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IntroductionThe Coastal and Continental Margin Zones (CCMZs) contribute to 40% of the total burial flux of biogenic silica (bSi) of the world ocean. However, the accurate determination of the bSi content (bSiO2%) in marine sediments remains a challenge. The alkaline methods commonly used to quantitatively determine bSiO2% can completely digest the amorphous silica of diatoms but are less effective at digesting radiolarians and sponge spicules. In addition, the lithogenic silica (lSi) found in sediments is partly digested during these alkaline extractions, which can bias the accuracy of the determined bSiO2%. This is of importance in CCMZs where the lSi:bSi ratio is high.MethodsIn this study, we examined sediments collected in the CCMZs of East China seas, an environment of peculiar interest given the large amount of lSi deposited by the Yellow River and the Yangtze River.Results and discussionThe results show that alkaline digestions using stronger solutions and pretreatment steps resulted in an overestimate of the bSiO2% due to increased leaching of silica mainly from authigenic silicates and clays, whereas weak digestions underestimated the bSiO2% owing to incomplete digestion of sponge spicules. We found that the use of the Si/Al method accurately corrects for the lSi fraction in marine sediments, and thereby improves the determinations of bSiO2% in the sediments of East China seas CCMZs. Ensuring full digestion of all bSi remainschallenging, in particular for sponge spicules, motivating both verifications via microscopy and longer extraction times. To emphasize the influence of these methodological differences, we revised the bSi burial flux in the East China seasand provide a new estimate of 253 (± 286) Gmol-SiO2 yr-1, which is one third of theprevious estimates. We discuss the potential contribution from radiolarian andsponges and we propose a new general protocol for the determination of bSi insediments that minimizes the methodological bias in bSi determination.

The dissolution efficiency of sedssimentary biogenic silica (bSiO2) 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 bSiO2 in the northern South China Sea (NSCS), one of the largest marginal sea continental shelves. The bSiO2 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 bSiO2 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 (bSiO2, OC, and TN) revealed a different preservation pattern of bSiO2 in finer (Φ > ~ 5.5) and coarser (Φ< ~ 5.5) sediments. The high concentration of Al in sea water and “Al – detrital – bSiO2” interactions in sediments significantly interfered with the apparent solubility and dissolution dynamics of bSiO2. We combined the regional characteristics (primary production, bottom current, and resuspension-deposition) and the reconstructed dissolution kinetics of bSiO2 explained the mismatch between the surface (diatom biomass)/(total phytoplankton biomass) ratio and the sedimentary bSiO2/OC ratio, and the mismatch between the surface bSiO2 primary productivity and the bSiO2 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 bSiO2/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 bSiO2 in the upper slope. Furthermore, through the comparison with other sea areas, the relatively lower reactivity (1.12 ± 0.3 yr-1) of bSiO2 in sediments supported the notion that the NSCS sediments may serve as an important silica sink in the world ocean silica cycle.