Late Precambrian tidal-flat deposits and algal stromatolites in the Båtsfjord formation,East Finnmark,North Norway

The Annijokka Member of the Late Precambrian Båtsfjord Formation consists of about 300 m of siliciclastic and carbonate deposits which accumulated on tidal flats, and includes several horizons of stromatolite biostromes. Seven main lithofacies are arranged in fining-up cycles a few metres thick. The cycles are terrigenous in the lower part and carbonate-rich higher up and are interpreted as reflecting shallowing-up conditions of deposition. Lithofacies distribution in the member as a whole also shows an upward decrease in the terrigenous component and grain size and increase in carbonates, thus suggesting that the Annijokka Member is regressive.The stromatolite biostromes of the member contain domal forms composed of calcite while the non-stromatolitic, though possibly in part algal-laminated, carbonate-rich beds of the member are dolomitic. This contrasting mineralogy suggests (1) penecontemporaneous supratidal dolomitization, and (2) a possibly freshwater-influenced origin of the domal stromatolites.     

Automatic digital recording of geomagnetic elements by means of a proton precession magnetometer

A simple system of automatic recording of geomagnetic field components by means of a proton vector magnetometer has been built and tested at L'Aquila Geomagnetic Observatory. The instrument is working with the use of a combination of the addition and compensation methods to directly measure absolute values of field components. A sophisticated equipment for the current generation and control is necessary to maintain the current flowing in the Helmholtz coils within 2 A for successive measurements. To maximize the signal coming from the sensor inside the coils for the different components, a simple arrangement of two orthogonal small coils in only one sensor has been made.After two years of experience and improvements the system has been further controlled comparing it with Ruska variometers regularly working in L'Aquila Observatory.Some practical problems found in operating the system are discussed. The automatic hourly mean computation is suggested to produce yearbooks.     

Long-term variation in mesohaline Chesapeake Bay macrobenthos: Spatial and temporal patterns

Macrobenthos, sediments, and environmental conditions were sampled in the mesohaline region of western Chesapeake Bay (1971–1984) and the Potomac River (1980–1984). The survey data were used to quantify variation in macrobenthos and the physicochemical environment due to seasonal dynamics, spatial pattern (regional and local), and annual as well as long-term trends. Field experiments were conducted to test hypotheses suggested by the analysis of the survey data. Long-term and regional changes in the physiochemical environment, particularly salinity and dissolved oxygen concentration, had major influences on regional and long-term abundance patterns of macrobenthos. Two major species groups were identified along the mesohaline salinity gradient: those characteristic of high and low mesohaline salinities. Salinity increased and dissolved oxygen concentration below the pycnocline declined over the 14 yr. Estuarine endemic and euryhaline marine species concomitantly decreased in abundance. Opportunist species responded to increasing salinity and declining oxygen levels with increases in abundance. Predation on macrobenthos by fish and crabs affected the amplitude of annual recruitment pulses. Food availability apparently determined the magnitude of summer macrobenthic mortality. Spring was a critical period for the establishment of distributional patterns. The macrobenthos of the upper Chesapeake Bay was relatively stable over the study period mainly due to the stability and predictability of physicochemical processes controlling recruitment patterns.     

Die Kristallstruktur des Polyhalits,K2Ca2Mg[SO4]4.2H2O

Zusammenfassung Die Kristallstruktur des triklin-pinakoidalen Polyhalits, K₂Ca₂Mg[SO₄]₄. 2H₂O wurde aus photographischen Daten mit Hilfe der 3-dimensionalen Patterson-Funktion ermittelt. Die Verfeinerung mit der Methode der kleinsten Quadrate führte mit individuellen, isotropen Temperaturfaktoren für die beobachteten Reflexe auf R=0,09.In der Atomanordnung sind Sulfatgruppen durch oktaedrisch koordinierte Mg²⁺, 8-koordinierte Ca²⁺ und 11-koordinierte K⁺ miteinander verknüpft. Die H₂O-Moleküle sind jeweils an ein Mg²⁺ und ein K⁺ koordiniert. Die wahrscheinliche Lage der H-Atome wird aus den interatomaren Abständen erschlossen.

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The crystal structure of polyhalite, K₂Ca₂Mg[SO₄]₄.2H₂O

Summary The crystal structure of the triclinic-pinacoidal mineral polyhalite, K₂Ca₂Mg[SO₄].2H₂O, is derived from photographic data by means of the 3-dimensional Patterson-function. The least squares-refinement resulted for the observed reflections and individual, isotropic temperature factors in R=0.09.In the atomic arrangement the sulfate groups are linked by octahedrally coordinated Mg²⁺, 8-coordinated Ca²⁺, and 11-coordinated K⁺. Each H₂O-molecule is coordinated to one Mg²⁺ and one K⁺. The probable positions of the H-atoms are derived from the interatomic distances.

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Mit 2 Abbildungen     

Versuch zur Bestimmung von μmax undK s mit dem Sapromat-Verfahren

Ohne Zusammenfassung Die Arbeit wurde im Rahmen eines WHO-Stipendiuma an der Eidg. Anstalt für Wasserversorgung, Abwasserreinigung und Gew?sserschutz (EAWAG), Dübendorf, durchgeführt.     

Estimating the timescale of fluvial response to anthropogenic disturbance using two generations of dams on the South River,Massachusetts, USA

Centuries-long intensive land-use change in the north-eastern United States provides the opportunity to study the timescale of geomorphic response to anthropogenic disturbances. In this region, forest-clearing and agricultural practices following EuroAmerican settlement led to deposition of legacy sediment along valley bottoms, including behind mill dams. The South River in western Massachusetts experienced two generations of damming, beginning with mill dams up to 6-m high in the eighteenth–nineteenth century, and followed by construction of the Conway Electric Dam (CED), a 17-m-tall hydroelectric dam near the watershed outlet in 1906. We use the mercury (Hg) concentration in upstream deposits along the South River to constrain the magnitude, source, and timing of inputs to the CED impoundment. Based on cesium-137 (¹³⁷Cs) chronology and results from a sediment mixing model, remobilized legacy sediment comprised % of the sediment load in the South River prior to 1954; thereafter, from 1954 to 1980s, erosion from glacial deposits likely dominated (63 ± 14%), but with legacy sediments still a substantial source (37 ± 14%). We also use the CED reservoir deposits to estimate sediment yield through time, and find it decreased after 1952. These results are consistent with high rates of mobilization of legacy sediment as historic dams breached in the early twentieth century, and suggest rapid initial response to channel incision, followed by a long decay in the second half of the century, that is likely dependent on large flood events to access legacy sediment stored in banks. Identifying sources of sediment in a watershed and quantifying erosion rates can help to guide river restoration practices. Our findings suggest a short fluvial recovery time from the eighteenth–nineteenth century to perturbation during the first half of the twentieth century, with subsequent return to a dominant long-term signal from erosion of glacial deposits, with anthropogenic sediment persisting as a secondary source. © 2020 John Wiley & Sons, Ltd.     

The 1669 eruption at Mount Etna: chronology,petrology and geochemistry,with inferences on the magma sources and ascent mechanisms

 Analysis of the petrochemical characters of the 1669 Etnean lavas shows that they can be grouped into two sets: SET1 lavas were erupted from 11 to 20 March and are more primitive in composition than SET2, erupted later until the end of activity. Both sets may be interpreted as the result of crystallization under different conditions of two primary magmas which are compositionally slightly distinct and which fractionate different volumetric proportions of minerals. To explain why more mafic lavas (SET1) were erupted earlier than more acid ones (SET2), we argue that new deeper magma rose up into a reservoir where residing magma was fractionating. Density calculations demonstrate that new magma is less dense and may originate a plume, rapidly rising through the residing magma which is cooler and more volatile-depleted than the new magma. Calculations of uprise velocity assuming laminar flow are consistent with this hypothesis. Received: 20 November 1995 / Accepted: 2 August 1996