Nitrate isotope dynamics in the lower euphotic-upper mesopelagic zones of the western South China Sea

The dual isotopes (N and O) of nitrate were measured using a denitrifier bacterial method in the western South China Sea (WSCS) during September 2015 to elucidate key information during N transformation in the lower euphotic zone (LEZ)-upper mesopelagic zone (UMZ, down to 500 m in this study) continuum, which is a vital sub-environment for marine N cycle and sequestration of atmospheric CO 2 as well. The N isotopic composition (δ 15 N) of nitrate generally decreased from 500 m toward the base of the euphotic zone (∼100 m), reaching a value of ∼4.6‰ (vs. air N 2 ) at the base of the LEZ, suggesting the imprint of remineralization (nitrification) of isotopically light N from atmospheric source. The δ 15 N and δ 18 O of nitrate only generally conform to a 1:1 line at 50 m and 75 m, suggesting that nitrate assimilation is a dominant process to shape nitrate isotope signature in this light-limited and relatively N-replete lower part of the euphotic zone. The fractionation factors of N and O isotopes during nitrate fractionation ( 15 ε ASSIM , 18 ε ASSIM ) using a steady-state model were estimated to be 4.0‰±0.39‰ and 5.4‰±0.39‰, respectively. The occurrence of nitrification at the base of the LEZ and most of the UMZ is corroborated by the decoupling of δ 15 N and the oxygen isotopic composition (δ 18 O) of nitrate. Our results will provide insights for better understanding N cycle in the South China Sea from a perspective of present and past.