Development of a correlated Fe‐Mn Crust stratigraphy using Pb and Nd isotopes and its application to paleoceanographic reconstruction in the Atlantic

Eight ferromanganese crust samples spanning the complete depth range of Tropic Seamount in the north‐east Atlantic were analysed for Pb and Nd isotopes to reconstruct water mass origin and mixing over the last 75 Ma. Pb isotopes were determined by LA‐MC‐ICP‐MS, which enables the rapid production of large, high spatial‐resolution datasets. This makes it possible to precisely correlate stratigraphy between different samples, compare contemporaneous layers, and create a composite record given the abundance of hiatuses in crusts. Pb and Nd isotope data show the influence of various oceanic and continental end‐members in the north‐east Atlantic Ocean. This reflects its evolution from a restricted, isolated basins in the Late Cretaceous with influxes from the Tethys Ocean, to an increasingly well‐mixed, large‐scale basin, with a dominant Southern Ocean signature until the Miocene. Less‐radiogenic Nd isotope signatures suggest Labrador Sea Water influenced the north‐east Atlantic basin as early as 17‐15 Ma, flowing through a northern route such as the Charlie‐Gibbs Fracture Zone. Pb and Nd isotopes highlight the increasing influence of Saharan aeolian dust input about 7 Ma, imparting a less‐radiogenic excursion to the binary mixing between North Atlantic water masses and riverine discharge from West and Central Africa. This highlights the influence of aeolian dust input on the open ocean Pb and Nd budget, and supports an early stage of North African aridification in the Late Miocene. This signature is overprinted about 3 Ma to the present by a strong North Atlantic Deep Water signature following the onset of Northern Hemisphere glaciation

Whole-mantle convection with tectonic plates preserves long-term global patterns of upper mantle geochemistry

The evolution of the planetary interior during plate tectonics is controlled by slow convection within the mantle. Global-scale geochemical differences across the upper mantle are known, but how they are preserved during convection has not been adequately explained. We demonstrate that the geographic patterns of chemical variations around the Earth’s mantle endure as a direct result of whole-mantle convection within largely isolated cells defined by subducting plates. New 3D spherical numerical models embedded with the latest geological paleo-tectonic reconstructions and ground-truthed with new Hf-Nd isotope data, suggest that uppermost mantle at one location (e.g. under Indian Ocean) circulates down to the core-mantle boundary (CMB), but returns within ≥100 Myrs via large-scale convection to its approximate starting location. Modelled tracers pool at the CMB but do not disperse ubiquitously around it. Similarly, mantle beneath the Pacific does not spread to surrounding regions of the planet. The models fit global patterns of isotope data and may explain features such as the DUPAL anomaly and long-standing differences between Indian and Pacific Ocean crust. Indeed, the geochemical data suggests this mode of convection could have influenced the evolution of mantle composition since 550 Ma and potentially since the onset of plate tectonics

Composition and evolution of the Ancestral South Sandwich Arc: implications for the flow of deep ocean water and mantle through the Drake Passage gateway

The Ancestral South Sandwich Arc (ASSA) has a short life-span of c.20 m.y. (Early Oligocene to Middle-Upper Miocene) before slab retreat and subsequent ‘resurrection’ as the active South Sandwich Island Arc (SSIA). The ASSA is, however, significant because it straddled the eastern margin of the Drake Passage Gateway where it formed a potential barrier to deep ocean water and mantle flow from the Pacific to Atlantic. The ASSA may be divided into three parts, from north to south: the Central Scotia Sea (CSS), the Discovery segment, and the Jane segment. Published age data coupled with new geochemical data (major elements, trace elements, Hf-Nd-Sr-Pb isotopes) from the three ASSA segments place constraints on models for the evolution of the arc and hence gateway development. The CSS segment has two known periods of activity. The older, Oligocene, period produced basic-acid, mostly calc-alkaline rocks, best explained in terms of subduction initiation volcanism of Andean-type (no slab rollback). The younger, Middle-Late Miocene period produced basic-acid, high-K calc-alkaline rocks (lavas and pyroclastic rocks with abundant volcanigenic sediments) which, despite being erupted on oceanic crust, have continental arc characteristics best explained in terms of a large, hot subduction flux most typical of a syn- or post-collision arc setting. Early-Middle Miocene volcanism in the Discovery and Jane arc segments is geochemically quite different, being typically tholeiitic and compositionally similar to many lavas from the active South Sandwich island arc front. There is indirect evidence for Western Pacific-type (slab rollback) subduction initiation in the southern part of the ASSA and for the back-arc basins (the Jane and Scan Basins) to have been active at the time of arc volcanism. Models for the death of the ASSA in the south following a series of ridge-trench collisions, are not positively supported by any geochemical evidence of hot subduction, but cessation of subduction by approach of progressively more buoyant oceanic lithosphere is consistent with both geochemistry and geodynamics. In terms of deep ocean water flow the early stages of spreading at the East Scotia Ridge (starting at 17-15 Ma) may have been important in breaking up the ASSA barrier while the subsequent establishment of a STEP (Subduction-Transform Edge Propagator) fault east of the South Georgia microcontinent (< 11 Ma) led to formation of the South Georgia Passage used by the Antarctic Circumpolar Current today. In terms of mantle flow, the subduction zone and arc root likely acted as a barrier to mantle flow in the CSS arc segment such that the ASSA itself became the Pacific-South Atlantic mantle domain boundary. This was not the case in the Discovery and Jane arc segments, however, because northwards flow of South Atlantic mantle behind the southern part of the ASSA gave an Atlantic provenance to the whole southern ASSA

Discovery of a meteoritic ejecta layer containing unmelted impactor fragments at the base of Paleocene lavas, Isle of Skye, Scotland

Evidence for meteorite impacts in the geological record may include the presence of shocked minerals, spherule layers, and geochemical anomalies. However, it is highly unusual to nd unmelted crystals from the actual impactor within an ejecta layer. Here we detail the rst recorded occurrence of vanadium-rich osbornite (TiVN) on Earth, from two sites on Skye, northwest Scotland, which are interpreted as part of a meteoritic ejecta layer. TiVN has only previously been reported as dust from comet Wild 2, but on Skye it has been identi ed as an unmelted phase. Both ejecta layer sites also contain niobium-rich osbornite (TiNbN), which has not previously been reported. An extraterrestrial origin for these deposits is strongly supported by the presence of reidite (a high-pressure zircon polymorph), which is only found naturally at sites of meteorite impact. Barringerite [(Fe,Ni)2P], baddeleyite (ZrO2), alabandite (MnS), and carbon-bearing native iron spherules, together with planar deformation features and diaplectic glass in quartz, further support this thesis. We demonstrate through eld relationships and Ar-Ar dating that the meteorite strike occurred during the mid-Paleocene. This is the rst recorded mid-Paleocene impact event in the region and is coincident with the onset of magmatism in the British Palaeogene Igneous Province (BPIP). The Skye ejecta layer deposits provoke important questions regarding their lateral extent at the base of the BPIP and the possibility of their presence elsewhere beneath the much larger North Atlantic Igneous Province

Tracking the tempo of a continental margin arc: insights from a forearc succession in West Antarctica

The Fossil Bluff Group of eastern Alexander Island records the exceptional preservation of more than 8 km of Mesozoic sedimentary rocks deposited into an accretionary forearc basin that developed unconformably above a late Paleozoic accretionary complex, and in proximity to a continental margin arc during a prolonged phase of enhanced magmatism. Through the Mesozoic, the Fossil Bluff Group evolved from a trench-slope environment to a forearc basin sourced from the continental margin arc. During this period, the Antarctic Peninsula’s convergent margin was characterized by episodes of magmatic flare-ups that developed during tectonic compression, crustal thickening, extension, and uplift. U-Pb and Lu-Hf detrital zircon data are used to determine the provenance of the forearc succession and as a monitor of arc magmatic tempos during the late Mesozoic. The magmatic record in the adjacent arc is poorly preserved or partially absent, but the sedimentary record of the forearc basin preserves a largely uninterrupted record of arc magmatism that can be studied with detrital zircon geochronology and geochemistry. The basal succession of the Fossil Bluff Group is sourced from the adjacent accretionary complex, but thereafter it is strongly controlled by the proximal arc in western Palmer Land and is characterized by a mixed arc/recycled signature during episodes of renewed sedimentation. However, the main phases of deposition during the Early Jurassic (ca. 180 Ma), Early Cretaceous (141–131 Ma), and mid-Cretaceous (125–102 Ma) are dominated by arc-only sources. The Lu-Hf isotopic record supports a transition from convergence to extension and a return to convergence during the Mesozoic, which is consistent with accretionary orogens from elsewhere along the West Gondwanan margin. The provenance record during the depositional history of the basin points overwhelmingly to an autochthonous origin; as such, models for parts of the western province of the Antarctic Peninsula being allochthonous are unsupported

Oceanic Anoxic Event 2 triggered by Kerguelen volcanism

Large Igneous Provinces (LIPs) are associated with global warming and carbon cycle perturbations during Oceanic Anoxic Event 2 (OAE2, ~94 Ma) and the Mid-Cenomanian Event (MCE, ~96.5 Ma). However, there is still no consensus on the role volcanism played as a trigger, or its source – previously ascribed to the Caribbean LIP or High Arctic LIP. Here, we use Mentelle Basin sedimentary mercury (Hg) concentrations to determine the timing of volcanism, and neodymium (Nd) and strontium (Sr) isotopes for sedimentary provenance. High Hg concentrations compared to Northern Hemisphere records, and a shift to radiogenic Nd isotopes, indicates Kerguelen LIP volcanic activity and plateau uplift occurred in the lead up to and within OAE2. Whilst we find limited evidence that a volcanic event caused the MCE, pulsed Hg spikes before and during OAE2 imply volcanic emissions were key in driving climate and carbon cycle changes and triggering OAE2

Exhumed hydrocarbon-seep authigenic carbonates from Zakynthos island (Greece): Concretions not archaeological remains

In Zakynthos Island (Greece), authigenic cementation of marine sediment has formed pipelike, disc and doughnut-shaped concretions. The concretions are mostly composed of authigenic ferroan dolomite accompanied by pyrite. Samples with >80% dolomite, have stable isotope compositions in two groups. The more indurated concretions have δ 18O around +4‰ and δ 13C values between -8 and -29‰ indicating dolomite forming from anaerobic oxidation of thermogenic methane (hydrocarbon seep), in the sulphate-methane transition zone. The outer surfaces of some concretions, and the less-cemented concretions, typically have slightly heavier isotopic compositions and may indicate that concretion growth progressed from the outer margin in the ambient microbially-modified marine pore fluids, inward toward the central conduit where the isotopic compositions were more heavily influenced by the seep fluid. Sr isotope data suggest the concretions are fossil features, possibly of Pliocene age and represent an exhumed hydrocarbon seep plumbing system. Exposure on the modern seabed in the shallow subtidal zone has caused confusion, as concretion morphology resembles archaeological stonework of the Hellenic period

A large-scale transcontinental river system crossed West Antarctica during the Eocene

Extensive ice coverage largely prevents investigations of Antarctica’s unglaciated past. Knowledge about environmental and tectonic development before large-scale glaciation, however, is important for understanding the transition into the modern icehouse world. We report geochronological and sedimentological data from a drill core from the Amundsen Sea shelf, providing insights into tectonic and topographic conditions during the Eocene (~44 to 34 million years ago), shortly before major ice sheet buildup. Our findings reveal the Eocene as a transition period from >40 million years of relative tectonic quiescence toward reactivation of the West Antarctic Rift System, coinciding with incipient volcanism, rise of the Transantarctic Mountains, and renewed sedimentation under temperate climate conditions. The recovered sediments were deposited in a coastal-estuarine swamp environment at the outlet of a >1500-km-long transcontinental river system, draining from the rising Transantarctic Mountains into the Amundsen Sea. Much of West Antarctica hence lied above sea level, but low topographic relief combined with low elevation inhibited widespread ice sheet formation

Measurement of the cross section for isolated-photon plus jet production in pp collisions at √s=13 TeV using the ATLAS detector

The dynamics of isolated-photon production in association with a jet in proton–proton collisions at a centre-of-mass energy of 13 TeV are studied with the ATLAS detector at the LHC using a dataset with an integrated luminosity of 3.2 fb−1. Photons are required to have transverse energies above 125 GeV. Jets are identified using the anti- algorithm with radius parameter and required to have transverse momenta above 100 GeV. Measurements of isolated-photon plus jet cross sections are presented as functions of the leading-photon transverse energy, the leading-jet transverse momentum, the azimuthal angular separation between the photon and the jet, the photon–jet invariant mass and the scattering angle in the photon–jet centre-of-mass system. Tree-level plus parton-shower predictions from Sherpa and Pythia as well as next-to-leading-order QCD predictions from Jetphox and Sherpa are compared to the measurements

A search for resonances decaying into a Higgs boson and a new particle X in the XH → qqbb final state with the ATLAS detector

A search for heavy resonances decaying into a Higgs boson (H) and a new particle (X) is reported, utilizing 36.1 fb−1 of proton–proton collision data at collected during 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. The particle X is assumed to decay to a pair of light quarks, and the fully hadronic final state is analysed. The search considers the regime of high XH resonance masses, where the X and H bosons are both highly Lorentz-boosted and are each reconstructed using a single jet with large radius parameter. A two-dimensional phase space of XH mass versus X mass is scanned for evidence of a signal, over a range of XH resonance mass values between 1 TeV and 4 TeV, and for X particles with masses from 50 GeV to 1000 GeV. All search results are consistent with the expectations for the background due to Standard Model processes, and 95% CL upper limits are set, as a function of XH and X masses, on the production cross-section of the resonance