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Biogeochemistry of silicon
Eric Struyf, Jack Middelburg, Wim Clymans
One of 118 elements…
… on THE table
Googlability
• 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.
A bit of wiki
Basic Chemistry of SiBasic Chemistry of Si
• Numerous Si-bearing minerals (mineralogy, Numerous Si-bearing minerals (mineralogy, petrology: disciplines within geology).petrology: disciplines within geology).– SiOSiO22: Quartz, glass : Quartz, glass
– Silicates:Silicates:• olivines: (Mg, Fe)olivines: (Mg, Fe)22SiOSiO44
• pyroxenes: Ca(Mg,Fe)Sipyroxenes: Ca(Mg,Fe)Si22OO66
• feldspars: (Na,K)AlSifeldspars: (Na,K)AlSi33OO88 to CaAl to CaAl22SiSi22OO88
• mica’s: KAlmica’s: KAl22(AlSi(AlSi33OO1010)(OH))(OH)22
• clay minerals: e.g. Alclay minerals: e.g. Al22SiSi22OO55(OH)(OH)44
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?
Quartz
Silicates
Sand Rock
Tight rasters
Quartz
The weathering is slow
Butcher et al. 1992
NaCl
CaSO4.2H2OCaSO4
CaCO3
CaMg(CO3)2
Sink for atmospheric CO2
CO2 + H2O H2CO3
CaSiO3 + 2H2CO3 Ca2+ + 2HCO3 + dissolved Si + H2O
DiatomsDiatoms
• Diatoms dominate coastal and Diatoms dominate coastal and oceanic biogenic Si productionoceanic biogenic Si production
• > 10.000 species> 10.000 species• Pelagic and benthic forms Pelagic and benthic forms
Thalassiosira
Diatoms take up dissolved Si (DSi),deposit it as amorphous (biogenic) Si
(ASi) in frustule
Calacademy.org
Astrographics.com
DiatomsDiatoms
• Centric forms:Centric forms:– radial or concentricradial or concentric– most pelagic are centricmost pelagic are centric
• Pennate forms:Pennate forms:– bilaterally symmetricalbilaterally symmetrical– more heavily silicifiedmore heavily silicified– most benthic are pennatesmost benthic are pennates
The ocean Si cycleFriedel, 1991
Diatoms control oceanic Si concentrations
Si, diatoms and the C cycleSi, diatoms and the C cycle
• Diatoms sink fast:Diatoms sink fast:– they are largethey are large– they aggregatethey aggregate
• An efficient transfer of labile C from photic An efficient transfer of labile C from photic zone to benthos and ocean interiorzone 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
Productivity240 Tmol y-1
Silica Burial6.3 Tmol y-1
Eolian input0.5 Tmol y-1
Seafloor input0.6 Tmol y-1
Groundwater0.4 Tmol y-1
Rivers: DSi: 6.2 Tmol y-1
ASi: 1.1 Tmol y-1
The Oceanic Si Cycle – Biological Si Pump
Weathering1.9 Tmol y-1
Sponges3.1 Tmol y-1
Reverse weathering: 1.5 Tmol y-1
(based on Tréguer & De La Rocha, 2012)
DSi: Dissolved Silica / ASi: Amorphous Silica
Estuarine ecosystemsHuman 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 PHumborg et al., Nature, 1997
Dams decrease
Si transport
Conley et al., L&O, 2000
The lake effect is observed worldwide!
The “dam-effect” is one of the best known human impacts on the Si cycle
Recapitulation
Dissolved silica essential for diatom growth in the ocean
Diatoms constitute 50+ % of ocean primary productivity
OCEAN SURFACE
DSi diatom ASi
240Tmole yr-1
After Struyf et al. 2010
The C-pump
OCEAN SURFACE
OCEAN FLOOR
diatom ASi in sea-floor sediment
ASi burial
DSi diatom ASi
240Tmole yr-1
6.5 Tmole yr-1
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
After Struyf et al. 2010
Ocean-continent link
OCEAN SURFACE
EARTH CRUST & SUBSOIL
primary and secondary silicate minerals
Weathering
eolian transport
Net riverine transport
OCEAN FLOOR
diatom ASi in sea- floor sediment
ASi burial
DSidiatom
ASi240
Tmole yr-1
6 Tmole yr-1 (*)
0.5 Tmole yr-1
6.5 Tmole yr-1
Terrestrial export of Si essential to sustain diatoms
Traditional view: export controlled by bedrock weathering
After Struyf et al. 2010
Tectonics
OCEAN SURFACE
EARTH CRUST & SUBSOIL
primary and secondary silicate minerals
Weathering
eolian transport
Net riverine transport
OCEAN FLOOR
diatom ASi in sea- floor sediment
ASi burial
DSidiatom
ASi240
Tmole yr-1
6 Tmole yr-1 (*)
0.5 Tmole yr-1
6.5 Tmole yr-1
Hydro- thermal
input
&
Seafloor weathering
plate tectonics
Tectonical processes close the cycle
After Struyf et al. 2010
A new paradigm
Recently discovered: bio-buffer between Si weathering and export
Regulates Si transport between land and ocean
ECOSYSTEM SOIL
60-200Tmole yr-1
OCEAN SURFACE
EARTH CRUST & SUBSOIL
primary and secondary silicate minerals
Weathering
eolian transport
Net riverine transport
OCEAN FLOOR
diatom ASi in sea- floor sediment
ASi burial
DSidiatom
ASi240
Tmole yr-1
6 Tmole yr-1 (*)
0.5 Tmole yr-1
6.5 Tmole yr-1
Hydro- thermal
input
&
Seafloor weathering
plate tectonics
After Struyf et al. 2010
Silica in terrestrial ecosystems
First...
May 2011, Lecture Dresden
Good for your bones,nervous system, hair and nails!
Humans and animals
May 2011, Lecture Dresden
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…
Even we are filters...
May 2011, Lecture Dresden
Schoelynck et al. 2013
58 mg Si L-1
12 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:
HerbivoresPlant disease
-Reduced water, salt, pollution, stress
- Enhanced productivity
A beneficial element!
For some it’s essential or crucial
Horsetails
Grasses and sedges
Si is beneficial – Crop YieldRice, Korndörfer & Leipsch, 2001
Strawberries, Crooks & Prentice, 2012
Guntzer et al, 2011
Biological stress
Chemical stress
Physical stress
Rice disease brown spot
Untreated
Silica
Commercial cure
Silica
Commercial cure + silica
Datnoff et al. 1997
Resistance disease
Vegetation-Soil continuum
Forest Arable
Return of plant litter, straw residue and dying roots
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
Cornelis et al. 2010
Si is accumulated in ecosystem soils
Plants stimulate the weathering
Hinsinger et al. 2001
Plants stimulate the weathering…
Hinsinger et al. 2001
The ecosystem Si filter
SOIL
Silicate minerals
Mineral weathering
bio-Si export
Dissolved Si export
VEGETATION
Riv
er E
stuary
Oce
an
Size of bio-Si stock
Land use
Bio-Si reactivity
HydrologyHydrology
February 2011, seminar Nottingham
Forests
Deforestation
Studies at Hubbard BrookExperimental 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 exportfollowing whole tree cutand removal (1983-84)
• 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
Deforestation
Scheldt watershed
52 river basins
Year-round (2008)
+ 500 observations
Goldschmidt, Knoxville, June 2010
But...
0
10
20
30
40
0 10 20 30 40 50
Forest coverage (%)
TS
i fl
ux
(µ
mo
l h
a-1 s
-1)
-50
0
50
100
150
200
250
300
350
0 10 20 30 40 50 60
% transformed into forest
% i
nc
rea
se
in
TS
i fl
ux
GrassHumanCrop
Si and long-term deforestation
Cultivation lowered base-flow Si fluxes
-50
0
50
100
150
200
250
300
350
0 10 20 30 40 50 60
% anthropogenic ecosystem transformed into forest
% in
cre
as
e in
TS
i flu
x
Forest vs. human
Forest vs. crop
Forest vs. grassland
Mixed multiple regression
(soil texture, lithology, drainage capacity,
land use)
‘forests vs. cropland’
(p < 0,001)
‘forest vs. grassland’
(p< 0.003)
‘forest vs. agriculture
(grassland + cropland)’ (p<
0,005) Goldschmidt, Knoxville, June 2010
1.0
10.0
100.0
0 10 20 30 40 50
Forestation (%)
TS
i fl
ux
(µm
ol.
ha-1
.s-1
)
New conceptual model
Si - fluxes
Soil ASi pool
0
100
0
100
TSi exportTSi-export
soil ASi-pool
mineralsilicates
DSi
ASi
mineralsilicates
DSi
ASi
exportmineralsilicates
DSi
ASi
exportmineralsilicates
ASi
export export
DSisoil
vegetation
Developing forest Climax forest Early deforested Climax cultivated
TSi-export
soil ASi-pool
mineralsilicates
DSi
ASi
mineralsilicates
DSi
ASi
exportmineralsilicates
DSi
ASi
exportmineralsilicates
ASi
export export
DSisoil
vegetation
Developing forest Climax forest Early deforested Climax cultivated
Goldschmidt, Knoxville, June 2010
New conceptual model
Si - fluxes
Soil ASi pool
0
100
0
100
TSi exportTSi-export
soil ASi-pool
mineralsilicates
DSi
ASi
mineralsilicates
DSi
ASi
exportmineralsilicates
DSi
ASi
exportmineralsilicates
ASi
export export
DSisoil
vegetation
Developing forest Climax forest Early deforested Climax cultivated
TSi-export
soil ASi-pool
mineralsilicates
DSi
ASi
mineralsilicates
DSi
ASi
exportmineralsilicates
DSi
ASi
exportmineralsilicates
ASi
export export
DSisoil
vegetation
Developing forest Climax forest Early deforested Climax cultivated
Goldschmidt, Knoxville, June 2010
New conceptual model
Si - fluxes
Soil ASi pool
0
100
0
100
TSi exportTSi-export
soil ASi-pool
mineralsilicates
DSi
ASi
mineralsilicates
DSi
ASi
exportmineralsilicates
DSi
ASi
exportmineralsilicates
ASi
export export
DSisoil
vegetation
Developing forest Climax forest Early deforested Climax cultivated
TSi-export
soil ASi-pool
mineralsilicates
DSi
ASi
mineralsilicates
DSi
ASi
exportmineralsilicates
DSi
ASi
exportmineralsilicates
ASi
export export
DSisoil
vegetation
Developing forest Climax forest Early deforested Climax cultivated
Goldschmidt, Knoxville, June 2010
Study Area
Arable Land
Pasture
Grazed Forest Continuous Forest
Clymans et al, 2011
Human impact on Si pools
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.
ca. 500 yrs human disturbance => 87±51 kg SiO2 ha-1
Clymans et al, 2011