Local Upwelling Research Articles

Quasi-permanent meanders of the Kuroshio Extension: Laboratory experiments with altimetry and numerical simulations

This is a study of gyres bounded by a barrier in the west and forced by a localized upwelling. It is motivated by observations of quasi-permanent meanders of the Kuroshio Extension in the North Pacific Subtropical Gyre. Laboratory experiments are performed in a rotating tank where a parabolic shape of the surface of water is used to model the latitudinal variation of the Coriolis parameter. Optical altimetry provides accurate records of circulations and allows us to observe the evolution of β-plumes (Rossby wave envelopes), including their reflection from the barrier. Experiments are supplemented by numerical simulations which extend the range of the control parameters. We demonstrate that meanders occur in the northern branch of the gyres, within an eastward current similar to those in the Kuroshio Extension. The meanders are due to the Rossby wave reflection from the boundary. The wavelength of the meanders is selected by the condition of stationarity of the westward propagating Rossby-wave phase within an eastward current.

Influence of the ITCZ and OMZ on the isotopic composition of suspended particulate matter in the western tropical North Atlantic

The tropical North Atlantic Ocean is a complex region where the intertropical convergence zone (ITCZ) and the oxygen minimum zone (OMZ) play important roles in the control of the physical and biogeochemical processes and affect the efficiency of the carbon pump. Changes in the composition of the particulate organic matter (POM) have been extensively studied in the eastern tropical North Atlantic and recent findings suggested less effective remineralization in the OMZ by comparison with well‑oxygenated waters. However, dynamics of the POM in the western tropical North Atlantic, including the effect of the hypoxic condition on the isotopic signature of the POM, has been poorly understood. This study uses data collected in October 2018 during the PIRATA XVIII cruise (38°W) to describe the biogeochemical variability and isotopic composition of suspended POM in the water column, with emphasis on the effect of the ITCZ and the OMZ on the POM. The ITCZ, located between 5°N and 10°N, was responsible for the low sea surface salinity and local upwelling, thus favoring the high biological productivity found in this band. Although a clear relation between δ13CPOC values and the ITCZ position was not found, low δ15NTPN values (∼2‰) and high silicate concentration observed on the surface between 4°N-6°N suggested that dust deposition and its associated enhancement of the diazotrophic activity may be important in the southernmost part of the ITCZ. Preferential remineralization of fractions rich in 13C (proteins, carbohydrates) and nitrogen compounds resulted in low δ13CPOC and high C:N values observed at depth. However, these alterations were attenuated in the OMZ, suggesting that reduced remineralization expected in waters with low oxygen concentrations may also affect the isotopic POM signature. Additionally, the minimum δ15NTPN value (∼1‰) observed in the core of the OMZ confirms that, although microbial activity may be important in this region, nitrogen-loss processes are not predominant in the western tropical Atlantic.

Seasonal variation in diet and isotopic niche of nominally herbivorous fishes in subtropical rocky reefs

Herbivorous fishes can be sensitive to environmental fluctuations, which influence both availability of food resources and metabolic rate, and thus nutritional requirements. Impacts on herbivore nutritional ecology may result from seasonal variations in temperature and meteoceanographic shifts such as wind-induced upwelling events. We evaluated the effects of seasonal changes on the nutritional ecology of 3 nominally herbivorous fishes (Acanthurus chirurgus, Sparisoma axillare and Kyphosus vaigiensis) in a subtropical rocky reef on the southeastern Brazilian coast using a combination of gut content analysis (at 2 scales of magnification) and stable isotope analysis. Sampling of in situ water temperature covered both patterns of seasonal variation in sea surface temperature, and seasonal occurrence of upwelling. Local upwelling occurred throughout the year but less frequently in winter. Diet and isotopic niche displayed little seasonal variation. Species-specific patterns of seasonal variation indicated distinct responses to environmental fluctuations. Temperature alone cannot explain the locality-specific variation in the nutritional ecology of herbivorous reef fish, and contrary to predictions that digestion in herbivorous fishes is impaired by cooler temperatures, no significant shifts in species’ nutritional ecology were detected.

Identifying Hydraulic Characteristics Related to Fishery Activities Using Numerical Analysis and an Automatic Identification System of a Fishing Vessel

Many countries worldwide promote artificial reef projects to increase and preserve fishery resources; however, how artificial reefs form fisheries is unclear. Nevertheless, specific hydraulic features of artificial reefs may attract fish. We selected an underwater reef as a research site to clarify this hypothesis. In this study, environmental conditions around the underwater reef were modeled and quantitatively assessed using numerical analysis. We identified two hydraulic features related to fish attraction: the wake region and the local upwelling region. Their spatial distributions were superimposed on the path of a fishing vessel that was monitored using an automatic identification system (AIS). We showed that various hydraulic characteristics (such as wake region, local upwelling region, and flow velocity) identified in the path of the fishing vessel can be quantitatively evaluated. Increasing amounts of information from the AIS can be used to identify the hydraulic features that attract the most fish and therefore improve the productivities of artificial reefs.

The Effects of IMF By on the Middle Thermosphere During a Geomagnetically “Quiet” Period at Solar Minimum

AbstractNumerical simulations using the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) are performed to elucidate the effects of the interplanetary magnetic field (IMF) on the middle thermosphere composition during a “geomagnetically quiet” period from the day of year (DOY) 110–111 in 2019 (when the Auroral electrojet (AE) index never exceeded 300 nT and the Kp never exceeded 2). In particular, this paper aims to examine how the Global‐scale Observations of the Limb and Disk (GOLD) mission observed daytime thermospheric O and N2 column density ratio (∑O/N2) depletion at mid‐latitudes originated under such a “geomagnetically quiet” condition. A comparison of electric potential, Joule heating rate per unit mass, ion velocity, neutral temperature and winds in the middle thermosphere (∼160 km) between real IMF and without the IMF east‐west component (By) indicates that a By dominant condition can enhance their strengths under this “geomagnetically quiet” condition. Consequently, ∑O/N2 depletion with a stronger magnitude (30% compared with ∼8% without By) and larger disturbed area was introduced in the post‐midnight sector at high‐latitudes due to strong and localized upwelling associated with the enhanced Joule heating rate per unit mass. The ∑O/N2 depletion was transported equatorward and corotated from local post‐midnight to early morning, and was observed by GOLD at middle latitudes during daytime.

Ring Structures on Baikal Ice and Local Upwelling Cases in Summer

Ring Structures on Baikal Ice and Local Upwelling Cases in Summer

Local winds and encroaching currents drive summertime subsurface blooms over a narrow shelf

AbstractThe ability to forecast the biological productivity of the coastal ocean relies on the quantification of the physical processes that deliver nutrients to the euphotic zone. Here we explore these pathways using observations of the coupled biological and physical variability of waters offshore of the east coast of Tasmania in the summertime. The observations include an array of moored autonomous profilers deployed over an 18‐d period—providing continuous, full‐depth measurements of turbulent microstructure, temperature, velocity, and chlorophyll a (Chl a) fluorescence, complemented by shipboard nutrient measurements. Local upwelling was driven by the encroaching East Australian Current (EAC) extension onto the shelf and to a lesser extent the local winds. The interaction of the local winds and the encroaching boundary current was reflected in the shelf nutrient budget and led to a rapid increase in subsurface Chl a. Diffusive vertical fluxes had minimal impact on subsurface Chl a in the mid‐shelf and outer‐shelf. Upwelling‐favorable winds were too weak to drive significant vertical mixing, and mixing associated with the current‐driven Ekman transport was too deep compared to the euphotic zone depth. The observed subsurface Chl a did not reflect the satellite estimates of productivity. Since the EAC extension transports warm, low‐nutrient surface waters from the subtropics, satellite chlorophyll measurements decreased during the same period the depth‐averaged Chl a increased. This seeming paradox illustrated how long duration, full water column sampling can elucidate the coupled biological and physical processes that aid our ongoing effort to forecast the biological state of the coastal ocean.

The Mediterranean Rhodes Gyre: modelled impacts of climate change, acidification and fishing

The Mediterranean Rhodes Gyre is a cyclonic gyre with high primary production due to local upwelling of nutrients and occasional deep overturning up to 1 km depth. This nutrient-rich state is in sharp contrast to other parts of the Eastern Mediterranean which are oligotrophic. Here we investigated the upwelling system central to the Rhodes Gyre and the impact of different stressors like meteorological changes, acidification and fishing pressure up to the year 2100. A water column model spanning the physical, chemical and biological system up to top predators (GOTM-ERSEM-BFM-EwE) was used to simulate the pelagic environment under single and combined stressors. Results show that due to increasing winter temperatures, deep overturning events will become more rare in the future until they stop occurring around 2060 under the Paris-agreement climate scenario (RCP4.5) or around 2040 under the business-as-usual climate scenario (RCP8.5). Stratification will become stronger as temperature effects outweigh salinity effects in the surface mixed layer. Together with the lack of deep overturning, this stronger stratification limits the nutrient supply to the euphotic zone, significantly reducing primary production. Phytoplankton species shift towards smaller species as nutrients become more scarce, mimicking the situation found currently on the edge of the gyre. Climatic changes and fishing pressure will affect higher trophic levels in an additive way for some species (sardines, dolphins), while in a synergistic way for others (anchovy, mackerel). Acidification impacts are negligible. Fish stocks will reduce significantly under all scenarios involving climate change effects: ~30% under scenarios imposing RCP4.5 and ~40% under scenarios imposing RCP8.5. The beneficial impact of maximum sustained yield-level fishing is very limited, indicating a need for mitigating measures beyond fleet control.

A CFD Analysis of Decommissioned Oil Platforms Jackets on the Brazilian Coast

Every facility reaches the last phase in its life cycle, which is decommissioning. Since the last decade, this subject has been gaining importance in Brazil’s offshore oil and gas companies. For jacket type rigs, one of the methods widely applied after idling the production is the conversion of these structures into artificial reefs (ARs). There are several critical aspects for choosing the best strategy for cutting and sinking a platform jacket, ensuring the success of an AR from a biological point of view. One of them is the influence of marine currents and their fluid-structure interaction which, by maximizing local upwelling and back vortex effects, favours the growth of aggregated flora and fauna. This study consists in the application of computational fluid dynamics (CFD) techniques for studying the marine flow around a disassembled and sunk jacket in the seabed for the purpose of converting it into an artificial reef. An FVM (Finite Volume Method) from a commercial software (most recent version of ANSYS FLUENT®) is applied with the upwind scheme. A k-ε turbulence model on steady state is chosen. Field data about Brazilian coastal currents are collected and analysed from the amount of information available on a Brazilian Navy's meteoceanographic program. Next, different combinations of cutting and sinking a jacket are studied, always keeping a minimum 55m free water column. The objective is to verify where the formation of local upwelling regions - that is, where the vertical velocity component reaches values equal or greater than 10% of the magnitude of the free flow velocity - is more significant, without decreasing back eddy formation. It is observed that the dismemberment of the jacket with the positioning of its parts in an increasing height sequence in the direction of the prevailing current is favourable to generate local upwelling while tipping the structure at 90° to the prevailing current results in the most voluminous back eddy region.

Permafrost Dynamics and Degradation in Polar Arctic From Satellite Radar Observations, Yamal Peninsula

We investigate permafrost surface features revealed from satellite radar data in the Siberian arctic at the Yamal peninsula. Surface dynamics analysis based on SRTM and TanDEM-X DEMs shows up to 2 m net loss of surface relief between 2000 and 2014 indicating a highly dynamic landscape. Surface features for the past 14 years reflect an increase in small stream channels and a number of new lakes that developed, likely caused by permafrost thaw. We used Sentinel-1 SAR imagery to measure permafrost surface changes. Owing to limited observation data we analyzed only 2 years. The InSAR time-series has detected surface displacements in three distinct spatial locations during 2017 and 2018. At these three locations, 60–120 mm/yr rates of seasonal surface permafrost changes are observed. Spatial location of seasonal ground displacements aligns well with lithology. One of them is located on marine sediments and is linked to anthropogenic impact on permafrost stability. Two other areas are located within alluvial sediments and are at the top of topographic elevated zones. We discuss the influence of the geologic environment and the potential effect of local upwelling of gas. These combined analyses of InSAR time-series with analysis of geomorphic features from DEMs present an important tool for continuous process monitoring of surface dynamics as part of a global warming risk assessment.

Patterns of microphytoplankton biomass and diversity in the southern Alboran Sea

The southern Alboran Sea is a highly dynamic region in the Mediterranean. However, there is few data on microphytoplankton which is, an important component of the marine ecosystem. We therefore collected microphytoplankton samples and related the biomass and diversity patterns to ambient variability, considering cross-shore and longitudinal gradients. There was a general eastward decrease in both species richness and biomass, with Cape Three Forks as a transitional point. Diversity increased in coastal areas and decreased with depth. High chlorophyll-a concentrations corresponded to low temperature and low-salinity waters, indicating the Atlantic origin. Microphytoplankton biomass was low in comparison with total chlorophyll, suggesting a dominance of nano- and picophytoplankton. Biomass values increased in the stretch between Cape Three Forks and Al Hoceima Bay, an area highly influenced by upwelled water originating from the northern Alboran Sea. We suggest that the Western Alboran Gyre enhances the development of dinoflagellates while local upwelling enhances the development of diatoms. A statistical relationship was found between microphytoplankton biomass and diversity, especially when diversity was estimated as species richness. These results are crucial for understanding microphytoplankton dynamics and trends in an area that is undergoing climate-derived changes and biodiversity losses.

A Recirculating Eddy Promotes Subsurface Particle Retention in an Antarctic Biological Hotspot

AbstractPalmer Deep Canyon is one of the biological hotspots associated with deep bathymetric features along the West Antarctic Peninsula. The upwelling of nutrient‐rich Upper Circumpolar Deep Water to the surface mixed layer in the submarine canyon has been hypothesized to drive increased phytoplankton biomass, attracting krill, penguins and other top predators to the area. However, observations in Palmer Deep Canyon lack a clear in‐situ upwelling signal, laboratory experiments do not illustrate a physiological response by phytoplankton to Upper Circumpolar Deep Water, and surface residence times are too short for phytoplankton populations to reasonably respond to any locally upwelled nutrients. This suggests that local upwelling may not be the mechanism that links Palmer Deep Canyon to increased biological activity. Previous observations of isopycnal doming within the canyon suggested that a subsurface recirculating feature may be present. Here, using in‐situ measurements and a circulation model, we demonstrate that the presence of a recirculating eddy may contribute to the maintenance of the biological hotspot by increasing residence times at depth and retaining a distinct layer of biological particles. Neutrally buoyant particle simulations showed that residence times increase to ∼175 days at 150 m within the canyon during the austral summer. In‐situ particle scattering, flow cytometry, and water samples from within the subsurface eddy suggest that retained particles are detrital in nature. Our results suggest that this seasonal, retentive feature in Palmer Deep Canyon is important to the retention of biological material and may contribute to the maintenance of this hotspot.

Planktic foraminiferal changes in the western Mediterranean Anthropocene

The increase in anthropogenic induced warming over the last two centuries is impacting marine environment. Planktic foraminifera are a globally distributed calcifying marine zooplankton responding sensitively to changes in sea surface temperatures and interacting with the food web structure. Here, we study two high resolution multicore records from two western Mediterranean Sea regions (Alboran and Balearic basins), areas highly affected by both natural climate change and anthropogenic warming. Cores cover the time interval from the Medieval Climate Anomaly to present. Reconstructed sea surface temperatures are in good agreement with other results, tracing temperature changes through the Common Era (CE) and show a clear warming emergence at about 1850 CE. Both cores show opposite abundance fluctuations of planktic foraminiferal species ( Globigerina bulloides , Globorotalia inflata and Globorotalia truncatulinoides), a common group of marine calcifying zooplankton. The relative abundance changes of Globorotalia truncatulinoides plus Globorotalia inflata describe the intensity of deep winter mixing in the Balearic basin. In the Alboran Sea, Globigerina bulloides and Globorotalia inflata instead respond to local upwelling dynamics. In the pre-industrial era, changes in planktic foraminiferal productivity and species composition can be explained mainly by the natural variability of the North Atlantic Oscillation, and, to a lesser extent, by the Atlantic Multidecadal Oscillation. In the industrial era, starting from about 1800 CE, this variability is affected by anthropogenic surface warming, leading to enhanced vertical stratification of the upper water column, and resulting in a decrease of surface productivity at both sites. We found that natural planktic foraminiferal population dynamics in the western Mediterranean is already altered by enhanced anthropogenic impact in the industrial era, suggesting that in this region natural cycles are being overprinted by human influences. • Western Mediterranean planktic foraminifera and sea surface temperature records. • Planktic foraminiferal changes explained by natural variability in pre-industrial times. • Anthropogenic warming depletes marine surface production.

Phytoplankton assemblages and optical properties in a coastal region of the South Brazil Bight

Phytoplankton assemblages in the inner portion of the South Brazil Bight are studied at a coastal time-series station (2012–2017). Diagnostic pigment analysis (DPA) coupled with a hierarchical cluster analysis were applied to generate and evaluate a locally tuned model to retrieve phytoplankton groups. These results are intercompared with phytoplankton taxonomy (microscopy analysis) and phytoplankton bio-optical properties (i.e., specific absorption coefficient aph∗(λ),package effect index Qa∗(440) and size indices Sfand SI). Overall, phytoplankton bio-optical properties were well related to the DPA phytoplankton groups and size fractions. Samples with high proportions of diatoms, dinoflagellates, and cryptophytes usually presented low or intermediate values of aph∗(λ),Qa∗(440),Sf, and SI. When cyanobacteria and green algae proportions were high, the values of these indices were also higher. The locally-tuned model overestimated the chlorophyll-a fraction in diatoms and underestimated that in nanophytoplankton and dinoflagellates, compared with the biomass estimated from microscopic size measurements. The area is characterized mainly by oligo-mesotrophic conditions, with a mixed assemblage, and episodic events of phytoplankton blooms. Dominance of diatoms were associated with low sea surface temperature and high total chlorophyll-a under the influence of nutrient-enriched waters mixed within the Coastal Water, especially in upwelling events during spring-summer. On the other hand, cyanobacteria were associated with high sea surface temperature and low total chlorophyll-a. Surface waters were dominated by the Coastal Water. Episodic events with the intrusion of low salinity and temperature waters suggested the presence of the Río de La Plata River Plume Water in autumn-winter. High salinity and low-temperature waters indicated surface intrusions of the South Atlantic Central Water, in spring-summer, either by local upwelling or the Cape Frio Upwelling Front, characterizing a highly dynamic environment influencing the phytoplankton assemblages.

The Driving Mechanisms on Southern Ocean Upwelling Change during the Last Deglaciation

We explore the change in Southern Ocean upwelling during the last deglaciation, based on proxy records and a transient climate model simulation. Our analyses suggest that, beyond a conventional mechanism of the Southern Hemisphere westerlies shift, Southern Ocean upwelling is strongly influenced by surface buoyancy forcing and the local topography. Over the Antarctic Circumpolar Current region, the zonal mean and local upwelled flows exhibited distinct evolution patterns during the last deglaciation, since they are driven by different mechanisms. The zonal mean upwelling is primarily driven by surface wind stress via zonal mean Ekman pumping, whereas local upwelling is driven by both wind and buoyancy forcing, and is tightly coupled to local topography. During the early stage of the last deglaciation, the vertical extension of the upwelled flows increased downstream of submarine ridges but decreased upstream, which led to enhanced and diminished local upwelling, downstream and upstream of the submarine ridges, respectively.

The long-term ichthyoplankton abundance summer trends in the coastal waters of the Black Sea under conditions of hydrometeorological changes

In this study a long-term literature and field-monitoring dataset collected from the Sevastopol coast in the Black Sea during summer seasons from 1960 to 2017 were examined. Principal component analysis (PCA) and ANOVA analysis clearly demonstrate changes in the dynamics of ichthyoplankton abundance due to wind conditions, temperature, and salinity. The results specifically showed significant relationship between the abundance of eggs (r = 0.82) and repeatability northeasterly winds and between larvae abundance and speed northeasterly wind (r = 0.64). As the wind speed increases, the surface water temperature decreases and salinity rises. According to the data on the temperature and wind speed, the period of occurrence of local upwellings in summer near the coast of Sevastopol was determined ~8 and 20–25 years. Based on analysis of linear trends for the summer periods from 1960 to 2017 in this area a decrease in the number of fish eggs and occurrences of larvae and a general reduction in temperatures, salinity, speed and repeatability northeasterly winds was observed. Long-term ichthyoplankton data monitoring can be used to validate model results and provide further predictions of changes in ecosystem components.

Occurrence and significance of smectite in the Pliensbachian (Lower Jurassic) at Lókút (Bakony Mts., Hungary)

AbstractAlthough the Mesozoic rocks of the Transdanubian Range have been the subject of a multitude of different studies, mineralogical research is largely underrepresented. The clay mineralogy of Lower Jurassic (especially the Pliensbachian and Toarcian) strata was broadly investigated earlier; however, systematic high-resolution clay mineralogical studies remain scarce. Here we present a mineralogical study focusing on the Upper Pliensbachian strata of the Lókút-Hosszúárok section, located near the Eplény Manganese Ore Field. We identified dioctahedral smectite, randomly interstratified illite/smectite, illite as 10 Å phyllosilicate, quartz and cristobalite. Based on our new results we propose that the smectite was formed by aging of Mg or Fe hydroxide-silica precipitates. The smectite and cristobalite were presumably formed from the siliceous tests of radiolarians, whose abundance was controlled by a local upwelling system. The occurrence of Pliensbachian smectite in the Lókút outcrop shows similarities with the Úrkút smectites known from both Pliensbachian and Toarcian strata, which implies that similar processes controlled the sedimentation during the Pliensbachian as well as during the black (gray) shale-hosted ore accumulation in the Eplény and Úrkút basins.

Stability range of localized subcritical Rayleigh–Bénard convection in temperature-dependent viscosity fluids: Constraints from two-dimensional simulations

We use a finite element code CITCOM to determine the stability range of a unique planform of Rayleigh–Bénard convection in temperature-dependent viscosity fluids—a spatially localized upwelling. Such a planform can only exist at Rayleigh numbers below the critical Rayleigh number for the linear onset of convection. Its initiation requires a finite-amplitude perturbation. We perform numerical simulations in long two-dimensional rectangular boxes with free-slip boundary conditions, which are relevant to convection in planetary interiors. The main result is a stability map of spatially localized single upwellings. Localized upwellings can exist at viscosity contrasts higher than about 150. The range of Rayleigh numbers at which localized upwellings are stable increases with the viscosity contrast. At the largest viscosity contrast studied, 1010, localized upwellings are stable in the range between about 0.5 and 0.9 of the critical Rayleigh number. The horizontal scale of localized upwellings is on the order of the layer thickness and decreases with the viscosity contrast. Spatially localized upwellings can occur on icy satellites and small planets where the conditions are likely to be subcritical due to the small size of these bodies. Localized upwellings might have industrial applications as well, such as patterning of polymers.

Early Oligocene environment of the Central Paratethys revealed by biomarkers and pyrite framboids from the Tarcău and Vrancea Nappes (Eastern Outer Carpathians, Romania)

The Menilite facies, representing the most prolific hydrocarbon source rocks in the Carpathian fold and thrust belt, has been studied in two outcrop sections of the Vrancea and Tarcău Nappes (Eastern Romania) with the aim of determining the depositional environment during the Early Oligocene in the Vrancea and Tarcău sub-basins of the Central Paratethys. The biomarker composition indicates spatial variability in organic matter sources with mainly algal (particularly diatoms and dinoflagellates) and bacterial contributions in both sub-basins. This is reflected by the occurrence of such biomarkers as triaromatic dinosteroids, C25 highly branched isoprenoid thiophenes, marine n-alkanes, hopanoids and monomethyl alkanes. Solely in the Vrancea domain, the input of terrigenous organic matter of higher plant origin can be anticipated from the presence of abietane-class biomarkers (conifer-derived) and oleanane (angiosperm-derived). Distinct nutrient availability related to different positions of individual sub-basins (i.e. shallower, temporarily eutrophic Vrancea sub-basin vs. open-marine mesotrophic Tarcău domain) is reflected by different paleoproductivity indicators, such as the total organic carbon content, hydrogen index and 17α-hopanes to steranes ratio.Water column stratification with bottom water anoxia enhanced by bacterial sulfate reduction stretching into the chemocline, at least intermittently, is expressed by a characteristic tiny pyrite framboid distribution and the lack of bioturbation. However, molecular indicators (Pr/Ph ratio, C35 homohopane index) imply fluctuations of redox conditions with the dominance of dysoxic conditions, which may suggest the occasional oxidation of bottom waters linked to local upwelling. Early diagenetic organic matter transformations such as photodegradation taking place in the oxidised upper part of the water column could also have affected the biomarker distributions. The presence of aryl isoprenoids and Me,i-Bu-maleimides indicates periods of euxinic conditions within the photic zone only in the Vrancea sub-basin during the deposition of the Lower Menilite Member. This is probably related to freshwater incursions from the adjacent, at least partly emerged forebulge, as suggested by a high MTTC ratio and the occurrence of higher plant-derived organic matter.

Evaluation of turbulence models for estimating the wake region of artificial reefs using particle image velocimetry and computational fluid dynamics

Many studies have evaluated the performance of turbulence models used for computational fluid dynamics (CFD) analysis of artificial reefs (ARs), but an optimal model remains elusive, particularly in terms of wake length and areal estimation. Only a few models were used in previous studies and no report has yet investigated wake shape. We compared five turbulence models and verified the CFD results using particle image velocimetry (PIV). A standard k-ε model, renormalization group k-ε model, k-ω model, shear stress transportation k-ω model, and Reynolds stress model (RSM) were used. A down-scaled half-ball-type AR was devised and CFD and PIV analyses were performed. Three different inlet velocities (three Reynolds numbers, Re) were considered in each model, and the CFD and PIV results were compared. Wake lengths obtained in the PIV experiments were 1.05 L (Re = 2632), 0.90 L (Re = 5655) and 0.85 L (Re = 8782), respectively. The RSM well-reflected this, especially when Re = 2632 (difference +1%) and Re = 5655 (difference −2%). PIV revealed that all wake regions had unique shapes, reflecting flow divergence (local upwelling) from the end of the wake. Considering such divergent flow, the RSM optimally predicted the overall characteristics of the wake region.