Hugh C. Jenkyns

The Toarcian Oceanic Anoxic Event (T‐OAE, ∼183 Ma) was characterized by globally enhanced organic‐carbon burial and a negative carbon‐isotope excursion (N‐CIE). However, the role of marine productivity at this time, and its spatiotemporal variability, is unclear. We present the first carbonate barium‐isotope (δ ¹³⁸ Ba _carb ) records across the T‐OAE from a shallow‐water platform (Nianduo, SE Tethys) and a pelagic basin (Dogna, Alpine‐Mediterranean Tethys) to reconstruct productivity dynamics. Both sites show positive δ ¹³⁸ Ba _carb shifts at the N‐CIE onset, indicating supra‐regional productivity enhancement. At Nianduo, δ ¹³⁸ Ba _carb rises through the onset, consistent with increased export production driven by weathering‐derived nutrient inputs. At Dogna, δ ¹³⁸ Ba _carb declines within the N‐CIE onset due to reduction‐driven barite dissolution, followed by a rise during the N‐CIE recovery. The Dogna data suggest protracted elevation of pelagic productivity supported by nutrient upwelling and aeolian fertilization. As such, pelagic basins may have acted as an important carbon sink regulating the T‐OAE carbon cycle.

The Bonarelli Level in the Umbria–Marche Basin (Italy) represents the most famous lithological expression of Oceanic Anoxic Event 2 (OAE 2), a major environmental perturbation that occurred c . 94 myr ago. The lack of carbonate prevents the use of carbonate carbon isotopes and the application of calcareous nannofossils and planktonic foraminifera for stratigraphic characterization. This limitation has led to contrasting interpretations of the stratigraphic significance of the unit, affecting correlation with coeval records. Here, we present new high-resolution organic carbon-isotope ratios from two key localities of the Bonarelli Level to compare with equivalent sections elsewhere. The data indicate that the entire OAE 2 interval, as defined by carbon-isotope chemostratigraphy, is potentially preserved in the Bonarelli Level, albeit in condensed form, challenging previous suggestions of a significant hiatus at the top of the unit. High-resolution chemostratigraphy reveals coeval variations in total organic carbon and lithological patterns in Umbria–Marche, indicating a basin-wide depositional response with local changes mainly controlled by winnowing and redeposition of radiolarian tests. This framework allows a clearer identification of the timing of the onset and termination of the dominant anoxic to euxinic conditions within the OAE 2 interval in the Umbria–Marche Basin.

The Carnian Pluvial Episode (CPE; 234–232 million years ago) is an iconic but poorly understood hyperthermal event. Here, we present an integrated high-resolution (~2–10 kyr) multi-proxy record from a Carnian lacustrine succession of the Junggar Basin of northwestern China. We find that the rapid CPE onset (~15.8 kyr) could have been the result of volcanism and subsequent surface carbon-cycle feedbacks. The CPE terrestrial carbon cycling, at a scale of ± 1‰ (δ¹³C_org), displays an in-phase relationship with the 405-kyr-long-eccentricity parameter, paralleling the warmhouse climate–carbon-cycle interactions throughout the Oligo–Miocene. The CPE hydrological cycle was typified by increased aridification in continental interiors and multiple precipitation centres at low-latitude eastern regions of Pangea and at the poles. The carbon and hydrological cycle changes of the CPE include features reminiscent of other warm events, suggesting they may share key characteristics and hold important clues to Earth system functioning.

Mesozoic oceanic anoxic events were characterized by relatively low seawater sulphate concentrations ([]), which likely regulated the development and evolution of these major palaeoceanographic phenomena. However, there is little reliable sedimentary evidence for low [] in ancient marine waters and understanding of how such a seawater chemistry potentially impacted oceanic anoxic events is limited. This study presents an integrated sedimentological, mineralogical and geochemical investigation of the mineral siderite hosted in dark grey shale and sideritic concretions of Early Aptian (coeval with Oceanic Anoxic Event 1a) from the Tibetan Himalaya. Siderite is present throughout the section and possesses similar morphological characteristics whether in dark grey shale or concretions. Siderite can be present as disseminated and rhombic crystals formed during early diagenesis, or minor spherical crystals formed during late diagenesis. The evidence from redox elements, middle rare‐earth element bulge patterns and extremely low carbon‐isotope values of the sideritic concretions indicates that the iron carbonate was formed in the Fe‐reduction zones by the process of dissimilatory iron reduction. This process would have required conditions of low [], reducing environment, abundant iron and high alkalinity. Additionally, the coexistence of siderite and pyrite may indicate that dissimilatory iron reduction occurred close to the microbial sulphate reduction zone, with seawater [] hovering around the tipping point at which pyrite could form once seawater sulphate increased. Such an increase during Oceanic Anoxic Event 1a could have resulted from basalt–seawater interaction and associated enhanced continental weathering, and/or hydrothermal activity. This study's observations support the previous hypothesis that low [] for Oceanic Anoxic Event 1a was probably caused by massive gypsum burial in the proto‐South Atlantic. Subsequently, enhanced sulphate input could have promoted microbial sulphate reduction and accompanying oxidation of organic matter, which likely further enhanced nutrient recycling, increased primary productivity and organic‐carbon burial, leading to more oxygen consumption and expansion of oxygen minimum zones, as reconstructed for many oceanic anoxic events.

Understanding the behavior of mercury (Hg) in organic‐rich sediments as they undergo thermal maturation is important, for example, because enrichment of Hg in sedimentary deposits has become a widely used proxy for volcanism from Large Igneous Provinces (LIPs). In this study, we evaluate the effects of such processes on sedimentary Hg concentrations by investigating a common stratigraphic interval in three drill cores with different levels of thermal maturity (immature, mature and post‐mature) in Toarcian sediments (Posidonienschiefer Formation) from the Lower Saxony Basin, Germany. We present Hg concentrations, bulk organic geochemistry, and total sulfur data. Mercury concentrations in the mature and post‐mature sediments are increased >2‐fold relative to the immature material, which is greater than any potential differences in original Hg concentrations in the studied successions prior to burial. Organic‐carbon and host‐rock mass loss during thermal maturation may have concentrated Hg in the mature sediments to some extent, provided Hg is considered effectively immobile. The increased Hg, TOC‐normalized Hg, and TS‐normalized Hg are most likely linked to the “closed system” behavior of Hg in sedimentary basins and the relatively low temperatures (70–260°C) during maturation that resulted in limited Hg mobility. More speculatively, a certain degree of redistribution of Hg within the mature sediments is suggested by its enrichment in distinct stratigraphic levels. Regardless of the exact mechanisms at play, the elevated Hg concentrations in mature sediments amplify both Hg/TOC and Hg/TS, implying that thermal effects must be considered when using normalized Hg as a proxy for far‐field volcanic activity.

For much of the pelagic sedimentary record, time control is limited to the resolution of precession cycles (ca 20 kyr): the Milankovitch parameter that forms the most detailed metronome for the Cenozoic and Mesozoic Eras. The influence of precession is often detected in lithological alternations, where the duration represented by individual lithologies is not well constrained. Here the novel technique of extraterrestrial helium abundance (³He_ET) is used to investigate the sedimentation dynamics and palaeoceanography within individual precessional cycles. High‐resolution ³He_ET timescales were produced for four precession cycles from the rhythmically bedded Scaglia Bianca Formation, a sequence of Upper Cretaceous (Cenomanian) deep‐marine pelagic limestones from central Italy that are well characterized by cyclostratigraphy. Using ³He_ET concentrations as a proxy for sedimentation rate allows instantaneous sedimentation rates and organic‐carbon mass accumulation rates to be calculated for each bed within a precession cycle. Eccentricity is known to modulate the amplitude of precession forcing, and precession cycles deposited under eccentricity maxima and minima were selected for comparison. Lithological changes through these chert–(black shale)–limestone cycles are explained using the concept of ‘palaeoenvironmental thresholds’; these timescale calculations indicate that when the amplitude of precessional insolation forcing was greatest (at eccentricity maxima) the palaeoenvironmental system spent longer in the more nutrient‐rich environment under which siliceous and organic‐rich sediments were deposited, reflecting increased time spent above a ‘threshold’ insolation level. Estimates of primary productivity are relatively elevated for organic‐rich beds. An increase in the flux of terrestrial helium (⁴He_terr) during the deposition of cherts may have been coincident with an increase in terrestrially derived nutrients. The presented results indicate great potential for the use of ³He_ET to understand past oceanographic, climatic and sedimentological processes at high temporal resolution.

In the mid-1850s, Thomas Henry Huxley, who coined the term ‘coccolith’, described their presence in oceanic sediments dredged from the sea floor. Subsequent recognition of their occurrence in the Upper Cretaceous English Chalk, largely unconsolidated pelagic sediment, and similar, more lithified limestones in the Jurassic of the Alps, led to their being considered major rock-forming elements through much of geological time, although they are now known to be limited to the Late Triassic–Recent interval. Unlike Charles Darwin, who did not travel abroad following the voyage of the Beagle, Huxley made a number of trips to Italy and Switzerland and is recorded as a guest in the exclusive Hotel Kursaal Maloja in the Engadin in 1893. While staying there, he made a number of excursions on foot and his presence was thought significant enough that it is recorded in an inscribed granitic stone tablet, erected in 1896, describing him as’the illustrious writer and naturalist’. His rambles would have led him past outcrops of tectonically emplaced true oceanic calcareous sediment of Early Cretaceous age, here shown to have originally contained coccoliths that were largely destroyed under the imprint of Alpine metamorphism of sub-greenschist to greenschist facies. To this extent, Huxley could have come close to recognizing true oceanic sediments exposed on land, but the dissimilarity between these Swiss Alpine deposits and the friable English Chalk would not obviously have led to the investigation of the former for the organisms he had christened as ‘coccoliths’.