[1] Concentrations of dissolved nitrous oxide (N2O) and. N2O flux densities were determined during the Lagrangian investigation of two filaments advected from ...
GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L21606, doi:10.1029/2011GL049322, 2011
The Lagrangian progression of nitrous oxide within filaments formed in the Mauritanian upwelling Andrew P. Rees,1 Ian J. Brown,1 Darren R. Clark,1 and Ricardo Torres1 Received 15 August 2011; revised 5 October 2011; accepted 10 October 2011; published 5 November 2011.
[1] Concentrations of dissolved nitrous oxide (N2O) and N2O flux densities were determined during the Lagrangian investigation of two filaments advected from the Mauritanian upwelling. The ratio between remineralization and N2O production, predominantly through nitrification was 0.11 ± 0.01%. This was linked to the hydrodynamic character of each filament, which was determined by the relative proportions of two central water masses from which they originated. Surface waters were saturated with N2O up to 150% when first upwelled, but following advective transport offshore and exchange with the atmosphere at rates between 5 and 27 mmol m−2d−1 were closer to equilibrium with the atmosphere (105%) when the filament was approximately two weeks old. Annual estimates of N2O emissions from transient filaments in this area are between 1.3 and 2.1 Gg N, but could be as high as 9.8 Gg N y−1 for the extended upwelling area in the vicinity of Cap Blanc. Citation: Rees, A. P., I. J. Brown, D. R. Clark, and R. Torres (2011), The Lagrangian progression of nitrous oxide within filaments formed in the Mauritanian upwelling, Geophys. Res. Lett., 38, L21606, doi:10.1029/2011GL049322.
Forster et al., 2009; Rhee et al., 2009] which have provided only a limited insight to this upwelling system, though a modeling study [Nevison et al., 2004] found that the NW African upwelling contributed up to 11% of N2O from coastal upwelling. The first rigorous investigation of the temporal and spatial estimates of N2O flux densities in the region [Wittke et al., 2010] revealed a strong seasonality and north‐south gradients in the transfer of N2O between the ocean and atmosphere, though they determined that the annual N2O flux of 1.0 Gg N to be a minor contribution to the atmosphere relative to other upwelling systems. [5] During a research cruise onboard RRS Discovery (D338) in April and May 2009, we deployed the tracer sulfur hexafluoride (SF6), in parallel with drogued drifter buoys in order to perform a Lagrangian investigation of the concentration and flux of N2O. Two experiments were performed in order to determine the extent of the ocean N2O source during the evolution and development of two upwelling filaments away from the African coastline.
2. Methods 1. Introduction [2] Nitrous oxide is a trace gas whose atmospheric concentration is increasing at a rate of 0.2–0.3% y−1 [Forster et al., 2007]. It is of paramount interest, in that it is radiatively active, with a global warming potential on a 100 year timescale of approximately 300 times that of CO2 [Ramaswamy et al., 2001] and contributes significantly to stratospheric ozone depletion [Ravishankara et al., 2009]. [3] Though most of the surface of the global ocean N2O is in close equilibrium with the atmosphere [Nevison et al., 1995], the oceans contribute about 30% of the natural N2O source to the atmosphere [Bange, 2006]. Much of this occurs in the coastal zones and in association with coastal upwelling as found in the Arabian Sea and eastern Pacific Ocean [Nevison et al., 2004]. The distribution of source and sink regions is largely influenced by the oxygen and nutrient status of a water body, as these impact on the release of N2O during microbial nitrification and denitrification so that regions overlying oxygen minima are likely to be strong net source areas [Yoshinari, 1976; Codispoti et al., 2001]. [4] The Mauritanian upwelling region is one of two major coastal upwellings in the Atlantic Ocean and globally one of the most biologically productive systems [Pauly and Christensen, 1995], despite this, the area remains largely un‐investigated. There have been a small number of observations of N2O in this area [Walter et al., 2004; 1
Plymouth Marine Laboratory, Plymouth, UK.
Copyright 2011 by the American Geophysical Union. 0094‐8276/11/2011GL049322
[6] Physical surveys of the two areas identified in Figure 1 using shipboard instrumentation and satellite derived sea surface temperature (SST) and chlorophyll‐a were performed in order to select starting positions for 2 Lagrangian experiments (Filament 1 and Filament 2). Each start point was chosen to reflect the position of recently upwelled waters, with the aim being to track an SF6 labeled water mass as it was propagated offshore during filament evolution. [7] SF6 deployment was made at 5m depth around a central drifter buoy [Nightingale et al., 2000], and onboard detection ensured that sampling was performed close to the centre of the labeled water mass until the tracer became too low to detect reliably. [8] Seawater samples were collected at stations identified in Figure 1 from Niskin bottles mounted on a CTD rosette frame using clean Tygon tubing into 1L borosilicate flasks. Samples were overfilled to three times volume in order to expel trapped air bubbles, poisoned with 200mL of saturated HgCl2 solution and temperature equilibrated at 25.0 ± 0.5°C. In all cases samples were analyzed within 8 hours of collection. N2O was determined by single‐phase equilibration gas chromatography with electron capture detection similar to that described by Upstill‐Goddard et al. [1996]. The analysis protocol involved determination of three certified (±2%) reference standards of 287, 402 and 511 ppb (Air Products) immediately before each sample and daily determination of the atmospheric mixing ratio versus the same standards. Mean instrument precision from daily, triplicate analyses of the three calibration standards (n = 81) was 0.95% (≤ ± 2.7%). Concentrations of N2O in seawater
L21606
1 of 6
L21606
REES ET AL.: N2O IN MAURITANIAN UPWELLING FILAMENTS
Figure 1. Remotely sensed imagery of Mauritanian upwelling: (a) MODIS – Aqua chlorophyll‐a, and AVHRR images of sea surface temperature for (b) Filament 1 (21–29 April 2009) and (c) Filament 2 (15–22 May 2009). Station positions are identified by white squares and direction of transit indicated by white arrows. Images courtesy of NEODAAS. were calculated from solubility tables of Weiss and Price [1980] at equilibration temperature (∼25°C) and salinity. Percent saturation of seawater with N2O was determined as the ratio of in‐situ N2O to atmospheric samples determined onboard (mean 322 ± 7 ppb, cf. 322.8 at Mace Head, Ireland and 322.4 Ragged Point, Barbados ‐ taken from the Advanced Global Atmospheric Gases Experiment (AGAGE) data set ‐ http://agage.eas.gatech.edu/). [9] The exchange of N2O between the ocean and the atmosphere, the sea ‐ air flux density (FN2O), was estimated from:
L21606
central waters: South Atlantic Central Water (SACW) and North Atlantic Central Water (NACW) [Tomczak, 1982; Walter et al., 2006]. Both have their origins in the sub‐ tropical gyres, SACW is generally warmer, fresher and has a higher nutrient and lower oxygen content than that of the NACW [Minas et al., 1982]. Filament 1 tracked a water parcel from its upwelling in surface waters for 9 days (21 to 29 April 2009) over a distance of approximately 200km, and Filament 2 from an estimated 7 day old upwelling for a further 8 days (15 to 22 May 2009) over a distance of approximately 160 km. The initial hydrodynamic conditions differed due to the relative proportions of each water mass present indicating that we traced filaments of distinct origin. [12] The percentage of each water mass was determined in each filament by applying optimum multiparameter (OMP) analysis [Tomczak and Large, 1989] to temperature, salinity, nitrate and silicate profiles using the OMP Package for MATLAB Version 2.0 (http://www.ldeo.columbia.edu/ ∼jkarsten/omp_std). The source water definitions were obtained from the OMP manual and are accessible at the OMP website. The upper 200m of Filament 1 were dominated by NACW (50–80%), as was the area west of ∼18.8°W of Filament 2, whilst similar depth waters in Filament 2 between ∼18°W and ∼18.8°W were dominated by SACW. SACW dominated depths between 200m and 500m beneath both filaments and contributed up to 100% in the eastern section of Filament 2. [13] The greatest concentrations of N2O (≤33.9 nmol L−1) were found in the depth range 200 to 500m coincident with concentrations of O2 < 100 mmol L−1 (≈40% saturation) associated with the area of ∼100% SACW below Filament 2. The upwelling of N2O from these intermediate depths to the surface (Figure 2) was reflected by an inverse correlation between N2O and temperature (r2 = −0.88, n = 176), as has been described for waters of the Atlantic equatorial upwelling [Walter et al., 2004]. During Filament 1, surface waters of the five eastern‐most stations, which were coincident with maximum upwelling, had similar concentrations (12.2 ± 0.4 nmol L−1) and saturations (146 ± 5%) of N2O.
FN2O ¼ Kw ðSc =600Þ0:5 ðCw Ca Þ
Where Kw is the gas exchange coefficient [Nightingale et al., 2000] as a function of wind speed determined at 18m above sea‐level on the ships foremast and which was normalized to a height of 10m above sea level [Large and Pond, 1982]. Sc is the Schmidt number for N2O [Wanninkhof, 1992], Cw the measured seawater concentration and Ca is the equilibrium concentration of N2O in seawater based on the measured atmospheric value. [10] The concentration of oxygen was determined by a Seabird SBE 43 oxygen sensor which was deployed on the CTD frame and calibrated by daily Winkler titrations. Nitrate concentrations were determined using a segmented flow, colorimetric autoanalyzer (Bran and Luebbe AAIII) according to Woodward and Rees [2001].
3. Results and Discussion 3.1. N2O Distribution [11] One of the defining oceanographic characteristics of this area is the density compensating mixing of two distinct
Figure 2. Contour profiles of N2O concentration (nmol L−1) during Lagrangian studies of Filaments 1 and 2 in the Mauritanian upwelling during April and May 2009. Contoured isolines are for temperature (°C), data points indicated by the solid circle. Image produced using Ocean Data View (http://odv.awi.de/).
2 of 6
L21606
REES ET AL.: N2O IN MAURITANIAN UPWELLING FILAMENTS
L21606
study were generally warmer (18.1°C compared to 16.7°C) than Filament 1 which was reflected in lower N2O concentrations, the mean of the first three stations occupied was 10.8 ± 0.1 nmol L−1 (133 ± 1.7% saturation) which decreased to 8.3 nmol L−1 (105% saturation) by the end of the experiment.
Figure 3. Relationship between (top) DN2O and AOU: open circle and solid square, from depths
