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Ecosystems of the 'mid' Cretaceous are significant but poorly understood, due in large part to a sparse fossil record. Existing fossils, particularly in North America, are not chronostratigraphically well-constrained, further hampering comparisons of species and ecosystems across Laramidia and Appalachia. Efforts to overcome this lack of temporal resolution typically involve systematic collection of ashfall deposits, geochemistry, and biostratigraphy. Here we describe a new, high-resolution palynological and sedimentologic dataset from the Arlington Archosaur Site (AAS) of the Lewisville Formation (Woodbine Group; Middle Cenomanian). The integration of these new data with existing biostratigraphic, macrofossil (vertebrate, invertebrate, botanical), lithologic, ichnologic, and geochemical data allows for a comprehensive paleoenvironmental reconstruction and assessment of paleoenvironmental evolution during AAS deposition. Depositional environments are paralic and include nearshore, shallow-marine tidal-flat, lagoonal, tidal-delta deposits, fluvial-distributary channels, and associated floodplain environments including wetlands (swamp-marsh-lakes) and paleosols. The presence of the Cyclonephelium compactum – C. membraniphorum (Ccm) morphological plexus throughout the AAS deposit suggests a younger minimum age of (early) late Cenomanian for deposits, indicating possible southward expansion of this group into the Cretaceous Western Interior Seaway (CWIS) coincident with the onset of the Plenus Cold Event (PCE) of OAE2. The revised age estimate for the AAS suggests that Woodbine deposition at more proximal clastic source areas continued into the late Cenomanian, coeval with down dip Eagle Ford Group sedimentation. This study has important implications for the biogeography of western Appalachia and the response of terrestrial and shallow marine ecosystems of the southeastern CWIS to the onset of OAE2.

The greenhouse world of the mid‐Cretaceous (~94 Ma) was punctuated by an episode of abrupt climatic upheaval: Oceanic Anoxic Event 2. High‐resolution climate records reveal considerable changes in temperature, carbon cycling, and ocean chemistry during this climatic perturbation. In particular, an interval of cooling has been detected in the English Chalk on the basis of an invasive boreal fauna and bulk oxygen‐isotope excursions registered during the early stages of Oceanic Anoxic Event 2—a phenomenon known as the Plenus Cold Event, which has tentatively been correlated with climatic shifts worldwide. Here we present new high‐resolution neodymium‐, carbon‐, and oxygen‐isotope data, as well as elemental chromium concentrations and cerium anomalies, from the English Chalk exposed at Dover, UK, which we evaluate in the context of >400 records from across the globe. A negative carbon‐isotope excursion that correlates with the original "Plenus Cold Event" is consistently expressed worldwide, and CO2 proxy records, where available, indicate a rise and subsequent fall in CO2 over the Plenus interval. However, variability in the timing and expression of cooling at different sites suggests that, although sea‐surface paleotemperatures may reflect a response to global CO2 change, local processes likely played a dominant role at many sites. Variability in the timing and expression of changes in water mass character, and problems in determining the driver of observed proxy changes, suggest that no single simple mechanism can link the carbon cycle to oceanography during the Plenus interval and other factors including upwelling and circulation patterns were locally important. As such, it is proposed that the Plenus carbon‐isotope event is a more reliable stratigraphic marker to identify the Plenus interval, rather than any climatic shifts that may have been overprinted by local effects. Key Points: This study is the first to review mechanistic interactions, and temporal leads and lags, between temperature, seawater chemistry, and CO2 within the Plenus intervalThe negative carbon‐isotope excursion during the Plenus interval was a global event driven by a CO2 increase, and appears decoupled from cooling and circulation changesCooling was not globally synchronous; local climatic and environmental responses to the CO2 change are more complex than previously understood [ABSTRACT FROM AUTHOR]

The Plenus Cold Event (PCE) temporarily interrupted the supergreenhouse conditions exacerbated during much of the Cenomanian–Turonian Oceanic Anoxic Event 2 (OAE 2). The PCE is coeval to the occurrence of Boreal benthic macroinvertebrates and of the nektonic belemnite Praeactinocamax plenus (after which the event is named) in Europe, and to the re-oxygenation of bottom waters in the Northern Hemisphere. However, its effects on the sea-surface circulation are unknown and evidence for changes in the biogeography of planktonic organisms are limited to the equatorward migration of the dinoflagellate cysts grouped in the Cyclonephelium compactum–membraniphorum morphological plexus. This study presents new planktonic foraminiferal quantitative data from two complete OAE 2 records of the Anglo-Paris (Eastbourne, SE England) and Vocontian (Clot Chevalier, SE France) basins that registered the equatorward pulse of Boreal macroinvertebrates during the PCE and have been extensively studied for bio- and chemostratigraphy. At the onset of OAE 2 (before the PCE), planktonic foraminifera are mainly represented by oligo-mesotrophic Tethyan taxa (rotaliporids and whiteinellids) in both localities, but this assemblage is sharply replaced by cold and meso-to-eutrophic species (praeglobotruncanids, dicarinellids and muricohedbergellids) during the PCE. The cold-water assemblage shows strong affinities with the coeval fauna of the Norwegian Sea and yields the Boreal endemic species Muricohedbergella kyphoma and Praeglobotruncana plenusiensis. This observation combined with previously published data collected in other localities of the Northern Hemisphere and on other fossil groups suggests a ~20° equatorward expansion of the Boreal marine bioprovince during the PCE. Moreover, contrarily to the nektonic belemnitellids that were able to move independently of ocean circulation, planktonic organisms are passively transported by currents and changes in the planktonic foraminiferal assemblages documented in the Anglo-Paris and Vocontian basins have been interpreted to reflect the transition from a dominant influence of warm, saline, and thermally stratified waters carried by the proto-Gulf Stream before the PCE to cold and low-saline Boreal waters originated in the Norwegian Sea during the PCE. We suggest that such changes were forced by the equatorward shift of the proto-Arctic Front (i.e., the boundary between warm saline Tethyan-Atlantic and cold low-saline Boreal waters) from offshore Norway to Southern England. In this southerly position, the proto-Arctic Front represented an oceanographic barrier that limited the influence of the proto-Gulf Stream in the Anglo-Paris Basin, and favored the inflow of Boreal waters from the North to the European epicontinental basins. The sea-surface cooling and equatorward expansion of Boreal planktonic assemblages during the PCE are of the same order of magnitude of those reconstructed between some glacial and interglacial intervals of the Plio-Pleistocene. Despite obvious differences between Cretaceous and Plio-Pleistocene paleogeography and climate dynamics, this study reviews the extent of environmental changes occurred during the PCE, provides evidence for a profound re-organization of the sea-surface circulation patterns and a more comprehensive overview of the equatorward migration of Boreal marine communities in the Northern Hemisphere. • Planktonic foraminiferal quantitative abundances across OAE 2 are provided for Eastbourne and Clot Chevalier. • Rotalipora and Whiteinella dominate the assemblages at the onset of OAE 2 and after the Plenus Cold Event (PCE). • Praeglobotruncana, Dicarinella and Muricohedbergella increase their abundance during the PCE • The assemblage of the PCE interval shows many affinities with the coeval Boreal fauna of the Norwegian Sea. • The biogeography of Boreal taxa and the proto-Arctic Front shifted equatorward of ~20° from the onset of OAE 2 to the PCE. • The shift of the proto-Arctic Front limited the inflow of the proto-Gulf Stream and favored that of Boreal waters in Europe. [ABSTRACT FROM AUTHOR]

The latest Cenomanian Oceanic Anoxic Event (OAE) 2 represents one of the most extreme perturbations of the global carbon cycle. Planktonic foraminiferal events, variations in the taxonomic composition of assemblages (e.g., appearances, extinctions, temporary crisis of certain taxa) and their correlation with changes in the physico-chemical properties of surface waters are essential to reconstructing the consequences of OAE 2 on this group of calcareous microfossils. We present the results of a high-resolution biostratigraphic and taxonomic study of planktonic foraminifera performed at Eastbourne (SE England), representing the most expanded, complete and well-calibrated OAE 2 record in Europe. In this stratigraphic section, we identify a sequence of step-wise extinctions (i.e., Thalmanninella and Rotalipora species, and " Globigerinelloides " bentonensis) that are followed by an eclipse (temporary disappearance) of planispiral taxa and of hedbergellids with radially elongated chambers. These events are consistently found in approximately coeval stratigraphic intervals across low to mid-latitudes, suggesting that they were controlled by wide-scale environmental perturbations. Moreover, this study suggests that the extinction of rotaliporids might have been influenced by climate changes (i.e., warming for Thalmanninella and cooling during the Plenus Cold Event for Rotalipora) at the onset of OAE 2, whereas the eclipse of planispiral taxa and hedbergellids with radially elongated chambers during the second half of OAE 2 was likely related to enhanced productivity and mixing of surface waters potentially associated to warming after the PCE for planispirals. Finally, we identify two short-range species (Muricohedbergella kyphoma and Praeglobotruncana plenusiensis n. sp.) that co-occur with boreal macrofossils at Eastbourne and might represent the first evidence for a planktonic foraminiferal PCE fauna. The long ranging species Praeglobotrucana gungardensis n. sp. is described as new. • Extinctions and eclipses of certain taxa are found in the same stratigraphic order at low to mid-latitudes. • Extinction of Thalmanninella spp. observed during a warming event at the onset of OAE 2. • Extinction of Rotalipora spp. occurred within the Plenus Cold Event. • Eclipse of planispirals and pseudoclavihedbergellids follows the onset of warming and increased fertility. • First evidence for a planktonic foraminiferal Plenus Cold Event fauna. [ABSTRACT FROM AUTHOR]