A pioneer geochemical approach applied on carbonate cements to decipher past environmental changes

For more than 40 years, carbonate cements have been the target of many geochemical studies whose focused mostly on the determination of carbon and oxygen isotopic ratios. However, this isotopic information can sometimes be insufficient to precisely define the cements' precipitation environment. The characterization of cement precipitation over time has also always been a source of interrogation since the diagenetic events can only be relatively dated, making impossible to outline the events with high temporal precision. To face these problems, we explored a multi-geochemical approach to characterize carbonate cements. The developed geochemical workflow relies on different and complementary methods: cathodoluminescence, REEY, δ18O and δ13C analyses of calcite cements directly measured on thin-sections, and in-situ dating of carbonate cements using U-Pb method. The combination of different environmental proxies, such as carbon and oxygen as well as REE content, associated with the first absolute dating of cements, permitted to solve the diagenetic evolution of the Upper Triassic Panthalassan Dalnegorsk atoll-type carbonate system (Russian Far East) into 10 diagenetic episodes. The acquired data allowed us to link each episode to specific environmental changes, including events never described in the Triassic and Jurassic of the Panthalassa Ocean. Thanks to these promising results, and to improve the very strong transversal potential of this geochemical approach, the depiction of geochemical aspects (i.e., δ18O, δ13C, REEY, U-Pb dating) of well-preserved calcite cements from an other Upper Triassic carbonate system from the Panthalassa Ocean (i.e., Sambosan Limestone, Sambosan Accretionary Complex, Japan) is currently investigated. The resulting interpretations will then be compared with the carbonate system previously studied using the same approach, such as Dalnegorsk Limestone, Taukha Terrane, Russian Far East. The analysis of similar and synchronous systems is an excellent way to refine the geochemical approach and to finalize the establishment of a new reference method of diagenetic analysis, applicable to any carbonate system regardless of its age, origin or composition and able to precise our knowledge of past climates.