Mesozoic climates and oceans – a tribute to Hugh Jenkyns and Helmut Weissert

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record.The study of past greenhouse climate intervals in Earth history, such as the Mesozoic, is an important, relevant and dynamic area of research for many sedimentary geologists, geochemists, palaeontologists and climate modellers. The Mesozoic sedimentary record provides key insights into the mechanics of how the Earth system works under warmer conditions, providing examples of natural climate change and perturbations to ocean chemistry, including anoxia, that are of societal relevance for understanding and contextualizing ongoing and future environmental problems. Furthermore, the deposition of widespread organic-carbon-rich sediments (‘black shales’) during the Mesozoic means that this is an era of considerable economic interest. In July 2015, an international group of geoscientists attended a workshop in Ascona, Switzerland, to discuss all aspects of the Mesozoic world and to celebrate the four-decade-long contributions made by Hugh Jenkyns (University of Oxford) and Helmut Weissert (ETH Zürich) to our understanding of this fascinating era in Earth history. This volume of Sedimentology arose from that meeting and contains papers inspired by (and co-authored by!) Hugh and Helmi. Here, a brief introduction to the volume is provided that reviews aspects of Hugh and Helmi's major achievements; contextualizes the papers of the Thematic Issue; and discusses some of the outstanding questions and areas for future research

Cretaceous oceanic anoxic events: causes and consequences

Organic carbon-rich sediments are globally developed in pelagic sedimentary sequences of Aptian-Albian and Cenomanian-Turonian age. They formed in a variety of paleo-bathymetric settings including oceanic plateaus and basins, continental margins and shelf seas. The widespread nature of these deposits suggests that they were not strictly controlled by local basin geometry but were a product of "Oceanic Anoxic Events". We interpret these events as the result of the interplay of two major geologic and climatic factors: firstly the Late Cretaceous transgression which increased the area and volume of shallow epicontinental and marginal seas and was accompanied by an increase in the production of organic carbon; and secondly the existence of an equable global climate which reduced the supply of cold oxygenated bottom water to the world ocean. This combination of climatic and hypsographic conditions favoured the ormation of an expanded oxygen-minimum layer and where this intersected the sediment-water interface, organic carbon-rich deposits could be formed, these being records of "Oceanic Anoxic Events"

Patterns of local and global redox variability during the Cenomanian–Turonian Boundary Event (Oceanic Anoxic Event 2) recorded in carbonates and shales from central Italy

Careful evaluation of the local geochemical conditions in past marine settings can provide a window to the average redox state of the global ocean during episodes of extensive organic-carbon deposition. These comparisons aid in identifying the interplay between climate and biotic feedbacks contributing to and resulting from these events. Well-documented examples are known from the Mesozoic Era, which is characterized by episodes of widespread organic-carbon deposition known as Oceanic Anoxic Events (OAEs). This organic-carbon burial typically leads to coeval positive carbon-isotope excursions. Geochemical data are presented here for several palaeoredox proxies (Cr/Ti, V, Mo, Zn, Mn, Fe speciation, I/Ca and sulfur isotopes) from a section exposed at Furlo in the Marche–Umbrian Apennines of Italy that spans the Cenomanian-Turonian boundary. Here, OAE 2 is represented by a ~1-m thick radiolarian-rich millimetre-laminated organic-rich shale known locally as the Bonarelli Level. Iron speciation data for thin organic-rich intervals observed below the Bonarelli Level imply a local redox shift going into the OAE, with ferruginous conditions (i.e., anoxic with dissolved ferrous iron) transiently developed prior to the event and euxinia (i.e., anoxic and sulfidic bottom waters) throughout the event itself. Pre-OAE enrichments of elements sensitive to anoxic water columns were due to initial development of locally ferruginous bottom waters as a precursor to the event. However, the greater global expanse of dysoxic to euxinic conditions during the OAE greatly reduced redox-sensitive trace-metal concentrations in seawater. Carbonate I/Ca ratios were generally low, suggesting locally reduced bottom water oxygen conditions preceding the event and relatively increased O2 concentrations post-event. Combined, the Furlo geochemical data suggest a redox-stratified water column with oxic surface waters and a shallow chemocline overlying locally ferruginous bottom waters preceding the event, globally widespread euxinic bottom waters during the OAE, followed by chemocline shallowing but sustained local redox stratification following the event