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

Zixuan Li; Chao Xu; Minfang Zheng; Mengya Chen; Yusheng Qiu; Hantao Zhou; Min Chen; Run Zhang

doi:10.1007/s13131-022-2091-4

Volume 42 Issue 1

Jan. 2023

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Zixuan Li, Chao Xu, Minfang Zheng, Mengya Chen, Yusheng Qiu, Hantao Zhou, Min Chen, Run Zhang. Nitrate isotope dynamics in the lower euphotic-upper mesopelagic zones of the western South China Sea[J]. Acta Oceanologica Sinica, 2023, 42(1): 1-11. doi: 10.1007/s13131-022-2091-4

Citation:

Zixuan Li, Chao Xu, Minfang Zheng, Mengya Chen, Yusheng Qiu, Hantao Zhou, Min Chen, Run Zhang. Nitrate isotope dynamics in the lower euphotic-upper mesopelagic zones of the western South China Sea[J]. Acta Oceanologica Sinica, 2023, 42(1): 1-11. doi: 10.1007/s13131-022-2091-4

Citation:

Zixuan Li, Chao Xu, Minfang Zheng, Mengya Chen, Yusheng Qiu, Hantao Zhou, Min Chen, Run Zhang. Nitrate isotope dynamics in the lower euphotic-upper mesopelagic zones of the western South China Sea[J]. Acta Oceanologica Sinica, 2023, 42(1): 1-11. doi: 10.1007/s13131-022-2091-4

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

doi: 10.1007/s13131-022-2091-4

College of Ocean and Earth Sciences, Xiamen University, Xiamen 361102, China

Funds: The National Natural Science Foundation of China under contract Nos 42076042 and 41721005; the Science and Technology Basic Resources Investigation Program of China under contract No. 2017FY201403.

More Information

Corresponding author: E-mail: zhangrun@xmu.edu.cn
Received Date: 2022-01-31
Accepted Date: 2022-07-12

Available Online: 2022-11-08

Publish Date: 2023-01-25

Abstract

Abstract

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 CO₂ as well. The N isotopic composition (δ¹⁵N) of nitrate generally decreased from 500 m toward the base of the euphotic zone (~100 m), reaching a value of ~4.6‰ (vs. air N₂) at the base of the LEZ, suggesting the imprint of remineralization (nitrification) of isotopically light N from atmospheric source. The δ¹⁵N and δ¹⁸O 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 (¹⁵ε_ASSIM, ¹⁸ε_ASSIM) using a steady-state model were estimated to be 4.0‰±0.3‰ and 5.4‰±0.3‰, respectively. The occurrence of nitrification at the base of the LEZ and most of the UMZ is corroborated by the decoupling of δ¹⁵N and the oxygen isotopic composition (δ¹⁸O) of nitrate. Our results will provide insights for better understanding N cycle in the South China Sea from a perspective of present and past.
- N and O isotopes,
- nitrate assimilation,
- nitrification,
- western South China Sea

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