Paul Dodsworth

The Cenomanian–Turonian Greenhouse-Hothouse interval, encompassing Oceanic Anoxic Event 2 (OAE-2), represents one of the most intensively studied episodes of global environmental change in Earth History. However, despite extensive research, nitrogen isotope records from this interval remain geographically limited with a recent compilation highlighting that some continental margins were under-represented (e.g. one location and/or limited stratigraphic coverage per margin). This prevents a comprehensive understanding of the local and regional variability in nitrogen cycling in these Greenhouse to Hothouse conditions. This study provides new nitrogen isotope data, integrated with an extensive multidisciplinary dataset from multiple cores and outcrops across a single margin to provide new insights into the spatial and temporal variability in marine nitrogen cycling and marine primary producer community response. This study demonstrates that nitrogen isotope depletion in the Cretaceous Western Interior Seaway began in the Early Cenomanian (approx 97-98 Ma) , significantly earlier than previously recognized. The occurrence of δ15N-depleted organic matter, coupled with biomarker and palynological evidence for abundant (cyano-) bacteria, indicates enhanced diazotrophy in oxygen-deficient surface waters resulting in ammonium as the dominant form of fixed nitrogen in the photic zone. These conditions favoured low diversity eukaryote communities dominated by blooms in Prasinophyceae and Peridiniales (Dinophaceae), likely due to their ability to more efficiently assimilate ammonium through alternative nitrogenases enzymes. However, the observed δ15N values, even under broadly similar environmental conditions impacted by periodic volcanic events, can vary significantly over relatively short distances suggesting that, despite large-scale controls, local physiographic conditions significantly influenced the nitrogen cycle and related biological responses. This highlights the need to integrate data from multiple stratigraphic sections across a basin to fully capture the complexity of nitrogen cycle dynamics and associated ecological feedback during extreme climate events.

Five Lower Albian (Lower Cretaceous) areoligeracean dinoflagellate cyst species are present in the Carstone Formation at Middlegate Quarry, North Lincolnshire, UK: Canninginopsis monile , Circulodinium deflandrei , Cyclonephelium compactum , Cyclonephelium intonsum and Cyclonephelium longispinatum . The taxonomy of Circulodinium deflandrei is reviewed with respect to 20 measured Carstone specimens. The taxonomy of Cyclonephelium longispinatum is reviewed with respect to 70 measured Carstone specimens and a study of 40 measured specimens from its type material in Site 361, Deep Sea Drilling Project Leg 40, offshore southwestern Africa. Circulodinium deflandrei and Cyclonephelium longispinatum have recently been considered by other workers to be junior synonyms, respectively, of Circulodinium hirtellum and Tenua anaphrissa . However, following detailed study of their morphologies, the synonymies are formally rejected here. An emended diagnosis is given for C. longispinatum . A re-assessment of the stratigraphical ranges of taxa co-occurring with the type material of C. longispinatum is consistent with a Late Albian to Early Turonian age range in Site 361.

At Middlegate Quarry, the Carstone Formation is a c. 0.8 m thick unit of oolitic ferruginous sandstone. It rests unconformably on the lower Kimmeridge Clay Formation (KCF) and is overlain by the Hunstanton Formation (Red Chalk) with an apparently gradational junction. Marine dinoflagellate cysts (dinocysts) are present at a concentration of 9520 to 13 600 specimens per gram in the 0.15 m of KCF below the unconformity, and include taxa that confirm ammonite evidence for the intra-Lower Kimmeridgian Substage (Upper Jurassic, Cymodoce Ammonite Zone). A rich recovery of well-preserved Lower Cretaceous palynomorphs is reported for the first time from the overlying Carstone. Reworking of mudstone from the KCF into the formation appears to have provided a local argillaceous preservation matrix at Middlegate. The basal 0.15 m is dominated by palynomorphs derived from the KCF but the interval from 0.15 to 0.55 m above the unconformity mainly contains indigenous Lower Cretaceous palynomorphs. Dinoflagellate cysts are present in the Carstone at an average concentration of 454 specimens per gram, and include taxa that probably have stratigraphical range bases above the Aptian–Albian stage boundary: Cyclonephelium compactum, Cyclonephelium intonsum, Endoscrinium heikeae , Leptodinium cancellatum (consistent), Stephodinium coronatum and Stephodinium spinulosum . The additional presence of taxa with range tops/event tops in the Lower Albian Tardefurcata Ammonite Zone (common Cauca parva , frequent Canninginopsis monile and Kleithriasphaeridium eoinodes , and rare Dingodinium albertii, Discorsia nannus and Kiokansium prolatum ) indicate probable assignment to this zone. The new palynological data support previous macrofossil (brachiopod) study of the Carstone at Middlegate and nearby Melton Bottom Quarry which tentatively assigns its highest part to the Tardefurcata Zone. The palynological and palynofacies assemblages are interpreted to confirm a relatively proximal to shoreline site of deposition, possibly inner to middle neritic. Supplementary material: Quarry photographs and a palynological distribution chart are available at https://doi.org/10.6084/m9.figshare.c.5538977

The lowermost 1.45 m of the Welton Chalk Formation, including the regional sedimentary record of Oceanic Anoxic Event 2 (OAE-2), has been sampled at Melton Ross Quarry in eastern England, UK. The section is investigated for organic geochemistry and stable isotopes for the first time, while a detailed palynological study follows previously published preliminary results. It comprises a condensed interval that spans the Cenomanian–Turonian Stage boundary. A locally preserved, lower ‘anomalous’ succession (Beds I–VII) and a ‘Central Limestone’ (Bed A) are shown to correlate respectively with the pre-Plenus sequence and Plenus Bed at Misburg and Wunstorf in the Lower Saxony Basin (LSB), NW Germany. They are overlain by a succession of variegated marls (Bed B to Bed H), including the Black Band (Beds C–E), that can be correlated across eastern England. Based on a carbon isotope (δ ¹³ C) profile and dinoflagellate cyst and acritarch bio-event correlation, Beds B–H appear to be a highly attenuated post-Plenus equivalent of the LSB succession, including part of the ‘Fish Shale’. The δ ¹³ C profile shows possible ‘precursor’/‘build-up’ events in the lower succession at Melton Ross, with the main OAE-2 δ ¹³ C excursion occurring in the Central Limestone and overlying Beds B–H. The darker coloured marls from the Black Band and Bed G contain 1.43–3.47% total organic carbon (TOC), hydrogen index values of 78–203 mg HC/g TOC and oxygen index values of 15–26 mg CO ₂ /g TOC, indicating type III and type II–III organic matter, of mixed terrigenous and marine algal sources. The corresponding palynological assemblages are dominated by marine dinoflagellate cysts, comprising mainly gonyaulacoid taxa, with subordinate terrigenous miospores, mainly gymnosperm bisaccate pollen, consistent with a distal marine setting. The interbedded lighter-coloured marls contain less than 0.4% TOC and lower proportions of miospores and peridinioid dinoflagellate cysts compared with the darker layers. This is suggestive of moderately raised levels of productivity during deposition of the darker layers, possibly related to greater nutrient availability from land-derived sources. The occurrence of the peridinioid taxa Eurydinium saxoniense and Bosedinia spp., together with higher proportions of prasinophyte phycomata in the darker layers, may also point to stimulation of organic-walled phytoplankton productivity by reduced nitrogen chemo-species encroaching the photic zone, possibly by expansion of an oxygen-minimum zone. Exceptionally high concentrations of palynomorphs (in the tens of thousands to lower hundreds of thousands per gramme range) in the darker layers at Melton Ross and eight other eastern England localities is consistent with increased quality of seafloor preservation in a low oxygen environment, coupled with a high degree of stratigraphic condensation. Two new dinoflagellate cyst species are described from Melton Ross, Canninginopsis ? lindseyensis sp. nov. and Trithyrodinium maculatum sp. nov., along with two taxa described in open nomenclature. Supplementary material : One pdf file, with detailed sample positions and descriptions, tables of supporting information (also available in Excel format), quarry photographs and a palynological distribution chart, is available at https://doi.org/10.6084/m9.figshare.c.4987205

This study documents the detailed palynology of the Bridge Creek Member, Greenhorn Limestone Formation, at the Rock Canyon anticline outcrop, west of Pueblo, Colorado, USA. The section is the formally ratified Global Boundary Stratotype Section and Point (GSSP) for the base of Turonian Stage and corresponding Lower Turonian Substage, and the only proposed GSSP for the base of the Middle Turonian Substage. The authors’ previously published palynological data from the underlying Cenomanian strata in the nearby US Geological Survey (USGS) Portland-1 core, and from cores and an outcrop of Cenomanian, Turonian and earliest Coniacian age in SW Texas, are reviewed for biostratigraphy. Dinoflagellate cyst ranges and events in both areas are calibrated against other published bio- and chemostratigraphical (carbon isotope) data and an astronomical age model. A regional dinoflagellate cyst zonation is proposed, adapted from a Central and Northern European scheme, comprising three zones and four subzones. Comparison with vintage published palynological data from other states, including Arizona, Kansas, Utah, Wyoming and Montana, indicates that the zonation is probably applicable to marine sections throughout the Cretaceous Western Interior Seaway of the USA. The new palynological data from around the Lower–Middle Turonian boundary at Pueblo support its proposal as a GSSP.

The Late Cretaceous Epoch was characterized by major global perturbations in the carbon cycle, the most prominent occurring near the Cenomanian–Turonian (CT) transition marked by Oceanic Anoxic Event 2 (OAE-2) at 94.9–93.7 Ma. The Cretaceous Western Interior Seaway (KWIS) was one of several epicontinental seas in which a complex water-mass evolution was recorded in widespread sedimentary successions. This contribution integrates new data on the main components of organic matter, geochemistry, and stable isotopes along a north–south transect from the KWIS to the equatorial western Atlantic and Southern Ocean. In particular, cored sedimentary rocks from the Eagle Ford Group of west Texas (∼ 90–98 Ma) demonstrate subtle temporal and spatial variations in palaeoenvironmental conditions and provide an important geographic constraint for interpreting water-mass evolution. High-latitude (boreal–austral), equatorial Atlantic Tethyan and locally sourced Western Interior Seaway water masses are distinguished by distinct palynological assemblages and geochemical signatures. The northward migration of an equatorial Atlantic Tethyan water mass into the KWIS occurred during the early–middle Cenomanian (98–95 Ma) followed by a major re-organization during the latest Cenomanian–Turonian (95–94 Ma) as a full connection with a northerly boreal water mass was established during peak transgression. This oceanographic change promoted de-stratification of the water column and improved oxygenation throughout the KWIS and as far south as the Demerara Rise off Suriname. In addition, the recorded decline in redox-sensitive trace metals during the onset of OAE-2 likely reflects a genuine oxygenation event related to open water-mass exchange and may have been complicated by variable contribution of organic matter from different sources (e.g. refractory/terrigenous material), requiring further investigation.

This study describes the detailed palynology of the Eagle Ford Group at Lozier Canyon, its principal outcrop reference section in west Texas. Prominent marine phytoplankton assemblages are consistent with a proposed depositional setting for Lozier Canyon on a submarine platform within the Cretaceous Western Interior Seaway (KWIS) of the USA. The Lower Eagle Ford Formation (middle to upper Cenomanian) was deposited under restricted marine conditions with oxygen-deficient, probably anoxic bottom waters, as indicated by the black, organic-rich (ca. 1 to > 6% total organic carbon) shale lithology. The formation comprises two fourth-order stratigraphical sequences, each showing sedimentary evidence of increasing water depth during deposition. Palynomorph assemblages from the sequences are dominated by green algal prasinophyte phycomata in the lower parts and peridinioid dinoflagellate cysts in the upper parts. The productivity of these marine phytoplankton groups may have been stimulated by availability of ammonium (prasinophytes) and nitrite (peridinioids) from upwelling, or vertical expansion of the oxygen-minimum zone. The transition from prasinophyte- to peridinioid-dominated assemblages may reflect elevation of the photic zone to a level less regularly encroached by deeper, ammonium-enriched waters of the denitrification zone. The overlying Upper Eagle Ford Formation (upper Cenomanian to upper Turonian or Coniacian) was deposited under deeper, relatively open marine conditions, as indicated by a decrease in organic carbon content and an increase in carbonate content. This is supported by the presence of diversified dinoflagellate cyst assemblages, including inter-regional (high- to mid-latitude) marker events; in the upper Cenomanian, base and acme Cyclonephelium compactum-membraniphorum, top consistent and frequent Litosphaeridium siphoniphorum, and top Adnatosphaeridium tutulosum; in the middle to upper Turonian, range bases of Heterosphaeridium difficile and Senoniasphaera rotundata. The stratigraphical resolution offered by palynology indicates potential application to wellsite ‘geosteering’ of commercial shale oil and shale gas wells drilled in the Eagle Ford Group.