We treat one-dimensional models of sedimentary systems separately in this book because of the added complication that they contain two phases – solid material and pore waters – that not only can interact biogeochemically, exchanging chemicals, but also can move in relation to one another. In fact, with a reference system fixed at the sediment–water interface, the solid phase is actually moving owing to a combination of sedimentation (addition of material at the interface) and compaction. If you're thinking that this makes the construction of models a little more complicated, those are exactly our sediments!

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 on the seafloor. This includes everything from bioturbation through chemical transformation to compaction and pore water extrusion. The general topic of diagenesis extends to even longer timescale processes that include metamorphism and weathering of sedimentary rocks after uplift, but we will focus on Early Diagenesis (Berner, 1980), which encompasses changes that occur at or near the sedimentary surface or in the upper portion of the sedimentary column.