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 δ 13 C 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 δ 13 C 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 δ 13 C 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 δ 13 C 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 δ 13 C 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.

ABSTRACT The Cretaceous-Paleocene (K/P) boundary intervals are rarely preserved in successions of shallow-water limestones. Here, we describe a shallow rocky shore on the active orogenic wedge of the eastern Alps (Austria) fringed by a carbonate platform that was largely cannibalized by erosion. We compared this succession with similar nearshore environments globally, as well as the deep sea, to gain a better understanding of the environmental response to the K/P boundary transition. In the eastern Alps, Cretaceous and Paleocene lithofacies across the K/P boundary transition are separated by a hardground that formed during subaerial exposure and that terminates Upper Maastrichtian limestone with planktic foraminiferal assemblages deposited at neritic depth during zone CF3 (ca. 66.500 Ma). Above the hardground, there are beachrocks with early Danian zone P1a(1) assemblages, which indicate the hardground spans about ~600 k.y. of nondeposition and/or erosion. During the early Danian, the marine transgressive fringe fluctuated between “shoreface to emersion” environments, depositing limestones rich in bryozoans, rhynchonellids, coralline algae, and rare planktic foraminifera along with abraded, bored, and/or encrusted clasts eroded from older rocks. Repeated short subaerial exposure is marked by vadose diagenesis and hardgrounds, including an ~1.5 m.y. interval between magnetochrons C29n to C28n and planktic foraminiferal zones P1b to P1c(2). Comparison with platform carbonate sequences from Croatia, Oman, Madagascar, Belize, and Guatemala, as well as nearshore siliciclastic environments of southern Tunisia, Texas, and Argentina, across the K/P boundary transition revealed surprisingly similar deposition and erosion patterns, with the latter correlative with sea-level falls and repeated subaerial exposure forming hardgrounds. Comparison with deep-sea depositional patterns revealed coeval but shorter intervals of erosion. This pattern shows a uniform response to the K/P boundary transition linked to climate and sea-level changes, whether in shallow nearshore or deep-sea environments, with climate change tied to Deccan volcanism in magnetochrons C29r-C29n.