ALBERT

Description: The complex deglacial to Holocene oceanographic development in the Gulf of Guayaquil (Eastern Equatorial Pacific) is reconstructed for sea surface and subsurface ocean levels from (isotope) geochemical proxies based on marine sediment cores. At sea surface, southern sourced Cold Coastal Water and tropical Equatorial Surface Water/Tropical Surface Water are intimately related. In particular since ~10 ka, independent sea surface temperature proxies capturing different seasons emphasize the growing seasonal contrast in the Gulf of Guayaquil, which is in contrast to ocean areas further offshore. Cold Coastal Water became rapidly present in the Gulf of Guayaquil during the austral winter season in line with the strengthening of the Southeast Trades, while coastal upwelling off Peru gradually intensified and expanded northward in response to a seasonally changing atmospheric circulation pattern affecting the core locations intensively since 4 ka BP. Equatorial Surface Water, instead, was displaced and Tropical Surface Water moved northward together with the Equatorial Front. At subsurface, the presence of Equatorial Under Current-sourced Equatorial Subsurface Water was continuously growing, prominently since ~10–8 ka B.P. During Heinrich Stadial 1 and large parts of the Bølling/Allerød, and similarly during short Holocene time intervals at ~5.1–4 ka B.P. and ~1.5–0.5 ka B.P., the admixture of Equatorial Subsurface Water was reduced in response to both short-term weakening of Equatorial Under Current strength from the northwest and emplacement by tropical Equatorial Surface Water, considerably warming the uppermost ocean layers.

Description: The North Brazil Current (NBC) constitutes a bottleneck for the mean northward return flow of the Atlantic Meridional Overturning Circulation (AMOC) in the tropical South Atlantic. Previous studies suggested a link between interannual to multidecadal NBC and AMOC transport variability and proposed to use NBC observations as an index for the AMOC. Here we use a set of hindcast, sensitivity, and perturbation experiments performed within a hierarchy of ocean general circulation models to show that decadal to multidecadal buoyancy-forced changes in the basin-scale AMOC transport indeed manifest themselves in the NBC. The relation is, however, masked by a strong interannual to decadal wind-driven gyre variability of the NBC. While questioning the NBC transport as a direct index for the AMOC, the results support its potential merit for an AMOC monitoring system, provided that the wind-driven circulation variability is properly accounted for.

Description: Glacial/interglacial changes in Southern Ocean's air-sea gas exchange have been considered as important mechanisms contributing to the glacial/interglacial variability in atmospheric CO2. Hence, understanding past variability in Southern Ocean intermediate to deep water chemistry and circulation is fundamental to constrain the role of these processes on modulating glacial/interglacial changes in the global carbon cycle. Our study focused on the glacial/interglacial variability in the vertical extent of southwest Pacific Antarctic Intermediate Water (AAIW). We compared carbon and oxygen isotope records from epibenthic foraminifera of sediment cores bathed in modern AAIW and Upper Circumpolar Deep Water (UCDW; 943-2066 m water depth) to monitor changes in water mass circulation spanning the past 350,000 years. We propose that pronounced freshwater input by melting sea ice into the glacial AAIW significantly hampered the downward expansion of southwest Pacific AAIW, consistent with climate model results for the Last Glacial Maximum. This process led to a pronounced upward displacement of the AAIW-UCDW interface during colder climate conditions and therefore to an expansion of the glacial carbon pool.

Description: This study explores simulations using the numerical Weather Research and Forecasting (WRF) model, with respect to the representation of the nocturnal low-level jet (LLJ) over the Sahel. Three sets of experiments are designed to investigate the sensitivity with respect to (i) the boundary-layer and surface-layer schemes including local and non-local closures, (ii) the horizontal grid spacing and the number of vertical levels within the lowest kilometre and (iii) the role of initial and boundary data. In total, 27 simulations are performed on one host domain and two nested domains for a representative LLJ case study on 9 November 2006. The ability of the individual simulations to represent the life cycle of the nocturnal LLJ is validated against observations carried out in the framework of the African Monsoon Multidisciplinary Analysis (AMMA) special observation periods: surface wind observations from Agoufou, Bamba and Banizoumbou, atmospheric wind profiles derived from Atmospheric Radiation Measurement Mobile Facility, wind radar measurements at Niamey and profiles from radiosondes launched at Niamey. All runs reproduce the general characteristics of the observed LLJs satisfactorily. In contrast to earlier studies, results are more sensitive to the choice of initial and boundary data (here GFS and ECMWF) than to the boundary-layer and surface schemes used or to model grid resolution. The sensitivity to the model grid resolution is surprisingly minor. Considerable differences between the individual stations suggest that local surface conditions such as roughness length, albedo or soil moisture may play an important role in the observed mismatch between model simulations and observations.

Description: Steel well casings in or near a hydrocarbon reservoir can be used as source electrodes in time-lapse monitoring using grounded line electromagnetic methods. A requisite component of carrying out such monitoring is the capability to numerically model the electromagnetic response of a set of source electrodes of finite length. We present a modelling algorithm using the finite-element method for calculating the electromagnetic response of a three-dimensional conductivity model excited using a vertical steel-cased borehole as a source. The method is based on a combination of the method of moments and the Coulomb-gauged primary–secondary potential formulation. Using the method of moments, we obtain the primary field in a half-space due to an energized vertical steel casing by dividing the casing into a set of segments, each assumed to carry a piecewise constant alternating current density. The primary field is then substituted into the primary–secondary potential finite-element formulation of the three-dimensional problem to obtain the secondary field. To validate the algorithm, we compare our numerical results with: (i) the analytical solution for an infinite length casing in a whole space, excited by a line source, and (ii) a three-layered Earth model without a casing. The agreement between the numerical and analytical solutions demonstrates the effectiveness of our algorithm. As an illustration, we also present the time-lapse electromagnetic response of a synthetic model representing a gas reservoir undergoing water flooding.

Description: Submarine permafrost degradation has been invoked as a cause for recent observations of methane emissions from the seabed to the water column and atmosphere of the East Siberian shelf. Sediment drilled 52 m down from the sea ice in Buor Khaya Bay, central Laptev Sea revealed unfrozen sediment overlying ice-bonded permafrost. Methane concentrations in the overlying unfrozen sediment were low (mean 20 µM) but higher in the underlying ice-bonded submarine permafrost (mean 380 µM). In contrast, sulfate concentrations were substantially higher in the unfrozen sediment (mean 2.5 mM) than in the underlying submarine permafrost (mean 0.1 mM). Using deduced permafrost degradation rates, we calculate potential mean methane efflux from degrading permafrost of 120 mg m−2 yr−1 at this site. However, a drop of methane concentrations from 190 µM to 19 µM and a concomitant increase of methane δ13C from −63‰ to −35‰ directly above the ice-bonded permafrost suggest that methane is effectively oxidized within the overlying unfrozen sediment before it reaches the water column. High rates of methane ebullition into the water column observed elsewhere are thus unlikely to have ice-bonded permafrost as their source.

Description: We compare selected marine electromagnetic methods for sensitivity to the presence of relatively thin resistive targets (e.g., hydrocarbons, gas hydrates, fresh groundwater, etc.). The study includes the conventional controlled-source electromagnetic method, the recently introduced transient electromagnetic prospecting with vertical electric lines method, and the novel marine circular electric dipole method, which is still in the stage of theoretical development. The comparison is based on general physical considerations, analytical (mainly asymptotic) analysis, and rigorous one-dimensional and multidimensional forward modelling. It is shown that transient electromagnetic prospecting with vertical electric lines and marine circular electric dipole methods represent an alternative to the conventional controlled-source electromagnetic method at shallow sea, where the latter becomes less efficient due to the air-wave phenomenon. Since both former methods are essentially short-offset time-domain techniques, they exhibit a much better lateral resolution than the controlled-source electromagnetic method in both shallow sea and deep sea. The greatest shortcoming of the transient electromagnetic prospecting with vertical electric lines and marine circular electric dipole methods comes from the difficulties in accurately assembling the transmitter antenna within the marine environment. This makes these methods significantly less practical than the controlled-source electromagnetic method. Consequently, the controlled-source electromagnetic method remains the leading marine electromagnetic technique in the exploration of large resistive targets in deep sea. However, exploring laterally small targets in deep sea and both small and large targets in shallow sea might require the use of the less practical transient electromagnetic prospecting with vertical electric lines and/or marine circular electric dipole method as a desirable alternative to the controlled-source electromagnetic method.

Description: Mud volcanism is commonly observed in Azerbaijan and the surrounding South Caspian Basin. This natural phenomenon is very similar to magmatic volcanoes but differs in one considerable aspect: Magmatic volcanoes are generally the result of ascending molten rock within the Earth's crust, whereas mud volcanoes are characterised by expelling mixtures of water, mud, and gas. The majority of mud volcanoes have been observed on ocean floors or in deep sedimentary basins, such as those found in Azerbaijan. Furthermore, their occurrences in Azerbaijan are generally closely associated with hydrocarbon reservoirs and are therefore of immense economic and geological interest. The broadside long-offset transient electromagnetic method and the central-loop transient electromagnetic method were applied to study the inner structure of such mud volcanoes and to determine the depth of a resistive geological formation that is predicted to contain the majority of the hydrocarbon reservoirs in the survey area. One-dimensional joint inversion of central-loop and long-offset transient electromagnetic data was performed using the inversion schemes of Occam and Marquardt. By using the joint inversion models, a subsurface resistivity structure ranging from the surface to a depth of approximately 7 km was determined. Along a profile running perpendicular to the assumed strike direction, lateral resistivity variations could only be determined in the shallow depth range using the transient electromagnetic data. An attempt to resolve further two-dimensional/three-dimensional resistivity structures, representing possible mud migration paths at large depths using the long-offset transient electromagnetic data, failed. Moreover, the joint inversion models led to ambiguous results regarding the depth and resistivity of the hydrocarbon target formation due to poor resolution at great depths (〉5 km). Thus, 1D/2D modelling studies were subsequently performed to investigate the influence of the resistive terminating half-space on the measured long-offset transient electromagnetic data. The 1D joint inversion models were utilised as starting models for both the 1D and 2D modelling studies. The results tend to show that a resistive terminating half-space, implying the presence of the target formation, is the favourable geological setting. Furthermore, the 2D modelling study aimed to fit all measured long-offset transient electromagnetic Ex transients along the profile simultaneously. Consequently, 3125 2D forward calculations were necessary to determine the best-fit resistivity model. The results are consistent with the 1D inversion, indicating that the data are best described by a resistive terminating half-space, although the resistivity and depth cannot be determined clearly.

Description: Ec' to.thi' o.rho' do.spi' ra. Gr. prep. ectos outside; Gr. n. thios sulfur; Gr. n. rhodon the rose; Gr. n. spira the spiral; M.L. fem. n. Ectothiorhodospira spiral rose with sulfur outside. Proteobacteria / Gammaproteobacteria / Chromatiales / Ectothiorhodospiraceae / Ectothiorhodospira Cells rod-shaped or vibrioid, also appearing as true spirals, 0.7–1.5 µm in diameter, motile by a polar tuft of flagella, multiply by binary fission, and may contain gas vesicles. Gram-negative and belong to the Gammaproteobacteria. Internal photosynthetic membranes are lamellar stacks that are continuous with the cytoplasmic membrane. Photosynthetic pigments are bacteriochlorophyll a and carotenoids. The mol% G + C of the DNA is: 61.4–68.4. Type species: Ectothiorhodospira mobilis Pelsh 1936, 120.

Description: Proteobacteria Alphaproteobacteria Rhodospirillales Acetobacteraceae A.ci.di.phi' li.um. M.L. n. acidum an acid; Gr. adj. philus loving; M.L. neut. n. Acidiphilium acid lover. Proteobacteria / Alphaproteobacteria / Rhodospirillales / Acetobacteraceae / Acidiphilium Cells are straight rods, 0.3–1.2 × 4.2 µm, multiply by binary fission and exhibit a pleomorphic tendency at varied pH values and in the presence of different carbon sources. Motile by means of polar, subpolar, and lateral flagella. Do not form spores or capsules. Gram negative. 16S rRNA structures and signatures conform to the Alphaproteobacteria . Cell suspensions and colonies are white to cream, yellow, pink, red, or brown. Strictly aerobic, chemoorganotrophic and chemolithotrophic bacteria containing photosynthetic pigments, categorized into the aerobic bacteriochlorophyll-containing bacteria. Main photosynthetic pigments are zinc-chelated bacteriochlorophyll (Zn-BChl) a and the carotenoid spirilloxanthin. Grow with simple organic compounds, such as sugars, as electron donors and carbon sources. Growth is inhibited in the presence of 0.6% yeast extract and low concentrations of acetate (0.25 mM) and lactate (2 mM). One species is a facultative chemolithotroph utilizing elemental sulfur as electron donor. Fe2+ does not serve as the electron donor for chemolithotrophic growth but has stimulatory effects on heterotrophic growth. Catalase is positive and oxidase negative or weakly positive. Mesophilic and obligately acidophilic bacteria growing in the pH range of 2.0–5.9 (but not at pH 6.1 and above). Straight-chain monounsaturated C 18:1 acid is the major component of cellular fatty acids. Ubiquinones with ten isoprene (Q-10) units are present. The mol% G + C of the DNA is: 62.9–68.1. Type species: Acidiphilium cryptum Harrison 1981, 331.