Visualising and quantifying Mg/Ca and Sr/Ca heterogeneity in the isochronous growth increments of bivalve shells (Tridacna)

Ultra-high (hourly to daily) and high (annual) resolution paleoenvironmental proxies derived from geological and archaeological sclerochronology archives provide us with a window into ancient times. As one of the most common and well-developed archives, bivalve shells play a very important role in reconstructing past environments and connecting palaeo-environmental records with past human behaviours. However, we still lack a basic understanding of whether bivalve shells have evenly distributed trace element abundances in shell portions that grew at the same time (i.e., isochronous growth increments). In this study, we present the first published application of laser-induced breakdown spectroscopy (LIBS) for qualitative two-dimensional elemental mapping of Tridacna shells. To complement the qualitative results, we also employed laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and wet-chemistry trace element analyses to acquire quantitative composition of Mg/Ca and Sr/Ca in Tridacna maxima shells, addressing the question of spatial heterogeneity in shell geochemistry. Our results reveal significant spatial heterogeneity in Mg/Ca and Sr/Ca ratios within contemporaneous increments, which could lead to a ± 4.3 to 5.5 °C variance in palaeo sea surface temperature reconstructions. This heterogeneous distribution may result from varying environmental exposure in different areas of a shell influencing the sensitivity and efficiency of minor and trace element incorporation in the different regions of the shell's epithelium tissue. By better understanding the natural heterogeneity of trace element uptake across isochronous growth increments, this study makes a fundamental step forward in assessing the reliability of sea surface temperature reconstructions and guiding future data acquisition strategies. • El/Ca distribution of Tridacna shell's isochronous growth increment was assessed. • Qualitative LIBS mapping intuitively shows heterogeneous distributions of El/Ca. • Quantitative LA- and Q-ICPMS results indicate significant heterogeneity of El/Ca. • Heterogeneous distributions can cause ±4.3 to 5.5 °C difference in SST reconstruction. • Consistent sampling location within the shell is essential when using El/Ca proxies.