Microscopic calcite dendrites in cold-water tufa: implications for nucleation of micrite and cement

Dendritic calcite forms in an active cold-water tufa system in association with extracellular polymeric substances (EPS) that discontinuously coat bryophytes and cyanobacteria. Dendrites consist of 100–200 nm thick calcite fibres that form 3D lattice-like domains. In each dendrite domain, fibres have three structurally equal orientations, which correspond in disposition to radii from the centre of a calcite unit cell to the convex triple face junctions on its surface. Fibres do not form in the orientation of the c-axis. The external form of each dendrite has the shape of half of a shortened octahedron, with an upper triangular surface parallel to the substrate. Dendrite nucleation takes place on or in microbial EPS, whether microbial cells are present or not, and is probably effected by attraction of Ca²⁺ cations to negatively charged EPS, together with CO₂-degassing and concomitant pH increase of supersaturated spring water in stream splash zones. Ensuing dendrite growth is abiogenic and controlled by diffusion. Dendrite c-axes are perpendicular to the substrate, probably because the negative charge of EPS forces the orientation of Ca²⁺ and CO planes within the developing dendrite crystal to be parallel to the EPS film surface. Dendrites are eventually filled and overgrown by solid, syntaxial calcite, which gradually and completely obliterates the dendrites as more familiar calcite crystal forms develop. No trace of the dendritic nucleus remains in the rock record. Calcite crystal nucleation may take place by this mechanism in many marine and meteoric settings, given that microbial EPS is now assumed to be virtually ubiquitous in these environments. This phenomenon could contribute to the development of familiar fabrics such as marine micrite cement and fibrous calcite cement, radial ooids, peloids, ‘abiogenic’ stromatolites, sea floor precipitates, microbialites, tufa, travertine, speleothems, and some meteoric cements. It may also contribute to the substrate-normal orientation of c-axes of common cement fabrics.     

EARLY MODERN EXPANSION AND THE POLITICIZATION OF OCEANIC SPACE*

ABSTRACT. The definition of oceans as international politicized space is an integral but little analyzed aspect of early modern European expansion, which took place between about 1450 and 1800. In this essay I explore the implications of thinking about the development of European imperialism and global dominance in oceanic terms. I argue that oceanic, rather than terrestrial, dominance characterized early modern European empires, particularly in relation to Africa and Asia, where indigenous political and economic control prevailed. The long apprenticeship in mastering oceanic space contributed to the ability of Europeans to build land-based empires in Asia and Africa in the nineteenth century. As well, the international relationships worked out by Europeans in the nonstate but militarized arena of the high seas contributed to an emergent global order.     

Barite in the ocean – occurrence,geochemistry and palaeoceanographic applications

The mineral barite (BaSO₄) can precipitate in a variety of oceanic settings: in the water column, on the sea floor and within marine sediments. The geological setting where barite forms ultimately determines the geochemistry of the precipitated mineral and its usefulness for various applications. Specifically, the isotopic and elemental composition of major and trace elements in barite carry information about the solution(s) from which it precipitated. Barite precipitated in the water column (marine or pelagic barite) can be used as a recorder of changes in sea water chemistry through time. Barite formed within sediments or at the sea floor from pore water fluids (diagenetic or cold seeps barite) can aid in understanding fluid flow and sedimentary redox processes, and barite formed in association with hydrothermal activity (hydrothermal barite) provides information about conditions of crust alteration around hydrothermal vents. The accumulation rate of marine barite in oxic‐pelagic sediments can also be used to reconstruct past changes in ocean productivity. Some key areas for future work on the occurrence and origin of barite include: fully characterizing the mechanisms of precipitation of marine barite in the water column; understanding the role and potential significance of bacteria in barite precipitation; quantifying parameters controlling barite preservation in sediments; determining the influence of diagenesis on barite geochemistry; and investigating the utility of additional trace components in barite.     

AREAL RAINFALL EVALUATION USING TWO SURFACE FITTING TECHNIQUES

ABSTRACT

Two trend surface fitting techniques, the use of bi-cubic splines and multiquadric analysis, have been applied to the determination of areal rainfall. On two mathematically defined test surfaces, the methods were applied using gridded data; the bi-cubic splines gave the better results. For non-gridded data, selected and random points, the multiquadric analysis gives satisfactory values. With real catchment and storm data, multiquadric analysis is shown to be a potentially very useful and flexible method for determining areal rainfall.     

SEISMIC REFRACTION MEASUREMENTS IN THE BALTIC SEA*

Seismic refraction measurements were made in two areas of the Baltic Sea in June 1967. The refraction data were obtained in the course of the transmission measurements program of Operation MILOC BALTIC 67. Three profile pairs were obtained, two in the area south of Öland Island, Sweden, the third to the east, north of the peninsula of Hel, Poland. The water depths vary from 60 to 90 meters between the areas. The receiving positions for the two profiles south of Oland Island are only 46 km apart but the structures differ markedly. The northern section, only 500 m thick, shows a rather thin sedimentary cover above 350 m of 3.7 km/sec material that in turn overlies 5.6 - 5.9 km/sec velocity material. The southern section, almost 2 km thick, has an equivalent amount of low velocity material, and a layer about 1 km thick having velocity 4.8 km/sec that overlies 6.0 km/sec velocity material. The eastern profile shows high velocity material, 5.6 km/sec, at 2.5 km depth. Correlation of the layers determined by seismic refraction with nearby geology suggests that the structural change south of Öland Island may represent the boundary of the Sarmatian Shield in this region.     

IDENTITY AND ASSIMILATION AMONG YOUNG ETHIOPIAN IMMIGRANTS IN METROPOLITAN WASHINGTON*

ABSTRACT. Ethiopians are a recent immigrant group in the United States, having entered the country in significant numbers during the 1980s and 1990s. This preliminary study examines the ethnic and racial identities of children of first‐generation Ethiopian immigrants living in the Washington, D.C. metropolitan area. The results of twenty in‐depth interviews demonstrate that race is a much more fluid and contested form of identification than is ethnicity to the young immigrants, who equate the latter unilaterally with their Ethiopian heritage. Immigrants also adopt different subject identities in various locales, favoring those that are most in accordance with their needs and sense of self.     

Rural Women's Self-concepts and Aspirations as Members of Selected Voluntary Organisations in New Zealand, Australia and Canada

Evidence from questionnaires completed by members of five rural women's organisations in 1994 and 1995 (three traditional and two younger organisations) clarifies the motives women have, and the identities they assume, in joining these organisations. The older organisations attract women who wish to make a commitment to their community but newer organisations attract women who see themselves as farmers, or as employees and professionals in the agricultural industry. This represents a major shift in gender dimensions of rurality that has taken place during the last twenty years or so. It indicates a ‘mobilisation’ of rural and farm women who seek to participate fully - in their productive role and in the public sphere - in the attempt to maintain the viability of family farms and rural communities. The role that organisations for men and women play in constituting the meaning of gender is discussed.