DISSOLVED CONSTITUENTS IN MARINE PORE WATER PORE WATER PROFILES DIFFUSIVE FLUXES... DATA EVALUATION... How to read pore water concentration profiles 1
How to read pore water concentration profiles Consumption of a reactant in pore water Release of a substance from the solid phase into the pore water fraction Diffusion transport of dissolved substances in pore water and across the sediment/bottom water boundary Non-reacting substance e.g. chloride 2
Substance that is depleted in the upper layers of the sediment e.g. dissolved oxygen Penetration depth Penetration depth Depth where concentration, gradient and diffusion simultanously reach zero Substance that is consumed in a particular reactive layer Constant gradient = Diffusive transport Process limited to one reactive layer e.g. oxygen consumption in a layer containing easily degradable organic matter or reductive solute species (Mn 2+, Fe 2+ ) 3
Substance that is released into pore water in the upper layers of the sediment Substance that is released into the pore water in specific reactive layers e.g. silica on account of the dissolution of sedimentary opal 4
Substance that is released into pore water in one discrete depth (reactive layer 1) and, in another depth (reactive layer 2) is removed from the pore water by consumption Gradient = Diffusive transport = 0 Constant gradient = Diffusive transport Gradient = Diffusive transport = 0 A few rules for reading and understanding pore water concentration profiles! Diffusive material fluxes always occur in the form of concentration gradients; concentration gradients always represent diffusive material fluxes! Reactions occuring in pore water always constitute changes in the concentration gradient; changes in the concentration gradient always represent reactions occuring in pore water 5
A few rules for reading and understanding pore water concentration profiles! A concave-shaped alteration in the concentration gradient profile signifies the depletion of a substance from pore water! A convex-shaped concentration gradient profile always depicts the release of a substance into the pore water Calculation of Diffusive Fluxes Steady State and Non-Steady State Situations 6
Steady State Situations (A)! Continuous consumption of a substance at a sprecific rate and within a reactive layer (B)! Constant concentration in the bottom water as infinite reservoir Constant concentration gradient between sediment surface and the reactive layers Same diffusive flux Steady State Situations No real steady-state situations in nature Term depends on a particular stretch of time, the dimension of the system, the accuracy of the measurements,!!! Seasonal variations!!! 7
Non-Steady State Situations The Steady State Situation and Fick s First Law of Diffusion Total diffusive flux J (mol m -2 s -1 ) of a given solute is proportional to the concentration gradient: D 0 Salinity- and substance-specific diffusion coefficient in seawater (m -2 s -1 )! C /! x Concentration gradient of the solute (mol m -3 m -1 ). 8
Molecular Diffusion! random movement of soluble particles or molecules! net transport from high to low concentration in the presence of a concentration gradient! faster for small molecules! depends on the temperature and salinity of the seawater! only effective over small distances (!m to mm scale) since the travel time of a molecule to a certain point increases with the square of the distance Only valid for free solutions, without the disturbing sedimentary solid phase Diffusion in sediments can only take place within the pore water volume (= porosity!) Diffusion coefficient is lower in the pore water volume of a sediment (D sed ) than in free solution (D 0 ) 9
! Porosity D sed Salinity- and substance-specific diffusion coefficient in seawater (m -2 s -1 )! C /! x Concentration gradient of the solute (mol m -3 m -1 ). Why is D sed lower than D 0? Why is D sed lower than D 0? Diffusion in the pore water volume cannot follow a straight way, but must take deviations around each single grain Tortuosity! = Degree of deviation = Mean ration between the real length of the pathway and the straight line distance 10
Why is D sed lower than D 0? With knowing D sw and " and the following approximation of #.... Dsed can be calculated from D0. Quantitative Evaluation grad =! C /! x Highest inclination located below the sediment surface! C /! x = 22.1 mol/m 3 m! = 0.8 T = 5 C D sw = 1.23*10-9 m 2 s -1 Calculation of D sed J sed, oxygen (mol m -2 s -1 ) (mol m -2 a -1 ) 11
How much C org is annually oxidised per m 2 assuming that all oxygen is used in the oxidation of organic matter (R ox,corg )? C:O ratio? Molecular weight of C? Elements or compounds do not exist or cycle individually but rather always interact and overlap with other geochemical cycles. Most important cycles: C, O, N, P, S (and Fe) Simplified molecular composition of living material The Redfield-ratio: C 106 :H 263 :O 110 :N 16 :P 1 :(S 1 ) 12
! C /! x = 5.5 mol/m 3 m! = 0.6 T = 5 C D sw? D sed? J sed? R ox,corg? J sed,up? J sed,down? 13