Distribution and dissolution kinetics of biogenic silica in sediments of the northern South China Sea
Yuwei Ma, Bin Yang, Nan Zhou, Jin Huang, Su Mei Liu, Dongdong Zhu, Wen Liang
Abstract
The dissolution efficiency of sedssimentary biogenic silica (bSiO 2 ) dramatically affects the regeneration of dissolved silicic acid (dSi) at the sediment-water interface, which is a crucial pathway to maintain Si balance and silicic productivity growth in marine environments. We conducted wet alkaline leach and flow-through experiments to explore the dissolution behaviors of sedimentary bSiO 2 in the northern South China Sea (NSCS), one of the largest marginal sea continental shelves. The bSiO 2 contents of surface sediments were 0.64 - 2.06%, with an average of 1.04 ± 0.35%, varying with isobath water depth. The solubility of bSiO 2 in surface sediments ranged from 227 μmol L -1 to 519 μmol L -1 , and the dissolution rate constants varied from 0.67 to 1.53 yr -1 under specific conditions in lab incubation. The correlation between the biogenic materials (bSiO 2 , OC, and TN) revealed a different preservation pattern of bSiO 2 in finer (Φ > ~ 5.5) and coarser (Φ< ~ 5.5) sediments. The high concentration of Al in sea water and “Al – detrital – bSiO 2 ” interactions in sediments significantly interfered with the apparent solubility and dissolution dynamics of bSiO 2 . We combined the regional characteristics (primary production, bottom current, and resuspension-deposition) and the reconstructed dissolution kinetics of bSiO 2 explained the mismatch between the surface (diatom biomass)/(total phytoplankton biomass) ratio and the sedimentary bSiO 2 /OC ratio, and the mismatch between the surface bSiO 2 primary productivity and the bSiO 2 sediment records in the NSCS. The resuspension-deposition, the higher reconstructed rate constants (0.94 ± 0.13 yr -1 ), and the dissolution rate (0.20 ± 0.01 yr -1 ) were responsible for the lower bSiO 2 /OC ratio (0.45 ± 0.28) at the inner shelf, and the winnowing process at the outer shelf with the lower reconstructed reactivity (0.30 yr -1 ) and dissolution rate (0.001 yr -1 ) led to the good preservation of bSiO 2 in the upper slope. Furthermore, through the comparison with other sea areas, the relatively lower reactivity (1.12 ± 0.3 yr -1 ) of bSiO 2 in sediments supported the notion that the NSCS sediments may serve as an important silica sink in the world ocean silica cycle.