A reappraisal of the 1835 eruption of Cosigüina and its atmospheric impact

In the great January 1835 eruption of Cosigüina volcano, Nicaragua, andesitic magma and lithic material were erupted over a period of at least three days. Proximal facies consist of clastogenic lava, scoria-fall, and lithic ash-fall produced by phreatomagmatic to vulcanian or plinian activity, together with surge deposits and lithic block-falls. Pyroclastic flow deposits covered some flanks of the volcano and entered the sea in the Gulf of Fonseca. Little record exists of the distal ash-fall, thus the total bulk volume erupted can only be roughly constrained to 2.9–5.6 km³. Furthermore, the amount of juvenile material is thought to be small. A recent study of volatiles in 1835 scoria suggests sulfur release from the magma was negligible. This reappraisal indicates that the Cosigüina eruption probably had little global climatic impact. Despite its violent nature, the magnitude of the eruption was modest. The eruption occurred too late to initiate the Northern Hemisphere cooling trend form 1828–1836. Dry fogs and other atmospheric optical phenomena usually observed after eruptions that contribute significantly to the stratospheric aerosol burden were not recorded after 1835.     

Symmetrical and segmented variation of physical and geochemical characteristics of the central american volcanic front

The regional variation of physical and geochemical characteristics of Central American volcanoes occurs in two fundamentally different patterns. The first pattern is symmetrical about Nicaragua. Crustal thickness, silica contents of mafic lavas and volcanic edifice heights are lowest in Nicaragua and increase smoothly toward Costa Rica to the south and Guatemala to the north. Magma density is maximum in Nicaragua and decreases smoothly outward. The regional variation in crustal thickness is just enough so that magma densities, calculated at appropriate Moho pressures, are the same at the base of the crust throughout the region. This is consistent with magma ponding at the base of the crust. The bulk compositions of Central American basalts show the same symmetrical variation. Suites of Nicaraguan basalts plotted in pseudo-ternary CMAS projections indicate large olivine and plagioclase primary-phase volumes. Toward Costa Rica and Guatemala the olivine and plagioclase fields inferred from suites of basaltic lavas are smaller, which is consistent with fractionation at increasing depth.The second pattern is the segmentation of the volcanic front and the plate margin in general. The segmentation strongly affects the spacing and size of volcanic centers. At segment boundaries volcanic centers are generally small and unusually widely spaced. Toward segment interiors volcano spacing and size increase systematically. The LIL element contents of lavas strongly reflect this pattern. For lavas with similar silica contents the larger the volcano, the higher the LIL element contents. The relationships between segmentation, volcano spacing and volcano size are compatible with diapiric rise of magma accumulated in narrow ribbons near the upper surface of the underthrust slab. The relationship between volcano volume and LIL element content is qualitatively in agreement with an open-system fractionation model.     

Decametric radio measurement of Jupiter's rotation period

A total of 26 measurements of Jupiter's 12-year average rotation period were made at frequencies of 18, 20, and 22.2 MHz at observatories in Florida and Chile. An improved method was employed in which histograms of occurrence probability vs central meridian longitude obtained at the same frequency and observatory during apparitions about 12 years (one Jovian year) apart were cross correlated. The longitude shift giving maximum cross correlation was used to correct the initially assumed rotation period value. The mean of the measurements is 9 hr 55 min 29.689 sec, with a standard deviation of the mean of 0.005 sec. This is about 0.02 sec, or 4 standard deviations, less than the System III (1965) value. The measurements indicate that the rotation period was not changing (linearly) at a rate in excess of 0.03 sec/yr. If the synoptic monitoring program is continued through the next maximum of the jovicentric declination of the Earth (D_E), we will probably be able to detect a rate of change in rotation period as small as 0.002 sec/yr. This accuracy might be sufficient to reveal a secular drift in Jupiter's magnetic field.     

Book Review: Geostatistics for Engineers and Earth Scientists,by Ricardo A. Olea. Kluwer Academic Publishers,Boston, 1999, 303 p., $135.00 (U.S.), hardcover,ISBN 0-7923-8523-3

Mathematical Geosciences -     

Feasibility Assessment of Long-Term Electrical Resistivity Monitoring of a Nitrate Plume

Long-term monitoring solutions at contaminated sites are necessary to track plume migration and evaluate the performance of remediation efforts. Electrical resistivity imaging (ERI) can potentially provide information about plume dynamics; however, the feasibility and likelihood of success are seldom evaluated before conducting a field study. Coupling flow and transport models with geoelectrical models provide a powerful way to assess the potential effectiveness of an actual ERI field campaign. We present a coupled approach for evaluating the feasibility of monitoring nitrate migration and remediation using 4D time-lapse ERI at a legacy nuclear waste facility. This kilometer-scale study focuses on depths below the water table (∼70 m). A flow and transport model is developed to perform simulations of nitrate migration and removal via a hypothetical pump-and-treat system. A tracer injection is also simulated at the leading edge of the nitrate plume to enhance the conductivity contrast between the native subsurface and the groundwater fluids. Images of absolute bulk conductivity provide limited information concerning plume migration while time-lapse difference images, which remove the static effects of geology, provide more useful information concerning plume dynamics over time. A spatial moment analysis performed on flow and transport and ERI models matches well during the tracer injection; however, inversion regularization smoothing otherwise limits the value in terms of locating the center of mass. We find that the addition of a tracer enables ERI to characterize plume dynamics during pump-and-treat operations, and late-time ERI monitoring provides a conservative estimate of nitrate plume boundaries in this synthetic study.     

Hydraulic Conductivity Calibration of Logging NMR in a Granite Aquifer,Laramie Range,Wyoming

In granite aquifers, fractures can provide both storage volume and conduits for groundwater. Characterization of fracture hydraulic conductivity (K) in such aquifers is important for predicting flow rate and calibrating models. Nuclear magnetic resonance (NMR) well logging is a method to quickly obtain near-borehole hydraulic conductivity (i.e., K_NMR) at high-vertical resolution. On the other hand, FLUTe flexible liner technology can produce a K profile at comparable resolution but requires a fluid driving force between borehole and formation. For three boreholes completed in a fractured granite, we jointly interpreted logging NMR data and FLUTe K estimates to calibrate an empirical equation for translating borehole NMR data to K estimates. For over 90% of the depth intervals investigated from these boreholes, the estimated K_NMR are within one order of magnitude of K_FLUTe. The empirical parameters obtained from calibrating the NMR data suggest that “intermediate diffusion” and/or “slow diffusion” during the NMR relaxation time may occur in the flowing fractures when hydraulic aperture are sufficiently large. For each borehole, “intermediate diffusion” dominates the relaxation time, therefore assuming “fast diffusion” in the interpretation of NMR data from fractured rock may lead to inaccurate K_NMR estimates. We also compare calibrations using inexpensive slug tests that suggest reliable K_NMR estimates for fractured rock may be achieved using limited calibration against borehole hydraulic measurements.     

Production regimes in four Eastern Boundary Current systems

High productivity (maxima 3 gCm⁻² day⁻¹) of the Eastern Boundary Currents (EBCs), i.e. the California, Peru-Humboldt, Canary and Benguela Currents, is driven by a combination of local forcing and large-scale circulation. The characteristics of the deep water brought to the surface by upwelling favorable winds depend on the large-scale circulation patterns. Here we use a new hydrographic and nutrient climatology together with satellite measurements of the wind vector, sea-surface temperature (SST), chlorophyll concentration, and primary production modeled from ocean color to quantify the meridional and seasonal patterns of upwelling dynamics and biological response. The unprecedented combination of data sets allows us to describe objectively the variability for small regions within each current and to characterize the governing factors for biological production. The temporal and spatial environmental variability was due in most regions to large-scale circulation, alone or in combination with offshore transport (local forcing). The observed meridional and seasonal patterns of biomass and primary production were most highly correlated to components representing large-scale circulation. The biomass sustained by a given nutrient concentration in the Atlantic EBCs was twice as large as that of the Pacific EBCs. This apparent greater efficiency may be due to availability of iron, physical retention, or differences in planktonic community structure.     

Last Interglacial palaeovegetation,palaeoenvironments and chronology: a new record from Lago Grande di Monticchio,southern Italy

The sediments of Lago Grande di Monticchio, southern Italy, extend continuously from the present back to the penultimate glacial stage and have an independent lamination-based chronology of high precision and accuracy. Results are presented here from a detailed palynological investigation of that part of the sediment sequence that extends from the last millennia of the penultimate glacial stage to the first stadial following the Last Interglacial (LI). Quantitative palaeovegetation and palaeoclimate reconstructions made from the palynological data are also presented. The onset of the LI is dated to 127.20 ka BP, a date that is consistent with other recent estimates; the duration of the LI is estimated to have been 17.70 ka. The palaeovegetation record indicates a transition period of 3.35 ka at the end of the penultimate glacial stage prior to the onset of the LI; no Younger Dryas-like oscillation is recorded, although the transition was interrupted by a brief event, lasting ca 250 years, during which pollen of woody taxa was reduced in abundance. Steppe vegetation dominated during the latter part of the penultimate full-glacial stage, but was replaced progressively by wooded steppe during the transition. Although the development of forest cover marked the onset of the LI, the forests were relatively open or discontinuous during the first 2.65 ka, closing progressively thereafter and generally dominating between 123.00 and 109.50 ka BP. The end of the LI is dated to 109.50 ka BP, after which date forest cover became discontinuous and wooded steppe or steppe dominated during the 1.90 ka of the subsequent stadial. As might be expected, given the location of the lake, the composition of the LI forests differs markedly from those recorded from northern Europe, as well as from those recorded at other localities in southern Europe. The palaeoclimate reconstructions reveal complex changes in seasonality, the maximum coldest month mean temperatures being between 125.70 ka BP and 123.00 ka BP, whereas maxima for both annual temperature sum and the ratio of actual to potential evapotranspiration were between 120.60 ka BP and 115.80 ka BP. Reconstructed zonal mean values for all three climatic variables in the zones in which they peak exceed values at the locality today. Comparison with other palaeovegetation records of the LI from Europe reveals that forest cover generally opened up north of the Alpine region probably ca 115 ka BP, coinciding with a marked decrease in sea surface temperatures in the Nordic Seas; this probably corresponds to a marked shift in forest composition at Lago Grande di Monticchio at 115.80 ka BP with an associated reconstructed decrease of ca 5 °C in coldest month mean temperature. Nonetheless, forest continued to dominate at Lago Grande di Monticchio until 109.50 ka BP. Such comparisons also reveal considerable complexity in the geographical and altitudinal patterns of change in palaeovegetation during the LI; such complexity is to be expected given the parallel complexity of Holocene changes. Systematic comparisons between reconstructions of palaeoclimate are hampered by a lack of consistency in approach and in the variables reconstructed. Further insight into this complexity of palaeoclimate development during the LI requires a synthesis of the available data and application of a consistent reconstruction approach that also provides robust estimates of the uncertainty in the reconstructed values.     

Developing a 150 ka luminescence chronology for the barrier dunes of the southern Cape, South Africa

The southern coastline of South Africa exhibits extensive aeolianites and spectacular barrier dunes; the ages and palaeoenvironmental significance of which have, until recently, received limited attention. We present preliminary results for samples taken from a single 6.5 m section in the seaward barrier dune near to the town of Wilderness. The reliability of these ages has been investigated using a series of dose recovery and pulsed irradiation experiments, as well as through analyses of single aliquot and single grain D_e distributions. Results indicate that the SAR protocol is providing accurate and precise D_e determinations and that adequate sensitivity correction is being made. The application of pulsed irradiations does not result in significant changes in the dose–response curves or averaged D_e's. Single grain D_e distributions are broad, with significant levels of overdispersion, which may reflect beta dose heterogeneity effects.

Peaks in aeolian activity date to 157–154 and 140–125 ka, with a slower rate of deposition recorded from 115 to 85 ka. Brief phases at 40 and 21 ka are also identified. Some ages, notably those from OIS 3 and OIS 2, are surprising from a geomorphological perspective, given the likely distance of the dune from the shoreline at such times, although they are not inconsistent with evidence from the east coast of South Africa. The results reveal multiple phases of Late Quaternary barrier dune accumulation, and suggest that aeolian activity in this region has been episodic and potentially more complex than previously thought.     

Re‐shaping hysteretic behaviour—spectral analysis and design equations for semi‐active structures

Semi‐active dampers offer significant capability to reduce dynamic wind and seismic structural response. A novel resetable device with independent valve control laws that enables semi‐active re‐shaping of the overall structural hysteretic behaviour has been recently developed, and a one‐fifth scale prototype experimentally validated. This research statistically analyses three methods of re‐shaping structural hysteretic dynamics in a performance‐based seismic design context. Displacement, structural force, and total base‐shear response reduction factor spectra are obtained for suites of ground motions from the SAC project. Results indicate that the reduction factors are suite invariant. Resisting all motion adds damping in all four quadrants and showed 40–60% reductions in the structural force and displacement at the cost of a 20–60% increase in total base‐shear. Resisting only motion away from equilibrium adds damping in quadrants 1 and 3, and provides reductions of 20–40%, with a 20–50% increase in total base‐shear. However, only resisting motion towards equilibrium adds damping in quadrants 2 and 4 only, for which the structural responses and total base‐shear are reduced 20–40%. The spectral analysis results are used to create empirical reduction factor equations suitable for use in performance based design methods, creating an avenue for designing these devices into structural applications. Overall, the reductions in both response and base‐shear indicate the potential appeal of this semi‐active hysteresis sculpting approach for seismic retrofit applications—largely due to the reduction of the structural force and overturning demands on the foundation system. Copyright © 2006 John Wiley & Sons, Ltd.