East Asian summer monsoon and human activity imprint on δ15Norg records at the northern coast of the South China Sea over the last 7500 years

In this study, we present high-resolution δ15N records of organic nitrogen (δ15Norg) from the northern inner shelf of the South China Sea (SCS) to reveal nitrogen processes in the coastal environment over the last 7500 years. To remove the bias effect from inorganic nitrogen (IN), mainly terrestrial clay lattice-fixed ammonia, on total/bulk nitrogen (TN) and its nitrogen isotopic composition (δ15Nbulk), a chemical method was used to separate organic nitrogen (ON) from TN. The results suggest that the IN accounts for a high proportion (35.79% on average) of TN in the coastal sediments from the northern SCS and has distinctive δ15N values that obscure δ15Nbulk signal and may reflect a climatic control (e.g., precipitation) on lithogenic materials sourced from land. Before 2000 cal yr BP, δ15Norg has a significant positive correlation with the sea surface temperature (SST) and a negative correlation with the (C/N)org. The variation of δ15Norg during this period mainly reflect the East Asian summer monsoon (EASM) intensity, as high SST and fluvial discharge leads to higher nitrate utilization and elevated primary productivity from phytoplankton (low (C/N)org), when the EASM prevails. However, over the past 2000 years, the EASM has been weaker and anthropogenic influences, particularly enhanced agricultural activity in the Pearl River Estuary (PRE), have replaced the EASM as the dominant driving force behind the increasing trend seen in δ15Norg. The δ15Norg signature preserved within the coastal sediments of the study area not only reflects the climate-controlled nitrogen processes but also the impacts of human activity.