Seismic stratigraphic framework and depositional history for Cretaceous and Cenozoic contourite depositional systems of the Mozambique Channel, SW Indian Ocean

International audienceThis study describes previously unrecognized contourite depositional systems (CDSs) in the Mozambique Channel which constrain palaeoceanographic models for this area. The stratigraphic stacking patterns record nine seismic units (SU1 to SU9) separated by eight major discontinuities (a to h, oldest to youngest). Key seismic markers in CDS evolutionary history occur during Aptian-Albian (~122 Ma), late Cenomanian (94 Ma), early (38.2–36.2 Ma) and late (25–23 Ma) Oligocene, and early-middle Miocene (~17–15 Ma) epochs. These record onset (~122 to 94 Ma), growth (94 to 25–23 Ma), maintenance (25–23 to 17–15 Ma), and burial (17–15 Ma to the actual time) stages for CDSs. CDSs first develop during the onset stage which coincides with the opening and deepening of the African-Southern Ocean gateway (at 122 and 100 Ma, respectively). The growth stage, beginning in the late Cenomanian (94 Ma), correlates with the opening and deepening of the Equatorial Atlantic gateway. During the growth stage, two major shifts in sedimentary stacking pattern occur which coincide with palaeoceanographic changes during the early (38.2–36.2 Ma) and late (25–23 Ma) Oligocene. These in turn coincide with the onset and local enhancement of Antarctic water masses. CDS growth continued until the early-middle Miocene during the maintenance stage (~17–15 Ma). Most CDS growth ceased at the end of the maintenance stage. Circulation of the North Atlantic water mass into the Southern Hemisphere led to a deepening of Antarctic water masses in the area

Contourite depositional systems along the Mozambique channel:The interplay between bottom currents and sedimentary processes

We present a combined study of the geomorphology, sedimentology, and physical oceanography of the Mozambique Channel to evaluate the role of bottom currents in shaping the Mozambican continental margin and adjacent Durban basin. Analysis of 2D multichannel seismic reflection profiles and bathymetric features revealed major contourite deposits with erosive (abraded surfaces, contourite channels, moats, furrows and scours), depositional (plastered and elongated-mounded drifts, sedimentary waves), and mixed (terraces) features, which were then used to construct a morpho-sedimentary map of the study area. Hydrographic data and hydrodynamic modelling provide new insights into the distribution of water masses, bottom current circulation and associated processes (e.g., eddies, internal waves, etc.) occurring along the Mozambican slope, base-of-slope and basin floor. Results from this work represent a novel deep-sea sedimentation model for the Mozambican continental margin and adjacent Durban basin. This model shows 1) how bottom circulation of water masses and associated sedimentary processes shape the continental margin, 2) how interface positions of water-masses with contrasting densities (i.e., internal waves) sculpt terraces along the slope at a regional scale, and 3) how morphologic obstacles (seamounts, Mozambique Ridge, etc.) play an essential role in local water mass behaviours and dynamics. Further analysis of similar areas can expand understanding of the global role of bottom currents in deep-sea sedimentation

Fluid Circulations at Structural Intersections through the Toro-Bunyoro Fault System (Albertine Rift, Uganda): A Multidisciplinary Study of a Composite Hydrogeological System

Regional fault structures along rift basins play a crucial role in focusing fluid circulation in the upper crust. The major Toro-Bunyoro fault system, bounding to the east of the Albertine Rift in western Uganda, hosts local fluid outflow zones within the faulted basement rocks, one of which is the Kibiro geothermal prospect. This major fault system represents a reliable example to investigate the hydrogeological properties of such regional faults, including the local structural setting of the fluid outflow zones. This study investigated five sites, where current (i.e., geothermal springs, hydrocarbon seeps) and fossil (i.e., carbonate veins) fluid circulation is recognized. This work used a multidisciplinary approach (structural interpretation of remote sensing images, field work, and geochemistry) to determine the role of the different macroscale structural features that may control each studied fluid outflow zones, as well as the nature and the source of the different fluids. The local macroscale structural setting of each of these sites systematically corresponds to the intersection between the main Toro-Bunyoro fault system and subsidiary oblique structures. Inputs from three types of fluid reservoirs are recognized within this fault-hosted hydrogeological system, with “external basin fluids” (i.e., meteoric waters), “internal basin fluids” (i.e., hydrocarbons and sediment formation waters), and deep-seated crustal fluids. This study therefore documents the complexity of a composite hydrogeological system hosted by a major rift-bounding fault system. Structural intersections act as local relative permeable areas, in which significant economic amounts of fluids preferentially converge and show surface manifestations. The rift-bounding Toro-Bunyoro fault system represents a discontinuous barrier for fluids where intersections with subsidiary oblique structures control preferential outflow zones and channel fluid transfers from the rift shoulder to the basin, and vice versa. Finally, this work contributes to the recognition of structural intersections as prime targets for exploration of fault-controlled geothermal systems

LA LAGUNE PLÉISTOCÈNE, À <em>GOBIUS</em> SP. DU MONTE TORRE (CALABRE MÉRIDIONALE, ITALIE): SIGNIFICATION PALÉOGÉOGRAPHIQUE

Le bassin plio-quaternaire du Monte Torre a livré une faune de milieu lagunaire, qui a permis de reconstituer une partie de son évolution paléogéographique. Pendant tout le Pliocène et vraisemblablement jusqu'à la base du Pléistocène, ce bassin a fonctionné en régime sédimentaire et hydrodynamique de détroit. A cette époque cet étroit couloir reliait les mers tyrrhénienne et ionienne. Les structures sédimentaires et l'exubérance de la faune bathyale rencontrée en témoignent. Le fonctionnement tectono-sédimentaire de ce basin est en de nombreux points comparable à celui du Détroit de Messine pliocène. Vers la base du Pléistocène le détroit du Monte Torre s'interrompt brutalement. Les communicarions entre bassin tyrrhénien et ionien sont interrompues. Une lagune, ouverte sur le bassin ionien, s'installe à la place de I'ancien détroit. Cet événement brutal dans I'histoire du bassin est à relier à une surrection d'ensemble de la Calabre méridionale

Exploring the impact of Cenomanian paleogeography and marine gateways on oceanic oxygen

International audienceThe Cenomanian-Turonian period recorded one of the largest disruptions to the oxygen and carbon cycles, the Oceanic Anoxic Event 2 (OAE2, 94 Ma). This event is global, yet paleo-reconstructions document heterogeneous ocean oxygenation states and sedimentary carbon contents, temporally and spatially, suggesting that several mechanisms are at play. To better understand the long-term controls on oceanic oxygen and the initial oxygenation conditions prevailing at the beginning of OAE2, we perform numerical simulations of the Cenomanian using the IPSCL-CM5A2 Earth System Model, which includes a marine biogeochemistry component. We examine the control of the biogeochemical states of the global and Central Atlantic oceans by the depth of the Central American Seaway (CAS). The simulations show that a vigorous ocean circulation existed during the Cenomanian before OAE2 and that dysoxia/anoxia was caused by paleogeography rather than by ocean stagnation. The existence of restricted basins, disconnected from the deep global circulation and supplied with oxygen-depleted waters from Oxygen Minimum Zones of the surrounding basins, played a key role in the development of dysoxic/anoxic regions. A comparison with redox-proxy data suggests that a deep connection existed between the Pacific and Central Atlantic prior to OAE2. A shallowing of the CAS may have contributed to the establishment of enhanced anoxia in the Central Atlantic, but cannot be the only OAE2 triggering factor. The paleogeographic configuration and that of gateways appear as major factors controlling the oceanic circulation and oxygen distribution, leading to extended low-oxygenated oceanic areas as prerequisite conditions necessary for the OAE2 to occur

Orbital-scale deoxygenation trends driven by ventilation in Cretaceous ocean

International audienceMechanisms driving cyclicity in the marine realm during hothouse climate periods in response to Earth's orbit variations remains debated. Orbital cycles fingerprint in the oceanographic records results from the effect of terrestrial (eg. weathering-derived nutrient supply, freshwater discharge) and oceanic (eg. productivity, oxygenation) processes, whose respective contribution remains to be defined. Here we investigate the effect of extreme orbital configurations on oxygenation state of the ocean using ocean biogeochemistry simulations with the IPSL-CM5A2 Earth System Model under (CT) Cenomanian-Turonian boundary conditions. We also use an additional inert artificial tracer allowing to compute the age of water masses, corresponding to the time spent since the last contact with the surface. Our simulations show that small ocean ventilation changes triggered by orbitally-induced variations in high latitude deep water formation have strong impact on the oceanic oxygen spatial distribution. It is particularly true for the proto-Atlantic basin which is the less oxygenated basin during the CT (Laugie et al., 2021). The eight sets of orbital parameters tested here imply changes in the Atlantic anoxic seafloor area going from 20 to 80%. All three parameters describing the Earth's orbit (eccentricity, precession and obliquity) show a substantial control on these fluctuations. We also note that orbital fluctuations result in important changes in continental runoff but the impact remains highly localized to coastal environments - the open ocean mainly responding to the ocean ventilation. Last but not least, changes in productivity induced by the orbital parameters remain spatially heterogeneous and could be responsible for more local signal within a single basin. Laugié, M., Donnadieu, Y., Ladant, J. B., Bopp, L., Ethé, C., & Raisson, F. (2021). Exploring the impact of Cenomanian paleogeography and marine gateways on oceanic oxygen. Paleoceanography and Paleoclimatology, 36(7):e2020PA004202

Seismic stratigraphy of Cretaceous eastern Central Atlantic Ocean: Basin evolution and palaeoceanographic implications

International audienceThe evolution and resulting morphology of a Cretaceous contourite drift in the eastern Central Atlantic oceanic basin is investigated in unprecedented detail using seismic imaging and age-calibrated cross-margin sections. The margin, from the shelf, slope to deep-water and abyssal plain is constructed by a succession of erosive and depositional mounded structures that relate to bottom-water currents and sediment winnowing. The regional mapping of these drifts, sediment waves and gravitational sedimentary systems allows us to test the Upper Cretaceous paleocirculation model. Combined with flexural backstripping of the regional cross section, it reveals the water-depth range at which the observed sedimentary features occur. A possible late Albian to Turonian contourite drift system is observed from Guinea to Mauritania. The development of a shallow to deep oceanic circulation system is a key element in the rock record, with implications for the palaeoceanography and layering of the Cretaceous ocean. The Cretaceous geological interval and oceanic model mirrors the stratification of the modern ocean and the morphology of its seafloor from offshore Morocco to Guinea

Stripping back the modern to reveal the Cenomanian-Turonian climate and temperature gradient underneath

International audienceDuring past geological times, the Earth experienced several intervals of global warmth, but their driving factors remain equivocal. A careful appraisal of the main processes controlling past warm events is essential to inform future climates and ultimately provide decision makers with a clear understanding of the processes at play in a warmer world. In this context, intervals of greenhouse climates , such as the thermal maximum of the Cenomanian-Turonian (∼ 94 Ma) during the Cretaceous Period, are of particular interest. Here we use the IPSL-CM5A2 (IPSL: In-stitut Pierre et Simon Laplace) Earth system model to unravel the forcing parameters of the Cenomanian-Turonian greenhouse climate. We perform six simulations with an in-cremental change in five major boundary conditions in order to isolate their respective role on climate change between the Cenomanian-Turonian and the preindustrial. Starting with a preindustrial simulation, we implement the following changes in boundary conditions: (1) the absence of polar ice sheets, (2) the increase in atmospheric pCO 2 to 1120 ppm, (3) the change in vegetation and soil parameters, (4) the 1 % decrease in the Cenomanian-Turonian value of the solar constant and (5) the Cenomanian-Turonian palaeo-geography. Between the preindustrial simulation and the Cre-taceous simulation, the model simulates a global warming of more than 11 • C. Most of this warming is driven by the increase in atmospheric pCO 2 to 1120 ppm. Palaeogeo-graphic changes represent the second major contributor to global warming, whereas the reduction in the solar constant counteracts most of geographically driven warming. We further demonstrate that the implementation of Cenomanian-Turonian boundary conditions flattens meridional temperature gradients compared to the preindustrial simulation. Interestingly , we show that palaeogeography is the major driver of the flattening in the low latitudes to midlatitudes, whereas pCO 2 rise and polar ice sheet retreat dominate the high-latitude response