DISSOLVED CONSTITUENTS
IN MARINE PORE WATER
PORE WATER PROFILES
DIFFUSIVE FLUXES
..... DATA EVALUATION .....
How to read pore water
concentration profiles
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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
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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
Process limited
reactive layer
to
Constant gradient
=
Diffusive transport
one
e.g. oxygen consumption in a
layer
containing
easily
degradable organic matter or
reductive
solute
species
(Mn2+ , Fe2+)
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
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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
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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
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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 !!!
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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:
D0
Salinity- and substance-specific diffusion
coefficient in seawater (m-2 s-1)
!C/!x
Concentration gradient of the solute (mol m-3 m-1).
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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 (Dsed)
than in free solution (D0)
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!
Porosity
Dsed
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 Dsed lower than D0?
Why is Dsed lower than D0?
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
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Why is Dsed lower than D0?
With knowing Dsw 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/m3m
! = 0.8
T = 5°C
Dsw = 1.23*10-9 m2s-1
Calculation of
Dsed
Jsed, oxygen
(mol m-2 s-1)
(mol m-2 a-1)
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How much Corg is annually oxidised per m2 assuming that all
oxygen is used in the oxidation of organic matter (Rox,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: C106:H263:O110:N16:P1:(S1)
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! C / ! x = 5.5 mol/m3m
! = 0.6
T = 5°C
Dsw ?
Dsed ?
Jsed ?
Rox,Corg ?
Jsed,up
?
Jsed,down ?
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