Man-made turbidity currents in Lake Superior

The discharge of taconite tailings into Lake Superior at Silver Bay, Minnesota, produces turbidity current flow. The silty fine-sand tailings fraction transported to the deepest part of the lake has formed a small fan with valleys similar in gross morphology to a submarine fan. Current meters anchored 5 m above the lake floor over the wall and over the levee of a distributary valley on the fan recorded intermittent turbidity current flows during 30 weeks in 1972–73. At least twenty-five discrete periods of observation of turbidity current flow were obtained; single episodes lasted 4?328+ h. Only flows thick enough to overflow the eastern levee of the valley could be observed, and this accounts for the intermittent nature of our observations, as flow within the valleys is expected to be continuous as long as tailings are discharged. Flow velocities were higher near the valley axis where the flow is thicker. Velocities measured over the valley wall averaged 10.8 cm/s for eleven episodes; velocities measured over the levee, more than 1/2 km from the valley axis, only 3.3 cm/s. The maximum velocity during 1300 h of observation did not exceed 31 cm/s. This agrees reasonably well with velocities calculated from channel properties, as commonly done for turbidity currents on deep-sea fans. Current meters tethered above the bottom meters indicate that lake currents normally parallel the shore throughout the water column. With the onset of a turbidity current, currents higher in the water column remain unchanged but velocities near the bottom go to zero, currents then change azimuth by 90° to parallel the downslope (down-valley) direction of the fan, then increase in velocity. During a turbidity current episode, the direction of bottom flow stays relatively constant (± 20° of the down-valley trend) but the velocity oscillates (commonly with 10 cm/s amplitude), periods being of 1/2 h or less to several hours. Turbidity currents generated on Reserve Mining Company's delta are effective in carrying essentially all tailings discharged into the lake into deeper water, where they are deposited.     

The microbiological formation of carbonate concretions in the Upper Lias of NE England

Carbonate concretions from the Jet Rock (Upper Lias, Lower Jurassic) of NE England grew in uncompacted sediment, close to the sediment surface. Microbiological activity created isolated microenvironments in which dissolved carbonate and sulphide species were produced more rapidly than they could be dispersed by diffusion, so establishing the localised supersaturation of calcite and metastable iron sulphides. Precipitation of these minerals in the microenvironment formed a single concretion.Mass-balance calculations demonstrate that at least two different microbiological processes participated in concretionary growth. The early growth stages had an unidentifiable microbiological source of carbonate which declined in importance relative to sulphate reduction as growth proceeded. It is suggested that the diffusion of dissolved organic material was important in sustaining microbiological activity.Mineralogical zonations in the concretions result from changes in the chemistry of the microenvironment due to variations in the rates of addition/removal Ca²⁺, Fe²⁺, HCO^?₃ and HS^? by microbiological activity, the crystallization of authigenic minerals and diffusion between the microenvironment and surrounding pore waters. Such changes are of only local significance and the resulting mineralogical zonations in a concretion cannot be used to deduce successive stages of diagenesis in the whole sediment.     

Variations in composition and δ18O values within the Kaffo albite—Riebeckite granite of Liruei Complex,Younger Granites of Nigeria

The peralkaline Kaffo albite—riebeckite granite is an albitised, low-temperature intrusion in Liruei Complex, one of the oldest of the ring-complexes in the Younger Granite province of Nigeria. Analyses of borehole samples from different parts of the intrusion show it to be compositionally heterogeneous, especially in respect of Si, Al, Na, K and F distribution and this, in part, can be correlated with the variable degree of albitisation. Isotopically the granite is a normal plutonic type with δ¹⁸ O values of $\text{+ 8.1 ± 0.2‰}$, and albitisation does not seem to have been accompanied by exchange of isotopes between albitising fluid and the granite. Co-existing riebeckitic-arfvedsonite and aegirine pairs from borehole samples show extreme enrichment in Na and Fe; the amphibole also shows considerable substitution of Fe by Ti, Zn and Mn, and of OH by F. Isotopically the amphibole and pyroxene are different from others, having low, variable δ¹⁸ O values (+5.3–+6.4 and $\text{+4.4–+5.1‰}$, respectively), and the fractionation value Δ Px — Am is always large, negative and constant (—$\text{1.2 ± 0.2‰}$). The low δ¹⁸ O values are considered to be due to special features of the crystal chemistry of the alkali amphiboles and pyroxenes, and the spread of each set of values may be due to sub-solidus isotope exchange between the minerals and albitising fluid.     

On the galactic plane discontinuity nearl=140°

Star counts, from measurements made by the GALAXY measuring machine, in unit B magnitude intervals were made in nine regions, each of 1.75 square degrees distributed about (l, b)= (140°, 0°). The mean density function and the mean variation of interstellar absorption with distance for the various areas were determined using a combination of theoretical and observed cumulative Wolf diagrams. For the same regions multi-colour (UBV) photographic photometry was used to identify early type stars. The distribution of these stars shows three main peaks which correspond well in distance with the Perseus, - and -spiral arms found by Verschuur from radio measurements of neutral hydrogen. The pattern made by the stellar arms in the (l, r) plane shows a discontinuity nearl=140°. From a consideration of both the star counts and the distribution of the reddened early type stars it is suggested that the cause of the discontinuity is a cloud with 0 _. ^m 7 total obscuration situated much nearer to the Sun than the Perseus arm and terminating on one side atl=140°.     

A maximal background spectrum of gravitational radiation

A maximal spectrum of gravitational radiation from sources outside our galaxy is calculated. The sources are galaxies, quasars and events that occur in the early history of the universe. The major contribution is from galaxies whose effect extends over the frequency region 10^–810⁺⁴Hz, peaking at 10^–110 Hz, with a spectral flux of 10 erg cm^–2, s^–1. The main processes of gravitational radiation in the galaxies are stellar collapse into a black hole and dying binary systems. In the region 10^–410⁴ Hz the background spectrum is well above the detection levels of currently proposed detectors. FromMinimal considerations of this spectrum it is determined that the density of gravitational radiation is 10^–39g cm^–3. This background spectrum is sensitive to galactic evolution and especially sensitive to the upper mass limits and mass distribution of stars in galactic models. Therefore, the spectrum could provide information about galactic evolution complementary to that obtained by electromagnetic investigations.     

A search for soft X-ray emission from stellar sources

With the soft X-ray detector (0.2–0.284 keV) aboard the Astronomical Netherlands Satellite (ANS) we have searched for X-ray emission from hot star coronae and peculiar stars. On Sirius ( CMa) and Capella ( Aur) X-ray emission has been measured at 6 and 5 level, respectively, above background. In all other cases the search revealed no evidence for soft X-ray emission. Upper limits to the luminosities of about 25 star coronae (main-sequence stars, (sub)giants, and supergiants) and of 4 peculiar stars ( Sco, Lyr, P Cyg, and Car) have been obtained.Paper presented at the COSPAR/IAU Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.