A numerical model for carbonate (shell) accumulation in marine sediments is proposed. Sediment shelliness is controlled by carbonate addition, taphonomic loss, carbonate reorganizing processes, and sedimentation rate. Using representative rates of carbonate production, taphonomic loss, and sedimentary carbonate content, the model shows that insufficient carbonate is produced today in many environments to explain sedimentary carbonate content and that most produced carbonate must be preseIVed despite a generally high capacity for taphonomic loss. An anthropogenically-produced decrease in carbonate production over the last ∼100 yr may explain the former. Representative rates of burial and sedimentation, and a temporal and spatial offset between carbonate production and organic matter decomposition can permit most produced carbonate to be preserved in sediments where taphonomic capacity greatly exceeds the carbonate production rate. The requirement that most carbonate be preserved, despite the observation that most individuals are not, indicates that most adults are preserved and reinforces the finding that biomass is a valuable community attribute for paleoecologic analysis. The requirement that most carbonate be preserved indicates that taphonomic loss must be restricted to the nearsurface in most habitats rather than being distributed throughout the bioturbated zone. The distribution and concentration of carbonate in sediments are partially decoupled from preservational processes because many processes affecting carbonate distribution have little effect on preservation. The time scales of the two differ. Preservational processes usually occur on time scales too short to be recorded as variations in carbonate content with depth. Evidence of preservational processes probably resides solely in the taphonomic signature of shells, hence emphasizing the importance of taphofacies analysis.