Faithful transfer of radiolarian silicon isotope signatures from water column to sediments in the South China Sea

Abstract. Radiolarian silicon isotopes (δ³⁰Si_rad) hold significant potential as a proxy for constraining past silicon cycling in seawater. However, the extent to which δ³⁰Si_rad signatures in sediments accurately represent the isotopic signals of the overlying water column remains unclear, particularly under the influence of radiolarian shell dissolution during sinking and burial in the sediment record. This study presents the first comparative analysis of δ³⁰Si_rad compositions and the radiolarian assemblage community using water column and surface sediment samples collected from the South China Sea. The results indicate that δ³⁰Si_rad values range from 1.56–1.83 ‰ (mean = 1.74 ‰) in the water column, and from 1.61–1.85 ‰ (mean = 1.73 ‰) in surface sediments. No significant discrepancies in δ³⁰Si_rad values were observed between plankton and sediment samples at each sampling station as evidenced by the paired t-test (p = 0.75), implying that dissolution has a minimal impact on δ³⁰Si_rad during the transfer of radiolarian shells to the sediment record. This finding may be enhanced by the dominance of more dissolution-resistant Spumellaria and Nassellaria taxa (>99 % relative abundance) within the radiolarian community, coupled with the scarcity or absence of the readily dissolvable radiolarian taxa in the analysed samples. This study demonstrates the faithful preservation of the δ³⁰Si_rad signature and its potential for studying past changes in the marine silicon cycle.