Seasonal geochemical and growth rate variabilities in a Miocene giant clam

Tridacna are important archives for (sub)tropical marine palaeoenvironmental conditions. Their longevity (up to 100 years), large aragonitic shells (up to 1m) and rapid shell accretion (mm-cm/year) make them ideal to give insights into region specific climate and environmental variability. Highly spatially-resolved analytical techniques such as LA-ICPMS mean that geochemical data can be retrieved at a high temporal resolution (subdaily). Tridacna, with their daily banding and growth rates of tens of µm/day, are ideal candidates for applying this methodology evaluating seasonality and extreme weather events in (sub-)tropical reefs since their appearance in the early Miocene. Studies of seasonal records and information on extreme weather events from past climate settings can help inform model assessment exercises regarding how seasonality might change in future climate scenarios. In this study we present a multiproxy record from a 250 mm large late Miocene Tridacna from East Borneo spanning several decades, with subseasonally resolved stable δ¹⁸O and δ¹³C data and sub-daily resolved elemental ratio data (B, Na, Mg, Sr, Ba to Ca). By applying Daydacna, a recently developed python script that enables daily cycle based internal age modelling, we can create an age model of the shell, which forms the basis for the temporal reassignment of the geochemical data. Displaying geochemical data against time rather than shell distance improves multi-annual as well as inter-organism comparisons for palaeoseasonality reconstructions. We reconstruct seasonal growth rate variability and compare it to the corresponding elemental and isotopic ratios to evaluate the relationships between geochemical signals, shell growth and environmental parameters.