A laboratory study on capillary sealing efficiency of Iranian shale and anhydrite caprocks

Caprock has the most important role in the long term safety of formation gas storage. The caprocks trap fluid accumulated beneath, contribute to lateral migration of this fluid and impede its upward migration. The rapid upward passage of invasive plumes due to buoyancy pressure is prevented by capillary pressure within these seal rocks. In the present study, two main seal rocks, from the Zagros basin in the southwest of Iran, a shale core sample of Asmari formation and an anhydrite core sample of Gachsaran formation, were provided. Absolute permeabilities of shale and anhydrite cores, considering the Klinkenberg effect, were measured as 6.09 × 10⁻¹⁸ and 0.89 × 10⁻¹⁸ m², respectively. Capillary sealing efficiency of the cores was investigated using gas breakthrough experiments. To do so, two distinct techniques including step by step and residual capillary pressure approaches were performed, using carbon dioxide, nitrogen and methane gases at temperatures of 70 and 90 °C, under confining pressures in the range 24.13–37.92 MPa. In the first technique, it was found that capillary breakthrough pressure of the cores varies in the range from 2.76 to 34.34 MPa. Moreover, the measurements indicated that after capillary breakthrough, gas effective permeabilities lie in range 1.85 × 10⁻²¹ – 1.66 × 10⁻¹⁹ m². In the second technique, the minimum capillary displacement pressure of shale varied from 0.66 to 1.45 MPa with the maximum effective permeability around 7.76 × 10⁻²¹ – 6.69 × 10⁻²⁰ m². The results indicate that anhydrite caprock of the Gachsaran formation provides proper capillary sealing efficacy, suitable for long term storage of the injected CO₂ plumes, due to its higher capillary breakthrough pressure and lower gas effective permeability.     

Effects of trophic ecology and habitat use on maternal transfer of contaminants in four species of young of the year lamniform sharks

Organic contaminant and total mercury concentrations were compared in four species of lamniform sharks over several age classes to examine bioaccumulation patterns and gain insights into trophic ecology. Contaminants found in young of the year (YOY) sharks were assumed to be derived from maternal sources and used as a proxy to investigate factors that influence maternal offloading processes. YOY white (Carcharodon carcharias) and mako (Isurus oxyrinchus) sharks had comparable and significantly higher concentrations of PCBs, DDTs, pesticides, and mercury than YOY thresher (Alopias vulpinus) or salmon (Lamna ditropis) sharks. A significant positive relationship was found between YOY contaminant loads and maternal trophic position, suggesting that trophic ecology is one factor that plays an important role in maternal offloading. Differences in organic contaminant signatures and contaminant concentration magnitudes among species corroborated what is known about species habitat use and may be used to provide insights into the feeding ecology of these animals.     

Behavioral responses of California sea lions to mid-frequency (3250–3450 Hz) sonar signals

Military sonar has the potential to negatively impact marine mammals. To investigate factors affecting behavioral disruption in California sea lions (Zalophus californianus), fifteen sea lions participated in a controlled exposure study using a simulated tactical sonar signal (1 s duration, 3250–3450 Hz) as a stimulus. Subjects were placed into groups of three and each group received a stimulus exposure of 125, 140, 155, 170, or 185 dB re: 1 μPa (rms). Each subject was trained to swim across an enclosure, touch a paddle, and return to the start location. Sound exposures occurred at the mid-point of the enclosure. Control and exposure sessions were run consecutively and each consisted of ten, 30-s trials. The occurrence and severity of behavioral responses were used to create acoustic dose–response and dose–severity functions. Age of the subject significantly affected the dose–response relationship, but not the dose–severity relationship. Repetitive exposures did not affect the dose–response relationship.     

Capturing channelized reservoir connectivity uncertainty with amalgamation curves

During reservoir characterization all the geological uncertainties affecting the quantity and distribution of hydrocarbons should be captured to assess the risks affecting final recovery.In a typical modeling workflow the geological uncertainties are accounted for through the construction of a sufficiently large set of 3-D static models. Out of this set, a few representative models are selected and dynamically simulated so as to correlate the geological characteristics of the reservoir with its dynamic performance and to propagate the uncertainty onto the final recovery factors, yet maintaining the computational run time acceptable. In channelized depositional environments, which are strongly heterogeneous, the selection approach must also account for channel connectivity, which plays a key role in the possibility of efficiently draining the reservoir for a reasonable number of wells.This study can be seen as a step forward in the assessment of the risks associated to the development of channelized reservoirs under the assumption that a way to express the concept of channel connectivity is channel amalgamation. Channel amalgamation is here defined through amalgamation curves which are numerically described using a set of indexes whose combination provide spatial information of channel intersections. These indexes were calculated for a full set of 3-D geological models and used to steer the selection of a representative model sub-set for subsequent fluid flow simulations.The validity of the index-based selection was verified on different sets of synthetic reservoir models through the evaluation of the representativeness of the model sub-set in reproducing the uncertainty of the original dataset. Eventually, the existence of a strong correlation between channel amalgamation and production performance was proved. From a practical perspective, the possibility to include channel amalgamation in the assessment of the geological models can considerably improve the representativeness of the selected models for uncertainty propagation thus reducing significantly the number of geological models to be considered.     

Quantitative evaluation of the reservoir potential and controlling factors of semi-deep lacustrine tempestites in the Eocene Lijin Sag of the Bohai Bay Basin,East China

Ancient lacustrine storm-deposits that act as petroleum reservoirs are seldom reported. The Lijin Sag, which is located in the southeastern corner of the Bohai Bay Basin in East China, is a NE–SW trending Cenozoic half-graben basin. Some of its Eocene deposits (Bindong deposits) were interpreted as lacustrine tempestites. The Bindong tempestites, which developed in the lower part of the upper fourth member of the Shahejie Formation (Es4¹), constitute a new kind of petroleum reservoir and are novel petroleum exploration targets in the Bindong Area. However, the characteristics of the Es4¹ Bindong tempestite reservoirs and their controlling factors are poorly understood. Point-count analyses of thin sections, scanning electron microscope image analyses, X-ray diffractometry data, and the petrophysical parameters of the Bindong tempestite reservoirs were utilized to estimate the reservoir quality. The reservoirs have undergone significant diagenetic alteration, which can be divided into negative and positive aspects. The negative alteration includes compaction, authigenic minerals, and cementation such as carbonates, clay minerals and overgrowths of quartz and feldspar. The uneven distribution of carbonate cement increased the reservoir’s heterogeneity, with carbonate cement commonly precipitating along the sandstone-mudstone contacts. The primary porosity was severely reduced because of compaction and extensive carbonate cementation. Positive alteration includes dissolution, carbonate cementation, undercompaction and fractures. Carbonate cementation exhibited both positive and negative effects on the reservoir quality. Overall, the objective reservoir quality is quite poor. A quantitative evaluation of the reservoirs’ potential was conducted. The cutoff values of several of the reservoir’s parameters were calculated. The lower limits of the porosity and permeability are 8.35–5.85% and 1.2587–0.2753 × 10^–3 μm², respectively, depending on the depth. The upper limits of the carbonate and mud content are approximately 18.5% and 9–10%, respectively. A fundamental understanding of these characteristics will provide necessary information for extracting hydrocarbons from analogous subsurface reservoirs.     

Salt tectonics and tear faulting in the central part of the Zagros Fold-Thrust Belt,Iran

The central part of the Zagros Fold-Thrust Belt is characterized by a series of right-lateral and left-lateral transverse tear fault systems, some of them being ornamented by salt diapirs of the Late Precambrian–Early Cambrian Hormuz evaporitic series. Many deep-seated extensional faults, mainly along N–S and few along NW–SE and NE–SW, were formed or reactivated during the Late Precambrian–Early Cambrian and generated horsts and grabens. The extensional faults controlled deposition, distribution and thickness of the Hormuz series. Salt walls and diapirs initiated by the Early Paleozoic especially along the extensional faults. Long-term halokinesis gave rise to thin sedimentary cover above the salt diapirs and aggregated considerable volume of salt into the salt stocks. They created weak zones in the sedimentary cover, located approximately above the former and inactive deep-seated extensional faults. The N–S to NNE–SSW direction of tectonic shortening during the Neogene Zagros folding was sub-parallel with the strikes of the salt walls and rows of diapirs. Variations in thickness of the Hormuz series prepared differences in the basal friction on both sides of the Precambrian–Cambrian extensional faults, which facilitated the Zagros deformation front to advance faster wherever the salt layer was thicker. Consequently, a series of tear fault systems developed along the rows of salt diapirs approximately above the Precambrian–Cambrian extensional faults. Therefore, the present surface expressions of the tear fault systems developed within the sedimentary cover during the Zagros orogeny. Although the direction of the Zagros shortening could also potentially reactivate the basement faults as strike-slip structures, subsurface data and majority of the moderate-large earthquakes do not support basement involvement. This suggests that the tear fault systems are detached on top of the Hormuz series from the deep-seated Precambrian–Cambrian extensional faults in the basement.     

King Canute muses in the South Seas: Why aren’t Pacific Islands transitioning to low carbon sea transport futures?

Transition to low carbon sea transport is a logical response to the extreme dependency of the Pacific Islands region on imported fossil fuel, its significant vulnerability to the effects of climate change and the critical shipping needs of Pacific Island countries (PICs). Building on previous work in low carbon sea transport in the Pacific, this paper further considers the barriers to achieving such transition by assessing, through a ‘post-Paris Agreement’ lens, the Intended Nationally Determined Contributions (INDCs) submitted by PICs and contrasting these to the near total lack of investment and planning for low carbon transition in the transport sector with the parallel occurrence in the electricity sector where ~USD 2 billion of donor investment is deployed or queued despite electricity using only ~20% of fossil fuel across the region. Consistent with recent international studies, inadequate and inappropriate financing and policy have been identified as dominant transition barriers for low carbon sea transport development in PICs. This paper further examines the regional level barriers to policy development, and finds them inhibited by the silo nature of the major regional actors. The implications that the Paris Agreement has for climate financing to support the essential research and capacity development needed to underpin a successful low carbon sea transport transition strategy at any useful scale and speed are also considered in this paper.     

The importance of missing strain in Deep Water Fold and Thrust Belts

Deep water fold and thrust belts (DWFTBs) are sedimentary wedges that accommodate plate-scale deformation on both active and passive continental margins. Internally, these wedges consist of individual structures that strongly influence sediment dispersal, bathymetry and fluid migration. Most DWFTB studies investigate basin- and intra-wedge- scale processes using seismic reflection profiles, yet are inherently limited by seismic resolution. Of critical importance is strain distribution and its accommodation on discrete faults compared to distributed deformation. Recent studies have considered strain distribution by investigating regional reflection DWFTBs profiles within coupled systems, which contain down-dip compression and up-dip extension. There is broad agreement of a mis-balance in compression versus extension, with ∼5% excess in the latter associated with horizontal compaction, yet this remains unproven.Using two exceptionally well exposed outcrops in the Spanish Pyrenees we consider deformation of DWFTB at a scale comparable to, and beyond, seismic resolution for the first time. By coupling outcrop observations (decametre to hectometre scale) with a re-evaluation of seismic profiles from the Orange Basin, South Africa, which contains one of the best imaged DWFTBs globally, we provide a unique insight into the deformation from metre to margin scale. Our observations reveal hitherto unrecognised second order structures that account for the majority of the previously recognised missing strain. This re-evaluation implies that ∼5% missing strain should be accounted for in all DWFTBs, therefore existing studies using restorations of the sediment wedge will have underestimated crustal shortening in active margins, or sedimentary shortening in gravity driven systems by this amount. In contrast to previous studies, our observations imply that the majority of this strain is accommodated on discrete fault surfaces and this can explain the occurrence and location of a range of intra-wedge processes that are intimately linked to structures including sediment dispersal, fluid migration pathways and reservoir compartmentalisation.     

Effects of rapid changes in temperature on two estuarine crustaceans

Weight specific oxygen consumption (Q_O2) patterns of the amphipod, Gammarus sp. (acclimated to 5°, 15° and 25°C) and of juvenile blue crabs, Callinectes sapidus (15° and 25°C) were used to evaluate the potential effect of exposure to rapid temperature changes simulating once-through power plant pumped entrainment. Amphipods at all acclimation temperatures and blue crabs at 15°C responded to the temperature changes by increasing Q_O2 above pre-exposure levels after the thermal increase and then returning to pre-exposure levels. The response was judged to be a normal physiological compensation response, not a thermal stress response, as suggested by some investigators. Significant differences were found among seasonal Q_O2, patterns in both species; Q_O2, increased with increasing acclimation temperature. However, no seasonal stress effects were found as a result of exposure to the temperature changes. This implies that the effects of ΔT's up to 10°C from power plants of this design should have no significant impact on these organisms.     

Identification of polycyclic sulfides hexahydrodibenzothiophenes and their implications for heavy oil accumulation in ultra-deep strata in Tarim Basin

Heavy oil accumulation in deep Ordovician carbonate stratum was discovered at present burial depths greater than 6600 m in the northern Tarim Basin, NW China. Density of the unusual ultra-deep heavy oils is greater than 0.92 g/cm³ at 20 °C. Crude oil produced from 6598 to 6710 m interval of the Ha9 well was selected for the thiophenic and sulfidic compounds characterization in order to understand the mechanism of heavy oil accumulation in the ultra-deep strata. In addition to the common thiophenic compounds, four homologues of novel polycyclic sulfides named as 1,1,4a,6-tetramethyl-9-alkyl-1,2,3,4,4a,9b-hexahydrodibenzothiophenes (H₆DBTs, 9-alkyl = H, methyl, ethyl, and propyl, respectively) were identified in Ha9 well crude oil, and it is the first time these biomarkers were detected in natural occurrence. H₆DBTs were generated from isoprenoid-related precursors reacted with reduced-state sulfur in early diagenesis stage by bacterial sulfate reduction. The occurrence of H₆DBTs further indicated biodegradation of the reservoir oil at a relatively mild temperature (60–65 °C), a favorable condition for microorganism survival. According to the history of reservoir forming, oil and gas accumulation occurred in reservoirs during the Late Permian period and then being uplifted, suffering biodegradation. Oil quality was significantly altered as a result of strong biodegradation since the Triassic. Heavy oil reservoir was buried deeper around. 5 Ma, leading to a rapid increase in reservoir temperature up to 150 °C at a burial depth of 6600 m. The quick burial and elevated temperature of the reservoir were favorable to the preservation of H₆DBTs.     

Long-Term Variability of UV Irradiance in the Moscow Region according to Measurement and Modeling Data

We have found distinct long-period changes in erythemal UV radiation (Q_er) characterized by a pronounced decrease at the end of the 1970s and a statistically significant positive trend of more than 5%/10 years since 1979 over the territory of the Moscow region according to the measurements and reconstruction model. The positive Q_er trend is shown to be associated mainly with a decrease in the effective cloud amount and total ozone content (TOC). Due to these variations, UV resources have significantly changed in spring for the population with the most vulnerable skin type I, which means a transition from the UV optimum to UV moderate excess conditions. The simulation experiments using the INM-RSHU chemistry climate model (CCM) for several scenarios with and without anthropogenic factors have revealed that the variations in the anthropogenic emissions of halogens have the most significant impact on the variability of TOC and Q_er. Among natural factors, noticeable effects are observed due to volcanic aerosol. The calculations of the cloud transmittance of Q_er are generally consistent with the measurements; however, they do not reproduce the observed value of the positive trend.     

Control of bacterial growth by temperature and organic matter in the Western Arctic

Temperature is thought to have a disproportionate role in controlling bacterial growth in perennially cold waters like the Western Arctic Ocean. One impact of temperature is that bacteria in cold waters may require more dissolved organic material (DOM) in order to approach growth rates observed at higher temperatures (the Wiebe–Pomeroy hypothesis). To explore these issues, this study examined the effect of DOM additions and temperatures shifts on bacterial assemblages during short (2 h) and long (up to 10 days) incubations. We found that the temperature response for bacterial assemblages in the Western Arctic was similar to that observed in temperate waters; the Q₁₀ values for leucine and thymidine incorporation were 3.1±2.6 and 1.9±0.56, respectively, not significantly different from values observed in the equatorial Pacific Ocean. In contrast to what would be predicted from the Wiebe–Pomeroy hypothesis, the impact of DOM additions on leucine incorporation either was the same or greater at higher, not lower temperatures. Increasing the incubation temperature did stimulate leucine incorporation more quickly than did DOM additions, but DOM seems as important as temperature in controlling bacterial growth. Leucine incorporation rates per cell (an index of community growth rates) observed in these experiments varied greatly and approached rates observed in waters warmer by 25 °C. These results suggest that the role of temperature in controlling bacterial growth in the Western Arctic is similar to that in low-latitude ocean.     

Thermal constraints on the respiration and excretion rates of krill,Euphausia hanseni and Nematoscelis megalops,in the northern Benguela upwelling system off Namibia

Rates of respiration and ammonia excretion of Euphausia hanseni and Nematoscelis megalops were determined experimentally at four temperatures representative of conditions encountered by these euphausiid species in the northern Benguela upwelling environment. The respiration rate increased from 7.7 µmol O₂ h^?1 g_ww ^?1 at 5 °C to 18.1 µmol O₂ h^?1 g_ww ^?1 at 20 °C in E. hanseni and from 7.0 µmol O₂ h^?1 g_ww ^?1 (5 °C) to 23.4 µmol O₂ h^?1 g_ww ^?1 (20 °C) in N. megalops. The impact of temperature on oxygen uptake of the two species differed significantly. Nematoscelis megalops showed thermal adaptations to temperatures between 5 °C and 10 °C (Q₁₀ = 1.9) and metabolic constraint was evident at higher temperatures (Q₁₀ = 2.6). In contrast, E. hanseni showed adaptations to temperatures of 10–20 °C (Q₁₀ = 1.5) and experienced metabolic depression below 10 °C (Q₁₀ = 2.6). Proteins were predominantly metabolised by E. hanseni in contrast to lipids by N. megalops. Carbon demand of N. megalops between 5 and 15 °C was lower than in E. hanseni versus equal food requirements at 20 °C. It is concluded that the two species display different physiological adaptations, based on their respective temperature adaptations, which are mirrored in their differential vertical positioning in the water column.     

Molecular iodine (I2) emission from two Laminaria species (Phaeophyceae) and impact of irradiance and temperature on I2 emission into air and iodide release into seawater from Laminaria digitata

Kelps of the genus Laminaria accumulate iodine at high concentrations, but the iodine retaining capacity can be affected by emersion and physiological stress. In this study, I₂ emission into the atmosphere from Laminaria digitata and Laminaria hyperborea was compared under controlled low irradiances and temperatures. The two species exhibited different I₂ emission rates as blades of L. digitata emitted I₂ at rates five times higher than those from newly-grown blades (current growth season) of L. hyperborea. I₂ emission was not detectable from old blades (previous growth season) of L. hyperborea. Additionally, effects of irradiance and temperature on both I₂ emission into air and net I⁻ release into seawater where assessed for L. digitata while monitoring photo-physiological parameters as stress indicators. Irradiances between 30 and 120 μmol photons m⁻² s⁻¹ had only marginal effects on both I₂ emission and I⁻ release rates, but physiological stress, indicated by photoinhibition, was observed. The results suggest that the irradiances applied here were not stressful enough to impact on the iodine release. By contrast, at elevated temperatures (20 °C), photoinhibition was accompanied by an increase in I₂ emission rates, but net I⁻ release rates remained similar at 10–20 °C. High I₂ emission rates into air and I⁻ release into seawater observed from L. digitata underpin the fundamental function of this kelp as mediator of coastal iodine fluxes.     

Molecular and isotopic compositions of bitumens in Silurian tar sands from the Tarim Basin,NW China: Characterizing biodegradation and hydrocarbon charging in an old composite basin

Biodegradation and oil mixing in Silurian sandstone reservoirs of the Tarim Basin, one of the largest composite basins in China, were investigated by analyzing the molecular characteristics and stable carbon isotopic signatures of low-molecular-weight (LMW) saturated hydrocarbons and high-molecular-weight (HMW) asphaltenes. Detection of 25-norhopanes and 17-nortricyclic terpanes in most Silurian tar sands from the Tabei Uplift in the Tarim Basin suggests a much greater degree of biodegradation here than in the Tazhong Uplift. This explains the relatively more abundant tricyclic terpanes, gammacerane, pregnane and diasteranes in tar sands from the Tabei Uplift than in those from the Tazhong Uplift. Hence, care must be taken when assigning oil source correlations using biomarkers in tar sands because of the biodegradation and mixing of oils derived from multiple sources in such an old composite basin. Asphaltenes in the tar sands seem to be part of the oil charge before biodegradation, depending on the relative anti-biodegradation characteristics of asphaltenes, the similarity in carbon isotopic signatures for asphaltenes and their pyrolysates, and the consistent product distribution for flash pyrolysis and for regular steranes in asphaltene pyrolysates, regardless of whether the tar sands were charged with fresh oil. According to the relative distributions of regular steranes and the relatively abundant 1,2,3,4-tetramethylbenzene significantly enriched in ¹³C, the oil sources for asphaltenes in the tar sands might be related to lower Paleozoic marine source rocks formed in euxinic conditions. Nevertheless, the relatively low abundance of gammacerane and C₂₈ regular steranes observed in asphaltene pyrolysates and residual hydrocarbons, within limited samples investigated in this work, made a direct correlation of oils originally charged into Silurian tar sands with those Cambrian source rocks, reported so far, seem not to be possible. Comparison of carbon isotopic signatures of n-alkanes in asphaltene pyrolysates with those of LMW saturated hydrocarbons is helpful in determining if the abundant n-alkanes in tar sands are derived from fresh oil charges after biodegradation. The limited carbon isotopic data for n-alkanes in LMW saturated hydrocarbons from the tar sands can be used to classify oils charged after biodegradation in the composite basin into four distinct groups.     

Source diagnostic and weathering indicators of oil spills utilizing bicyclic sesquiterpanes

Bicyclic Sesquiterpanes (BSs) are ubiquitous components of crude oils and ancient sediments. BSs in crude oils and diesel oil were identified and characterized, the effects of moderate weathering on BS distributions were discussed, and a methodology using diagnostic indices of BSs was developed for oil correlation and differentiation. The results showed that crude oils and oil products had different abundances and distributions of BSs and, consequently, resulted in different diagnostic ratios. The selected diagnostic ratio indices of BSs, such as BS4/BS5 (ratio of 4,4,8,9,9-pentamethyl-decahydronaphthalene to 8β (H)-drimane), BS6/BS5 (ratio of 4,4,9,9,10-pentamethyl-decahydronaphthalene to 8β (H)-drimane), BS8/BS9 (ratio of the second peak to the third peak of C 16 sequiterpane), and BS8/BS10 (ratio of C 16 sequiterpane to 8β (H)-homodrimane), still maintained better stabilities (%RSD < 5%) after weathering for 30 d. The longer weathering process (150 d) had some effect on such ratios (5% < %RSD < 10%). The facts of the uniqueness, abundance in petroleum, and chemical stability of BSs enable them to be suitable as an effective diagnostic means for identifying spilled oil with moderate weathering, particularly for lighter refined product samples that are difficult to identify by current techniques.     

Robust control for an autonomous underwater vehicle that suppresses pitch and yaw coupling

Attitude control systems for autonomous underwater vehicles are often implemented with separate controllers for pitch motion in the vertical plane and yaw motion in the horizontal plane. We propose a novel time-varying model for a streamlined autonomous underwater vehicle that explicitly displays the coupling between yaw and pitch motion due to nonzero roll angle and/or roll rate. The model facilitates the use of a multi-input multi-output H_∞ control design that is robust to yaw-pitch coupling. The efficacy of our approach is demonstrated with field trials.     

Sediment retention at the Changjiang sub-aqueous delta over a 57 year period,in response to catchment changes

The growth of a river delta system is controlled mainly by fluvial sediment discharge and sediment retention in the deltaic areas. In the present study, we attempt to define a sediment retention index, R, and its relation to the deposition rate of the delta. Based upon two assumptions that the amount of the sediment that escapes from the deltaic areas, Q_E, is stationary with minor fluctuations, and that there is a linear relationship between the deposition rate averaged over the sub-aqueous delta (D_av) and the deposition rate at accretion sites within the delta (D_R), the changes in the accretion/erosion patterns of the Changjiang sub-aqueous delta during the period of 1951–2007, in response to river sediment discharge changes, are analyzed. The results show that the sediment retention index can be related to the deposition rate of the sub-aqueous delta; the spatial-temporal distribution pattern of the deposition rate reveals the behaviour of sediment retention of a delta system. For the Changjiang sub-aqueous delta, fluvial sediment discharge data, together with Pb-210 based deposition rates, provide useful information on sediment retention. Changes in the sediment retention index and the accretion/erosion patterns of the sub-aqueous delta have taken place in response to river input changes. In order to improve our understanding of the processes associated with estuarine sediment retention, sediment cores with sufficient spatial coverage may be collected and analyzed to establish accurate Q_E–R and D_av–D_R relationships. In combination with numerical modeling of sediment transport, these relationships form a basis for the analysis of sediment retention mechanisms.     

On the vertical lifting of dust in a convective unstable atmospheric boundary layer

A model for the density Q of vertical mass flux of sand (dust) in the convective atmospheric boundary layer as a function of the number density N of convective elements (including vortices), friction velocity u _*, and vertical (turbulent) buoyancy flux B is proposed. It is shown that the flux Q is proportional to the product of the square root of B and the sixth power of u _*. This finding is consistent with empirical dependences Q(u _*) reported in the literature. We discuss two methods for experimentally determining density N when the lifting of dust occurs, mainly due to (terrestrial and Martian) dust devils.     

Surface layer mixing during the SAGE ocean fertilization experiment

Vessel-based observations of the oceanic surface layer during the 14-day 2004 SAGE ocean fertilization experiment were conducted using ADCP, CTD and temperature microstructure in a frame of reference moving with a patch of injected SF₆ tracer. During the experiment the mixed layer depth z_mld ranged between 50 and 80 m, with several re-stratifying events that brought z_mld up to less than 40 m. These re-stratifying events were not directly attributable to local surface-down development of stratification and were more likely associated with horizontal variation in density structure. Comparison between the CTD and a one-dimensional model confirmed that the SAGE experiment was governed by 3-d processes. A new method for estimating z_mld was developed that incorporates a component that is proportional to density gradient. This highlighted the need for well-conditioned near-surface data which are not always available from vessel-based survey CTD profiles. A centred-displacement scale, L_c, equivalent to the Thorpe lengthscale, reached a maximum of 20 m, with the eddy-centroid located at around 40 m depth. Temperature gradient microstructure-derived estimates of the vertical turbulent eddy diffusivity of scalar (temperature) material yielded bin-averaged values around 10⁻³ m² s⁻¹ in the pycnocline rising to over 10⁻² m² s⁻¹ higher in the surface layer. This suggests transport rates of nitrate and silicate at the base of the surface layer generate mixed layer increases of the order of 38 and 13 mmol/m²/day, respectively, during SAGE. However, the variability in measured vertical transport processes highlights the importance of transient events like wind mixing and horizontal intrusions.