The nitrogen isotope archived in sediment core serves as a proxy for productivity. However, isotopic compositions of planktonic foraminifera tests (δ¹⁵N_FB) and bulk nitrogen (δ¹⁵N_TN) (both are sourced from the bottom of euphotic zone) in South China Sea (SCS) show inconsistent temporal patterns (variable offsets (Dδ¹⁵N_TN-FB) through time) with debatable causes. Here we applied a novel method to measure organic nitrogen content (ON) and its isotope composition (δ¹⁵N_ON) directly for the sediment core MD972142 in Southeastern SCS. Results show the contribution of clay-fixed inorganic nitrogen to δ¹⁵N_TN was ignorable and the offsets between δ¹⁵N_TN and δ¹⁵N_FB (Dδ¹⁵N_TN-FB) oscillated from -3 to 2‰ over the past 500ky, with negative values in glacial and positive in interglacial period. We synthesized modern observations of nitrogen for sinking particles (PN%, δ¹⁵N_sinking) over the SCS basin to simulate the isotope alteration, of which selective decomposition (depth <1500 m, PN% > 0.5) and Rayleigh fractionation (depth >1500 m, PN% < 0.5) domain were classified, for particles during sinking in water column from export to burial. We hypothesized that Rayleigh fractionation during the decomposition of export production below 1500 m deep had governed the variations of sedimentary δ¹⁵N_TN and thus Dδ¹⁵N_TN-FB. Accordingly, more negative values of Dδ¹⁵N_TN-FB represent better preservation of sinking organics, which was supported by the positive correlation between Dδ¹⁵N_TN-FB and Mn%, a redox sensitive element, showing better preservation in reducing glacial. We concluded that nitrogen isotope offsets between bulk sedimentary nitrogen and planktonic foraminifera tests can be an excellent proxy in oligotrophic ocean for the preservation efficiency of export production.

export production; organic nitrogen isotope