The impact of vertebrate herbivores on wetland vegetation in Atchafalaya Bay, Louisiana

Delta islands in the Atchafalaya and Wax Lake deltas in Atchafalaya Bay, Louisiana, are in an extremely dynamic successional phase. These islands initially supported large marshes dominated by the pioneering plant species Sagittaria latifolia and Sagittaria platyphylla. A general decrease in vegetated areas has occurred in the delta island marshes in the Atchafalaya Delta since about 1980, while in the Wax Lake Delta portion of the complex the vegetation still flourished. The Atchafalaya Delta provides an interesting setting for the study of herbivory because of the complex interaction of biotic and physical factors operating in this delta. We hypothesized that grazing by herbivores has a marked effect on vegetation in these developing marshes. To test this hypothesis, exclosures were erected on islands in both deltas in September 1985 and January 1986. Each set of exclosure treatments included an openly-grazed control area, an ungrazed area, an area allowing nutria grazing, and one allowing waterfowl grazing in each site. Results of the experiment, based on field sampling of vegetation, indicated decreases in plant biomass and changes in plant species composition in grazed treatments. Waterfowl and nutria reduced biomass aboul equally, but there was a more marked effect in the openly grazed areas. These findings may be extrapolated to sediment diversion areas along the Mississippi River.     

Gamma-Ray Bursts: Where are We Now?

There has been significant progress recently in our understanding of gamma-ray bursts. The long-sought counterparts at other wavelengths have finally been found for a few bursts. This breakthrough is the result of coordinated observations involving several satellites and ground-based optical and radio observatories. In one case, GRB970508, redshifted absorption lines have been detected, finally settling the debate about the distance scale. The consensus is that the burst sources lie at cosmological distances, requiring at least ∼ 10⁵¹ergs to be emitted in gamma rays in just a few seconds. The gamma radiation is thought to be produced by shocks in a highly relativistic fireball. Many mysteries remain. There is no consensus on the nature of the sources, although coalescing neutron stars are the leading candidate. There is evidence that the sources of the faintest bursts may be at redshifts above 2. If so, gamma-ray bursts may ultimately tell us something about the early Universe. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cumberland Falls chondritic inclusions: Mineralogy/petrology of a forsterite chondrite suite

As previously found for a chondritic inclusion of unknown affinity, mineralogic and petrologic properties of 9 inclusions in the Cumberland Falls enstatitc achondrite are primitive members of the forsterite (F) chondrite group, hitherto defined by 4 meteorites of similar redox state. The inclusions define a primitive suite with properties indicating 8 as F3 and one of even lower petrologic type. The abundant minerals include: low-Ca pyroxene, olivine, plagioclase, kamacite, taenite, schreibersite, troilite, ferroan alabandite and daubreelite. Diopside, oldhamite and a Ti-rich sulfide are present in one or two inclusions. Petrologic textures and jadeitic pyroxene, hitherto unidentified in meteorites, indicate substantial degree of shock. The inclusions acquired their chemical characteristics during nebular condensation and accretion over a broad redox range (metal-silicate trends in them verify Prior's Rules): their parent body later impacted the enstatite meteorite parent body. During impact, the inclusions were shocked and incorporated with enstatite achondrite host as a breccia that would become Cumberland Falls.     

The kinetics of dissolution of muscovites at 25°C and 1 atm CO2 partial pressure

Two different dry-ground muscovites were placed separately in deionized water saturated with 1 atm partial pressure of carbon dioxide at room temperature and pressure for about 1000 hr. The carbonic acid formed buffered the bulk solution between 4.6 and 5.1 during the whole time of the experiment. K and Na were rapidly released by a reaction with hydrogen ions in the solution, which is usually attributed to a surface exchange reaction. The cation exchange capacity is close to the amount of K and Na released to the solution. However, measurement of surface areas of the micas both before and after the rapid release of K and Na, indicate that only one third of the cation exchange capacity can be attributed to release from the outer surfaces, whereas two thirds must be attributed to removal from within deeper parts of the structure.Approximately 2.5% of total K and Na from the interlayer regions, 1.5% of Mg from the octahedral sheets, and 0.5% of Si from the tetrahedral sheets were released by the end of the experiment, indicating that dissolution was incongruent in both cases. Si showed the slowest total release rate of all the ions. In the later stages (after 400 hr), the release rate of Si becomes a linear function of time. The linear release rate of Si was found to be 2.4 × 10^?17 mol/cm²/sec for the ruby mica, and 2.7 × 10^?17 mol/cm²/sec for the green mica. The rate-controlling mechanism in the dissolution of muscovite in the later stages is probably related to the rate of destruction of the tetrahedral sheets (Si-O bonds) which in turn is reflected by the rate of release of Si into the solution.     

The impact of drilling fluid and well cuttings on polychaete feeding guilds from the US northeastern continental shelf

The effect of recent drilling operations (fluid and well cuttings) on polychaete feeding guilds from the continental shelf off Atlantic City, New Jersey, was examined. Although there were some adverse effects on macrobenthos from 2160 metric tons of cuttings and mud solids discharged into the marine environment, the composition of polychaete feeding guilds remained essentially unchanged. This key trophic relationship between polychaetes and the changing environment due to the drilling operation was apparently uninterrupted. This relationship remains to be examined in other natural and perturbed habitats.     

Pillow talk

Three distinct types of pillows and pillow lava sequences with different modes of origin have been recognized in the extrusive sequences comprising the upper parts of ophiolite complexes that represent the mafic portion of the floor of an Early Cretaceous back-arc basin in southern Chile. One type of pillow formed by non-explosive submarine effusion. A second type formed by magmatic intrusion into pre-existing aquagene tuff formed by explosive eruption. The third type of pillow occurs within dikes, forming pillowed dikes, possibly as a result of vapor streaming within a cooling dike. Where studied in southern Chile, aquagene tuffs and intrusive pillows decrease and water-lain pillows increase in relative abundance from north to south. This variation corresponds with a north-to-south decrease in both the relative volume of extrusives to extensional dikes and the range and volume of differentiated rocks, suggesting a southward increase in rate of extension relative to rate of magma supply within the spreading ridges at which the ophiolites formed. In the northern part of the original basin where the rate of extension was small relative to the rate of magma supply, magma remained in magma chambers longer, resulting in a greater range and volume of differentiated rocks. The larger volume of more differentiated, cooler and more viscous magmas, in conjunction with the likelihood of more violent eruption of volatile-rich differentiates, may have been responsible for the large volume of aquagene tuff in the northern part of the original basin. These observations in southern Chile suggest that ophiolites which contain a great abundance of aquagene tuffs and intrusive pillow lavas formed in tectonic environments in which the rate of extension was small relative to the rate of magma supply (island arcs, embryonic marginal basins). Ophiolites with predominantly water-lain pillowed and massive lavas formed in tectonic environments in which the rate of extension was large relative to the rate of magma supply (mid-ocean ridges, mature back-arc basins). Thus geologic field data may supplement geochemical data as a tool in distinguishing the original igneous-tectonic environments in which ophiolites originate.     

A general mixing equation with applications to Icelandic basalts

The mixing equation applied by Vollmer [1] to Pb and Sr isotope ratios is shown to be a general equation applicable to consideration of element and isotope ratios. The mixing equation is hyperbolic and has the form:Ax + Bxy + Cy + D = 0where the coefficients are dependent on the type of plot considered: i.e. ratio-ratio, ratio-element, or element-element. Careful use of this equation permits testing whether mixing is a viable process, places constraints on end member compositions, allows distinction between mixing of sources and mixing of magmas, and should allow distinction between recent mixing and long-term evolution of sources.The available chemical data for postglacial basalts from Iceland and along the Reykjanes Ridge are not consistent with either mixing of magmas or simple mixing of an enriched ocean island source with a depleted ocean ridge source. If the available analyses for basalts are representative of the source regions, the data are consistent with at least two models neither of which can be properly tested with the available data.(1) There are two separate mixing trends: one beneath Iceland with the alkali basalt source and a depleted Iceland source as end members; the second along the Reykjanes Ridge with a heterogeneous ocean ridge basalt source and a source similar to that for intermediate basalts on Iceland as end members. The depleted Iceland source and the depleted ocean ridge source are not the same.(2) The chemistry of the basalts is not determined by mixing. Instead the basalts are derived from a multiplicity of sources with a similar history which have been isolated for hundreds of millions of years.     

Geologic assessment of environmental impact in Lake Macatawa,Michigan

A historical record of the effects of human development upon Lake Macatawa is established using geologic criteria. Significant cultural events are recognized using the pollen record, particularly ambrosia/vesiculate pollen ratios, and occurrences of detrital and chemical contaminants within the more recent sedimentary column. Detrital contaminants include such things as cinder and ash from coal burning power plants and animal hair from early tanning operations. Trace metal contaminants, frequently associated with specific industrial events, are particularly useful in establishing a modern sedimentary chronology. The impact of human activity upon Lake Macatawa is measured using fossil diatoms. The advantage of this method of environmental impact assessment is that it provides a historical perspective in which changes within an environment are related to its natural conditions.     

Modelling of major elements in mantle-melt systems using trace element approaches

Major elements can be modelled in ways similar to the quantitative petrogenetic modelling used for trace elements. In contrast to modelling with trace elements, however, modelling with major elements is constrained by the stoichiometry of the solid phases. Within these constraints, the same equations for partial melting and crystallization which have been used to such advantage for trace elements may be used for major elements.Calculated MgO and FeO abundances in a mantle-melt system are used as an example of the modelling technique. Such modelling yields limited fields of permissible melts and residues for a given parent composition, but does not give the paths of melting. It does allow the temperature and extent of melting which gave rise to a melt to be determined from the MgO and FeO abundances of the melt or residual solid. Applying the results of the modelling to igneous rocks and ultramafic nodules leads to the following conclusions, which are subject to the uncertainties in the available distribution coefficients. Least differentiated basalt glasses from the ocean floor are derived from parent melts with less than 15.5 weight % MgO and 8.2 wt. % FeO. Komatiites may be derived by less than 60% melting of a pyrolite source leaving a residue of olivine and pyroxene. Many nodules from the subcontinental mantle appear to be residues of large fractions of melting (>30%) at high temperature and pressure, whereas ultramafic nodules from oceanic basalts appear to be residues of smaller fractions of melting (<30%) at lower temperatures and pressures.     

A numerical investigation of the thermal state of the earth's mantle

An idealized convecting mantle with internal heat generation and viscosity dependent on temperature and pressure is examined with numerical calculations. Temperature and viscosity are coupled and self-regulating in the quasi-steady solutions. The lack of any tendency for upwelling flow to constrict itself to narrow channels argues against the existence of plumes. Unsteadiness is an essential feature of mantle convection, not only for mixing at large Rayleigh numbers, but also to prevent the flow from being impeded by continuous rigid regions.