Short-term variation of ooid mineralogy in the Triassic-Jurassic boundary interval and its environmental implications: Evidence from the equatorial Ghalilah Formation, United Arab Emirates

Abstract In the Triassic-Jurassic (T-J) interval, only a few continuous carbonate sections have been reported, and detailed studies about ooids and their significance are scarce. This study focuses on abundant ooids in potentially continuous T-J carbonate sections representing equatorial, shallow marine environments. Mineralogical changes of ooids are proposed as a marker for transitional marine chemistry including carbonate saturation after ocean acidification and provide information about crisis and recovery scenarios for the initial CIE (carbon isotope excursion) and subsequent positive CIE. In the Ghalilah Formation, United Arab Emirates (UAE), the Sakhra Member is deposited immediately above the T-J boundary. Based on field work and thin section observation, the Sakhra Member can be divided into three coarsening-upward cycles (in ascending order, named C1–C3), each of which consists of peloidal mudstone/wackestone in the lower part and oolitic packstone/grainstone in the upper part. Petrological observation (thin section, SEM), stable isotope (inorganic carbon and oxygen) and elemental analysis suggest temporal change of original mineralogy from C1 to C3 ooids: from high-Mg calcite in C1 ooids to aragonite in C2 and C3 ooids. The mineralogical change of ooids is possibly related to variations in seawater carbonate saturation. The lower carbonate saturation indicated by C1 ooids reflects a transitional period before recovery from ocean acidification due to massive and rapid release of acidic gases (CO2 and SO2) by CAMP eruptions. Subsequently, from C1 to C3 ooids, seawater gradually experienced increasing carbonate saturation and increasing microbial carbonate precipitation. Increased microbial activities combined with elevated terrestrial influx may have significantly reduced the atmospheric CO2 concentration and restored carbonate saturation, which laid the foundation for full biotic recovery.