1,663 research outputs found
Towards an unbiased estimate of fluctuations in reef abundance and volume during the Phanerozoic
The globally preserved number and volume of ancient biogenic reefs is strongly biased by two factors: geological history and research intensity. These biases are sufficiently strong to cast doubts on the biological meaning of the recorded raw pattern. Without adjustment, it is hard to reliably identify factors potentially controlling the waxing and waning of this important ecosystem through time. Although it is currently impossible to completely compensate for the biases, I demonstrate here, based on a comprehensive database of ancient reefs, that spatiotemporal heterogeneities of the biases can largely be evened out by: (1) omitting oceanic reef sites and reef sites only known from subsurface exploration; (2) standardizing for economic factors known to affect research intensity; and (3) adjusting for sedimentary cycling processes. The resulting curves of fossil reef abundance and volume appear quite different from the original ones but the patterns of waxing and waning of the time series are not significantly altered and the overall volatility is not reduced. This suggests that both the raw curves and the adjusted curves correctly depict the basic timing of major reef blooms and declines. Nevertheless, the general pattern of the new curves (maximum proliferation in the middle Paleozoic, decline thereafter) is more in line with patterns of global cratonic carbonate sedimentation than the original curves. The adjusted curves should thus be preferred over the original ones for future tests of potential extrinsic controls of Phanerozoic reef development
Radiolarian faunal characteristics in Oligocene of the Kerguelen Plateau, Leg 183, Site 1138
Three sites from Ocean Drilling Program (ODP) Leg 183 (Kerguelen Plateau) have been analyzed to document faunal change in high-latitude radiolarians and to compare the faunal change to Eocene-Oligocene climatic deterioration. Radiolarians are not preserved in Eocene sediments. In Oligocene sediments, radiolarian preservation improves in a stepwise manner toward the Miocene. A total of 115 species were found in lower Oligocene samples from Site 1138; all are documented herein. Radiolarian preservation is presumably linked to productivity triggered by climatic cooling during the early Oligocene. Similar patterns of improving preservation through the Eocene/Oligocene boundary are documented from several Deep Sea Drilling Project and ODP sites in the Southern Ocean, indicating a general pattern. In contrast to the Southern Kerguelen Plateau, however, proxies for productivity are more divergent at Site 1138 (Central Kerguelen Plateau). Whereas carbonate dissolution, as indicated by poor preservation of foraminifers and common hiatuses, is very pronounced in the upper Eocene-lowermost Oligocene, the quality of radiolarian and diatom preservation does not significantly increase until the uppermost lower Oligocene. Multiple measures of radiolarian diversity in the Oligocene from Site 1138 closely parallel radiolarian preservation, indicating that preserved radiolarian diversity is controlled by productivity
Optical spin pumping of modulation doped electrons probed by a two-color Kerr rotation technique
We report on optical spin pumping of modulation electrons in CdTe-based
quantum wells with low intrinsic electron density (by 10^10 cm^{-2}). Under
continuous wave excitation, we reach a steady state accumulated spin density of
about 10^8 cm^{-2}. Using a two-color Hanle-MOKE technique, we find a spin
relaxation time of 34 ns for the localized electrons in the nearly unperturbed
electron gas. Independent variation of the pump and probe energies demonstrates
the presence of additional non-localized electrons in the quantum well, whose
spin relaxation time is substantially shorter
Quantum-dot-based optical polarization conversion
We report circular-to-linear and linear-to-circular conversion of optical
polarization by semiconductor quantum dots. The polarization conversion occurs
under continuous wave excitation in absence of any magnetic field. The effect
originates from quantum interference of linearly and circularly polarized
photon states, induced by the natural anisotropic shape of the self assembled
dots. The behavior can be qualitatively explained in terms of a pseudospin
formalism.Comment: 5 pages, 3 figures; a reference adde
The topological Faraday effect cannot be observed in a realistic sample
A striking feature of 3 dimensional (3D) topological insulators (TIs) is the
theoretically expected topological magneto-electric (TME) effect, which gives
rise to additional terms in Maxwell's laws of electromagnetism with an
universal quantized coefficient proportional to half-integer multiples of the
fine structure constant . In an ideal scenario one therefore expects
also quantized contributions in the magneto-optical response of TIs. We review
this premise by taking into account the trivial dielectric background of the TI
bulk and potential host substrates, and the often present contribution of
itinerant bulk carriers. We show that (i) one obtains a non-universal
magneto-optical response whenever there is impedance mismatch between different
layers and (ii) that the detectable signals due to the TME rapidly approach
vanishingly small values as the impedance mismatch is detuned from zero. We
demonstrate that it is methodologically impossible to deduce the existence of a
TME exclusively from an optical experiment in the thin film limit of 3D TIs at
high magnetic fields
A Maastrichtian microbial reef and associated limestones in the Roca Formation of Patagonia (Neuquén Province, Argentina)
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