While microplastics are known to pervade the global seafloor, the processes that control their dispersal and concentration in the deep sea remain largely unknown. Here we show that ...thermohaline-driven currents, which build extensive seafloor sediment accumulations, can control the distribution of microplastics and create hotspots of up to 1.9 million pieces m
This is the highest reported value for any seafloor setting, globally. Previous studies propose that microplastics are transported to the seafloor by vertical settling from surface accumulations; here we demonstrate that the spatial distribution and ultimate fate of microplastics is strongly controlled by near-bed thermohaline currents (bottom currents). These currents are known to supply oxygen and nutrients to deep sea benthos, suggesting that deep sea biodiversity hotspots are also likely to be microplastic hotspots.
Introduction
Submarine canyons are incisive morphologies that play an important role in the exchange between shallow and deep waters. They interact with the general circulation and induce a specific ...circulation locally oriented by the morphology. The characteristics of the physical processes at play, the way they interact with each other and the influence of extreme events is still an open question as few observations are available. To answer this question and to improve the representation of submarine canyons in numerical models, it is key to understand the specific circulation patterns and their transitions in these specific environments.
Methods
This paper presents observations of currents, temperature and turbidity along the Cassidaigne canyon, northwestern Mediterranean Sea. Two oceanographic cruises carried out in 2017 and 2019 gathered data from the outer shelf and canyon head at 100-400 m depth to the base of the continental slope at 1900 m depth.
Results and Discussion
The circulation in the Cassidaigne area is subject to upwelling and downwelling-favorable winds, to the Northern Current and its associated mesoscale structures and is oriented by the local morphology. Upwellings occur both during stratified and non-stratified conditions. They are triggered by a wind forcing higher than 14 m s
–1
and their consecutive relaxations are marked by a counter-current. Near the canyon head and on the shelf, the current orientation depends on the stratification, the wind, the bottom morphology and the general circulation. The mesoscale variability of the Northern Current can lead to its intrusion over the shelf leading to barotropic cross currents over the canyon. At 1700 m depth, a quasi-permanent residual up-canyon flow is observed in a narrow gorge area and can be extrapolated to the canyon body. Finally, turbidity currents were observed for the first time in connection with upwelling events, suggesting the key role of canyons’ internal hydrodynamics on shelf sedimentary processes.
We derive a spectral element framework to compute the price of vanilla derivatives when the dynamic of the underlying follows a
general
exponential Lévy process. The representation of the solution ...with Legendre polynomials allows one to naturally approximate the convolution integral with high order quadratures. The method is spectrally accurate in space for the solution and the greeks, and third order accurate in time. The spectral element framework does not require an approximation of the Lévy measure nor the lower truncation of the convolution integral as commonly seen in finite difference approximations. We show that the spectral element method is ten times faster than Fast Fourier Transform methods for the same accuracy at strike, and two hundred times faster if one reconstructs the greeks from the solution obtained by FFT. We use the SEM approximation to derive the
Δ
and
Γ
in a variance gamma model, for which there is no closed form solution.
This research paper proposes two different methods for removing biases from geo-spatial trajectories obtained from AIS, enabling the creation of machine learning models that generalize well to areas ...with no training data. We focus here on the task of behavior detection such as moored, underway or drifting from AIS. The first method utilizes data augmentation techniques specifically designed for augmenting geo-spatial trajectories that include angled information such as direction, while the second method is based on engineering features that eliminate these geographical biases. These two methods are compared using different types of machine learning models, including random forests, hidden Markov models, LSTMs, and transformer networks.
•Vessel behavior detected from AIS with machine learning models.•Data augmentation or engineering of unbiased features allow us to generate global models.•Comparison of different models for behavior detection such as HMM, LSTM and transformer networks.
Contourites are common morphological features along continental margins where currents encounter the seafloor. They can provide long-term archives of palaeoceanography, may be prone to sediment ...instability, and can have a great potential for hydrocarbon exploration. Despite their importance and increasingly recognised ubiquitous occurrence worldwide, the link between oceanographic processes and contourite features is poorly constrained. In particular, it is unclear under which specific conditions sediments are mobilised, modified and deposited by bottom currents. Here, we aim to determine key bottom current characteristics (velocity and bottom shear stress) affecting contourite deposition, by assuming that recent oceanographic regimes may be extended back in time over the past glacial-interglacial cycles, with strong winter circulation assumed similar to glacial conditions and weak summer circulation to interglacials. We present an integrated study from the NW Mediterranean Sea that couples results of the MARS3D hydrodynamic model with high-resolution sedimentological and geophysical data (piston cores, multibeam bathymetry and high resolution seismic data). Near bottom circulation was modelled during winter and summer 2013 as representative of past periods of high and low current intensity, respectively. Model results match well with the extent of contourite depositional systems and their different localised morphologic elements. We deduce that higher intensity events control the formation of erosional features such as moats and abraded surfaces. The heterogeneous distribution of bottom-current intensity on slopes explains the development of different types of contourite drifts. Plastered drifts form in zones of low bottom-current velocities constrained upslope and downslope by higher current velocities. Separated elongated mounded drifts develop where fast bottom-currents decelerate at foot of the slope. In contrast, no mounded contourite morphologies develop when the current velocity is homogeneous across the slope, especially in margins prone to downslope sediment transport processes. In confined basins, gyres may transport sediment in suspension from a margin with a high sediment supply to an adjacent starved margin, favouring the development of fine-grained contourites in the latter. Our results provide new insights into how detailed bottom-circulation modelling and seafloor geomorphological analyses can improve the understanding of palaeoflow-regimes, at least over time spans when the overall paleogeography and the distribution of contourite drifts is comparable to present-day conditions. The approach of coupled hydrodynamic models and geomorphological interpretations proposed here for depositional, erosional and mixed contourite features may be used to understand other areas affected by bottom currents, and for a better conceptual understanding of bottom-current processes and their interactions with the seafloor.
•Hydrodynamic modelling is a useful tool to understand the formation of contourites.•Contourite drifts develop in zones of minimum bottom currents.•Plastered drifts develop in a zone of weak currents between two zones of stronger currents.•High intensity events control the formation of erosional features as moats.•Gyres favour the formation of contourites along starved margins in confined basins.
•Thermal and haline fronts differ in the central part of the Algero-Provençal basin.•Thermal fronts form a seasonal frontal zone from the Pyrenees almost toward Corsica.•Salinity fronts form a ...quasi-permanent frontal zone from Mallorca to South Sardinia.•The main salinity frontal zone shows a marked inter-annual variability.•Deep Water Formation and Algerian Eddies spreading shift the haline front.
We focus on the characterization of thermohaline fronts in the Western Mediterranean, with a particular focus on the North Balearic Front (NBF), separating the Atlantic Waters that spread into the Algerian Basin from the saltier and colder waters of the Liguro-Provençal area. We use a simple gradient-based method of front detection applied to a 20-year (from June 1993 to June 2013) reanalysis of the Mediterranean. Statistics of daily frontal indices are used to identify areas of recurrent fronts, i.e., frontal zones. Comparisons with data from glider transects and remotely sensed sea surface temperature and altimetry data have been used to validate our approach from daily to seasonal and interannual time scales. Our method yielded two co-existing frontal zones in the area of the NBF. One with an almost permanent haline surface frontal zone extending southeastward from the Balearic Islands to Sardinia, representing the northern limit of the fresher Atlantic Water that spreads via instabilities of the Algerian Current and associated Algerian Eddies. Between years, its position shifts by about 1° of latitude, possibly due to processes associated with both the deep water formation in the Provençal Basin and the spreading of southern Algerian Eddies. The second frontal zone is seasonal and thermally driven extending off the northeast of Menorca to the northwest of Corsica. The Pyrenees Front, a sharp thermal front off Cap de Creus which marks the boundary between the warm surface waters of the Balearic Sea and the cooler waters in the Gulf of Lion in late summer, appears to facilitate the formation of the aforementioned seasonal thermal front through the northeastward advection of its waters toward the West Corsican Current. The divergent eastward extensions of the two frontal zones, and their differences in nature, structure, and spatio-temporal variability, call for a revisit of the NBF appellation and a clarification of its dynamics.
Microplastics (MPs) were sampled in three seasons from 2016 to 2018 in the Bay of Marseille, northwestern Mediterranean Sea, adjacent to a highly urbanized area. Six sites were selected according to ...their different characteristics (river mouth, treatment plants, protected marine area). Surface floating MPs were characterized (number, weight, typology and polymer) as was zooplankton. In addition, mussels were submerged and used to investigate ingestion. Finally, a hydrodynamic model was used to improve understanding of dispersion mechanisms. The annual averages of floating MPs values ranged from 39,217 to 514,817 items/km2. The MPs collected were mainly fragments principally composed of polyethylene and polypropylene. The mean abundance ratio (MPs/zooplankton) was 0.09. On average 87% of mussel pools were contaminated and ingested 18.73 items/100 g of flesh. Two hydrodynamic patterns were identified: the first retaining the MPs in the harbor, and the second dispersing them outside.
•The distribution and fate of microplastics were evaluated in the bay of Marseille (French Northwestern Mediterranean Sea).•The floating MP reached more than 3.6 million items·km2, mainly composed of polyethylene and polypropylene.•87% of mussels ingested microplastics, with concentrations at 18.73 items/100 g of tissues on the average.•The ratio microplastics/zooplankton is a potential indicators of interactions between microparticles and zooplankton.•Two hydrodynamic patterns were identified through modelling studies, including retention in harbor and dispersion offshore.
Validation of oil spill forecasting systems suffers from a lack of data due to the scarcity of oil slick in situ and satellite observations. Drifters (surface drifting buoys) are often considered as ...proxy for oil spill to overcome this problem. However, they can have different designs and consequently behave in a different way at sea, making it not straightforward to use them for oil spill model validation purposes and to account for surface currents, waves and wind when modelling them. Stemming from the need to validate the MEDESS4MS (Mediterranean Decision Support System for Marine Safety) multi-model oil spill prediction system, which allows access to several ocean, wave and meteorological operational model forecasts, an exercise at sea was carried out to collect a consistent dataset of oil slick satellite observations, in situ data and trajectories of different type of drifters. The exercise, called MEDESS4MS Serious Game 1 (SG1), took place in the Elba Island region (Western Mediterranean Sea) during May 2014. Satellite images covering the MEDESS4MS SG1 exercise area were acquired every day and, in the case an oil spill was observed from satellite, vessels of the Italian Coast Guard (ITCG) were sent in situ to confirm the presence of the pollution. During the exercise one oil slick was found in situ and drifters, with different water-following characteristics, were effectively deployed into the oil slick and then monitored in the following days. Although it was not possible to compare the oil slick and drifter trajectories due to a lack of satellite observations of the same oil slick in the following days, the oil slick observations in situ and drifters trajectories were used to evaluate the quality of MEDESS4MS multi-model currents, waves and winds by using the MEDSLIK-II oil spill model. The response of the drifters to surface ocean currents, different Stokes drift parameterizations and wind drag has been examined. We found that the surface ocean currents mainly drive the transport of completely submerged drifters. The accuracy of the simulations increases with higher resolution currents and with addition of the Stokes drift, which is better estimated when provided by wave models. The wind drag improves the modelling of drifter trajectories only in the case of partially emerged drifters, otherwise it leads to an incorrect reproduction of the drifters׳ direction, which is particularly evident in high speed wind conditions.
We derive a spectral element framework to compute the price of vanilla derivatives when the dynamic of the underlying follows a general exponential LACOvy process. The representation of the solution ...with Legendre polynomials allows one to naturally approximate the convolution integral with high order quadratures. The method is spectrally accurate in space for the solution and the greeks, and third order accurate in time. The spectral element framework does not require an approximation of the LACOvy measure nor the lower truncation of the convolution integral as commonly seen in finite difference approximations. We show that the spectral element method is ten times faster than Fast Fourier Transform methods for the same accuracy at strike, and two hundred times faster if one reconstructs the greeks from the solution obtained by FFT. We use the SEM approximation to derive the $\Delta $ I and $\Gamma $ I in a variance gamma model, for which there is no closed form solution.
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