Distributions of hydrothermal activity along the Mid-Atlantic Ridge: interplay of magmatic and tectonic controls

Hydrothermal activity has been investigated along three different sections of the slow-spreading Mid-Atlantic Ridge (MAR): 11°20′–30°N, 36–38°N and the Reykjanes Ridge, 57°45′–63°06′N. When considered in total, the incidence of venting along these three sections of the MAR compares well with the predictions of a model in which frequency of venting is linearly related to ridge-crest spreading-rate. At the scale of individual study areas, however, departure from the model is observed by up to an order of magnitude. Venting is anomalously rare along the Reykjanes Ridge but anomalously abundant along the MAR 36–38°N. Whilst such variability may be within the error of the linear spreading-rate model, we note that the interplay between magmatic and tectonic processes also differs between the three study areas. In the case of the Reykjanes Ridge we propose that the low incidence of venting reported may reflect a limitation of the sampling/investigative strategy because the style of venting which predominates may not give rise to conventional black-smoker hydrothermal plumes. Along the oblique and broadly segmented MAR 36–38°N, we propose that vigorous hydrothermal venting in broad segment-end non-transform discontinuities may be focussed along deeply penetrating active faults with the requisite heat supply being supported through some combination of along-axis magmatic intrusions and thermal release associated with the serpentinisation of crustal peridotites.     

Evaluation and analysis of surface deformation in west Chapala basin,central Mexico

Vertical deformation was measured at 14 benchmarks within the urban area of Jocotepec Mexico using first-order leveling methods and then spatially analyzed in relation to land subsidence and soil discontinuity patterns. The study area is located within the western portion of the Chapala basin, middle-west Mexico. Observations of vertical surficial deformation were made at each benchmark (September and November 2012) relative to a fixed station and compared to an initial survey of each benchmark (April 2012). Results indicate that a maximum subsidence of 7.16 cm over the 8-month measurement interval occurs near downtown coincident with the largest levels of drawdown and translates to a maximum subsidence rate of 0.89 cm/month for the sampling period. Two benchmarks located northwest and southeast of the urban area exhibited uplift of 2.8 and 0.76 cm, respectively, suggesting a complex mechanical response between the sedimentary soil units and the factors causing deformation. A potential spatial relationship exists between subsidence patterns and soil discontinuities. Four separate cones of groundwater depression were observed with two being coincident with subsidence bowls downtown and south of the urban area; however, there is no clear relationship between drawdown and subsidence in the remaining areas. Hydrogeologic reconstructions reveal alternating sequences of alluvial aquifers and highly deformable lacustrine aquitards. An analysis of the soil discontinuities reveals that they are directly aligned with the patterns of vertical deformation.     

Seismoacoustic effects of the Hovsgol earthquake (M w = 4.9) of December 5, 2014

Doklady Earth Sciences - The results of study of the Hovsgol earthquake with M w = 4.9, which occurred on December 5, 2014, in the northern part of Hovsgol Lake in Mongolia, are presented. An...     

Isotopen-geochemische Untersuchungen an Mollusken-Schalen verschiedener Seen Mitteleuropas

Zusammenfassung Die Schalen limnischer Gastropoden und Pelecypoden aus verschiedenen mitteleuropäischen Seen zeigen neben der zu erwartenden, von marinen Mollusken grundsätzlich abweichenden Isotopenzusammensetzung beim¹⁸O, ortsspezifische Unterschiede, die auf unterschiedlich isotopisch zusammengesetzte Wässer in den einzelnen Seen zurückzuführen sind.So spiegeln die Mollusken des Bodensees die isotopische Zusammensetzung des schmelzwasserführenden Alpenrheins wider und sind entsprechend arm an¹⁸O, während beim sehr flachen Neusiedler See durch starke Evaporation das¹⁸O/¹⁶O-Verhältnis in Richtung des schwereren Isotops verschoben ist.

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Isotope geochemistry of mollusc shells from central European lakes

Summary Carbonate shells of limnic gastropods and pelecypods from various lakes of central Europe have an isotopic composition which is principally asimilar—as to be expected—to the composition of marine molluscs.Differences in the¹⁸O concentration can be related to the specific isotopic composition of the water in which the molluscs live. The low¹⁸O concentration of-for example-Lake Constance molluscs reflects the isotopic composition of the Rhine Rive, which carries abundant melting water from the Alps with a correspondingly low¹⁸O concentration. In the Neusiedler See, strong evaporation leads to an enrichment in the heavy isotope¹⁸O in the water of the very shallow lake and consequently in the mollusc shells.

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Mit 2 Abbildungen     

Heavy metals in sediments of the rhine and elbe estuaries: Mobilization or mixing effect?

On their way from the Rhine estuary into the North Sea and Dutch Wadden Sea, Rhine sediments “lose” large portions of their original heavy metal concentrations. Until now these losses were explained by a mobilization process, solubilization — the decomposition products of organic matter form soluble organometallic complexes with the metals of the sediment. Our investigations of the sediments of the Elbe clearly indicate that a mixing process, whereby highly polluted Elbe sediments mix with relatively non-polluted North Sea sediments, rather than solubilization, is the cause of the dilution of heavy metals in the sediments of the Elbe estuary. Because of the similarity of the Elbe data with those from the Rhine River, we propose that a mixing process is also effective in the Rhine estuary and adjacent North Sea areas. The mechanism by which heavy metals are “diluted” is important to the marine ecosystem. In the mixing process proposed in this paper, the heavy metals fixed to the suspected material are trapped in bottom sediments of the marine environment, whereas solubilization would increase the concentration of heavy metals in the sea water and thus they would be more available for uptake by aquatic organisms.     

Vorkommen und Entstehung von „Salz-Schirmen“ im Tuz Gölü („Salzsee“), Türkei

Zusammenfassung Aus randlichen Teilbecken des Tuz Gölü (Salzsee), Türkei, werden bis zu 40 cm große, über dem Wasserspiegel des Salzbeckens liegende Salzschirme beschrieben, deren einzelne Anwachsstreifen verschiedenen Wasserständen bei der Evaporation des Beckens entsprechen. Die Schirmform kann durch bevorzugtes Horizontal-Wachstum der NaCl-Kristalle in der obersten Wasserschicht bei gleichzeitig sinkendem Wasserspiegel gedeutet werden. Ansatzpunkte für die Schirmbildung sind im Sediment steckende NaCl-inkrustierte Pflanzenstengel, die in das Becken eingeschwemmt wurden.

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Occurrence and origin of Salt-umbrellas in the Tuz Gölü (Salt-Lake), Turkey

Salt umbrellas with a maximum diameter of 40 cm are a special form of NaCl growth. They occur above the water surface of evaporating Tuz Gölü (Salt Lake) in Central Anatolia, Turkey. The different growth rims of the umbrella correspond to the different water levels during evaporation of the basin.The umbrella-like shape is best to be explained by the fact that within the uppermost water layer (highest NaCl-concentration) NaCl-crystals grow preferentially in horizontal direction while the water level is steadily sinking.Small plant-stems having been brought into the basin and incrusted by NaCl act as crystallization centers for the formation of the umbrellas.

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Herrn R. H. Okan, Direktor der Salzwerke von Yavsan Tuzlasi, sowie Herrn Georg Irion, Heidelberg, danke ich herzlich für die wertvolle Mithilfe bei der Probenahme.     

High strontium contents and Sr/Ca-ratios in Lake Constance waters and carbonates and their sources in the drainage area of the Rhine River (Alpenrhein)

Algal and mollusk carbonates in Lake Constance sediments proved Sr/Ca-ratios unusually high for fresh water environments. In most cases the observed Sr/Ca-ratios for mollusk shells fall into the range of marine mollusks. These high ratios can be attributed to a high Sr/Ca-ratio (6 ± 0.5) in Lake Constance water. In the Alpenrhein River, the main influent of Lake Constance, Sr/Ca-ratios were found to vary between 4.7 (at high water discharge) and 10.7 (at low water level). High Sr/Ca-ratios correspond to high sulfate contents. From all tributaries of the Alpenrhein, the Ill River revealed to have the highest Sr-contents and Sr/Ca-ratios. A detailed study of its drainage area led to the detection of exceptionally high Sr-contents (up to 11.6 mg/l) and Sr/Ca-ratios (up to 116.5) in springs of the Triassic Ladinian stage of the Austroalpine nappe zone (Ostalpine Deckenzone). The fact that high Sr-concentrations occur together with high sulfate contents leads to the assumption that celestite is leached within Ladinian rocks. This assumption is confirmed by the evidence that celestite deposits have been reported from the same stratigraphic section (Wettersteinkalk) from other localities in the Northern Calcareous Alps.

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Zusammenfassung In Bodensee-Sedimenten auftretende biogene Carbonate zeigen ein für Süßwasserbildungen ungewöhnlich hohes Sr/Ca-Verhältnis; die meisten bei Mollusken gefundenen Werte fallen in den marinen Bildungsbereich. Diese hohen Sr/Ca-Verhältnisse können durch das hohe Sr/Ca-Verhältnis des Bodenseewassers (6 ± 0,5) gedeutet werden, das seinerseits vom noch höheren Sr/Ca-Verhältnis des wichtigsten Zuflusses, des Alpenrheins (4,7 bei Hochwasser, 10,7 bei Niedrigwasser) geprägt wird. Hohe Sr/Ca-Verhältnisse fallen mit hohen Sulfatgehalten zusammen. Von allen Nebenflüssen des Alpenrheins zeigt die Ill die höchsten Sr-Gehalte und Sr/Ca-Verhältnisse. Eine Untersuchung des Einzugsgebietes der Ill führte zur Auffindung von Bächen und Quellen mit ungewöhnlich hohen Sr-Gehalten (bis 11,6 mg/l) und Sr/Ca-Verhältnissen (bis 116,5), die im Ladin der Alpinen Trias der Ostalpinen Deckenzone entspringen. Da hohe Konzentrationen mit hohen Sulfatgehalten zusammenfallen, kann angenommen werden, daß Coelestin aus Gesteinen des Ladins ausgelaugt wird und die hohen Sr/Ca-Verhältnisse der Wässer bedingt. Diese Annahme wird durch die Tatsache erhärtet, daß Coelestin im gleichen stratigraphischen Horizont (Wettersteinkalk) an mehreren anderen Stellen der Nördlichen Kalkalpen beobachtet wurde.