File last modified 26 November 1996
18.1 General Theory
We will be talking about the process of diagenesis, i.e. the sum total of all processes that bring about changes to sediments after they have been deposited. This includes everything from compaction and bioturbation to metamorphism and weathering after uplift. But of particular interest to marine geochemists, chemical oceanographers, biological oceanographers, marine geologists, etc. is Early Diagenesis, that is to say, changes that occur during burial and down to a few hundred meters.
It is important to keep in mind that in sedimentary diagenesis, your coordinate frame is sinking. For example, consider some property of the sediment column, p:
where z represents depth (positive downwards) and t represents time. But which level do we choose
for 
?a particular layer within the sediment column? or the seawater-sediment interface? We can
write:
where 
is simply the rate of deposition without any of the complicating factors. Since, with the
exception of paleo-types, we are interested in processes operating on time scales commensurate with
depths no greater than a few hundred meters, we generally choose the seawater-sediment interface as
z=0, positive downward. In this fashion equation (18.1.2) becomes:
Now if we assume (for the moment) that 
is constant with time and depth, we can examine two
scenarios that cover most of the processes we are interested in: burial without diagenesis and burial
with diagenesis.
In the first case, no diagenesis, 
and we can write:
and we are talking about historical changes that occurred at the time of deposition. If you look at Fig. 18.1.1 the consequences are displayed.
Figure 18.1.1 shows a situation that typically applies to the solids of the sedimentary column.
Now suppose the sedimentation remained constant, historically, but the property in the sediment column underwent chemical changes as it was buried, i.e. with the passage of time. This gives rise to the concept of steady state diagenesis:
which implies:
that is to say, the source at the seawater-sediment interface has been historically constant. Figure 18.1.2 displays the consequences of this situation.
When we consider the mass balance of material in a box of sediment as simply the difference between what goes in, what goes out, and what is produced/consumed on the premises we can write:
where 
is concentration of component i per volume of total sediment, Fi is the flux of component i
per unit area per unit time, and Ri is the rate of diagenetic change in mass per total sediment volume
per unit time. Ans as we know, fluxes can come in two forms: diffusive and advective:
where in this case v is the flow relative to the seawater-sediment interface. For solids 
, that is,
the rate of burial. For pore waters, v is generally different than 
. Now we can put it all together
and obtain:
which is the general diagenetic equation and it should look familiar by now.
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