Excess nitrate and nitrogen fixation in the North ... - Semantic Scholar

Marine Chemistry 84 (2004) 243 – 265 www.elsevier.com/locate/marchem

Excess nitrate and nitrogen fixation in the North Atlantic Ocean Dennis A. Hansell a,*, Nicholas R. Bates b, Donald B. Olson a a

Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA b Bermuda Biological Station for Research, Inc., St. Georges, GE 01 Bermuda Received 20 September 2002; received in revised form 7 August 2003; accepted 8 August 2003

Abstract The process of nitrogen fixation in the subtropical North Atlantic has received considerable study over the last few decades. The findings have highlighted a large discrepancy in estimates for the locations and rates of nitrogen fixation when results from biological techniques are compared to geochemical techniques. Here, we evaluated the distribution and rates of excess nitrate development in the North Atlantic using World Ocean Circulation Experiment (WOCE) nutrient data. These data indicate that excess nitrate development is largely confined to depths of f 150 – 400 m in the region of 15 – 25jN by 25 – 75jW, an area considerably smaller than that employed by Gruber and Sarmiento [Glob. Biogeochem. Cycles 11 (1997) 235] (10 – 50jN by 10 – 90jW) to estimate rates of nitrogen fixation in the North Atlantic. The areally integrated nitrogen fixation rate for the subtropical North Atlantic was 0.045 mol N m 2 year 1, or 62% of the geochemical estimate by Gruber and Sarmiento [Glob. Biogeochem. Cycles 11 (1997) 235]. The regional rate of fixation was 3.1 1011 mol N year 1 (1.5 – 4.6 1011 mol N year 1 given a 50% uncertainty), similar to rates expected from biological measures of fixation, but only 15% of the areal rate estimated by Gruber and Sarmiento [Glob. Biogeochem. Cycles 11 (1997) 235]. An accurate assessment of the region over which excess nitrate accumulates is critical to the estimate of nitrogen fixation, but remains prone to large uncertainty because of the gaps in spatial coverage. Additional survey work in the North Atlantic must be done to lessen the uncertainty. With this work, we reduce the differences between the biological and geochemical rate estimates, and describe a conceptual model for the location and dynamics of nitrate excess development in the North Atlantic. D 2003 Elsevier B.V. All rights reserved. Keywords: Nitrate; Phosphate; N/P ratio; Subtropical North Atlantic; Nitrogen fixation

1. Introduction Nitrogen is the limiting nutrient for phytoplankton growth and export production in many regions of the global ocean (Codispoti, 1989). The biological pump removes N from the euphotic zone with high efficiency.

* Corresponding author. Tel.: +1-305-361-4078. E-mail address: [email protected] (D.A. Hansell). 0304-4203/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2003.08.004

The N-bearing, sinking biogenic particles carry carbon away from the surface ocean, thus sequestering carbon into the ocean interior and away from the atmosphere on time scales of years to centuries. The oceanic nitrogen budget is not static, but set by the balance of sources and sinks. Fixed nitrogen is lost from the oceans during the process of denitrification. In order to maintain the global budget, sources of new nitrogen are required. These sources include wet and dry deposition of N from the atmosphere, riverine inputs of

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D.A. Hansell et al. / Marine Chemistry 84 (2004) 243–265

inorganic and organic N, and N2 fixation by marine diazotrophs. The new N supply most difficult to accurately quantify is N2 fixation by pelagic phytoplankton such as Trichodesmium. The global N2 fixation rate in the ocean has been recently estimated at 8 F 3 1012 mol N year 1, with c 2 1012 mol N year 1 fixed in the North Atlantic Ocean (Gruber and Sarmiento, 1997, hereafter GS97). A similar global rate has been estimated by Lee et al. (2002) who evaluated net carbon production in nitratedepleted tropical and subtropical waters. The GS97 rate of N2 fixation could remove c 5 1013 mol C year 1 from the upper ocean, or 10% of marine new production estimated by Chavez and Toggweiler (1995). Because of the high N/P ratio of marine diazotrophs, mineralization of these sinking particles below the euphotic zone can result in nitrate concentrations in excess of those predicted from phosphate concentrations and N/P element stoichiometry (e.g., Redfield et al., 1963; Takahashi et al., 1985; Anderson and Sarmiento, 1994). N2 fixation was invoked by Fanning (1987, 1992) to explain high N/P ratios, for example, in the thermocline waters of the western North Atlantic subtropical gyre (or Sargasso Sea). More recently, the parameter N* was introduced by Michaels et al. (1996) and Gruber and Sarmiento (1997) to indicate the degree to which the nitrate concentration is in excess of that expected from the remineralization of phosphate at stoichiometries of 16:1 (Redfield et al., 1963). N* spatial distributions (and concentration gradients) in the main thermoclines of the world’s oceans broadly reflect the global distribution of N2 fixation and denitrification. Elevated N* values indicate a history of net additions of N relative to P, while low values indicate net removal of N due to denitrification. Over the past decade, both geochemical and direct biological techniques have been employed to determine the distribution and rates of N2 fixation in the North Atlantic. Based on depth integrated excess nitrate (N*) and rates of thermocline ventilation, annual rates of N2 fixation in the subtropical gyre of the North Atlantic have been calculated to lie in the range of 72 mmol N m 2 year 1 (GS97) to 133 – 230 mmol N m 2 year 1 (Michaels et al., 1996). The region of the N* maximum was located

in the western Sargasso Sea, largely between 20j and 40jN in the waters of the main thermocline (between 200 and 600 m). These geochemical estimates of N2 fixation in the western Sargasso Sea (Michaels et al., 1996; GS97) are several fold higher than most direct biological measurements of N2 fixation by dominant diazotrophic phytoplankton (i.e., Trichodesmium spp.) in the overlying water. Carpenter (1983) reviewed studies of directly determined N2 fixation in the entire Atlantic and found relatively low rates of 0.25 – 2.26 mmol N m 2 year 1. Capone et al. (1997), summarizing work in the North Atlantic over the past few decades, reported a broad range of N2 fixation rates, from 0.51 mmol N m 2 year 1 in the NW Sargasso Sea to 101 mmol N m 2 year 1 in the NE Caribbean Sea. In the SW North Atlantic, Carpenter et al. (1999) reported elevated nitrogen fixation rates of 3 mmol N m 2 day 1 in an extensive bloom of a colonial diatom containing an N2-fixing cyanobacterial endosymbiont. They estimated that 41010 mol N could be added to the euphotic zone by the N2 fixers over the lifetime of the bloom (10 days). More recently, Orcutt et al. (2001) measured low mean annual rates of N2 fixation (15 mmol N m 2 year 1) at the Bermuda Atlantic Time-series Study (BATS) site. They found that the surface waters of the western Sargasso Sea near Bermuda have relatively low densities of Trichodesmium colonies and trichomes, but seasonally elevated biomass was found during the stratified summer months in some, but not all, years. South of Bermuda (26jN) the fixation rate was higher at 34 mmol N m 2 year 1. Except for the single very high rate reported by Capone et al. (1997) in the NE Caribbean, the N2 fixation rates determined using direct methods are considerably lower (0.25 –34 mmol N m 2 year 1) than the geochemical estimates of 72 –230 mmol N m 2 year 1 (Michaels et al., 1996; GS97). It appears that, even with interannual variability, the biologically measured fixation rates do not account for more than 20% of the minimum rate determined geochemically. Some of the biological rates reported may be underestimates of the true rates due to undersampling of the complete suite of diazotrophs present (Zehr et al., 2001), but the extent of the underestimate is not known.


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The goals of this paper are to develop an improved understanding of the spatial distribution and production of the excess nitrate found in the subtropical gyre of the North Atlantic. Our approach was to quantify geochemically the rate of excess nitrogen production using end-member analyses and isopycnal mixing considerations. This approach narrows the discrepancy between geochemical and biological estimates of nitrogen fixation rates. The temporal variability for excess nitrate in the Sargasso Sea is large. An analysis of that variability can be found in Bates and Hansell (2003).

2. Data analysis 2.1. Data sources In this analysis, we investigated the distribution and rates of formation of excess nitrate in the North Atlantic. Data employed for end-member analysis, mixing and excess nitrate formation were taken from the World Ocean Circulation Experiment (WOCE) hydrographic and nutrient data collections maintained by the World Hydrographic Program Data Archive Center (http://www-ocean.tamu.edu/WOCE/ Progress/data.html). North Atlantic WOCE sections (and dates of occupation) evaluated are AR01 (February 1998), A02 (November 1992), A03 (October 1993), A16N (August 1988), A16C (April 1989), A17N (March 1994), A20 (July 1997), A22 (August 1997) and A24 (June 1997). Section locations are shown in Fig. 1A. Nutrient data from A03 showed unreasonably high station-to-station variability, vertical variability and variability in deep water values, so these data were not used in this analysis. Phosphate data are not available on AR01 from c 21.5j to 45.5jW, so our analysis of nutrient distributions is absent that portion of the line. Salinity data from the Reid/Mantalya data set (Reid, 1994) were employed for evaluating subtropical water mixing in the North Atlantic. Water mass ages were determined from WOCE North Atlantic pCFC-12 data (e.g., Doney et al., 1997) using standard solubility considerations (Warner and Weiss, 1985) and time-series data of the atmospheric pCFC-12 transient (Walker et al., 2000). It was assumed that thermocline waters at the outcrop were in equilibrium with the present atmo-

Fig. 1. (A) Locations and designations of WOCE sections and stations evaluated. Locations of mixed water regions (MWR) I and II are shown. (B) Schematic of major currents in the region under study (AC, Azores Current; GC, Guyana Current; NBC, North Brazil Current; SEC, South Equatorial Current; NECC, North Equatorial Counter Current; NEC, North Equatorial Current; GSE, Gulf Stream Extension; NAC, North Atlantic Current).

sphere. Most figures shown here were constructed with Ocean Data View (Schlitzer, 2002). We used the concentrations of excess nitrate (DINxs) as a tracer, where DINxs ¼ N 16P

ð1Þ

and N is the concentration (Amol kg 1 N) of nitrate (plus nitrite where available) and P is the

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concentration of soluble reactive phosphate (Amol kg 1 P). DINxs is the excess in nitrate concentration relative to that expected from Redfield et al. (1963) stoichiometry. The coefficient 16 is the Redfield ratio of N/P, as confirmed by Takahashi et al. (1985) and Anderson and Sarmiento (1994). The coefficient was likewise employed by GS97 in their evaluation of N* distributions. The GS97 definition of N* is: N * ¼ ðN 16P þ 2:90Þ 0:87

ð2Þ

The constant 2.90 and the 0.87 multiplier were used in the N* equation to force the global mean N* value to be zero. More recently, the equation has been modified with removal of the multiplier 0.87 (Deutsch et al., 2001). The relationship between N*, as modified by Deutsch et al. (2001), and DINxs is N * ¼ DINxs þ 2:9

ð3Þ

giving a constant offset of 2.9 Amol kg 1 N between the two variables. We elected to use the variable DINxs instead of N* since we focused on a single ocean basin and hence did not require the constant to force the global mean N* value to be zero (GS97). Given the constant offset, spatial gradients of the excess nitrate, whether calculated as N* or DINxs, would be identical. 2.2. Water mass composition determination In this paper, the distribution of DINxs was calculated primarily for the tropical and subtropical North Atlantic. From the distributions and water mass ages, we estimated the rate of DINxs development, as well as identified regions of DINxs decrease due to water mass mixing. In order to account for binary mixing in modifying the distribution of DINxs in the subtropical gyre, the two end-member compositions were determined for rh surfaces 26.0, 26.5 and 27.0. These potential density surfaces encompass the part of the water column showing excess nitrate development in the North Atlantic. The formation of upper thermocline waters in the North Atlantic subtropical gyre involve water mass formation and subduction in the North Atlantic,

then mixing with South Atlantic water advected across the equator as part of the global thermohaline circulation (Stommel, 1965; Schmitz and Richardson, 1991). The thermocline can be analyzed using a two component potential temperature (h)-salinity mixing calculation. The h-salinity diagram from the Reid/Mantalya data set (Reid, 1994) and the distribution of temperature on density surfaces from the WOCE data are shown in Fig. 2. The main feature in the h/S curve is the pronounced salinity maxima to the right of the figure. This defines the Subtropical Underwater (STUW), a water mass formed in the eastern subtropical gyre that is then mixed with southern hemisphere water and deeper thermocline waters as it is carried around the subtropical gyre. If waters in the STUW along the right hand side of the h/S diagram (labeled NC for northern component) are defined as 100% STUW on density surfaces, and the left hand side of the cloud is defined as the southern component end-member (i.e., 0% STUW), then the other points in the h/S diagram can be quantified as a percentage of the component formed in the gyre interior (% STUW). The Reid data were used to produce a map of the percentage of STUW formed entirely in the North Atlantic and found on the 26.5 potential density surface (Fig. 3). This analysis identifies a region in the eastern limb of the gyre as the source of newly formed gyre water. This water formed in the gyre is mixed with southern component water to create the mixed waters of the western gyre. Potential temperature on rh density surfaces (Fig. 2B) was used to compute water mass fractions in the mixed water regions (discussed below). The northern components of the three potential density surfaces considered here were taken from WOCE section A16N (Table 1). The northern component of the rh 27.0 surface was taken from a region with low apparent oxygen utilization (AOU, Fig. 4) and low pCFC-12 age lying between 42j and 45jN (Fig. 5). This zone is a few degrees south of the 46jN winter outcrop position for this density surface (Jenkins, 1987; Robbins et al., 2000). The northern component of the rh 26.5 surface was determined in the region of maximum subtropical water content (Fig. 3), low AOU (Fig. 4) and minimum water mass pCFC-12 age (Fig. 5),


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Fig. 2. (A) Potential temperature (jC)/salinity diagram from the Reid/Mantalya data set (Reid, 1994). The two spline fits for gyre end points (STUW, or northern component (NC) water, and southern component (SC) water) were used to produce the chart of percent North Atlantic subtropical gyre water over the basin shown in Fig. 3. (B) Scatterplot of potential temperature (h, jC) and sigma theta (rh, kg/m3) in the North Atlantic (0 – 60jN) from WOCE data. At each potential density, the coldest and warmest waters are SC and NC, respectively. Mixing of these waters largely occurs along isopycnal surfaces, so temperatures intermediate to NC and SC indicate degree of mixing.

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Fig. 3. Distribution of subtropical underwater formed entirely in the North Atlantic, as indicated by percent fraction of the water, on the rh 26.5 surface. Values >90% indicate the region of formation.

broadly centered near 28jN, 25jW. The outcrop position for this potential density layer is near 33jN in the northeast Atlantic (Jenkins, 1987). For the rh 26.0 surface, the region of 19– 24jN (Table 1) exhibited low apparent oxygen utilization (AOU; Table 2) as well as young pCFC-12 age (Fig. 5) and thus was taken as representative of northern component water on that density surface. For the southern component, we chose equatorial waters taken to be representative of southern hemisphere water transiting into the North Atlantic (Fig. 1B). The southern component waters were evaluated in the North Equatorial Counter Current using data from WOCE section A17N (1– 5.5jN; Fig. 1B), with the rh 26.0 surface near 110 m, the rh 26.5 surface near 150 m and rh 27.0 near 370 m.

We also evaluated waters of the Guyana Current (Fig. 1B) as the southern component end-member, but found that where the WOCE sections (A17N and A20) crossed the current, salinity and potential temperature already showed significant mixing of southern and northern component waters. The waters of the North Equatorial Counter Current still retained the low salinities and low temperatures expected of southern component waters (Fig. 2, Table 2). As these waters have the same source as the Guyana Current (i.e., the North Brazil Current; Fig. 1B), they are reasonable representatives of southern component water prior to mixing with the northern waters. Water mass composition was determined using regression analyses (Fig. 6). WOCE CTD data were

D.A. Hansell et al. / Marine Chemistry 84 (2004) 243–265 Table 1 WOCE sections employed for generating regressions to determine concentrations of variables on rh 26.0, 26.5 and 27.0 in the northern and southern component end-members and the mixed water regions under evaluation Northern component

Southern component

MWRI

rh 26.0 WOCE A16N A17N – section Latitude 19 – 24jN 1 – 5.5jN – rh 25.75 – 26.25 25.75 – 26.25 – rh 26.5 WOCE A16N section Latitude 26 – 30jN rh 26.4 – 26.7 rh 27.0 WOCE A16N section Latitude 42 – 45jN rh 26.9 – 27.2

MWRII

A22 19 – 24jN 25.8 – 26.3

A17N

–

A22

1 – 5.5jN 26.3 – 26.7

– –

19 – 24jN 26.45 – 26.8

A17N

A16N

A22

1 – 5.5jN 26.9 – 27.25

20.5 – 24jN 19 – 24jN 26.75 – 27.25 26.8 – 27.2

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The waters at MWR I and II are a mix of northern and southern components so it was necessary to first calculate the preformed values of the variables of interest. Preformed values are the concentrations expected from mixing of the end-members alone. Mixing proportions of the northern and southern components in the mixed water regions were estimated with the presumption that the waters on a density horizon can be approximated by binary mixing. To calculate preformed values we followed the protocol of Takahashi et al. (1985), employing potential temperature as the conservative tracer. Potential temperatures in the northern and southern end-members are separated by c 7 – 10 jC on the density surfaces evaluated here (Fig. 2B). The fraction of the northern ( fn) and southern ( fs)

Horizontal (latitudinal) and vertical (rh) ranges of data selected for each region are shown.

used for determining potential temperature at specific potential densities, while WOCE bottle data were employed for nutrient and other determinations (i.e., nitrate, phosphate, silicate, AOU, pCFC-12 age). In each region (Table 1), rh was plotted against the relevant variables. The rh range bounding each potential density surface (rh 26.0, 26.5 and 27.0) that showed linearity with the variable of interest in property/property plots (e.g., Fig. 6) was characterized by linear regression. The regression equations were used to estimate property values for the variables at rh 26.0, 26.5 and 27.0 in each region. End-member compositions determined by this protocol are shown in Table 2. Specific regions of the gyre evaluated for DINxs development were designated mixed water regions I and II (MWR I and II; Fig. 1A). The highest DINxs values on rh 27.0 were located in MWR I. It was also the region taken as representative of northern component water on rh 26.0. MWR II was chosen for analysis because it had elevated DINxs values on rh 26.0 and it served for analysis of DINxs development across the North Atlantic on rh 26.5.

Fig. 4. Distribution of AOU (Amol kg 1) on rh surfaces (A) 26.5 and (B) 27.0.

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Fig. 5. Distribution of pCFC12 ages in years on rh surfaces (A) 26.0, (B) 26.5 and (C) 27.0.

component end-members in the mixed water region were obtained from O O fs ¼ ðhO n hÞ=ðhn hs Þ

ð4Þ

and fn ¼ 1 fs

ð5Þ

where hnO and hOs are the potential temperatures of the northern and southern components, respectively, and h is potential temperature measured in the region being evaluated. The preformed concentrations (or age) in the mixed water region were obtained by C O ¼ fn CnO þ fs CsO

ð6Þ

where CnO and COs are the preformed quantities in the northern and southern end-members, respectively. Values for CnO and COs were computed using the

rh- property regressions established (e.g., Fig. 6). Measured and preformed concentrations in MWR I and II are reported in Table 2. The differences between preformed and measured values are a measure of the concentration changes due to processes other than mixing, i.e., nitrogen fixation and the biogeochemical processes of interest. The largest contributor to error in estimating the modification of water properties by mixing is the local heterogeneity of water masses due to mesoscale and sub-mesoscale spatial variability. The two component mixing analysis is particularly robust in the present case since we are considering a water mass formed in the North Atlantic subtropical gyre and its modification through the admixture of southern hemisphere water that is well characterized in the North Brazil Current. Errors in the estimation of properties expected from mixing alone were assessed using the root mean square variability in regional observations (measured values). These errors were then propagated through the equations for determin-

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Table 2 Potential temperature, composition and ages of end-members and mixed water regions I and II on rh 26.0, 26.5 and 27.0 Northern component

Southern component

MWR II Measured c

rh 26.0—mixing into MWR II between northern (at MWR I) (87 F 0.3%) h (jC) 21.4 F 0.1 17.8 F 0.2 Silicate (Amol kg 1) 0.2 F 0.2 3.6 F 1.2 0.9 F 0.9 8.3 F 3.6 Nitrate (Amol kg 1) Phosphate (Amol kg 1) 0.06 F 0.05 0.6 F 0.2 AOU (Amol kg 1) 8.1 F 11.0 62.3 F 27.1 pCFC-12 age (years) 1.3 F 2.9 11.9 F 1.5 DINxs (Amol kg 1) 0.1 F 0.3 0.2 F 1.1 Northern component

Southern component

Preformeda

and southern (13%) components 20.9 F 0.2 – 0.5 F 0.1 0.7 F 0.4 1.4 F 0.7 1.9 F 1.3 0.06 F 0.04 0.14 F 0.1 27.6 F 6.0 15.4 F 13.3 2.4 F 0.9 2.7 F 2.7 0.4 F 0.2 0.3 F 0.4

Southern component

Measured

Preformed

Southern component

Difference – 0.3 F 0.1 1.9 F 0.3 0.1 F 0.1 17.2 F 1.7 5.2 F 1.0 0.5 F 0.5

MWR I Measured

Preformed

rh 27.0—mixing into MWR I between northern (102 F 1%) and southern ( 2%) components h (jC) 12.7 F 0.1 8.7 F 0.2 12.8 F 0.1 – Silicate (Amol kg 1) 2.6 F 0.5 16.3 F 0.6 6.5 F 0.9 2.6 F 0.5 Nitrate (Amol kg 1) 7.4 F 0.9 28.6 F 0.9 16.9 F 1.7 7.4 F 1.3 0.4 F 0.0 1.9 F 0.0 1.0 F 0.9 0.4 F 0.0 Phosphate (Amol kg 1) AOU (Amol kg 1) 18.4 F 4.8 155.0 F 8.3 98.4 F 11.4 18.4 F 6.8 pCFC-12 age (years) 0.3 F 1.6 30.0 F 1.5 22.1 F 3.2 0.3 F 2.0 DINxs (Amol kg 1) 0.4 F 0.8 1.8 F 0.5 1.2 F 0.4 0.4 F 0.9 MWR I component

– 0.2 F 0.3 0.5 F 0.6 0.1 F 0.1 12.2 F 0.7 0.3 F 1.8 0.7 F 0.2

MWR II

rh 26.5—mixing into MWR II between northern (90 F 0.3%) and southern (10%) components h (jC) 18.0 F 0.2 14.0 F 0.1 17.6 F 0.1 – Silicate (Amol kg 1) 0.6 F 0.1 7.7 F 0.4 1.6 F 0.1 1.3 F 0.2 Nitrate (Amol kg 1) 2.3 F 0.6 17.2 F 1.6 5.7 F 0.5 3.8 F 0.8 Phosphate (Amol kg 1) 0.1 F 0.0 1.2 F 0.1 0.3 F 0.0 0.2 F 0.1 16.0 F 5.3 108.5 F 14.7 42.5 F 4.9 25.2 F 6.6 AOU (Amol kg 1) pCFC-12 age (years) 2.7 F 1.0 13.9 F 1.5 9.0 F 2.1 3.8 F 1.1 DINxs (Amol kg 1) 0.7 F 0.6 1.9 F 0.3 0.9 F 0.1 0.4 F 0.6 Northern component

Differenceb

c

Difference – 3.9 F 0.4 9.5 F 0.4 0.5 F 0.9 80.0 F 4.6 21.8 F 1.2 0.9 F 0.5

MWR II Measured

rh 27.0—mixing into MWR II between MWR I (81 F1%) and southern (19%) component h (jC) 12.8 F 0.1 8.7 F 0.2 12.0 F 0.5 Silicate (Amol kg 1) 6.5 F 0.9 16.3 F 0.6 8.9 F 1.5 Nitrate (Amol kg 1) 16.9 F 1.7 28.6 F 0.9 19.7 F 2.0 Phosphate (Amol kg 1) 1.0 F 0.9 1.9 F 0.0 1.2 F 0.1 AOU (Amol kg 1) 98.4 F 11.4 155.0 F 8.3 124.7 F 11 pCFC-12 age (years) 22.1 F 3.2 30.0 F 1.5 27.1 F 2.1 DINxs (Amol kg 1) 1.3 F 0.4 1.7 F 0.5 0.3 F 0.3

Preformed

Difference

– 8.3 F 1.1 19.1 F 2.0 1.1 F 0.7 108.9 F 13.3 23.6 F 3.5 0.7 F 0.1

– 0.6 F 0.4 0.6 F 0.1 0.1 F 0.6 15.8 F 2.3 3.5 F 1.4 0.3 F 0.2

Changes in composition on rh 27.0 between MWR I and MWR II, with preformed values expected from mixing between southern component waters and those of MWR I, are also considered. a Preformed values in the mixed waters are those expected from mixing of the northern and southern component waters. b Differences between measured and preformed concentrations are due to processes other than mixing. c Percentages of the northern and southern components in the mixed water regions considered.


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ing preformed concentrations and rates using product and quotient rules given in Rade and Westergren (1992). The assumption was that the largest uncertainty was in determining the end points, due to local variability at those sites. The errors increase for very young waters and those with concentrations approaching the level of the regional variance.

3. Results and discussion 3.1. Distribution of DINxs Contoured sections of DINxs on three meridional sections (WOCE lines A16, A20 and A22) are shown in Fig. 7. The highest concentrations of DINxs lie between rh surfaces 26 and 27, with the southern regions showing the greatest accumulations. The horizontal distributions of DINxs on the rh 26, 26.5 and 27 surfaces are shown in Fig. 8. The zero isoline indicates the locations of Redfield ratios of N and P, whereas values >0 Amol kg 1 indicate the presence of excess nitrate relative to phosphate. On the rh 26 surface, the most elevated DINxs values (>0.5 Amol kg 1) were located in the southwestern North Atlantic. The highest DINxs values on the two other density surfaces were largely in the southeastern sector of the North Atlantic basin, where values reached >1 and >1.5 Amol kg 1 on the rh 26.5 and 27.0 surfaces, respectively (Fig. 8). The lowest values on all surfaces were in the northeast of the basin and in the equatorial region, reflective of low DINxs northern and southern components, respectively. The DINxs distributions shown in Fig. 8 are based on section occupations that took place over a 10-year period, so some temporal variations exist that are not shown in the distribution maps. For example, DINxs values on the 26.5 surface, west of 60jW on the AR01 line, were >1 Amol kg 1 during the February 1998 occupation, as shown in Fig. 8B. Values on A22 (occupied August 1997), at the crossover point with

Fig. 6. Representative data regressions employed to determine endmember and mixed water region compositions on the potential density surfaces of interest. Data shown are potential temperature (h, jC) and potential density (rh, kg/m3) at 26.5 from the region of (A) the northern component, (B) MWR II and (C) the southern component.

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Fig. 7. Contours of DINxs (Amol kg 1) on WOCE sections (A) A16, (B) A20 and (C) A22.

AR01, were < 1 Amol kg 1, or as much as 0.6 Amol kg 1 less than the values found during AR01. Temporal variability of DINxs in the Sargasso Sea is rapid, with large changes occurring over seasonal time scales (Bates and Hansell, 2003). We chose to map the elevated values present on AR01, but the meridional extent of those elevated values at the time of the AR01 occupation is unknown. Gyre scale circulation patterns, the mixing proportions of low DINxs northern and southern component waters, in situ DINxs development, and the vertical sloping of the isopycnals across the basin combine to

control the distributions of DINxs. Taking the rh 26.5 surface (Fig. 8B) as an example for the impact of these processes, the northern limb of the gyre undergoes mixing with low-DINxs northern water as it flows to the east (Line A, Fig. 9). During southward flow (Line B, Fig. 9), DINxs builds to its highest levels in the gyre. The southern limb of the gyre (Line C, Fig. 9) experiences a net decrease in DINxs by dilution with low-DINxs southern component water. A distinctive difference in the DINxs distributions is the southwestern location of the DINxs maximum on the shallowest potential density surface (rh 26.0)


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Fig. 9. Schematic of subtropical gyre circulation, with inputs of SC and NC waters (the latter introduced vertically in the region identified by the dashed oval). Circulation streamlines overlay the contoured distribution of DINxs on the rh 26.5 surface. Lines A, B and C are discussed in the text.

f 0.25 Amol kg 1 across the basin, while on the rh 27.0 surface DINxs decreases by >1 Amol kg 1. The distribution of density across the North Atlantic shows that the three surfaces considered here are relatively shallow in the eastern basin and deepen in the western basin (Fig. 10). We suggest that if the process responsible for DINxs development is occurring somewhat uniformly across the basin, and if the depth of DINxs development across the basin is similar, then the unequal distributions of DINxs shown in Fig.

Fig. 8. Contoured distributions of DINxs (Amol kg 1) on rh surfaces (A) 26.0, (B) 26.5 and (C) 27.0.

versus the southeastern locations of maxima for the deeper surfaces. A difference in the DINxs distributions (Fig. 8) between the 26.5 and 27.0 density surfaces emerges as the gyre circulation moves the high DINxs waters from east to west on the southern gyre limb (Fig. 8B,C). DINxs on rh 26.5 decreases by

Fig. 10. Sigma theta in the upper 1000 m across the North Atlantic at 24.5jN (WOCE section AR01).

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8 would be expected due to deepening of the isopycnals. We can test for the depths of incipient DINxs development by evaluating the distribution of the depth where DINxs = 0 Amol kg 1 in the North Atlantic. Since DINxs in the surface mixed layer is generally < 0 Amol kg 1, while DINxs at the depth of the DINxs maximum is normally >0.5 Amol kg 1, the transition from negative to positive DINxs (i.e., the depth where DINxs = 0 Amol kg 1) is a useful indicator of the depth at which DINxs begins to develop. DINxs increases to >0 Amol kg 1 at depths generally >150 m (Figs. 11 and 12) across the entirety of the gyre. Because the isopycnals are relatively shallow in the east (Fig. 10), development of DINxs in that part of the basin will be weak on the shallowest surfaces (rh 26.0 was < 150 m deep) and stronger on the deeper two surfaces (rh 26.5 and 27.0 at 150– 450 m deep). In the west, the rh 26.0 surface deepens to intersect the depths of DINxs production, while the rh 27.0 surface may lie below the depth of high DINxs production, and therefore be more strongly impacted by dilution with low DINxs southern component water. The effect of sloping isopycnals and dilution is depicted schematically in Fig. 13. The result of deepening of density surfaces from east to west (Fig. 10), the observed basin wide uniformity in the depths of DINxs development (Fig. 12), and dilution by low

Fig. 11. Depth profiles of DINxs at WOCE stations in the area bounded by 15 – 30jN across sections A22 (66jW), A20 (52jW) and A16N (27jW).

Fig. 12. Distribution of the shallowest depths where DINxs = 0 Amol kg 1.

DINxs southern component waters in the west (Fig. 9), would be unequal east –west DINxs distributions and gradients on the rh 26.0, 26.5 and 27.0 surfaces, as seen in Fig. 8. 3.2. Development of DINxs It appears that DINxs develops most strongly on the rh 26.0 surface during transit from the southeast to the southwest portions of the gyre (Fig. 8A), and on the rh 26.5 and 27.0 surfaces during transit from the northeast (near sites most impacted by northern component ventilation) to the southeast of the basin (Figs. 8B,C and 9). On the deeper two surfaces, with subsequent flow to the west, DINxs is apparently diluted with southern component water, with the strongest dilution effect seen on the rh 27.0 surface. The regions of DINxs development and dilution can be further studied by plotting DINxs against a variable such as AOU. The AOU maxima (Fig. 4) are located in the circulation shadow zone (Luyten et al., 1983) off NW Africa (these waters are not included in the following analysis of DINxs/AOU). Low AOU in recently ventilated northern component water (NE basin) and high AOU in older southern component water (equatorial region) are evident (Fig. 4). Property/property plots of DINxs and AOU show low DINxs values both at low AOU (northern component water) and at high AOU (southern component water) (Figs. 14 and 15). DINxs builds on the rh 26.5 surface between AOU 15 and 50 Amol kg 1, holds relatively


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Fig. 13. Conceptual model depicting development and distribution of DINxs on three rh surfaces on a zonal section in the subtropical North Atlantic. The relative rates and depths of DINxs formation are indicated by the horizontally aligned gray scale figure, with darkest gray indicating the highest formation rate. Arrows indicate the contributions of northern (from the northeast) and southern (from the southwest) component waters, which carry low DINxs values. The highest DINxs values on the rh 26.0 surface are in the west (Fig. 8), where that isopycnal deepens to intersect the depths of high DINxs production. In the east, the rh 26.0 surface is too shallow to experience strong DINxs accumulation. The highest DINxs values on both the rh 26.5 and 27.0 surfaces, however, are in the eastern North Atlantic, where these isopycnal surfaces intersect the depths of maximum DINxs production. As the waters at these densities are transported with the gyre circulation to the west and the density surfaces deepen, the shallower density surface (rh 26.5) continues to experience DINxs accumulation, but the deeper density surface lies below the depths where DINxs development is important. DINxs on this deepest surface will decrease as it is diluted with low DINxs southern component water. On the rh 26.5 surface, dilution with southern component water occurs as well, but DINxs development continues so the values remain elevated.

constant at values < 1 Amol kg 1 to AOU of 100 Amol kg 1, then decreases as southern component water becomes quantitatively more important (Fig. 14). DINxs is less well correlated with AOU development (AOU < 60 Amol kg 1) on the rh 27.0 surface (Fig. 15). Dilution with low DINxs southern component water is strong across the basin (note decreasing DINxs with increasing AOU). Rates of DINxs development were determined after correcting for the effects of mixing between the endmembers. End-member compositions are given in Table 2. Also shown in Table 2 are the preformed and measured values in the mixed water regions. The regions evaluated for DINxs development (MWR I and II; Fig. 1A) were taken from the areas of maximum DINxs along the A16N section (for rh 27.0) and from the southern Sargasso Sea along the A22 section (for rh 26.0 and 26.5, where DINxs development is strong, and for rh 27.0, where dilution with southern component water is strong). DINxs development on rh 26.5 was evaluated at MWR II alone. The measured concentrations of most variables in MWR II on rh 26.0 were less than predicted by conservative mixing of the two end-members, resulting in negative changes in concentrations (Table 2). This potential density surface was at depths of 100 m (at MWR I and southern component sites; Table 1) to 200 m (MWR II), so non-conservative concentration

decreases could be due to biological utilization of the nutrients while in the euphotic zone. AOU increased, however, suggestive of a net heterotrophic system. Because two end-member mixing resulted in negative production rates for several of the variables, we chose to assume single end-member mixing (i.e., no other mixing end-members involved in transport and mixing) on this surface (from MWR I, the northern component of rh 26.0, to MWR II). The rates of change in the variables are given in Table 3. DINxs production at 0.43 Amol kg 1 year 1 was very similar to that of GS97 (dDN*/dDs in their Table 2) who determined a rate of 0.41 Amol kg 1 year 1 on rh 25.9. Adjustment of the GS97 estimate for the Deutsch et al. (2001) modification of the N* equation would increase their rate to 0.47 Amol kg 1 year 1. During flow from the northeast gyre to the southwestern Sargasso Sea on rh 26.5 (to MWR II; or Lines B and C in Fig. 9) DINxs increased at a rate of 0.09 Amol kg 1 year 1 (Table 3). Nitrate increased at 0.37 Amol kg 1 year 1, with most of this accompanied by an increase in the phosphate concentrations at the Redfield ratio. The rate of DINxs change can be compared to the rate estimated by GS97 for that surface (using interpolation of their estimates on bounding surfaces and adjusted for the Deutsch et al. (2001) modification of the N* equation). The corrected GS97 rate of N* development was 0.15 Amol kg 1 year 1, or 65%

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On rh 27.0, the rate of change in DINxs during circulation from the northeast to the southeast sector of the gyre (MWR I; Line B in Fig. 9) was 0.04 Amol kg 1 year 1 or 53% of the (adjusted) rate estimated by GS97. During flow from the east to the west along the southern limb of the gyre (from MWR I to MWR II; Line C in Fig. 9), the rate of DINxs change was negative at 0.10 Amol kg 1 year 1. The other nutrient variables increased at rates more or less similarly on both Lines B and C, but DINxs showed

Fig. 14. (Upper) Plot of AOU (Amol kg 1) vs. DINxs (Amol kg 1) on the rh surface 26.5. Data are from the rh range 26.4 < rh < 26.5, on the hydrographic sections shown in the map (lower plot). Northern component (data from north of Line A on the map; AOU < 25 Amol kg 1 and DINxs < 0.3 Amol kg 1) and southern component (south of Line B; high AOU and low DINxs) data are plotted within the concavities of Lines A and B in the data plot, respectively. AOU develops most strongly between Lines A and B, the region of the subtropical gyre circulation, commensurate with c 0.5 Amol kg 1 increase in DINxs.

higher than the DINxs rate of change estimated here. The oxygen utilization rate (OUR; estimated from the gradient in AOU divided by the gradient in age) of 3.33 Amol kg 1 year 1 on the rh 26.5 surface is somewhat low compared to earlier estimates. Jenkins (1987) estimated OUR in the b-triangle area to be 8.9 Amol kg 1 year 1, while Sarmiento et al. (1990) estimated a rate for the broader subtropical gyre to be 4.1 Amol kg 1 year 1.

Fig. 15. (Upper) Plot of AOU (Amol kg 1) vs. DINxs (Amol kg 1) on the rh surface 27.0. Data are from the rh range 26.9 < rh < 27.1, on the hydrographic sections shown in the map (lower plot). Data from the ellipse A in the data plot are from ellipse A in the map. Data from ellipse B are from the region encompassed by ellipse B on the map. Strong DINxs accumulation in region A is not well correlated to AOU development. Between regions A and B, DINxs decreases in part due to mixing.

D.A. Hansell et al. / Marine Chemistry 84 (2004) 243–265 Table 3 Estimated rates of change in composition (Amol kg 1 year 1) along path of flow after correction for mixing between end-members Rate (A) MWR II on rh 26.0a Silicate 0.228 F 0.407 Nitrate 0.374 F 1.185 Phosphate 0.003 F 0.089 OUR 16.25 F 10.63 DINxs 0.434 F 0.259

DO2/DProperty ratio 71.3 43.4 5413 – –

(B) MWR II on rh 26.5b Silicate 0.056 F 0.030 Nitrate 0.369 F 0.128 Phosphate 0.017 F 0.023 OUR 3.330 F 0.963 DINxs 0.091 F 0.115

59 9 196 – –

(C) MWR I on rh 27.0c Silicate 0.179 F 0.028 Nitrate 0.436 F 0.042 Phosphate 0.025 F 0.043 OUR 3.670 F 0.413 DINxs 0.039 F 0.025

20 8 147 – –

(D) MWR II on rh 27.0d Silicate 0.159 F 0.183 Nitrate 0.173 F 0.097 Phosphate 0.017 F 0.183 OUR 4.471 F 2.463 DINxs 0.096 F 0.091

28 26 263 – –

26) relative to Line B. The DO2/DPO4 3 ratio had a narrow range in the east ( 147 to 196) and increased by approximately one half to 263 in the southern gyre. Takahashi et al. (1985) reported DO2/ DNO3 and DO2/DPO4 3 ratios on rh 27.0 in the North Atlantic of 9.4 and 165, respectively, in agreement with those we report in the eastern gyre. Both phosphate and nitrate increased in the southern gyre at rates lower than expected given the oxygen utilization, but of the two properties nitrate growth was the weakest relative to OUR. The true cause of the DINxs concentration decrease is unknown. With the exception of the rh 26 surface, the rates given in Table 3 have been corrected for binary mixing of northern and southern components in the gyre. On the rh 26 surface, northern component water strongly dominates in this analysis (i.e., at MWR I, where the northern component made up essentially 100% of the water) so single end-member mixing is reasonable. On deeper surfaces production rates assuming single end-member mixing overestimate the rates corrected for binary mixing by up to three-fold (Table 4). GS97 did not correct for binary mixing on Table 4 Rates of change (Amol kg 1 year 1) assuming binary and single end-member mixing, as well as the ratio of the rates

a strong negative growth in the southern portion of the gyre that cannot be explained by mixing alone. The OUR on rh 27.0 ranged from 3.67 to 4.47 Amol kg 1 year 1 for the eastern and southern portions of the gyre, somewhat lower than the OUR of 6 Amol kg 1 year 1 determined by Doney and Bullister (1992) in the eastern North Atlantic on that surface. Property/property ratios give some insight as to which variables (nitrate or phosphate) may have caused the negative DINxs production on rh 27.0 (Table 3). In the eastern gyre (Line B, Fig. 8), the DO2/DNO3 had a narrow range of 8.4 to 9.0 on the two potential density surfaces, but in the southern section (Line C) the ratio increased three-fold (to

Ratioa

Mixing

a

Rates between northern component and MWR II on rh 26.0. Single end-member mixing was assumed on the rh 26 surface; two end-members mixing was assumed on the other surfaces. b Rates between northern component and MWR II on rh 26.5. c Rates between northern component and MWR I on rh 27.0. d Rates between MWR I and MWR II on rh 27.0.

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Binary

Single

Not corrected/ corrected

(A) MWR II on rh 26.5 Silicate 0.06 Nitrate 0.37 Phosphate 0.03 OUR 3.33 DINxs 0.09

0.16 0.54 0.03 4.10 0.07

2.8 1.5 1.9 1.2 0.7

(B) MWR I on rh 27.0 Silicate 0.18 Nitrate 4.36 Phosphate 0.02 OUR 3.67 DINxs 0.04

0.18 4.36 0.03 3.67 0.04

1.0 1.0 1.1 1.0 1.0

(C) MWR II on rh 27.0 Silicate 0.16 Nitrate 0.17 Phosphate 0.02 OUR 4.47 DINxs 0.10

0.48 0.56 0.05 5.20 0.20

3.0 3.2 2.7 1.2 2.1

a

Calculated prior to rounding of rates.

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the two potential density surfaces we have considered, and this difference in treatment contributes to the differences in the rates estimated. A second difference between the calculations here and by GS97 is the choice of age tracers. There exists an offset between water mass ages estimated by pCFC-12 age (used here) and the 3H/3He used by GS97. Doney et al. (1997) reported a relatively constant, positive offset in the CFC ages in the thermocline (older CFC age compared to the 3H/3He age), and a non-linear offset at 3H/3He ages >12.5 years, with much younger 3H/3He ages relative to CFC ages. The rates of DINxs development on the rh 26.0 and 26.5 surfaces are unaffected by the offset because the ages in MWR II are < 9 years (Table 2), so both tracers should give similar age gradients. On the deeper isopycnal, however, the offset is significant and nonlinear, thus affecting calculated rates. In MWR I on the rh 27.0 surface, the pCFC-12 age of 22 years (Table 2) is approximately equal to a 3H/3He age of 15 years, estimated from the age offset in Fig. 8A of Doney et al. (1997). Using the younger 3H/3He age on the rh 27.0 surface rather than the pCFC-12 age would raise the DINxs production rate by f 50%. This difference can account for the discrepancy in rates on the rh 27.0 surface estimated here with that by GS97. van Aken (2001) suggests that the main cause for the offset in age between the two tracers is that the surface layer water can remain undersaturated with respect to the atmospheric CFC concentrations, causing an underestimate in the OUR. He recommended an upward revision of CFC-derived estimates of OUR in the main thermocline by a factor of 1.3. Applying this factor to the DINxs production rates on rh 27.0 in Table 3 would reduce the difference with the estimates of GS97. Finally, variability in the strength, timing and location of nitrogen fixation in the North Atlantic is likely reflected by interannual or decadal changes in the location and gradients of excess nitrate. Bates and Hansell (2003) have demonstrated strong temporal variability in DINxs in the Sargasso Sea, which correlates well with the North Atlantic Oscillation (NAO). Likewise, as indicated above, DINxs values at the cross over point of AR01 and A22 varied by as much as 0.6 Amol kg 1, suggesting temporal variability in that region. GS97 employed data collected from several oceanographic programs (such as the

Geochemical Sections [GEOSECS] program and Transient Tracers in the Ocean [TTO]) taking place over the course of c 2 earlier decades. The WOCE program data employed in this analysis were collected later, between 1988 and 1998, so the offset in time of collections may be important. 3.3. Regional rates of nitrogen fixation GS97 calculated the rate of nitrogen fixation in the North Atlantic by applying their rates of N* production to the volumes of the isopycnal layers of interest. The rates of N* development were adjusted for the effect of elevated N/P ratios in the nitrogen-rich organic matter produced by diazotrophic organisms (taken to be 125), thus providing estimates of regional N2 fixation (with application of Eq. (14) from GS97). We used the GS97 equations for calculating rates, so if there is an error in the assumption about the ratio, then our rates are likewise affected. Analyses are required to accurately confirm the appropriate ratios to be used in the rate calculations. The region over which GS97 applied their volumetric rates was 10– 50jN and 10 –90jW, an area bounding the subtropical gyre and a portion of the Gulf of Mexico. We also estimated the volumes bounding the rh 26.0, 26.5 and 27.0 surfaces where DINxs development was evident (Table 5). We caution that WOCE coverage of the North Atlantic is inadequate to provide high confidence in the actual areas over which the rates should be applied, particularly for the rh 27.0 surface. Our findings, however, do not support the application of the DINxs development rates to the entirety of the subtropical gyre as was done in GS97; we found that the areas where DINxs development takes place were regional and not gyre wide (Fig. 8). This difference in regional distribution of excess nitrate development results in a large difference in estimated regional rates of nitrogen fixation. Volumes of the 26.0, 26.5 and 27.0 potential density intervals were calculated from the areas bounded by 10– 25jN, 25 – 75jW; 15 – 25jN, 25– 75jW; and, 15 – 40jN, 20– 40jW, respectively. Selection of the regions over which excess nitrate develops on density surfaces is of central importance to the resulting regional estimates of nitrogen fixation. We used the distributions depicted in Figs. 8, 14 and 15) to guide the selections. Fig. 8A provided the


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Table 5 Estimated rates of change of DINxs and rates of nitrification of nitrogen-rich organic material from diazotrophic organisms (N-rich Nitr.) on three isopycnal surfaces in the North Atlantic Potential density interval

Volumea (1014 m3)

DDINxs/Dage (Amol kg 1 year 1)

DDINxs/Dage (1011 mol year 1)

N-rich Nitr.b (Amol kg 1 year 1)

N-rich Nitr. (1011 mol year 1)

GSV/Vc

GSN/Nc

25.9 – 26.1 26.4 – 26.6 26.9 – 27.1

1.0 2.6 5.1

0.434 F 0.259 0.091 F 0.115 0.039 F 0.025

0.43 0.24 0.20

0.580 F 0.350 0.123 F 0.155 0.052 F 0.034

0.58 0.34 0.26

5.1 6.9 6.9

4.6 9.1 11.5

a

Volumes of the 26.0, 26.5 and 27.0 potential density intervals are from the areas bounded by 10 – 25jN, 25 – 75jW; 15 – 25jN, 25 – 75jW; and 15 – 40jN, 20 – 40jW, respectively. b Calculated using Eq. (14) of GS97 and assuming zero denitrification. c GSV/V and GSN/N are the ratios of the GS97 volumes and nitrification rates of nitrogen-rich organic material to volumes and rates determined in this analysis. The GS97 volumes and rates were recalculated for the potential density intervals employed here.

rationale for the choice of region for the rh 26.0 surface. Fig. 8B demonstrates that excess nitrate development on the rh 26.5 surface is spread widely across the gyre; hence, the wide region chosen (25 – 75jW). Figs. 8C and 15 demonstrate that excess nitrate development on the deepest density surface considered here was largely restricted to the eastern portion of the gyre. The westernmost boundary of the box defining the rh 27.0 surface lies in a particularly data sparse region between the A20 and A16 sections. Given the paucity of data between those widely spaced lines (i.e., recall the gap in nutrient data on AR01), we chose the approximate midpoint between them (40jW) as the western boundary. The volumes applied here are five to seven times smaller (Table 5) than the volumes employed by GS97 over the same potential density intervals (we do not include the Gulf of Mexico). Our estimates for the rates of nitrification of nitrogen-rich organic matter (assuming that nitrogen fixation is the primary source of the nitrate excess) were 4.6 –11.5 times less than the rates estimated by GS97 because our DINxs production rates and the volumes over which production occurred were smaller (Table 5). The sum of the rates of nitrification of nitrogen enriched material estimated here for the North Atlantic subtropical gyre is 13.5% of the GS97 rate summed over the same three potential density intervals. Applying this percentage to the total rate of nitrogen fixation estimated by GS97 for the North Atlantic Ocean (23 1011 mol N year 1) results in our areally integrated estimate of 3.1 1011 mol N year 1. An assigned 50% uncertainty in our choice of areas under which excess nitrate accumulation occurs gives a range of 1.5–

4.6 1011 mol N year 1. This range, determined here from DINxs spatial variability, is comparable to the range by Bates and Hansell (2003) based on an analysis of DIN xs temporal variability (0.9 – 4.6 1011 mol N year 1). With most of the DINxs development occurring in the area 15– 25jN by 25 – 75jW (6.83 1012 m2), the mean nitrogen fixation rate is 0.045 mol N m 2 year 1, or 62% of the GS97 estimate. We do not know if N2 fixation is the sole source of excess nitrate in the subtropical North Atlantic. GS97 corrected for atmospheric deposition of N to the North Atlantic using the Duce et al. (1991) estimate of 11 mmol N m 2 year 1, thus reducing their estimate to 20 1011 mol N year 1. Prospero et al. (1996) estimated a higher rate of 17 – 20 mmol N m 2 year 1, which includes wet and dry deposition of inorganic and organic N. If this N accumulates at the depths of the nitrate excess maximum in the areas considered here, then our rate of N2 fixation would be an overestimate. A brief note is included here on a unique region of high DINxs on potential density surfaces greater than rh 27.0. Throughout most of the North Atlantic, DINxs values at depths greater than the base of the main thermocline are < 0 Amol kg 1, indicating a nitrate deficiency relative to phosphate. An exception exists in the depth range of 1000 – 4000 m on the A16 line, centered near 38jN (data not shown), where elevated values for DINxs (>0.5 Amol kg 1) were present. These high DINxs values are associated with elevated salinity, indicative of the Mediterranean Sea outflow. GS97 plots of N* on rh 27.1 also show evidence of this potential Mediterranean source of

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excess N. Kress and Herut (2001) reported N/P ratios in the deep water and the Levantine Intermediate Water of the eastern Mediterranean Sea to be >24 and >18, respectively. Outflow of similarly high N/P water from the Mediterranean Sea would result in the DINxs distribution found at >1000 m on section A16.

4. Concluding statements Geochemical and direct biological estimates of nitrogen fixation in the tropical and subtropical North Atlantic converge in this analysis. Our areal rate (0.045 mol N m 2 year 1) is three times the mean rate of nitrogen fixation measured by Orcutt et al. (2001) at 31jN in the western Sargasso Sea near Bermuda, supporting the finding that most of the N2 fixation occurs at latitudes south of Bermuda. Our rate is consistent with the global mean rate of N2 fixation for the tropical ocean of 0.039 mol m 2 year 1 and twice the rate of 0.026 mol m 2 year 1 measured between 14jN and 22jN in the SW North Atlantic (Capone et al., 1997). The Capone et al. (1997) global mean rate of tropical nitrogen fixation, applied over the area in which we found excess nitrate in the North Atlantic (6.83 1012 m2), gives a nitrogen fixation rate of 2.7 1011 mol N year 1. Given the uncertainties in both estimates, our rate based on geochemical methods (3.1 1011 mol N year 1) is indistinguishable from the rate derived from biological measurements. The distributions of N* in GS97 suggested that most of the N2 fixation in the Sargasso Sea was in the west, including the waters around Bermuda. The low rates measured at the BATS site by Orcutt et al. (2001), as well as the absence of seasonal variability in the isotopic composition of DON there (Knapp and Sigman, 2003), challenge those findings. Lipschultz and Owens (1996) listed several inconsistencies between observations of excess nitrate at the BATS site and measured rates of nitrogen fixation, all suggesting a weak linkage between N* and nitrogen fixation at the site. Hansell and Carlson (2001) reported summer time TON concentrations of 4 –5 Amol kg 1 at BATS, values that are both relatively low and static. Such characteristics are not expected if N2 fixation is a dominant process there. Karl et al. (1992), for example, reported a 3.1-fold increase in DON in the presence of a Trichodesmium spp. bloom near Station

ALOHA in the North Pacific. Because Hansell and Carlson (2001) found no substantive summer time increases in TON, they suggested that N2 fixation rates were unlikely to be as high at the BATS site as indicated by the GS97 findings. Instead, if N2 fixation was responsible for elevated N/P ratios in the main thermocline of the Sargasso Sea, then the process must take place south of Bermuda and the excess nitrate transported with the gyre circulation. The results presented here support that hypothesis. Here, we showed particularly high excess nitrate concentrations to the south of Bermuda and in the eastern portion of the gyre. If the high excess nitrate signals are truly indicative of high nitrogen fixation rates in those locales, then it is intriguing to consider that the regional imprint of nitrogen fixation is largely aligned with the track of dust transport and deposition in the North Atlantic (Ginoux et al, 2001). Bates and Hansell (2003) proposed a link between the temporal variability of DINxs in the western Sargasso Sea and variations in dust inputs that were tied to the North Atlantic Oscillation. Mahaffey et al. (2003) likewise suggested that biogeochemical proxies for nitrogen fixation (i.e., high excess nitrate in the thermocline, cyanobacterial phytopigments and stable isotope signatures), particularly high between 26jN and 32jN on 20jW, are coincident with enhanced atmospheric dust deposition. Their findings, like those reported here, indicate that nitrogen fixation may occur at high rates at mid latitudes in the eastern basin where dust inputs are high. Support for this comes from Ka¨hler et al. (personal communication) who measured both high N2 fixation rates (up to 4.5 mmol N m 2 day 1) ) and high rates of DINxs accumulation (0.5 mmol N m 2 day 1) in the thermocline along 30jW (15 – 35jN). Tyrrell et al. (2003) reported that high Trichodesmium spp. abundance in the eastern North Atlantic was correlated with shallow mixed layer depths and high iron deposition. Interestingly, they also found the highest abundances between 0jN and 15jN in late summer and more moderate abundances to 30jN. The distributions of the diazotrophs were spatially offset from the distribution of the excess nitrate shown here. Our findings for the distribution and production of excess nitrate in the North Atlantic have similarities and dissimilarities to the GS97 results. We concur that the northern and southern components contribute


water with low excess nitrate, and that nitrate excess develops below the mixed layer in the subtropical North Atlantic. The GS97 findings suggest, though, that N* develops uniformly on the isopycnal surfaces, regardless of location or depth in the gyre. Our contrasting findings indicate that excess nitrate develops at depths of 150 –400 m, and so it is only when an isopycnal lies at those depths will it gain excess nitrate (Fig. 10). Where an isopycnal such as rh 27.0 is deeper than 400 m, the surface does not gain additional excess nitrate, thus restricting the areas in which that surface should be included for estimating the regional rate of N2 fixation. Finally, we cannot explain the differences in the distributions of excess nitrate as shown by GS97 and in this analysis. It may be that data quality was different, with GS97 using data sets from the 1970s, the 1980s and the early 1990s, and having to apply corrections to some of the data. The most important cause for the difference between our estimate of areally integrated nitrogen fixation in the North Atlantic and that of GS97 is the area over which the process is thought to occur. Our regional estimates for N2 fixation retain uncertainty because the WOCE sections we used, while providing coverage adequate to roughly outline the regions of DINxs development, were inadequate to closely define the spatial boundaries for estimating volumes of the potential density intervals. The DINxs distributions shown in Fig. 8 are consistent with gyre circulation (e.g., Fig. 9), accounting for the inflow of DINxs depleted northern and southern components and the accumulation of DINxs signal over similar depth ranges, as expected for remineralization of common sinking particles. Not quantified are processes other than N2 fixation, and listed by Fanning (1992) and GS97, that may contribute to the development of excess nitrate in the subtropical Sargasso Sea. We have reduced the differences between biological and geochemical estimates of N2 fixation. If additional processes contribute to the geochemical signal, the gap may be reduced further. We conclude that N2 fixation is an important part of the nitrogen cycle in the subtropical North Atlantic, but it occurs at considerably lower rates than reported in some of the recent literature where it was evaluated using geochemical data. If the rate of nitrogen fixation in the North Atlantic is indeed considerably lower than estimated by GS97, then recently considered global marine nitrogen budg-

263

ets are impacted. Gruber and Sarmiento (2002) estimated that present day sources of new nitrogen to the global ocean (nitrogen fixation, riverine and atmospheric inputs), summing to 171012 mol N year 1, are in approximate balance with the global marine rate of denitrification (the sink term for fixed nitrogen). Codispoti et al. (2001), however, reports a very large imbalance between fixed nitrogen sources and sinks in the ocean, with sinks (i.e., denitrification) exceeding sources by a factor of two. Both the Gruber and Sarmiento (2002) and the Codispoti et al. (2001) budgets rely on the GS97 global estimate of nitrogen fixation, and that estimate is highly reliant on the rates GS97 reported for the North Atlantic. If the global marine nitrogen fixation rate is less than suggested by GS97, then the imbalance reported by Codispoti et al. (2001) increases and the balanced budget suggested by Gruber and Sarmiento (2002) is challenged. Acknowledgements Our deepest appreciation goes to the numerous P.I.’s, scientists, technicians and ship’s officers and crews who completed the enormous task of occupying and analyzing the WOCE sections and samples. The products of their efforts will resound through our science for many years to come. Rana Fine and Debra Willey calculated water ages and provided plots from the North Atlantic, and we thank them for this help. Geoff Daniels is thanked for calculating volume data for water masses of interest in the Atlantic. Mick Follows joined our early discussions on the Sargasso Sea and participated in our seagoing activities. Niki Gruber, Fred Lipschultz and Associate Editor Mark Altabet suggested several improvements on the manuscript. We thank an anonymous reviewer for extensive comments as well. This work was supported by the National Science Foundation. DAH and NRB were supported by NSF OCE-0196143. DBO was supported by NSF OCE-9981500. Associate editor: Dr. Mark Altabet. References Anderson, L.A., Sarmiento, J.L., 1994. Redfield ratios of remineralization determined by nutrient data analysis. Global Biogeochemical Cycles 8, 65 – 80.

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Bates, N.R., Hansell, D.A., 2003. Temporal variability of excess nitrate in the subtropical mode water of the North Atlantic Ocean. Marine Chemistry 84, 225 – 241. Capone, D.G., Zehr, J.P., Paerl, H.W., Bergman, B., Carpenter, E.J., 1997. Trichodesmium, a globally significant marine cyanobacterium. Science 276, 1221 – 1229. Carpenter, E.J., 1983. Nitrogen fixation by marine Oscillatoria (Trichodesmium) in the world’s oceans. In: Carpenter, E.J., Capone, D.G. (Eds.), Nitrogen in the Marine Environment. Academic Press, New York, pp. 65 – 104. Carpenter, E.J., Montoya, J.P., Burns, J., Mulholland, M.R., Subramaniam, A., Capone, D.G., 1999. Extensive bloom of a N2fixing diatom/cyanobacterial association in the tropical Atlantic Ocean. Marine Ecology Progress Series 185, 273 – 283. Chavez, F.P., Toggweiler, J.R., 1995. Physical estimates of global new production: the upwelling contribution. In: Summerhayes, C.P., Emeis, K.-C., Angel, M.V., Smith, R.L., Zeitschel, B. (Eds.), Upwelling in the Ocean: Modern Processes and Ancient Records. Wiley, New York, pp. 313 – 320. Codispoti, L.A., 1989. Phosphorus vs. nitrogen limitation of new and export production. In: Berger, W.H., Smetacek, V.S., Wefer, G. (Eds.), Productivity of the Ocean: Present and Past. Wiley, NY, pp. 377 – 394. Codispoti, L.A., Brandes, J.A., Christensen, J.P., Devol, A.H., Naqvi, S.W.A., Paerl, H.W., Yoshinari, T., 2001. The oceanic fixed nitrogen and nitrous oxide budgets: moving targets as we enter the Anthropocene? Scientia Marina 65 (Suppl. 2), 85 – 105. Deutsch, C., Gruber, N., Key, R.M., Sarmiento, J.L., Ganachaud, A., 2001. Denitrification and N2 fixation in the Pacific Ocean. Global Biogeochemical Cycles 15, 483 – 506. Doney, S.C., Bullister, J.L., 1992. A chlorofluorocarbon section in the eastern North Atlantic. Deep-Sea Research I 39, 1857 – 1883. Doney, S.C., Jenkins, W.J., Bullister, J.L., 1997. A comparison of ocean tracer dating techniques on a meridional section in the eastern North Atlantic. Deep-Sea Research I 44, 603 – 626. Duce, R.A., Merrill, J.T., Atlas, E.L., Buat-Menard, P., Hicks, B.B., Miller, J.M., Prospero, J.M., Arimoto, R., Church, T.M., Ellis, W., Galloway, J.N., Hansen, L., Jickells, T.D., Knap, A.H., Reinhardt, K.H., Schneider, B., Soudine, A., Tokos, J.J., Tsunogai, S., Wollast, R., Zhou, M., 1991. The atmospheric input of trace species to the world ocean. Global Biogeochemical Cycles 5, 193 – 259. Fanning, K., 1987. Anomalous NO3/PO4 ratios in the west central North Atlantic Ocean (abstract). Eos Transactions AGU 68, 1754. Fanning, K., 1992. Nutrient provinces in the sea: concentration ratios, reaction rate ratios, and ideal covariation. Journal of Geophysical Research 97, 5693 – 5712. Ginoux, P., Chin, M., Tegen, I., Prospero, J.M., Holben, B., Dubovic, O., Lin, S.-J., 2001. Sources and distributions of dust aerosols simulated with the GOCART model. Journal of Geophysical Research 106, 20255 – 20273. Gruber, N., Sarmiento, J.L., 1997. Global patterns of marine nitrogen fixation and denitrification. Global Biogeochemical Cycles 11, 235 – 266. Gruber, N., Sarmiento, J.L., 2002. Large scale biogeochemical-

physical interactions in elemental cycles. In: Robinson, A.R., McCarthy, J.J., Rothschild, B.J. (Eds.), The Sea, vol. 12. Wiley, New York, pp. 337 – 399. Hansell, D.A., Carlson, C.A., 2001. Biogeochemistry of total organic carbon and nitrogen in the Sargasso Sea: control by convective overturn. Deep-Sea Research II 48, 1649 – 1667. Jenkins, W.J., 1987. 3H and 3He in the beta triangle: observations of gyre ventilation and oxygen utilization rates. Journal of Physical Oceanography 17, 763 – 783. Karl, D.M., Letelier, R., Hebel, D., Bird, D.V., Winn, C.D., 1992. Trichodesmium blooms and new production in the North Pacific gyre. In: Carpenter, E.J., Capone, D.G., Rueter, J.G. (Eds.), Marine Pelagic Cyanobacteria: Trichodesmium and other Diazotrophs. Kluwer Academic Publishers, Netherlands, pp. 219 – 237. Knapp, A.N., Sigman, D.M., 2003. Stable isotopic composition of dissolved organic nitrogen from the surface waters of BATS. 2003 Abstract Book ASLO Aquatic Sciences Meeting, p. 78. Kress, N., Herut, B., 2001. Spatial and seasonal evolution of dissolved oxygen and nutrients in the Southern Levantine Basin (Eastern Mediterranean Sea): chemical characterization of the water masses and inferences on the N:P ratios. Deep-Sea Research I 48, 2347 – 2372. Lee, K., Karl, D.M., Waninkhof, R., Zhang, J.-Z., 2002. Global estimates of net carbon production in the nitrate-depleted tropical and subtropical oceans. Geophysical Research Letters 29, 1907 (dio: 10.1029/2001GL014198, 2002). Lipschultz, F., Owens, N., 1996. An assessment of nitrogen fixation as a source of nitrogen to the North Atlantic Ocean. In: Howarth, R.W. (Ed.), Nitrogen Cycling in the North Atlantic Ocean and Its Watersheds. Kluwer Academic Publishers, Boston, MA, pp. 261 – 274. Luyten, J.R., Pedlosky, J., Stommel, H., 1983. The ventilated thermocline. Journal of Physical Oceanography 13, 292 – 309. Mahaffey, C., Williams, R.G., Wolff, G.A., Mahowald, N., Anderson, W., Woodward, M., 2003. Biogeochemical signatures of nitrogen fixation over the eastern North Atlantic. Geophys. Res. Letters 30 (6), doi:10.1029/2002GL016542. Michaels, A.F., Olson, D., Sarmiento, J.L, Ammerman, J.W., Fanning, K., Jahnke, R., Knap, A.H., Lipschultz, F., Prospero, J.M., 1996. Inputs, losses and transformations of nitrogen and phosphorus in the pelagic North Atlantic. In: Howarth, R.W. (Ed.), Nitrogen Cycling in the North Atlantic Ocean and Its Watersheds. Kluwer Academic Publishers, Boston, MA, pp. 181 – 226. Orcutt, K.M., Lipschultz, F., Gundersen, K., Arimoto, R., Michaels, A.F., Knap, A.H., Gallon, J.R., 2001. Seasonal pattern and significance of N2 fixation by Trichodesmium spp. at the Bermuda Atlantic Time-Series Study (BATS) site. Deep-Sea Research II 48, 1583 – 1608. Prospero, J.M., Barrett, K., Church, T., Dentener, F., Duce, R.A., Galloway, J.N., Levy II, H., Moody, J., Quinn, P. 1996. Atmospheric deposition of nutrients to the North Atlantic basin. Biogeochemistry 35, 27 – 73. Rade, L., Westergren, B., 1992. Beta Mathematics Handbook, 2nd ed. CRC Press, Boca Raton, p. 329.

D.A. Hansell et al. / Marine Chemistry 84 (2004) 243–265 Redfield, A.C., Ketchum, B.H., Richards, F.A., 1963. The influence of organisms on the composition of seawater. In: Hill, M.N. (Ed.), The Sea, Ideas and Observations on Progress in the Study of the Seas, vol. 2. Interscience, New York, pp. 26 – 77. Reid, J.L., 1994. On the total geostrophic circulation of the North Atlantic Ocean: flow patterns, tracers, and transports. Progress in Oceanography 33, 1 – 92. Robbins, P.E., Price, J.F., Owens, W.B., Jenkins, W.J., 2000. The importance of lateral diffusion for the ventilation of the lower thermocline in the subtropical North Atlantic. Journal of Physical Oceanography 30, 67 – 89. Sarmiento, J.L., Thiele, G., Key, R.M., Moore, W.S., 1990. Oxygen and nitrate new production and remineralization in the North Atlantic subtropical gyre. Journal of Geophysical Research 95, 18303 – 18315. Schlitzer, R., 2002. Ocean Data View, WWW page, http://www. awi-bremerhaven.de/GEO/ODV (version 1.1a). Schmitz Jr., W.J., Richardson, P.L., 1991. On the sources of the Florida Current. Deep-Sea Research 38 (Suppl. 1), S389 – S409. Stommel, H., 1965. The Gulf Stream: A Physical and Dynamical Description, 2nd ed. Univ. of California Press, Berkeley and Los Angeles, California. 248 pp.

265

Takahashi, T., Broecker, W.S., Langer, S., 1985. Redfield ratio based on chemical data from isopycnal surfaces. Journal of Geophysical Research 90, 6907 – 6924. Tyrrell, T., Maran˜o´n, E., Poulton, A.J., Bowie, A.R., Harbour, D.S., Woodward, E.M.S., 2003. Large-scale latitudinal distribution of Trichodesmium spp. in the Atlantic Ocean. Journal of Plankton Research 25, 405 – 416. van Aken, H.M., 2001. The hydrography of the mid-latitude Northeast Atlantic Ocean: Part III. The subducted thermocline water mass. Deep-Sea Research I 48, 237 – 267. Walker, S.J., Weiss, R.F., Salameh, P.K., 2000. Reconstructed histories of the annual mean atmospheric mole fractions for the halocarbons CFC-11, CFC-12, CFC-113, and carbon tetrachloride. Journal of Geophysical Research 105, 14285 – 14296. Warner, M.J., Weiss, R.F., 1985. Solubilities of chlorofluorocarbons 11 and 12 in water and seawater. Deep-Sea Research 32, 1485 – 1497. Zehr, J.P., Waterbury, J.B., Turner, P.J., Montoya, J.P., Omoregie, G.F., Steward, G.F., Hansen, A., Karl, D.M., 2001. Unicellular cyanobacteria fix N2 in the subtropical North Pacific Ocean. Nature 412, 635 – 638.