Carbon and deuterium isotopes and C ratios in gas hydrates of ODP Leg 204 sites

Sediments at the southern summit of Hydrate Ridge display two distinct modes of gas hydrate occurrence. The dominant mode is associated with active venting of gas exsolved from the accretionary prism and leads to high concentrations (15%-40% of pore space) of gas hydrate in seafloor or near-surface sediments at and around the topographic summit of southern Hydrate Ridge. These near-surface gas hydrates are mainly composed of previously buried microbial methane but also contain a significant (10%-15%) component of thermogenic hydrocarbons and are overprinted with microbial methane currently being generated in shallow sediments. Focused migration pathways with high gas saturation (>65%) abutting the base of gas hydrate stability create phase equilibrium conditions that permit the flow of a gas phase through the gas hydrate stability zone. Gas seepage at the summit supports rapid growth of gas hydrates and vigorous anaerobic methane oxidation. The other mode of gas hydrate occurs in slope basins and on the saddle north of the southern summit and consists of lower average concentrations (0.5%-5%) at greater depths (30-200 meters below seafloor [mbsf]) resulting from the buildup of in situ-generated dissolved microbial methane that reaches saturation levels with respect to gas hydrate stability at 30-50 mbsf. Net rates of sulfate reduction in the slope basin and ridge saddle sites estimated from curve fitting of concentration gradients are 2-4 mmol/m**3/yr, and integrated net rates are 20-50 mmol/m**2/yr. Modeled microbial methane production rates are initially 1.5 mmol/m**3/yr in sediments just beneath the sulfate reduction zone but rapidly decrease to rates of <0.1 mmol/m**3/yr at depths >100 mbsf. Integrated net rates of methane production in sediments away from the southern summit of Hydrate Ridge are 25-80 mmol/m**2/yr. Anaerobic methane oxidation is minor or absent in cored sediments away from the summit of southern Hydrate Ridge. Ethane-enriched Structure I gas hydrate solids are buried more rapidly than ethane-depleted dissolved gas in the pore water because of advection from compaction. With subsidence beneath the gas hydrate stability zone, the ethane (mainly of low-temperature thermogenic origin) is released back to the dissolved gas-free gas phases and produces a discontinuous decrease in the C1/C2 vs. depth trend. These ethane fractionation effects may be useful to recognize and estimate levels of gas hydrate occurrence in marine sediments.

Sapientia, 1998, Vol. LIII, nº 204 (número completo)

Contenido: El tiempo como medida de la primera esfera en la Física de Aristóteles / Marcelo L. Imperiale – Verdad, tiempo y justicia. Un tema de la filosofía de Anselmo de Canterbury / Ubaldo Pérez Paoli – La aportación de Tomás de Aquino a la filosofía de la justicia / Carlos Ignacio Massini Correas – Libertad y naturaleza: la voluntad como naturaleza y como razón según Tomás de Aquino / Guillermo Eduardo Spiegel Sosa – La representación como desvelamiento en Tomás de Aquino / Patrica Moya C. – Las condiciones de una nueva teoría del ente natural tras la condena de 1277. La Physica de Walter Burley y sus peculiaridades / Olga L. Larre – Gewibheit und Wahrheit. Descartes’ Grundlegung der Ersten Philosophie / Wilhelm Metz – La noción aristotélico-tomista de verdad y su interpretación en “El ser y el tiempo” de Martin Heidegger / Silvana Filippi – Subjetivistas radicales y hermenéutica en la escuela austríaca de economía / Ricardo F. Crespo – Examen de los principios de la bioética contemporánea predominantes / Camilo Tale – Sguardi sulla fine dei tempi e sulle cose ultime. A proposito Della struttura escatologica del cristianísimo / Vittorio Possenti – Naturaleza teleológica: articulación entre ser, deber ser y virtud / Jorge Peña Vial -- ¿Qué es un doctor de la Iglesia? / Francisco Canals Vidal – Anotaciones sobre la recepción y la crítica de la metafísica tomista en el pensamiento contemporáneo / Mario Enrique Sacchi – Notas y comentarios -- Bibliografía

Primary cosmic-ray positrons and negatrons in 1968 at energies between 11 and 204 MeV

The cosmic-ray positron and negatron spectra between 11 and 204 MeV have been measured in a series of 3 high-altitude balloon flights launched from Fort Churchill, Manitoba, on July 16, July 21, and July 29, 1968. The detector system consisted of a magnetic spectrometer utilizing a 1000-gauss permanent magnet, scintillation counters, and a lucite Čerenkov counter.

Launches were timed so that the ascent through the 100 g/cm² level of residual atmosphere occurred after the evening geomagnetic cutoff transition. Data gathered during ascent are used to correct for the contribution of atmospheric secondary electrons to the flux measured at float altitude. All flights floated near 2.4 g/cm².

A pronounced morning intensity increase was observed in each flight. We present daytime positron and negatron data which support the interpretation of the diurnal flux variation as a change in the local geomagnetic cutoff. A large diurnal variation was observed in the count rate of positrons and negatrons with magnetic rigidities less than 11 MV and is evidence that the nighttime cutoff was well below this value.

Using nighttime data we derive extraterrestrial positron and negatron spectra. The positron-to-total-electron ratio which we measure indicates that the interstellar secondary, or collision, source contributes ≾50 percent of the electron flux within this energy interval. By comparing our measured positron spectrum with the positron spectrum calculated for the collision source we derive the absolute solar modulation for positrons in 1968. Assuming negligible energy loss during modulation, we derive the total interstellar electron spectrum as well as the spectrum of directly accelerated, or primary, electrons. We examine the effect of adiabatic deceleration and find that many of the conclusions regarding the interstellar electron spectrum are not significantly altered for an assumed energy loss of up to 50 percent of the original energy.

Internationale Jungheringsuntersuchungen auf der Georges-Bank 204

86. Reise FFS "Anton Dohrn", 13. Februar - 9. April 1978