Advances in Polar Science  2012, Vol. 23 Issue (2): 87-94

  The article information

CHEN Liqi, ZHAN Liyang, XU Suqing, ZHANG Jiexia, ZHANG Yuanhui, XU Guojie
Multiple processes affecting surface seawater N2O saturation anomalies in tropical oceans and Prydz Bay, Antarctica
Advances in Polar Science, 2012, 23(2): 87-94
10.3724/SP.J.1085.2012.00087

Article history

Received: 14 March 2012
Accepted: 5 April 2012
Multiple processes affecting surface seawater N2O saturation anomalies in tropical oceans and Prydz Bay, Antarctica
CHEN Liqi1, 2 , ZHAN Liyang1, 2, XU Suqing1, 2, ZHANG Jiexia1, 2, ZHANG Yuanhui1, 2, XU Guojie1, 2    
1 Key Laboratory of Global Change and Marine-Atmospheric Chemistry, State Oceanic Administration (SOA), Xiamen 361005, China;
2 Third Institute of Oceanography, SOA, Xiamen 361005, China
Received 14 March 2012; accepted 5 April 2012
Corresponding author: CHEN Liqi (email: Lqchen@soa.gov.cn)
Abstract: We analyzed the N2O content of surface seawater sampled from Prydz Bay, Antarctica, on a cruise track between 30°S and 30°N during the twenty-second Chinese National Antarctic Research Expedition during austral summer, 2006. The surface water showed an average pN2O value of 311.9±7.6 nL·L-1 (14.1±0.4 nmol·L-1), which was slightly undersaturated. The air-sea N2O flux in the region was -0.3±0.8 μmol·m-2·d-1; however, N2O in the surface water was oversaturated in most stations along the cruise track. Saturation anomalies were greater than 10%, with a maximum of 54.7% being observed at the Equator, followed by 31% at 10°N in the Sulu Sea. The air-sea fluxes at these locations were 12.4 and 4 μmol·m-2·d-1, respectively. Overall, the results indicated that surface water in Prydz Bay was near equilibrium with atmospheric N2O, and that ocean waters in lower latitudes acted as a N2O source. Physical processes such as stratification, ice-melt water dilution, and solar radiation dominate the factors leading to N2O saturation of surface water of Prydz Bay, while biological production and upwelling are primarily responsible for the N2O oversaturation of surface water observed in subtropical and tropical regions along the cruise track.
Keywords: dissolved N2O    saturation anomalies    tropic    subtropic    Prydz Bay    

0 Introduction The atmospheric concentration and budget of N2O is receiving increasing attention due to its effects on ozone depletion[1] and because it is a strong greenhouse effect with a molecule-for-molecule radiative forcing efficiency about 200-300 times greater than that of CO2. The atmospheric N2O mixing ratio has increased from a pre-industrial level of about 270±1 nL·L-1 to the current value of around 318-319 nL·L-1[2] and continues to increase at about 0.25% per year[3]. The sharply increasing atmospheric N2O mixing ratio in the past century may be in response to various an thropogenic activities,which contribute about 4.5±0.6 Tg·N·a-1 (Tg=1012g,this unit means 1012g per year). However,natural N2O emissions still account for two thirds of the total emissions worldwide. For example,emissions from the ocean account for about 4 Tg·N·a-1 of N2O[4]. These data indicate that oceanic N2O exchange contributes significant amounts of N2O to the atmospheric reservoir and acts as a net source for atmospheric N2O.

Since the first study of N2O by Craig and Gordon[5],considerable research has been conducted to investigate N2O in the Pacific[6, 7, 8, 9, 10, 11, 12, 13],Atlantic[14, 15, 16, 17, 18],Indian[19, 20, 21, 22, 23, 24, 25, 26, 27] and Southern Oceans[28, 29]. Specifically,the studies that have been conducted to date were designed to better understand the possible mechanisms of N2O production and factors that influence the distributions of N2O in the ocean[12, 30, 31]. Isotopic techniques used to investigate the N2O production mechanism have shown that nitrification could be a dominant mechanism in N2O production in open oceans[32, 33]. However,other studies have suggested that denitrification could also be an important process in N2O production[34]. More recently,the improvement of isotopic analytical techniques has enabled measurement of isotopomers,which could be used as evidence of the existence of nitrifier denitrification. Using this method,Charpentier et al.[35] suggested that nitrifier denitrification could be an important N2O production pathway,even in oligotrophic and well-oxygenated waters.

Model studies contribute to our understanding of the distributions of N2O. Nevison et al.[4] reported a N2O flux of 1.2-6.8 Tg·N·a-1 with an average of about 4 Tg·N·a-1 from the world ocean,which agrees with the results of Suntharalingam and Sarmiento (3.85 Tg·N·a-1)[36]. Taken together,these findings suggest that the ocean between 40° -60°S is a significant N2O source,accounting for about one third of the global oceanic N2O. Nevertheless,Nevison et al.[37] suggested that her previous work conducted in 1995 may overestimate the source strength of this area. This deviation might be due to a lack of data pertaining to the area. Clearly,further studies are necessary to better predict the N2O air-sea flux and its variability throughout the world ocean.

Here,we present data obtained from Prydz Bay,Antarctica and along the cruise track between Fremantle,Australia and Shanghai,China during the twenty-second Chinese National Antarctic Research Expedition (22nd CHINARE) from January 15 to 25,and March 13 to 24,2006. We also examine the differences in the distributions of surface water N2O concentrations,saturation anomalies,and air-sea fluxes of different regions on the cruise based on these data.

1 Method 1.1 Study areas and methods The sampling regions were located in Prydz Bay and on the cruise track between Fremantle and Shanghai (Figure 1a,1b). The cruise track was divided into the following regions: West Coast of Australia (Stations 1-4),North Australian Basin (Stations 5-8),Lombok Strait (Stations 9-10),Makassar Strait (Stations 11-12),Celebes Sea (Stations 13-14),Sulu Sea (Stations 15-16),South China Sea (Stations 17-18),East China Sea (Stations 19-20).

Figure 1 Location of study site and stations. a , Stations located in Prydz Bay. b , Stations located on the cruise track between Fremantle and Shanghai.

Seawater from a depth of about 6 m was continuously pumped. Triplicate water samples were collected and then treated and analyzed in the laboratory as described by Zhan and Chen[29].

1.2 Calculation and data processing According to IPCC 2007 AR4,the mixing ratio of the N2O atmospheric boundary layer is 318.5 nL·L-1[38]. N2O Concentrations in the sea water were derived from the methods described by Weiss and Price[39]. Air-sea fluxes were calculated from equation (1):

where,F is the flux (μmol·m-2·d-1),the seawater N2O equilibrated concentration Ca is calculated from the boundary layer N2O mixing ratio,and Cw is the observed in situ N2O concentration in surface water. Additionally,the transfer velocity kw was determined from the following equation (2):

where,kCO2 and Sc were calculated according to Liss and Merlivat[40] and Wanninkhof[41],respectively.

1.3 Other data SST (surface seawater temperature) and SSS (surface seawater salinity) data with intervals within 1/4° of the study areas were selected from data available at http://www.nodc. noaa.gov/OC5/SELECT/woaselect/woaselect.html,and interpolated to obtain the SST and SSS data for all stations on the cruise tracks. The surface water salinity and temperature in Prydz Bay were obtained from the Mark III CTD cast’s measurement. The atmospheric pressure along the cruise tracks was measured every 10 min using the MILOS 500 meteorological monitoring system (Vaisala® Helsinki,Finland).

2 Results and discussion According to Weiss and Price[39],the solubility of N2O in surface seawater is strongly correlated with SST. However,deviation from equilibrium was observed in Prydz Bay and along the cruise track. The distribution patterns of N2O in Prydz Bay and along the cruise track plotted against SST are shown in Figures 2 and 3. The in situ concentrations do not appear to be in equilibrium,which indicates that the distribution of N2O may be influenced by other factors in addition to SST. To investigate these effects,we defined the saturation anomaly (SA) as equation (3):

Figure 2 N2O concentration (solid circles) in surface water vs. SST. An exponential function was fitted to the equilibrium concentrations (open triangles) for both Prydz Bay and the cruise track.

Figure 3 N2O concentration (solid circles) in surface water vs. SST. An exponential function was fitted to equilibrium concentrations (open triangles) along the cruise track.

where,the equilibrium concentration Ce (nmol·L-1) is derived according to Weiss and Price[39],the proportion between Cs and Ce is the saturation of N2O (S) in surface seawater,and the difference between this proportion and 1 (100%) is the SA. A value of 0 indicates that N2O in the atmosphere and surface water is in equilibrium,while positive or negative values indicate that N2O is oversaturated or undersaturated,respectively,in the surface water relative to the atmosphere.

2.1 Pattern of dissolved N2O in Prydz Bay and its air-sea fluxes As shown in Figure 3,about two thirds of the measurements taken at stations in Prydz Bay showed an N2O concentration lower than the equilibrium value,which suggests that the surface water of Prydz Bay is undersaturated with N2O. Two areas with minimum values were found near 72°E and 74.5°E,where the N2O concentration was about 12.5 nmol·L-1. This concentration agrees well with data obtained from the Bellingshausen Sea[28] and McMurdo Sound[42]. N2O production in the euphotic layer is inhibited by sunlight[43] ; therefore,surface N2O can only come from air-sea and surface-subsurface exchange. Phytoplankton blooming and subsequent remineralization in summer[44] probably enhance subsurface N2O production in Prydz Bay. However,exchange between surface and subsurface waters is hampered in the bay by strong stratification during summer [44, 45]; accordingly,N2O can only be exchanged by eddy diffusion between these fractions during this period. It must also be assumed that subsurface biological processes contribute only a negligible amount of N2O to surface water. Furthermore,the SST and SSS in Prydz Bay varied from -1.13℃-3.48℃ and 29.8-34.6,respectively (Figure 4a,4b),which will have a significant impact on N2O saturation. Zhan et al.[29] suggested that undersaturation may result from melting ice,and a similar phenomenon has been reported by Hamme and Emerson for inert gases[46]. However,as shown in Figure 5,no obvious relationship was observed between N2O and SSS in Prydz Bay.

Figure 4 SST (a) and SSS (b) of Prydz Bay surface seawater during the expedition. The low salinity of Prydz Bay suggests that sea-ice melt water dilutes the surface water of Prydz Bay, especially around 74°E.

Figure 5 Saturation vs. salinity in Prydz Bay surface water. Three groups of water mass are divided and denoted as type A (SSS≤34, SST≤0℃), type B (SSS≤34, SST>0℃), and type C (SSS>34, SST>0℃). Type A was best fitted with an exponential growth equation f (x)= exp[a(x-x0)] (R2 = 0.985 4).

Dilution of surface water by sea-ice melt water may not be the only process that controls the surface water N2O distribution pattern. To understand the processes controlling the distribution pattern,surface water was divided into three types of water masses in terms of SSS and SST,for which 34℃ and 0℃ were selected as the critical values,respectively. The three water masses were denoted type A (SSS≤34,SST≤0℃),type B (SSS≤34,SST>0℃),and type C (SSS>34,SST>0℃). As shown in Figure 5,N2O saturation of water mass type A was well correlated with SSS (saturation is used instead of SA because SA could be negative,which cannot be further fitted against SSS using an exponential growth equation). Below a certain temperature,saturation of dissolved N2O should grow exponentially with increasing salinity. Since the temperature range of type A is within 1℃,we assumed that the effect of temperature on N2O in water mass type A is negligible. An exponential curve was fitted to the saturation and salinity of type A water (R2=0.985 4),suggesting that the N2O saturation in this water type was dominated by ice-melt water.

To quantify the impact of ice melting on the saturation anomaly,the following mass balance equations were set:

where,f1 and f2 are fractions of two end members: Sea-ice meltwater and seawater. One end member consisted of the salinity S1 and temperature T1 of ice-melt water,which are 4 and -1.9℃,respectively. The other end member consisted of S2 and T2,which are the SSS and SST of end member water from various stations. Because variations in salinity only introduce marginal error to calculation of the N2O concentration in seawater end members,salinity S2 is taken as 34,which is typical of surface water south of the Polar Front,which is unaffected by sea-ice melt water. Temperature T2 can be calculated from equation (6),C1 is the concentration of N2O in sea-ice melt water,which is assumed to be zero because the formation is a degassing process,and C2 is the equilibrium concentration of the other end member before it mixes with ice-melt water.

Tmix and Smix are the SST and SSS after mixing of the two end members,or the in situ SST and SSS. Saturation anomalies of different stations of type A can be calculated using the following equation:

where,Cmix is calculated from equation (5),and C (Tmix,Smix) is the equilibrium concentration of the two-end-member mixture,which is derived from in situ temperature Tmix and salinity Smix. All of the calculated saturation anomalies listed in Table 1 are within 5% of their observed counterparts. The calculated and observed values are correlated with each other and have R2 values as high as 0.991 9 (Figure 6). These results also suggest that N2O in water mass type A is dominated by sea-ice dilution.

Table 1 Observed saturation and simulated saturation anomalies
StationsObserved saturation anomaly/%Simulated saturation anomaly/% Absolute deviation /%
PLZW-3-1-a-21.9-17.14.8
II-7-a-8.5-8.50.0
I-9-a-8.0-7.80.2
III-6-1-a-5.1-5.20.1
III-9-a-1.2-3.82.6

Figure 6 Correlation between simulation saturation anomalies and observed saturation anomalies in Prydz Bay (R2 = 0.991 9).

The distribution of N2O in water mass type B shows a different pattern. Specifically,no relationship was observed between N2O and salinity (Figure 5),and the saturation value was around 100%,suggesting that this type of water is near saturation. Additionally,this water mass has lower salinity,which suggests that it underwent ice-melt water dilution,was then heated by solar radiation,and finally re-equilibrated with atmospheric N2O.

The SSS of water mass type C is greater than 34. This water mass may be unaffected or only mildly affected by ice-melt seawater. The increasing SST and SSS in type C water and their positive correlation suggest that this water mass may have been warmed by solar radiation,resulting in some evaporation. Salinity only changed within a range of 0.6,suggesting a negligible contribution to N2O saturation anomalies; therefore,the anomaly would primarily be affected by SST. The SST of type C water ranges within 2.7℃ (0.7℃—3.4℃),which corresponds to a difference of 9.7% between saturation anomalies. These findings agree well with the in situ difference of 10% observed between the highest and lowest SA in water mass type C and suggest that N2O in water mass type C is primarily controlled by solar radiation.

Based on equations (1) and (2),the calculated average flux of N2O in Prydz Bay was -0.3±0.8 μmol·m-2·d-1 during the survey period. These results indicate that the N2O in the surface seawater of Prydz Bay is nearly in equilibrium with the atmospheric mixing ratio during summer,while there is nearly no sea to air input of N2O in this area.

2.2 Pattern of dissolved N2O and its air-sea fluxes The N2O saturation anomalies of surface seawater measured at most stations along the cruise track were greater than 10%,with the exception of a value of -3.5% recorded at 16.72°N. Maximum values greater than 50% and 30% were found at the equator and around 10°N,respectively (Figure 7). Variations in the saturation of N2O in the surface water measured along the cruise track could have many origins,such as differences in hydrographical or biogeochemical properties at different latitudes.

Figure 7 Saturation anomaly measured along the cruise track between Fremantle and Shanghai. Maximum values were observed between the equator and 10°N.
2.2.1 North Australia Basin Data pertaining to the south end of the cruise track near the coast of Western Australia (Figure 8) suggest that the saturation anomaly could be explained by seasonal thermal deviation of surface seawater[29]. Variations in the temperature of coastal surface water at around 30°S can be as high as 7℃. Such variations would result in a saturation anomaly of approximately 21%,which is consistent with the observations of the present study.

Figure 8 SST (open squares) and SSS (solid squares) measured along the cruise track. Three obvious hydrographic regimes are marked from left to right with straight lines.

Fieux et al.[47] revealed that a sharp front existed during both summer and winter at around 13°S to 14°S,which is also demonstrated by the SSS turning point at approximately the same latitude (Figure 8). South of this front are high salinity subtropical waters produced by higher evaporation rates,while low salinity water from Indonesia flows to the north. However,the measured saturation anomaly does not appear to be different from that other stations in the North Australia Basin.

2.2.2 Makassar Strait The highest N2O saturation anomaly was observed in the Makassar Strait,around the Equator. Since seasonal thermal effects could only account for 6% of the saturation anomaly in equatorial regions,the remaining 44% of the anomaly must be explained by other oceanic processes such as variations in SST,SSS,ocean currents,and possible biological activity.

The hydrographic characteristics of the Makassar Strait could influence the saturation anomaly. The Makassar Strait is a westerly path for the Indonesia Through Flow (ITF). The vertical profile of the Makassar Strait reflects characteristics of different origins. The surface water is freshened by Mahakam River runoff during the Northwest Monsoon (NWM),when samples of this region were collected. This could explain why the lowest salinity was observed between 30°S and 30°N (Figure 8)[48]. The highest N2O saturation anomaly found on the salinity front along the cruise track suggests a possible strong biological N2O contribution during mixing processes.

2.2.3 Sulu Sea Another N2O saturation anomaly maximum was observed in the north Sulu Sea around 10°N. The saturation anomaly value was as high as 31%,which was the largest value measured during cruise. This anomaly may have resulted from upwellings in this region,as indicated by SST and SSS data along the cruise track (Figure 8). The SST and SSS curves show a trough and a crest,respectively,at around 10°N. Additionally,this region had lower temperature and higher salinity,suggesting that upwelling was occurring. These findings are consistent with those of Wang et al.[49],who found that during austral winter (November to April),strong northeast winds drive the upwelling of cold and nutrient rich water from lower depths to the upper layer in the north Sulu Sea. N2O could also be brought to the surface during this process.

2.2.4 South China Sea and East China Sea The lowest saturation anomaly of -3.5% was observed at 16.7°N,at the eastern boundary of the South China Sea,indicating that the surface water N2O was nearly in equilibrium with the atmosphere. Relatively low salinity values were also found at this latitude (Figure 8) corresponding to river runoff. However,no obvious increase in the saturation anomaly related to runoff or other biological or physical processes was observed in this region.

The saturation anomaly values measured at stations located in the east of Taiwan and in the East China Sea were both around 10%. The SST east of Taiwan showed no significant changes between March and December,suggesting that there is a source at this latitude during early spring.

2.3 Air-sea fluxes along the cruise track The air-sea fluxes measured between Fremantle and Shanghai fluctuated along the cruise track (Figure 9),with the highest value of 12.4 μmol·m-2·d-1 being observed at around 10°N,possibly reflecting the combined effects of upwelling in the north Sulu Sea and high wind speed. The second highest value was observed at around 30°N,which likely resulted from the high wind speed at this latitude. No significant air-sea N2O flux was observed between 0°— 10°S,although the highest saturation anomaly of the entire cruise track was observed at the Equator. Another low value was observed at 16.7°N,resulting from both low saturation anomaly and low wind speed at this latitude.

Figure 9 Air-sea fluxes of N2O between Fremantle and Shanghai.
3 Conclusions Analyses of surface seawater samples collected in Prydz Bay during the 22nd CHINARE cruise between January and March 2006 indicated that the seawater was nearly in equilibrium with the atmospheric N2O mixing ratio,with an average concentration of 14.1±0.4 nmol·L-1,or 311.9± 7.6 nL·L-1 after conversion to partial pressure under in-situ conditions. When compared with the atmospheric mixing ratio of 318.5 nL·L-1,the surface seawater is slightly undersaturated,with an average air-sea N2O flux of -0.3± 0.8 μmol·m-2·d-1 during the period. The complicated distribution pattern of N2O saturation anomalies in the seawater may have resulted from a combination of physical processes. The dilution of surface water by sea ice meltwater with a lower N2O would result in N2O undersaturaton. The mixture of sea ice meltwater and surface seawater in open water was further warmed by solar radiation,which enabled re-equilibration with the atmosphere. Some surveyed regions with an SST of greater than 3℃ SST showed no significant impact from sea ice meltwater and resulted in N2O oversaturation in this region.

In contrast to Prydz Bay,surface seawater samples collected between Fremantle and Shanghai were supersaturated with N2O,indicating that this area behaved as a source for atmospheric N2O. Saturation anomalies in most of the surface water samples were greater than 10%,with the highest value of 54.7% being observed at the Equator,and the second highest value of 31% occurring at about 10°N in the Sulu Sea. No significant sources of N2O were observed in the North Australia Basin or East China Sea. The highest values may have primarily resulted from subsurface production,upwelling or both of these factors; however,the air-sea fluxes of these two locations were reversed relative to their saturation anomalies,with values of 12.4 μmol·m-2·d-1 and less than 4 μmol·m-2·d-1,respectively. This difference may be attributed to wind speed.

Acknowledgements Data issued by the Data-sharing Platform of Polar Science (http://www.chinare.org.cn) maintained by Polar Research Institute of China (PRIC) and Chinese National Arctic & Antarctic Data Center (CN-NADC). This research was jointly sponsored by the National Natural Science Foundation of China (Grant nos. 40671062,41106168),the National High Technique Research & Development Program of China (Grant no. 2008AA121703),the Ministry of Science and Technology of China (Grant nos. 2004DIB5J178,2009DFA22920) and the Chinese Arctic and Antarctic Administration (CAA) Cooperation Program (Grant nos. IC2010013,IC2011114,IC201201).

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