Comparison of Nitrate Isotopes Between the South China Sea and Western North Pacific Ocean: Insights Into Biogeochemical Signals and Water Exchange

Reconstructing past changes in the oceanic nitrate inventory with sedimentary N records in the South China Sea (SCS), which is the terminal of North Pacific Intermediate Water (NPIW), has regional and global implications. However, water-column nitrate cycling that affects N isotope preservation remains poorly understood in the SCS. We present a new data set of nitrate isotopes (delta 15NNO3 ${\delta }<^>{15}{\mathrm{N}}_{{\text{NO}}_{3}}$ and delta 18ONO3 ${\delta }<^>{18}{\mathrm{O}}_{{\text{NO}}_{3}}$) to elucidate nitrate dynamics in the SCS and the adjoining western North Pacific Ocean (wNPO). Greater increases in delta 18ONO3 ${\delta }<^>{18}{\mathrm{O}}_{{\text{NO}}_{3}}$ than in delta 15NNO3 ${\delta }<^>{15}{\mathrm{N}}_{{\text{NO}}_{3}}$ are observed in the SCS euphotic zone, suggesting a combined effect of partial nitrate assimilation and nitrification. In the subsurface and thermocline waters of both regions, upward disproportional decreases in delta 15NNO3 ${\delta }<^>{15}{\mathrm{N}}_{{\text{NO}}_{3}}$ and delta 18ONO3 ${\delta }<^>{18}{\mathrm{O}}_{{\text{NO}}_{3}}$ accompanied by elevated nitrate anomalies (N*) indicate an accumulation of external N. Such changes are less significant in the SCS due to higher nitrate concentrations therein, although external N influxes are comparable in both regions. High delta 15NNO3 ${\delta }<^>{15}{\mathrm{N}}_{{\text{NO}}_{3}}$ and delta 18ONO3 ${\delta }<^>{18}{\mathrm{O}}_{{\text{NO}}_{3}}$ values in the wNPO intermediate water result from the lateral transport of NPIW with isotopically more enriched nitrate from the remote denitrification zones followed by mixing with overlying water containing isotopically depleted nitrate. As NPIW flows into the SCS, its isotopically enriched signal is further diluted by strong vertical mixing with overlying and underlying waters in the interior. Compared to its source water from the wNPO, the SCS deep water has consistent nitrate isotopic compositions but significantly lower N*, indicating increased benthic denitrification at the wNPO margins with an estimated rate of 0.26-0.41 mmol N m(-2) day(-1).

Automated summary

This study utilizes sedimentary N records in the South China Sea to reconstruct past changes in the oceanic nitrate inventory and provides insights into nitrate dynamics in the region. The findings suggest that the changes in nitrate isotopic ratios in the euphotic zone of the South China Sea are influenced by both nitrate assimilation and nitrification. In subsurface and thermocline waters, disproportional decreases in nitrate isotopes and elevated nitrate anomalies indicate the accumulation of external nitrogen. The study also reveals increased benthic denitrification at the western North Pacific Ocean margins, leading to consistent nitrate isotopic compositions but lower nitrate anomalies in the deep water of the South China Sea.