Pollen representation,source area,and basin size: Toward a unified theory of pollen analysis

The concepts of pollen source area and of production and dispersal biases in pollen representation are quantified by means of a simple theoretical model. Source areas and relative pollen representation are shown to depend on basin size according to functions that describe the amount of pollen remaining airborne at increasing distances from single pollen sources. The form of these functions is determined by physical processes. Standard formulas for elevated sources do not apply, but the integrated form of Sutton's equation for particle dispersal from a ground-level source gives useful approximations applicable to pollen transport over a forest canopy. Simulations using this equation yielded source areas that increased realistically with basin size, showed substantial differences between source areas for pollen grains with different deposition velocities, and predicted that lighter pollen grains should become better represented with increasing basin size. All of these predictions are qualitatively consistent with present knowledge of the characteristics of pollen assemblages in different depositional environments. The model further allows parameters that can be estimated by statistical calibration methods to be predicted from underlying physical quantities. This extension suggests procedures for testing the theory with quantitative data on surface pollen and forest composition. Preliminary results showed reasonable agreement between estimated and predicted values of dispersal indices for the most abundant taxa in pollen spectra from the northern midwestern United States.     

On the estimation of earthquake maximum magnitude based on a joint analysis of seismological and geothermal parameters

To improve the reliability of earthquake prediction, it is important to determine those layers in the crust of the earth in which the foci of most strong earthquakes occur. An analysis of regional earthquake catalogues shows that in depressions strong events originate within a specific layer in the middle part of the crust. An investigation of the geothermal regime in the depressions under study shows that the thickness and location of this layer depend on geothermal parameters which correlate with the strongest earthquake magnitude in particular areas. Thus, it seems possible to use geothermal and seismological parameters for estimating the maximum magnitude of earthquakes for at least some areas.     

The difference between oceanic and continental tholeiites: a fluid dynamic explanation

Continental tholeiites have higher SiO₂, K₂O and light rare earth element contents and more evolved isotopic characteristics than their oceanic counterparts. These differences can be explained if the compositions of the parent magmas to both types of tholeiites are similar but if continental magmas assimilate significant amounts of continental crust en route to the surface. Although there is little doubt that most continental tholeiites have assimilated crustal material, the lcoation and mechanism of assimilation remain uncertain. Longhi (1981) has argued that magmas derived directly from the mantle should crystallize little orthopyroxene. The abundance of orthopyroxene in most continental layered intrusions suggests that they have crystallized from magmas which have assimilated continental crust. Since orthopyroxene is an early crystallizing phase in layered intrusions, this assimilation must occur early, before the magma enters the chamber. Assimilation can occur at the margins of the dykes which feed magma chambers, depending on the nature of the flow. If the flow is turbulent the high temperatures at the centre of the dyke will extend to the margins and the magma will erode the dyke walls. If the flow is laminar, a conductive profile develops at the margin and the flowing magma chills against the walls, protecting them from thermal erosion. The nature of flow in a dyke can be predicted from the Reynolds number, the criteria for turbulence. Reynolds number calculations suggest that the flow of primitive magmas in continental dykes will be fully turbulent and, if this is the case, assimilation of low melting point components in the walls of the dyke is inevitable. It is therefore suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust.     

Morphology,perfection and growth of natural pyrite whiskers and thin platelets

Whiskers and thin platelets of pyrite have been found growing coherently on the face of small cubic pyrite crystals from the Strashimir and Gradishte hydrothermal lead-zinc deposits. The host crystals formed framboids and spheroids of macroscopic size inside large crystals of chalcopyrite from which they are separated by concentric gaps. The whiskers, one to several μm in width, are elongated along [001] and bounded by {100}, partly by {210} and {111} faces. Scanning electron microscopic (SEM) observation revealed longitudinal steps and grooves, flat rectangular pits, drop-like and elongated bulges on their side faces. High-voltage electron microscopic (HVEM) studies showed that the thin whiskers are perfect and dislocation-free, although some ribbon-like whiskers and platelets contain internal longitudinal channels and small isometric fluid inclusions. Based on these observations, the following growth mechanism is proposed. Whiskers grow in the restricted volume of the internal cavities in chalcopyrite from stagnant solutions under a diffusion regime. The growth proceeds in two stages: (1) Initially, pyramidal “pedestals” are formed due to morphological instability of the host crystal surfaces and multiapex growth, (2) Whiskers grow rapidly by unidirectional supply of material and competition between neighbouring individual whiskers, the growth taking place at the tip by a two-dimensional nucleation mechanism. The whiskers and thin platelets are considered to be peculiar highly anisometric skeletal crystals.     

Lamprophyric lavas in the Colima graben,SW Mexico

The Colima graben, located in SW Mexico, is one of three grabens which intersect about 50 km SSW of Guadalajara, forming a triple junction. The 90 km long, 20–60 km wide Colima graben represents a N-S rift of the E-W trending Mexican Volcanic Belt. Since the Early Pliocene, the Colima graben has served as a locus for the eruption of alkaline lavas, the most recent of which are basanites and minettes erupted from Late Pleistocene cinder cones (Luhr and Carmichael 1981). In this paper, we report on older alkaline lavas which crop out in the graben's walls. These rocks include phlogopite- and hornblende-bearing lamprophyres, a phlogopite-kalsilite-ankaratrite, and high-K andesites. These lavas crop out throughout the Colima graben area, and are intimately associated with calc-alkaline lavas in the field. Compared to these, the alkaline rocks are strikingly enriched in the incompatible elements, particularly Ba, Sr, P, and the LREE. Unlike the younger Late Pleistocene alkaline cinder cone lavas, most of the graben wall lamprophyres and the high-K andesites represent magmas that appear to have undergone significant evolution since their generation, including fractionation, crustal contamination, and possible magma mixing. Least-squares modeling indicates that the cinder cone minettes represent reasonable parental magmas for the graben lamprophyres. The occurrence of these alkaline lavas in an active calc-alkaline volcanic arc is unusual, and we suggest that they are a manifestation of the rifting processes which produced the Colima graben.     

Tritium in the Dead Sea

Tritium data in the Dead Sea for the period 1960–1979 are given. Tritium levels have increased until 1965 in the upper layers of the Dead Sea reaching a level of 170 TU, in response to the atmospheric buildup of tritium from thermonuclear testing. The levels have been decreasing ever since, both because of rapidly declining atmospheric concentrations of tritium and because of mixing of the surface layers with tritium deficient, deeper water masses. The depth of penetration of the tracer delineated the depth of meromictic stratification and successfully monitored the deepening of the pycnocline, until the overturn in 1979 homogenised the entire tritium profile. Modelling the changing tritium inventory over this period showed the predominance of the direct exchange across the air/sea interface, both in the buildup of tritium in the lake and also in its subsequent removal from it. The good fit between calculated and measured tritium inventories confirmed the evaporation estimate of 1.46 m/yr (the mean value for the period) with a precision unattained by other methods.     

U-Pb Pan-African ages of two charnockite-granite associations from Southwestern Nigeria

U-Pb dating of zircons from two separate charnockite granite associations from S.W. Nigeria, previously considered to be of Kibaran or Archean age, yield PanAfrican emplacement ages of 631±18 Ma and 586±5 Ma respectively when all the data points of each association are regressed together. These U-Pb data on zircons do not permit a confirmation of the field-established sequence of rock emplacement within each charnockite-granite association; thus synchronous ages of 620±20 Ma (charnockite NIG 4), 634±21 Ma (charnockite NIG 7) and 621±10 Ma (granite NIG 12) are obtained for the three analyzed rocks of the Akure-Ikerre-Ado Ekiti association while those of the Idanre association yield younger but synchronous ages of 580±10 Ma (charnockite NIG 2), 593±13 Ma (charnockite NIG 3) and 587±10 Ma (granite NIG 11). The ages obtained for the rocks of the Akure-Ikerre-Ado Ekiti association very well fit the existing data on the tectonic evolution of the basement complex of Nigeria, whereas those from the Idanre area are difficult to reconcile with the current interpretation of tectonic data in this area.     

Carbohydrate sources in a coastal marine environment

Individual neutral sugars in sediments, sediment trap materials and major biological sources of a coastal marine environment (Dabob Bay, Washington State) were analyzed by capillary gas chromatography of equilibrated isomeric mixtures. Plankton, bacteria, and vascular plant tissues of different types yielded reproducible and biochemically consistent compositional patterns. These patterns, when expressed in simple parameters, allowed distinctions between marine and terrestrial carbohydrate sources as well as among the major different types of vascular plant tissues. Plankton and bacteria, due to their compositional diversity, were not further distinguishable by carbohydrate compositions alone. Carbohydrate compositions of Dabob Bay sediments and sediment trap materials, interpreted using source-indicator parameters, indicate a predominantly marine origin with increased relative input of terrestrially-derived carbohydrates in winter periods of low phytoplankton productivity. Both plankton and grasses are indicated as major carbohydrate sources during spring. Glucose yield enhancement factors, determined by comparative acid pretreatments, confirm the general predominance of α-cellulose-poor marine polysaccharides and increased levels of α-cellulose-rich vascular plant remains in winter sediment trap samples.