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

Abstract

The nitrogen (N) cycle in marginal seas, as an important component of global marine biogeochemistry and the Earth's climate system, is dynamic and complicated (Gruber & Galloway, 2008;Voss et al., 2013). Marginal seas receive considerable amounts of human-derived N via atmospheric transport and river input (Jickells et al., 2017); on the other hand, waters in the marginal sea communicate with the open ocean, which substantially affects the nutrient distribution, budget and cycling in its interior (Chen, 2010;Dai et al., 2013;Xu et al., 2018). The impacts of anthropogenic activities and communication with the open ocean have been imprinted into nutrient

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

“…With regard to the NPIW, its δ 15 N is primarily affected by changes in the global mean ocean nitrate δ 15 N, which undergoes a deglacial maximum but does not change substantially between the last ice age and the interglacial (Deutsch et al., 2004; Ren, Sigman, Chen, & Kao, 2012). Note that, along the path of the high‐latitude North Pacific, where NPIW formed, to the WNP and eventually the SCS, the originally isotopic signal of NPIW was firstly overprinted by the production/regeneration cycling of sinking OM together with ocean circulation that transmits the isotopically heavy nitrate from the WCD zones in the ETNP, then the δ 15 N nitrate became lighter due to the strong vertical mixing with overlying and underlying waters (with isotopically light δ 15 N nitrate from the regeneration of regional NF) in the interior (Kao et al., 2008; Liu et al., 1996; Yang et al., 2022). Modeling and the neodymium (Nd) isotopes of fish debris (ε Nd‐FD ) records illustrate that NPIW formation was enhanced in the subarctic and northwestern Pacific during cold intervals (i.e., HS1 and YD) of the last deglaction period (Figure 5c), in response to the slowdown of the Atlantic Meridional Overturning Circulation via teleconnection between the Atlantic and Pacific Oceans (Gong et al., 2019; Okazaki et al., 2010).…”

Upper Ocean Biogeochemistry of the Oligotrophic North Pacific Subtropical Gyre: From Nutrient Sources to Carbon Export

“…In parallel, nitrate isotopic analysis reveals a ubiquitous δ 15 N minimum at the base of the EZ in subtropical gyres, which is interpreted as regenerated nitrate sourced from N 2 fixation (Knapp et al., 2005; J.‐Y. T. Yang et al., 2022). These results indicate that a large fraction of DDN in the NDL must be released and recycled in both the NDL and NRL before its export.…”

High-resolution record of temporal change in organic matter burial over the past ∼8,600 years on the northwestern continental slope of the South China Sea

“…Bulk sediment δ 15 N records from modern water column denitrification zones like the eastern tropical Pacific show strong links between hydrography and denitrification rates (Dubois et al, 2011). Hydrographic variability in the NSCS is influenced predominantly by the western Pacific Ocean (Du et al, 2021;Yang et al, 2022). In particular, the nature of the surface and subsurface waters of the NSCS depends on the intensity of intrusion of the Kuroshio Current through the Luzon Strait (Liu et al, 2016).…”

“…The lateral intrusion of the Kuroshio Current affects heat and salinity in the SCS, and also the biogeochemical cycling of N and C owing to the low salinity and high content of dissolved OC in Kuroshio water . Although few studies have examined the evolution of the Kuroshio intrusion in the NSCS, especially regarding the intensity and location of the intrusion (Du et al, 2021;Yang et al, 2022), It has been shown that the geochemical record of northern continental slope of the SCS is strongly influenced by the intensity of the Kuroshio (Li et al, 2021;Wang et al, 2023a). Records from cores B-3gc and B-255 from the western Pacific Ocean show a pronounced increase in the abundance of the planktonic foraminifer P. obliquiloculata, a marker species of the Kuroshio Current, at 7.3 cal kyr BP, followed by an abrupt decrease at ~4.6 cal kyr BP-Termed as the Pulleniatina Minimum Event (Figures 6D, E; Jian et al, 2000).…”

“…Recent research conducted in the NSCS has revealed similar δ 15 N values for nitrate in the EZ and subsurface layer, demonstrating that subsurface nitrate serves as the primary source of new N to support export production (Yang et al, 2017). The nitrogen isotopic signature of upwelled nitrate in the subsurface may become imprinted on settling particulate organic matter that is ultimately buried in sediment (Yang et al, 2022). This altered nature of subsurface water may explain the variation in δ 15 N values of core G02 sediments.…”