Examining N cycling in the northern South China Sea from N isotopic signals in nitrate and particulate phases

Abstract

Nitrogen sources and dynamics, one of the key issues in marine biogeochemical cycles, remain poorly constrained in marginal seas. Here we examine the nitrogen cycle in the northern South China Sea (SCS) by combining data from previous reports with a new data set of N isotopic compositions (δ15N) of nitrate, zooplankton, and sinking particles. Average δ15N in subsurface nitrate is 4.8 ± 0.3‰, similar to that of sinking particles (δ15Nsink of 4.4‰) through the euphotic zone (EZ) collected by floating traps and to documented mean (4.6‰) for long‐term mooring traps at 200 m. This along with oft‐observed shallow nitracline (<100 m) suggests that subsurface nitrate is the primary source of new N to support export production. Moreover, δ15Nsink at the bottom of the EZ resembles those of suspended particles (4.2 ± 1.0‰) and zooplankton (5.4 ± 1.0‰) inside the EZ. High similarity in δ15N among various types of particles including zooplankton in different size fractions in the EZ implies rapid N turnover in the ecosystem. In deep waters at 2000–3000 m, however, additional particulate N fluxes are found due to lateral transport, which contain 15N‐depleted particles, resulting in a downward decreasing trend of δ15Nsink. Incorporation of lighter N by bacteria and selective export of picoplankton are proposed as alternative mechanisms contributing to low δ15Nsink in the deep waters. The significant δ15Nsink change in the deep water column makes the SCS different from most other marginal seas; thus, caution should be made when using sedimentary δ15N to reconstruct paleonitrogen processes.

Distribution of Concentration and Stable Isotopic Composition of N2O in the Shelf and Slope of the Northern South China Sea: Implications for Production and Emission

“…δ 18 O‐N 2 O (53.4‰–54.7‰) in the deep water was almost constant and higher than that in the intermediate and near‐surface waters (Figure ) but are consistent with those at similar depths reported for other regions of the North Pacific, including subtropical western North Pacific, ALOHA and Eastern Tropical North Pacific (ETNP; Figure c). δ 15 N bulk ‐N 2 O measured in the deep water ranged between 9.2‰ and 10.3‰ with a mean of 9.7‰ ± 0.4‰, which is higher than δ 15 N bulk ‐N 2 O values observed for the waters above this depth interval and the δ 15 N of sinking particles at this depth interval (3.5‰; Yang et al, ). δ 15 N bulk ‐N 2 O in the deep water is consistent with those reported for ETNP (Figure d).…”

“…Strong positive correlations ([N 2 O, nM] = 2.47[N*, μM] + 2.55, n = 40, r 2 = 0.71; Figure ) were observed between the N 2 O concentration and N* (the values of N* was omitted when either nitrate or phosphate concentrations were below the detection limit), suggesting that remineralization of nitrogen‐rich and 15 N‐depleted organic matter formed by nitrogen fixation may have partially contributed to the production of 15 N‐depleted N 2 O and the decline of δ 15 N‐N 2 O. This result is supported by low δ 15 N (4.9‰ at 100 m and 3.3‰ at 150 m) in the sinking particles observed near the bottom of the euphotic zone and the presence of δ 15 N‐NO 3 subsurface minima (4.5‰–5.1‰) at 100–150 m in the SCS (Yang et al, ).…”

“…The difference of δ 15 N bulk and δ 18 O values of N 2 O between the SCS and ALOHA increased upward from the N 2 O maximum to the isotopic minimum (Figure ). δ 15 N‐NO 3 in the northern SCS increased continuously downward from the subsurface minimum (4.8‰ ± 0.3‰) and reached a maxima of ~6.0‰ at 300–500 m (Yang et al, ). δ 15 N of sinking particles collected at ~400 m at the northern SCS was about 5.2‰–5.5‰ (Yang et al, ).…”

Nitrogen Isotope Variations in the Northern South China Sea Since Marine Isotopic Stage 3: Reconstructed From Foraminifera‐Bound and Bulk Sedimentary Nitrogen

“…It is noted that when the DOT and vertical mixing were relatively deep and strong during the LGM, the FB‐δ 15 N values of G. ruber and G. sacculifer in this study and all three species of MD97‐2142 (Ren, Sigman, Chen, et al, ) were generally higher, and the DOT became deeper earlier than 20 ka as FB‐δ 15 N values became larger. Therefore, it is suggested that stronger diapycnal mixing during the last glacial time could have brought nitrate with higher δ 15 N from deeper water (Wong et al, ; Yang et al, ) to the euphotic zone. In a study in the modern northern SCS, the annual cycle of particulate nitrogen δ 15 N collected in the upper 200 m resembled that of the mixing layer depth variation with the highest δ 15 N values during periods of the deepest mixing layer depth, indicating that the seasonal pattern of particulate isotopic composition might be associated with nutrient supply via diapycnal mixing (Kao et al, ).…”

Physical Forcing-Driven Productivity and Sediment Flux to the Deep Basin of Northern South China Sea: A Decadal Time Series Study

“…Our result is in line with previous studies. Previous studies have indicated that the lateral transport or resuspension contributes some sinking particles onto the continental slopes of the nSCS [14,[19][20][21]. Using the TJ-A-1 moored system, Zhang et al [14] indicated that the southwestward propagation of two mesoscale eddies from the western Luzon Strait carried a considerable amount of suspended particles for the whole water column during January to March 2012 and November 2012 to January 2013, respectively, suggesting that the transportation of sinking particles from southwest of Taiwan.…”