Silicon (Si) plays a central role in regulating marine primary productivity and mediating interactions between carbon (C) and nitrogen (N) cycling through its control on diatom growth. The stable Si isotopic composition (δ³⁰Si) of biogenic silica (BSi) preserved in sediments has therefore been widely used to reconstruct past nutrient utilization and productivity. However, its applicability in low-productivity ocean regions, where diatoms contribute only modestly to total primary production, remains poorly constrained. Here, we present the first high-resolution, seasonally resolved datasets of coupled Si, C, and N isotope systems from the upper 200 m at two stations in the oligotrophic South China Sea (SCS) basin.
Surface waters exhibit relatively heavy δ³⁰Si signatures of dissolved silicic acid (DSi; δ³⁰Si_DSi; +2.6‰ to +3.2‰) and of BSi (δ³⁰Si_BSi; +1.9‰ to +2.1‰), reflecting near-complete DSi utilization by diatoms. Both Rayleigh-derived fractionation factors (³⁰ℇ_DSi) based on δ³⁰Si_DSi data and apparent fractionation factors (∆³⁰Si, δ³⁰Si_{BSi_obs.} - δ³⁰Si_{DSi_obs.}) show a strong seasonal variability. ³⁰ℇ_DSi in the euphotic zone ranging from -1.1‰ to -1.3‰ in summer and -0.6‰ to -0.8‰ in winter, and ∆³⁰Si in the surface mixed layer ranging from -1.0‰ to -1.1‰ in summer and -0.5‰ to -0.8‰ in winter, with no clear spatial variability. Seasonal shifts in diatom productivity influence the distribution and export of biogenic particles. While the overall contribution to bulk primary production remains limited in the picophytoplankton-dominated SCS basin, diatoms can enhance BSi export and particulate organic carbon and nitrogen (POC and PON) production in winter. Coupled analyses of δ³⁰Si_BSi and C and N isotopic compositions of POC (δ¹³C_POC) and PON (δ¹⁵N_PON) in the euphotic zone reveal a significant positive correlation between δ³⁰Si_BSi and δ¹³C_POC during the winter mixing period, indicating a strong linkage between the Si and C cycles under favorable environmental conditions. In contrast, no correlations are observed between δ³⁰Si_BSi and δ¹⁵N_PON in either season, suggesting that Si and N cycles are largely decoupled due to complex N cycling processes. These findings indicate that δ³⁰Si_BSi holds potential as a proxy for tracing primary production in oligotrophic settings and highlight the value of integrating multiple isotope systems to disentangle cycling of different nutrients and improve palaeoceanographic reconstructions in low-productivity ocean regions.

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