Jin-Yu Terence Yang

Cyclone-driven deep sea injection of freshwater and heat by hyperpycnal flow in the subtropics

China (973 Program) [2009CB421200]; Program of Introducing Talents of Discipline to Universities [B07034]; Academia Sinica Thematic Program AFOBi, Taiwan [NSC 98-2116-M-001-005]

Isotope constraints on particulate nitrogen source and dynamics in the upper water column of the oligotrophic South China Sea

Taiwan [NSC 98-2116-M-001-005]; Academia Sinica Thematic Program AFOBi; China (973 Program) [2009CB421200]; Introducing Talents of Discipline to Universities [B07034]

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

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 dataset 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 paleo- nitrogen processes.