RESUMO
The Triassic/Jurassic boundary, 208 million years ago, is associated with widespread extinctions in both the marine and terrestrial biota. The cause of these extinctions has been widely attributed to the eruption of flood basalts of the Central Atlantic Magmatic Province. This volcanic event is thought to have released significant amounts of CO2 into the atmosphere, which could have led to catastrophic greenhouse warming, but the evidence for CO2-induced extinction remains equivocal. Here we present the carbon isotope compositions of pedogenic calcite from palaeosol formations, spanning a 20-Myr period across the Triassic/Jurassic boundary. Using a standard diffusion model, we interpret these isotopic data to represent a rise in atmospheric CO2 concentrations of about 250 p.p.m. across the boundary, as compared with previous estimates of a 2,000-4,000 p.p.m. increase. The relative stability of atmospheric CO2 across this boundary suggests that environmental degradation and extinctions during the Early Jurassic were not caused by volcanic outgassing of CO2. Other volcanic effects-such as the release of atmospheric aerosols or tectonically driven sea-level change-may have been responsible for this event.
RESUMO
A beam of laser light enters the camera. The light diffusely reflected by the film re-emerges, and the speckle pattern formed at the plane of the lens diaphragm is observed. A traversing lens produces slight movements of the light spot at the film plane. If the camera is out of focus these cause bodily movements of the speckle pattern. As the focal position is approached the speckle size and rate of movement increase and finally give way to a random change of structure. The focal position can be estimated to within 0.1 or less of the Rayleigh limit. Using this it is possible to measure all the primary and secondary aberrations (other than distortion) of the camera, including any errors of film position.
