The four major Isotopes of Dolomite (C, O, Mg, Ca): Closing the Calcium Isotope (d44/42Ca) Gap

白云石是一种由Ca2+、Mg2+和CO32-组成的造岩矿物，自然存在于多种沉积成岩环境中。由于其在深部记录中的优势，各种白云岩的元素和同位素代用资料得到了广泛的研究。相比之下，白云岩钙同位素特征的数据仍然很少。这与非化学计量白云岩有关，因为地球历史上大部分白云岩通常是富钙的（Ca:Mg的摩尔比为1），因此，Ca同位素（d44/40Ca或d44/42Ca）特征显然值得关注。支持者和同事汇编的先前数据集表明，白云岩的Ca同位素特征是可变的，重要的分馏过程影响了早期成岩白云岩。同样，（替代和沉淀）白云岩Ca同位素比值数据反映了固流交换过程。本文认为，由于几种关键元素的高浓度，早期海相成岩白云岩是一种复杂但有前途的多同位素代理方法靶材料。因此，前人的研究表明，不同早成岩白云岩的d44/42Ca、d26Mg、d13C和d18O同位素特征代表了不同的成岩域和成岩时间间隔。本项目旨在大大超越这一点，并采用双重方法：(i)分析选定野外地区和时间间隔的白云岩/白云岩样品。重点放在早期成岩阶段：海洋蒸发，即sabkha，回流，非海洋蒸发和paluine环境。考虑到动力学和微生物地质的限制，我们打算测试这些早期成岩结构作为其原始和演化的海洋孔隙流体档案的潜力。这需要在众所周知的物理化学条件下进行白云岩形成实验进行校准（格拉茨科技大学）。低温实验（25-60℃）下无定形碳酸钙/镁，高温实验（100-250℃）下文石将作为种子材料。这两种实验装置都是对天然白云岩环境的合理模拟。该建议的目标是：(i)建立早期成岩白云岩类型钙同位素特征的严格限定特征；（ii）结合多代理Ca、Mg、C、O（和稳定的Sr）同位素数据，通过评估协方差模式来降低数据集的噪声，增强数据集的信号；（三）评价不同化学计量白云岩类型的形成和成岩蚀变及其相对于同沉积非化学计量相的同位素和元素模式。总的来说，我们的目标是为社区提供基础，以建立一个约束良好的古海水档案。

Dolomite is a rock forming mineral that consists of Ca2+, Mg2+, and CO32- and naturally occurs in a variety of sedimentological and diagenetic environments. Owing to its predominance in the deep time record, elemental and isotopic proxy data from various dolomites have been extensively studied. In contrast, data on the calcium isotope signatures of dolomites remain scarce. This is relevant as non-stoichiometric dolomites, the bulk of dolomites in Earth’s history, are often Ca-rich (molar ratio of Ca:Mg>1) and hence, Ca isotope (d44/40Ca or d44/42Ca) signatures clearly deserve attention. Previous data sets compiled by the proponents and coworkers document that dolomite Ca isotope signatures are variable and significant fractionation processes affect early diagenetic dolomite. Along similar lines, (replacive and precipitative) dolostone Ca isotope ratio data reflect solid-fluid exchange processes. Here we argue that due to high concentrations of several key elements, early marine diagenetic dolomites are a complex but promising target material for multi-isotope-proxy approaches. Accordingly, previous work of the applicants documented that the d44/42Ca, d26Mg, d13C, and d18O isotope signatures of a variety of early diagenetic dolomites/dolostones represent distinct diagenetic domains and time intervals. This project aims to go significantly beyond this point and follows a dual approach: (i) analysis of dolomite/dolostone samples from selected field areas and time intervals. The focus is placed on early diagenetic phases: marine evaporative, i.e. sabkha, reflux, non-marine evaporative, and palustrine environments. We intend to test the potential of these early diagenetic fabrics as archives of their primary and evolved marine pore-fluids considering kinetics and microbiogeological constraints. (ii) This requires calibration by means of dolomite formation experiments at well-known physicochemical conditions (Graz University of Technology). Amorphous calcium/magnesium carbonate at low temperature experiments (25-60°C), and aragonite at elevated temperatures (100-250°C) will be used as seed material. Both experimental setups are reasonable analogues to natural dolomite environments. The objectives of the proposal are to: (i) Establish a well-constrained characterization of Ca isotope signatures for early diagenetic dolomite types; (ii) Combine multi-proxy Ca, Mg, C, O (and stable Sr) isotope data in order to reduce noise and enhance signal in data sets by assessing covariance patterns; and (iii) Assess the formation and diagenetic alteration of different stoichiometric dolomite types and their isotopic and elemental patterns relative to syn-sedimentary non-stoichiometric phases. Overall, we aim to provide the community with the foundations to establish a well-constrained palaeo-seawater archive.