Nourishment of perched sand dunes and the issue of erosion control in the Great Lakes

Although limited in coverage, perched sand dunes situated on high coastal bluffs are considered the most prized of Great Lakes dunes. Grand Sable Dunes on Lake Superior and Sleeping Bear Dunes on Lake Michigan are featured attractions of national lakeshores under National Park Service management. The source of sand for perched dunes is the high bluff along their lakeward edge. As onshore wind crosses the bluff, flow is accelerated upslope, resulting in greatly elevated levels of wind stress over the slope brow. On barren, sandy bluffs, wind erosion is concentrated in the brow zone, and for the Grand Sable Bluff, it averaged 1 m³/yr per linear meter along the highest sections for the period 1973–1983. This mechanism accounts for about 6,500 m³ of sand nourishment to the dunefield annually and clearly has been the predominant mechanism for the long-term development of the dunefield. However, wind erosion and dune nourishment are possible only where the bluff is denuded of plant cover by mass movements and related processes induced by wave erosion. In the Great Lakes, wave erosion and bluff retreat vary with lake levels; the nourishment of perched dunes is favored by high levels. Lake levels have been relatively high for the past 50 years, and shore erosion has become a major environmental issue leading property owners and politicians to support lake-level regulation. Trimming high water levels could reduce geomorphic activity on high bluffs and affect dune nourishment rates. Locally, nourishment also may be influenced by sediment accumulation associated with harbor protection facilities and by planting programs aimed at stabilizing dunes.     

Basalt geochemistry and tectonic discrimination within continental flood basalt provinces

Continental flood basalts are usually regarded as a single tectonomagmatic entity but frequently quoted examples exhibit a variety of tectonic settings. In one well-studied, classic, flood basalt province, the Mesozoic Karoo province of southern Africa, magmatism occurred in the following tectonic settings: (a) continental rifting leading to ocean-floor spreading in the South Atlantic Ocean (Etendeka suite of Namibia); (b) stretched continental lithosphere and rifting not leading directly to ocean-floor formation (Lebombo suite of southeastern Africa); and (c) an a-tectonic, within-plate, continental setting characterized by an absence of faulting or warping (Lesotho highlands and Karoo dolerites of South Africa). By means of spidergrams of the elements Rb, Ba, Th, Nb, K, La, Ce, Sr, Nd, P, Hf, Zr, Sm, Ti, Tb, Y, V, Ni and Cr, uncontaminated tholeiites from (c) above [i.e. the Lesotho-type continental flood basalts (LTCFB)] are compared with mid-ocean ridge basalts (MORB), ocean-island tholeiites (OIT), and tholeiites and calc-alkali basalts from subduction environments. The comparison reveals the LTCFBs are geochemically distinct. The differences are reflected in relative enrichments or depletions of the more incompatible elements (Rb-Ce) to less incompatible elements (Ce-Y), i.e. the overall slope of the spidergrams, and in anomalous enrichments or depletions of one or more of the elements Th, K, Nb, Sr, Ti, Hf, and Zr. The distinctive geochemical character of the Lesotho LTCFBs is interpreted in terms of a lithospheric mantle source for the basalts. This is supported by isotopic data. There are no major geochemical differences between Lesotho CFBs and basalts of the rift-related Etendeka and Lebombo suites, although the latter are somewhat enriched in Rb, Ba and K. However, unlike the Lesotho basalts, the Lebombo and Etendeka basalts are associated with voluminous silicic volcanics or intrusive centres and late-stage dolerites having MORB/OIT (i.e. asthenospheric) geochemical characteristics. The flood basalt/silicic magmatism/late-stage dyke swarm association is characteristic of several rift or thinned lithosphere environments (e.g., Ethiopia, Skye, eastern Greenland) but in many of these the flood basalts have ocean-island basalt (OIT) geochemical characteristics. The Lesotho-type CFB geochemistry is exhibited by the Grande Ronde Basalt of the Columbia River Group (a possible subduction-related flood basalt province) and the basic rocks associated with Mesozoic rifting in the North and South Atlantic. Basalt geochemistry alone is unhelpful in determining the tectonic setting of CFBs although the rift-related environments may be identified by the petrology and geochemistry of the whole igneous suite. A two-source model is proposed for the mantle-derived basic rocks in rift-related CFB provinces. Early enriched basalts are derived from the lithosphere and, following pronounced lithospheric attenuation or rifting, later MORB-like melts are emplaced from the rising asthenosphere. The presence of both Lesotho- and OIT-type geochemical patterns in rift-related CFBs suggests that the lithosphere exhibits different styles of enrichment.     

Petrogenesis of Cenezoic volcanic rocks from the Aegean island arc

The Aegean volcanic arc formed in response to northeasterly subduction of the Mediterranean sea floor beneath the Aegean Sea. The active arc lies over 250 km from the Hellenic Trench in a region which has suffered considerable extension and subsidence since the mid-Tertiary. Suites of samples from the different volcanic centres making up the arc have been studied geochemically in order to assess lateral variations and to constrain the contribution of crustal contamination and sediment subduction in their petrogenesis.Lavas from all the major volcanic centres exhibit typical calc-alkaline major-element characteristics, and show enrichment in light REE and LIL elements but low contents of HFS elements. The enrichment in light REE is greater in the eastern (Nisyros, Kos) and western (Milos, Poros, Methana, Aegina) sectors of the arc (Ce_n/Yb_n=4) than in the central Santorini sector (Ce_n/Yb_n=2). All lavas have significant negative Eu anomalies and many have slight negative Ce anomalies. Less coherence is observed in the abundances and ratios of the other LIL elements, compared with the REE, along the island chain.Whereas the effects of crystal fractionation are evident in the trace-element patterns of lavas from individual islands, and are particularly well marked for Santorini, it is clear that there are consistent differences in trace-element abundances and ratios in the lavas of the various islands which reflect compositional differences in the mantle source and/or in melting conditions. Lavas from the eastern and western sectors have much higher levels of Ba and Sr but relatively lower Th, K and Rb than those from Santorini. Although some geochemical features could be explained through involvement of a component of subducted sediment in the source regions of the volcanoes, other element abundances and ratios indicate that this component must be very small. Detailed consideration of the inter-island geochemical variations suggests a complex make-up of the underlying lithosphere, resulting from a long history of subduction. In the region of Santorini, where crustal stretching is greatest, the underlying asthenosphere may be involved in magma production.     

On melting of the subducted oceanic crust: Effects of subduction induced mantle flow

Based on petrological and geochemical arguments, it is possible that arc magma is derived from subducted oceanic crust. In this paper, regional thermal models have been constructed to study the feasibility of melting cold subducted oceanic crusts at shallow depth (i.e. at depths of about 100 km) by a dynamic mantle. Calculated results suggest that plate subduction will generate an induced flow in the wedge above the subducting slab. This current continuously feeds hot mantle material into the corner and onto the slab surface. A high temperature thermal environment can be maintained in the vicinity of the wedge corner, immediately beneath the over-riding plate. Our regional models further demonstrate quantitatively that production of local melting is possible just about 30 km down dip from the asthenosphere wedge corner. Additional geological processes such as reasonable amounts of shear heating and minor dehydration (which will lower the local melting temperature) will further increase the probability of melting a cold subducted oceanic crust at shallow depth.     

Accuracy of ozonesonde measurements in the troposphere

Measurements of atmospheric ozone at concentrations typical of the free troposphere have been compared for ECC sondes and a UV-absorption photometer, using a Bendix chemiluminescent analyser as a transfer standard. Comparisons were conducted in the laboratory and in the tropospheric part of the atmosphere. It was found that the measurements agreed to within 4% provided that the background current for the ECC sonde was measured before exposure to ozone in the preparation procedure, and was assumed to be constant throughout the sonde flight. These results confirm those of earlier experiments and mean that the methods currently used to correct for the background current in the troposphere need to be revised.     

On the crystallinity,probability of occurrence,and rheology of lava and magma

Given a set of comagmatic lavas of similar composition but varying crystallinity, a diagram can be constructed using only the modes of the phenocrysts that quantitively shows the sequence of crystallization. This is done by plotting the amount of each phenocryst against the total crystallinity or percentage of melt of the lava itself. A histogram of the total phenocryst content measures the probability of the magma to be erupted as lava. This eruption probability (P _E ) is the product of the probability of finding the magma at any state of crystallinity (thermal probability, P _T ) and the rheological probability (P _R ) of the magma being physically able to erupt (i.e. P _E =P _T P _R ). It is shown that P _E is given by dX/dT, where X is the crystallinity of the magma as a function of temperature (T). Because crystal production is generally nonlinear—in most rocks it is step-like—P _E is a bellshaped curve stradling the temperature at which the magma is one half crystallized. Near the liquidus it is most favorable rheologically for the magma to erupt. But the probability is small of sampling a magma near its liquidus, because it cools quickly there. It is maximum when there are high rates of crystal production, because it then cools slowly. As the crystallinity increases, it reaches a critical point of maximum packing (i.e. lowest porosity) around 50–60% crystals where it becomes rheologically impossible to erupt. The magma looses its potential to become a lava and it becomes a pluton. From a histogram of crystallinity and P _T ,P _R can be found. This technique, as well as the construction of the mode-crystallization (M-C) diagram, is illustrated using a set of Aleutian lavas. These lavas also show that the point of critical crystallinity decreases with increasing silica content of the lava. Because this critical crystallinity is much lower for granitic magmas, they are much more probable than basaltic magmas to become plutons. Beyond this point, granitic magmas can only erupt as ash flows. This correlation of critical crystallinity and silica content is used to show a method by which the viscosity of the magma can be estimated as a function of crystallinity. This variation is found to compare favorably with Roscoe's equation of the dependence of viscosity on the concentration of suspended solids. These results show that differentiation probably can not normally take place beyond this critical crystallinity. The extraction of melt beyond this critical point by filter pressing is unlikely because the assemblage dilates upon stressing. Only if the phenocrysts deform viscously can additional melt be extracted, and this can probably only occur with large (–30km) bodies.     

Effect of suspension and sedimentation on the amount of microbial colonization of salt marsh microdetritus

The relative density of bacterial colonization of small (5 to 40 μ diameter) suspended and sedimented detritus particles in salt marsh creeks was studied using an europium chelate-fluorescent brightener stain coupled with epifluorescent microscopy. Particles were obtained from four substations at each of three sites in salt marshes bordering the York River, Virginia. Coverage of detrital particles by bacteria was uniformly low, averaging 1.54%±1.83% of the particle surface area. Suspended particles larger than 10 μ diameter had lower bacterial densities than did sedimented particles in the same size ranges. It is speculated that microbial growth may be enhanced on sedimented particles both through lack of motion and access to greater nutrient concentrations.     

Global detailed geoid computation and model analysis

This paper presents a survey of recent work on the gravimetric geoid. The gravity models considered are those published in the past few years by the Goddard Space Flight Center (GSFC), the Smithsonian Astrophysical Observatory (SAO) and the Ohio State University (OSU). Comparisons and analyses have been carried out through the ose of detailed gravimetric geoids which we have computed by combining the above-mentioned models with a set of 26 000, 1^ox1^o mean free air gravity anomalies. The accuracy of the detailed gravimetric geoid computed using the most recent Goddard Earth Model (GEM-6) in conjunction with the set 1^ox1^o mean free air gravity anomalies is assessed at 2 m on the continents of North America, Europe And Australia, 2 to 5 m in the North-East Pacific and North Atlantic areas and 5 to 10 m in other areas where surface gravity data are sparse. Rms differences between this detailed geoid and the detailed geoids computed using the other satellite gravity fields in conjunction with same set of surface data range from 3 to 7 m. The maximum differences in all cases occurred in the Southern Hemisphere where surface data and satellite observations are sparse. These differences exhibited wavelengths of approximately 30^o to 50^o in longitude. Detailed geoidal heights were also computed with models truncated to 12th degree and order as well as 8th degree and order. This truncation resulted in a reduction of the rms differences to a maximum of 5 m. Comparisons have been made with the astrogeodetic data of Rice (United States), Bomford (Europe), and Mather (Australia) and also with geoidal heights from satellite solutions for geocentric station coordinates in North America and the Caribbean.