INTERPRETING LIFECYCLE BEHAVIOR OF EXTINCT BIVALVE SPECIES THROUGH STABLE AND CLUMPED ISOTOPIC VARIATIONS

Isotopic analysis of bivalve shell material can be used to determine climate conditions in the past, but potential biases may be introduced due to changing lifecycle behavior through time (e.g. seasonal biases) and which portion of the shell is sampled. Shell growth has been shown to decrease or ceases altogether during the season of reproduction (which occurs only later in life), as well as when environmental temperatures are above or below thermal thresholds. This can bias the isotopic composition recorded in shell material precipitated later in life and can therefore skew inferred temperatures, depending on the location of sampling. In this study, we investigate these biases in two extinct, Cretaceous-age bivalve species, Lahillia larseni and Cucullaea sp., from Seymour Island, Antarctica. We perform low-, medium-, and high-resolution sampling and analyze the carbonate powder for its stable (δ¹⁸O, δ¹³C) and clumped isotopic composition (Δ⁴⁷), from which we calculate past temperature and the isotopic composition of the inhabited seawater (δ¹⁸O_water). With this sampling method, isotopic differences between shell positions can be interpreted to determine the season and age of onset of reproduction. For L. larseni, we find reproduction occurred in the summer and began when the bivalve was ~3-5 years old. For Cucullaea sp., we find reproduction occurred in late winter to early spring. This work highlights the need to understand bivalve lifecycle behavior and potential seasonal biases in order to better select shell sampling locations. This knowledge can be applied to bivalves living before, during, and after the K-Pg extinction to investigate the effects of environmental stresses on lifecycle behavior.