The electronic structure of amorphous silica: A numerical study

We present a computational study of the electronic properties of amorphous SiO2. The ionic configurations used are the ones generated by an earlier molecular dynamics simulations in which the system was cooled with different cooling rates from the liquid state to a glass, thus giving access to glass-like configurations with different degrees of disorder [Phys. Rev. B 54, 15808 (1996)]. The electronic structure is described by a tight-binding Hamiltonian. We study the influence of the degree of disorder on the density of states, the localization properties, the optical absorption, the nature of defects within the mobility gap, and on the fluctuations of the Madelung potential, where the disorder manifests itself most prominently. The experimentally observed mismatch between a photoconductivity threshold of 9 eV and the onset of the optical absorption around 7 eV is interpreted by the picture of eigenstates localized by potential energy fluctuations in a mobility gap of approximately 9 eV and a density of states that exhibits valence and conduction band tails which are, even in the absence of defects, deeply located within the former band gap.Comment: 21 pages of Latex, 5 eps figure

Stable-isotope stratigraphy for the last 750,000 years : "Meteor" core 13519 from the eastern equatorial Atlantic

From a 10.7 m long gravity core from the Sierra Leone Rise (5° 39,5' N, 19° 51'W) a detailed oxygen and carbon isotope record of both planktonic and benthonic foraminifera species was obtained extending from the Recent to the Jaramillo event. The analysis yielded six major results. 1. Benthos oxygen isotopes varied by 1.8-2.2‰ from interglacial to glacial times and may indicate a synglacial cooling of North Atlantic Deep Water at 2800 m depth by 1-3° C. 2. Variable anomalies between the benthos and plankton δ18 O record indicate a cooling of sea-surface temperatures (SST) by up to 6° C during some glacial stages. 3. Southerly trade winds and equatorial upwelling may exert the primary control of SST variations, in particular of extreme values of cold and warm stages and of the abrupt character of climate transitions and their leads and lags, and finally, of variable sedimentation rates. 4. The benthos δ13C record correlates well with the flux and preservation of organic matter. 5. A new time scale, CARPOR, was established from the assumption that terrigenous sediment supply was ± constant but CaC03 varied considerably. When applied to the δ18O record, three major and numerous short-term variations of sedimentation rates (0.8 to 4.0 cm/kyr) can be distinguished. 6. The climatic record was modified by bioturbation much more strongly during cold than during warm stages

Centennial-to-millennial-scale periodicities of Holocene climate and sediment injections off the western Barents shelf, 75°N

At the western continental margin of the Barents Sea, 75°N, hemipelagic sediments provide a record of Holocene climate change with a time resolution of 10–70 years. Planktic foraminifera counts reveal a very early Holocene thermal optimum 10.7–7.7 kyr BP, with summer sea surface temperatures (SST) of 8°C and a much enhanced West Spitsbergen Current. There was a short cooling between 8.8 and 8.2 kyr BP. In the middle and late Holocene summer, SST dropped to 2.5°-5.0°C, indicative of reduced Atlantic heat advection, except for two short warmings near 2.2 and 1.6 kyr BP. Distinct quasi-periodic spikes of coarse sediment fraction (with large portions of lithic grains, benthic and planktic foraminifera) record cascades of cold, dense winter water down the continental slope as a result of enhanced seasonal sea ice formation and storminess on the Barents shelf over the entire Holocene. The spikes primarily cluster near recurrence intervals of 400–650 and 1000–1350 years, when traced over the entire Holocene, but follow significant 885–/840– and 505–/605-year periodicities in the early Holocene. These non-stationary periodicities mimic the Greenland-10Be variability, which is a tracer of solar forcing. Further significant Holocene periodicities of 230, (145) and 93 years come close to the deVries and Gleissberg solar cycles

Marine Palaeoklimatologie: Rapiden Klimawechseln auf der Spur Schlussbericht 1990-1993

SIGLEAvailable from TIB Hannover: F95B2048 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Forschung und Technologie (BMFT), Bonn (Germany)DEGerman

Deglacial subarctic Pacific surface water hydrography and nutrient dynamics and links to North Atlantic climate variability and atmospheric CO2

The glacial-to-Holocene evolution of subarctic Pacific surface water stratification and silicic acid (Si) dynamics is investigated based on new combined diatom oxygen (delta O-18(diat)) and silicon (delta Si-30(diat)) isotope records, along with new biogenic opal, subsurface foraminiferal delta O-18, alkenone-based sea surface temperature, sea ice, diatom, and core logging data from the NE Pacific. Our results suggest that delta O-18(diat) values are primarily influenced by changes in freshwater discharge from the Cordilleran Ice Sheet (CIS), while corresponding delta Si-30(diat) are primarily influenced by changes in Si supply to surface waters. Our data indicate enhanced glacial to mid Heinrich Stadial 1 (HS1) NE Pacific surface water stratification, generally limiting the Si supply to surface waters. However, we suggest that an increase in Si supply during early HS1, when surface waters were still stratified, is linked to increased North Pacific Intermediate Water formation. The coincidence between fresh surface waters during HS1 and enhanced ice-rafted debris sedimentation in the North Atlantic indicates a close link between CIS and Laurentide Ice Sheet dynamics and a dominant atmospheric control on CIS deglaciation. The Bolling/Allerod (B/A) is characterized by destratification in the subarctic Pacific and an increased supply of saline, Si-rich waters to surface waters. This change toward increased convection occurred prior to the Bolling warming and is likely triggered by a switch to sea ice-free conditions during late HS1. Our results furthermore indicate a decreased efficiency of the biological pump during late HS1 and the B/A (possibly also the Younger Dryas), suggesting that the subarctic Pacific has then been a source region of atmospheric CO2