Early diagenesis of corals generated by variable activity of endolithic algae and its impact on geochemical proxies.

文石珊瑚被广泛认为是其过去海洋环境的合适档案和海平面演变的监测仪。骨骼的化学和同位素特征的变化可以根据珊瑚分泌生物矿物质时的海水温度、盐度和pH等环境参数进行校准。核状珊瑚的地球化学代理方法受到了挑战，因为处理早期珊瑚成岩作用的实际研究表明，矿物学和化学变化可能在死后立即开始，甚至在活珊瑚的骨骼下，在埋葬或暴露在空中之前。这方面的证据来自其他微量元素比例，它们在最初共生骨骼部分的代理信息中存在显著差异。前人主要讨论了原生生物文石的成岩作用、非生物文石的长入、原生珊瑚骨架的溶解、再结晶和向次生方解石的转变等物理化学过程。我们认为这些过程应该从早期生物成因因素，特别是内生藻类在珊瑚碳酸盐岩成岩作用中的作用的角度来重新审视。内生藻构成了热带珊瑚的共同特征，生活在骨骼孔隙空间内。珊瑚内的藻类活动包括通过光合作用溶解骨架和增加pH值。后者是在珊瑚特定分馏系统之外具有化学特征的次生相的诱导沉淀。因此，这两个过程可能强烈影响碳酸盐稳定性和替代记录的可靠性，可能对早期珊瑚成岩作用和替代校准方法产生重大影响。这项研究得益于低分辨率和高分辨率结构和矿物学调查技术的创新组合，以及元素和同位素地球化学分析。获得的数据将全面了解孔隙体积特征和矿物组成，为解释元素（Li/Ca、B/Ca、Mg/Ca、Sr/Ca、I/Ca Ba/Ca U/Ca）和同位素地球化学指标（delta11B、delta13C、delta18O、delta26Mg、delta44/40Ca、delta88/86Sr、87Sr/86Sr、U- th地质年代学）的空间变化提供基础。根据这些结果，我们的目标是详细了解内生藻类活动对珊瑚早期成岩作用的影响。

Aragonitic corals are widely considered as suitable archives of their past marine environment and as monitors of sea-level evolution. Variations in the chemical and isotope signatures of the skeleton can be calibrated to environmental parameters such as seawater temperature, salinity, and pH at time of coral biomineral secretion. The geochemical proxy approach on scleractinian corals is challenged because actual studies dealing with early coral diagenesis revealed that mineralogical and chemical alteration may commence immediately post mortem or even in the skeleton underneath living corals, prior to burial or sub-aerial exposure.. Evidence for this comes among others from trace element ratios significantly differing in their proxy information for initially cogenetic skeleton parts. Previous workers have discussed mainly physico-chemical processes including diagenesis of primary biogenic aragonite, ingrowth of abiogenic aragonite, dissolution of the primary coral skeleton, recrystallization and transformation to secondary calcite. We argue that these processes should be readdressed from the viewpoint of early biogenic factors, specifically the role of endolithic algae on coral carbonate diagenesis. Endolithic algae constitute a common feature of tropical corals, living within the skeleton pore space. Algae activity within the coral includes skeleton dissolution and pH increase by photosynthesis. The latter is inducing precipitation of secondary phases with chemical signatures outside coral specific fractionation systematics. Consequently, both processes may strongly influence the carbonate stability and proxy record reliability, potentially representing a significant impact on early coral diagenesis and proxy calibration approaches.This study benefits from an innovative combination of low- and high-resolution techniques for structural and mineralogical investigation, as well as for element and isotope geochemical analyses. The obtained data will provide a comprehensive understanding of pore volume characteristics and mineralogical composition as basis for the interpretation of spatial variations in elemental (Li/Ca, B/Ca, Mg/Ca, Sr/Ca, I/Ca Ba/Ca U/Ca,) and isotope geochemical proxies (delta11B, delta13C, delta18O, delta26Mg, delta44/40Ca, delta88/86Sr, 87Sr/86Sr, U-Th geochronology). From these results we aim to achieve a detailed understanding of the impact of endolithic algae activity on coral early diagenesis.