Silicon balance in the South China Sea
2022
Ma, Yuwei | Zhang, Lanlan | Liu, Sumei | Zhu, Dongdong
The South China Sea (SCS) is one of the largest marginal seas in the world, but the processes that control the silicon cycle are not well understood. Here, we analyse the factors that affect the distribution of biogenic silica (bSi) in SCS sediments and discuss the silicon cycle based on a box model. Our results show that the bSi content (~ 1.2%) in the shelf-slope (< 2000 m) sediments is lower overall than that (~ 2%) in the basin (> 2000 m) sediments. The dilution by terrestrial input, degradation rate of biogenic materials and lateral transport by current patterns are the main factors influencing the bSi content in SCS sediments. The bSi burial flux in the shelf-slope region (161 ± 156 Gmol-Si yr⁻¹) is six times greater than that in the basin region (27 ± 18 Gmol-Si yr⁻¹) due to higher burial rates, larger sea area, and coarser sediments. The estimated bSi burial efficiency (ratio of the bSi burial rate divided by the sum of the benthic silicic acid (dSi) diffusion rate and the burial rate) in SCS sediments is 27%, comparable to those of other marginal seas. Therefore, at the sediment–water interface, 73% of Si is recycled back to the water column. From compiled datasets of water fluxes and dSi concentrations over the past decades, we calculate a total output silicon flux of dSi of 2259 ± 397 Gmol-Si yr⁻¹ for the SCS, which is not significantly different from the total input flux (2196 ± 458 Gmol-Si yr⁻¹), suggesting that the Si cycle of this marginal sea is in a steady state. Interestingly, the submarine groundwater discharge (SGD) flux (477 ± 140 Gmol-Si yr⁻¹) in the SCS is 2.5 times higher than the riverine dSi and dissolvable amorphous silica input (194 ± 40 Gmol-Si yr⁻¹).
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