and Boulahdid [1987] ranging between 9.1:1 inthe shallower layers and 8.7:1 at r = 1027.8 kg m�3. TheRedfield AOU:N ratio of 8.6:1 [Redfield et al., 1963] is alsoclose to the present findings.

6. Water Column Inventories of RemineralizedDIC, NO3/2, and PO4

[24] In order to assess the contributions of remineraliza-tion of organic matter to the observed concentrations ofDIC, NO3/2, and PO4, an absolute measure is required, sincethe above ratios describe only relative changes. This abso-lute measure for the remineralization processes is provided

by AOU, which in turn can be used to assess the requiredabsolute changes [e.g., Hupe et al., 2001]. Thus, AOUconcentrations have been related to the above remineraliza-tion ratios in order to obtain the remineralization invento-ries. The remineralized contributions of all four parameters(PO4,rem, NO3/2,rem, AOU, and DICrem; Figures 8a–8d)clearly reflect the separation between the western and east-ern basins. Lower and rather homogeneous values areobtained for the western basin due to its younger age andthus shorter ‘‘biological history.’’ The concentrations in thewestern basin vary between 0.2 and 0.3 mmol kg�1 PO4,rem,3 and 4 mmol kg�1 NO3/2,rem, 25 and 40 mmol kg�1 AOU,and 30 and 40 mmol kg�1 DICrem. In the eastern basin theIW originating from the subsurface waters of the equatorialAtlantic Ocean clearly can be identified by high concen-trations of the remineralization products PO4,rem, NO3/2,rem,and AOU (0.4–0.6 mmol kg�1, 6–9 mmol kg�1, and 60–80mmol kg�1, respectively), whereas a clear fingerprint ofDICrem cannot be obtained due to the low carbon-relatedratios in the shallower layers (r < 1027.7 kg m�3). Asindicated above, the ‘‘missing carbon’’ in the shallowerlayers can be interpreted as a consequence of the combus-tion of N-and P-rich material under high oxygen demand.The intrusion of the LSW into the intermediate layers of thewater column of the eastern basin is shown by lowerconcentrations of PO4,rem, NO3/2,rem, and AOU. In the

Figure 7. Remineralization ratios of DIC nutrients and oxygen obtained for the deeper water columnbelow r = 1027.7 kg m�3. The plots are shown with the same scale as Figure 6 to enable better comparison.


deeper layers the concentrations of all parameters show anincrease to values higher than 0.6 mmol kg�1PO4,rem,9 mmol kg�1NO3/2,rem, 80 mmol kg�1AOU, and 100 mmolkg�1DICrem, respectively.

[25] The water column inventories (given in the top ofFigures 8a–8d) show as a general trend an increase fromwestto east, which can be seen as a function of water columndepth, but most notably as a consequence of the longer

Figure 8. (bottom) Distributions and (top) water column inventories of remineralized (a) PO4, (b) NO3/2,(c) AOU, and (d) DIC observed along the WOCE 1A/E section obtained as function of AOU and thecorresponding remineralization ratios. (e) For comparison, the distribution of DDICant is given. Figures 8cand 8e (redrawn here) are reprinted from Thomas and Ittekot [2001] with permission from Elsevier Science.


‘‘biological history’’ of the waters in the eastern basincompared to the younger waters in the western basin. Here,the PO4,rem inventory is slightly below 1 mol m�2 PO4,rem,while for the eastern basin, approximately 2 mol m�2 PO4,rem

are calculated. The same patterns are obtained for NO3/2,rem,AOU, and DICrem revealing inventories of 10 mol m�2 NO3/

2,rem, 100 mol m�2 AOU, and 100 mol m�2 DICrem, in thewestern basin and 30 mol m�2 NO3/2,rem, 280mol m�2 AOU,and 280 mol m�2 DICrem, in the eastern basin, respectively.For convenience, the distribution of DDICant [Thomas andIttekkot, 2001] is shown in Figure 8e. While the remineral-ized compounds show a general increase with depth, theDDICant concentrations are highest at the surface. Theseexpected different features are caused by the different inputways: DDICant enters the ocean via the air-sea interface,whereas the remineralized compounds are released within theocean and show the highest concentrations in those parts ofthe water column which are exposed for long time to theremineralization of POM. Expect for the IW, those aregenerally the deeper parts of the water column. In contrastto the compounds released by organic mater remineraliza-tion, DDICant shows similar inventories in both basins. Thewestern basin has been ventilated more recently and thus ischaracterized by higher DDICant concentrations throughoutthe water column. These higher concentrations cause thecomparable inventories, although the eastern basin is deeper.[26] In order to obtain remineralization ratios for the

entire investigation area, the concentrations of PO4,rem,NO3/2,rem, AOU, and DICrem are integrated over the watercolumn and linearly interpolated along the sampling section.The averaged values are given in Table 3. The variability ofthe individual ratios is caused by the varying water columndepth along the section and the subsequent application ofthe two different ratios according to the two density layers.Consequently, the AOU:P and AOU:N ratios show rathersmall variability, since the ratios are almost constant overthe entire water column. In contrast, the carbon-relatedratios show greater variability since the water column depthand thus the applied ratios vary along the section. Even theintegrated C:N, C:P, and AOU:C ratios show that the carbonexport to the water column is higher than predicted by theobserved Redfield ratios of POM. The trend might also holdtrue for the export of P compared to N as indicated by theN:P, AOU:N, and AOU:P ratios.

7. Discussion

[27] The present study in the North Atlantic Ocean con-firms the picture of variable remineralization ratios. This hasbeen shown for other regions of the world ocean [e.g., Hupeand Karstensen, 2000], but also for the North Atlantic Oceanrestricted to N, P and AOU. Notably the carbon-related ratiosobtained above, which play a key role in assessing thebiological pump, now complete this picture and thus enablea comprehensive description of the relationships between

carbon, nitrogen, phosphorus, and oxygen in the watercolumn of the Atlantic Ocean. Attempts have been madeto provide explanations for the variability as reviewed, forexample, by Azam [1998] or Thomas et al. [1999]. Thefindings of lower C:N and C:P ratios in the shallower levelsmight thus be explained by higher N and P demand finallyconstituting a preferential recycling of nutrients. The highAOU to carbon ratios also fit into the picture of the earlyremineralization of organic nitrogen and phosphorus com-pounds like amino acids or phospholipids. The ratios inte-grated over the water column show that, on average, theexport of carbon is higher than predicted by the elementalcomposition of POM. When applying these ratios indynamic modeling experiments, more emphasis would belaid on the differences between the different depth levels.This means that carbon export relative to the nutrients alongthe deep currents would be described by the deeper watercolumn ratio rather than by the water column average. Theremineralization obtained here ratios might also have strongimpact on the determination of anthropogenic CO2 follow-ing the above separation concepts, since the quantification ofthe biological component of the total DIC would be affected.

8. Conclusions

[28] The proposed normalization of DIC with respect tohydrographic and atmospheric conditions provides a cor-rected term DICbio which reflects biological changes of DICand thus can be seen as carbon analogy to the apparentoxygen utilization (AOU). DICbio enables the description ofcarbon-related remineralization ratios of POM in the watercolumn and thus provides a direct tool for assessing the‘‘biological CO2 pump.’’ Two density levels can be identi-fied, characterized by different remineralization ratios indi-cating different microbial key processes. Notably, theobtained carbon-related remineralization ratios are on aver-age higher (for oxygen: lower) than the correspondingcomposition ratios of POM and thus imply a higher carbondrawdown of the ‘‘biological CO2 pump’’ than predicted bythe Redfield ratios of POM.

[29] Acknowledgments. The excellent cooperation of the crew and thescientific staff during the METEOR cruise M30/3 under wintry conditions isgratefully acknowledged. Special thanks to Malte Mobius for performingDIC analysis during the cruise. I express my thanks to A. Sy for providingnutrient, oxygen, and hydrographic data. The comments of Peter Croot andtwo anonymous reviewers greatly helped improve an earlier manuscript.This work was supported by the German Ministry of Education, Science,Research and Technology (BMBF, 03F0108F) and the German ResearchFoundation (DFG, IT6/9-1). This is NIOZ publication 3692.

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Table 3. Integrated Water Column Remineralization Ratios Obtained for the WOCE A1/E Section in the North Atlantic Ocean

Ratio C:N C:P AOU:C N:P AOU:P AOU:N

Water column average 8.8 ± 1.7 124 ± 22 1.1 ± 0.3 14.2 ± 0.3 131 ± 1 9.3 ± 0.1Redfield ratio 6.6 106 1.3 16 138 8.6


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Department of Marine Chemistry and Geology, P.O. Box 59, NL-1790 ABDen Burg, Texel, Netherlands. ([email protected])


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