Reconstructing seasonal growth rate changes in fossil giant clams using sub-daily resolved LA-ICPMS

Bivalves are important environmental archives that can provide past "climate snapshots" at high temporal resolution. The long-lived giant clam Tridacna builds large (< 1 m), dense aragonite shells in which seasonal to daily bands are recorded both structurally and chemically. Therefore, Tridacna have been key archives of seasonality and extreme weather events in (sub)tropical reefs since their appearance in the early Miocene. Detailed ontogenetic ages and growth rates can be assessed by counting the daily bands and measuring their intervals, which range from a few micrometres to tens of micrometres. However, in many areas of our late Miocene Tridacna shell, daily bands were not continuously countable. Continuous optical analyses of daily bands are also time-consuming in organisms that can live for up to a century. A more efficient approach is to utilize daily geochemical cycles, measured via ultrahigh-resolution Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). First, daily geochemical cycles were recognizable even in areas where we failed to identify daily bands optically. Second, through computational evaluation of the daily cycles’ wavelengths (e.g., wavelet transform) we can develop daily growth rate models. Consequently, we can track the ontogenetic age and the growth-rate variability and compare it to the corresponding (sub)seasonal geochemical data (δ18O, El/Ca). This allows us to evaluate the relationships between geochemical signals, environmental parameters, and shell growth to tackle growth rate and ontogenetic effects as well as seasonal biases. Displaying geochemical data against time rather than shell distance further improves multi-annual and inter-organism comparisons of palaeoseasonality reconstructions.