Lead-210 and pollen geochronologies on Lake Superior sediments

Rates of sedimentation in two Lake Superior deposits were determined by both ragweed pollen and ²¹⁰Pb geochronologies. The former yields an average rate over the time since the first appearance of enhanced concentrations of the pollen as a consequence of human settlement. Sedimentation rates derived on these two bases can be brought into accord if the first appearance of ragweed pollen in the sediments was around 1830 and if the sedimentation rates have been uniform over the past century.     

MELIMEX,an experimental heavy metal pollution study: Particle size distributions in limno-corrals

The particle size distributions in three limno-corrals, located in Baldeggersee, Switzerland, have been determined by means of a Zeiss Micro-Videomat image analyzer as a function of depth and time. The distributions were measured biweekly over a period of 1 year at depths of 0 m, 2.5 m, 5 m, 7.5 m and 10 m (=above bottom). Two of the limno-corrals were charged with heavy metals, whereas the third was uncharged and served as a reference. The shape of the distributions as well as the particle concentrations in the uncharged container did not differ from the ones in the charged limno-corrals. The distributions were found to be self-preserving and independent on heavy metal load, time, depth and particle concentrations. As an average, the dependence of the distributions on the particle diameter was found to be ?1.5 to ?2.5.     

Crustal evolution of the early earth: The role of major impacts

Impact cratering was an important — even dominant — process affecting the crustal evolution of the small terrestrial planets. The fundamental highlands/maria dichotomy of the Moon's surface can be traced to a late heavy bombardment by basin-forming, asteroid-sized bodies which produced not only a topographic division in the lunar crust but also localized the later eruptions of mare basalts. Major impact basins with diameters in excess of 200 km are recognized throughout the inner solar system from Mars to Mercury. Similar craters must have formed on the Earth prior to 4 Ga ago, and the minimum number of such basin-forming impacts can be calculated by scaling from the observed (minimum) number preserved on the Moon. When allowance is made for differences in impact velocity, gravitational cross-section and the effects of gravity on crater diameter, it is found that at least 50% of a presumed global sialic crust would have been converted into impact basins by 4 Ga ago. Among the effects resulting from the impact of an asteroidal object on the early crust were: (a) establishment of a topographic dichotmy of 3–4 km (after isostatic adjustment), (b) pressure-release partial melting of the upper mantle and rapid flooding of the basin floor by basalt, and (c) enhancement of thermal gradients in the sub-basin lithosphere and upper asthenosphere. Comparative planetary data such as impact scaling can be used as important constraints on models of the early terrestrial crust. For example, the topography resulting from impact bombardment produced discrete oceans and dry land by 4 Ga ago, making unreasonable models of a globe-encircling ocean on the Earth after that time.     

Radium isotopes in sub-Arctic waters

Measurements of the²²⁸Ra/²²⁶Ra activity ratio in the waters of the Greenland, Norwegian and Labrador Seas and Baffin Bay reveal strong horizontal gradients in the surface waters. The coastal waters are dominated by²²⁸Ra injection from nearshore sediments. There is an inverse correlation between the²²⁸Ra/²²⁶Ra activity ratio and salinity in the 30–36‰ salinity range. Vertical profiles indicate that the²²⁸Ra/²²⁶Ra activity ratio is also strongly coupled toσ_θ except for some regions where²²⁸Ra is being injected into higher density water as these isopycnals intersect coastal areas. We use these measurements in the area of formation of North Atlantic Deep Water to estimate that this water mass forms with a²²⁸Ra/²²⁶Ra activity ratio of 0.10.     

Some aspects of the advance in the knowledge of hurricanes

Several topics discussed byBergeron (1954) are reviewed and compared with present-day positions. They are: conditions for formation; maintenance of the hurricane circulation; modification; and cyclones with hurricane winds outside the tropics.On some of these subjects, notably hurricane formation, Bergeron's views in many respects approach those of the present day. A rarely-occurring event is needed in the tropics besides the climatically favorable circumstances for formation. He postulated baroclinity in the form of a surface tropical front. If the baroclinity is shifted from the low to the upper troposphere, his and present-day viewpoints agree that release of baroclinic instability is the main mechanism of initial kinetic energy generation, in contrast to the CISK theory.Major additions can be made especially on the subject of the role of air-sea interaction in hurricane maintenance through downward momentum transport and establishment thereby of a thermal cyclostrophic wind which keeps the warm core in place. Furthermore, many new data exist on extratropical hurricanes; here, a serious research effort has still to be carried out in the future.     

La theorie des deux magmas fondamentaux dans le volcanisme intracontinental et l'evolution geochimique des laves du Mont-Dore (France)

Résumé Les auteurs utilisent toutes les données chimiques, volcanologiques, chronologiques et tectonophysiques fournies pour un certain nombre de massif continentaux (Mont-Dore et autres). La synthèse de toutes ces études montre qu'il existe deux magmas continentaux indépendants: le magma basaltique venant du manteau (sima) et le magma rhyolitique fourni par la croûte supérieure (sial). Le mélange de ces deux magmas et l'action secondaire de la' gravité et de la pneumatolyse expliquent logiquement la totalité des phénomènes géochimiques des volcans continentaux.

---

The authors used all the chemical, volcanological, chronological and tectonophysical data supplied for a certain number of continental mountain massifs (Mont-Dore and others). The synthesis of these studies shows that there are two independent continental magmas: the basaltic magma coming from the mantle (sima) and the rhyolitic magma supplied by the upper crust (sial). The mixing of these two magmas, with the secondary action of gravity and pneumatolysis logically explains the totality of the geochemical phenomena in continental volcanoes.

---

Zusammenfassung Die Autoren benutzten alle chemischen, vulkanologischen, chronologischen und tektonisch-physikalischen Daten, die von einer bestimmten Anzahl von kontinentalen Massiven (Mont-Dore und andere) entnommen wurden.Das Ergebnis dieser Studien zeigt, daß es zwei voneinander unabhängige kontinentale Magmen gibt.Das basaltische Magma, vom Sima stammend, und das rhyolitische Magma aus der oberen Sial-Kruste. Die Vermischung dieser beiden Magmen und die zusätzliche Wirkung der Gravitation und Pneumatolyse erklären auf natürliche Weise die Gesamtheit der geochemischen Phänomene des kontinentalen Vulkanismus.

---

. , .

     

Greatlakean Substage: A replacement for Valderan Substage in the Lake Michigan basin

New evidence from recent field and seismic investigations in the Lake Michigan basin and in the type areas of the Valders, Two Creeks and Two Rivers deposits necessitates revision of late-glacial ice-front positions, rock- and time-stratigraphic nomenclature and climatic interpretations and deglaciation patterns for the period ca. 14,000–7,000 radiocarbon years B.P. The previously reported and long accepted pattern of deglaciation for the Lake Michigan basin started with a regular retreat from the Lake Border Morainic System, with a minor oscillation marked by the Port Huron moraine(s) and then an extensive Twocreekan deglaciation followed by a major (320 km) post-Twocreekan advance (Valders). However, we now record a major retreat between the times of the Lake Border and Port Huron moraines, followed by a gradual retreat from the Port Huron limit and interrupted by a minor standstill (deposition of Manitowoc Till), a retreat (Twocreekan) and a readvance (Two Rivers Till). No Woodfordian or younger readvance was as extensive as had been the preceding one. This sequence argues for a normal, climatically controlled, progressive deglaciation rather than one interrupted by a major post-Twocreekan (formerly Valderan) surge. This revision appears finally to harmonize the geologic evidence and the palynological record for the Great Lakes region. Our investigations show that Valders Till from which the Valderan Substage was named is late-Woodfordian in age. We propose the term “Greatlakean” as a replacement for the now misleading time-stratigraphic term “Valderan”. The type section and the definition of the upper and lower boundaries of the Greatlakean Substage remain the same as those originally proposed for the Valderan Substage but the name is changed.