7.4. Atmosphere-Ocean Interaction Model
133
The structure of the Fe module of this model follows that of the Nand C
modules and is shown in Figure 7.4. Analogous to Figure 7.3, this module
uses the Michaelis-Menten function to calculate particulate Fe fluxes to the
deep ocean, using an average Fe : N ratio. The value for the Fe : N ratio applied in the model O.65e-4 mol mol" ) has been determined for species of
marine phytoplankton maintained in laboratory culture (Morel & Hudson
1984).
In the model, the major particulate flux of carbon from the surface water
to the deep ocean and sediments occurs in the form of CaC0 3 burial. We
incorporate in the model a constant background flux (i.e. independent of
Fe availability) of C, N and Fe from calcite-forming organisms (Broecker &
Peng 1982). Calculating downward particulate flux of C, N, and Fe in this
manner assumes that :
(a) ocean nutrient inventorie s remain constant;
(b) the downward flux of particulate material results largely from the
growth and sinking of diatoms (Goldman 1993);
(c) the relative elemental ratios of C : N : Fe in sinking biogenic material
are constant over the time scales considered in the model;
(d) the downward flux of biogenic material or export production in this
model is set by the input of Fe into the surface ocean;
(e) when Fe concentrations are sufficient for maximum N0 3
- uptake the
downward flux of particulate C and, therefore, atmospheric CO 2 concentrations are controlled by, and equal to, the upwelling flux of N0 -
from deep water to the surface ocean. This is also the definition of new
production (Dugdale & Goering 1967; Eppley & Peterson 1979).
Additionally, the model assumes that:
CD downwelling and upwelling fluxes of the dissolved chemical species
are calculated based on the concentration of the species (mol) and the
mean ocean downwelling and upwelling rates of 0.39 m (day)-l
(Broecker & Peng 1982);
(g) particulate matter burial equals 1% of the flux of the particulate matter
to the deep ocean;
(h) particulate matter remineralization occurs on the time scale of ocean
mixing -3000 years (Broecker & Peng 1982). This implies that on
these time scales there is no organic matter preservation (i.e, steadystate) .
Assumptions (a)-(h) link the C, N and Fe portions of the model. Despite a
fixed relation between the incorporation of C, N and Fe into particulate
matter the relative drawdown of carbon from the atmosphere and surface
ocean into the deep ocean is a function of the concentrations of the two
major plant nutrients N and Fe. Changing either the ratio of C : N : Fe in
particulate matter or the absolute uptake of N (mol yr") does not alter the
model 's function-it will only change the relative drawdown or accumulation of CO 2 in the atmosphere and surface ocean. Fluxes of dissolved C, N
3
133
The structure of the Fe module of this model follows that of the Nand C
modules and is shown in Figure 7.4. Analogous to Figure 7.3, this module
uses the Michaelis-Menten function to calculate particulate Fe fluxes to the
deep ocean, using an average Fe : N ratio. The value for the Fe : N ratio applied in the model O.65e-4 mol mol" ) has been determined for species of
marine phytoplankton maintained in laboratory culture (Morel & Hudson
1984).
In the model, the major particulate flux of carbon from the surface water
to the deep ocean and sediments occurs in the form of CaC0 3 burial. We
incorporate in the model a constant background flux (i.e. independent of
Fe availability) of C, N and Fe from calcite-forming organisms (Broecker &
Peng 1982). Calculating downward particulate flux of C, N, and Fe in this
manner assumes that :
(a) ocean nutrient inventorie s remain constant;
(b) the downward flux of particulate material results largely from the
growth and sinking of diatoms (Goldman 1993);
(c) the relative elemental ratios of C : N : Fe in sinking biogenic material
are constant over the time scales considered in the model;
(d) the downward flux of biogenic material or export production in this
model is set by the input of Fe into the surface ocean;
(e) when Fe concentrations are sufficient for maximum N0 3
- uptake the
downward flux of particulate C and, therefore, atmospheric CO 2 concentrations are controlled by, and equal to, the upwelling flux of N0 -
from deep water to the surface ocean. This is also the definition of new
production (Dugdale & Goering 1967; Eppley & Peterson 1979).
Additionally, the model assumes that:
CD downwelling and upwelling fluxes of the dissolved chemical species
are calculated based on the concentration of the species (mol) and the
mean ocean downwelling and upwelling rates of 0.39 m (day)-l
(Broecker & Peng 1982);
(g) particulate matter burial equals 1% of the flux of the particulate matter
to the deep ocean;
(h) particulate matter remineralization occurs on the time scale of ocean
mixing -3000 years (Broecker & Peng 1982). This implies that on
these time scales there is no organic matter preservation (i.e, steadystate) .
Assumptions (a)-(h) link the C, N and Fe portions of the model. Despite a
fixed relation between the incorporation of C, N and Fe into particulate
matter the relative drawdown of carbon from the atmosphere and surface
ocean into the deep ocean is a function of the concentrations of the two
major plant nutrients N and Fe. Changing either the ratio of C : N : Fe in
particulate matter or the absolute uptake of N (mol yr") does not alter the
model 's function-it will only change the relative drawdown or accumulation of CO 2 in the atmosphere and surface ocean. Fluxes of dissolved C, N
3