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Biogeochemistry of silicon Eric Struyf, Jack Middelburg, Wim Clymans One of 118 elements… … on THE table Googlability A bit of wiki • Silicon is the eighth most common element in the universe by mass • Silicon the second most abundant element in the Earth's crust (about 28% by mass) after oxygen • Silicon has a large impact on the world economy. Highly purified silicon is used in semiconductor electronics: a great deal of modern technology depends on it. Basic Chemistry of Si • Numerous Si-bearing minerals (mineralogy, petrology: disciplines within geology). – SiO2: Quartz, glass – Silicates: • • • • • olivines: (Mg, Fe)2SiO4 pyroxenes: Ca(Mg,Fe)Si2O6 feldspars: (Na,K)AlSi3O8 to CaAl2Si2O8 mica’s: KAl2(AlSi3O10)(OH)2 clay minerals: e.g. Al2Si2O5(OH)4 Weathering of silicates is ultimate source of all dissolved Si in water: mineral Si to dissolved Si. Transport and cycling in riverine continuum Butcher et al. 1992 Who are these minerals? Silicates Quartz Sand Rock Tight rasters Quartz The weathering is slow CaMg(CO3)2 CaCO3 CaSO4.2H2O CaSO4 NaCl Butcher et al. 1992 Sink for atmospheric CO2 CO2 + H2O CaSiO3 + 2H2CO3 H2CO3 Ca2+ + 2HCO3 + dissolved Si + H2O Diatoms • Diatoms dominate coastal and oceanic biogenic Si production • > 10.000 species • Pelagic and benthic forms Thalassiosira Diatoms take up dissolved Si (DSi), deposit it as amorphous (biogenic) Si (ASi) in frustule Calacademy.org Astrographics.com Diatoms • Centric forms: – radial or concentric – most pelagic are centric • Pennate forms: – bilaterally symmetrical – more heavily silicified – most benthic are pennates The ocean Si cycle Friedel, 1991 Diatoms control oceanic Si concentrations Si, diatoms and the C cycle • Diatoms sink fast: – they are large – they aggregate • An efficient transfer of labile C from photic zone to benthos and ocean interior • Diatom-frustules buried on ocean floor: 1,5 – 3.0 Gton C y-1 • +/- 25 % of yearly anthropogenic CO2 output The Oceanic Si Cycle – Biological Si Pump (based on Tréguer & De La Rocha, 2012) Rivers: DSi: 6.2 Tmol y-1 ASi: 1.1 Tmol y-1 Eolian input 0.5 Tmol y-1 Reverse weathering: 1.5 Tmol y-1 Groundwater 0.4 Tmol y-1 Sponges 3.1 Tmol y-1 Weathering 1.9 Tmol y-1 Seafloor input 0.6 Tmol y-1 DSi: Dissolved Silica / ASi: Amorphous Silica Productivity 240 Tmol y-1 Silica Burial 6.3 Tmol y-1 Estuarine ecosystems Human interference (global change, habitat loss, pollution) - Expansion of agricultural activities - Reservoir construction - Urbanization - Industrialization - ... Si and eutrophication Expansion of agricultural activities - Increased input of N and P - Ratio of N/Si/P disturbed - Ratio determines composition of phytoplankton - Ideal molar ratio 16/16/1 Changes in composition of coastal phytoplankton Si-limitation: shift to non-diatom species Risk of collapse of foodwebs (supported by diatoms) Eutrophication Phaeocystis sp. blooms: “foam algae” Gonyaulax sp. blooms Toxic “red tides” Cloern, 2001 Not only increase in N and P Not only increase in N and P Humborg et al., Nature, 1997 Dams decrease Si transport The lake effect is observed worldwide! The “dam-effect” is one of the best known human impacts on the Si cycle Conley et al., L&O, 2000 Recapitulation After Struyf et al. 2010 OCEAN SURFACE DSi 240 Tmole yr-1 diatom ASi Dissolved silica essential for diatom growth in the ocean Diatoms constitute 50+ % of ocean primary productivity The C-pump After Struyf et al. 2010 OCEAN SURFACE DSi 240 Tmole yr-1 diatom ASi ASi burial 6.5 Tmole yr-1 OCEAN FLOOR diatom ASi in seafloor sediment Diatom shells buried on ocean floor: 1,5 – 3.0 Gton C y-1 Ocean C-pump ~ Si-pump: +/- 25 % of yearly human CO2 output Ocean-continent link eolian transport After Struyf et al. 2010 0.5 Tmole yr-1 OCEAN SURFACE Net riverine transport DSi 240 Tmole yr-1 diatom ASi 6 Tmole yr-1 (*) Weathering EARTH CRUST & SUBSOIL primary and secondary silicate minerals ASi burial 6.5 Tmole yr-1 OCEAN FLOOR diatom ASi in seafloor sediment Terrestrial export of Si essential to sustain diatoms Traditional view: export controlled by bedrock weathering Tectonics eolian transport After Struyf et al. 2010 0.5 Tmole yr-1 OCEAN SURFACE Net riverine transport DSi 240 Tmole yr-1 diatom ASi 6 Tmole yr-1 (*) Weathering EARTH CRUST & SUBSOIL primary and secondary silicate minerals Hydrothermal input & Seafloor weathering ASi burial 6.5 Tmole yr-1 OCEAN FLOOR diatom ASi in seafloor sediment plate tectonics Tectonical processes close the cycle A new paradigm ECOSYSTEM SOIL eolian transport After Struyf et al. 2010 0.5 Tmole yr-1 OCEAN SURFACE 60-200 Tmole yr-1 Net riverine transport DSi 240 Tmole yr-1 diatom ASi 6 Tmole yr-1 (*) Weathering EARTH CRUST & SUBSOIL primary and secondary silicate minerals Hydrothermal input & Seafloor weathering ASi burial 6.5 Tmole yr-1 OCEAN FLOOR diatom ASi in seafloor sediment plate tectonics Recently discovered: bio-buffer between Si weathering and export Regulates Si transport between land and ocean Silica in terrestrial ecosystems First... Good for your bones, nervous system, hair and nails! May 2011, Lecture Dresden Humans and animals May 2011, Lecture Dresden Even we are filters... Good for students and scientists? Anderson, I.W., Molzahn, S.W., Roberts, N.B., Bellia, J. and Birchall, J.D., Proc. Eur. Brew. Conv., Brussels, 1995, 543-551 Silica gives stronger bones… And is good for the brain… May 2011, Lecture Dresden Schoelynck et al. 2013 12 mg Si L-1 58 mg Si L-1 May 2011, Lecture Dresden +Si -Si Silicon and siliceous structures in biological systems (1981). Simpson, T.L. and B.E. Volcani (eds.), Springer-Verlag May 2011, Lecture Dresden Plants and Si BIOgeochemistry Vegetation stores Si “phytoliths” “silica sheets cells” - Enhanced strength - Resistence to: Herbivores Plant disease -Reduced water, salt, pollution, stress - Enhanced productivity A beneficial element! For some it’s essential or crucial Grasses and sedges Horsetails Si is beneficial – Crop Yield Rice, Korndörfer & Leipsch, 2001 Strawberries, Crooks & Prentice, 2012 Biological stress Physical stress Chemical stress Guntzer et al, 2011 Resistance disease Silica Rice disease brown spot Commercial cure Commercial cure + silica Untreated Silica Datnoff et al. 1997 Vegetation-Soil continuum Return of plant litter, straw residue and dying roots Forest Arable Vegetation-Soil continuum Phytoliths Diatoms Sponges Clarke, 2003 The Si in biota is AMORPHOUS Not ordered in a tight crystal raster, like minerals It dissolves more than 1000 times faster Amorphous matrix of hydrated silica (SiO2•nH2O). A bit like: The “bio” in Si biogeochemistry - Yearly production of plant ASi, 60 – 200 Tmole comparable to ocean ASi production (Conley 2002) a multitude is in soil organic matter - High solubility range, REACTIVE on biological timescales NEW CONCEPT: Ecosystems control Si-concentrations in rivers Large stock in ecosystems Si is accumulated in ecosystem soils Cornelis et al. 2010 Plants stimulate the weathering Hinsinger et al. 2001 Plants stimulate the weathering… Hinsinger et al. 2001 The ecosystem Si filter Land use Size of bio-Si stock Hydrology Mineral weathering Dissolved Si export Ocean SOIL Estuary Hydrology bio-Si export River VEGETATION Silicate minerals Bio-Si reactivity February 2011, seminar Nottingham Forests Deforestation Studies at Hubbard Brook Experimental Forest Large-scale experiments! American beech (Fagus grandifolia) Sugar maple (Acer saccharum) Yellow birch (Betula alleghaniensis). 3-yr running mean of average volume weighted “Excess” dissolved silicate Large increase in export following whole tree cut and removal (1983-84) Deforestation • Release from the biologically derived BSi pool • Highest Si fluxes when plant materials left on the soil surface after deforestation • Deforestation appears to enhance land-ocean flux of biogenically reactive Si But... Scheldt watershed 52 river basins Year-round (2008) + 500 observations Goldschmidt, Knoxville, June 2010 40 -1 -1 TSi flux (µmol ha s ) Si and long-term deforestation 30 20 10 0 0 10 20 30 40 50 Forest coverage (%) % increase in TSi flux 350 300 Grass Human Crop 250 200 150 100 50 0 -50 0 10 20 30 40 % transformed into forest 50 60 Cultivation lowered base-flow Si fluxes Mixed multiple regression (soil texture, lithology, drainage capacity, land use) 100.0 -1 TSi flux (µmol.ha .s ) 350 % increase in TSi flux -1 300 250 200 150 100 ‘forests vs. cropland’ (p < 0,001) Forest vs. human Forest vs. crop Forest vs. grassland 10.0 ‘forest vs. grassland’ (p< 0.003) ‘forest vs. agriculture (grassland + cropland)’ (p< 0,005) 50 0 0 -50 1.0 10 0 20 10 30 40 20 50 30 60 40 50 Forestation % anthropogenic ecosystem transformed into forest(%) Goldschmidt, Knoxville, June 2010 New conceptual model Developing forest Climax forest ASi Early deforested ASi Climax cultivated ASi ASi vegetation Si - fluxes DSi soil mineral silicates 0 DSi export mineral silicates DSi export mineral silicates DSi export mineral silicates export soil ASi-pool Soil ASi pool 100 0 TSi export TSi-export 100 Goldschmidt, Knoxville, June 2010 New conceptual model Developing forest Climax forest ASi Early deforested ASi Climax cultivated ASi ASi vegetation Si - fluxes DSi soil mineral silicates 0 DSi export mineral silicates DSi export mineral silicates DSi export mineral silicates export soil ASi-pool Soil ASi pool 100 0 TSi export TSi-export 100 Goldschmidt, Knoxville, June 2010 New conceptual model Developing forest Climax forest ASi Early deforested ASi Climax cultivated ASi ASi vegetation Si - fluxes DSi soil mineral silicates 0 DSi export mineral silicates DSi export mineral silicates DSi export mineral silicates export soil ASi-pool Soil ASi pool 100 0 TSi export TSi-export 100 Goldschmidt, Knoxville, June 2010 Study Area Arable Land Pasture Grazed Forest Continuous Forest Clymans et al, 2011 Human impact on Si pools ca. 500 yrs human disturbance => 87±51 kg SiO2 ha-1 Fig. Representation of the land use sequence in the study area, southern Sweden. Values indicate measured means (±standard errors) for total biogenic silica pool (PSia) and easily soluble silica pool (PSie) in the soils. Clymans et al, 2011