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

中-中新世气候过渡期(MMCT，~14.5~13 Ma)是地球晚新生代降温的关键时期，其特征是全球气温下降约3℃，并伴随着冰盖、大气和海洋环流的变化。我们的目标是通过在MMCT之前和之后获取几张长达十年的“快照”，以巨蛤(Tridacna)作为气候档案，研究这种气候转变对高时间分辨率热带印度尼西亚Throughflow的影响。这样的重建很重要，因为深部(前更新世)热带季节是否相对于现代增加了，这一问题仍未得到解决。此外，Tridacna记录了与极端天气事件(EWE)相关的亚季节极端天气事件(EWE)相关的环境变异性，由于预期人类导致的变暖导致EWE增加，因此对高(Er)CO2期间的环境变异性的研究令人感兴趣。然而，过去通过地球化学指标进行的温度重建可能会受到盐度、海水成分和动力学的影响。我们计划使用最近发展起来的双束同位素分析(Δ47，Δ48)和微量元素分析来提供不受这些影响的古温度。巨大的文蛤有很好的“深层”保存潜力，由于它们的大小，非常适合通过块状同位素等物质密集型方法进行(次)季节性分解分析。Tridacna提供高达每日分辨率的气候快照和记录母羊，特别是它们的元素组成(EL/Ca比率)。为了能够可靠地重建古气候，我们将通过在受控环境条件下养殖文蛤来评估贝壳沉淀动力学对Δ47和Δ48的影响，以及盐度和海水化学对EL/Ca比值的影响。在我们之前成功培养的基础上，将进行三个实验来研究不同的海水钙浓度、盐度和温度对藻类生长的影响。随后，我们将使用LA-ICPMS、稳定和块状同位素分析，在(亚)年到日分辨率上对这些壳体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沉淀动力学对巨型蛤壳(Δ47，Δ48)的影响，以及海水成分(钙浓度和盐度/δ18O)对Tridacna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率的年平均温度变化。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，随着MMCT的降温，羊群频率和季节降水变率减少，而温度变率增加。总体而言，这些人类时间尺度上的古气候数据与气候模拟测试有关。

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.