Giant clams as multidecadal deep-time climate snapshots – mineralization, calibration & application to the Indonesian Throughflow across the mid-Miocene transition

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The transition from the Miocene Climate Optimum to later Miocene conditions, the Mid-Miocene Climate Transition (MMCT, ~14.5-13 Ma), represents a key interval in Earth’s late Cenozoic cooling, characterized by ~3°C global temperature decrease and associated changes in ice cover, atmospheric and oceanic circulation. We aim to investigate the effects of this climate transition for the tropical Indonesian Throughflow at high time resolution by obtaining few decade-long ‘snapshots’ before and after the MMCT using giant clams (Tridacna) as climate archives. Such reconstructions are important as it remains unresolved whether deep-time (pre-Pleistocene) tropical seasonality was increased relative to modern. In addition, Tridacnas record sub-seasonal extreme weather event (EWE)-related environmental variability, the study of which during high(er) CO2 intervals is of interest due to the expected EWE-increase from anthropogenically-induced warming. Past temperature reconstructions via geochemical proxies, however, can be biased by salinity, seawater composition and kinetics. We plan to provide palaeotemperatures unbiased by these influences by using recently-developed dual-clumped isotopic analysis (Δ47, Δ48) and trace element analysis. Giant clams have good ‘deep-time’ preservation potential and, due to their size, are ideally suited for (sub-)seasonally-resolved analysis by material-intense methods such as clumped isotopes. Tridacna provide up to daily-resolved climate snapshots and record EWEs especially in their elemental composition (El/Ca-ratios). To enable reliable palaeoclimate reconstructions we will evaluate the influence of shell precipitation kinetics on Δ47, Δ48, and the impact of salinity and seawater chemistry on El/Ca ratios by culturing clams under controlled environmental conditions. Following on from our previous successful culturing, three experiments will investigate the influence of varying seawater Ca-concentration, salinity and temperature. Subsequently we will conduct multiproxy geochemical analyses (El/Ca, δ18O, δ13C, Δ47, Δ48, 87Sr/86Sr) of these shells at (sub)annual to daily time resolution, using LA-ICPMS, stable and clumped isotope analysis. This will allow us to assess the impact of precipitation kinetics on giant clam shells (Δ47, Δ48) and the effect of seawater compositions (Ca-concentration and salinity/δ18O) on the elemental and isotopic compositions of Tridacna. Applying these results to exceptionally-preserved fossil samples, we will reconstruct mean annual temperature variations across the MMCT and Miocene tropical seasonal temperature variability. Multiproxy data on seasonal to daily time scales will reveal the frequency of extreme e.g. runoff or upwelling events. We hypothesize a decrease in EWE-frequency and seasonal precipitation variability and an increase in temperature variability with cooling across the MMCT. Overall, such palaeoclimate data on human time-scales are relevant for testing climate simulations.