Geochemical stability of fine-grained silicic Holocene tephra in Iceland and Scotland

This paper tests two assumptions fundamental to the use of fine-grained silicic tephras from Iceland for creating chronological frameworks in northwest Europe. It is shown firstly, that glass shards can retain their overall chemical integrity on at least a four millennial time-scale in contrasting depositional environments in Iceland and Scotland, and secondly, that an acid digestion process, the most practical method for extracting tephra from peat, does not significantly bias the results of major element analysis by electron microprobe. The implication is that there is great potential and an appropriate method for developing both the resolution and the spatial coverage of chronologies based on silicic Icelandic tephras in northwest Europe.     

Venus cloud microphysics

Because sulfuric acid does not wet sulfur, composite drops in the atmosphere of Venus cannot have sulfur “cores,” but must instead have sulfur coats. Both components then communicate with the vapor phase. Drops that are fully coated with sulfur are immune to coalescence; this sets a limit to growth that may explain “Mode 3” particles. The sulfur coating is probably responsible for the anomalously low refractive indices derived from entry-probe nephelometer data. There appears to be about an order of magnitude less elemental sulfur than sulfuric acid in the clouds.     

Dynamics of co-ignimbrite plumes generated from pyroclastic flows of Mount St. Helens (7 August 1980)

 Four co-ignimbrite plumes were generated along the flow path of the pyroclastic flow of 7 August 1980 at Mount St. Helens. Three of the plumes were generated in discrete pulses which can be linked to changes in slope along the channel. One plume was generated at the mouth of the channel where the flow decelerated markedly as it moved onto the lower slopes of the pumice plain. Plume generation here may be triggered by enhanced mixing due to a hydraulic jump associated with an abrupt slope change. Measurements of plume ascent velocity and width show that the co-ignimbrite plumes increased in velocity with height. The plumes have initial velocities of 1–2 m/s. Two of the plumes reached a velocity maximum (4.6 and 8.8 m/s, respectively, at heights of 270 and 315 m above the flow) and thereafter decelerated. The other plumes reached velocities of 6.2 and 13 m/s. The four plumes become systematically less energetic downstream as measured by their ascent rates, which can be interpreted as a consequence of decreasing interaction of the pyroclastic flow front with the atmosphere. Theoretical models of both co-ignimbrite plumes and discrete co-ignimbrite clouds assume that there is no initial momentum, and both are able to predict the observed acceleration stage. The rising plumes mix with and heat air and sediment out particles causing their buoyancy to increase. Theoretical models agree well with observations and suggest that the initial motion of the ascending material is best described as a discrete thermal cloud which expands as it entrains air, whereas the subsequent motion of the head may become influenced by material supplied from the following plume. The models agree well with observations for an initial temperature of the ash and air mixture in the range of 500–600 K, which is in turn consistent with the measured initial ash temperature of around 920 K. Ash masses of 3.4×10⁵ to 1.8×10⁶ kg are estimated. Received: 11 January 1996 / Accepted: 7 October 1996     

SUMMER MEASUREMENTS OF DISSOLVED ION CONCENTRATIONS IN ALPINE STREAMS,BLANCA PEAK REGION,COLORADO*

Chemical weathering is commonly assumed to be negligible in the denudation of alpine landscapes. Spot samples of dissolved ions from alpine streams on the Blanca Massif had minimum average concentrations in 1978 of 35.53 ppm (range: 14.9–75.6 ppm) and in 1979, 20.23 ppm (range: 7.35–35.0 ppm). Correlations between the variation in the concentration of dissolved ions (silica, pH, and minimum ion concentration) and the elevations and drainage-basin areas of sample sites were significant at the 0.05 level.     

Archipelagos and the law of the sea: Island straits states or island-studded sea space?

At first sight, the regime suggested for archipelagos at UNCLOS seems to recognize to a large extent the concept of the archipelago as a single unit of islands and water. The author examines closely whether in fact the archipelagic states' view of their maritime jurisdiction has been upheld, or whether the negotiations have led to a position based on traditional concepts of law of the sea. The archipelago doctrine and interests underlying it, opposition to it, and the considerations by the international legal community are discussed. Finally, developments on archipelagos in the Seabed Committee and UNCLOS III are traced in order to understand the language of the proposed archipelagic regime.     

Solar velocity fields: 5-min oscillations and supergranulation

One dimensional magnetograph scans have been used to study the 5-min photospheric velocity oscillations and the supergranulation. The oscillations in wing brightness lead the oscillations in velocity by less than 90° in the photosphere, and about 90° in the chromosphere, suggesting that they are traveling waves at lower levels and standing waves at higher levels. Downward flows have been observed to be coincident with the chromospheric network confirming the hypothesis that material is flowing downward at supergranular boundaries.     

Paleolimnological investigations of anthropogenic environmental change in Lake Tanganyika: VII. Carbonate isotope geochemistry as a record of riverine runoff

Evaporation dominates the removal of water from Lake Tanganyika, and therefore the oxygen isotope composition of lake water has become very positive in comparison to the waters entering the lake. The surface water in Lake Tanganyika has remained relatively unchanged over the last 30 years with a seasonal range of +3.2 to +3.5 VSMOW. Water from small rivers entering the lake seems to have a ¹⁸O value between –3.5 and –4.0, based on scattered measurements. The two largest catchments emptying into the lake deliver water that has a ¹⁸O value between these two extremes. This large contrast is the basis of a model presented here that attempts to reconstruct the history of runoff intensity based on the ¹⁸O of carbonate shells from Lake Tanganyika cores. In order to use biogenic carbonates to monitor changes in the ¹⁸O of mixing-zone water, however, the oxygen isotope fractionation between water and shell carbonate must be well understood. The relatively invariant environmental conditions of the lake allow us to constrain the fractionation of both oxygen and carbon isotope ratios. Although molluskan aragonitic shell ¹⁸O values are in agreement with published mineral-water fractionations, ostracode calcite is 1.2 more positive than that of inorganic calcite precipitated under similar conditions. Ostracode shell ¹⁸O data from two cores from central Lake Tanganyika suggest that runoff decreased in the first half of this millennium and has increased in the last century. This conclusion is poorly constrained, however, and much more work needs to be done on stable isotope variation in both the waters and carbonates of Lake Tanganyika. We also compared the ¹³C of shells against predicted values based solely on the ¹³C of lake water dissolved inorganic carbon (DIC). The ostracode Mecynocypria opaca is the only ostracode or mollusk that falls within the predicted range. This suggests that M. opaca has potential for reconstructing the carbon isotope ratio of DIC in Lake Tanganyika, and may be a useful tool in the study of the history of the lakes productivity and carbon cycle.     

Improvement of photomultiplier performance in astronomical applications

The physics of an EMI 9558 Å photomultiplier has been investigated in some detail, and this study has led to a gain of more than thirty times in the signal-to-noise obtainable in a given observation time when used in receiver noise limited conditions. The importance of giant pulses in limiting performance is demonstrated.     

The distribution of aluminum-26 in the early Solar System—A reappraisal

Abstract— A compilation of over 1500 Mg-isotopic analyses of Al-rich material from primitive solar system matter (meteorites) shows clearly that ²⁶Al existed live in the early Solar System. Excesses of ²⁶Mg observed in refractory inclusions are not the result of mixing of “fossil” interstellar ²⁶Mg with normal solar system Mg. Some material was present that contained little or no ²⁶Al, but it was a minor component of solar system matter in the region where CV3 and CO3 carbonaceous chondrites accreted and probably was a minor component in the accretion regions of CM chondrites as well. Data for other chondrite groups are too scanty to make similar statements. The implied long individual nebular histories of CAIs and the apparent gap of one or more million years between the start of CAI formation and the start of chondrule formation require the action of some nebular mechanism that prevented the CAIs from drifting into the Sun. Deciding whether ²⁶Al was or was not the agent of heating that caused melting in the achondrite parent bodies hinges less on its widespread abundance in the nebula than it does on the timing of planetesimal accretion relative to the formation of the CAIs.