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

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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.