Examination of ontogenetic variation in the stable isotopic composition of accretionary biogenic hard parts is becoming a commonplace technique for gaining insights into organism age, changes in growth rate through ontogeny, and environmental conditions under which organisms lived. However, the application of isotopic data to life-history studies often is impeded by the noisy nature of environmental signals, as well as by seasonal cessation of hardpart accretion. A computational approach is presented that quantitatively resolves secular (seasonal) change in environmental parameters such as temperature and salinity with spatial (hard part) variation in isotopic composition. Output consists of determinations of mean annual isotopic composition, seasonal range in isotopic composition, and variation in hard-part growth rate for any interval of data in an accretionary transect. Quantitative estimates of absolute growth rates, as well as the proportion of each season represented by hard-part accretion, are derived for every season of the organism's life recorded in their hard parts.

This methodology is applied to δ18O data from a valve of the surf clam, Spisula solidissima, collected live from the New Jersey shelf. Based on the saw-toothed pattern of δ18O variation in this clam, and on the fact that observed amplitudes of δ18O variation are only a fraction of those anticipated from measurements of ambient water salinity and temperature, it is apparent that growth cessation occurred during each of the dozen summers and dozen winters of ontogeny. Determination of best-fit sinusoids for each season of growth reveals that: (1) rate of growth decreased exponentially during the ontogeny of this individual; (2) net spring season accretion was initially twice that in the fall; and (3) ontogenetic decrease in net accretion was accomplished in part by reduction in rate of accretion, and in part by reduction in the number of spring and fall days during which growth occurred. Isotope thermometry suggests that all growth intervals span an optimum temperature; shorter (or longer) durations of seasonal shell accretion occur over narrower (or wider) temperature ranges centered on about 11°C.

Beyond the obvious paleoenvironmental applications, the advantage of this method for biologists and paleontologists is that rate and duration of growth can be approximated at seasonal to subseasonal levels of resolution for any living or fossil organism. Such information from a suite of individuals can reveal how growth within a species is affected by changes in latitude, depth, productivity, anthropogenic influence, and other environmental gradients. In addition, because changes in rate and timing of growth within a lineage are the mainstay of heterochrony studies, quantitative methods for assessing such variables through time in fossil specimens will allow more rigorous testing of evolutionary hypotheses.

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