Early alteration of interstitial water can appreciably hasten sediment diagenesis; this is especially true for calcareous deposits. Thus, from a theoretical standpoint, it is of interest to determine if "original" interstitial water can be preserved as a paleosalinity in areas where post-depositional bottom-water salinity changes characteristically take place, e.g., coastal swamps and deltas.

A model based on the coastal mangrove swamps of southwest Florida, an area where marine swamps recently have replaced fresh-water swamps, was constructed and an equation derived to evaluate the effect of long-term and short-term salinity variations in swamp water on the salinity of water entrapped in the 1-3 meters of sediment underlying the swamps. The model considers ionic diffusion in a homogeneous sediment column overlying an impervious basement and underlying swamp water that seasonally (short term) fluctuates in salinity and that systematically decreases or increases in mean water salinity over periods measured in years (long term).

The model predicts that, in areas where the bottom water has an annual sinusoidal-salinity fluctuation with an amplitude of 35 ^pmil (parts per mil), the interstitial water will be affected significantly (more than 0.2 ^pmil) in the upper 35 cm. of the underlying sediment. This prediction is borne out by field data. If the mean water salinity about which the short-term fluctuations take place is increased or decreased, most of the effect of the drift in the mean is transmitted to the bottom of the sediment column within a few hundred years. Normal sedimentation rates in the swamps have little effect on the rate at which the interstitial water is modified.

Preservation of paleosalinities in the coastal swamps of southwest Florida, or in similar environments, evidently requires special condition, e.g., a recent change in mean bottom-water salinity combined with rapid deposition of sediment. Deposition of sediment isolates underlying deposits from diffusion contact with the changed bottom water.

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