Harmonic Analysis of Sedimentary Cyclic Sequences in Kansas, Midcontinent, USA

Several stratigraphic sequences in the Upper Carboniferous (Pennsylvanian) in Kansas (Midcontinent, USA) were analyzed quantitatively for periodic repetitions. The sequences were coded by lithologic type into strings of datasets. The strings then were analyzed by an adaptation of a one-dimensional Fourier transform analysis and examined for evidence of periodicity. The method was tested using different states in coding to determine the robustness of the method and data. The most persistent response is in multiples of 8–10 ft (2.5–3.0 m) and probably is dependent on the depositional thickness of the original lithologic units. Other cyclicities occurred in multiples of the basic frequency of 8–10 with persistent ones at 22 and 30 feet (6.5–9.0 m) and large ones at 80 and 160 feet (25–50 m). These levels of thickness relate well to the basic cyclothem and megacyclothem as measured on outcrop. We propose that this approach is a suitable one for analyzing cyclic events in the stratigraphic record.     

Trough structures in the Western syenite of Kûngnât, S Greenland: mineralogy and mechanism of formation

In the Western layered syenite stock of the Kûngnât complex, close to the intrusion wall, there are two groups of regularly spaced, horizontally aligned, stacks of infilled troughs. These features resemble, to some extent, the trough layering within the UZa trough zone of the gabbroic Skaergaard¹ intrusion. Such features have never before been described in syenitic rocks. The lower group comprises six stacks of infilled troughs; the upper group comprises seven. The two groups are vertically separated by about 10 m of sparsely layered syenite. The infilled troughs in the upper group are stacked vertically; those in the lower group gradually shift laterally and appear to converge over an antiformal structure. It is not possible to measure the plunge of the troughs due to outcrop geometry. Troughs are up to about 80 cm deep. The material filling the troughs is more melanocratic than the syenite that surrounds them. Individual troughs exhibit normal grading. The syenite within the troughs contains large amphibole and biotite oikocrysts. Faintly layered syenite occurs laterally between stacks. Minerals in the host syenite are more ferroan than those in the troughs, this is thought to represent re-equilibration with residual melt. Feldspar textures, irregular pyroxene and apatite zoning, and the presence of irregular shaped amphibole patches within pyroxene grains all imply alteration by a pervasive deuteric fluid in both troughs and host syenite. We suggest the following hypothesis for the formation of the troughs. A mafic crystal sheath which grows on the walls of the magma chamber becomes detached and flows downwards. The descending slurry forms a series of regularly spaced streams which erode trough shaped channels into the crystal pile at the base of the chamber. Uneroded crystal material separates the channels. Crystals are deposited from these streams. Successive streams flow over each other to generate stacks of troughs.     

Erosion rates of alpine bedrock summit surfaces deduced from in situ Be and Al

We have measured the concentration of in situ produced cosmogenic ¹⁰Be and ²⁶Al from bare bedrock surfaces on summit flats in four western U.S. mountain ranges. The maximum mean bare-bedrock erosion rate from these alpine environments is 7.6 ± 3.9 m My⁻¹. Individual measurements vary between 2 and 19 m My⁻¹. These erosion rates are similar to previous cosmogenic radionuclide (CRN) erosion rates measured in other environments, except for those from extremely arid regions. This indicates that bare bedrock is not weathered into transportable material more rapidly in alpine environments than in other environments, even though frost weathering should be intense in these areas. Our CRN-deduced point measurements of bedrock erosion are slower than typical basin-averaged denudation rates ( 50 m My⁻¹). If our measured CRN erosion rates are accurate indicators of the rate at which summit flats are lowered by erosion, then relief in the mountain ranges examined here is probably increasing.

We develop a model of outcrop erosion to investigate the magnitude of errors associated with applying the steady-state erosion model to episodically eroding outcrops. Our simulations show that interpreting measurements with the steady-state erosion model can yield erosion rates which are either greater or less than the actual long-term mean erosion rate. While errors resulting from episodic erosion are potentially greater than both measurement and production rate errors for single samples, the mean value of many steady-state erosion rate measurements provides a much better estimate of the long-term erosion rate.     

Artificial periodic irregularities in the auroral ionosphere

Artificial periodic irregularities (API) are produced in the ionospheric plasma by a powerful standing electromagnetic wave reflected off the F region. The resulting electron-density irregularities can scatter other high-frequency waves if the Bragg scattering condition is met. Such measurements have been performed at mid-latitudes for two decades and have been developed into a useful ionospheric diagnostic technique. We report here the first measurements from a high-latitude station, using the EISCAT heating facility near Troms0, Norway. Both F-region and lower-altitude ionospheric echoes have been obtained, but the bulk of the data has been in the E and D regions with echoes extending down to 52-km altitude. Examples of API are shown, mainly from the D region, together with simultaneous VHP incoherent-scatter-radar (ISR) data. Vertical velocities derived from the rate of phase change during the irregularity decay are shown and compared with velocities derived from the ISR. Some of the API-derived velocities in the 75–115-km height range appear consistent with vertical neutral winds as shown by their magnitudes and by evidence of gravity waves, while other data in the 50–70-km range show an unrealistically large bias. For a comparison with ISR data it has proved difficult to get good quality data sets overlapping in height and time. The initial comparisons show some agreement, but discrepancies of several metres per second do not yet allow us to conclude that the two techniques are measuring the same quantity. The irregularity decay time-constants between about 53 and 70 km are compared with the results of an advanced ion-chemistry model, and height profiles of recorded signal power are compared with model estimates in the same altitude range. The calculated amplitude shows good agreement with the data in that the maximum occurs at about the same height as that of the measured amplitude. The calculated time-constant agrees very well with the data below 60 km but is larger above 60 km by a factor of up to 2 at 64 km. The comparisons with the model are considered to be a good basis for more refined comparisons.     

Observations of flux rope – associated particle bursts with GEOTAIL in the distant tail

Geotail energetic particle, magnetic field data and plasma observations (EPIC, MGF and CPI experiments) have been examined for a number of energetic particle bursts in the distant tail (120Re < |X_GSM| < 130 Re), associated with moving magnetic field structures, following substorm onsets. The features obtained from this data analysis are consistent with the distant magnetotail dynamics determined first by ISEE3 observations and explained in terms of the neutral line model. At the onset of the bursts, before plasma sheet entrance, energetic electrons appear as a field-aligned beam flowing in the tailward direction, followed by anisotro-pic ions. Within the flux rope region, suprathermal ions exhibit a convective anisotropy, which allows determination of the plasma flow velocity, assuming that the anisotropy arises from the Compton-Getting effect. The velocities thus determined in the plasma sheet are estimated to be 200–650 km/s, and compare favourably with the velocities derived from the CPI electron and proton experiment. The estimated length of magnetic field structures varies between 28 and 56 Re and depends on the strength of the westward electrojet intensification. Finally, the three structures reported here show clear magnetic field signatures of flux rope topology. The existence of a strong magnetic field aligned approximately along the Y-axis and centred on the north-to-south excursion of the field, and the bipolar signature in both By and/or Bz components, is consistent with the existence of closed field lines extending from Earth and wrapping around the core of the flux rope structure.     

A 22-year cycle in the F layer ionization of the ionosphere

The double-sunspot-cycle variation in terrestrial magnetic activity has been well known for about 30 years. In 1990 we examined and compared the low-solar-activity (LSA) part of two consecutive cycles and predicted from this database and from published results the existence of a double-sunspot-cycle variation in total electron content (TEC) of the ionosphere too. This is restricted to noontime when the semi-annual component is well developed. Since 1995 we have had enough data for the statistical processing for high-solar-activity (HSA) conditions of two successive solar cycles. The results confirm the LSA findings. The annual variation of TEC shows a change from an autumn maximum in cycle 21 to a spring maximum during the next solar cycle. Similar to the aa indices for geomagnetic activity the TEC data show a phase change in the 1-year component of the Fourier transform of the annual variation. Additionally we found the same behaviour in the F-layer peak electron density (N_max) over four solar cycles. This indicates that there exists a double-sunspot-cycle variation in the F-layer ionization over Europe too. It is very likely coupled with the 22-year cycle in geomagnetic activity.