Probable Fertile Region of Asteroxylon mackiei K. and L.
Asteroxylon mackiei was first described from the Rhynie Chert by Kidston and Lang1, but the nature of its reproductive region has remained uncertain, since the peculiar leafless axes and small pear-shaped sporangia, which they provisionally assigned to the plant, have never been found connected to its leafy stems.
The Rhynie cherts: an early Devonian ecosystem preserved by hydrothermal activity.
The Rhynie cherts contain a remarkable early Devonian terrestrial to freshwater biota preserved in siliceous sinter by the action of a precious-metal-bearing hot spring system. Arthropods, vascular and non-vascular plants, algae, fungi and cyanobacteria are present. Preservation ranges from perfect 3D cellular permineralization to compacted coalified films, and can be related to both silicification processes and stages of biological and physical degradation of the plants at the time of silicification. Plants occasionally have original subaerial vertical axes preserved in growth position, and rhizomes bearing rhizoids. The plant litter of the substrate is also partly silicified. Silicification of organic material took place in hot spring pools, by surface flooding of areas with growing plants, and by permeation of the substrate. Sinters recognized include botryoidal geyserite typical of vent margins, and laminated sinter comparable with that of modern sinter terraces. Massive, vuggy, brecciated and nodular sinter textures are also present. At the microscopic level, textures associated with filamentous elements of the biota, and with the preservation of plants, closely match those present in modern sinters. Oxygen isotope and organic geochemical data from the Rhynie cherts indicate a temperature of 90-120 degrees C. This is apparently greater than the temperature at which elements of the biota were preserved and represents subsequent shallow burial in the hot spring system. The range of temperature and chemistry present at the surface provided high local environmental gradients. Current work attempts to identify thermophilic elements of the biota and document environmental zonation of biota relative to hot spring vents.
The microscale obstacle-resolving meteorological model MITRAS v2.0: model theory
Abstract. This paper describes the developing theory and underlying
processes of the microscale obstacle-resolving model MITRAS version 2. MITRAS
calculates wind, temperature, humidity, and precipitation fields, as well as
transport within the obstacle layer using Reynolds averaging. It explicitly
resolves obstacles, including buildings and overhanging obstacles, to
consider their aerodynamic and thermodynamic effects. Buildings are
represented by impermeable grid cells at the building positions so that the
wind speed vanishes in these grid cells. Wall functions are used to calculate
appropriate turbulent fluxes. Most exchange processes at the obstacle
surfaces are considered in MITRAS, including turbulent and radiative
processes, in order to obtain an accurate surface temperature. MITRAS is also
able to simulate the effect of wind turbines. They are parameterized using
the actuator-disk concept to account for the reduction in wind speed. The
turbulence generation in the wake of a wind turbine is parameterized by
adding an additional part to the turbulence mechanical production term in the
turbulent kinetic energy equation. Effects of trees are considered
explicitly, including the wind speed reduction, turbulence production, and
dissipation due to drag forces from plant foliage elements, as well as the
radiation absorption and shading. The paper provides not only
documentation of the model dynamics and numerical framework but also a solid
foundation for future microscale model extensions.
