Easterly denitrification signal and nitrogen fixation feedback documented in the western Pacific sediments

Abstract

A sedimentary δ15N record in the equatorial western Pacific (WP) shows glacial‐interglacial variability from 6.2 to 11.2‰ during the last two climatic cycles, similar to the denitrification record in the eastern tropical Pacific (ETP). Contrastively, a record in the South China Sea (SCS) exhibits less changes from 4.4 to 6.4‰ and is quite alike previously published results in marginal seas in the WP. By ruling out several possible causes for theδ15N variability, the δ15N record in the equatorial WP is interpreted as the source nitrate δ15N signals advected from the ETP. Comparison of several δ15N records for the last 25 ka distributed in the WP brings out a pattern of northward decrease in δ15N values and variability from the equator to off Mindano and then to marginal seas, supposed to be caused by the northward increase of local N2fixation. Therefore, the less glacial‐interglacial changes in someδ15N records in the WP could imply that the glacial decrease in subsurface δ15N due to less denitrification in source waters from the ETP would have been isotopically compensated by a synchronous decrease in local N2 fixation.

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

“…The SCS is not the only region that presents connections between N cycle and upper ocean structure on precession cycle. This schema also applies to the δ 15 N records from the equatorial western Pacific (Figure f; Jia & Li, ) and the tropical North Atlantic (Figure g; Straub et al, ). The δ 15 N signals of the western Pacific originate from WCD in eastern tropical Pacific (Jia & Li, ; Kienast et al, ), which is very likely constrained by upwelling intensity (Kienast et al, ; Martinez & Robinson, ).…”

“…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.…”

“…(e) The Gaussian band‐pass‐filtered (filter = 0.0435, bandwidth = 0.008) records of MD05‐2901 δ 15 N Src (pink), MD97‐2142 FB‐δ 15 N (green), and MD05‐2891 δ 15 N org (light blue). (f) MD01‐2386 δ 15 N bulk record from the western Pacific (yellow; Jia & Li, ). (g) ODP999A FB‐δ 15 N record from the Caribbean Sea (dark blue; Straub et al, ).…”

“…Higher δ 15 N bulk during interglacials indicates enhanced WCD, which is modulated either by high‐latitude process or proximal subtropical ocean circulation (Dubois et al, ; Hendy & Pedersen, ; Kienast et al, ; Martinez & Robinson, ; Robinson et al, ). Sedimentary δ 15 N bulk records from the equatorial western Pacific are similar to those from the eastern tropical Pacific, which suggests the advection of δ 15 N signals along the equatorial Pacific (Jia & Li, ; Rafter et al, ). However, the application of δ 15 N bulk proxy has encountered problems in marginal seas and some oligotrophic areas.…”

“…Terrestrial input in the northern and southern SCS reflects sea level-related variations over the last million years, with coarser grains and higher accumulation rates during glacials (Boulay et al, 2007;Wang et al, 1999). However, the mass accumulation rate of terrestrial material in core MD05-2901 shows lower values in glacial times (Figure 4a), different from that of the northern and southern SCS (Jian et al, 2001;Wang et al, 1999). This implies that changes in terrestrial input are variable in different parts of the SCS basin.…”