Biogenic%20Silica%20in%20Marine%20Sediments

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Biogenic Silica in Marine Sediments

• Inputs and Outputs
• Distribution
• Burial Efficiency
• Mechanisms of dissolution preservation

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Biogenic opal
What is it? Amorphous silica
( 10 water)
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Biogenic opal
What is it? Amorphous silica
( 10 water)
Measurement Leach solid in Na2CO3 make a
correction for detrital Si
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Biogenic opal
What is it? Amorphous silica
( 10 water)
Precipitated in the surface ocean by --
phytoplankton diatoms, silicoflagellates --
protozoans radiolaria
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Biogenic opal
What is it? Amorphous silica
( 10 water)
Precipitated in the surface ocean by --
phytoplankton diatoms, silicoflagellates --
protozoans radiolaria
A fraction fall of this opal falls to the sea
floor -- its efficiently recycled, in water
column and sediments -- overall, 3 of opal
production is preserved in sediments
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A marine Si budgetDeMaster, 2002, DSR II 49,
3155-3167
Residence time 15,000 years
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opal in sediments -- from Sarmiento and Gruber
2006
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Opal Diagenesis in the deep sea at Porcupine
Abyssal PlainRagueneau et al., 2001, Prog.
Oceanog. 50, 171-200
Porcupine Abyssal Plain 4850N, 1625W
water depth 4800m 1. Sediment trap data
Mean flux 43 mmol/m2/y
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2. Benthic flux chamber data
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3. Pore water data
Organized by time of sampling lots of profiles!
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3. Cont pore water / bfc comparison
Summary Dissolution fluxes
0.35
From sed. Traps mean rain 0.12
0.2
Fall
Spring
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4. Solid phase data -- burial rate
Sed. Rate 7.5 cm/ky Burial flux 0.008
mol/m2/y
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5. Summary Rain rate a) sed trap 0.043
mol/m2/y b) dissburial 0.065 Dissolution
flux 0.057 Burial 0.008 Recycling
efficiency 12
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Opal preservation efficiency in sediments
summarySources Ragueneau et al., 2001Nelson et
al., 2002, DSRII 49, 1645-1674
Summary of fluxes
Summary of burial efficiencies
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Biogenic opal
What is it? Amorphous silica
( 10 water)
Biogenic opal is soluble in seawater,
Si(OH)4 is a weak acid, pK 9 gt its mostly
protonated in seawater
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The solubility of biogenic opal in
seawaterInitial Studies -- Hurd, 1973, GCA 37,
2257-2282
Experiment -- separate opal from cores -- clean
with acid -- place in sw at controlled temp
pH After days
2000
23C
1500
Si(OH)4 µmol/l
3C
1000
500
Note T dependence Solubility 900 µM at
2C !
pH
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New solubility studiesFlow-through reactors
V volume C conc of solute m conc of solid R
reaction rate
CR , F
at steady state
So
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Results
Example of one experiment
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Results
T dependence of solubility - KTB06
Example of one experiment
Conc. In outflow water Filled precipitation Open
dissolution
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Deep water 20-120
Highest values 200
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The kinetics of biogenic opal dissolution in
seawaterHurd, 1972, EPSL 15, 411-417
(log scale)
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A mineral,undersaturated in seawaterapparently
simple dissolution kinetics
What do we expect Si(OH)4 in pore water to look
like?
Concentration
CBW
CSAT
Diagenesis of a solid, undersaturated in bw
Asymptotic approach to Saturation in pore water
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Observations Si(OH)4 In pore waters
N. Atlantic (Bermuda)
Csat 600
Csat 100-120
Southern Ocean
Peru Margin
Csat 500-750
Csat 550-830
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Comparing asymptotic pore water Si(OH)4 tothe
equilibrium value
Why?
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Dependence of solubility on Al/Si in diatom
testsvan Bennekom et al., 1988, Paleo3 67, 19-30
Batch-dissolution experiments diatoms from
different regions in sw
Both dissolution rate and asymptotic value
decrease as Al content of test increases
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Dependence of solubility on Al/Si in diatom
testsvan Bennekom et al., 1988, Paleo3 67, 19-30
And added to these results Al/Si ratio in
diatoms vs. Si(OH)4 in surrounding pore waters
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-2
Squares data from previous plot Closed circles
diatoms from sediment / pore water SiO2
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(No Transcript)
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North
South
South opal-dominated
Higher detrital content
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Further studies1. Effect of Al on solubility
Southern ocean sediments, 0-0.5 cm measure
Al/Si ratio determine solubility (note
Al/Si in diatoms found to covary with detrital
of sediments)
Result significant effect but not enough to
explain all asymptotic SiO2 variation
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Further studies2. Experiment on effect of
det/opal on equilibrium Si(OH)4
Mix detrital material and biogenic opal in
variable proportions observe the dissolved
Si(OH)4 At steady state Detrital material (a)
kaolinite -- Al2Si2(OH)4 (b)
basalt (48.5 wt SiO2 14.4 wt Al2O3
21-month batch experiments
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Further studies2. Experiment on effect of
det/opal on equilibrium Si(OH)4
Mix detrital material and biogenic opal in
variable proportions observe the dissolved
Si(OH)4 At steady state Detrital material (a)
kaolinite -- Al2Si2(OH)4 (b)
basalt (48.5 wt SiO2 14.4 wt Al2O3
21-month batch experiments
Dependence of steady-state Si(OH)4 on det/opal
ratio significantly greater than Al/Si effect on
solubility
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Further studies3. Are authigenic
aluminosilicates precipitating?
Experiment flow through reactors, containting
50 mg biosiliceous ooze Inflow 500 nM Al,
variable Si(OH)4
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Further studies3. Are authigenic
aluminosilicates precipitating?
Experiment flow through reactors, containting
50 mg biosiliceous ooze Inflow 500 nM Al,
variable Si(OH)4
No SiO2 in inflow high Al in outflow
Highest SiO2 in inflow lowest Al in outflow
gt removal of Al(III) from solution When Si(OH)4
is present
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2
1
1
3
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Summary
Inputs and Outputs Distribution Burial
Efficiency Mechanisms of dissolution
preservation