Permanent Thermal and Chemical Stratification in a Restored Urban Meromictic Lake

Meromictic lakes are unique aquatic ecosystems that occur extremely rarely. The phenomenon of meromixis can result from both natural and anthropogenic factors. The aim of this study was to analyse thermal and chemical stratification in a small, deep (6 ha, H max = 24.5 m) lake. The evaluated lake had a typical summer thermal profile with a shallow epilimnion, a sharp thermocline, and a distinct monimolimnion layer in the hypolimnion, which was also maintained during circulation. The lake had a clinograde oxygen profile, with an oxygen deficit in the metalimnion and permanent anoxic conditions in the deeper layers, including during circulation. A redox zone was identified during summer stagnation. The monimolimnion formed a thermally isolated layer at a depth of around 15 m, and the chemocline was situated above the monimolimnion. In the chemocline, the EC gradient ranged from 61 to 77 μS·cm−1 per meter of depth in the summer and from 90 to 130 μS·cm−1 per meter of depth during circulation. EC was significantly correlated with Ca2+ concentration (r2 = 0.549). Chemical stratification, particularly with regard to organic matter distribution, was observed in the chemocline. The monimolimnion severely limited nutrient internal loading.

The mic-mac connection inside stars: the interdependence of atomic diffusion and hydrodynamic instabilities

The interdependence of microscopic (atomic) and macroscopic (hydrodynamic) processes inside stars and their consequences for stellar structure and evolution were recognized by Jean-Paul Zahn several decades ago. He was a pioneer in that respect, discussing the importance of the macroscopic motions related to stellar rotation, in competition with the chemical stratification induced by gravitational settling and radiative accelerations. This has been much developed in recent years, in connection with the improvement of observational data, including asteroseismology. Morover, it has been recently discovered that the microscopic atomic diffusion processes can lead to macroscopic results which may infuence in a non negligible way the internal stellar structure, independently of the abundances observed at the surface.

Vertical shear mixing in stellar radiative zones

Jean-Paul Zahn’s formalism for vertical shear mixing is used in several stellar evolution codes, but the physics of the shear instability in stellar radiative zones is still not completely understood. Over the last few years, numerical simulations have provided new constraints on the shear instability, including the effect of thermal diffusion and chemical stratification. We present here new simulations that show the effect of viscosity on the vertical turbulent transport due to the shear instability.

LS IV — 14°116 : A Time-Resolved Spectroscopic Study

LSIV-14 116 is a very unusual subdwarf B star. It pulsates non-radially with high-order g-modes, these pulsations are unexpected and unexplained, as the effective temperature is 6 000K hotter than the blue edge of the hot subdwarf g-mode instability strip. Its spectrum is enriched in helium which is not seen in either the V361 Hya (p-mode pulsators) or the V1093 Her stars (g-mode pulsators). Even more unusual is the 4 dex overabundance of zirconium, yttrium, and strontium. It is proposed that these over-abundances are a result of extreme chemical stratification driven by radiative levitation. We have over 20hrs of VLT/UVES spectroscopy from which we have obtained radial velocity curves for individual absorption lines. We are currently exploring ways in which to resolve the photospheric motion as a function of optical depth.

1. The basic idea

‘The basic idea’ presents the principles of plate tectonics and describes how this revolutionary theory took hold. It begins with Alfred Wegener in 1912, who proposed the concept of continental drift and a former huge continent, Gondwanaland. In the face of strong opposition, this theory was supported by the development of palaeomagnetism in the 1950s and, in the 1960s, became subsumed within the broader framework of plate tectonics. Three major events precipitated this change: a switch in emphasis from continents to ocean basins and their exploration; rapid growth in seismology; and a shift in perspective from the chemical stratification of the Earth, in terms of crust and mantle, to another that emphasized strength—a strong lithosphere, some 100–200 km thick, overlying a weak asthenosphere.