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

中中新世气候过渡（MMCT, ~14.5 ~ 13 Ma）是中新世晚期气候变冷的关键时期，其特征是全球气温下降~3°C，并伴随冰盖、大气和海洋环流的变化。我们的目标是通过使用巨蛤（砗磲）作为气候档案，获得MMCT前后几十年的“快照”，以高时间分辨率研究这种气候转变对热带印度尼西亚通流的影响。这种重建是重要的，因为它仍然没有解决是否深时间（前更新世）热带季节性增加相对于现代。此外，三叉带记录了与亚季节极端天气事件（EWE）相关的环境变率，在高（er） CO2间隔期间对其进行研究是有意义的，因为人为引起的变暖预计会增加EWE。然而，过去通过地球化学指标重建的温度可能受到盐度、海水成分和动力学的影响。我们计划利用最近发展的双团块同位素分析（Δ47， Δ48）和微量元素分析来提供不受这些影响的古温度。巨型蛤蜊具有良好的“深时间”保存潜力，并且由于它们的大小，非常适合通过聚集同位素等材料密集方法进行（亚）季节性分辨分析。Tridacna提供每日解析的气候快照，并记录ewe，特别是其元素组成（El/ ca比）。为了实现可靠的古气候重建，我们将通过在受控环境条件下养殖蛤蜊，评估贝壳降水动力学对Δ47， Δ48的影响，以及盐度和海水化学对El/Ca比的影响。在我们之前成功培养的基础上，我们将进行三个实验来研究海水钙浓度、盐度和温度的变化对海藻的影响。随后，我们将利用LA-ICPMS、稳定和团块同位素分析，在（亚）年至日的时间分辨率上对这些壳进行多代理地球化学分析（El/Ca、δ18O、δ13C、Δ47、Δ48、87Sr/86Sr）。这将使我们能够评估降水动力学对巨型蛤壳的影响（Δ47， Δ48）以及海水成分（ca浓度和盐度/δ18O）对砗磲元素和同位素组成的影响。将这些结果应用于保存完好的化石样本，我们将重建MMCT和中新世热带季节温度变化的年平均温度变化。从季节到每日时间尺度的多代理数据将揭示极端事件的频率，例如径流或上升流事件。我们假设ewe频率和季节性降水变异性降低，而随着整个MMCT的冷却，温度变异性增加。总的来说，这些人类时间尺度上的古气候数据与测试气候模拟有关。

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.