Marine aragonite evolution in the oxygen-decreasing interval before the Cenomanian-Turonian Ocean anoxic event (OAE2) in the southeastern Neo-Tethys

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Environmental scenarios around the Cenomanian-Turonian ocean anoxic event (OAE2) interval are scarcely studied in the southeastern Neo-Tethys and not discussed in terms of variation of primary mineralogies of marine carbonates. Based on petrological and geochemical analyses, this study focuses on the increased aragonite abundance in the oxygen-decreasing interval before the OAE2 and its environmental implications in the southeastern Neo-Tethys. In central Iraq, the upper Cenomanian-lower Turonian Mishrif Formation represents marine carbonate records in the southeastern, tropical Neo-Tethys and includes five members from the lowermost Mi-5 to the uppermost Mi-1. From Mi-5 to Mi-1, dominant carbonate components and primary carbonate mineralogies vary. Mi-5 is dominated by primarily calcitic grains (mainly calcite shells of rudists and echinoids); Mi-4 and Mi-3 dominated by primarily aragonitic grains (mainly green algae and aragonitic bivalves) and planktonic calcitic foraminifera; and Mi-1 dominated by primarily calcitic ooids. Paleoenvironmental interpretations suggest: i) open marine, offshore settings for Mi-5 to Mi-3, and a shallow marine shoal setting for Mi-1; ii) a sea level rise from Mi-5 to Mi-3, followed by a sea level fall to Mi-1. The increasing aragonite abundance in Mi-4 and Mi-3 is exceptional and suggested to most likely reflect enhanced preservation of shallow-marine aragonite transported into open marine offshore settings. This enhanced aragonite preservation is likely related to an oxygen-decreasing level before the OAE2 in the offshore settings. Remarkably, these primarily aragonitic components are commonly preserved now as moldic pores in Mi-4 and Mi-3. The moldic pores are interpreted to be produced via deep burial dissolution of recrystallized aragonitic grains, based on petrological and geochemical analyses. Moreover, the primary carbonate compositional and mineralogical changes from Mi-5 to Mi-1 provide some potential clues for the environmental scenarios around the OAE2 interval in the southeastern, tropical Neo-Tethys: i) from Mi-5 to Mi-4 and Mi-3, the increasing abundance of primarily aragonitic components is accompanied by a sea level rise, increasing seawater temperature and decreasing oxygen level of bottom water; ii) from Mi-3 to Mi-1, the decreasing abundance of primarily aragonitic components is accompanied by a sea level fall, decreasing seawater temperature and enhanced microbial carbonate production.