Biogenic Silica in Marine Sediments

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

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

3. Biogenic opal What is it? Amorphous silica: (~ 10% water) Measurement: Leach solid in Na2CO3 ; make a correction for detrital Si

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

5. 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

6. A marine Si budgetDeMaster, 2002, DSR II 49, 3155-3167 Residence time ~ 15,000 years

7. % opal in sediments -- from Sarmiento and Gruber 2006

8. Opal Diagenesis in the deep sea at Porcupine Abyssal PlainRagueneau et al., 2001, Prog. Oceanog. 50, 171-200 Porcupine Abyssal Plain 48°50’N, 16°25’W water depth ~ 4800m 1. Sediment trap data Mean flux: 43 mmol/m2/y

9. 2. Benthic flux chamber data

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

11. 3. Cont : pore water / bfc comparison Summary: Dissolution fluxes 0.35 From sed. Traps: mean rain = 0.12 0.2 Fall Spring

12. 4. Solid phase data -- burial rate Sed. Rate = 7.5 cm/ky Burial flux = 0.008 mol/m2/y

13. 5. Summary Rain rate a) sed trap 0.043 mol/m2/y b) diss+burial 0.065 Dissolution flux 0.057 Burial 0.008 Recycling efficiency = 12%

14. Opal preservation efficiency in sediments : summarySources: Ragueneau et al., 2001Nelson et al., 2002, DSRII 49, 1645-1674 Summary of fluxes Summary of burial efficiencies

15. 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

16. 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 23°C 1500 [Si(OH)4] µmol/l 3°C 1000 500 Note: T dependence Solubility ~ 900 µM at 2°C ! pH

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

18. Results Example of one experiment

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

20. Deep water: 20-120 Highest values: ~200

21. The kinetics of biogenic opal dissolution in seawaterHurd, 1972, EPSL 15, 411-417 (log scale)

22. 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

23. 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

24. Comparing asymptotic pore water [Si(OH)4] tothe “equilibrium” value Why?

25. 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

26. 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: 2 -2 Squares: data from previous plot; Closed circles: diatoms from sediment / pore water SiO2

28. North South South: opal-dominated Higher detrital content

29. 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

30. 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

31. 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

32. Further studies3. Are authigenic aluminosilicates precipitating? Experiment: flow through reactors, containting 50 mg “biosiliceous ooze” Inflow: 500 nM Al, variable Si(OH)4

33. 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 ==> removal of Al(III) from solution When Si(OH)4 is present

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