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Interpreting profiles of pore water solutes

Interpreting profiles of pore water solutes. First, solute transport (simple). Diffusive Transport:. 2. Sediment Burial Generally: Assume a constant mass flux (…not always true). Mass accumulation rate:. Solute burial, cont. Below the “compacting layer”.

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Interpreting profiles of pore water solutes

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  1. Interpreting profiles of pore water solutes

  2. First, solute transport (simple) Diffusive Transport: 2. Sediment Burial Generally: Assume a constant mass flux (…not always true) Mass accumulation rate:

  3. Solute burial, cont. Below the “compacting layer” Which must be the same as MAR at shallower depths… That’s for solids…

  4. For solutes… The burial rate for solutes is slightly smaller than for solids… AND

  5. Interpreting pore water profiles… In general: we consider rates of change of concentration over space and time We derive descriptive expressions by considering the mass balance in a layer of sediment F1 x1 R x2 F2 In the box:

  6. Interpreting, cont. (x2 - x1) --> small… Since…

  7. Interpreting, cont… Then… Simplify: steady state ; constant  ; constant D ; burial << transport:

  8. ==> A simple interpretation of pw profiles

  9. Interpretation of profile shapes : quantitative Steady-state mass balance in a sediment layer: Rate of reaction within the layer = net flux out of the layer Diffusive flux : 1 oxic 2 Flux at pt. 1 (x=0) : gives total, net NO3 Production in sediment column Flux at pt. 2 : gives rate of NO3 consump. By denitrification Sum of absolute values of Flux at 1 + Flux at 2: Gives rate of NO3 production by oxic Decomposition of organic matter denitrification

  10. But we can get more information… What else do we need to solve this equation?

  11. But we can get more information… Boundary conditions! At sediment-water interface (x=0) At depth in the sediments:

  12. What about R ? Example : organic matter oxidation by O2 What solutes could you measure to define reaction rate?

  13. What about R ? For O2, it has been convenient to use R = P(x) i.e., with no dependence on [O2], even though it’s a reactant Can that be justified? Devol (1978) DSR 25, 137-146 Cultured marine bacteria from low-O2 waters… Found O2 consumption followed Michaelis-Menten kinetics: And found a “Critical O2 Concentration” below Which rate depended on [O2] Of ~ 2.4 µmol/l

  14. Pore water profiles :O2all done by in situ microelectrode profiling Total Corg ox. Rate (µmol/cm2/y) 350 45 14

  15. Continental margin sediments:* large organic matter flux* electron acceptors other than O2 Let’s consider a sediment dominated by sulfate reduction: Defining P as the production rate of a solute, What would we predict pore water profiles of these 3 solutes to look like? Solution:

  16. Solve the equation for each solute: Assume P0 = 100, p1 = 0.2 Assume porosity = 0.8 and Dsed = Dsw x (porosity)2 … then DHCO3- = 323 cm2/y , DNH4+ = 543 cm2/y, DHPO42- = 208 cm2/y

  17. Plotting the concentration of one solute vs. another… Interpreting the slopes: At any depth, Therefore, the slopes imply

  18. 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

  19. 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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