Integrating high-resolution Sr/Ca and ultrastructural analyses of the Tridacna squamosa shell to reconstruct sub-daily seawater temperature variation

Abstract

Modern and fossil bivalves record environmental variability in their shells in the form of chemical and ultrastructural properties as well as changes in growth rate. These proxy data can be placed in precise temporal context based on growth pattern analysis. Some species such as tridacnids grow particularly fast providing unique insights into environmental changes on the time scale of weather which opens new opportunities for paleoclimate research. Here, we assessed the potential use of the fluted giant clam (Tridacna squamosa) to reconstruct sub-daily sea surface temperature (SST) fluctuations from shell Sr/Ca ratios. Through a combined μm-scale analysis of shell Sr/Ca (NanoSIMS) and ultrastructure (SEM) it was possible to study shell material produced during daytime (growth increments) and nighttime (growth lines) separately. Unlike coarser resolution chemical analysis (LA-ICP-MS and ICP-OES), this approach revealed a significant positive correlation between SST and Sr/Ca during daytime (R2 = 0.36, p < 0.001). The correlation further increased when the NanoSIMS data of several consecutive daily increments were combined to match the sampling resolution of LA-ICP-MS and ICP-OES data, i.e., four days to two weeks (R2 of up to 0.86, p < 0.001). With an uncertainty of at least ± 1.5 °C, the applicability of the Sr/Ca thermometer remains limited considering that T. squamosa only occurs in ecosystems with minimal seasonal temperature amplitudes. Consistent daily Sr/Ca cycles were observed with local maxima at growth lines. This cyclic pattern was found even when the ultrastructure morphology varied or when the complex crossed-lamellar ultrastructure of the shell deviated from its typical configuration during extreme weather events. Therefore, Sr/Ca is likely not directly linked to the shell ultrastructure, but instead both properties are driven by underlying physiological factors.