Clams are biological chart recorders: their shells contain a record of environmental conditions in the form of geochemical profiles. Are these records always easily interpretable? Modeled stable oxygen isotope profiles based on several patterns of growth suggest the answer is no. They indicate that through a clam's life the recorder runs at different speeds and there are changing thresholds at which the recorder turns on and off.

I generated three sets of hypothetical inter-annual oxygen isotope profiles. First, I generated a profile in which the full range of environmental conditions was represented. Second, profiles were modeled with an ontogenetic decrease in growth rate, a feature shared by most bivalve mollusks. This growth rate decline has long been recognized and can result from changes in the growth period, daily growth rate, or both. Altering the growth period simulates the effects of thermal thresholds, above or below which no shell material is deposited. Changes in the daily growth rate produce slower annual shell growth while keeping the growth period constant. Combining the two provides a more accurate representation of bivalve shell growth in many subtropical and temperate species. Third, profiles were modeled based on actual patterns of daily growth increment width variation observed in northern Gulf of California bivalves. These models indicate that geochemical profiles are strongly influenced by changes in subannual growth patterns.

Comparison of modeling results with observed oxygen isotope profiles indicates that the fullest range of environmental conditions is only reflected in the earliest years of growth and profiles from successive years can have reduced amplitudes, sample resolutions, or both. Within-shell trends in annual isotopic amplitudes and averages may reflect decreases in growth rate rather than environmental trends especially in long-lived species. Sampling strategies should account for changes in growth rate, otherwise comparisons between different shells should be based on profiles from the same ontogenetic year. Furthermore, because clams of the same species, living in different places, can record conditions differently, care should be taken when comparing profiles from different regions or climatic regimes.