The effects of anoxia on Si dynamics in sediments

Abstract

Purpose This study explored the effects of anoxia on adsorbed Si dynamics in surface sediments. While the effects of anoxia on the adsorption and release of phosphorus (P) have been studied frequently, its effects on adsorbed silicon (Si) are commonly ignored, despite the similarities between the elements. The study addresses the theory that as both Si and P produce anionic ligands, processes that affect adsorbed P and its release could also affect Si and its release. Materials and methods Two main procedures were conducted: A sequential extraction procedure composed of (1) water, (2) 0.46 M sodium chloride, (3) 0.11 M sodium dithionite, (4) 0.1 M sodium hydroxide and (5) 0.5 M hydrochloric acid extractions, and a long-term release dynamics experiment (RDE) lasting 28 days. Both were conducted using seven sediment samples of different composition. Results and discussion Silicon behaved differently than P under anoxic conditions. Although anoxia generally causes increased release of Fe-bound P due to Fe 3+ reduction, the release of Si decreased under anoxic conditions in the loosely sorbed and redox-sensitive fractions of the sediments. The long-term experiment (RDE) showed that Si release was time-dependent and slower under anoxic than oxic conditions. Resorption to Al oxides and the formation of colloidal mixed Fe-Si oxides are possible reasons for these results. There were, however, also some indications that anoxic conditions enhanced biogenic Si (BSi) and mineral Si dissolution. Conclusions Anoxia clearly affects Si sorption processes, but it may have both negative (e.g. increased resorption preventing P adsorption) and positive (e.g. enhanced BSi dissolution, increasing dissolved Si concentration in water) effects on the release of Si. More investigations dealing with the subject are needed.

Nutrient fluxes from reduced Baltic Sea sediment: effects of oxygenation and macrobenthos

“…The present experimental study corroborates the hypothesis that oxic bottom water conditions reduce benthic release of DSi in the Baltic Sea (Danielsson 2014), while previous reports focusing on redoxdependent benthic DSi cycling are few and contradictive (e.g. Kamp-Nielsen 1974, Belias et al 2007, Ekeroth et al 2016, Siipola et al 2016). The mechanistic explanation for the lower DSi flux after oxygenation may be the same as for the lower DIP flux, namely scavenging by iron oxides sorbents in the surficial sediment (e.g.…”

Biogeochemical and climate drivers of wetland phosphorus and nitrogen release: Implications for nutrient legacies and eutrophication risk

“…Release of Si can also be driven by high availability of labile C sources. Prolonged periods of anoxia, where microbial activity is uninhibited by C source availability, can result in accelerated dissolution of mineral and biogenic Si, including siliceous diatom frustules and the silica‐rich emergent wetland plant Phragmites australis , which is abundant along the pond margins (Siipola, Lehtimaki, & Tallberg, 2016; Struyf & Conley, 2009; Tallberg, Heiskanen, Niemisto, Hall, & Lehtoranta, 2017; Villnas, Norkko, Lukkari, Hewitt, & Norkko, 2012). Silicate anions released during prolonged periods of anoxia can compete with PO 4 3− for sorption sites (e.g., on Al oxides), thus increasing SRP availability (Hartikainen, Pitkanen, Kairesalo, & Tuominen, 1996).…”

“…Silicate anions released during prolonged periods of anoxia can compete with PO 4 3− for sorption sites (e.g., on Al oxides), thus increasing SRP availability (Hartikainen, Pitkanen, Kairesalo, & Tuominen, 1996). However, in this case, the high concentrations of Si at the time of termination of the nutrient release event may have helped to accelerate subsequent SRP removal by inhibiting formation of crystalline Fe oxides and promoting the production of amorphous Fe oxides, which have a higher surface area and high P sorption capacity (Siipola et al., 2016). Indeed, the formation of Fe‐Si colloids and the aggregation and sedimentation of Fe‐Si particulates provides a plausible mechanism to explain the observed removal of Si during the nutrient release termination phase.…”

Si cycling in transition zones: a study of Si isotopes and biogenic silica accumulation in the Chesapeake Bay through the Holocene

“…Since both sediment cores come from the deep channel, an overestimation of BSi accumulation rate is possible. Further, anoxic water conditions in the deep channel may increase BSi redissolution during summer (Villnäs et al 2012;Siipola et al 2016), however, at the same time less bioturbation would enhance the preservation of diatom frustules in the sediments. Consequently the spatial extrapolation to the whole bay area may cause an The calculation for the northern CB is further biased by the arbitrary definition of the surface area.…”