Quantifying Sub-Seasonal Growth Rate Changes in Fossil Giant Clams Using Wavelet Transformation of Daily Mg/Ca Cycles

Shells of the giant clam Tridacna can provide decade-long records of past environmental conditions via their geochemical composition and structurally through growth banding. Counting the daily bands can give an accurate internal age model with high temporal resolution, but daily banding is not always visually retrievable, especially in fossil specimens. We show that daily geochemical cycles (e.g., Mg/Ca) are resolvable via highly spatially resolved laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS; 3 x 33 mu m laser slit) in our Miocene (similar to 10 Ma) specimen, even in areas where daily banding is not visually discernible. By applying wavelet transformation on the measured daily geochemical cycles, we quantify varying daily growth rates throughout the shell. These growth rates are thus used to build an internal age model independent of optical daily band countability. Such an age model can be used to convert the measured elemental ratios from a function of distance to a function of time, which helps evaluate paleoenvironmental proxy data, for example, regarding the timing of sub-seasonal events. Furthermore, the quantification of daily growth rates across the shell facilitates the evaluation of (co)dependencies between growth rates and corresponding elemental compositions. Plain Language Summary Shells of giant clams exhibit growth bands, similar to tree rings, which form in both seasonal (visible by eye) and daily (resolvable by microscope) increments. However, the optical visibility of daily bands in fossil giant clam shells can be poor. Fortunately, growth bands are often accompanied by changes in the chemical composition of the shell. The incorporation of trace elements into the shell depends on environmental factors (like temperature and light) and biological controls, which are both characterized by cyclic daily variation. With our Python script Daydacna, we present a tool that enables daily resolution scale changes in growth rate to be evaluated using daily geochemical cycle lengths, that is, how much the shell has grown each day. Daydacna then creates an internal age model and converts the respective element compositions from being expressed over distance to being expressed over time. This information enables an unambiguous estimate of growth rate to be compared to elemental compositions, enabling (e.g.) potential (co) dependencies of these parameters to be identified. Time-resolved data also allow to determine the timing of seasonal environmental changes, affecting the shell composition, with higher confidence and thus form an important basis for research on the seasonal aspects of the (paleo)climate.

Automated summary

The geochemical composition of giant clam shells can provide records of past environmental conditions, and their growth bands can serve as a proxy for daily and seasonal increments. However, visually identifying daily growth bands in fossil specimens can be challenging. In this study, the authors demonstrate that highly spatially resolved laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) can be used to resolve daily geochemical cycles, even in areas where daily banding is not visually discernible. By quantifying varying daily growth rates, an independent internal age model can be built, which helps evaluate paleoenvironmental proxy data and investigate the timing of sub-seasonal events.