ALBERT

Description: Oxygen isotope geochemistry is a powerful tool for investigating rocks that interacted with fluids, to assess fluid sources and quantify the conditions of fluid-rock interaction. We present an integrated modelling approach and the computer program PTLOOP that combine thermodynamic and oxygen isotope fractionation modelling for multi-rock open systems. The strategy involves a robust petrological model performing on-the-fly Gibbs energy minimizations coupled to an oxygen fractionation model both based on internally consistent databases. This approach is applied to subduction zone metamorphism to predict the possible range of δ18O values for stable phases and aqueous fluids at various pressure-temperature (P-T) conditions in the subducting slab. The modelled system is composed by a sequence of oceanic crust (mafic) with sedimentary cover of known initial chemical composition and bulk δ18O. The evolution of mineral assemblage and δ18O values of each phase is calculated along a defined P-T path. Fluid-rock interactions may occur as consequence of (1) infiltration of an external fluid into the mafic rocks or (2) transfer of the fluid liberated by dehydration reactions occurring in the mafic rocks into the sedimentary rocks. The effects of interaction with externally-derived fluids on the mineral and bulk δ18O of each rock are quantified for two typical compositions of metabasalts and metasediments with external fluid influx from serpentinite. The dehydration reactions, fluid loss and mineral fractionation produce minor to negligible variations in bulk δ18O values, i.e. within 1 ‰. By contrast, the interaction with external fluids may lead to shifts in δ18O up to one order of magnitude larger. Such variations can be detected by analysing in-situ oxygen isotope in key metamorphic minerals such as garnet, white mica and quartz. The simulations show that, when the water released by the slab infiltrates the forearc mantle wedge, it can cause extensive serpentinization within fractions of a Myr and significant oxygen isotope variation at the interface. This technique opens new perspectives to track fluid pathways in subduction zones, to distinguish porous from channelized fluid flows, and to determine the P-T conditions and the extent of fluid/rock interaction.

Description: Projected future shoaling of the wintertime mixed layer in the Northeast (NE) Atlantic has been shown to induce a regime shift in the main nutrient supply pathway from the Atlantic to the Northwest European Shelf (NWES) near the end of the 21st century. While reduced winter convection leads to a substantial decrease in the vertical nutrient supply and biological productivity in the open ocean, vertical mixing processes at the shelf break maintain a connection to the subpycnocline nutrient pool and thus productivity on the shelf. Here we investigate how meltwater discharge from the Greenland ice sheet (GIS) not yet taken into account impacts the mixed layer shoaling and the regime shift in terms of spatial distribution and temporal variability. To this end we have downscaled sensitivity experiments by a global earth system model for various GIS melting rates with a regionally coupled ocean-atmosphere climate system model. The model results indicate that increasing GIS meltwater discharge leads to a general intensification of the regime shift. Atlantic subpycnocline water masses mixed up at the shelf break become richer in nutrients and thus limit the projected nutrient decline on the shelf. Moreover, the stronger vertical nutrient gradient through the pycnocline results in an enhanced interannual variability of on-shelf nutrient fluxes which, however, do not significantly increase variations in nutrient concentrations and primary production on the shelf. Moreover, due to the impact of the GIS meltwater discharge on the NE Atlantic mixed layer depth, the regime shift becomes initiated earlier in the century by about 1–2 decades, depending on the discharge rate. The effect on the onset timing, though, is found to be strongly damped by the weakening of the Atlantic meridional overturning circulation. A GIS melting rate that is even 10 times higher than expected for emission scenario RCP8.5 would lead to an onset of the regime shift not until the 2070s.

Description: The Humboldt Large Marine Ecosystem (HLME) and Patagonian Large Marine Ecosystem (PLME) are the two largest marine ecosystems of the Southern Hemisphere, respectively located along the Pacific and Atlantic coasts of southern America. This work investigates the exchange between these two LMEs and its variability, employing numerical model results and offline particle tracking algorithms. 27 years of a 1/12° ROMS configuration (CMM) show a general poleward transport on the Southern region of HLME, and equatorward on the Patagonian Shelf (PS). A mean transport across Cape Horn's shelf (68.1° W) is 0.95 Sv. Lagrangian simulations show that the majority of the southern PS waters originate from the upper layer in the southeast South Pacific (〉 200 m), mainly from the southern Chile and Cape Horn shelves. The exchange takes place through Le Maire Strait, Magellan Strait, and the shelf-break. These inflows account to a net northeastward transport of 0.88 Sv at 51° S in the southern PLME. The transport across Magellan strait is small (0.1 Sv) but due to its relatively low salinity it impacts greatly the density and surface circulation of the coastal waters of the southern PLME. The water masses flowing into the Malvinas Embayment eventually reach the PLME through the Malvinas Shelf and occupy the outer part of the shelf. The seasonal and interannual variability of the transport are also addressed. On the southern PLME, the interannual variability of the shelf exchange is partly explained by the large-scale wind variability, which in turn is partly associated with the SAM index (r = 0.52).

Description: Anelasticity may decrease the shear modulus of the asthenosphere by 8–10 % at semi-diurnal tidal periods compared with the reference 1 s period of seismological Earth models. We show that such anelastic effects are likely to be significant for ocean tide loading displacement at the M2 tidal period around the East China Sea. By comparison with tide gauge observations, we establish that NAO99Jb is the most accurate numerical ocean tide model in this region, and that related errors in the predicted M2 vertical ocean tide loading displacements will be 0.2–0.5 mm. In contrast, GPS observations on the Ryukyu Islands (Japan), with uncertainty 0.2–0.3 mm, show discrepancies of over 1.5 mm with respect to ocean tide loading displacements predicted using the purely elastic radial Preliminary Reference Earth Model. We show that the use of an anelastic PREM-based Earth model reduces these discrepancies to no more than 0.8 mm, which is of the same order as the sum of the remaining errors due to uncertainties in the ocean tide model and the GPS observations. Use of a regional Earth model based on the laterally-varying S362ANI, with or without further empirical tuning, results in minor additional improvements in fit.

Description: The Kuroshio Current System in the North Pacific displays path transitions on a decadal time scale. It is known that both internal variability involving barotropic and baroclinic instabilities and remote Rossby waves induced by North Pacific wind-stress anomalies are involved in these path transitions. However, the precise coupling of both processes and its consequences for the dominant decadal transition time scale are still under discussion. Here, we analyse the output of a multi-centennial long high-resolution global climate model simulation and study phase synchronisation between Pacific zonal wind-stress anomalies and Kuroshio Current System path variability. We apply the Hilbert transform technique to determine the phase and find epochs where such phase synchronisation appears. The physics of this synchronisation is shown to occur through the effect of the vertical motion of isopycnals, as induced by the propagating Rossby waves, on the instabilities of the Kuroshio Current System.

Description: Soil position in the landscape reveals its history of formation and genesis. Therefore, the landscape is the combination of features of the surface of the earth with subsurface components (parent material), while the soil is a three-dimensional, dynamic natural body inserted in the landscape. This research aimed to study the soil-landscape relationship in a sandstone-gneiss topolithosequence in Amazonas, Brazil. The study was carried out along a 9.253-meter transect from the top downwards the softer slope. Soil profiles were selected in five landscape compartments (top, upper third, lower third, transport foothill, and deposition foothill). Morphological, mineralogical, physical, chemical, and ray diffraction characterizations were performed. Soils had different morphological, physical, chemical, and mineralogical attributes due to the variations of the geological substrate and landscape position. The mineralogy of the clay fraction is composed of kaolinite, goethite, hematite, and gibbsite, with goethite being the predominant iron oxide. A sand fraction dominance was observed in relation to the other fractions in all the profiles, being related to the alluvial nature of the parent material, with the highest values occurring in the lower third. The separation of the landscape into geomorphic surfaces and identification of the parent material were effective for understanding the variation of soil attributes along the landscape.

Description: A dynamically passive inert tracer was released in the interior South Pacific Ocean at latitudes of the Antarctic Circumpolar Current. Observational cross sections of the tracer were taken over four consecutive years as it drifted through Drake Passage and into the Atlantic Ocean. The tracer was released within a region of high salinity relative to surrounding waters at the same density. In the absence of irreversible mixing a tracer remains at constant salinity and temperature on an isopycnal surface. To investigate the process of irreversible mixing we analysed the tracer in potential density versus salinity-anomaly coordinates. Observations of high tracer concentration tended to be collocated with isopycnal salinity anomalies. With time an initially narrow peak in tracer concentration as a function of salinity at constant density, broadened with the tracer being found at ever fresher salinities, consistent with diffusion-like behaviour in that coordinate system. The second moment of the tracer as a function of salinity suggested an initial period of slow spreading for approximately 2 years in the Pacific, followed by more rapid spreading as the tracer entered Drake Passage and the Scotia Sea. Analysis of isopycnal salinity gradients based on the Argo programme suggests that part of this apparent change can be explained by changes in background salinity gradients while part of the change may be explained by geographical changes in background mixing.

Description: Surface currents are poorly known over most of the oceans. Satellite-borne Doppler Waves and Current Scatterometers (DWCS) can be used to fill this observation gap. The Sea surface KInematics Multiscale (SKIM) proposal, is the first satellite concept built on a DWCS design at near-nadir angles, and now one of the two candidates to become the 9th mission of the European Space Agency Earth Explorer program. As part of the detailed design and feasibility studies (phase A) funded by ESA, airborne measurements were carried out with both a Ku-Band and a Ka-Band Doppler radars looking at the sea surface at near nadir-incidence in a real-aperture mode, i.e. in a geometry and mode similar to that of SKIM. The airborne radar KuROS was deployed to provide simultaneous measurements of the radar backscatter and Doppler velocity, in a side-looking configuration, with an horizontal resolution of about 5 to 10 m along the line of sight and integrated in the perpendicular direction over the real-aperture 1-way 3-dB footprint diameter (about 580 m). The KaRADOC system has a much narrower beam and footprint that only about 45 m in diameter. The experiment took place in November 2018 off the French Atlantic coast, with sea states representative of the open ocean and a well known tide-dominated current regime. The data set is analyzed to explore the contribution of non-geophysical velocities to the measurement and how the geophysical part of the measured velocity combines wave-resolved and wave-averaged scales. We find that the measured Doppler velocity contains a characteristic wave phase speed, called here C0 that is analogous to the Bragg phase speed of coastal High Frequency radars that use a grazing measurement geometry, with little variations ΔC associated to changes in sea state. The Ka-band measurements at an incidence of 12° are 10 % lower than the theoretical estimate C0 ~ 2.4 m/s for typical oceanic conditions defined by a wind speed of 7 m/s and a significant wave height of 2 m. For Ku-band the measured data is 30 % lower than the theoretical estimate 2.8 m/s. ΔC is of the order of 0.2 m/s for a 1 m change in wave height, and cannot be confused with a 1 m/s change in tidal current. The actual measurement of the current velocity from an aircraft at 4 to 18° incidence angle is, however, made difficult by uncertainties on the measurement geometry, which are much reduced in satellite measurements.

Description: The Coastal-Ocean Carbon Exchange in the Canary Region Project (COCA) arises in order to analyse and get to understand the impact of lateral export of nutrients and organic matter from the highly productive Coastal Upwelling System off NW Africa in the biogeochemical cycles during two different seasons. The circulation patterns off NW African Upwelling System are examined by applying an inverse model to two hydrographic datasets gathered in fall 2002 and spring 2003. The mass transports estimated by model are consistent with the thermal wind equation and the conservation of mass in a closed volume. Besides, the Ekman transport and the freshwater flux are also considered. These estimates show a seasonal variability in the circulation patterns at central levels, particularly in the southern boundary of the domain, where the Cape Verde Frontal Zone is located. In the beginning of fall, this circulation is deeper and northward with a net transport of 6 ± 3 Sv and, in the late spring, it is shallower and southward with a similar intensity. At intermediate levels important differences are also observed between the two seasons. In fall, the Antarctic Intermediate Waters reaches higher latitudes with 2 ± 2 Sv flowing northward. During spring, there is no significant northward flow of AAIW. However, there is a moderate westward mass transport which impacts both the lateral transports of inorganic nutrients and organic matter at intermediate layers and also the shallowest lateral transports of organic matter. Seasonal variability in circulation patterns are also reflected in lateral transports of inorganic nutrients and dissolved organic carbon. Therefore, the changes in the circulation patterns between the two seasons have allowed us to assess the variability in the contributions of SiO2, NO3, PO4 and DOC from the first to the second season. In fall, the transports are mainly northward from the south with −0.80 ± 0.34, −1.11 ± 0.47 and −0.07 ± 0.03 kmol s-1 of SiO2, NO3 and PO4, respectively. In spring, however, lateral transports off-shore are favoured with 0.75 ± 0.37, 1.34 ± 0.66 and 0.08 ± 0.04 kmol s-1 of SiO2, NO3 and PO4, respectively. This westward transport stimulates in turn an intensified westward DOC transport at shallow layers, specifically 0.50 ± 0.25 x 108 mol C day-1.

Description: Tectonic nappes are observed for more than a hundred years. Although geological studies often refer to a nappe theory, the physical mechanisms of nappe formation are still incompletely understood. We apply two-dimensional numerical simulations of shortening of a passive margin, to investigate the thermo-mechanical processes of detachment, transport and stacking of nappes. We use a visco-elasto-plastic model with standard creep flow laws and Drucker-Prager yield criterion. We consider tectonic inheritance with two initial mechanical heterogeneities: (1) lateral heterogeneity of the basement-cover interface due to half-grabens and horsts and (2) vertical heterogeneities due to layering of mechanically strong and weak sedimentary units. The model shows detachment and horizontal transport of a thrust nappe and stacking of this thrust nappe above a fold nappe. The detachment of the thrust sheet is triggered by stress concentrations around the sediment-basement contact and the resulting brittle-plastic shear band formation. The horizontal transport is facilitated by a basal shear zone just above the basement-cover contact, composed of thin, weak sediments. Fold nappe formation occurs by a dominantly ductile closure of a half-graben and the associated extrusion of the half-graben fill. We apply our model to the Helvetic nappe system in Western Switzerland, which is characterized by stacking of the Wildhorn thrust nappe above the Morcles fold nappe. The modeled structures and temperature field agree with data from the Helvetic nappe system. The mechanical heterogeneities must generate contrasts in effective viscosity (i.e. ratio of stress to strain rate) of four orders of magnitude to model nappe structures similar to the ones of the Helvetic nappe system.