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ABSTRACT

The Guerrero-Morelos carbonate platform (southwestern Mexico) is one of the rare platforms that persisted throughout the Cenomanian–Turonian oceanic anoxic event 2 (OAE 2). Two sections from this carbonate platform exhibit the typical δ13C positive excursion characterizing the OAE 2. This enables the precise distribution of larger benthic foraminifers and their biotic response to paleoenvironmental changes to be characterized during this event. At Axaxacualco, oligotrophic conditions prevailed during the δ13C positive excursion in the distal part of the carbonate platform. In the more proximal part, at Barranca del Cañon, OAE 2 impact was more significant and is marked by thick laminated microbialite deposition, indicating mesotrophic conditions. Low phosphorus and trace-element contents confirm the persistence of oligotrophic to mesotrophic conditions throughout OAE 2 in the Central Mexico carbonate platform despite the proximity of the Caribbean-Colombian oceanic plateau. Before the δ13C positive excursion, which characterizes the OAE 2, the microfauna assemblage was well diversified, including corals, rudists, and large benthic foraminifera (Pseudorhapydionina chiapanensis, Pseudorhapydionina dubia, Cuneolina parva, Dicyclina sp., Chrysalidina gradata, and large miliolids). During the δ13C positive excursion, significant biotic changes occurred, with increasing dominance of organisms adapted to high-stress conditions. Endobenthic assemblages such as Nezzazatinella sp. and Nezzazata sp. were still present. The green symbiotic benthic foraminifera nearly disappeared, except C. parva and Dicyclina sp. During the basal Turonian, the carbonate platform returned to a more open and oxygenated environment, with the reappearance of pre-OAE microfauna, but without the large benthic foraminifera, which did not survive. The definitive drowning of the Guerrero-Morelos carbonate platform took place after the early Turonian, well above the end of the δ13C shift. The deposition of black shale and turbidites, which indicate deeper, anoxic environments, precluded large benthic foraminifera reestablishment. Correlation with the pelagic environments of the Eastbourne section (UK) reveals a synchronicity of biotic responses between basin and platform environments. Low-oxygen conditions are marked by multiple blooms of Heterohelix species in the basin, corresponding to an assemblage dominated by ?Decastronema, Thaumatoporella, and Istriloculina on the carbonate platforms.

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