Side-scan and acoustic subbottom characterization of the sea floor near the Dry Tortugas, Florida

 Between 2 and 6 February, 1995, a 25 km² area at the Dry Tortugas (Florida Keys) was surveyed with a 100 kHz side-scan sonar system and 3.5-kHz subbottom profiler. The side-scan system revealed a pattern of alternating high and low backscatter. The subbottom profiler showed areas with no acoustic penetration between sediment troughs. The combination of both methods allowed delineation of the boundaries in high-backscatter regions, and sediment samples allowed correlations between high backscatter and coarser-grained sediments.     

Towards high‐resolution 3D marine seismic surveying using Boomer sources

The use of Boomer sources for 3D seismic imaging of shallow marine structures was investigated in a feasibility study. Boomers show sufficient stability to be used in multichannel seismic surveys. The acquisition of a high-frequency, densely sampled seismic data volume was successfully performed in the Baltic Sea. A Pleistocene fluvial channel system and shallow gas accumulations were revealed beneath the unconsolidated sediments which constitute the sea-floor in the southern Kiel Bay.     

Redistribution of HFSE elements during rutile replacement by titanite

Titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon. We investigated an amphibolite sample from the Tromsø eclogite facies terrain in Northern Norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite. Patchy compositional zoning of Al, Ti, and F in titanite relates to the presence of a fluid with variable Ti/Al and/or F during its growth. Laser ablation ICP–MS and electron microprobe data for high field strength elements (HFSE: Nb, Zr, Ta, and Hf) of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal (~8 mm) and a systematic decrease towards uniform HFSE contents in the large core. HFSE contents of smaller rutile grains (~0.5 mm) and rutile inclusions (<100 μm) in the titanite overgrowth are similar or higher than in the rims of large rutile crystals. Element profiles from the rim inward demonstrate that HFSE enrichment in rutile is controlled by diffusion. HFSE ratios in diffusion-altered rutile show systematic variations compared with the uniform core composition of the large rutile. Modelling of Zr and Nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid-dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems. Variations of HFSE contents in the newly formed titanite show no systematic spatial distribution. HFSE ratios in titanite and the rims of rutile are different, indicating different solid/fluid distribution coefficients in these minerals. Element fractionation by diffusion into the relict rutile and during fluid-mediated growth of new titanite could substantially change the HFSE budget of these minerals and could affect their use for geochemical tracing and other applications, such as Zr-based geothermobarometry.     

Pre-Cenozoic intra-plate magmatism along the Central Andes (17–34°S): Composition of the mantle at an active margin

Sr–Nd–Pb isotope ratios of alkaline mafic intra-plate magmatism constrain the isotopic compositions of the lithospheric mantle along what is now the eastern foreland or back arc of the Cenozoic Central Andes (17–34°S). Most small-volume basanite volcanic rocks and alkaline intrusive rocks of Cretaceous (and rare Miocene) age were derived from a depleted lithospheric mantle source with rather uniform initial ¹⁴³Nd/¹⁴⁴Nd ( 0.5127–0.5128) and ⁸⁷Sr/⁸⁶Sr ( 0.7032–0.7040). The initial ²⁰⁶Pb/²⁰⁴Pb ratios are variable (18.5–19.7) at uniform ²⁰⁷Pb/²⁰⁴Pb ratios (15.60 ± 0.05). A variety of the Cretaceous depleted mantle source of the magmatic rocks shows elevated Sr isotope ratios up to 0.707 at constant high Nd isotope ratios. The variable Sr and Pb isotope ratios are probably due to radiogenic growth in a metasomatized lithospheric mantle, which represents the former sub-arc mantle beneath the early Palaeozoic active continental margin. Sr–Nd–Pb isotope signatures of a second mantle type reflected in the composition of Cretaceous (one late Palaeozoic age) intra-plate magmatic rocks (¹⁴³Nd/¹⁴⁴Nd  0.5123, ⁸⁷Sr/⁸⁶Sr  0.704, ²⁰⁶Pb/²⁰⁴Pb  17.5–18.5, and ²⁰⁷Pb/²⁰⁴Pb  15.45–15.50) are similar to the isotopic composition of old sub-continental lithospheric mantle of the Brazilian Shield.

Published Nd and Sr isotopic compositions of Mesozoic to Cenozoic arc-related magmatic rocks (18–40°S) represent the composition of the convective sub-arc mantle in the Central Andes and are similar to those of the Cretaceous (and rare Miocene) intra-plate magmatic rocks. The dominant convective and lithospheric mantle type beneath this old continental margin is depleted mantle, which is compositionally different from average MORB-type depleted mantle. The old sub-continental lithospheric mantle did not contribute to Mesozoic to Cenozoic arc magmatism.     

Sm–Nd ages of mafic rocks from the Coastal Cordillera at 24°S, northern Chile

We investigated the Sm–Nd systematics of mafic granulite and undeformed layered gabbro, which form a midcrustal section of the Jurassic magmatic belt in the North Chilean Coast Range, south of Antofagasta. Mineral isochrons indicate ages between ca. 171 and 150?Ma for the granulite and an age of ca. 161 Ma for the gabbro. These ages are interpreted as closure time of the Sm–Nd system in the area. The age of metamorphism is Late-Jurassic. The minimum intrusion age of the protolith of the granulite is likely Early Jurassic (ca. 200?Ma), but an exact intrusion age could not be derived from the data. The intrusion age of the gabbro is ca. 185 Ma. Granulite generated from mantle-derived gabbroic magmas has ?_Nd200 between 6.28 and 7.05 and the gabbro that intruded the granulite has ?_Nd185 between 5.89 and 6.09.     

Numerical modelling of the seasonal variability of the North and Tropical Atlantic currents using an adaptation approach

The paper reports computed data on the seasonal variability of the Atlantic Ocean currents which were obtained through numerical integration (using conservative difference schemes) of non-linear equations of hydrothermodynamics taking into account an upper quasi-homogeneous layer and the bottom topography. The seasonal variability of the meridional heat transfer is discussed. The results were obtained using an adaptation approach.Translated by Vladimir A. Puchkin.     

Effect of polychromatic X‐ray microtomography imaging on the amino acid content of the Murchison CM chondrite

X‐ray microcomputed tomography (μCT) is a useful means of characterizing cosmochemical samples such as meteorites or robotically returned samples. However, there are occasional concerns that the use of μCT may be detrimental to the organic components of a chondrite. Small organic compounds such as amino acids comprise up to ~10% of the total solvent extractable carbon in CM carbonaceous chondrites. We irradiated three samples of the Murchison CM carbonaceous chondrite under conditions akin to and harsher than those typically used during typical benchtop X‐ray μCT imaging experiments to determine if detectable changes in the amino acid abundance and distribution relative to a nonexposed Murchison control sample occurred. After subjecting three meteorite samples to ionizing radiation dosages between ~300 Gray (Gy) and 3 kGy with bremstrahlung X‐rays, we analyzed the amino acid content of each sample. Within sampling and analytical errors, we cannot discern differences in the amino acid abundances and amino acid enantiomeric ratios when comparing the control samples (nonexposed Murchison) and the irradiated samples. We conclude that a polychromatic X‐ray μCT experiment does not alter the abundances of amino acids to a degree greater than how well those abundances are measured with our techniques and therefore any damage to amino acids is minimal.     

Mikroskopische und mikroanalytische Untersuchungen an metallischen Kügelchen aus Tiefseesedimenten des Zentralen Pazifik

Metallic spherules from Central Pacific Ocean sediments (“Valdivia” expedition VA 05-1, 1973) were studied by means of the ore microscope and by electron microprobe. The data suggest that most of the spherules are of meteoritic origin, i.e. ablation drops from meteorites as suggested in former studies on spherules from marine environment by Schmidt and Keil (1966), Finkelmann (1970, 1972) and others. The metallic spherules contain up to about 60% Ni, 37% Fe and 3% Co. Unusually high contents of Cr (up to 12%) were noted in some spherules.     

Die Westwärts-Intensivierung des Polarfront-Jetstream in den Winkel zwischen Polarfront und Tropopause an der Ostseite eines polaren Kälteausbruchs,eine Erweiterung des klassischen Grenzflächenproblems von Margules

Zusammenfassung Es wird eine auf rein dynamischen Grundlagen beruhende Erklärung für die starke Westwärts-Intensivierung des Polar-front-Jetstream in den Winkel zwischen der geneigten Polarfrontfläche und der gegengeneigten Tropopause an der Ostflanke eines polaren Kälteausbruches gegeben.Eine gewisse Ähnlichkeit des Atmosphärenaufbaues mit dem Aufbau des Ozeans im Stromstrich des Golfstromes führt dazu, die Trägheitsgrenzschichten-Theorie vonJ. Charney [1] auf ein solches atmosphärisches System anzuwenden. Ein dreigeschichtetes Atmosphärenmodell wird entwickelt, und für dieses werden Bewegungsgleichungen abgeleitet, die es erlauben, das Problem entsprechend dem LösungsgangCharneys zu behandeln.Der Vergleich mit den aus den Beobachtungen abgeleiteten Neigungen der Polarfront und der Tropopause und der beobachteten Westwärts-Intensivierung des Stromes fällt sehr befriedigend aus. Diese Theorie erfaßt selbstverständlich nur die rein dynamische Seite der Erscheinung und berücksichtigt den sicher wichtigen Beitrag der thermodynamischen Prozesse durch die Verwendung des rohen Modells nur näherungsweise.

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Summary Based on purely dynamical reasoning an explanation for the strong westward-intensification of the polarfront jetstream into the angle between the inclined polarfront surface and the opposite inclined tropopause on the eastern side of an outbreak of cold polar air is presented.Certain similarity of the structure of the atmosphere with that of the ocean in the main core of the gulf stream has guided me to apply the inertial boundary layer theory ofJ. Charney [1] to such an atmospheric system. A three-layered model of the atmosphere has been developed and corresponding equations of motion were derived, which allow to handle the problem according to the solution ofCharney.The comparison with the slopes of polarfront and tropopause according to the observations and with the observed westward intensification of the flow is rather satisfactory. However, we must understand that this theory only considers the dynamical side of the phenomenon and does only approximately account for the important contribution of thermodynamic processes by the use of the rather simple model.

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Résumé L'auteur tente d'expliquer du point de vue purement dynamique l'intensification vers l'Ouest du jet situé dans l'angle compris entre le front polaire et la tropopause d'inclinaison contraire, sur le flanc oriental d'un afflux d'air polaire.Une certaine analogie entre la structure de l'atmosphère et celle de l'océan à l'endroit du Gulf-stream incite à appliquer au cas évoqué ici la théorie de la couche limite d'inertie deJ. Charney [1]. On imagine un type d'atmosphère à trois couches pour lequel on établit les équations du mouvement permettant de traiter le problème en cause à l'aide du procédé deJ. Charney.Les observations concernant l'inclinaison du front polaire et de la tropopause ainsi que celles de l'intensification du jet vers l'Ouest confirment la théorie de façon très satisfaisante. Cette dernière, purement dynamique, ne tient compte que d'une manière approchée des processus thermodynamiques sans doute importants.

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