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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