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Stable isotope ratio of atmospheric and seawater nitrate in the East Sea in the northwestern Pacific ocean
Marine Pollution Bulletin 149 (2019) 110610
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Baseline
Stable isotope ratio of atmospheric and seawater nitrate in the East Sea in the northwestern Pacific ocean
T
Haryun Kima, Geun-Ha Parkb, Seon-Eun Leeb, Young-il Kimc, Kitack Leed, Yeo-Hun Kime, Tae-Wook Kimf,g,∗ a
Fundamental Research Division, National Marine Biodiversity Institute of Korea, Janghang, 33662, South Korea Marine Environmental Research Center, Korea Institute of Ocean Science & Technology, Busan, 49111, South Korea c East Sea Research Institute, Korea Institute of Ocean Science & Technology, Uljin, 36315, South Korea d Division of Environmental Science and Engineering, Pohang University of Science & Technology, Pohang, 37673, South Korea e Global Ocean Research Center, Korea Institute of Ocean Science & Technology, Busan, 49111, South Korea f Division of Environmental Science and Ecological Engineering, Korea University, Seoul, 02841, South Korea g OJeong Eco-Resilience Institute, Korea University, Seoul, 02841, South Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nitrogen and oxygen isotope Anthropogenic nitrogen Atmospheric pollution Marginal sea Ocean nitrogen cycle East Asia
The nitrogen and oxygen isotopic values of anthropogenic NOx− (including NO2− and NO3−) are preserved in precipitation and airborne total suspended particles. Hence, isotope analyses of these atmospheric samples allow for the identification of the NOx− sources. In this study, atmospheric δ15N- and δ18O-NOx− values were measured in an eastern coastal site of South Korea over a three-year period, and compared with seawater values observed from coastal waters of the East Sea. The highest seasonal atmospheric δ15N-NOx− values were associated with wintertime air masses blowing from northeastern China, and similar to those produced by coal and biomass burning. By contrast, the lowest atmospheric δ15N-NOx− values were observed in summer, and the major source appeared to be vehicle exhaust. In addition, the reductions in the seawater δ15N-NOx− and Δ(15−18) (i.e., δ15N-NOx− minus δ18O-NOx−) values in the East Sea could be at least partially attributed to atmospheric deposition.
The amount of anthropogenic nitrogen (NANTH) transferred from the atmosphere to the oceans was estimated to be 54 Tg N yr−1 in 2000, a ten-fold increase from 1860 (5.7 Tg N yr−1) (Duce et al., 2008). It is very likely that atmospheric NANTH exerts ecological impacts on remote ocean regions where the input of riverine nitrogen (N) is negligible and the availability of N is limited. Among atmospheric NANTH species, NOx− (including NO2– and NO3–) is mainly produced through the photochemical oxidation of NOy (i.e., NO + NO2) co-emitted with carbon dioxide (CO2) from fossil fuel combustions (Duce et al., 2008). Atmospheric NOx− deposition may relieve nitrogen limitation to primary production and thus partially counterbalance the atmospheric increase in CO2. In this context, many studies have estimated the NOx– flux deposited to the ocean and investigated its potential effect on ocean primary productivity (e.g. Kim et al., 2014a, 2014b; Liu et al., 2019; Park et al., 2019; Wu et al., 2018a; Yan and Kim, 2015; Zhang et al., 2019). Among them, however, few studies have attempted to link atmospheric NOx– deposition to seawater NOx− availability using dual stable isotopic values of nitrogen and oxygen atoms in NOx− (δ15N- and
∗
δ18O-NOx−) (e.g., Liu et al., 2017a; Luo et al., 2018; Wu et al., 2018b; Yang et al., 2014). The δ15N- and δ18O-NOx− values are widely used to identify pollution sources and chemical processes in the atmosphere (Chang et al., 2018; Chen et al., 2019; Nelson et al., 2018), coastal lagoons (Mwaura et al., 2017), and estuaries (Wiegner et al., 2016). The isotopic signatures of NANTH sources are not affected by photochemical processes, so they are preserved in δ15N-NOx− in precipitation and airborne total suspended particles (TSP) (Elliott et al., 2009; Hastings et al., 2003). Further, the δ15N-NOx− transferred from the atmosphere to the oceans can alter NOx− isotopic values in the water column, which provides insight into the effect of atmospheric NOx− on oceanic N biogeochemistry. By contrast, atmospheric δ18O-NOx− values are mainly determined by photochemical processes and thus highly sensitive to temperature and day-time duration (Hastings et al., 2003). The East Sea (or Japan Sea), which is surrounded by Korea, Japan, and Russia, has been subject to increasing emissions of NANTH over the past 30 years, as a result of the growing economies of the East Asian
Corresponding author. Fundamental Research Division, National Marine Biodiversity Institute of Korea, Janghang, 33662, South Korea. E-mail address: [email protected] (T.-W. Kim).
https://doi.org/10.1016/j.marpolbul.2019.110610 Received 2 July 2019; Received in revised form 6 September 2019; Accepted 18 September 2019 Available online 22 October 2019 0025-326X/ © 2019 Elsevier Ltd. All rights reserved.
Marine Pollution Bulletin 149 (2019) 110610
H. Kim, et al.
Fig. 1. Location map of the East Sea (a), sampling sites of the precipitation and airborne TSP (ESRI, Uljin, South Korea), and the seawater (circle) samples (b).
countries (Kim et al., 2011, 2017). Anthropogenic NOx− sources can be optimally traced in this marine region, as it receives the outflows of three countries. For example, during winter and spring, the winds blowing from China toward the East Sea carry Asian dust and atmospheric pollutants generated from populated cities (Anderson et al., 2019; Park, 2015), while in summer, prevailing southwesterly winds transport pollutants produced in Japan (Ishizaka and Yamada, 2019; Son et al., 2006). In addition, the East Sea is minimally affected by riverine discharge because all major rivers discharge into the west and south coasts. In this study, we investigated δ15N- and δ18O-NOx− values (including NO3− and NO2−, which is virtually equal to NO3− due to a relatively negligible NO2− concentration; Park et al., 2019) in atmospheric (precipitation and particulate matters) and seawater samples collected in coastal regions in the East Sea in order to determine the sources of atmospheric anthropogenic NOx− and assess the impact on the East Sea N pool. The study site, Uljin, is a rural area located on the central eastern coast of South Korea (Fig. 1). There are no significant large-scale industrial and agricultural activities in Uljin (e.g., Park et al., 2019). Thus it was an optimal region for our study purpose. For this study, a precipitation sampler and a high-volume air sampler were installed on the rooftop of the East Sea Research Institute (ESRI; 37.08°N, 129.41°E; Fig. 1b) in Uljin. Precipitation and airborne TSP samples were then collected between October 2013–June 2017 (n = 108) and between March 2014–May 2017 (n = 273), respectively. Further detail regarding the atmospheric sampling and NOx– deposition estimation can be found in Park et al. (2019). Park et al. (2019) mainly described the factors affecting atmospheric NANTH deposition and its impact on ocean productivity, but they did not focus on δ15N- and δ18O-NOx− values. As this study included a new dataset collected between March 2016–June 2017, we briefly presented the updated results in the supplementary text. In addition, the annual and seasonal δ15N-NOx− mean values between October 2013 and October 2014 were previously presented in Kim et al. (2017). Seawater samples were collected from the upper 300 m depth at 25-m intervals four times between March and April 2013 in the East Sea near Uljin (Fig. 1b). These seawater samples were filtered using a 0.45-μm HDPE syringe filter prior to chemical analysis. NOx− concentrations were measured using a nutrient auto-analyzer (Seal Analytical Inc., QuAAtro 39, USA) and an ion chromatograph (861 Advanced Compact IC, Metrohm, Germany; only for atmospheric samples). NOx− isotopic values were measured using a bacterial denitrification assay (Sigman et al., 2001); in this assay, NOx− was converted into N2O gas using denitrifying bacteria, then the isotopic composition of the analyte gas (N2O) was measured with a Delta V Advantage continuous flow isotope ratio mass spectrometer and interfaced with a Gas Bench II (Thermo Scientific, Bremen, Germany) at the Stable Isotope Facility Center, University of California, Davis, USA. N
and O isotope values were expressed in δ (‰) with respect to the 15/ 14 N2 gas in the air and to the 18/16O in Vienna Standard Mean Ocean Water (VSMOW), respectively. They were calibrated against reference materials (USGS-32, USGS-34, and USGS-35). The measurement errors were < 0.4‰ for 15N and < 0.5‰ for 18O. Flux-weighted δ15N- and δ18O-NOx− values were calculated using the following equation (Fry, 2006): d
FΣ =
∑i = 1 (mi × Fi ) d
∑i = 1 mi
FΣ = monthly flux-weighted δ15N- and δ18O-NOx− values (‰) mi = NOx− deposition flux collected on the ith day Fi = δ15N- and δ18O-NOx− collected on the ith day (‰) mi × Fi = Product of the ith day NOx− deposition flux with δ15N- and δ18O-NOx− values collected on the ith day d = number of airborne TSP samplings and precipitation events per month d ∑i = 1 mi is calculated by summing all NOx− deposition flux for one month Air mass backward trajectories were calculated using the Hybrid Single-Particle Lagrangian Integrated Trajectory model, which is distributed by the National Oceanic and Atmospheric Administration Air Resource Laboratory (Stein et al., 2015). The backward trajectories started every 6 h during the study period from 500 m above ground at the ESRI, and these contained the hourly positions over the previous three days. A cluster analysis was performed to determine the general atmospheric transport routes (trajectory clusters) for each season (Winter; December to February, Spring; March to May, Summer; June to August, and Fall; September to November). In addition, the δ15N- and δ18O-NOx− values were assigned to backward trajectories generated during the corresponding sampling periods: mean δ15N- and δ18O-NOx− values were calculated in the determined trajectory clusters. However, we did not analyze the backward trajectories of the δ15N- and δ18ONOx− values obtained from the precipitation samples due to the variability of precipitation duration and intensity. Nonetheless, it is likely that NOx− source regions for TSP were similar to those for precipitation because of the significant contribution of below-cloud scavenging of TSP during precipitation events (Park et al., 2019). The flux-weighted δ15N-NOx− in precipitation (3.2 ± 0.9‰) and airborne TSP (8.2 ± 0.4‰) showed their peak values in winter in the study area (Table 1, Fig. 2). In comparison, high δ15N-NOx− values were recorded during winter (dry season) in cities adjacent to the South China Sea (SCS) and Beijing, for both precipitation (average in SCS = 4.9‰) and airborne TSP (average in SCS = 2.2‰; range in Beijing = 7.6–19.2‰) (Chen et al., 2019; He et al., 2018; Xiao et al., 2
Marine Pollution Bulletin 149 (2019) 110610
H. Kim, et al.
Table 1 Average NOx− flux, flux weighted δ15N-NOx− and δ18O-NOx−, and absolute δ15N-NOx− and δ18O-NOx− values in precipitation and airborne TSP, measured at the ESRI station near to the East Sea. Year or seasona
Precipitation
Airborne TSP
Winter Spring Summer Fall Annual Winter Spring Summer Fall Annual
NOx− flux
Flux weighted δ15N-NOx−
(mmol m−2 month−1)
(‰)
1.0 1.6 2.2 1.3 1.5 3.3 3.4 1.4 1.6 2.5
( ± 0.2)b ( ± 0.3) ( ± 0.4) ( ± 0.3) ( ± 0.2) ( ± 0.3) ( ± 0.4) ( ± 0.2) ( ± 0.2) ( ± 0.2)
Flux weighted δ18O-NOx−
δ15N-NOx−
δ18O-NOx−
(‰)
3.2 ( ± 0.9) –0.2 ( ± 0.7) –2.4 ( ± 1.0) 0.6 ( ± 0.8) 8.2 ( ± 0.4) 1.8 ( ± 0.4) 1.6 ( ± 0.4) 3.6 ( ± 0.5) -
84.1 78.6 72.8 75.8 88.0 76.4 69.8 78.1 -
( ± 1.4) ( ± 0.9) ( ± 0.7) ( ± 1.6) ( ± 0.8) ( ± 1.1) ( ± 1.2) ( ± 1.9)
3.4 ( ± 0.9) –0.2 ( ± 0.7) –2.1 ( ± 1.0) 0.2 ( ± 0.7) 0.4 ( ± 0.5) 8.6 ( ± 0.4) 2.0 ( ± 0.4) 1.6 ( ± 0.4) 4.1 ( ± 0.5) 3.9 ( ± 0.5)
83.9 78.3 72.0 75.6 77.8 86.6 75.4 68.6 77.0 76.8
( ± 1.3) ( ± 0.9) ( ± 0.7) ( ± 1.7) ( ± 0.9) ( ± 0.7) ( ± 1.2) ( ± 1.2) ( ± 1.8) ( ± 1.2)
a
Seasonal averages were calculated using data collected in winter (December, January, and February), spring (March, April, and May), summer (June, July, and August), and fall (September, October, and November) over a period of three years (October 2013–June 2017 for precipitation and March 2014–May 2017 for airborne TSP). b Standard errors are indicated in parentheses.
occurrence of typhoons: the δ15N-NOx− values produced from lightning are ∼0‰ (ranging from –0.5‰ to 1.4‰) (Hastings et al., 2003). However, the East Sea is exposed to typhoons less often than the SCS, where the contribution of NOx− from lightning would be < 4% (Zhao et al., 2015); thus, this scenario is unlikely. Spring was characterized by four distinct air mass transport routes: Two of them represented ∼ 61% of the total backward trajectories (C1SPR = 16%, C2SPR = 45%), and these stayed over the Korean peninsula for a relatively long time. The remaining routes, representing 40% of the total backward trajectories (C3SPR = 27%, C4SPR = 13%), originated from northeastern China (Fig. 3). During spring, the isotopic values ranged between 1.93 and 2.76‰ for δ15N-NOx− (Fig. 3). The low δ15N-NO3− values for airborne TSP during March might be attributable to the seasonal reduction of coal combustion and biomass burning. The overall wind direction in fall was similar to that in spring. Of the backward trajectories, 46% (C1FAL) were mainly derived from the Korean peninsula, while the remaining 54% were significantly affected by China (C2FAL = 22%, C3FAL = 14%, and C4FAL = 18%) (Fig. 3). During the fall, among the determined transport routes, the mean isotopic values ranged between 3.63 and 4.96‰ for δ15N-NOx− (Fig. 3), which were higher than those in summer and spring but lower than those in winter. In China, winter heating typically starts in November. The increased coal combustion for heating in late fall might increase the production of NOx−, resulting in relatively high δ15NNOx− values in the study site. The δ18O-NOx− values were also higher in winter and lower in summer (Fig. 2); however, this is likely due to photochemical processes, not changes in the pollution sources. Generally, NO2 is oxidized to HNO3 via a hydroxyl radical produced by photolysis (OH pathway). In summer, this process is prevalent due to the higher concentration of photochemically-produced OH radicals (Calvert et al., 1985). During nighttime, NO2 tends to react with O3 and produce the NO3 radical; this radical subsequently combines with NO2 and produces dinitrogen pentoxide (N2O5). N2O5 is hydrolyzed by H2O to produce HNO3 (N2O5 pathway). This process is dominant in winter due to the stability of N2O5 under low temperatures (Calvert et al., 1985). Thus, the increased contribution of O3 (which has high isotopic values, between 90 and 120‰) to HNO3 formation (e.g., through the N2O5 pathway) might increase the δ18O-HNOx value (Hastings et al., 2003; Johnston and Thiemens, 1997; Krankowsky et al., 1995). In accordance with our results, several previous studies have shown the prevalence of higher δ18O-NOx− values in winter compared to summer (Altieri et al., 2013; Gobel et al., 2013; Wu et al., 2018b; Yang et al., 2014). The occurrence of a strong negative correlation between temperature and δ18O-NOx− in the precipitation and airborne TSP samples observed in our study
2015). During winter, the ESRI station was strongly affected by the northwesterly routes (C1win + C2win + C3win = 66%) passing through northeastern China, including the Beijing-Tianjin-Hebei regions (Fig. 3). These populated urban areas have been found to have the worst air quality in China (Ma et al., 2017), and coal consumption in China was estimated to be ∼3.8 billion tones in 2011, accounting for ∼47% of the global coal consumption (US Energy Information Administration). Thus, the highest δ15N-NOx− values during winter observed in the study site appeared to include the effects of coal and biomass combustions in China, based on the fact that their δ15N-NOx− values (6–26‰) were higher than those released from internal combustion engines that convert atmospheric N2 and O2 into NOx− (median value = –4.7‰; ranging from –19 to 9.8‰) (Felix et al., 2012; Wang et al., 2016; Park et al., 2018) (Fig. 4). Liu et al. (2017b) reported that industrial sectors and power plants consuming coal contributed to ∼62% of total anthropogenic NOy (including HNO3) emission in China, and the 15N-NOx− values from these sectors tended to be higher than the 15N-NOx− values generated from transportation. Because the northwesterly routes were not significantly associated with South Korea, the impacts of coal and biomass combustions in China on atmospheric δ15N-NOx− values deposited into our site (and the East Sea) could be greater than those in South Korea. In addition, total NOy emission in China is 20-fold greater than that in South Korea (Liu et al., 2017b). Previous studies have also reported that the NOx− pollutant generated in China has been transported to Korea (Itahashi et al., 2015; Park et al., 2018). The lowest seasonal means of flux-weighted δ15N-NOx− values for precipitation (–2.4 ± 1.0‰) and airborne TSP (1.6 ± 0.4‰) were observed in summer (Table 1, Fig. 2), which was consistent with previous results obtained at sites near the SCS (average for precipitation = 0.2‰; average for TSP = –0.3‰) (Chen et al., 2019; Xiao et al., 2015). During summer, 40% (C2SUM) and 17% (C1SUM = 13%, C3SUM = 4%) of the backward trajectory routes traveled over middle and southern Korea, respectively; one of these routes represented air masses from the East Sea (C4SUM = 33%). The average isotopic values in these transport routes ranged from −0.63 to 2.00‰ for δ15N-NOx−, which were much lower than those in winter (Fig. 3). These low δ15NNOx− values were similar to those found in vehicles exhaust (Fig. 4) accounting for ∼58% of total anthropogenic NOy emission in South Korea (National Institute of Environmental Research, 2017), suggesting that vehicle exhaust produced in South Korea is a summertime pollution source. Additionally, the C5SUM route (11%) blowing from northeastern China also showed reduced δ15N-NOx− values (−0.63‰) (Fig. 3), which was associated with diminished heating demand in summer. Alternatively, these low values might have been caused by the 3
Marine Pollution Bulletin 149 (2019) 110610
H. Kim, et al.
Fig. 2. Monthly averages of the NOx− flux (light gray bars), flux-weighted δ15N-NOx− (red circles), and δ18O-NOx− (blue circles) values in the precipitation (a) and airborne TSP (b) samples collected at the ESRI station near the East Sea over a period of three years (October 2013–June 2017 for precipitation and March 2014–May 2017 for airborne TSP). Whiskers indicate the standard errors. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
complete nitrification (hereafter, remineralized NOx−) will lower δ15NNOx− and Δ(15−18) values (Sigman et al., 2005; Rafter et al., 2013; Yoshikawa et al., 2018). During this process, the δ18O-NOx− values approach the δ18O values in ambient water (δ18O-H2O ≈ 2‰). In our data, the δ15N-NOx− and Δ(15−18) values of the remineralized NOx− in the subsurface layer in the study site appeared to be lower than those (∼5‰ and ∼3‰ for δ15N-NOx− and Δ(15−18), respectively) that have been reported in the global deep ocean (Sigman et al., 2000, 2005; Rafter et al., 2013). In addition, the dual NOx− isotope data were located above a slope of 1:1 (Fig. 5d). Thus, these results could imply potential contributions from the two external nitrogen sources. However, the low δ15N-NOx− and Δ(15−18) in the remineralized NOx− could alternatively be explained by a partial assimilation of NOx− because surface NOx− was not completely consumed during the study period (mean residual NOx− concentration = 2.5 ± 0.4 μmol L−1, n = 11) (Sigman et al., 2005; Rafter et al., 2013; Yoshikawa et al., 2018). Phytoplankton tends to preferentially take up NOx− with low δ15N-NOx− values. This subsequently lowers the δ15N-NOx− and
supports the above hypothesis (Fig. S3). In the East Sea water column during spring, the average δ15N- and δ18O-NOx− values for the surface layer (0–75 m depth) were ∼8.9‰ (Fig. 5a; standard error = 0.3, n = 16) and ∼11.1‰ (Fig. 5b; standard error = 1.3, n = 16), respectively. In subsurface waters (200–300 m depth), the average isotopic values dropped to ∼4.5‰ for δ15N (Fig. 5a; standard error = 0.1, n = 20) and ∼2.3‰ for δ18O in NOx− (Fig. 5b; standard error = 0.3, n = 20), respectively. The difference in value between δ15N- and δ18O-NOx−, Δ(15−18), in the subsurface layer ranged from –1.5‰ to 3.5‰, with an average of 2.1‰ (Fig. 5c; standard error = 0.3‰, n = 20). The Δ(15−18) values were further reduced in the surface layer. These dual isotope data could be used to infer the nitrogen sources, although the data were only collected from one season. When external nitrogen sources (i.e., N2 fixation or atmospheric deposition) with low δ15N-NOx– values are significant N sources, organic matter with reduced δ15N values is formed: the NOx− resulting from the ammonification of this organic matter and subsequent
4
Marine Pollution Bulletin 149 (2019) 110610
H. Kim, et al.
Fig. 3. Clusters (C1 to C5) of air mass backward trajectories starting from the ESRI station and associated δ15N- and δ18O-NOx− values in winter (WIN; December to February), spring (SPR; March to May), summer (SUM; June to August), and fall (FAL; September to November). Color shadings represent a fractional contribution (%) of each bin to clustered trajectory endpoints. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Fig. 4. NOx− isotopic values originating from different sources (gray, modified from Wang et al. (2016)), precipitation (blue), and airborne TSP (red) samples collected at the ESRI station between 2013 and 2017. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Δ(15−18) value in the remineralized NOx−, which complicates the interpretation of δ15N values in the remineralized NOx− existing in the subsurface layer. In the euphotic surface layer, as we observed, the partial consumption enriches the δ15N- and δ18O-NOx− in ‘residual’ NOx−. However, it should not affect the Δ(15−18) value in the residual NOx−, because the fractionation effects on δ15N- and δ18O-NOx− cancel out each other. Nonetheless, the Δ(15−18) values were negative and located above the 1:1 δ15N:δ18O line in the surface layer (Fig. 5c and d), suggesting an extra factor affecting the Δ(15−18) value in the residual NOx−. The atmospheric deposition of NOx− with very low Δ(15−18) values (< −70 in spring; Table 1) can immediately and substantially reduce Δ(15−18) in the surface layer. N2 fixation only, which is
expected to produce a Δ(15−18) value of approximately −2, cannot produce very low Δ(15−18) values (up to −12.8‰) observed in the study site. In addition, N2 fixation is unusual during the nutrient-replete spring period as well as in cold water (favor temperature for N2 fixation is > 25 °C in surface) (Guerrero and Jones, 1996; Karl et al., 2002). Thus, it is difficult for N2 fixation to be a main factor in reducing the δ15N-NOx− and Δ(15−18) values of the East Sea, where the surface temperature generally remains under 25 °C, except during the summer season. Thus, it was more reasonable to conclude that the atmospheric deposition reduced the Δ(15−18) value of the residual NOx− in the euphotic surface layer, while the low δ15N-NOx− and Δ(15−18) in the remineralized NOx− were mainly caused by both partial consumption and atmospheric deposition. However, in a longer time scale (> 1 5
Marine Pollution Bulletin 149 (2019) 110610
H. Kim, et al.
Fig. 5. Seawater isotopic values for δ15NNOx− (a), δ18O-NOx− (b), and Δ(15−18) (c) in the East Sea, collected four times between March–April 2013. The relationship between δ15N-NOx− and δ18O-NOx− is shown in (d). The central red line indicates the median, and edges indicates the 25th and 75th percentile in each box. The whiskers represent a range of available data. The dashed lines in (a–c) indicate the global averages of each variable in the deep ocean, while the dashed line in (d) represents a theoretical change (1:1) between δ15N- and δ18O-NOx− resulting from phytoplankton assimilation. Dual isotopic data should be distributed along this 1:1 line if partial consumption is the only factor affecting seawater concentration of NOx−. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
year), the effect of partial consumption would be eventually negligible because of the complete nitrate consumption every summer. The reduction of δ15N values caused by NANTH input were previously observed in sediment samples in the East Sea (Kim et al., 2017). Umezawa et al. (2014) also provided a similar description to explain the relatively deficient δ15N-NO3− in the East China Sea. In summary, the greatest flux-weighted δ15N-NOx− value at the ESRI station adjacent to the East Sea was observed in winter, and this was attributed to coal combustion and biomass burning for heating. By contrast, the lowest flux-weighted δ15N-NOx− value was observed during summer; atmospheric NOx− mainly originated from vehicle exhaust. Finally, the low Δ(15−18) values observed in the East Sea suggest that the atmospheric deposition of NOx− has affected N pool and biogeochemistry in the East Sea water column. Our study provided the first baseline record for dual NOx− stable isotope ratios of atmospheric and seawater samples in the East Sea. Any future changes in anthropogenic NOx− sources will lead to a significant deviation in δ15N-NOx− and Δ(15−18) from our record.
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Acknowledgements This work was supported by Basic Science Research Program (2012R1A6A3A04038883/2017R1C1B2009189) and Global Research Laboratory Program (2013K1A1A2A02078278) funded by the National Research Foundation of Korea, a Research Program (No. 2019M00400) by the National Marine Biodiversity Institute of Korea, a research project by OJeong Eco-Resilience Institute, and a project (PE99712) funded by the Korea Institute of Ocean Science & Technology. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.marpolbul.2019.110610. References Johnston, J.C., Thiemens, M.H., 1997. The isotopic composition of tropospheric ozone in
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Baseline
Stable isotope ratio of atmospheric and seawater nitrate in the East Sea in the northwestern Pacific ocean
T
Haryun Kima, Geun-Ha Parkb, Seon-Eun Leeb, Young-il Kimc, Kitack Leed, Yeo-Hun Kime, Tae-Wook Kimf,g,∗ a
Fundamental Research Division, National Marine Biodiversity Institute of Korea, Janghang, 33662, South Korea Marine Environmental Research Center, Korea Institute of Ocean Science & Technology, Busan, 49111, South Korea c East Sea Research Institute, Korea Institute of Ocean Science & Technology, Uljin, 36315, South Korea d Division of Environmental Science and Engineering, Pohang University of Science & Technology, Pohang, 37673, South Korea e Global Ocean Research Center, Korea Institute of Ocean Science & Technology, Busan, 49111, South Korea f Division of Environmental Science and Ecological Engineering, Korea University, Seoul, 02841, South Korea g OJeong Eco-Resilience Institute, Korea University, Seoul, 02841, South Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nitrogen and oxygen isotope Anthropogenic nitrogen Atmospheric pollution Marginal sea Ocean nitrogen cycle East Asia
The nitrogen and oxygen isotopic values of anthropogenic NOx− (including NO2− and NO3−) are preserved in precipitation and airborne total suspended particles. Hence, isotope analyses of these atmospheric samples allow for the identification of the NOx− sources. In this study, atmospheric δ15N- and δ18O-NOx− values were measured in an eastern coastal site of South Korea over a three-year period, and compared with seawater values observed from coastal waters of the East Sea. The highest seasonal atmospheric δ15N-NOx− values were associated with wintertime air masses blowing from northeastern China, and similar to those produced by coal and biomass burning. By contrast, the lowest atmospheric δ15N-NOx− values were observed in summer, and the major source appeared to be vehicle exhaust. In addition, the reductions in the seawater δ15N-NOx− and Δ(15−18) (i.e., δ15N-NOx− minus δ18O-NOx−) values in the East Sea could be at least partially attributed to atmospheric deposition.
The amount of anthropogenic nitrogen (NANTH) transferred from the atmosphere to the oceans was estimated to be 54 Tg N yr−1 in 2000, a ten-fold increase from 1860 (5.7 Tg N yr−1) (Duce et al., 2008). It is very likely that atmospheric NANTH exerts ecological impacts on remote ocean regions where the input of riverine nitrogen (N) is negligible and the availability of N is limited. Among atmospheric NANTH species, NOx− (including NO2– and NO3–) is mainly produced through the photochemical oxidation of NOy (i.e., NO + NO2) co-emitted with carbon dioxide (CO2) from fossil fuel combustions (Duce et al., 2008). Atmospheric NOx− deposition may relieve nitrogen limitation to primary production and thus partially counterbalance the atmospheric increase in CO2. In this context, many studies have estimated the NOx– flux deposited to the ocean and investigated its potential effect on ocean primary productivity (e.g. Kim et al., 2014a, 2014b; Liu et al., 2019; Park et al., 2019; Wu et al., 2018a; Yan and Kim, 2015; Zhang et al., 2019). Among them, however, few studies have attempted to link atmospheric NOx– deposition to seawater NOx− availability using dual stable isotopic values of nitrogen and oxygen atoms in NOx− (δ15N- and
∗
δ18O-NOx−) (e.g., Liu et al., 2017a; Luo et al., 2018; Wu et al., 2018b; Yang et al., 2014). The δ15N- and δ18O-NOx− values are widely used to identify pollution sources and chemical processes in the atmosphere (Chang et al., 2018; Chen et al., 2019; Nelson et al., 2018), coastal lagoons (Mwaura et al., 2017), and estuaries (Wiegner et al., 2016). The isotopic signatures of NANTH sources are not affected by photochemical processes, so they are preserved in δ15N-NOx− in precipitation and airborne total suspended particles (TSP) (Elliott et al., 2009; Hastings et al., 2003). Further, the δ15N-NOx− transferred from the atmosphere to the oceans can alter NOx− isotopic values in the water column, which provides insight into the effect of atmospheric NOx− on oceanic N biogeochemistry. By contrast, atmospheric δ18O-NOx− values are mainly determined by photochemical processes and thus highly sensitive to temperature and day-time duration (Hastings et al., 2003). The East Sea (or Japan Sea), which is surrounded by Korea, Japan, and Russia, has been subject to increasing emissions of NANTH over the past 30 years, as a result of the growing economies of the East Asian
Corresponding author. Fundamental Research Division, National Marine Biodiversity Institute of Korea, Janghang, 33662, South Korea. E-mail address: [email protected] (T.-W. Kim).
https://doi.org/10.1016/j.marpolbul.2019.110610 Received 2 July 2019; Received in revised form 6 September 2019; Accepted 18 September 2019 Available online 22 October 2019 0025-326X/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Location map of the East Sea (a), sampling sites of the precipitation and airborne TSP (ESRI, Uljin, South Korea), and the seawater (circle) samples (b).
countries (Kim et al., 2011, 2017). Anthropogenic NOx− sources can be optimally traced in this marine region, as it receives the outflows of three countries. For example, during winter and spring, the winds blowing from China toward the East Sea carry Asian dust and atmospheric pollutants generated from populated cities (Anderson et al., 2019; Park, 2015), while in summer, prevailing southwesterly winds transport pollutants produced in Japan (Ishizaka and Yamada, 2019; Son et al., 2006). In addition, the East Sea is minimally affected by riverine discharge because all major rivers discharge into the west and south coasts. In this study, we investigated δ15N- and δ18O-NOx− values (including NO3− and NO2−, which is virtually equal to NO3− due to a relatively negligible NO2− concentration; Park et al., 2019) in atmospheric (precipitation and particulate matters) and seawater samples collected in coastal regions in the East Sea in order to determine the sources of atmospheric anthropogenic NOx− and assess the impact on the East Sea N pool. The study site, Uljin, is a rural area located on the central eastern coast of South Korea (Fig. 1). There are no significant large-scale industrial and agricultural activities in Uljin (e.g., Park et al., 2019). Thus it was an optimal region for our study purpose. For this study, a precipitation sampler and a high-volume air sampler were installed on the rooftop of the East Sea Research Institute (ESRI; 37.08°N, 129.41°E; Fig. 1b) in Uljin. Precipitation and airborne TSP samples were then collected between October 2013–June 2017 (n = 108) and between March 2014–May 2017 (n = 273), respectively. Further detail regarding the atmospheric sampling and NOx– deposition estimation can be found in Park et al. (2019). Park et al. (2019) mainly described the factors affecting atmospheric NANTH deposition and its impact on ocean productivity, but they did not focus on δ15N- and δ18O-NOx− values. As this study included a new dataset collected between March 2016–June 2017, we briefly presented the updated results in the supplementary text. In addition, the annual and seasonal δ15N-NOx− mean values between October 2013 and October 2014 were previously presented in Kim et al. (2017). Seawater samples were collected from the upper 300 m depth at 25-m intervals four times between March and April 2013 in the East Sea near Uljin (Fig. 1b). These seawater samples were filtered using a 0.45-μm HDPE syringe filter prior to chemical analysis. NOx− concentrations were measured using a nutrient auto-analyzer (Seal Analytical Inc., QuAAtro 39, USA) and an ion chromatograph (861 Advanced Compact IC, Metrohm, Germany; only for atmospheric samples). NOx− isotopic values were measured using a bacterial denitrification assay (Sigman et al., 2001); in this assay, NOx− was converted into N2O gas using denitrifying bacteria, then the isotopic composition of the analyte gas (N2O) was measured with a Delta V Advantage continuous flow isotope ratio mass spectrometer and interfaced with a Gas Bench II (Thermo Scientific, Bremen, Germany) at the Stable Isotope Facility Center, University of California, Davis, USA. N
and O isotope values were expressed in δ (‰) with respect to the 15/ 14 N2 gas in the air and to the 18/16O in Vienna Standard Mean Ocean Water (VSMOW), respectively. They were calibrated against reference materials (USGS-32, USGS-34, and USGS-35). The measurement errors were < 0.4‰ for 15N and < 0.5‰ for 18O. Flux-weighted δ15N- and δ18O-NOx− values were calculated using the following equation (Fry, 2006): d
FΣ =
∑i = 1 (mi × Fi ) d
∑i = 1 mi
FΣ = monthly flux-weighted δ15N- and δ18O-NOx− values (‰) mi = NOx− deposition flux collected on the ith day Fi = δ15N- and δ18O-NOx− collected on the ith day (‰) mi × Fi = Product of the ith day NOx− deposition flux with δ15N- and δ18O-NOx− values collected on the ith day d = number of airborne TSP samplings and precipitation events per month d ∑i = 1 mi is calculated by summing all NOx− deposition flux for one month Air mass backward trajectories were calculated using the Hybrid Single-Particle Lagrangian Integrated Trajectory model, which is distributed by the National Oceanic and Atmospheric Administration Air Resource Laboratory (Stein et al., 2015). The backward trajectories started every 6 h during the study period from 500 m above ground at the ESRI, and these contained the hourly positions over the previous three days. A cluster analysis was performed to determine the general atmospheric transport routes (trajectory clusters) for each season (Winter; December to February, Spring; March to May, Summer; June to August, and Fall; September to November). In addition, the δ15N- and δ18O-NOx− values were assigned to backward trajectories generated during the corresponding sampling periods: mean δ15N- and δ18O-NOx− values were calculated in the determined trajectory clusters. However, we did not analyze the backward trajectories of the δ15N- and δ18ONOx− values obtained from the precipitation samples due to the variability of precipitation duration and intensity. Nonetheless, it is likely that NOx− source regions for TSP were similar to those for precipitation because of the significant contribution of below-cloud scavenging of TSP during precipitation events (Park et al., 2019). The flux-weighted δ15N-NOx− in precipitation (3.2 ± 0.9‰) and airborne TSP (8.2 ± 0.4‰) showed their peak values in winter in the study area (Table 1, Fig. 2). In comparison, high δ15N-NOx− values were recorded during winter (dry season) in cities adjacent to the South China Sea (SCS) and Beijing, for both precipitation (average in SCS = 4.9‰) and airborne TSP (average in SCS = 2.2‰; range in Beijing = 7.6–19.2‰) (Chen et al., 2019; He et al., 2018; Xiao et al., 2
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Table 1 Average NOx− flux, flux weighted δ15N-NOx− and δ18O-NOx−, and absolute δ15N-NOx− and δ18O-NOx− values in precipitation and airborne TSP, measured at the ESRI station near to the East Sea. Year or seasona
Precipitation
Airborne TSP
Winter Spring Summer Fall Annual Winter Spring Summer Fall Annual
NOx− flux
Flux weighted δ15N-NOx−
(mmol m−2 month−1)
(‰)
1.0 1.6 2.2 1.3 1.5 3.3 3.4 1.4 1.6 2.5
( ± 0.2)b ( ± 0.3) ( ± 0.4) ( ± 0.3) ( ± 0.2) ( ± 0.3) ( ± 0.4) ( ± 0.2) ( ± 0.2) ( ± 0.2)
Flux weighted δ18O-NOx−
δ15N-NOx−
δ18O-NOx−
(‰)
3.2 ( ± 0.9) –0.2 ( ± 0.7) –2.4 ( ± 1.0) 0.6 ( ± 0.8) 8.2 ( ± 0.4) 1.8 ( ± 0.4) 1.6 ( ± 0.4) 3.6 ( ± 0.5) -
84.1 78.6 72.8 75.8 88.0 76.4 69.8 78.1 -
( ± 1.4) ( ± 0.9) ( ± 0.7) ( ± 1.6) ( ± 0.8) ( ± 1.1) ( ± 1.2) ( ± 1.9)
3.4 ( ± 0.9) –0.2 ( ± 0.7) –2.1 ( ± 1.0) 0.2 ( ± 0.7) 0.4 ( ± 0.5) 8.6 ( ± 0.4) 2.0 ( ± 0.4) 1.6 ( ± 0.4) 4.1 ( ± 0.5) 3.9 ( ± 0.5)
83.9 78.3 72.0 75.6 77.8 86.6 75.4 68.6 77.0 76.8
( ± 1.3) ( ± 0.9) ( ± 0.7) ( ± 1.7) ( ± 0.9) ( ± 0.7) ( ± 1.2) ( ± 1.2) ( ± 1.8) ( ± 1.2)
a
Seasonal averages were calculated using data collected in winter (December, January, and February), spring (March, April, and May), summer (June, July, and August), and fall (September, October, and November) over a period of three years (October 2013–June 2017 for precipitation and March 2014–May 2017 for airborne TSP). b Standard errors are indicated in parentheses.
occurrence of typhoons: the δ15N-NOx− values produced from lightning are ∼0‰ (ranging from –0.5‰ to 1.4‰) (Hastings et al., 2003). However, the East Sea is exposed to typhoons less often than the SCS, where the contribution of NOx− from lightning would be < 4% (Zhao et al., 2015); thus, this scenario is unlikely. Spring was characterized by four distinct air mass transport routes: Two of them represented ∼ 61% of the total backward trajectories (C1SPR = 16%, C2SPR = 45%), and these stayed over the Korean peninsula for a relatively long time. The remaining routes, representing 40% of the total backward trajectories (C3SPR = 27%, C4SPR = 13%), originated from northeastern China (Fig. 3). During spring, the isotopic values ranged between 1.93 and 2.76‰ for δ15N-NOx− (Fig. 3). The low δ15N-NO3− values for airborne TSP during March might be attributable to the seasonal reduction of coal combustion and biomass burning. The overall wind direction in fall was similar to that in spring. Of the backward trajectories, 46% (C1FAL) were mainly derived from the Korean peninsula, while the remaining 54% were significantly affected by China (C2FAL = 22%, C3FAL = 14%, and C4FAL = 18%) (Fig. 3). During the fall, among the determined transport routes, the mean isotopic values ranged between 3.63 and 4.96‰ for δ15N-NOx− (Fig. 3), which were higher than those in summer and spring but lower than those in winter. In China, winter heating typically starts in November. The increased coal combustion for heating in late fall might increase the production of NOx−, resulting in relatively high δ15NNOx− values in the study site. The δ18O-NOx− values were also higher in winter and lower in summer (Fig. 2); however, this is likely due to photochemical processes, not changes in the pollution sources. Generally, NO2 is oxidized to HNO3 via a hydroxyl radical produced by photolysis (OH pathway). In summer, this process is prevalent due to the higher concentration of photochemically-produced OH radicals (Calvert et al., 1985). During nighttime, NO2 tends to react with O3 and produce the NO3 radical; this radical subsequently combines with NO2 and produces dinitrogen pentoxide (N2O5). N2O5 is hydrolyzed by H2O to produce HNO3 (N2O5 pathway). This process is dominant in winter due to the stability of N2O5 under low temperatures (Calvert et al., 1985). Thus, the increased contribution of O3 (which has high isotopic values, between 90 and 120‰) to HNO3 formation (e.g., through the N2O5 pathway) might increase the δ18O-HNOx value (Hastings et al., 2003; Johnston and Thiemens, 1997; Krankowsky et al., 1995). In accordance with our results, several previous studies have shown the prevalence of higher δ18O-NOx− values in winter compared to summer (Altieri et al., 2013; Gobel et al., 2013; Wu et al., 2018b; Yang et al., 2014). The occurrence of a strong negative correlation between temperature and δ18O-NOx− in the precipitation and airborne TSP samples observed in our study
2015). During winter, the ESRI station was strongly affected by the northwesterly routes (C1win + C2win + C3win = 66%) passing through northeastern China, including the Beijing-Tianjin-Hebei regions (Fig. 3). These populated urban areas have been found to have the worst air quality in China (Ma et al., 2017), and coal consumption in China was estimated to be ∼3.8 billion tones in 2011, accounting for ∼47% of the global coal consumption (US Energy Information Administration). Thus, the highest δ15N-NOx− values during winter observed in the study site appeared to include the effects of coal and biomass combustions in China, based on the fact that their δ15N-NOx− values (6–26‰) were higher than those released from internal combustion engines that convert atmospheric N2 and O2 into NOx− (median value = –4.7‰; ranging from –19 to 9.8‰) (Felix et al., 2012; Wang et al., 2016; Park et al., 2018) (Fig. 4). Liu et al. (2017b) reported that industrial sectors and power plants consuming coal contributed to ∼62% of total anthropogenic NOy (including HNO3) emission in China, and the 15N-NOx− values from these sectors tended to be higher than the 15N-NOx− values generated from transportation. Because the northwesterly routes were not significantly associated with South Korea, the impacts of coal and biomass combustions in China on atmospheric δ15N-NOx− values deposited into our site (and the East Sea) could be greater than those in South Korea. In addition, total NOy emission in China is 20-fold greater than that in South Korea (Liu et al., 2017b). Previous studies have also reported that the NOx− pollutant generated in China has been transported to Korea (Itahashi et al., 2015; Park et al., 2018). The lowest seasonal means of flux-weighted δ15N-NOx− values for precipitation (–2.4 ± 1.0‰) and airborne TSP (1.6 ± 0.4‰) were observed in summer (Table 1, Fig. 2), which was consistent with previous results obtained at sites near the SCS (average for precipitation = 0.2‰; average for TSP = –0.3‰) (Chen et al., 2019; Xiao et al., 2015). During summer, 40% (C2SUM) and 17% (C1SUM = 13%, C3SUM = 4%) of the backward trajectory routes traveled over middle and southern Korea, respectively; one of these routes represented air masses from the East Sea (C4SUM = 33%). The average isotopic values in these transport routes ranged from −0.63 to 2.00‰ for δ15N-NOx−, which were much lower than those in winter (Fig. 3). These low δ15NNOx− values were similar to those found in vehicles exhaust (Fig. 4) accounting for ∼58% of total anthropogenic NOy emission in South Korea (National Institute of Environmental Research, 2017), suggesting that vehicle exhaust produced in South Korea is a summertime pollution source. Additionally, the C5SUM route (11%) blowing from northeastern China also showed reduced δ15N-NOx− values (−0.63‰) (Fig. 3), which was associated with diminished heating demand in summer. Alternatively, these low values might have been caused by the 3
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Fig. 2. Monthly averages of the NOx− flux (light gray bars), flux-weighted δ15N-NOx− (red circles), and δ18O-NOx− (blue circles) values in the precipitation (a) and airborne TSP (b) samples collected at the ESRI station near the East Sea over a period of three years (October 2013–June 2017 for precipitation and March 2014–May 2017 for airborne TSP). Whiskers indicate the standard errors. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
complete nitrification (hereafter, remineralized NOx−) will lower δ15NNOx− and Δ(15−18) values (Sigman et al., 2005; Rafter et al., 2013; Yoshikawa et al., 2018). During this process, the δ18O-NOx− values approach the δ18O values in ambient water (δ18O-H2O ≈ 2‰). In our data, the δ15N-NOx− and Δ(15−18) values of the remineralized NOx− in the subsurface layer in the study site appeared to be lower than those (∼5‰ and ∼3‰ for δ15N-NOx− and Δ(15−18), respectively) that have been reported in the global deep ocean (Sigman et al., 2000, 2005; Rafter et al., 2013). In addition, the dual NOx− isotope data were located above a slope of 1:1 (Fig. 5d). Thus, these results could imply potential contributions from the two external nitrogen sources. However, the low δ15N-NOx− and Δ(15−18) in the remineralized NOx− could alternatively be explained by a partial assimilation of NOx− because surface NOx− was not completely consumed during the study period (mean residual NOx− concentration = 2.5 ± 0.4 μmol L−1, n = 11) (Sigman et al., 2005; Rafter et al., 2013; Yoshikawa et al., 2018). Phytoplankton tends to preferentially take up NOx− with low δ15N-NOx− values. This subsequently lowers the δ15N-NOx− and
supports the above hypothesis (Fig. S3). In the East Sea water column during spring, the average δ15N- and δ18O-NOx− values for the surface layer (0–75 m depth) were ∼8.9‰ (Fig. 5a; standard error = 0.3, n = 16) and ∼11.1‰ (Fig. 5b; standard error = 1.3, n = 16), respectively. In subsurface waters (200–300 m depth), the average isotopic values dropped to ∼4.5‰ for δ15N (Fig. 5a; standard error = 0.1, n = 20) and ∼2.3‰ for δ18O in NOx− (Fig. 5b; standard error = 0.3, n = 20), respectively. The difference in value between δ15N- and δ18O-NOx−, Δ(15−18), in the subsurface layer ranged from –1.5‰ to 3.5‰, with an average of 2.1‰ (Fig. 5c; standard error = 0.3‰, n = 20). The Δ(15−18) values were further reduced in the surface layer. These dual isotope data could be used to infer the nitrogen sources, although the data were only collected from one season. When external nitrogen sources (i.e., N2 fixation or atmospheric deposition) with low δ15N-NOx– values are significant N sources, organic matter with reduced δ15N values is formed: the NOx− resulting from the ammonification of this organic matter and subsequent
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Fig. 3. Clusters (C1 to C5) of air mass backward trajectories starting from the ESRI station and associated δ15N- and δ18O-NOx− values in winter (WIN; December to February), spring (SPR; March to May), summer (SUM; June to August), and fall (FAL; September to November). Color shadings represent a fractional contribution (%) of each bin to clustered trajectory endpoints. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Fig. 4. NOx− isotopic values originating from different sources (gray, modified from Wang et al. (2016)), precipitation (blue), and airborne TSP (red) samples collected at the ESRI station between 2013 and 2017. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Δ(15−18) value in the remineralized NOx−, which complicates the interpretation of δ15N values in the remineralized NOx− existing in the subsurface layer. In the euphotic surface layer, as we observed, the partial consumption enriches the δ15N- and δ18O-NOx− in ‘residual’ NOx−. However, it should not affect the Δ(15−18) value in the residual NOx−, because the fractionation effects on δ15N- and δ18O-NOx− cancel out each other. Nonetheless, the Δ(15−18) values were negative and located above the 1:1 δ15N:δ18O line in the surface layer (Fig. 5c and d), suggesting an extra factor affecting the Δ(15−18) value in the residual NOx−. The atmospheric deposition of NOx− with very low Δ(15−18) values (< −70 in spring; Table 1) can immediately and substantially reduce Δ(15−18) in the surface layer. N2 fixation only, which is
expected to produce a Δ(15−18) value of approximately −2, cannot produce very low Δ(15−18) values (up to −12.8‰) observed in the study site. In addition, N2 fixation is unusual during the nutrient-replete spring period as well as in cold water (favor temperature for N2 fixation is > 25 °C in surface) (Guerrero and Jones, 1996; Karl et al., 2002). Thus, it is difficult for N2 fixation to be a main factor in reducing the δ15N-NOx− and Δ(15−18) values of the East Sea, where the surface temperature generally remains under 25 °C, except during the summer season. Thus, it was more reasonable to conclude that the atmospheric deposition reduced the Δ(15−18) value of the residual NOx− in the euphotic surface layer, while the low δ15N-NOx− and Δ(15−18) in the remineralized NOx− were mainly caused by both partial consumption and atmospheric deposition. However, in a longer time scale (> 1 5
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Fig. 5. Seawater isotopic values for δ15NNOx− (a), δ18O-NOx− (b), and Δ(15−18) (c) in the East Sea, collected four times between March–April 2013. The relationship between δ15N-NOx− and δ18O-NOx− is shown in (d). The central red line indicates the median, and edges indicates the 25th and 75th percentile in each box. The whiskers represent a range of available data. The dashed lines in (a–c) indicate the global averages of each variable in the deep ocean, while the dashed line in (d) represents a theoretical change (1:1) between δ15N- and δ18O-NOx− resulting from phytoplankton assimilation. Dual isotopic data should be distributed along this 1:1 line if partial consumption is the only factor affecting seawater concentration of NOx−. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
year), the effect of partial consumption would be eventually negligible because of the complete nitrate consumption every summer. The reduction of δ15N values caused by NANTH input were previously observed in sediment samples in the East Sea (Kim et al., 2017). Umezawa et al. (2014) also provided a similar description to explain the relatively deficient δ15N-NO3− in the East China Sea. In summary, the greatest flux-weighted δ15N-NOx− value at the ESRI station adjacent to the East Sea was observed in winter, and this was attributed to coal combustion and biomass burning for heating. By contrast, the lowest flux-weighted δ15N-NOx− value was observed during summer; atmospheric NOx− mainly originated from vehicle exhaust. Finally, the low Δ(15−18) values observed in the East Sea suggest that the atmospheric deposition of NOx− has affected N pool and biogeochemistry in the East Sea water column. Our study provided the first baseline record for dual NOx− stable isotope ratios of atmospheric and seawater samples in the East Sea. Any future changes in anthropogenic NOx− sources will lead to a significant deviation in δ15N-NOx− and Δ(15−18) from our record.
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