The late Pleistocene geomagnetic field as recorded by sediments from Fargher Lake,Washington, U.S.A.

Measurement of the remanent magnetization of a 6.88-m oriented core of soft sediments and tephras from Fargher Lake near Mount St. Helens in southwestern Washington State shows that no significant geomagnetic reversals were recorded in the sediments of the lake. Radiocarbon and palynological dating of the tephra layers from the lake bed indicates deposition during the interval 17, 000–34, 000 years B.P. although geochemical correlation of a prominent tephra layer in the core with tephra set C of Mount St. Helens could mean that the maximum age of the sediments may be at least 36, 000 years B.P. The core was divided into specimens 0.02 m long, each representing approximately 55 years of deposition assuming a constant rate of sedimentation. Pilot alternating field demagnetization studies of every tenth specimen indicated a strong, stable remanence with median destructive field of 15 mT, and the remaining specimens were subsequently demagnetized in fields of this strength. The mean inclination for all specimens exclusive of the unstably magnetized muck and peat from near the surface is 56.1° which is 8° shallower than the present axial dipole field at this site, perhaps because of inclination error in the detrital remanent magnetization of the sediments, although because of the variability in the data, this departure from the axial dipole field may not be significant. The ranges of inclination and declination are comparable to those of normal secular variation at northern latitudes. Although three isolated specimens have remanence with negative inclination, these anomalous directions are due to sampling and depositional effects. Measurement of a second core of 6.86 m length also revealed only normal magnetic polarity, but this result is of little stratigraphic value as this core failed to penetrate the distinctive tephra found near the base of the former core.Studies of a concentrate of the magnetic minerals in the sediments by optical microscopy and X-ray diffraction indicate that the primary magnetic constituent is an essentially pure magnetite of detrital origin. The magnetite occurs in a wide range of grain sizes with much of it of sub-multidomain size (< 15 μm).As a whole, this study provides substantial evidence against the existence of large-scale worldwide geomagnetic reversals during the time interval of Fargher Lake sedimentation, a segment of geological time for which many excursions and reversals have been reported elsewhere.     

A Late Eocene paleomagnetic pole for the Pacific plate

Seamount magnetic anomaly inversions as well as DSDP paleomagnetic and equatorial sediment facies data constrain a paleomagnetic pole for the Pacific plate of Late Eocene age. The location of the pole at 77.5°N, 21.2°E implies 12.5 ± 1.6° of apparent polar wander for the Pacific plate during the last 41 ± 5 m.y. The Late Eocene pole is significantly different from the Pacific Maastrichtian pole at the 95% confidence level and indicates 7.2° of apparent polar motion of the Pacific between 69 and 41 m.y. B.P. The data source locations for the Late Eocene pole are scattered over a large area of the North Pacific and thus the consistency of the data supports the hypothesis that the north central Pacific plate has been rigid since the Eocene. The agreement of the Late Eocene pole with the motion predicted for the Pacific from hotspot models suggests that relative motion between the spin axis and hotspots has been small since that time. Additionally, this finding dictates that the significant amounts of hotspot versus spin axis motion inferred by other authors to have occurred since the Cretaceous must have instead occurred at a faster rate and concluded before the Eocene.     

Manganese in the North Pacific

A quantitative and precise method for determination of dissolved Mn at the nanomole(nmol)/kg level in seawater has been developed and used to study the distribution of Mn in the northeast Pacific. Mn concentrations in the surface mixed layer decrease from 1.0 to 0.6 nmol/kg between the central gyre and the western boundary of the California Current, then increase to values from 2 to 6 nmol/kg near the coastal boundary (in contrast to the distribution of²¹⁰Pb). Particulate Mn in the surface waters accounts for only about 1% of the total.Vertical distributions of Mn are characterized by surface maxima, minima near 300 m, maxima at mid-depth coinciding with the oxygen minimum and the labile nutrient maxima, and concentrations in Pacific bottom waters of approximately 0.2 nmol/kg. The oceanic distribution of Mn appears to be dominated by external inputs superimposed upon overall scavenging which can lead to Mn maxima in (1) the surface waters due to riverine and atmospheric sources; (2) the deep ocean as a result of hydrothermal injection and/or sediment resuspension; and (3) the oxygen minimum region resulting from in-situ breakdown of organic matter, in-situ MnO₂ reduction, and/or advective-diffusive transport of dissolved Mn from anoxic slope sediments.     

Trace metal fluxes to nearshore Long Island Sound sediments

Sediments are very important as trace metal ‘sinks’ in coastal areas. In this paper we present data on the concentrations and calculated fluxes of the trace metals, Fe, Mn, Cu, Pb, Zn, Cd and Ag at nearshore sites in Long Island Sound. In both areas a percentage of the metal accumulation can be ascribed to local human activities with the highest anthropogenic metal fluxes occurring closest to their source.     

Changing settlement patterns in the German Democratic Republic: 1945–1976

The settlement pattern in the German Democratic Republic has undergone dramatic changes since 1945, largely as a consequence of the socialist government's aim of concentrating population into fewer but larger central places. The impact of this policy has been greatest in rural areas, most especially in the northern part of the country. This paper examines the character of the changes experienced by both urban and rural areas in northern G.D.R. since 1945.     

On the rigidity of the North American plate

Recently reported Very Long Baseline Interferometer (VLBI) observations between Green Bank, West Virginia, Haystack, Massachusetts and Owens Valley, California, show that the linear distance between these sites has not changed for at least three years (i.e., smooth rate of change less than 1 cm/yr). This level of stability is logically consistent with the hypothesis that the continental U.S. North American plate is geodetically rigid at least on the 2 cm level. Model studies show that this result is in accord with present knowledge of the intra-plate stress field and the elastic properties of typical continental lithosphere.     

Transcrystalline shear fracturing and pseudotachylite generation in a meta-anorthosite (Harris, Scotland)

The meta-anorthosite is locally deformed by brittle shear fracturing, which progressively increases from isolated fractures with little cataclasite to many generations of closely spaced fractures, the intervening rock being highly deformed, in both a plastic and brittle way. In most cases an E-W compression on gently dipping to steep reverse shear planes occurs, which we relate to a Caledonian thrust zone.In places, the highly deformed rock is cut by pseudotachylite veins, which locally form networks. The pseudotachylite is generally intrusive, but does not appear to be related to movement on major slip surfaces. Very locally it may have formed in situ. Pseudotachylite only occurs in highly deformed rock, is only very occasionally deformed itself and, thus, generally represents at each locality the last stage of a complex deformation history, as if its presence welded the rock and prevented further deformation. These striking differences from the country-rock gneisses (in which pseudotachylite occurs on well developed fractures in very slightly deformed rock) are considered to be due to the low anisotropy of the meta-anorthosite, to its lower shear strength and to the easy propagation and branching of the shear fractures in plagioclase. The source of the heat necessary to generate the pseudotachylite melt is not clear—it may come from crack propagation as well as frictional sliding.