summary of basalt-seawater interaction - soest · 2013-02-22 · summary of basalt-seawater...

Summary of Basalt-Seawater Interaction• Mg2+ is taken up from seawater into clay minerals, chlorite,

and amphiboles, in exchange for• Ca2+, which is leached from silicates into solution.• K+ is taken up into clay minerals and zeolites at low T, but

is leached from basalt into solution at high T (>150°C).• Na+ can go either way depending on conditions.• Sulfate is precipitated as anhydrite (CaSO4 ) and reduced to

sulfide.• HCO3

- is converted to CO2 and alkalinity drops, due to H+

production.• Chloride increases from uptake of H2 O into solids, and it

may increase or decrease as a result of phase separation.

Note: Volcanogenic sediment will behave similarly to basalt.

Role of Sediments in Mass Fluxes• Convection is slow or absent because sediments have

low permeability, preventing rapid transport of mass (e.g., flow of seawater).

• Diffusion is slow:

Entire river input of Mg2+ would be taken up into sediments by diffusion if Mg2+ went to zero at 18 mbsf over the entire area of the seafloor.(This is outside the realm of possibility!)

• Reaction is therefore the key, but most sediment is not very reactive, as it formed not far from equilibrium with seawater (e.g. clay minerals, CaCO3 , SiO2 ).

Diagenesis

--reactions in modern sediments--the sum total of processes that bring about changes in

a sediment or sedimentary rock subsequent to deposition in water, but excluding weatheringand metamorphism

Weathering occurs after contact with the atmosphere.

Metamorphism occurs after burial, at elevated T and P.

Early Diagenesis--during burial to < few 100 m--T not elevated much.--Uplift has not occurred.

pore spaces constantly filled with water

Includes: --compaction and dewatering--bioturbation--diffusion of dissolved salts--microbial decomposition of organic matter--dissolution and precipitation of minerals,

including cementation and replacement

Use of Interstitial (Pore) WatersTypical surficial marine sediment:

--porosity ~70%--wet-bulk density ~1.5 g/cm3

= 0.7 g H2 O + 0.8 g solids

Elements other than H and O are overwhelminglypresent in the solid phases rather than in solution.

Effects of chemical reactions are much more readily detected as changes in composition of the pore waterthan of the solids!

Reactions in the Sediment Column

1) Oxidation of organic C by sulfate reduction

SO4= + 2”CH2 O” = H2 S + 2HCO3

-

2) Precipitation of CaCO3

Ca2+ + 2HCO3- = CaCO3 + H2 O + CO2

Cumulative number distributions of DSDP sites with and withoutchemical gradients in sediment pore water, vs. sediment thickness

Chemical Gradients in Pore Waters

McDuff (1981)

Chemical Gradients in Pore WatersCan be caused by:1. Diffusion

– changing concentrations at boundaries(seafloor, basement)

– to and from reaction zones2. Reaction

– in the sediment column– in basement (diffusing or advecting into sediment)

3. Advection (flow)– through the sediment column

– vertical, upward or downward– horizontal

– through basement (diffusing or advecting into sediment)

1-d Advection and Diffusion (with reaction in basement)

Dsed depends onT, porosity, and tortuosity.

Rapid upwelling produces a spring.

Diffusion dominates as

gradient increases toward seafloor.

Sediment pore water frompush cores collected onBaby Bare, an isolatedbasement outcrop on 3.0 Macrust on the eastern flank of the Juan de Fuca Ridge

Chemical Gradients in Pore WatersAre typically absent:a) in slowly deposited sediments such as

red clays, where deposition is slower than diffusion.

Mean diffusion path z, over which an original concentration gradient would be more than 90% eliminated, is:

z = (Dt)1/2

where D = diffusion coefficient (for bulk sediment)t = time

Dsed = Dw /(

x F) where = porosityand F = formation factor (a measure of tortuosity)

Chemical Gradients in Pore WatersAre typically absent:a) in slowly deposited sediments such as

red clays, where deposition is slower than diffusion.

Mean diffusion path z, over which an original concentration gradient would be more than 90% eliminated, is:

z = (Dt)1/2

where D = diffusion coefficient (for bulk sediment)t = time

For Dsed = 10-6 cm2/s, diffusional communication with the overlying ocean is possible to depths of:

Sed. rate (cm/1000 yrs): 500 50 5 1Depth for diffusion (mbsf): 2 18 180 900

shelf rise biogenic

Chemical Gradients in Pore WatersAre typically absent:b) in sediment sections <200 m thick in which sulfate

reduction is not an important process, because of:

a) on young crust: rapid convection of seawater through young crust, such that the basement formation water closely resembles seawater, and

b) on older crust: slow sedimentation rates.

Chemical Gradients in Pore WatersAre typically present:

in sediment sections >200 m thick, because of:a) reactions within the sediment, the effects of

which cannot diffuse away as rapidly as sediment is being deposited, and

b) reactions within basement, which change the composition of basement formation water away from that of seawater when throughput of seawater is small relative to reaction rates.

The General Diagenetic Equation (one-dimensional!)

dC/dt = d(D.(dC/dx))/dx - d(vC)/dx + w.(dC/dx) + R

In a layer-based coordinate system:

diffusionadvection

sedimentationreaction

Derivation of Fick’s Second Law of Diffusion (Berner, 1980)

.Site 505

Location of DSDPSites 501/504 and 505on the southern flank of the Costa Rica Rift(the easternmost segment ofthe Galapagos Rift)

Heat flow and seismic reflection profiles on the southern flank of theCosta Rica Rift. The Galapagos Rift is ~110 km north of the left edgeof this figure.

Sediment layer (transparent)

Basaltic basement (opaque)

Heat flow in mW/m2Site505

Sites501/504

4 Ma 6 Ma

90% advective

100% conductive

Location of the five holes at Site 501/504 on 6 Ma crust