Controlled-source marine electromagnetic 2-D modeling gas hydrate studies

Research on gas hydrate has increased recently as an alternative to fossil fuel. This study of marine controlled source electromagnetics (CSEM) is motivated by this increase, particularly in deep waters, and examines representative models. We present 2D models and test their efficacy in detection and characterization of gas hydrates. Earlier modeling studies used a horizontal transmitter to study the CSEM response??two electrical and one magnetic component??for resistive subsurface layers. Here we use six components??three electrical and three magnetic??and show that the proposed method reduces ambiguity in interpretation. Additionally, we show results utilizing the transmitter dipole in a borehole and receivers at the sea bottom. We found that CSEM response from a vertical transmitter helps us characterize resistive layers more confidently than from a transmitter moving horizontally at sea bottom. We conclude that in a complex environment, combining horizontal and vertical movements of the transmitter with sea-bottom receivers helps us delineate the subsurface structure more clearly and may help reduce drilling costs. Our models closely match the gas hydrate region in the Gulf of Mexico??Walker Ridge Block-313. Although this study examines gas hydrate, the methodology is applicable to other areas??for example, in monitoring gas diffusion at subsurface depths, which may help in CO₂ sequestration.     

Thioploca spp. sheaths as niches for bacterial and protistan assemblages

Sulfide oxidizing bacterial mats are common in regions of the continental shelves characterized by high primary production and the resultant oxygen minimum zone. These mats are made up of several species of Beggiatoa and/or Thioploca, which oxidize sulfide that is generated in the sediment. Thioploca spp. inhabit a large polysaccharide sheath that encompasses bundles of 1–20 filaments (trichomes), each ranging from 3 to 60 μm in diameter. This sheath has been shown to be a critical component of the autecology of Thioploca. Analysis of Thioploca from cold seeps in Monterey Bay using light and transmission electron microscopy identified new and diverse microbial assemblages associated with interstitial spaces between trichomes, inside the sheath. Small diameter, non‐vacuolate, filamentous prokaryotes were numerous. Amoebae, euglenozoa, ciliates and other protists of unknown affiliation were observed in sheaths. Most of the protists possessed food vacuoles and some protists showed ultrastructural evidence of endosymbionts. These observations suggest that Thioploca sheaths may serve as oases on the sea floor, providing nutritional and detoxification services to previously unrecognized microbial partners.     

Influence of the vertical distribution of cyanobacteria in the water column on the remote sensing signal

An increased intensity of cyanobacterial blooms and their potentially harmful effects have attracted the attention of environmental agencies, water authorities and the general public worldwide. Reliable operational monitoring methods of coastal waters, lakes and ponds are needed. Mapping of the surface extent of cyanobacterial blooms with remote sensing is straightforward, but recognizing waters dominated by cyanobacteria throughout the water column and quantitative mapping of cyanobacterial biomass with remote sensing is more complicated. Unlike most algae, cyanobacteria can regulate their buoyancy and move vertically in the water column. We used the Hydrolight 4.2 radiative transfer model and the specific optical properties of three species of cyanobacteria to study the impact of vertical distribution of cyanobacteria on the remote sensing signal. The results show that the vertical distribution of cyanobacteria in the water column has a significant impact on the remote sensing signal. This result indicates that developing remote sensing methods for quantitative mapping of cyanobacterial biomass is much more complex than quantitative mapping of an algal biomass that is uniformly distributed in the top mixed layer of water column.     

Mesoscale physical processes and zooplankton transport-retention in the northern Norwegian shelf region

Northern Norwegian shelf regions are highly productive, supporting fisheries rich in commercially important species such as cod, herring and capelin. It has been long recognized that the mesoscale jets, meanders and eddies associated with interactions between the North Atlantic Current, Norwegian Coastal Current and regional bottom topographic features such as troughs, banks and shelfbreaks play important roles in transporting and retaining zooplankton. To investigate zooplankton distributions and their correspondence with the physical fields, three large-scale surveys with mesoscale resolutions on physical and biological fields were conducted in northern Norwegian shelf regions between latitudes 68°15′N and 70°15′N in springs of 2000–2002. Survey results provide insights into the relationships between zooplankton distributions and the physical features such as fronts, the Norwegian Coastal Current and eddies related to topographic features. The physical and biological data are integrated and analyzed focusing on water types, estimation of geostrophic currents from direct current measurements, along-shelf transport of zooplankton, and retention of zooplankton by the mesoscale meander–eddy over a typical bank area on the shelf. The estimated mean transport in the upper 100 m on the shelf in the survey region is approximately 6.4×10³ tonnes wet weight day⁻¹ northward. High zooplankton abundances were found over both Malangsgrunnen and Sveinsgrunnen banks. The specific accumulation rate from northward–southward transport in the upper 100 m over Malangsgrunnen was approximately 0.08 day⁻¹, while variable currents with an offshore gradient of zooplankton abundance over Sveinsgrunnen implies an offshore dispersion of coastal-originated zooplankton cohort.     

On the geologic structure of the Explora Escarpment (Weddell Sea,Antarctica) revealed by satellite and Shipboard data evaluation

Bathymetric, gravity, and magnetic data from Antarctic expeditions with RV POLARSTERN and satellite altimeter data from the Geosat Geodetic Mission are analysed using methods from geostatistics and geophysical inverse theory.The Explora Escarpment represents the edge between the Antarctic Continental Shelf and the Weddell Abyssal Plain. It is an important link in the reconstruction of Gondwana breakup, but a feature as large as the 2000 m deep Wegener Canyon was only discovered in 1984, when extensive bathymetric, gravimetric, and magnetic surveys with RV POLARSTERN began.Geostatistics, the theory of regionalized variables, is applied to integrate dense surveys of Wegener Canyon and sparse observations in adjacent areas into maps with full coverage of the 230 km by 330 km area at 10°–20° W/70°–72° S. The resultant highresolution bathymetric and gravity maps reveal detailed structures of the Explora Escarpment. Using geophysical inversion, the gravity terrain effect is calculated. Satellite data are used for their better coverage, but have much lower resolution. Nevertheless, the structures of Wegener Canyon and other more prominent features appear with surprisingly good correlation also in the Geosat altimeter data. While it was initially supposed that Wegener Canyon is purely an erosional structure, the magnetic map now provides evidence of the canyon's tectonic origin.     

Röntgenographische Gefügeanalyse mit dem Texturgoniometer am Beispiel von Quarziten aus kaledonischen Überschiebungszonen

Zusammenfassung Quantitative Gefügediagramme können röntgenographisch mit einem Zählrohr-Textur-Goniometer gemessen werden. Diese Methode kommt besonders in Frage bei sehr feinkörnigen Gesteinen oder aber dann, wenn etwa bei einachsigen Mineralen auch die Regelung anderer Gitterrichtungen außer der c-Achsen-Richtung interessiert. Das beim Quarz anzuwendende Verfahren wird behandelt und gezeigt, wie man hier im Einzelfall Prismenregelungen nachweisen kann. Als praktische Beispiele werden Quarzitgefüge mit Prismenregelung aus kaledonischen Überschiebungszonen Schottlands und Norwegens abgebildet und ihre Entstehung diskutiert. Die Messungen scheinen darauf hinzuweisen, daß beim Quarz bevorzugt Translation mit (11¯20) als Gleitfläche und c als Gleitgerade maßgebend war.     

Naming of undersea features

The Intergovernmental Oceanographic Commission (IOC), the International Hydrographic Organization (IHO), and the joint IOC/IHO Guiding Committee for the General Bathymetric Chart of the Oceans (GEBCO) have expressed considerable concern about the indiscriminate and unregulated naming of undersea features which often go into print without any close scrutiny. An author may not realize that the feature has a name already, maybe in another language, or that his terminology conflicts with established definitions.Geo-Marine Letters wants to follow the IOC/IHO/GEBCO recommendations and requests that its authors follow the set forth guidelines. Examples of terms and definitions are given and addresses of national authorities provided.     

Influence of relative sea level changes on the construction of the Mississippi Fan

A conceptual sea-level-driven depositional model for individual fanlobes (channel-overbank systems) of the Mississippi Fan does not permit direct application of the sequence stratigraphic principles of Vail and colleagues. Deep Sea Drilling Project Leg 96 results suggest that, during initial relative lowering of sea level, the canyon and upper fan channel were formed; excavated fine-grained slope sediments may have formed a debris flow deposit base for the fanlobe. Continued lowering produced constructional channel-levee-overbank deposits. Rising relative sea level inhibited input of coarse clastics, and channel depressions filled with muds. A blanket of (hemi)pelagics represents relative high sea level stand.