capacity, oxidation reserve and oxygen debt The sediment’s hydrogen sulphide buffering capacity, oxidation
2 Construction and functioning of the sediment flux model
2.7 The calculation flow of the model
2.8.1 Input determined before the model is run in 1 Porosity
A porosity (ϕ) is assigned to each model layer. Therefore, the porosity
measured in the sediment is approximated by an exponentially decreasing function. The exponential function is then used to calculate porosity in each model layer (Figure 2.5, Table 2.2).
2.8.1.2 Density
Likewise, the density of the solid components of the sediment (ρs) is
calculated from sediment measurements. This value is not affected by variations in porosity, which is why density only rarely varies with depth.
2.8.1.3 Diffusivity
Diffusivities of free-water solutes are temperature dependent. Thus, good descriptions of diffusivity can be obtained by adjusting values found in the literature using either a second order polynomial or a straight line. The temperature dependency of diffusivity is shown in Table 2.2.
2.8.1.4 Q10 value
The Q10 value determined for the primary reactions is based on
sulphate reduction rates measured in Aarhus Bay. Apart from temperature, sulphate reduction in the upper seabed is influenced by the load of organic matter and the freshness of that matter. Therefore, only sulphate reduction rates in the depth interval 7-10 cm were used
to determine the Q10 value. As shown in Figure 2,6 the rates are
clearly correlated with temperature and by fitting the measured
values we arrive at a Q10 value of 3.8, which we then use for all
primary reactions. For the secondary reactions we use an estimated
Q10 value of 2 based on values found in the literature.
2.8.1.5 Biodiffusivity
The biodiffusivity of solids (Eq.5) is found by interpretation of 210
Pb profiles from Aarhus Bay, concentration profiles and data on organic matter consumption. These interpretations show that biodiffusivity is more or less constant in the upper 12 cm, approximately, of the seabed, below which it decreases exponentially. Comparison of measured profiles and flux in the seabed shows that the
biodiffusivity of solutes (DB w in Eq.4) is approximately 10 times
greater than that of solids.
2.8.1.6 Sedimentation
Sedimentation rates in Aarhus Bay are found by interpretation of
210
Pb profiles. These interpretations are based on more recent and slightly more accurate mathematical modelling than used previously and, therefore, we have calculated a sedimentation rate
20 15 10 5 0 0,50 0,75 1,00 1,25 ϕ= 0,763+0,086e-0,216x Porosity Depth (cm)
Figure 2.5. Porosity (A) measured in the sediment of Aarhus Bay and
approximated by an exponentially decreasing function. 4 6 8 10 12 14 16 1 10 100 7 - 10 cm n = 3 Fit: Q10 = 3,8 SRR (nmol cm -3 da y -1) Temp (oC)
Figure 2.6. The Q10 value of 3.8 calculated for the primary reactions is based on the correlation between temperature and sulphate reduction rates in the depth interval 7-10 cm in Aarhus Bay. Note the logarithmic SRR scale. See text for further discussion of the figure.
approximately 2.5 times higher than that reported by the HAV90 studies in Aarhus Bay. In a modelling context sedimentation rate is a very important parameter, because permanent burial of solids in the seabed is directly proportional to this rate.
2.8.1.7 Irrigation
Irrigation is generally the dominant form of transport, for instance
when it comes to removal of CO2 from the seabed. Therefore, the
irrigation parameter (α, see equation Eq.6) is initially assessed on the
basis of profiles and production of CO2. It should be noted, however,
that sediment profiles of CO2 were not determined in Aarhus Bay in
the HAV90 programme. For use in the model it was therefore necessary to determine the irrigation parameter on the basis of a few
CO2 measurements made in 2000, which makes this determination
somewhat uncertain.
Instead of using the irrigation parameter calculated for 2000 uncritically in the sediment flux model, we have verified the
irrigation parameter by interpreting profiles of MnO2 and Mn
2+
, which were measured frequently during the HAV90 programme (1990-1991). In the model manganese can exist in the sediment in two
different states; solid state (MnO2) and dissolved state (Mn
2+
). It is obvious, therefore, that production of one pool at a given depth must be balanced by a corresponding consumption of the other pool. The
shape of the concentration profile of MnO2 and Mn
2+
, respectively, does in fact reflect production and consumption of the substances,
R2=1,00 R2=1,00 -0,005 0 0,005 0 0,005 7,5 6,0 4,5 3,0 1,5 0 -1,5 0 15000 30000 0 50 100 1 100 10000 Depth (cm)
MnO2 (nmol g-1) Mn2+ (µM) α (year-1)
MnO2 production
(nmol cm-3 s-1)
Mn2+ production
(nmol cm-3 s-1)
A B C
Figure 2.7. The irrigation parameter (<) as a function of depth based on concentration profiles of MnO2, Mn
2+
and CO2 (Aarhus Bay). A: MnO2 concentration and calculated MnO2 consumption (negative rate). B: Mn
2+
concentration and calculated Mn2+
production (positive rate) by reduction of MnO2. C: Irrigation parameter (circles) calculated on the basis of consumption and production, respectively, of MnO2 and Mn
2+
and compared with calculations of CO2 concentrations and production in the seabed (solid line). See text for discussion of preconditions for calculating the irrigation parameter.
and because irrigation only influences the Mn2+
ion, any imbalance in this account can be used to quantify the significance of irrigation and hence the value of the irrigation parameter. Figure 2,7 shows the result of interpretations such as these and the irrigation parameter derived from them. The figure demonstrates a fair agreement
between interpretation of Mn2+ and MnO2 profiles and the somewhat
uncertain calculation of the irrigation parameter based on
concentration profiles of CO2 from 2000.
2.8.1.8 Adsorption
The adsorption constants of Mn2+
and Fe2+
were determined on the basis of specific measurements made in Aarhus Bay. The constants
for NH4
+
and PO4
3-
were determined on the basis of mean values found in the literature, and are considerably lower than the
corresponding constants for Mn2+
and Fe2+
.
2.8.1.9 The diffusive boundary layer
Sediment oxygen concentrations were measured using microelectrodes with a depth resolution of 100 µm. This makes it possible to determine the extent of the diffusive boundary layer, i.e. the width of the part of the oxygen profile exhibiting a linear decline in oxygen concentration. Microelectrode measurements show that the thickness of the diffusive boundary layer is approximately 0.03 cm.
2.8.1.10 Flux of manganese, iron and organic matter
As described previously, the external fluxes of MnO2 and FeOOH to
the sediment surface are equivalent to the amounts of manganese and iron, respectively, buried in the seabed and thus disappearing from the model. Sediment concentrations of both manganese and iron have been measured, and the sedimentation rate is known, and the fraction of the two compounds that is buried in the seabed is thus easy to calculate.
The ratio of the flux of undegraded organic matter (FOM n) to total
organic matter flux (FOM total) is calculated from the concentration of
organic matter at the bottom of the model (at a depth of 20 cm) and the sedimentation rate. Therefore, it is a prerequisite to the calculation that all degradable organic material has been degraded when this depth is reached. The ratio of the flux of readily degradable
organic matter (FOM f) to the total organic matter flux (FOM total) was
determined from literature values.
2.8.2 Input determined to some extent during the run-in phase