Biogenic Silica in Marine Sediments Inputs and Outputs Distribution Burial Efficiency Mechanisms of dissolution & preservation

Biogenic opal What is it? Amorphous silica: (~ 10% water)

Biogenic opal What is it? Amorphous silica: (~ 10% water) Measurement: Leach solid in Na 2 CO 3 ; make a correction for detrital Si

Biogenic opal What is it? Amorphous silica: (~ 10% water) Precipitated in the surface ocean by: -- phytoplankton diatoms, silicoflagellates -- protozoans radiolaria

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 -- it’s efficiently recycled, in water column and sediments -- overall, ~ 3% of opal production is preserved in sediments

A marine Si budget DeMaster, 2002, DSR II 49, Residence time ~ 15,000 years

% opal in sediments -- from Sarmiento and Gruber 2006

Opal Diagenesis in the deep sea at Porcupine Abyssal Plain Ragueneau et al., 2001, Prog. Oceanog. 50, Porcupine Abyssal Plain 48°50’N, 16°25’W water depth ~ 4800m 1. Sediment trap data Mean flux: 43 mmol/m 2 /y

2. Benthic flux chamber data

3. Pore water data Organized by time of sampling … lots of profiles!

3. Cont : pore water / bfc comparison From sed. Traps: mean rain = 0.12 FallSpring Summary: Dissolution fluxes

4. Solid phase data -- burial rate Sed. Rate = 7.5 cm/ky Burial flux = mol/m 2 /y

5. Summary Rain rate a) sed trap0.043 mol/m 2 /y b) diss+burial0.065 Dissolution flux0.057 Burial Recycling efficiency = 12%

Opal preservation efficiency in sediments : summary Sources: Ragueneau et al., 2001 Nelson et al., 2002, DSRII 49, Summary of fluxesSummary of burial efficiencies

Biogenic opal What is it? Amorphous silica: (~ 10% water) Biogenic opal is soluble in seawater, Si(OH) 4 is a weak acid, pK ~9 ==> it’s mostly protonated in seawater

The solubility of biogenic opal in seawater Initial Studies -- Hurd, 1973, GCA 37, Experiment: -- separate opal from cores -- clean with acid -- place in sw at controlled temp & pH After ~ days: Note: T dependence Solubility ~ 900 µM at 2°C ! °C 3°C [Si(OH) 4 ] µmol/l pH

New solubility studies: Flow-through reactors V = volume C = conc of solute m = conc of solid R = reaction rate C R, F … at steady state… So

Results Example of one experiment

Results Example of one experiment T dependence of solubility - KTB06 Conc. In outflow water Filled: precipitation Open: dissolution

Deep water: Highest values: ~200

The kinetics of biogenic opal dissolution in seawater Hurd, 1972, EPSL 15, (log scale)

A mineral, undersaturated in seawater apparently simple dissolution kinetics… What do we expect [Si(OH) 4 ] in pore water to look like? C BW C SAT Concentration Diagenesis of a solid, undersaturated in bw Asymptotic approach to Saturation in pore water

Csat = Csat = 600 Csat = Csat = N. Atlantic ( Bermuda) Peru Margin Southern Ocean Observations: Si(OH) 4 In pore waters

Comparing asymptotic pore water [Si(OH) 4 ] to the “equilibrium” value Why?

Dependence of solubility on Al/Si in diatom tests van Bennekom et al., 1988, Paleo 3 67, Batch-dissolution experiments: diatoms from different regions in sw Both dissolution rate and asymptotic value decrease as Al content of test increases

Dependence of solubility on Al/Si in diatom tests van Bennekom et al., 1988, Paleo 3 67, And added to these results: Al/Si ratio in diatoms vs. [Si(OH)4] in surrounding pore waters: Squares: data from previous plot; Closed circles: diatoms from sediment / pore water SiO

South: opal- dominated Higher detrital content South North

Further studies 1. Effect of Al on solubility Southern ocean sediments, 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

Further studies 2. 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 -- Al 2 Si 2 (OH) 4 (b) basalt (48.5 wt% SiO 2 ; 14.4 wt % Al 2 O 3 21-month batch experiments

Further studies 2. 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 -- Al 2 Si 2 (OH) 4 (b) basalt (48.5 wt% SiO 2 ; 14.4 wt % Al 2 O 3 21-month batch experiments Dependence of steady-state Si(OH) 4 on det/opal ratio significantly greater than Al/Si effect on solubility

Further studies 3. Are authigenic aluminosilicates precipitating? Experiment: flow through reactors, containting 50 mg “biosiliceous ooze” Inflow: 500 nM Al, variable Si(OH )4

Further studies 3. Are authigenic aluminosilicates precipitating? Experiment: flow through reactors, containting 50 mg “biosiliceous ooze” Inflow: 500 nM Al, variable Si(OH )4 No SiO 2 in inflow: high Al in outflow Highest SiO 2 in inflow : lowest Al in outflow ==> removal of Al(III) from solution When Si(OH) 4 is present

Summary Inputs and Outputs Distribution Burial Efficiency Mechanisms of dissolution & preservation