922 / 2024-09-19 20:20:12
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
nitrogen cycle,paleoceanography,south china sea
Session 8 - Modern and past processes of ocean-atmosphere-climate interactions in the low-latitude Pacific and Indian Ocean
Abstract Accepted
In the oligtrophic South China Sea (SCS), both nitrogen isotopes in bulk sediment (δ15NTN) and co-deposited planktonic foraminifera tests (δ15NFB) document the nitrate dynamics in the upper ocean, yet exhibit inconsistent temporal offsets (Δδ15NTN-FB). This study investigates the potential drivers of these offsets, including terrigenous input, diagenetic effects on δ15NTN, and interspecies isotope differences in δ15NFB. By analyzing various nitrogen speciations in sediment core MD972142 from Southeastern SCS, we identified periodic variations in Δδ15NTN-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 Δδ15NTN-FB with redox-sensitive manganese contents points to redox-driven degradation as the primary factor shaping these Δδ15NTN-FB variations. Since sedimentary δ15NFB is is shielded from diagenetic alteration, it serves as a baseline indicator of initial δ15N of sinking particulate nitrogen (PN) at export, thus Δδ15NTN-FB recording the relative changes in δ15NTN caused by degradation from export to burial.To understand how this degradation influences Δδ15NTN-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. Base on these variation, we propose that interglacial sea-level rises improved water circulation coupled with low productivity, creating more oxic conditions that prolonged oxygen exposure and enhanced δ15NTN enrichment, resulting in positive offsets of Δδ15NTN-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. Overall, this study emphasizes the utility of Δδ15NTN-FB as a valuable proxy for understanding the preservation of export production in oligotrophic ocean environments.