Temporal variations in nitrogen isotope offsets between bulk sediment and planktonic foraminifera in the South China Sea linked to redox changes over the past 500 kyr

Abstract

In the oligotrophic South China Sea (SCS), both nitrogen isotopes in bulk sediment (δ¹⁵N_TN) and co-deposited planktonic foraminifera tests (δ¹⁵N_FB) document the nitrate dynamics in the upper ocean, yet exhibit inconsistent temporal offsets (Δδ¹⁵N_TN-FB). This study investigates the potential drivers of these offsets, including terrigenous input, diagenetic effects on δ¹⁵N_TN, and interspecies isotope differences in δ¹⁵N_FB. By analyzing various nitrogen speciations in sediment core MD972142 from Southeastern SCS, we identified periodic variations in Δδ¹⁵N_TN-FB, ranging from −3 to +2‰ over the past 500 kyr, with negative offsets during glacial periods and positive during interglacial periods. Our findings suggest minimal influence from clay-fixed inorganic nitrogen, terrestrial organic nitrogen inputs, and interspecies differences among foraminifera. Instead, significant co-variation of Δδ¹⁵N_TN-FB with redox-sensitive manganese contents points to redox-driven degradation as the primary factor shaping these Δδ¹⁵N_TN-FB variations. Since sedimentary δ¹⁵N_FB is shielded from diagenetic alteration, it serves as a baseline indicator of initial δ¹⁵N of sinking particulate nitrogen (PN) at export, thus Δδ¹⁵N_TN-FB recording the relative changes in δ¹⁵N_TN caused by degradation from export to burial. To understand how this degradation influences Δδ¹⁵N_TN-FB, we synthesized modern observations of sinking PN in the SCS, revealing that δ¹⁵N of sinking PN consistently declines from the euphotic zone to constant, low values in deep water, illustrating negative isotopic offsets relative to initial export. As sinking PN undergoes further decomposition at the sediment-water interface—described by Rayleigh fractionation—it becomes isotopically enriched, potentially leading to positive offsets relative to initial export. Considering these patterns, we propose that interglacial sea-level rises improved water circulation coupled with low productivity, creating more oxic conditions that prolonged oxygen exposure and enhanced δ¹⁵N_TN enrichment, resulting in positive offsets of Δδ¹⁵N_TN-FB. Conversely, negative offsets occurred during glacial periods when lower sea levels reduced circulation and high productivity led to low-oxygen conditions and shorter oxygen exposure times.