Deep-water Regions Research Articles

Water Mass Structure and Mixing Fractions in the Deepwater Region of the Gulf of Mexico

AbstractHere, we estimate water mass fractions in the deepwater region of the southern Gulf of Mexico from samples collected during June 2016. Past water mass characterizations have relied mainly on temperature, salinity, and dissolved oxygen concentrations, although the degree of mixing and the effects on the distribution of nutrients and other dissolved constituents remain uncertain. Based on nine water types defined in the eastern Caribbean Sea and North Atlantic and excluding surface waters, five dominant water masses with contributions between 50% and 93% in their cores were identified. North Atlantic Subtropical Underwater contributed 42% ± 9% between 49 and 105 m. Modal water with origins in the northern subtropical gyre (Eighteen Degree Water) were ubiquitous in the gulf, with average contributions of 62% ± 25% between 110 and 153 m and 51% ± 12% between 248 and 306 m, while the core of Tropical Atlantic Central Water (TACW; 70% ± 5%) dominated between 355 and 510 m. Water at the nitrate maximum consisted of TACW (48% ± 13%) and Antarctic Intermediate Water (AAIW; 32 ± 14%), with the local remineralization of organic matter adding ∼10% to the nitrate pool. Below ∼800 m, water transitioned from AAIW to a domain of two components of Labrador Seawater. Mesoscale eddies controlled the observed depth variability of the water masses from the surface down to ∼1,000 m. This study provides a quantitative approximation to evaluate the addition of nutrients to the different water masses by biogeochemical processes driven by an epipelagic ecosystem generally considered to be oligotrophic.

Impacts of mesoscale eddies on the nitrate distribution in the deep-water region of the Gulf of Mexico

Mesoscale eddies play an important role in modulating the vertical distributions of nutrients and primary productivity in the ocean. In this study, we evaluated the role that eddies play in determining the vertical distributions of Nitrate + Nitrite (N + N), dissolved oxygen (DO), conservative temperature (CT), and absolute salinity (S A ) in the deep-water region of the Gulf of Mexico (GoM). We compared water-column profiles up to 1000 m depth and calculated the N + N stock in the euphotic layers within cyclonic eddies (CEs), anticyclonic Loop Current eddies (LCEs), and waters free of eddy influence (NOE). Stations were sampled based on a fixed grid during two oceanographic campaigns carried out under winter mixing and thermal stratification conditions and were classified as CE, LCE, or NOE stations based on a cluster analysis using dynamic height (DH) and the depth of the 25.5 kg m −3 isopycnal. Average profiles of the stations closest to the eddy cores (those with the highest or lowest dynamic heights and isopycnal depths of 25.5 kg m −3 ) were compared with those of NOE stations. Differences between structures were observed in multiple variables up to 1000 m, although the most notable differences occurred in the euphotic and upper mesopelagic zones (< 250 m). The average concentration of N + N between 0 and 50 m was ~0.03 μmol kg −1 , which was close to the detection limit, regardless of the presence or absence of eddies. The onset of the nitracline, defined as the depth at which the N + N concentration was 0.5 μmol kg −1 , was observed at ~56 m, ~ 82 m, and between ~100–120 m in CE, NOE, and LCE waters, respectively, which resulted in contrasting N + N stocks in the intermediate euphotic layer (50–100 m) among CE (158–208 mmol m −2 ), LCE (6–9 mmol m −2 ), and NOE (~ 28 mmol m −2 ) stations. As a result, fluorescence in the deep chlorophyll maximum was markedly higher in CE waters compared to that in NOE and LCE waters. In the summer campaign, a subsurface DO maximum between 30 and 50 m, which is characteristic of oligotrophic regions, was observed as a product of photosynthesis most likely associated with a positive anomaly of preformed N + N and positioned around the 24.3 kg m −3 isopycnal. In subsurface CE, LCE, and NOE waters, negative anomalies of preformed N + N were observed at different depths that remained between the same isopycnals (24.8–26.2 kg m −3 ), reaching up to −3.4 μmol kg −1 in CEs. As has been suggested for other oligotrophic regions, this anomaly could be due to the uptake of N + N by migrating phytoplankton without DO production, the use of N + N by heterotrophic bacteria that are limited by dissolved organic nitrogen, the remineralization of transparent exopolymeric particles (TEPs) with high C:N ratios, or to a combination of these processes, which remain to be studied within the GoM. • N + N stocks in the Gulf of Mexico are strongly modulated by mesoscale eddies. • Shallow/deep nitraclines within cyclonic (56 m)/Loop Current (LC;100–120 m) eddies. • N + N stocks (mmol m −2 ) in cyclonic eddies (158–208) greater than in LC (6–9) eddies. • Deep chlorophyll maximum fluorescence markedly high in cyclonic eddies. • Subsurface preformed N + N negative anomaly at 24.8–26.2 kg m −3 isopycnals in summer.

Bathymetric and Capacity Relationships Based on Sentinel-3 Mission Data for Aswan High Dam Lake, Egypt

Aswan High Dam Lake (AHDL) is one of the most relevant hot spots at both local and global levels after construction of the Grand Ethiopian Renaissance Dam (GERD) was announced. The management of AHDL is a vital task, which requires the input of reliable information such as the lake bathymetry, water level, and the water surface area. Traditional, bathymetric methods are still very expensive and difficult to operate. Nowadays, satellite data and remote sensing techniques are easily accessible. In particular, datasets produced by operational missions are freely and globally available, and may provide efficient and inexpensive solutions for the retrieval of quantitative parameters concerning strategic water bodies, such as AHDL. This work identifies the performance of Sentinel-3A optical imagery data in the visible and NIR bands from the two optical instruments SLSTR and OLCI, and proposes the integration with Sentinel-3A radar altimetry from SRAL instrument applied to AHDL. This preliminary and first study investigated the relationship between the reflectance data and in situ data for water depth after a bathymetric campaign in the deep-water region using statistical regression models. These statistical models showed promising results in terms of correlation value (R2 &gt; 0.8) and normalized root mean square errors (NRMSE &lt; 0.4). Also, Heron’s formula was applied to combine optical imagery and Sentinel-3 altimetry water level datasets to estimate water storage variations in AHDL. In addition, equations governing the relationship between water level, water surface area, and water volume were analyzed. The work is very useful for all authorities and stakeholders dealing with large water bodies.

Impact of highest maximum sustained wind speed and its duration on storm surges and hydrodynamics along Krishna–Godavari coast

The storm surge and hydrodynamics along the Krishna–Godavari (K–G) basin are examined based on numerical experiments designed from assessing the landfalling cyclones in Bay of Bengal (BoB) over the past 38 years with respect to its highest maximum sustained wind speed and its duration. The model is validated with the observed water levels at the tide gauge stations at Visakhapatnam during 2013 Helen and 2014 Hudhud. Effect of gradual and rapid intensification of cyclones on the peak water levels and depth average currents are examined and the vulnerable locations are identified. The duration of intensification of a rapidly intensifying cyclone over the continental shelf contributed to about 10–18% increase in the peak water levels, whereas for the gradually intensifying cyclone the effect is trivial. The inclusion of the wave-setup increased the peak water levels up to 39% compared to those without wave-setup. In the deep water region, only rapidly intensifying cyclones affected the peak MWEs. Intensification over the continental slope region significantly increases the currents along the shelf region and coast. The effect on peak maximum depth averaged current extends up to 400 km from the landfall location. Thus, it is necessary to consider the effect of various combinations of the highest cyclone intensity and duration of intensification for identifying the worst scenarios for impact assessment of coastal processes and sediment transport. The study is quite useful in improving the storm surge prediction, in preparedness, risk evaluation, and vulnerability assessment of the coastal regions in the present changing climate.

Hydrocarbon Systems and Estimation of the Probability of Discovery of Oil and Gas Accumulations in the Bering Sea

The results of studying the hydrocarbon systems with application of the modelling techniques using the Schlumberger PetroMod software complexes are considered. Modeling of sedimentary basins of the Bering Sea and their hydrocarbon systems is performed. Estimation of the probability of discovery of oil and gas accumulations related to the marine hydrocarbon systems showed that the highest probabilities of the discovery characterize the GAHS correspondent to the Eocene-Miocene (Mainitsky-Sobolkov) sediments. Probabilities of discovery of the deposits within the younger Gagarin-Avtatkul hydrocarbon systems (Lower to Middle Miocene) are substantially lower. The perspectives of discovery of oil and gas accumulations are related to oil and gas generation and accumulation capacities of insufficiently studied and distributed along the area of the deep-water region Cretaceous-Cenozoic or more ancient sloping deep sea sedimentary complexes, total capacity of which is higher in the northern and eastern near-slope zones of the Aleutian basin comprising 6 to 8 km or more. Studying of the above complexes would apparently facilitate a substantial increase of total hydrocarbon potential of the Bering Sea.

Variability of fish larvae assemblages relative to mesoscale features in the deep water region of the southern Gulf of Mexico

Variability of fish larvae assemblages relative to mesoscale features in the deep water region of the southern Gulf of Mexico

Nitrogen sources (NO3− vs N2 fixation) inferred from bulk δ15N values of zooplankton from the deep water region of the Gulf of Mexico

Abstract Dinitrogen (N2) fixation is an important source of new N in oceanic oligotrophic waters, but its contribution has not been quantified in the central and southern Gulf of Mexico (GM), where mesoscale anticyclonic (ACE) and cyclonic eddies (CE) influence NO3− availability for phytoplankton. We investigated the temporal and spatial variation in zooplankton δ15N values as a proxy for baseline nitrogen sources and estimated the contribution of N2 fixation and subsurface nitrate to secondary production. Copepods, euphausiids and two size classes of zooplankton were collected from 0 to 200 m during five cruises. Zooplankton δ15N values were higher during winter due to wind-driven deepening of the mixed layer, reflecting NO3− transport toward the surface. There was a higher estimated contribution of N2 fixation during the highly stratified summer months. Zooplankton δ15N values from ACEs were significantly lower than in CE, associated with a deepening of the 25.5 isopycnal and lower depth-integrated [NO3−]. N2 fixation contributed 60–80% of the N in ACEs. Some stations within the Bay of Campeche showed higher δ15N values due to NO3− transport to the euphotic layer due to a semipermanent CE, regional upwelling and river discharge. Our results highlight the importance of fixed nitrogen over an extensive region of theGM.

Marine Heatwaves in Siberian Arctic Seas and Adjacent Region

We used a satellite-derived global daily sea surface temperature (SST) dataset with resolution 0.25 × 0.25∘ to analyze interannual changes in the Arctic Shelf seas from 2000 to 2020 and to reveal extreme events in SST distribution. Results show that the second decade of the 21st century for the Siberian Arctic seas turned significantly warmer than the first decade, and the increase in SST in the Arctic seas could be considered in terms of marine heatwaves. Analyzing the spatial distribution of heatwaves and their characteristics, we showed that from 2018 to 2020, the surface warming extended to the northern deep-water region of the Laptev Sea 75∘ to 81∘N. To reveal the most important forcing for the northward extension of the marine heatwaves, we used three-dimensional numerical modeling of the Arctic Ocean based on a sea-ice and ocean model forced by the NCEP/NCAR Reanalysis. The simulation of the Arctic Ocean variability from 2000 to 2020 showed marine heatwaves and their increasing intensity in the northern region of the Kara and Laptev seas, closely connected to the disappearance of ice cover. A series of numerical experiments on the sensitivity of the model showed that the main factors affecting the Arctic sea-ice loss and the formation of anomalous temperature north of the Siberian Arctic seas are equally the thermal and dynamic effects of the atmosphere. Numerical modeling allows us to examine the impact of other physical mechanisms as well. Among them were the state of the ocean and winter sea ice, the formation of fast ice polynias and riverine heat influx.

Numerical study on wave attenuation of extreme waves by emergent rigid vegetation patch

Attributed to the processes of wave focusing, extreme waves often occur unexpectedly with huge wave height in both deep and shallow water regions, which can cause serious damages to the coastal infrastructures, and threaten the intactness of coastal communities. Field observations after extreme weather events have revealed that as a type of natural barrier, coastal vegetations can effectively attenuate the intensity of extreme surges and waves and thus play a good role in naturally protecting the coast. Usually, focused wave is used as a typical extreme wave model to study the great impact of extreme waves on the coast. To numerically evaluate the wave attenuation of extreme waves by the emergent rigid vegetation patch, a high-resolution numerical wave tank has been established by applying the nonhydrostatic numerical wave model (NHWAVE), where both 2nd-order crest and trough focused waves can be generated by considering wave-wave interactions. The efficiencies of the rigid emergent vegetation patch in mitigating the maximum wave amplitude and total wave energy of both crest and trough focused waves as function significant wave height, peak wave period, water depth, vegetation density and width have been well analyzed and discussed. The results indicate that the emergent rigid vegetation patch can effectively reduce the maximum wave amplitude of focused wave by averagely 31% and the total wave energy by averagely 45%, thus, showing that coastal vegetation can effectively protect the coast from extreme waves. It is hoped that the research findings drawn from this study can further broaden our understanding on the wave attenuation of extreme waves by coastal vegetations.

Computer Aided Coupled Smart Prediction Model of Cement Temperature During Pressure Cementing in Deep-Water

Managed Pressure Cementing (MPC) is a new technology, which can precisely control the annular fluid pressure profile. Accurate calculation of wellbore temperature and pressure is the key to MPC. This paper focus on coupled models of temperature and pressure for MPC in deep-water region. On basis of the fluid rheology model, a coupled model of temperature and pressure in wellbore is established considering the transient flow characteristics during cementing displacement stage. A series of numerical simulations are conducted. Simulation results show that neglecting the interactions between temperature and pressure can cause a big error. During the well cementing, higher temperature in the deep part of wellbore reduces the fluid viscosity, which leads to a smaller friction. On the contrary, larger friction is observed near seabed as a result of the low temperature in deep-water environment. The pressure in wellbore changes frequently due to the coexistence of multiple fluids in wellbore.

Remineralization dominating the δ13C decrease in the mid-depth Atlantic during the last deglaciation

δ13C records from the mid-depth Atlantic show a pronounced decrease during the Heinrich Stadial 1 (HS1), a deglacial episode of dramatically weakened Atlantic Meridional Ocean Circulation (AMOC). Proposed explanations for this mid-depth decrease include a greater fraction of δ13C-depleted southern sourced water (SSW), a δ13C decrease in the North Atlantic Deep Water (NADW) end-member, and accumulation of the respired organic carbon. However, the relative importance of these proposed mechanisms cannot be quantitatively constrained from current available observations alone. Here we diagnose the individual contributions to the deglacial Atlantic mid-depth δ13C change from these mechanisms using a transient simulation with carbon isotopes and idealized tracers. We find that although the fraction of the low-δ13C SSW increases in response to a weaker AMOC during HS1, the water mass mixture change only plays a minor role in the mid-depth Atlantic δ13C decrease. Instead, increased remineralization due to the AMOC-induced mid-depth ocean ventilation decrease is the dominant cause. In this study, we differentiate between the deep end-members, which are assigned to deep water regions used in previous paleoceanography studies, and the surface end-members, which are from the near-surface water defined from the physical origin of deep water masses. We find that the deep NADW end-member includes additional remineralized material accumulated when sinking from the surface (surface NADW end-member). Therefore, the surface end-members should be used in diagnosing mechanisms of δ13C changes. Furthermore, our results suggest that remineralization in the surface end-member is more critical than the remineralization along the transport pathway from the near-surface formation region to the deep ocean, especially during the early deglaciation.

Nonlinear hydrodynamic characteristics of multi-body platform system

Along with the technology development of ocean resources, offshore platforms are gradually becoming larger and more complex. Recent development of the oil and gas field in the deep-water region often involves multiple floating platforms adjacent to each other to perform more complex functions for oil and gas production. This paper describes the investigation carried out on the dynamic responses of a two platforms system containing a Tension Leg Platform (TLP) and a tender assisted drilling (TAD) with a flexible connection between the two platforms. The mooring lines and tendons are taken into consideration in the coupled analysis of the multi-body platform’s system. The motion responses and wave load characteristics on the two platforms in multi-body coupled model are investigated in the numerical simulation. Compared to the situation of the two platforms in isolation, it is revealed that the motion responses for the two platforms in coupled model are altered by the combined effects of the platforms’ interaction and constrain by the connection. The interaction between platforms can increase both the first- and second-order wave force on platforms in the arrangement direction of two platforms. Quantitative analysis demonstrates their relative importance and thus provides much-needed guidance in practical design of the coupled system.

Light Regulation of Phytoplankton Growth in San Francisco Bay Studied Using a 3D Sediment Transport Model

In San Francisco Bay (SFB), light availability is largely determined by the concentration of suspended particulate matter (SPM) in the water column. SPM exhibits substantial variation with time, depth, and location. To study how SPM influences light and phytoplankton growth, we coupled a sediment transport model with a hydrodynamic model and a biogeochemical model. The coupled models were used to simulate conditions for the year of 2011 with a focus on northern SFB. For comparison, two simulations were conducted with ecosystem processes driven by SPM concentrations supplied by the sediment transport model and by applying a constant SPM concentration of 20 mg l–1. The sediment transport model successfully reproduced the general pattern of SPM variation in northern SFB, which improved the chlorophyll-a simulation resulting from the biogeochemical model, with vertically integrated primary productivity varying greatly, from 40 g[C] m–2year–1over shoals to 160 g[C] m–2year–1in the deep channel. Primary productivity in northern SFB is influenced by euphotic zone depth (Ze). Our results show thatZein shallow water regions (&amp;lt;2 m) is mainly determined by water depth, whileZein deep water regions is controlled by SPM concentration. As a result,Zehas low (high) values in shallow (deep) water regions. Large (small) differences in primary productivity exist between the two simulations in deep (shallow) water regions. Furthermore, we defined a new parameterFlightfor “averaged light limitation” in the euphotic zone. The averaged chlorophyll-a concentration in the euphotic zone andFlightshare a similar distribution such that both have high (low) values in shallow (deep) water regions. Our study demonstrates that light is a critical factor in regulating the phytoplankton growth in northern SFB, and a sediment transport model improves simulation of light availability in the water column.

An integrated study of the petrographic and geochemical characteristics of organic-rich deposits of the Wufeng and Longmaxi formations, western Hubei Province, South China: Insights into the co-evolution of paleoenvironment and organic matter accumulation

Enhanced productivity and/or widespread anoxia have been postulated as the dominant controls on organic matter (OM) accumulation in South China during the Ordovician-Silurian (O-S) transition. However, their roles appear to have varied in space and time, and causal mechanisms linking OM accumulation with changing paleoenvironmental conditions remain poorly constrained. Here we report on results of a multi-faceted investigation of the Wufeng and Longmaxi formations in a drilled core from Xianfeng County, western Hubei Province, South China. The roles played by paleoredox conditions, paleoproductivity, paleoclimate, terrigenous input, upwelling, and volcanism on OM accumulation and preservation during the O-S transition are considered. Our results suggest that the depositional history of OM accumulation as recorded by the studied succession can be subdivided into five phases, each one represented by a lithostratigraphic unit. Unit 1 (average TOC = 3.28%) comprises siliceous shale deposited under generally suboxic conditions interrupted by brief episodes of anoxia beneath very productive surface water during the late Katian time (~447.62 Ma). Seasonal upwelling-driven productivity appears to have exerted fist-order control on OM accumulation. Overlying siliceous organic-rich shale of unit 2 (average TOC = 4.93%) records anoxic bottom water conditions from the latest Katian to Hirnantian time. Enhanced productivity and OM preservation may have been maintained by phosphorous cycling. Calcareous shale of the overlying Guanyingqiao unit 3 (TOC = 2.8%) accumulated under suboxic conditions and low to moderate water column productivity during the late Hirnantian Stage. Climate-driven sea level fall was responsible for diminished OM accumulation at this time. Siliceous clay-rich shale of unit 4 (average TOC = 5.53%) was deposited beneath highly productive surface water under anoxic bottom water conditions at the beginning of the Rhuddanian. The presence of volcanic ash and enriched Hg concentrations in these deposits suggests that productivity was enhanced by volcanism. Overlying silty shale of unit 5 (average TOC = 1.71%) was deposited in the middle to upper Rhuddanian under dominantly suboxic conditions, declining productivity levels, and increased delivery of terrigenous sediments. Accumulation of these deposits was contemporaneous with a period of falling sea level related to the Kwangsian Orogeny that would have curbed OM accumulation. Our results, considered in tandem with those of studies from shallow to deep water regions of the Yangtze Block, reveal a level of spatial heterogeneity of redox conditions and productivity levels across the Yangtze Sea during the O-S transition manifested by regional differences in OM accumulation mechanisms and histories.

Strong Motions on Land and Ocean Bottom: Comparison of Horizontal PGA, PGV, and 5% Damped Acceleration Response Spectra in Northeast Japan and the Japan Trench Area

ABSTRACT A large-scale permanent ocean-bottom seismograph network, named S-net, has been established in the Japan Trench area and consists of 150 observatories equipped with seismometers and tsunamimeters. Most stations at water depths &amp;lt;1500 m were buried to a depth of about 1 m while they were sited freely on the seafloors at greater water depths. To understand the characteristics of strong ground motions on the offshore area, we compared the horizontal vector peak ground accelerations (PGA), peak ground velocities (PGVs), and acceleration response spectra (ARS) between the land and S-net sites for nine earthquakes (5.3≤Mw≤7.1) using ground-motion prediction equations developed for Japan. We found that the observed values of PGAs and short-period (&amp;lt;0.5 s) ARS were generally similar between the land and S-net sites, whereas the PGVs and ARS for the periods longer than 0.5 s were apparently larger at the S-net sites. These results based on data covering a wide area on the seafloors were generally similar to the previous results based on limited ocean-bottom stations. However, analysis of the residuals, within the source-to-site distance of 200 km, revealed that the residual values were smaller in the shallow water region compared to those toward the Japan Trench, which is characterized by proximity to high Qs in the Pacific plate, the presence of thick unconsolidated sediments on the upper crust, and increasing heights of water columns. The difference of station settings in the shallow and deep water regions may also have contributed to the biased distribution of residuals at the short periods. Quantifications of these results are expected to contribute to the predictions of ground motions for earthquake early warning and seismic demand analysis of offshore facilities and await further analysis of a larger data set.

The effect of gas production from deeper conventional gas reservoirs on shallower gas hydrate layer stability: A case study in the conditions of the Sakarya gas field, Western Black Sea

Gas hydrate deposits are generally found in the shallow deepwater regions and permafrost regions. The presence of water, methane, and thermodynamic conditions (low temperature and high pressure) is critical for gas hydrate accumulation. Disturbing thermodynamic conditions such as depressurizing and thermal treatment are the primary processes for gas hydrate dissociation. In this study, we investigate the stability of a shallow depth gas hydrate layer in the conditions of conventional gas production from a newly discovered deep gas field, Sakarya gas field, Western Black Sea. The study focuses on thermal conduction generated by conventional gas production across the wellbore profile along the gas hydrate section. The study includes wellbore simulations using the academic version of PIPESIM to obtain temperature and pressure profiles across the wellbore and numerical simulations for thermal transition for various cases in the hydrate zone using TOUGH + HYDRATE v.1.5 (T + H). The temperature elevation in the hydrate zone was found to change between 19 K and 25 K. The critical temperature elevation for hydrate dissociation and hydrate dissociation front were determined using the simulations, which might cause wellbore stability problems in the long term. We increase the impact of the study by suggesting appropriate solutions to minimize the adverse effects of production from the hot and deep conventional gas on gas hydrate stability found at shallower depths.

Distribution and assessment of heavy metal contents in surface sediments of the western Sunda Shelf

The heavy metal contents (Cr, Cu, Zn, Cd, Pb, Hg, and As) of 88 surface sediment samples from the western Sunda Shelf were analyzed to determine their spatial distribution patterns and contamination status. The results demonstrated that high enrichment regions of heavy metals were focused in the Kelantan, Pahang, and Ambat river estuaries, and deep water regions of the study area. These high enrichment regions were mainly controlled by riverine inputs and their hydrodynamic conditions. The enrichment factor (EF), geoaccumulation index (Igeo), and potential ecological risk index (PERI) were used to assess heavy metal accumulation. The results indicated that the study area was not significantly contaminated overall at the time of the study; however, Cd, As, and Hg were at levels corresponding to moderate contamination at many stations located in the Pahang River estuary, Kelantan River estuary, and north-eastern region of the study area, primarily because of anthropogenic activities.

Stability of Floating Wind Turbine Wakes

Floating offshore wind turbines (FOWTs) are the next frontier in offshore wind energy, allowing exploration of deep-water regions previously unavailable to fixed-foundation turbines. Since offshore turbines operate in lower turbulence levels, the intrinsic hydrodynamic unstable modes of the tip vortices can have even more relevance than in onshore turbines. For floating turbines, platform motion induced by wind and wave loads can trigger vortex instabilities, modifying the wake structure, possibly influencing the flow reaching downstream wind turbines. In the present paper, we study those effects by the means of numerical simulations and their comparison with analytical studies. In our simulations, the wind turbine blades are modeled as actuator lines in the incompressible Navier–Stokes equations. Heave motion with different amplitudes and frequencies are studied. The effect of increasing amplitude is to advance the onset of vortex interaction. For the lower frequency of heave motion, several vortices coalesce to form a large flow structure. High amplitude of oscillations in the streamwise velocity were observed due to these flow structures, which may increase fatigue or induce high amplitude motion on downstream turbines. The number of vortices that interact, as other qualitative phenomena of the numerical simulation, were well predicted by a simple stability model of two-dimensional row of vortices. The disturbances imposed by the heave motion were also compared to the eigenvectors resulting from linear stability theory for helical vortices and the predicted growth rates for the wavenumbers resulting from this comparison were consistent with the model of a row of vortices. These results motivate further studies to understand the impact of the larger flow structures on downstream turbines.

Submarine topography-related spatial variability of the southern Taiwan Strait sands (East Asia)

The Taiwan Strait serves as a link between the East China Sea and South China Sea in East Asia. Complex ocean dynamics, huge sediment inputs and distinct tectonic, climatic and bedrock lithological settings of the two sides of the strait make it ideal for sediment source-to-sink studying. While mud sediments in the strait have been well investigated, sand composition and provenance remain understudied. Here, we present framework petrography and heavy mineral data of sands from the southern Taiwan Strait and the adjacent representative rivers to characterize sand provenance and depositional mechanisms. As expected, the SE China river sands are dominated by quartz and feldspar, whereas sands from the westward-flowing mountainous rivers in Taiwan are rich in lithic fragments and heavy minerals of metamorphic origin. The southern Taiwan Strait sands show significant spatial variations in composition and texture of the framework grains and heavy minerals. Framework grain-based provenance modeling results show that sands in the southwest margin of Taiwan Strait (water depth of 30–60 m) are mainly supplied by SE China rivers. Taiwan mountainous rivers made prominent contributions to the central-western Taiwan Strait (40–60 m) and the south of Taiwan Shoal (below 50 m), both of which are far away from the Taiwan island (ca. 100–300 km away). Furthermore, sands from the Taiwan Shoal (20–30 m) show extremely high compositional maturity and are mainly composed of coarse, rounded quartz. These sands, previously proposed as relict sediments, have been intensely altered by modern high-energy hydrodynamic conditions and can also be interpreted as palimpsest sediments. These results demonstrate that modern river-derived sands are eventually deposited in relatively deep-water regions in the strait, rather than the shallow regions (Taiwan Shoal). We propose that the sand composition and distribution are closely related to the submarine topography of the Taiwan Strait. Combining previous mud belt investigations, we suggest that sands and muds tend to have different fates in shallow continental shelves with complex climate, ocean current and seafloor landform conditions. This study also highlights the importance of both modern and relict sands in the strait and our findings are important to better understanding of shelf sedimentary systems with huge river-sediment-input and high wave/tidal-current-energy.

Sequence stratigraphy and organic geochemistry: An integrated approach to understand the anoxic events and paleoenvironmental evolution of the Ceará basin, Brazilian Equatorial margin

An organic geochemical investigation combined with sequence stratigraphy was performed in the Ceará Basin, an offshore basin located in Northeastern Brazil. The information available from 30 well logs (gamma-ray, resistivity, density), besides geochemical (TOC, and pyrolysis indexes) and isotopic (δ13C) data, aided the preparation of a dataset for this study. The application of sequence-stratigraphic methods helped classify and correlate seismic and organic facies. Four key petroleum source-rock units were identified, from the oldest to the youngest: (1) Mundaú Formation – top of the Rift Sequence (Berriasian-Aptian); (2) Paracuru Formation – Breakup Sequence (Aptian-Albian); (3) Itapajé Member of the Ubarana Formation – Continental Drift Sequence (Albian-Turonian), and (4) Uruburetama Member of the Ubarana Formation – Continental Drift Sequence (Turonian-Maastrichtian). The geochemical characteristics of the Mundaú Formation (high total organic carbon (TOC), hydrogen index (HI), relative hydrocarbon potential (RHP = (S1 + S2)/TOC)) point to a typical transgressive sequence. Six transgressive-regressive (T-R) cycles were recognized in the entire Paracuru Formation. The best geochemical marker is related to the top of the Paracuru Formation. This stratigraphic unit can be correlated to a major anoxic event and is the best source rock of this basin. Evaporitic facies found in this top section, maximum RHP values (anoxic conditions), and maximum flooding surfaces related to transgressive events characterize this interval. Moreover, the wide spatial cover of organic-rich rocks, carbon isotopic data, and the recognition of favorable characteristics for anoxia in other basins of the Equatorial Margin are suggestive of the Aptian-Albian Oceanic Anoxic Event (OAE-1b) occurring in the Ceará Basin. The Ubarana Formation represented by the Uruburetama Member and the Itapajé Member yields the least promising source rocks. However, high TOC values suggest the occurrence of the late Cenomanian–early Turonian Oceanic Anoxic Event (OAE-2), when organic-rich strata started to deposit in deep-water regions. The predominance of a regressive interval in the Uruburetama Member points to oxic or sub-oxic conditions. Additionally, the correlations between the Brazilian Equatorial Margin and its African counterpart, and the organic geochemical characterization allied to the definition of depositional systems for these regions proved to be useful for oil exploration.