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.     

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.     

FUZZY QMODEL—A new approach for linear unmixing

Many data sets can be viewed as a collection of samples representing mixtures of a relatively small number of end members. When end members are present in the sample set, the algorithm QMODEL by Klovan and Miesch can efficiently determine proportionate contributions. EXTENDED QMODEL by Full, Ehrlich, and Klovan was designed to deduce the composition of realistic end members when the end members are not represented by samples. However, in the presence of high levels of random variation or outliers not belonging to the system of interest, EXTENDED QMODEL may not be reliable inasmuch as it is largely dependent on extreme values for definition of an initial mixing polyhedron. FUZZY QMODEL utilizes the fuzzy c-means algorithm of Bezdek to provide an alternative initial mixing polyhedron. This algorithm utilizes the collective property of all the data rather than outliers and so can produce suitable solutions in the presence of noisy or “messy” data points.     

Simulating the lateral transport of ichthyoplankton in the Potomac estuary

Previous simulations of potential ichthyoplankton entrainment by power generating stations on the Potomac estuary have not included the influence of lateral transport in distributing eggs and larvae over the nursery area. Therefore, two-dimensional, vertically-averaged hydrodynamic and kinematic models of passive organism transport were developed to represent advective and dispersive processes near the proposed Douglas Point Nuclear Generating Station. Although the more refined model did not substantially alter the estimate of ichthyoplankton entrainment, it did reveal that lateral inhomogeneities in hydrodynamics could engender severalfold differences in entrainment probabilities on opposite sides of the estuary. Models of higher resolution and greater biological detail did not project greater total entrainment by the Douglas Point plant, because the volume of nontidal flow past the site was large in comparison to the proposed rate of cooling water withdrawal.     

Cataclasites along the Saltville thrust,U.S.A. and their implications for thrust-sheet emplacement

Cataclasis and frictional wear are the primary bulk deformation mechanisms along steeply dipping portions of the Saltville thrust in the southern Appalachian foreland zone, U.S.A. Fault character ranges from a single discrete sliding surface with negligible gouge, to a zone of several discrete sliding surfaces or a zone (up to 0.3 m thick) of pervasive cataclasite. Marked fracturing occurs up to 20 m above the fault, whereas minimal deformation is found in the footwall rocks. Hanging wall dolomites range from crush breccias (less than 5% matrix) to ultracataclasites (with 90% matrix), although cataclasites (50–70% matrix) are predominant. Foliated cataclasites occur where dolomite is thrust over shale. Progressive development of cataclastic fabrics is due to comminution by fracturing and grinding along intersecting fractures. Continued frictional grinding results in complete disruption of the original fabric to produce cataclasite and minor ultracataclasite. Grain alignment occurs by rigid body rotation with subsequent local enhancement by pressure-solution. Microstructural relations of the fault gouge suggest periodic fluctuations in fluid pressure, where λ_v (ratio of fluid to overburden pressure) probably ranged between 0.45 and 1. The Saltville thrust-sheet emplacement must have occurred in a caterpillar-like fashion involving aseismic and seismic shear. Shear stresses accompanying fault motion as determined from dolomite twin lamellae are in the order of 65 mPa.     

Theories for the origin of lunar magnetism

This paper reviews the major theories which have been proposed to explain the remanent magnetism found in the lunar crust. A total of nine different mechanisms for lunar magnetism are discussed and evaluated in the light of the theoretical and experimental constraints pertinent to lunar magnetism. We conclude that none of these theories in their present state of development satisfy all the known constraints. However, the theories which agree best with our present understanding of the Moon are meteorite impact magnetization, thermoelectric dynamo field generation, and an early solar wind field.     

Seismic gaps and source zones of recent large earthquakes in coastal Peru

The earthquakes of central coastal Peru occur principally in two distinct zones of shallow earthquake activity that are inland of and parallel to the axis of the Peru Trench. The interface-thrust (IT) zone includes the great thrust-fault earthquakes of 17 October 1966 and 3 October 1974. The coastal-plate interior (CPI) zone includes the great earthquake of 31 May 1970, and is located about 50 km inland of and 30 km deeper than the interface thrust zone. The occurrence of a large earthquake in one zone may not relieve elastic strain in the adjoining zone, thus complicating the application of the seismic gap concept to central coastal Peru. However, recognition of two seismic zones may facilitate detection of seismicity precursory to a large earthquake in a given zone; removal of probable CPI-zone earthquakes from plots of seismicity prior to the 1974 main shock dramatically emphasizes the high seismic activity near the rupture zone of that earthquake in the five years preceding the main shock. Other conclusions on the seismicity of coastal Peru that affect the application of the seismic gap concept to this region are: (1) Aftershocks of the great earthquakes of 1966, 1970, and 1974 occurred in spatially separated clusters. Some clusters may represent distinct small source regions triggered by the main shock rather than delimiting the total extent of main-shock rupture. The uncertainty in the interpretation of aftershock clusters results in corresponding uncertainties in estimates of stress drop and estimates of the dimensions of the seismic gap that has been filled by a major earthquake. (2) Aftershocks of the great thrust-fault earthquakes of 1966 and 1974 generally did not extend seaward as far as the Peru Trench. (3) None of the three great earthquakes produced significant teleseismic activity in the following month in the source regions of the other two earthquakes. The earthquake hypocenters that form the basis of this study were relocated using station adjustments computed by the method of joint hypocenter determination.