Dissolution mechanisms of silicate minerals yielded by intercomparison with glasses and radiation damage studies

The behaviour of two silicates (albite and olivine) toward aqueous dissolution has been investigated with emphasis to two aspects: (1) similarities and differences with silicate glasses; and (2) influence of ion bombardment which transforms the initially crystalline structure to a glass-like one. The occurrence on leached materials of hydrated layers enriched with metallic elements is checked by using high-energy ion beam analytical techniques.

It is shown that for both amorphous and crystalline silicates, dissolution involves competing processes responsible for either hydrated-layer generation (hydration by ion exchange and/or water permeation), or its destruction (hydrolysis of siloxane bonds and detachment of silica units). At moderate temperature ( 100°C), the latter processes dominate for crystalline silicates and the resulting dissolution is congruent. However, these silicates tend to behave like glasses upon ion bombardment above a critical dose. At higher temperature (> 200°C), hydration is markedly enhanced and thick altered layers are generated even on undamaged minerals such as albite and orthoclase, where alkali cations are easily exchangeable.     

The two-step shape and timing of the last deglaciation in Antarctica

The two-step character of the last deglaciation is well recognized in Western Europe, in Greenland and in the North Atlantic. For example, in Greenland, a gradual temperature decrease started at the Bölling (B) around 14.5 ky BP, spanned through the Alleröd (A) and was followed by the cold Younger Dryas (YD) event which terminated abruptly around 11.5 ky BP. Recent results suggest that this BA/YD sequence may have extended throughout all the Northern Hemisphere but the evidence of a late transition cooling is still poor for the Southern Hemisphere. Here we present a detailed isotopic record analyzed in a new ice core drilled at Dome B in East Antarctica that fully demonstrates the existence of an Antarctic cold reversal (ACR). These results suggest that the two-step shape of the last deglaciation has a worldwide character but they also point to noticeable interhemispheric differences. Thus, the coldest part of the ACR, which shows a temperature drop about three times weaker than that recorded during the YD in Greenland, may have preceded the YD. Antarctica did not experienced abrupt changes and the two warming periods started there before they started in Greenland. The links between Southern and Northern Hemisphere climates throughout this period are discussed in the light of additional information derived from the Antarctic dust record.     

Soluble and insoluble impurities along the 950 m deep Vostok ice core (Antarctica) — Climatic implications

Simultaneous measurements of soluble and insoluble impurities were made on the 950 m deep Vostok (78°30′S, 106°54′E, 3420 m a.s.l.) ice core, spanning roughly 50000 yr, using various analytical techniques. We observed higher continental (×37) and marine (×5.1) inputs during the last glacial age than during the Holocene stage. A study of microparticle compositions and of volcanic indicators (Zn, H₂SO₄), shows that the high observed crustal input is not due to enhanced volcanism, but is rather of continental eolian origin. For the first time, the ionic balance along a deep ice core is established, mainly used in discussing the evolution of the Cl to Na ratio over central East Antarctica with changing climatic conditions: the presence of relatively high amounts of Na₂SO₄ in the marine aerosol at the Vostok site during the Holocene is demonstrated. Comparison with the Dome C (74°39′S, 124°10′E, 3040 m a.s.l.) results confirms the chronology of the major events: (i) maximum terrestrial input around the last glacial maximum (～18 ka BP); (ii) end of the high continental flux over Antarctica near 13 ka BP; (iii) marine input varying in an opposing manner to isotopic fluctuations with rather high concentrations beginning to decrease when isotopic values increase and reaching Holocene values at the end of the transition between cold and warmer climate conditions. Detailed comparison with results provided by deep ice cores from other sites which are probably more influenced by oceanic air masses seems to indicate that most of the aerosol reaching central East Antarctica travel over large distance probably at rather high altitude through the troposphere. We can consider that central East Antarctica is well representative of the upper part of the troposphere (higher than i.e., 3000 m) and should, therefore, provide valuable data for global and Antarctic paleoclimatological models.     

Ice core evidence for secular variability and 200-year dipolar oscillations in atmospheric circulation over East Antarctica during the Holocene

Two Holocene ice core records from East Antarctica (Vostok and EPICA-Dome C) were analysed for dust concentration and size distribution at a temporal resolution of 1 sample per ~50 years. A series of volcanic markers randomly distributed over the common part of the ice cores (from 9.8 to 3.5 kyear BP) ensures accurate relative dating (±33 years). Dust-size records from the two sites display oscillations structured in cycles with sub-millennial and secular scale frequencies that are apparently asynchronous. The power spectra of the composite sum (Σ) of the two dust-size records display spectral energy mostly for 150- to 500-year periodicities. On the other hand, the 200-year band is common to both records and the 200 year components of the two sites are out-of-phase (100-year lead or lag) over ~5.5 kyear, a phenomenon also reflected by a significant (>99% conf. lev.) band in the power spectra of the composite difference (Δ) of the two size records. During long-range transport, mineral dust originating from the Southern Hemisphere continents is graded to a variable extent depending on the altitude and duration of atmospheric transport. Relatively coarse dust is associated with air mass penetration from the middle–lower troposphere and conversely relatively fine dust with upper troposphere air masses or the influence of subsidence over the Antarctic plateau, a hypothesis already proposed for the changes that occurred during the Last Glacial Maximum to Holocene transition (Delmonte et al. 2004b). Moreover, we assume that the overall fluctuation of air mass advection over Antarctica depends on the meridional pressure gradient with respect to low latitudes, i.e. the Antarctic Oscillation (AAO). We therefore suggest a regional variability in atmospheric circulation over East Antarctica. The 150–500 year power spectrum of the composite (Σ) parameter represents the long term variability of the AAO, imprinted by secular internal oscillations probably related to the southern ocean-climatic system. On the other hand, the Δ dust composite parameter suggests a persistent atmospheric dipole over East Antarctica delivering coarser (finer) dust particles alternatively to Vostok and Dome C regions with a bi-centennial periodicity. Indeed, a seesaw phenomenon in dust size distribution was already observed at three East Antarctic sites during the last deglaciation (Delmonte et al. 2004b) and was interpreted as a progressive reduction of the eccentricity of the polar vortex with respect to the geographic south pole. Interestingly, the Δ parameter shows a pronounced 200-year oscillation mode, throwing new light on the unresolved question of a possible relationship between climate and solar activity.     

Progress in Carrier Phase Time Transfer

The progress of the joint Pilot Project for time transfer, formed by the International GPS Service (IGS) and the Bureal International des Poids et Mesures (BIPM), was recently reviewed. Three notable milestones were set. (1) The IGS will implement, at least in a test mode, an internally realized time scale based on an integration of combined frequency standards within the IGS network. This will eventually become the reference time scale for all IGS clock products (instead of the current GPS broadcast time). (2) A new procedure for combined receiver and satellite clock products will be implemented officially in November 2000. Receiver clocks are an entirely new product of the IGS. (3) The BIPM will coordinate an effort to calibrate all Ashtech Z12-T (and possibly other) receivers suitable for time transfer applications, either differentially or absolutely. Progress reports will be presented publicly in the spring 2001. ? 2001 John Wiley & Sons, Inc.     

Planetary Embryos Never Formed in the Kuiper Belt

We study the orbital evolutions of various systems of planetary embryos in the transneptunian region, undergoing mutual scattering and perturbations from the giant planets. We show that about 15-20% of the original embryos should survive in the transneptunian region at the current epoch. The orbital dispersion of the surviving embryos depends on their individual mass, so that only lunar mass embryos could survive with semimajor axis smaller than 50 AU. In all cases, we show by a Monte Carlo model that at least one of the surviving embryos should have already been discovered by one of the most effective Kuiper-belt surveys. This implies that planetary embryos did not form in the transneptunian region (or have been removed by some external and unknown mechanism). Therefore, we conclude that the Kuiper belt was not excited by resident planetary embryos, unlike the asteroid belt. We also compute with the Monte Carlo model that a significant number (order 10) of Pluto-size bodies could exist only on very eccentric and long-periodic orbits, typical of the scattered disk, while the existence of about 30 bodies brighter than absolute magnitude 4 in the classical belt is compatible with the discovery of Varuna by the Spacewatch survey.     

The conditions for biodegradation of petroleum hydrocarbons at sea

The hydrocarbon degradation potential of seawater appears higher in zones chronically polluted by these materials; they are characterized by an abundant bacterial development. The salinity of the effluent from a refinery and also its hydrocarbon content are factors affecting the speed of degradation.