The magnitude of global fresh-water transports of importance to ocean circulation

Water-vapor transport from low to high latitudes in a given ocean and from one ocean to another must be compensated by a net flow of salt through the sea. A comparison is presented which shows that water-vapor fluxes derived from meteorological information, from an atmospheric general circulation model and from a radiocarbon-calibrated ocean box model are in first-order agreement. Offprint requests to: WS Broecker     

The statistical magnetic signature of magnetospheric substorms

The characteristic magnetic signatures of magnetospheric substorms both on the ground and in space have been determined from the analysis of ～1800 substorm events. The timing and properties of these events were objectively determined according to explicit mathematical criteria by a computer pattern-recognition program. This program processed daily magnetograms from a mid-latitude network of geomagnetic observatories.Ground data analyzed, using onsets determined in this manner, included the AE indices and individual magnetograms at different local times in the auroral zone and at midlatitudes. Superposed epoch averages of these data confirm the local time magnetic substorm signatures, determined in earlier studies of fewer events, and demonstrate the validity of the computerized onset determination procedure.Superposed epoch averages of the interplanetary magnetic field (IMF) associated with the onsets demonstrates both a distinct southward component prior to the onsets and a dependence of the substorm amplitude on the integrated preceding southward IMF flux. Superposed epoch averages of the tail lobe magnetic field magnitude and vector components demonstrates field magnitude changes and rotations in association with the substorm onsets. These lobe field changes are consistent with the growth-phase model of substorm activity and with variations in the magnetopause flaring angle.     

Origin of July Antarctic precipitation and its influence on deuterium content: a GCM analysis

The NASA/GISS GCM is used to estimate the evaporative contributions of several oceanic regions (defined by temperature) to Antarctica's July precipitation. Tracer diagnostics in the GCM suggest that the weighted average evaporative source temperature for Antarctic precipitation as a whole is about 12°C. The average source temperature for local precipitation there varies from 9° C to 14° C. To examine the effect of evaporative source on water isotope concentration, the GCM also follows a global deuterium (HDO) tracer and deuterium tracers evaporating from each oceanic region. The results suggest that although evaporative source temperature does affect the concentrations of the individual HDO tracers, differences in evaporative source do not explain the scatter in the roughly linear relationship between condensation temperature and isotope concentration. Offprint requests to: R Koster     

Origin and chronology of chondritic components: A review

Mineralogical observations, chemical and oxygen-isotope compositions, absolute ²⁰⁷Pb-²⁰⁶Pb ages and short-lived isotope systematics (⁷Be-⁷Li, ¹⁰Be-¹⁰B, ²⁶Al-²⁶Mg, ³⁶Cl-³⁶S, ⁴¹Ca-⁴¹K, ⁵³Mn-⁵³Cr, ⁶⁰Fe-⁶⁰Ni, ¹⁸²Hf-¹⁸²W) of refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)], chondrules and matrices from primitive (unmetamorphosed) chondrites are reviewed in an attempt to test (i) the x-wind model vs. the shock-wave model of the origin of chondritic components and (ii) irradiation vs. stellar origin of short-lived radionuclides. The data reviewed are consistent with an external, stellar origin for most short-lived radionuclides (⁷Be, ¹⁰Be, and ³⁶Cl are important exceptions) and a shock-wave model for chondrule formation, and provide a sound basis for early Solar System chronology. They are inconsistent with the x-wind model for the origin of chondritic components and a local, irradiation origin of ²⁶Al, ⁴¹Ca, and ⁵³Mn. ¹⁰Be is heterogeneously distributed among CAIs, indicating its formation by local irradiation and precluding its use for the early solar system chronology. ⁴¹Ca-⁴¹K, and ⁶⁰Fe-⁶⁰Ni systematics are important for understanding the astrophysical setting of Solar System formation and origin of short-lived radionuclides, but so far have limited implications for the chronology of chondritic components. The chronological significance of oxygen-isotope compositions of chondritic components is limited. The following general picture of formation of chondritic components is inferred. CAIs and AOAs were the first solids formed in the solar nebula ∼4567-4568 Myr ago, possibly within a period of <0.1 Myr, when the Sun was an infalling (class 0) and evolved (class I) protostar. They formed during multiple transient heating events in nebular region(s) with high ambient temperature (at or above condensation temperature of forsterite), either throughout the inner protoplanetary disk (1-4 AU) or in a localized region near the proto-Sun (<0.1 AU), and were subsequently dispersed throughout the disk. Most CAIs and AOAs formed in the presence of an ¹⁶O-rich (Δ¹⁷O ∼ −24 ± 2‰) nebular gas. The ²⁶Al-poor [(²⁶Al/²⁷Al)₀ < 1 × 10⁻⁵], ¹⁶O-rich (Δ¹⁷O ∼ −24 ± 2‰) CAIs - FUN (fractionation and unidentified nuclear effects) CAIs in CV chondrites, platy hibonite crystals (PLACs) in CM chondrites, pyroxene-hibonite spherules in CM and CO chondrites, and the majority of grossite- and hibonite-rich CAIs in CH chondrites—may have formed prior to injection and/or homogenization of ²⁶Al in the early Solar System. A small number of igneous CAIs in ordinary, enstatite and carbonaceous chondrites, and virtually all CAIs in CB chondrites are ¹⁶O-depleted (Δ¹⁷O > −10‰) and have (²⁶Al/²⁷Al)₀ similar to those in chondrules (<1 × 10⁻⁵). These CAIs probably experienced melting during chondrule formation. Chondrules and most of the fine-grained matrix materials in primitive chondrites formed 1-4 Myr after CAIs, when the Sun was a classical (class II) and weak-lined T Tauri star (class III). These chondritic components formed during multiple transient heating events in regions with low ambient temperature (<1000 K) throughout the inner protoplanetary disk in the presence of ¹⁶O-poor (Δ¹⁷O > −5‰) nebular gas. The majority of chondrules within a chondrite group may have formed over a much shorter period of time (<0.5-1 Myr). Mineralogical and isotopic observations indicate that CAIs were present in the regions where chondrules formed and accreted (1-4 AU), indicating that CAIs were present in the disk as free-floating objects for at least 4 Myr. Many CAIs, however, were largely unaffected by chondrule melting, suggesting that chondrule-forming events experienced by a nebular region could have been small in scale and limited in number. Chondrules and metal grains in CB chondrites formed during a single-stage, highly-energetic event ∼4563 Myr ago, possibly from a gas-melt plume produced by collision between planetary embryos.     

On early Solar System chronology: Implications of an heterogeneous spatial distribution of Al and Mn

Early Solar System chronology is usually built with the assumption that the distribution of short-lived radionuclides was homogeneous through the solar accretion disk. At present, there is no unambiguous evidence for a homogeneous distribution of short-lived radionuclides in the solar accretion disk, while some data point to a heterogeneous distribution of short-lived radionuclides. In this paper, we explore a possible chronology based on a heterogeneous distribution of ²⁶Al and ⁵³Mn in the accretion disk. Our basic assumption is that the different abundances of extinct short-lived radionuclides in calcium-aluminium-rich inclusions (CAIs) and chondrules are due to spatial rather than temporal differences. We develop a simple model where CAIs and chondrules form contemporaneously, in different spatial locations, and are characterised by distinct initial ²⁶Al and ⁵³Mn abundances. In this model, all evolved bodies are supposed to be originally chondritic, i.e., to be made of a mixture of CAIs, chondrules, and matrix. This mixture determines the initial content in ²⁶Al and ⁵³Mn of a chondritic parent-body as a function of its CAI and chondrule abundance fraction. This approach enables us to calculate coherent ²⁶Al and ⁵³Mn ages from the agglomeration of the parent-body precursors (CAIs and chondrules) until the isotopic closure of ²⁶Al and ⁵³Mn, thereafter called ²⁶Al-⁵³Mn age. We calculate such ²⁶Al-⁵³Mn ages for a diversity of evolved objects, with the constraint that they should be found for realistic chondritic parent-body precursors, i.e., objects having similar or identical petrograpy to the existing chondrite groups. The so defined age of the d’Orbigny angrite is 4.3 ± 1.1 Myr, for the Asuka-881394 eucrite 2.8 ± 1.0 Myr, for the H4 chondrite Sainte Marguerite ∼3 Myr, and for H4 Forest Vale ∼5 Myr. The calculated ²⁶Al-⁵³Mn ages give timescales for the evolution of the respective parent-bodies/meteorites that can be investigated in the light of further petrographic studies. We anchor the calculated relative chronology to an absolute chronology using absolute Pb-Pb ages and relative Hf-W ages of the objects under scrutiny. The precursors of Sainte Marguerite and Forest Vale agglomerated at the same time (∼4565.8 ± 1.2 Ma ago). The precursors of eucrites (Asuka-881394) agglomerated 4564.8 ± 1.2 Ma ago. The precursors of angrites agglomerated late (4561.5 ± 1.8 Ma ago). Our model provides a fully compatible Al-Mg/Mn-Cr/Pb-Pb chronology, and is shown to be robust to reasonable changes in the input parameters. The calculated initial ²⁶Al/²⁷Al ratios are high enough to have ²⁶Al as a possible heat source for differentiation.     

Ozonation of seawater from different locations: formation and decay of total residual oxidant--implications for ballast water treatment

Ballast water is a likely cause for worldwide transfer of non-indigenous aquatic species because of the large volumes and frequency of possible inoculations. Ozone is one treatment option being considered for eliminating non-indigenous species in ballast water. When ozone is applied to seawater, secondary disinfectants are formed, commonly measured and expressed as total residual oxidant (TRO). The goal of this study was to determine those variables most likely to affect the rate of TRO increase during ozonation and the subsequent TRO decline that occurs over time. These parameters strongly influence the efficacy of ozone treatments aimed to eliminate organisms present in ballast water. Seawater was obtained from Puget Sound, Washington; Cape Fear, North Carolina; and San Francisco Bay. Results indicated that seawater characteristics, including the organic content and ammonia, affect the amount of ozone required to achieve a desired TRO level and rate of TRO decay, and therefore need to be considered in determining ozone requirements for ballast water treatment.     

Regional ash fall hazard I: a probabilistic assessment methodology

Volcanic ash is one of the farthest-reaching volcanic hazards and ash produced by large magnitude explosive eruptions has the potential to affect communities over thousands of kilometres. Quantifying the hazard from ash fall is problematic, in part because of data limitations that make eruption characteristics uncertain but also because, given an eruption, the distribution of ash is then controlled by time and altitude-varying wind conditions. Any one location may potentially be affected by ash falls from one, or a number of, volcanoes so that volcano-specific studies may not fully capture the ash fall hazard for communities in volcanically active areas. In an attempt to deal with these uncertainties, this paper outlines a probabilistic framework for assessing ash fall hazard on a regional scale. The methodology employs stochastic simulation techniques and is based upon generic principles that could be applied to any area, but is here applied to the Asia-Pacific region. Average recurrence intervals for eruptions greater than or equal to Volcanic Explosivity Index 4 were established for 190 volcanoes in the region, based upon the eruption history of each volcano and, where data were lacking, the averaged eruptive behaviour of global analogous volcanoes. Eruption histories are drawn from the Smithsonian Institution’s Global Volcanism Program catalogue of Holocene events and unpublished data, with global analogues taken from volcanoes of the same type category: Caldera, Large Cone, Shield, Lava dome or Small Cone. Simulated are 190,000 plausible eruption scenarios, with ash dispersal for each determined using an advection–diffusion model and local wind conditions. Key uncertainties are described by probability distributions. Modelled results include the annual probability of exceeding given ash thicknesses, summed over all eruption scenarios and volcanoes. A companion paper describes the results obtained for the Asia-Pacific region     

Evaluation and reduction of the dynamic coupling between amanipulator and an underwater vehicle

The formulation of the dynamic coupling between a manipulator and an underwater vehicle is presented. Results from a simulation of a particular manipulator-vehicle configuration illustrate the nature and extent of the dynamic coupling. The modeling processes for the underwater vehicle and the manipulator are described with an evaluation of the simple hydrodynamic effects that can be incorporated in the dynamic equations of the manipulator. The equations are formulated for the combination of a 6-degrees-of-freedom vehicle and a 3-degrees-of-freedom manipulator. The effect of the manipulator motion, assuming perfect manipulator joint angle tracking, on the vehicle's position/orientation and consequently the manipulator end-effector position is investigated assuming no vehicle control. Slotine's sliding mode approach has been used to reduce the effect of the manipulator disturbances. This technique allows the expressions developed for the manipulator disturbances to be incorporated in the control law. Control of the vehicle's yaw angle, in this particular manipulator-vehicle configuration, has been determined to be the single most important factor in reducing the end-effector error variation. This is shown to be beneficial in the regulation of the vehicle's yaw angle and offers improved performance compared to a sliding mode controller that does not incorporate the manipulator disturbances. This technique also demonstrates superior performance and insensitivity to parameter variations compared to a fixed-gain controller