Diagenesis in estuarine sediments is affected by broad physical and chemical environmental patterns, the nature of which is becoming clear. The bottom environment is influenced most fundamentally by physical conditions, such as the pattern of water movements and the stability of the water column from place to place. Physical parameters are distinctly different in three major regimes of estuaries, the distributary reaches, the mixing zone, and the saline basin. Location and extent of each of these regimes are determined in any system by the interaction among fresh-water discharge, tidal range, channel geometry, and wind effects. The Rappahannock River estuary and the Po River delta illustrate contrasts in such physical differentiation.

In estuaries the environment for diagenesis is imposed by a determining physical background and the supplemental operation of chemical and biological processes. Variations in pH, Eh, and in dissolved silica and attainment of sulfide equilibrium illustrate the interaction between physical milieu and bottom chemistry. There is a class of chemical interactions (pH adjustment, ion exchange) which proceed instantaneously when fresh-water sediment enters the marine zone. There is another class that requires physical stability in the bottom sediments before equilibrium is approached (dissolution of silica, sulfide equilibrium). This latter class of reactions is the starting point for diagenesis. The initial stages of diagenesis begin just below the sediment-water interface in estuaries.

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