Summary of Basalt-Seawater Interaction Mg 2+ is taken up from seawater into clay minerals, chlorite, and amphiboles, in exchange for Ca 2+, 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 (CaSO 4 ) and reduced to sulfide. HCO 3 - is converted to CO 2 and alkalinity drops, due to H + production. Chloride increases from uptake of H 2 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 Mg 2+ would be taken up into sediments by diffusion if Mg 2+ 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, CaCO 3, SiO 2 ).
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 weathering and 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) Waters Typical surficial marine sediment: --porosity ~70% --wet-bulk density ~1.5 g/cm 3 = 0.7 g H 2 O + 0.8 g solids Elements other than H and O are overwhelmingly present in the solid phases rather than in solution. Effects of chemical reactions are much more readily detected as changes in composition of the pore water than of the solids!
Reactions in the Sediment Column 1) Oxidation of organic C by sulfate reduction SO 4 = + 2 CH 2 O = H 2 S + 2HCO 3-2) Precipitation of CaCO 3 Ca 2+ + 2HCO 3 - = CaCO 3 + H 2 O + CO 2
Chemical Gradients in Pore Waters McDuff (1981) Cumulative number distributions of DSDP sites with and without chemical gradients in sediment pore water, vs. sediment thickness
Chemical Gradients in Pore Waters Can be caused by: 1. Diffusion changing concentrations at boundaries (seafloor, basement) to and from reaction zones 2. 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)
Rapid upwelling produces a spring. Diffusion dominates as gradient increases toward seafloor. D sed depends on T, porosity, and tortuosity. 1-d Advection and Diffusion (with reaction in basement)
Sediment pore water from push cores collected on Baby Bare, an isolated basement outcrop on 3.0 Ma crust on the eastern flank of the Juan de Fuca Ridge
Chemical Gradients in Pore Waters Are 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: where z = (Dt) 1/2 D = diffusion coefficient (for bulk sediment) t = time D sed = D w /( x F) where = porosity and F = formation factor (a measure of tortuosity)
For D sed = 10-6 cm 2 /s, diffusional communication with the overlying ocean is possible to depths of: Sed. rate (cm/1000 yrs): 500 50 5 1 Depth for diffusion (mbsf): 2 18 180 900 shelf rise biogenic Chemical Gradients in Pore Waters Are 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: where z = (Dt) 1/2 D = diffusion coefficient (for bulk sediment) t = time
Chemical Gradients in Pore Waters Are 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 Waters Are 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!) In a layer-based coordinate system: dc/dt = d(d.(dc/dx))/dx - d(vc)/dx + w.(dc/dx) + R diffusion advection sedimentation reaction
Derivation of Fick s Second Law of Diffusion (Berner, 1980)
. Site 505 Location of DSDP Sites 501/504 and 505 on the southern flank of the Costa Rica Rift (the easternmost segment of the Galapagos Rift)
Site 505 Heat flow in mw/m 2 Sites 501/504 4 Ma 6 Ma 100% conductive 90% advective Sediment layer (transparent) Basaltic basement (opaque) Heat flow and seismic reflection profiles on the southern flank of the Costa Rica Rift. The Galapagos Rift is ~110 km north of the left edge of this figure.
Location of the five holes at Site 501/504 on 6 Ma crust