Observations of the sodium layer at high latitudes in summer
At altitudes between 80 and 100 km the upper atmosphere contains a layer of free sodium atoms presumably maintained by the almost continuous inflow and vaporization of cosmic dust particles. This sodium layer has been studied by ground-based photometers and lidar (laser radar), rocket-borne instrumentation, and satellite-borne photometers. Sodium measurements have been carried out at low, middle and high geographic latitudes. Lidar observations show that the peak of the layer commonly occurs close to the altitude of 90 km, with maximum densities of ˜5,000 atoms cm−3. In the 1960s photometric measurements established that the sodium layer shows seasonal variations which become more pronounced towards high latitudes (for a review of these early measurements, see ref. 1). Later lidar measurements yielded the following ratios of midsummer over midwinter column densities: 4.5 to 1 at 40° N (ref. 2), 3 to 1 at 44° N (ref. 3), and 6 to 1 at 51° N (ref. 4). Polar latitudes, however, are constantly sunlit in summer and the measurement of Na density profiles under these particular conditions have not yet been possible by ground-based methods. The state of our theoretical understanding of the Na layer has recently been reviewed by Kirchhoff5. Here we report on results of the first lidar measurements of the Na layer performed at polar latitudes in summer. Our sodium lidar instrument located at the And0ya Rocket Range, Norway (69° N, 16° E) gave measurements over ten days during July 1986 and July 1987. We find the sodium layer confined entirely to the lower thermosphere. The mean values for the layer parameters are as follows: the sodium column density is 6×l08cm−2; the number density at the layer peak is 1.2×103 cm−3; the altitude of the layer peak is 93 km, and the halfwidth of the layer is 6 km. Comparing these values with previous lidar measurements made at the same site in winter we obtain the following ratio of winter to summer densities: 10 to 1 for the sodium column density and 5 to 1 for the sodium number density at the layer peak.
Lidar observations of substantial sodium depletion in the summertime Arctic mesosphere
The chemistry and dynamics of the mesospheric sodium layer have been studied extensively with lidar (laser radar) techniques since the late 1960s. Meteoric ablation is generally regarded as the dominant source of all mesospheric alkali metals including sodium. The Na layer is typically confined to the region between 80 and 110 km with a peak near the mesopause at 90 km, where the density ranges from ∼103 to 104 cm−3. The Na column abundance at mid-latitudes in the Northern Hemisphere varies from a summer minimum of ∼ 3x 109 cm−2 to a winter maximum of ∼1010 cm−2 in December and January1. The seasonal and geographical variations in Na abundance are now believed to be related to changes in the mesopause temperature that affect the reaction rates of the main chemical loss processes for Na. In addition to chemical activity, the dynamic effects of the tides, gravity waves and the mean winds have a significant influence on the vertical structure of the layer. In June, the University of Illinois (UIUC) sodium lidar system was installed at Nordlysstasjonen, Svalbard (78° 12' N, 15°50' E) on Spitsbergen Island and we conducted more than 27 h of observations during the period 1015 July 1987. The average Na layer column abundance was ∼ 6 x 108 cm−2, a value that is almost five times lower than typical summertime Na abundances measured at mid-latitudes. Additional observations were conducted during the period 711 September 1987 and the measured Na abundances varied between ∼ 5 x l09 and 14 x 109 cm−2.
Preservation in microbial mats: mineralization by a talc-like phase of a fish embedded in a microbial sarcophagus
Microbial mats have been repeatedly suggested to promote early fossilization of macroorganisms. Yet, experimental simulations of this process remain scarce. Here, we report results of 5 year-long experiments performed onfish carcasses to document the influence of microbial mats on mineral precipitation during early fossilization. Carcasses were initially placed on top of microbial mats. After two weeks, fishes became coated by the mats forming a compact sarcophagus, which modified the microenvironment close to the corpses. Our results showed that these conditions favoured the precipitation of a poorly crystalline silicate phase rich in magnesium. This talc-like mineral phase has been detected in three different locations within the carcasses placed in microbial mats for more than 4 years: 1) within inner tissues, colonized by several bacillary cells; 2) at the surface of bones of the upper face of the corpse buried in the mat; and 3) at the surface of several bones such as the dorsal fin which appeared to be gradually replaced by the Mg-silicate phase. This mineral phase has been previously shown to promote bacteria fossilization. Here we provide first experimental evidence that such Mg-rich phase can also be involved in exceptional preservation of animals.