The life and time of clams: Derivation of intra-annual growth rates from high-resolution oxygen isotope profiles

The paired analysis of subannual microincrements and serially-sampled oxygen isotope data affords insights into the physiological and environmental conditions controlling bivalve growth. However, daily microincrements are faint or absent in many taxa, difficult or ambiguous to count under the best conditions, and are often constrained to the earliest years of ontogeny, limiting the practicality of intra-annual comparisons between growth rate and environmental parameters. We present a computational approach to derive growth rates using only serially-sampled oxygen isotope data, thereby allowing for broader application of bivalve growth rate studies. Variation in the isotopic composition of shell carbonate along an ontogenetic sampling profile reflects temporal variation in ambient water temperatures, while variation in the position of isotope values in the distance domain reflects intra-annual variation in rates of shell accretion. Thus, the shape of the isotope profile in distance space over a given year records the concomitant influence of subannual variation in temperature (y-axis) and growth rate (x-axis). Presumption that annual variation in temperature is sinusoidal allows for the determination of an intra-annual growth rate function that best approximates observed δ¹⁸O_carb data. The fidelity of the approach is affirmed using synthetically generated isotope datasets and previously published isotope profiles. The method offers a variety of applications to sclerochronologic studies. Using only oxygen isotope values determined along an ontogenetic trajectory, the approach can be used to quantify spatial and temporal patterns of intra-annual accretion within populations. Additionally, the model can be applied to long time series such that year-to-year variations in the growth profiles can be used to identify ontogenetic and climatic trends. While we focus here on isotope-distance records from bivalve shell carbonate, this method is equally applicable to similar data from other accretionary biogenic skeletal materials.