Transient ocean warming and shifts in carbon reservoirs during the early Danian

A long-standing question in Paleogene climate concerns the frequency and mechanism of transient greenhouse gas-driven climate shifts (hyperthermals). The discovery of the greenhouse gas-driven Paleocene–Eocene Thermal Maximum (PETM; ～ 55 Ma) has spawned a search for analogous events in other parts of the Paleogene record. On the basis of high-resolution bulk sediment and foraminiferal stable isotope analyses performed on three lower Danian sections of the Atlantic Ocean, we report the discovery of a possible greenhouse gas-driven climatic event in the earliest Paleogene. This event – that we term the Dan-C2 event – is characterized by a conspicuous double negative excursion in δ¹³C and δ¹⁸O, associated with a double spike in increased clay content and decreased carbonate content. This suggests a double period of transient greenhouse gas-driven warming and dissolution of carbonates on the seafloor analogous to the PETM in the early Paleocene at ～ 65.2 Ma. However, the shape of the two negative carbon isotope excursions that make up the Dan-C2 event is different from the PETM carbon isotope profile. In the Dan-C2 event, these excursions are fairly symmetrical and each persisted for about ～ 40 ky and are separated by a short plateau that brings the combined duration to ～ 100 ky, suggesting a possible orbital control on the event. Because of the absence of a long recovery phase, we interpret the Dan-C2 event to have been associated with a redistribution of carbon that was already in the biosphere. The Dan-C2 event and other early Paleogene hyperthermals such as the short-lived early Eocene ELMO event may reflect amplification of a regular cycle in the size and productivity of the marine biosphere and the balance between burial of organic and carbonate carbon.     

Innovative Contaminant Mass Flux Monitoring in an Aquifer Subject to Tidal Effects

Exposure from groundwater contamination to aquatic receptors residing in receiving surface water is dependent upon the rate of contaminated groundwater discharge. Characterization of groundwater fluxes is challenging, especially in coastal environments where tidal fluctuations result in transient groundwater flows towards these receptors. This can also be further complicated by the high spatial heterogeneity of subsurface deposits enhanced by anthropogenic influences such as the mixing of natural sediments and backfill materials, the presence of subsurface built structures such as sheet pile walls or even occurrence of other sources of contaminant discharge. In this study, the finite volume point dilution method (FVPDM) was successfully used to characterize highly transient groundwater flows and contaminant mass fluxes within a coastal groundwater flow system influenced by marked tides. FVPDM tests were undertaken continuously for more than 48 h at six groundwater monitoring wells, in order to evaluate groundwater flow dynamics during several tide cycles. Contaminant concentrations were measured simultaneously which allowed calculating contaminant mass fluxes. The study highlighted the importance of the aquifer heterogeneity, with groundwater fluxes ranging from 10⁻⁷ to 10⁻³ m/s. Groundwater flux monitoring enabled a significant refinement of the conceptual site model, including the fact that inversion of groundwater fluxes was not observed at high tide. Results indicated that contaminant mass fluxes were particularly higher at a specific monitoring well, by more than three orders of magnitude, than at other wells of the investigated aquifer. This study provided crucial information for optimizing further field investigations and risk mitigation measures.     

Particle transport in gravel-bed rivers: Revisiting passive tracer data

Data from tracer experiments were compiled and analysed in order to explore the role of geomorphological, hydrological and sedimentological constraints on fluvial gravel transport in gravel-bed rivers. A large data set from 217 transport episodes of tagged stones were compiled from 33 scientific papers. Our analyses showed that while magnitude of peak discharge is a major control on gravel transport and mobility, tracer travel distances show some scale dependence on the morphological configuration of the channel. Our results also highlight differences in the way tracers are displaced between step–pool and riffle and pool channels. The riffle–pool sequence seems to be a more efficient trap for travelling gravels than the step–pool pair. In addition, in step–pool channels there are clear differences in tracer transport between observations of first displacements after tracer seeding (unconstrained-stone conditions), and second and subsequent observations of tracer displacements (constrained-stone conditions). The comparison between tracer experiments under constrained conditions and those under unconstrained conditions also highlights the importance of bed state and structures in gravel mobility. The results of this study confirm that sediment transport in gravel-bed rivers is a complex process, whereby sedimentological and geomorphological controls are superimposed on the hydraulic forcing. © 2018 John Wiley & Sons, Ltd.     

Tracing the formation and differentiation of the Earth by non-traditional stable isotopes

The Earth has grown from chaotically mixed small dusts and gases to its present highly differentiated layered structure over the past 4.567 billion years. This differentiation has led to the formation of the atmosphere, hydrosphere,biosphere, crust, mantle, and core. The timing and mechanism for the formation and evolution of these different layers are still subjects of intense debate. This review brings together recent advances in using non-traditional stable isotopes to constrain major events and processes leading to the formation and differentiation of the Earth, including the Moon-forming giant impact, crustmantle interactions, evolution of life, the rise of atmospheric oxygen, extreme paleoclimate changes, and cooling rate of magmas.     

EUREC<Superscript>4</Superscript>A: A Field Campaign to Elucidate the Couplings Between Clouds,Convection and Circulation

Trade-wind cumuli constitute the cloud type with the highest frequency of occurrence on Earth, and it has been shown that their sensitivity to changing environmental conditions will critically influence the magnitude and pace of future global warming. Research over the last decade has pointed out the importance of the interplay between clouds, convection and circulation in controling this sensitivity. Numerical models represent this interplay in diverse ways, which translates into different responses of trade-cumuli to climate perturbations. Climate models predict that the area covered by shallow cumuli at cloud base is very sensitive to changes in environmental conditions, while process models suggest the opposite. To understand and resolve this contradiction, we propose to organize a field campaign aimed at quantifying the physical properties of trade-cumuli (e.g., cloud fraction and water content) as a function of the large-scale environment. Beyond a better understanding of clouds-circulation coupling processes, the campaign will provide a reference data set that may be used as a benchmark for advancing the modelling and the satellite remote sensing of clouds and circulation. It will also be an opportunity for complementary investigations such as evaluating model convective parameterizations or studying the role of ocean mesoscale eddies in air–sea interactions and convective organization.     

MAritime RISk Assessment (MARISA), a fuzzy approach to define an individual ship risk factor

This paper presents a fuzzy approach for the MAritime RISk Assessment (MARISA) applied to safety at sea. The aim of this work is to define automatically an individual ship risk factor which could be used in a decision making system. To achieve this purpose, a modular and hierarchical structure using fuzzy logic has been developed. It allows us to obtain a fuzzy risk factor (FRF) composed of a static risk factor (SRF) and a dynamic risk factor (DRF). The static risk factor assessment takes into account several static data relative to the ship (age, flag, gross tonnage, number of companies, duration of detention and type). The dynamic risk factor is evaluated by considering the meteorological conditions (sea state, wind speed and visibility) and the moment of the day. Moreover, the MARISA graphic interface developed with the Labview software is presented. This interface allows several simulations to be carried out to validate the fuzzy method proposed. Simulation results are presented.     

Quaternary thermokarst and thermal erosion features in northern France: origin and palaeoenvironments

A thermokarst is a collapse feature resulting from the thawing of ice‐rich permafrost or of massive ice of various origins. Little attention has been paid to the sedimentary fabric resulting from this type of collapse, except for glaciotectonic features. In western Europe, two palaeo‐forms are commonly studied: lithalsas and ice‐wedge casts. Collapsed pingos are much rarer. Very few papers have compiled present‐day and fossil data. Here, field data collected from quarries in the eastern Paris Basin were analysed, providing useful records of thermokarst collapses in alluvial calcareous silts, sands, and gravels. These forms have a circular shape when viewed on satellite images. Permafrost is attested regionally by the recurrent occurrence of meter‐sized pattern grounds at the surface of the chalk and of ice‐wedge casts. Traces of segregation and reticulate ice are common. These features are primarily connected to a major interstadial, c. 150 ka BP, orbitally forced and commonly associated with a major glacial retreat. They occur both in drained and waterlogged situations, resulting in a specific pattern of deformation. They are controlled by the brittle and plastic behaviour of sediments and resemble passive glaciotectonism. Normal and reverse faults, with the offset decreasing downward, are common, and those with local shear are reported. Lithalsas, seasonal frost blisters, spring frost blisters and perhaps pingos seem to have formed. Most of these deformations correspond to thermokarst sinkholes bordered by gravitational collapse faults. The offset of these faults increases towards the surface, and the faults have been recurrently confused with neotectonism triggered by palaeo‐earthquakes. However, there are no faults beneath the observed deformation features, and the region lacks recorded seismic activity over the last century. Our data may be helpful in interpreting similar structures elsewhere.     

Thermo‐tectonic evolution of the south‐central Pyrenees from rifting to orogeny: insights from detrital zircon U/Pb and (U‐Th)/He thermochronometry

Constraining the thermal and denudational evolution of continental margins from extensional episodes to early orogenic stages is critical in the objective to better understand the sediment routing during the growth of orogenic topography. Here, we report 160 detrital zircon U/Pb ages and 73 (U‐Th)/He ages from Albian, Upper Cretaceous and Eocene sandstones from the south‐central Pyrenees. All samples show dominant zircon U/Pb age peaks at 310–320 Ma, indicating a primary contribution from Variscan granites of the central Pyrenean Axial Zone. A secondary population at 450–600 Ma documents zircon grains sourced from the eastern Pyrenees. Zircon (U‐Th)/He ages recovered from older samples document, a Triassic age peak at ca. 241 Ma, corresponding to denudation coeval with the initiation of Atlantic rifting. An Early Cretaceous cooling event at ca. 133 Ma appears consistent with rift‐related exhumation and thermal overprint on the Iberian margin. The (U‐Th)/He age peaks from ca. 80 Ma to ca. 68 Ma with decreasing depositional ages are interpreted to reflect the southward‐migrating thrust‐related exhumation on the pro‐wedge side of the Pyrenean orogen. The increase in lag times, from ca. 15 Ma in the Tremp Formation (ca. 65 Ma) to 28 Ma in the Escanilla Formation (ca. 40 Ma), suggests decreasing exhumation rates from 0.4 km Myr^–1 to 0.2 km Myr^–1. The apparent inconsistency with convergence rates is used to infer that rocks cooled at 68 Ma may have resided in the crust before final exhumation to the surface. Finally, the cooling event observed at 68 Ma provides support to the inferred acceleration of convergence, shortening and exhumation during Late Cretaceous times.     

Reversible and irreversible impacts of greenhouse gas emissions in multi-century projections with the NCAR global coupled carbon cycle-climate model

The legacy of historical and the long-term impacts of 21st century greenhouse gas emissions on climate, ocean acidification, and carbon-climate feedbacks are investigated with a coupled carbon cycle-climate model. Emission commitment scenarios with zero emissions after year 2100 and 21st century emissions of 1,800, 900, and 0 gigatons of carbon are run up to year 2500. The reversibility and irreversibility of impacts is quantified by comparing anthropogenically-forced regional changes with internal, unforced climate variability. We show that the influence of historical emissions and of non-CO₂ agents is largely reversible on the regional scale. Forced changes in surface temperature and precipitation become smaller than internal variability for most land and ocean grid cells in the absence of future carbon emissions. In contrast, continued carbon emissions over the 21st century cause irreversible climate change on centennial to millennial timescales in most regions and impacts related to ocean acidification and sea level rise continue to aggravate for centuries even if emissions are stopped in year 2100. Undersaturation of the Arctic surface ocean with respect to aragonite, a mineral form of calcium carbonate secreted by marine organisms, is imminent and remains widespread. The volume of supersaturated water providing habitat to calcifying organisms is reduced from preindustrial 40 to 25% in 2100 and to 10% in 2300 for the high emission case. We conclude that emission trading schemes, related to the Kyoto Process, should not permit trading between emissions of relatively short-lived agents and CO₂ given the irreversible impacts of anthropogenic carbon emissions.