Examination of intertidal mud flat and tidal creek sediments from Sapelo Island, Georgia, and Indian Neck, Connecticut, indicates that variations in the degree of calcium carbonate preservation exist between these localities. Although the preservation of calcium carbonate in modern nearshore terrigenous marine muds has previously been shown to be strongly dependent on the physical and biological attributes of the depositional environment (Berger and Soutar 1970; Sholkovitz 1973; Aller 1982), the phenomenon documented in this study occurs in the absence of significant variations in benthic faunas or physical conditions. Evidence for variations in the degree of calcium carbonate preservation is provided by analyses of sediment chemistry and observations of mollusc shells recovered from the sediments. The pattern of sediment chemistry and calcium carbonate preservation is shown to be related to seasonal climatic variations and the metabolizability of organic matter supplied to the sediments. During the summer the presence of readily metabolizable organic matter in the Connecticut sediments results in high rates of microbial decay, establishment of anoxic-sulfidic conditions at the sediment-water interface, and the production of large amounts of ferrous sulfide minerals. During periods of reduced microbial activity (winter), ferrous sulfide minerals in the surficial sediments are oxidized, resulting in dramatically lower in situ pH values, and undersaturation with respect to calcium carbonate. In Georgia, temperatures remain relatively warm throughout the year and therefore macrofaunal and microbial activity do not exhibit the extreme seasonal variations present in Connecticut. Because the destruction of reactive organic matter continues throughout the year, the organic matter supplied to the Georgia sediments is less readily metabolizable. As a result, the inception of anoxic-sulfidic conditions is depressed below the sediment-water interface; only limited amounts of ferrous sulfide minerals are formed in the surficial sediments; and saturation or supersaturation with respect to calcium carbonate is maintained throughout the year. This relationship between organic matter metabolizability and calcium carbonate preservation has the potential to be used as an indicator of the environmental conditions present at the time of sediment deposition. Application of the results of this modern sediment study to facies-dependent variations in calcium carbonate preservation in ancient marine shales suggests that prior interpretations of the paleoenvironmental significance of these variations may be in error.