Sedimentary Organic Nitrogen Isotopic Constraints on the Mid-Holocene Transition in the Nitrogen Dynamics of the Northern South China Sea

Millennial-scale nitrogen (N) cycling processes in marginal seas and their response to climate change have not been well understood. Here, we present high-resolution (ca. 110 years) organic nitrogen isotope (delta N-15(org)) data since the last deglaciation (16.1 ka) derived from a highly resolved sediment core in the northern South China Sea, aiming to explore millennial-scale N cycling processes in this area. Unlike most bulk nitrogen isotope (delta N-15(bulk)) records from the South China Sea, the delta N-15(org) records show a clear response to well-defined climatic episodes during the last deglaciation and early Holocene (EH, similar to 11.7 to 9 ka), but exhibit a gradually decreasing trend in mid-to-late Holocene (since ca. 9 ka). During the last deglaciation and EH, the upper water column N dynamics are controlled by the lateral transport of surface nitrate from eastern tropical Pacific (ETP) presumably via the North Pacific Intermediate Water (NPIW) and to some extent, influenced by the altered input of terrigenous matter driven by sea level change. The significant decrease in delta N-15(org) since the mid-Holocene (at ca. 9 ka) can be best explained by the increase in local N-2 fixation forced by enhanced El Ni & ntilde;o. This mechanism is consistent with modern observations. Overall, our results may reflect the main controlling factors of surface ocean N dynamics have shifted from zonal transport of nitrate from the ETP to El Ni & ntilde;o since the mid-Holocene. Plain Language Summary The South China Sea (SCS) is an ideal area in which to study past changes in the low-latitude oceanic nitrogen (N) pool using sedimentary N records. However, reconstructions of past oceanic N cycling in the SCS based on different delta N-15 proxies reveal distinct characteristics through the last deglaciation. Based on sedimentary organic nitrogen isotopic values, our results show that there is a clear response to well-defined climatic episodes over the last 16.1 ka. Global mean ocean nitrate delta N-15 change, mainly through the NPIW intrusion, is the primary control of upper water column N dynamics during climate episodes of the last deglaciation and EH, while the strengthened N-2 fixation (via the Kuroshio Intrusion into the SCS) forced by the El Ni & ntilde;o is proposed as the main cause for the significant decrease in delta N-15(org) since the mid-Holocene.