Nitrogen Isotope Variations in the Northern South China Sea Since Marine Isotopic Stage 3: Reconstructed From Foraminifera‐Bound and Bulk Sedimentary Nitrogen

Abstract

We report the differences between δ15N of organic nitrogen bound in foraminifera tests (foraminifera‐bound δ15N, or FB‐δ15N), which is considered an effective recorder of thermocline nitrate δ15N in the oligotrophic areas, and bulk sedimentary δ15N (δ15Nbulk) of core MD12‐3433, located in the northern South China Sea (SCS) to explore possible causes of the offset between these two proxies in the SCS. The offset can be best explained by the influence of the terrigenous input on δ15Nbulk, as evidenced by its general correlation with sea level changes, C:N ratios, and inorganic nitrogen contribution to total nitrogen. Moreover, our new record is compared to six previously published δ15Nbulk records of the SCS based on their geographical proximity to the mainland or large rivers, which all show being influenced by terrigenous input. The effect of changes in vertical mixing is evaluated and believed to play a minor role on the general pattern of FB‐δ15N, while N2 fixation is the most important impact on FB‐δ15N records in the SCS as previously studies revealed. A deeper mixed layer during the last glacial period could result in nitrate with higher δ15N from the subsurface being mixed into the euphotic zone. FB‐δ15N of Orbulina universa have higher values than Globigerinoides ruber and Globigerinoides sacculifer, with a maximum difference in the last glacial maximum, which could be accounted for by its deeper habitat where it is sensitive to changes in the contributions of vertically mixed subsurface nitrate relative to nitrate from N2 fixation.

Nitrogen Fixation Changes Regulated by Upper Water Structure in the South China Sea During the Last Two Glacial Cycles

“…The MD05‐2901 δ 15 N Src record shows significant similarities with the FB‐δ 15 N records of MD97‐2142 from Luzon coast (Ren et al, ) and MD12‐3433 from northern SCS (Wang et al, ; Figure a). The δ 15 N records from these three sites all present glacial‐interglacial variations of ~3‰, with distinct lower values in interglacial periods.…”

“…Thus, the subsurface nitrate in the SCS is unlikely fueled by nitrate from the western Pacific and probably mainly originates from the deeper SCS via vertical mixing during both glacials and interglacials. Second, western Pacific bulk δ 15 N varied in a similar pattern as the eastern tropical Pacific denitrification records with higher values during interglacials (Jia & Li, 2011), but the SCS nitrate δ 15 N proxies show an opposite trend with lower values during interglacials (Figures 3a-3c; Dong et al, 2019;Ren et al, 2017;Wang et al, 2018). As a result, changes in the subsurface δ 15 N in SCS is supposed to be dominated by regional N cycle within the SCS, namely, the changing rates of marine N fixation.…”

“…Proxy records of the South China Sea (SCS) δ 15 N and upper water thermal structure. (a) FB‐δ 15 N from MD12‐3432 (light green) and MD97‐2142 (dark green; Ren et al, ; Wang et al, ). (b) Organic δ 15 N (δ 15 N org ) from MD05‐2897 (blue) and MD01‐2392 (dark purple; Dong et al, ).…”

“…δ 15 N bulk from the adjacent SCS are relatively low with no systematic correlation to climatic cycles (Kienast, ). In contrast, recently published foraminifera‐bound δ 15 N (FB‐δ 15 N) records in the SCS show distinct lower FB‐δ 15 N during interglacials (Ren et al, ; Ren et al, ; Wang et al, ). The disagreement between δ 15 N bulk and FB‐δ 15 N is very likely due to influences of terrestrial inputs and diagenetic effects.…”

N and C Isotope Variations Along an Extreme Eutrophication and Salinity Gradient in the Coorong Lagoon, South Australia

“…The isotopic compositions of these elements can provide source and process-related information for both modern and past depositional environments (Casciotti 2016;DeNiro and Epstein 1978;Mettam and Zerkle 2021;Miyake and Wada 1967;Ohkouchi, et al, 2015;Sigman and Casciotti 2001). Nitrogen stable isotope (δ 15 N) composition in sedimentary archives are used to infer changes in the biogeochemical nitrogen cycle, water column redox, anoxia/euxinia, and nutrient conditions in past marine and coastal environments (e.g., Sigman and Casciotti, 2001;Wang et al, 2018;Cox et al, 2019;Davis et al, 2019;Isaji et al, 2019;Obrist-Farner et al, 2019;Tuite et al, 2019;Zhu et al, 2020). The stable isotope composition of carbon (δ 13 C) in marine sedimentary archives, including both carbonates and organic sediments, is also interpreted to reflect aquatic productivity and carbon cycling from local to global scales, as well as to assess the relative contributions of terrestrial and marine organic matter, and marine vs. freshwater mixing in past aquatic environments (e.g., Popp et al, 1997;Meyers and Lallier-Verges 1999;Wilson et al, 2005;Lamb et al, 2006).…”