Ocean Circulation Research Articles

Microplastic abundance, characteristics, and heavy metal contamination in coastal environments of Western Sri Lanka

This study was conducted to assess the abundance of microplastics and associated metal contamination at selected beaches in the Western Province of Sri Lanka. Beach sand samples were collected from four beaches: Modera, Negombo, Mount Lavinia, and Panadura. Microplastics were extracted from dried sand samples using a saturated NaCl solution, followed by sieving. Particles were identified using Fourier Transform InfraRed Spectrophotometer, and associated heavy metals; Cr, Pb, Cu, Zn, and Ni were subjected to acid digestion for 24 h before analysis using Microwave Plasma Atomic Emission Spectrometry. More than half of the extracted plastics (56.31%) were identified as microplastics. The average microplastic abundance in beach sand samples ranged from 42.0 to 91.3 items/kg. The sand collected at Mount Lavinia exhibited the lowest sbundance, whereas those from Panadura beach revealed the highest. Hydrodynamic factors like ocean currents, wave patterns, associated with Southwest monsoon period, and human activities may have caused the variability in microplastic abundances and metal contamination. The majority of the microplastics (52.29%) were polyethylene, followed by polypropylene (35.18%), resembling the records of the most common plastic waste types in the country. Most of the microplastics were found to be fragments (87.95%), while white being the prominent color (53.49%). The toxic trace element concentration ranged from 5.0 × 10−3 to 1.8 × 102 μg/g in beaches. This study establishes a baseline for the west coastline prior to the X-press Pearl Ship Disaster in 2021. Future studies are encouraged to assess the beach microplastic pollution across the- Sri Lankan coastline.

Fate and mass budget of microplastic in the Beibu Gulf, the northern South China sea

This study investigated the distribution, abundance, and mass budget of microplastics (MPs) in the Beibu Gulf, Northern South China Sea, focusing on their ecological significance and anthropogenic influence. Microplastics were found in all water and sediment samples, with concentrations in surface water ranging from 0.10 to 0.89 items/m³, and in bottom water from 0 to 0.85 items/m³. Sediment samples exhibited higher levels, ranging from 13.12 to 155.59 items/kg. The spatial distribution revealed higher concentrations along the northern Gulf and Guangxi Province, suggesting significant human influence from coastal activities, such as mariculture and industrial discharges. The study utilized both field data and a mass balance box model to estimate the MPs mass budget, revealing that oceanic flow and riverine discharge are major contributors, accounting for 49% and 52% of MPs, while Atmospheric deposition and sedimentation represents 8% and 1%, respectively. The inventory estimation calculated that 0.24 tons of MPs exist in the water column, and 137 tons in the sediment, emphasizing the substantial environmental burden posed by these pollutants. The significance of this research lies in its comprehensive assessment of MPs in a semi-enclosed gulf, providing critical insight into the role of coastal and hydrodynamic processes in MP distribution. This study highlights the urgent need for better waste management practices in coastal regions to mitigate microplastic pollution and its detrimental effects on marine ecosystems.

Prediction of Sea Surface Current Around the Korean Peninsula Using Artificial Neural Networks

AbstractThe prediction of sea surface currents is essential for various marine activities such as disaster monitoring, fishing industries, and search and rescue operations. Continuous improvements in numerical models have made it possible to predict more realistic oceans using data assimilation and fine spatial resolution. However, such complicated ocean models require high computational power and time, making them difficult to use for near‐real time forecasting. To compensate for these high computational costs, novel approaches with efficient computational costs combined with numerical model outputs need to be developed. Artificial neural networks could be one of the solutions as they require low computational power for prediction, owing to pre‐trained networks. Here, we present a prediction framework applicable to surface current prediction in the seas around the Korean Peninsula using three‐dimensional (3‐D) convolutional neural networks. The network is based on a 3‐D U‐shaped network structure and is modified to predict sea surface currents using oceanic and atmospheric variables. The forecast procedure is optimized to minimize the error of the next day's sea surface current field with a spatial resolution of 1/24° and its recursively predicting structure allows more days to be predicted. The network performance is evaluated by changing the input days and variables to find the optimal surface‐current‐prediction artificial neural network model. The optimized model shows a root mean squared error of 2.3 cm/s for the first forecast day, demonstrating its strong potential for practical use in the near future.

The first Mud Dragons (Kinorhyncha) from the Emperor Seamount Chain (Northwestern Pacific) with notes on their biogeography and distribution patterns in the Pacific Deep-Sea

Seamounts and oceanic currents play crucial roles in shaping the genetic diversity of species by either acting as barriers or pathways for species dispersal. In the meiofaunal samples collected using ROV facilities at the slope of the Koko Guyot (2172 m) in the southernmost part of the Emperor Seamount Chain in the Northwestern Pacific, we found representatives of three kinorhynch genera, Echinoderes cf. lupherorum Sørensen et al. 2018, Campyloderes cf. vanhoeffeni Zelinka, 1913 and Sphenoderes sp. 1., previously known from other locations in the Pacific. Kinorhynchs were studied and illustrated using light (LM) and scanning electron (SEM) microscopy to prove the species identity and to compare morphological variations with representatives of these species collected from other, very distant localities in the Pacific. We illustrate the pan-oceanic distribution of the collected species with a system of abyssal currents in the Pacific. The probable distribution pattern corresponds with the northward pathway of Antarctic Bottom Water out of the Southern Ocean in the Pacific through the deep-sea area off New Zealand. Echinoderes lupherorum appears to be the first representative of the Echinoderidae with the pan-Pacific distribution. Discussions on the biogeography of Campyloderes cf. vanhoeffeni Zelinka, 1913 and Sphenoderes in the Pacific are also provided.

Advancements to generalized equivalence theory for preserving cross-sections of whole core simulations

The conventional two-step approach, without the use of an equivalence method, can introduce significant error for the simulation of non-LWRs (Light Water Reactors), especially fast reactors, due to the large leakage and high anisotropic neutron distribution. To improve upon the accuracy of the two-step approach, a 3D whole core model is simulated with a transport method to generate region-wise homogenized cross-sections (XS). These XS can then be used in a diffusion whole core solver during the second step to extend the application to cycle and transient analysis. Discontinuity factors (DFs) are then introduced to improve the accuracy during the simulation of the second step. With properly generated DFs from Generalized Equivalence Theory (GET), the region-averaged solutions from the first transport step can then be reproduced by the second step diffusion solver. The Monte Carlo method was selected to perform the whole core transport simulation to generate region-wise XS. However, simulating whole core problems with Monte Carlo may result in poor statistics near the peripheral region especially for partial current tallies. This paper introduces an advancement to GET to reproduce region-wise solutions for select regions when the reaction rates and surface currents have good statistics in these regions but poor statistics in other regions. A reference high-fidelity model was constructed using the Serpent 2 Monte Carlo code based on the EBR-II benchmark evaluation and verification was carried out using the TriPEN-4 method in PARCS. The results show that it is possible to reproduce the exact eigenvalue and power distributions of whole core problems in a feasible manner.

Research on Submersible Position Prediction Based on Kalman Filter Algorithm

The global situation of the world's submersible technology is experiencing a boom, with countries competing to develop advanced submersible equipment to explore the deep sea, undersea resources, and uncharted territories. With the wide range of applications of submersibles in various fields, the risks of communication loss and mechanical failures faced during their missions require the establishment of accurate predictive models of submersible positions to ensure safe and efficient rescue. In this study, a model containing equations of motion is developed from the motion characteristics of a submersible on the seafloor. Uncertainties such as sea currents, topography, and seawater density are considered, and a random walk model is used to simulate these effects to ensure that the model is closer to reality. Subsequently, combined with the Kalman filter algorithm, a time-dependent position prediction model is constructed in this study. The model can predict the position of the submersible in real-time, and after error analysis, it shows that the average variance and root mean square error are at a low level, which proves that its prediction is accurate and reliable, and provides strong technical support for the safe rescue of the submersible.

Three-layer circulation in the world deepest hadal trench

The Challenger Deep (CD) is the deepest known hadal trench in the world. Due to challenges in data sampling at extreme ocean depths, the Lower Circumpolar Deep Water (LCDW) transport and ocean circulation structure in the CD remain unclear. By analyzing data from an extra-deep current meter mooring array, here we find a three-layer circulation in the CD, transitioning downward from westward LCDW flow (about −1.866 ± 2.953 Sv, 1 Sv = 106 m3/s) to cyclonic circulation, and then to anticyclonic circulation. The westward flow reverses its direction during summer, giving evidence for bidirectional connectivity of deep-sea basins, while the cyclonic-anticyclonic circulation is relatively steady. The LCDW intrusion, local topography and turbulent mixing are crucial for modulating the three-layer circulation. Turbulent mixing plays a key role in driving the anticyclonic circulation. Our findings provide insights for understanding the hydrodynamic environment in the ocean’s deepest areas.

Evaluation of GOCE/GRACE and combined global geopotential models using GNSS/levelling data over Nigeria

AbstractGlobal Geopotential Models (GGMs) provide valuable information about Earth’s gravity field functionals, such as geoid heights and gravity anomalies. However, ground-based datasets are required to validate these GGMs at the regional and local scales. In this study, we validated the accuracy of GGMs by comparing them with ground-based Global Navigation Satellite System (GNSS)/levelling data for the first time in Nigeria. We employed two validation scenarios: with and without considering spectral consistency using the spectral enhancement method (SEM) to incorporate high and very high frequencies of the gravity field spectrum from the combined global gravity field model (XGM2019e_2159) and the residual terrain model (RTM) derived from the Shuttle Radar Topography Mission (SRTM) data, respectively. The results of this evaluation confirmed that the application of SEM improved the assessment of the GGM solutions in an unbiased manner. Integrating XGM2019e_2159 and SRTM data to constrain the high-frequency component of geoid heights in Gravity Field and Steady-State Ocean Circulation Explorer (GOCE)-based GGMs led to an improvement of approximately 10% in reducing the standard deviation (STD) relative to when SEM was not applied. GO_CONS_GCF_2_TIM_R6 at spherical harmonics (SH) of up to degree and order 260 demonstrated the lowest STD when compared to GO_CONS_GCF_2_DIR_R6 and GO_CONS_GCF_2_SPW_R5, with a reduction from 0.380 m without SEM application to 0.342 m with SEM implementation. In addition, four transformation models, namely, linear, four-parameter, five-parameter, and seven-parameter models, were evaluated. The objective is to mitigate the reference system offsets between the GNSS/levelling data and the GGMs and to identify the particular parametric model with the smallest STD across all GGMs. This effort reduced the GGMs misfits to GNSS/levelling to 0.30 m, representing a 15.3% decrease in STD. Notably, the XGM2019e_2159 model provides this improvement.

Towards the construction of regional marine radiocarbon calibration curves: an unsupervised machine learning approach

Abstract. Radiocarbon may serve as a powerful dating tool in palaeoceanography, but its accuracy is limited by the need to calibrate radiocarbon dates to calendar ages. A key problem is that marine radiocarbon dates must be corrected for past offsets from either the contemporary atmosphere (i.e. “reservoir age” offsets) or a modelled estimate of the global average surface ocean (i.e. delta-R offsets). This presents a challenge because the spatial distribution of reservoir ages and delta-R offsets can vary significantly, particularly over periods of major marine hydrographic and/or carbon cycle change such as the last deglaciation. Modern reservoir age and delta-R estimates therefore have limited applicability. While forward modelling of past R-age variability has been proposed as a means of resolving this problem, this requires accurate a priori knowledge of past global radiocarbon budget closure (i.e. production, and cycling), which we currently lack. In this context, the construction of empirical regional marine calibration curves could provide a way forward. However, the spatial reach of such calibrations and their robustness subject to (uncertain) temporal changes in climate and ocean circulation would need to be tested. Here, we use unsupervised machine learning techniques to define distinct regions of the surface ocean that exhibit coherent behaviour in terms of their radiocarbon age offsets from the contemporary atmosphere (R ages), regardless of the causes of R-age variability. We apply multiple algorithms (k-means, k-medoids, and hierarchical clustering) to outputs from two different numerical models spanning a range of climate states, forcings, and timescales of adjustment. Comparisons between the cluster assignments across model runs confirm some robust regional patterns that likely stem from constraints imposed by large-scale ocean and atmospheric physics. At the coarsest scale, regions of coherent R-age variability correspond to the major ocean basins. By further dividing basin-scale shape-based clusters into amplitude-based subclusters, we recover regional associations, such as increased high-latitude R ages, or the propagation of R-age anomalies from regions of deep mixing in the Southern Ocean to upwelling sites in the eastern equatorial Pacific, which cohere with known modern oceanographic processes. We show that the medoids (i.e. the most representative locations) for these regional sub-clusters provide significantly better approximations of simulated local R-age variability than constant offsets from the global surface average. This remains true when cluster assignments obtained from one model simulation are applied to simulated R-age time series from another. Further, model-based clusters are found to be broadly consistent with existing reservoir age reconstructions that span the last ∼30 kyr. We therefore propose that machine learning provides a promising approach to the problem of defining regions for which empirical marine radiocarbon calibration curves may eventually be generated.

Stokes drift and particle trajectories induced by surface waves atop a shear flow

Surface waves and currents are crucial to the mass transfer in the air-sea interaction as they can drive a variety of dynamical processes. How mass can be transported by surface waves and current coupling is addressed through a study of their induced motions of fluid parcels. To this end, a weakly nonlinear wavetrain is imposed on the background flow whose direction and magnitude are permitted to vary with water depth and second-order features of this configuration are investigated. A leading-order approximation to the Stokes drift is derived, correct to the second order in wave steepness, and applicable to an arbitrarily depth-dependent background flow. The reduced forms of the approximate Stokes drift are provided in a few limiting cases such as a current with an exponential profile or propagating in an orthogonal direction to the wave propagation. Novel features related to the Stokes drift and particle trajectories have been reported for the first time as a result of the rotation induced by the wave and current coupling. A non-vanishing component of the Stokes drift velocity and net-mean displacement of fluid parcels in the span-wise direction to the wave propagation are observed in the cases where a shear current propagates obliquely to the waves direction. A non-monotonic dependence on water depth of the stream-wise component of the Stokes drift is shown, and thereby the largest mass transport induced no longer occurs on the still water surface but some depth beneath. The non-monotonic behavior occurs beyond the regime of the near-irrotational assumption of wave-induced motions. It can also lead to the change of the signs for the stream-wise Stokes drift throughout the water column, and thus an overall cancellation of the integrated mass transport by waves over the water column, indicating that the depth-integrated models can likely lead to underestimated effects of the mass transport which is non-trivial at a local depth. The results from this study have far-reaching impact. The Stokes drift profile is a direct input to the parametrization of the surface waves forcing in ocean circulations and the obliquely propagating Stokes drift can be plausibly responsible for the formation of oblique Langmuir rolls to wave propagation in the open ocean.

Water budget of the Caspian Sea by numerical experiments with ocean circulation model INMIO-CICE in the last glacial maximum and pre-industrial period

We used the hydrodynamic model of the Caspian Sea, INMIO-CICE, to calculate equilibrium river runoff and evaporation from the sea surface over a wide range of sea levels (from –85 to +50 m asl) for different climatic conditions: the Last Glacial Maximum (about 21kyr) and pre-industrial climate (~1850 CE). Data from the climate model INMCM4.8 were used as boundary conditions. It was found that to maintain sea level at 35–50 m asl, corresponding to the maximum values of the Khvalynian transgression, a river runoff of about 400 km3/year was required in the Last Glacial Maximum. In the Last Glacial Maximum evaporation from the sea surface decreased by 105–170 mm (12–22%), and precipitation, according to the INMCM4.8 model, by 50–70 mm (15–30%). This caused the equilibrium runoff to decrease by about 10–20% compared to pre-industrial conditions. Smaller absolute and relative changes correspond to lower sea levels. The maximum decrease in evaporation occurred at 5 m asl.

Evolution of a buried moat-drift system within the Ewing Terrace uncovering highly dynamic bottom currents at the Argentine Margin during Early Oligocene to Middle Miocene

The Ewing Terrace is a relatively flat surface formed by the action of bottom currents and part of a Contourite Depositional System (CDS) at the Argentine continental slope. It is situated in a highly complex oceanographic setting at the Brazil-Malvinas Confluence Zone. Located in water depths of approx. 1000 – 1200 m and incised by the Mar del Plata Canyon, the Ewing Terrace is separated into the Northern Ewing Terrace (NET) and the Southern Ewing Terrace (SET). The long-term variations in ocean circulation led to a complex internal architecture of the terrace. As a result, this region represents a unique archive for studying sedimentary features that were eroded, transported, and deposited by along- and down-slope processes. An in-depth data analysis of high-resolution multichannel seismic profiles exhibits a complex sequence of erosional and depositional contouritic features, namely buried moat-drift systems identified in depths of approx. 370-750 m below the seafloor. They are arranged in migrating sequences and clustered in the Early Oligocene to Middle Miocene. This pattern is probably attributable to the vertical shift of water masses and to a highly dynamic oceanographic setting with spatial changes influenced by the Brazil-Malvinas Confluence Zone over this particular geological time. The moat-drift systems reveal significant lateral changes from north to south. In the southern area of the SET the moats are constructional and the associated separated mounded drifts are well developed. In contrast, the northern area exhibits two types of moats, reminiscent of cut-and-fill structures that mirror the significant and rapid changes in bottom current dynamics. With these new insights, this study contributes to a better understanding of moat-drift systems and improves the knowledge about past oceanographic dynamics and sediment deposition at the northern Argentine margin.

Mitochondrial DNA patterns describe the evolutionary history of the bonnethead shark Sphyrna tiburo (Linneus 1758) complex in the western Atlantic Ocean.

The apparent lack of physical barriers in the marine realm has created the conception that many groups have a constant gene flow. However, changes in ocean circulation patterns, glacial cycles, temperature, and salinity gradients are responsible for vicariant events in many fish species, including sharks. The bonnethead shark, Sphyrna tiburo, is an endangered small coastal shark species. Although considerable efforts have recently been undertaken, little remains known about the possible biogeographic scenario that can explain its actual distribution within the western Atlantic (WA). Here, we used 599 mitochondrial sequences to assess the phylogeographic structure and implement Bayesian phylogenetic analyses to obtain divergence times and reconstruct the ancestral geographic range. This allowed us to infer processes responsible for the diversification of S. tiburo into major divergent lineages. Our results indicated that S. tiburo in the WA represents three independent lineages, with Brazilian samples differentiated into a distinct genetic cluster. The posterior probability of ancestral range analysis indicated that the species likely originated in the northern region (Carolina Province and the southern Gulf of Mexico), where it colonized southward through the uplifting of the Central American Isthmus (CAI). The Northern and Caribbean genetic clusters appear to have arisen from the intensification of the Loop Current, which currently flows northward passing the Yucatan Peninsula, Gulf of Mexico, and east Florida. Following initial colonization, the Northeastern Brazil group differentiated from the Caribbean region due to the sediment and freshwater discharge of the Amazon-Orinoco Plume. Thus, the evolutionary history of the S. tiburo complex can be explained by a combination of dispersal and vicariance events that occurred over the last ~5 million years (MY). We established and confirmed the species and population limits, demonstrating that the Amazon-Orinoco Plume constitutes a significant dispersal barrier for coastal sharks. Finally, we discuss some recommendations for the conservation of the bonnethead shark.

Noisy Dueling Double Deep Q-Network algorithm for autonomous underwater vehicle path planning.

How to improve the success rate of autonomous underwater vehicle (AUV) path planning and reduce travel time as much as possible is a very challenging and crucial problem in the practical applications of AUV in the complex ocean current environment. Traditional reinforcement learning algorithms lack exploration of the environment, and the strategies learned by the agent may not generalize well to other different environments. To address these challenges, we propose a novel AUV path planning algorithm named the Noisy Dueling Double Deep Q-Network (ND3QN) algorithm by modifying the reward function and introducing a noisy network, which generalizes the traditional D3QN algorithm. Compared with the classical algorithm [e.g., Rapidly-exploring Random Trees Star (RRT*), DQN, and D3QN], with simulation experiments conducted in realistic terrain and ocean currents, the proposed ND3QN algorithm demonstrates the outstanding characteristics of a higher success rate of AUV path planning, shorter travel time, and smoother paths.

'Oevaru' 2023: Ocean currents and the journey of an inaugural mental health conference in the Maldives.

This paper describes an important mental health initiative in Maldives - the inaugural national mental health research conference. Amid socio-cultural challenges in the background of climate change, amplified by COVID-19, mental health issues in Maldives surged. National Centre for Mental Health (NCMH), acknowledging data gaps, has prioritized evidence-based care since 2019 with a vision of building a research culture. Oevaru 2023, Maldives' inaugural mental health research conference, was convened by NCMH and held on October 15-16, 2023, addressing holistic care, prevention, innovation, and investment in mental health as sub-themes. Pre-conference sessions provided skill-building, followed by workshops on specialized topics. Oevaru featured keynotes, expert talks, panel discussions, and poster presentations, including a hybrid session with the Creating Futures Conference. It culminated in a white paper, emphasizing research-driven strategies for a resilient mental health system in Maldives.

Propulsion and energy extraction performance of wave-powered mechanism considering wave and current

Wave-powered mechanism can extract wave energy as propulsion, enabling long-term ocean observation. The kinematic properties of wave propulsion mechanisms are susceptible to ocean current loads in addition to wave effects, presenting complex dynamic behaviors. In this paper, to understand the effects of ocean currents on the kinematic performances of wave propulsion mechanism, the dynamic performance of a wave propulsion mechanism in the combined condition of wave and current is analyzed based on numerical simulation method for fluid-rigid body coupling and experimental methods. The performance of the propulsion mechanism against the current under counter-current conditions and the contribution of flow to the propulsion performance under co-current conditions are discussed. The intrinsic relationship between wave height, current velocity and current direction is investigated, and the energy extraction performance of wave propulsion mechanism is analyzed. According to the results, the ability of propulsion mechanism to resist counter-current is obtained, and it is found that as the counter-current velocity increases, the hydrofoil will stall and the propulsion force and efficiency decreases significantly. Whilst, the co-current has a promoting effect on the forward of the mechanism, but within a certain range of co-current speed, the mechanism will be negatively affected by the shedding vortex. And the discrimination mechanisms for wave-dominated propulsion and current-dominated propulsion are obtained. This work reveals the combined effect of waves and currents on the motion performance of wave propulsion mechanism, providing a reference for the development of surface vehicles that utilize multiple ocean energies for synergistic propulsion.

Dynamic event-triggered integrated task and motion planning for process-aware source seeking

The process-aware source seeking (PASS) problem in flow fields aims to find an informative trajectory to reach an unknown source location while taking the energy consumption in the flow fields into consideration. Taking advantage of the dynamic flow field partition technique, this paper formulates this problem as a task and motion planning (TAMP) problem and proposes a bi-level hierarchical planning framework to decouple the planning of inter-region transition and inner-region trajectory by introducing inter-region junctions. An integrated strategy is developed to enable efficient upper-level planning by investigating the optimal solution of the lower-level planner. In order to leverage the information acquisition and computational burden, a dynamic event-triggered mechanism is introduced to enable asynchronized estimation, region partitioning and re-plans. The proposed algorithm provides guaranteed convergence of the trajectory, and achieves automatic trade-offs of both exploration-exploitation and accuracy-efficiency. Simulations in a highly complicated and realistic ocean surface flow field validate the merits of the proposed algorithm, which demonstrates a significant reduction in computational burden without compromising planning optimality.

Blocking effects on mean ocean currents by offshore wind farm foundations

Blocking effects on mean ocean currents by offshore wind farm foundations

Archaeopteris trees at high southern latitudes in the late Devonian

During the Devonian Period plants first reached forest stature, impacting chemical weathering of rocks, fluvial systems, atmospheric composition and possibly aquatic eutrophication. Hypothetically these factors contributed to increasing climatic instability culminating in the End Devonian Mass Extinction Event. Understanding the timing of the spread of forests is however a prerequisite to correlation with its proposed consequences. Though evidence for forests at low palaeolatitudes demonstrates their emergence by the mid Devonian, sparse high-palaeolatitude records almost entirely comprise herbaceous lycopods. By the Famennian forest ecosystems are widely evidenced at low palaeolatitudes, however high latitude palaeofloras are almost exclusively represented by a single locality, the Waterloo Farm lagerstätte from South Africa (approximate palaeolatitude, 70°S). Understanding climatic and ecological conditions at this locality is doubly important as it also hosts diverse vertebrate taxa, including the only high latitude Devonian tetrapods. Archaeopteris, the quintessential Late Devonian woody tree, has previously been identified at this locality on the basis of leafy branch system fragments, though some uncertainty has remained as to whether these represent tree sized organisms. Here we present co-occurring large axes, including a trunk base, attributable to Archaeopteris trees inferred to be in excess of 20 m height, the first demonstration of forest stature at high latitudes in the Devonian. This possibly reflects high latitude climatic amelioration, resultant from warm ocean currents circulating southwards in response to progressive closure of the Iapetus Sea. As such, changing continental configurations may have indirectly facilitated the spread of forest ecosystems and helped to drive climatic instability and ultimately extinctions towards the end of the Devonian.

A Model-Free Adaptive Positioning Control Method for Underactuated Unmanned Surface Vessels in Unknown Ocean Currents

Aiming to address the problem of underactuated unmanned surface vehicles (USVs) performing fixed-point operations at sea without dynamic positioning control systems, this paper introduces an original approach to positioning control: the virtual anchor control method. This method is applicable in environments with currents that change slowly and does not require prior knowledge of current information or vessel motion model parameters, thus offering convenient usability. This method comprises four steps. First, a concise linear motion model with unknown disturbances is proposed. Then, a motion planning law is designed by imitating underlying principles of ship anchoring. Next, an adaptive disturbance observer is proposed to estimate uncertainties in the motion model. In the last step, based on the observer, a sliding-mode method is used to design a heading control law, and a thrust control law is also designed by applying the Lyapunov method. Numerical simulation experiments with significant disturbances and tidal current variations are conducted, which demonstrate that the proposed method has a good control effect and is robust.