New field and laboratory evidence for the origin of hyaloclastite flows on seamount summits

New field observations with the submersible ALVIN and photographic evidence from a study of the summits of seamounts near the East Pacific Rise show that hyaloclastite deposits occur commonly. Hyaloclastite outcrops were found on six volcanoes at depths from 1240 to 2500 m. These new observations plus laboratory study of new hyaloclastite specimens extend the results of previous studies. Most of the hyaloclastite samples are of hydrovolcanic eruptive origin, but a few show evidence of a predominantly sedimentary origin. Primarily from morphology, we identify several vent areas from which hyaloclastite presumably erupted. The surface appearance of the hyaloclastite deposits varies with distance to these vents, leading us to propose a facies model for deep-sea hyaloclastites on seamount summits. Hyaloclastites of hydromagmatic origin exhibit weak normal grading and bedding-parallel alignment of platy shards. They consist of blocky, sliver and fluidal basalt glass shards and lithics in a matrix that contains pelagic sediment. The shards themselves are remarkably free of even the tiniest crystals and are usually chemically homogeneous. We propose that the shards form mainly by cooling-contraction granulation, but cannot rule out the possibility of limited steam explosivity. Hyaloclastites are closely associated with submarine pahoehoe and we propose that a very rapid eruption rate, promoting clastic-dominated versus flow-dominated eruptive behavior, is the dominant control on hyaloclastite formation. We propose that shard formation occurs during submarine lava fountaining. Gravitational instability of the resulting slurry of shards, sea water and possibly steam causes gravity flow that carries the shards outward from the vent. Further field and modelling studies are needed to test these ideas and more quantitatively constrain the ascent mechanism, eruption dynamics and deposition of deep-sea hyaloclastites.     

The Grand Brûlé exploration drilling: New data on the deep framework of the Piton de la Fournaise volcano. Part 2: Secondary minerals

The Grand Brûlé borehole intersects a thick pile of basaltic lavas, down to 1010 m, and a basic-ultrabasic intrusive complex, from 1010 to 3003.50 m.The lavas are, in general, unaltered except in two fractured zones, where hydrothermal fluids circulated at temperatures not exceeding 350 ° C. The main secondary minerals there are pyroxene, feldspar, epidote, actinolite and chlorite.The entire thickness of the intrusive body intersected contains secondary minerals representing three stages of cooling:

1. 1. A late magmatic episode (600–900°C) characterized by biotite, kaersutite, edenite and pargasite.

2. 2. A hydrothermal episode (T<- 350°C) characterized by epidote, albite, biotite, actinolite and chlorite.

3. 3. A phase of serpentinization (T≤ 350°C).

It is very likely that the two later events occurred simultaneously, with physical and chemical interference.     

Shape analysis of Pacific seamounts

Shape statistics have been compiled from 85 profiles of well-surveyed Pacific seamounts in the height range 140–3800 m. A flat-topped cone was fit to each seamount's cross-sectional profile maintaining the slopes of the sides as closely as possible. On each profile a basal widthd_b, a summit widthd_t, and a maximum heighth, were measured. The height-to-basal-radius ratio isξ_h is estimated by the ratio2hd_b and flatnessf by the ratiod_td_b. Slope angleφ = arctan(ε) is estimated fromε =2h(d_b − d_t). Summit height and basal radius are found to be highly correlated (r = 0.93). The 85-point sample mean of the height-to-basal-radius ratio isξ_h = 0.21 ± 0.08 implying that a seamount's summit height is typically one fifth its basal radius. Despite the high correlation, individual points show some scatter, and there may be groupings into different morphological types. For example, all but one of the seamounts with summit heights above 1000 m have values ofξ_h that are larger than the sample mean. The 85-point sample mean of flatness isf = 0.31 ± 0.18. Data points show a large scatter with values off varying between 0 (a pointy cone) and 0.69 (a flat-topped cone). A histogram representation of flatness, however, indicates that certain values off may be more common than others: the histogram shows a bimodal distribution with maxima occurring at values off in the ranges 0.10–0.20 and 0.35–0.50. Moreover, there is some evidence that the mean flatness decreases with summit height so that the preferred shape of a large-sized seamount may be a pointy cone. Slope angle has an 85-point sample mean ofφ = 18 ± 6°; individual values ofφ vary between 5° and 36°. In addition to having a lower than average mean flatness seamounts with heights above 2600 m also have a lower than average mean slope angle (15°). To determine which variables account for most of the observed variation in the seamount shapes, a multivariate principal component analysis was performed on the data using five shape variables (summit height, basal radius, summit radius, flatness, and slope). The analysis indicates that most of the variation is described by two variables: flatness and summit height.     

Magnotospheric imaging of high latitude ion outflows

High latitude ion outflows mostly consist of upward streaming O⁺ and He⁺ emanating from the ionosphere. At heights above 1000 km, these flows consist of cold and hot components which resonantly scatter solar extreme ultraviolet (EUV) light, however, the ion populations respond differently to Doppler shifting resulting from the large relative velocities between the ions and the Sun. The possibility of optical detection of the Doppler effect on the scattering rate will be discussed for the O⁺ (83.4 nm) ions. We have contrasted the EUV solar resonance images of these outflows by simulations of the 30.4 nm He⁺ and 83.4 nm O⁺ emissions for both quiet and disturbed geomagnetic conditions. Input data for the 1000 km level has been obtained from the EICS instrument aboard the Dynamics Explorer satellite. Our results show emission rates of 50 and 56 milli-Rayleighs at 30.4 nm for quiet and disturbed conditions and 65 and 75 milli-Rayleighs at 83.4 nm for quiet and disturbed conditions, respectively, obtained for a polar orbiting satellite and viewing radially outward. We also find that an imager at an equatorial distance of 9 R_E or more is in a favorable position for detecting ion outflows, particularly when the plasmapause is depressed in latitude. However, an occultation disk is necessary to obscure the bright plasmaspheric emissions.     

Within-storm variations in runoff and sediment export from a rapidly eroding coal-refuse deposit

Measurements of rainfall, runoff and sediment export from a barren deposit of coal mine refuse in south-western Indiana were collected during three storms in the summer and autumn of 1990. Interfluve sheetwash, sediment mass flux, sediment concentration and, to a lesser extent, trunk gully discharge all responded quickly to changes in rainfall intensity. Grain-size distributions varied considerably during storms, containing exclusively fine-grained sediment at low sediment discharges but very large quantities of coarse (> 2mm) sediment at peak sediment discharges. Although data from a fairly long, multipulsed storm indicate that sediment production is limited by supply, the imbricated layer of flat chips that exists at the surface of the deposit is apparently mobilized during most high-intensity pulses of rainfall, thereby producing large volumes of coarse sediment during summer thunderstorms.     

Nutrient enrichments and phytoplankton growth in the surface waters of the Louisiana Bight

Nitrate concentrations have increased twofold in the Mississippi River during the past three decades. The increased nitrogen loading to the Louisiana shelf has been postulated as a factor leading to eutrophication and the subsequent development of hypoxia west of the Mississippi River delta. While ratios of nitrogen:phosphorus and nitrogen:silica are relatively high in surface waters on the western Louisiana shelf, nitrogen has been posed as the ‘limiting’ nutrient in this region. Bioassays were performed with nutrient additions to surface waters collected from the Louisiana shelf to examine the potential for specific nutrient limitation. Experiments were conducted in March and September 1991, and May 1992. The growth responses of natural and cultured phytoplankton populations were determined by measuring the time course of in vivo and 3-(3,4 dichlorophenyl)-1, 1-dimethylurea (DCMU)-induced fluorescence, as well as initial and final chlorophylla concentrations. The results suggest that phosphate and silicate potentially limit phytoplankton growth during the winter-spring, particularly at low salinities. In late summer, in contrast, nitrogen limitation may be prominent at higher salinities.     

Differences in the crab fauna of mangrove areas at a southwest Florida and a northeast Australia location: Implications for leaf litter processing

Existing paradigms suggest that mangrove leaf litter is processed primarily via the detrital pathway in forests in the Caribbean biogeographic realm whereas herbivorous crabs are relatively more important litter processors in the Indo-West Pacific. To test this hypothesis, we used pitfall traps to collect intertidal crabs to characterize the crab fauna in a mangrove estuary in southwest Florida. We also tethered mangrove leaves to determine if herbivorous crabs are major leaf consumers there. We compared the results with previously published data collected in an analogous manner from forests in northeastern Australia. The crab fauna in Rookery Bay, Florida, is dominated by carnivorous xanthid and deposit-feeding ocypodid crabs whereas that of the Murray River in northeastern Australia is dominated by herbivorous grapsid crabs. No leaves tethered at five sites in the forests in Southwest Florida were taken by crabs. This contrasts greatly with reported values of leaf removal by crabs in Australian forests of 28–79% of the leaves reaching the forest floor. These differences in the faunal assemblages and in the fate of marked or tethered leaves provide preliminary support for the hypothesis that leaf litter is in fact processed in fundamentally different ways in the two biogeographic realms.     

A comparison of the reactions to government taxation measures of twentieth century mining companies and nineteenth century pastoralists

This paper compares pastoralists in South Australia in the nineteenth century with Australian mining companies in the twentieth century as they resist the efforts of government to extract a greater proportion of the wealth created in the exploitation of natural resources. In effect, in both cases the struggle relates to the allocation of economic rent between private and corporate capital on the one hand, and the owners of the resources, the people of Australia, on the other. It is difficult not to be impressed by the similarities in the ideologies, objectives and strategies of these two groups of resource exploiters, separated though they are by over one hundred years.