91 research outputs found
Fazies-Analyse im Paläozoikum des Oslo-Gebietes
No full textAlle vorhandenen bathymetrischen Kriterien (Lithologie, Sedimentstrukturen aller Art, Fossilorientierung, Stromrichtungen, benthonische Fauna und fossile Lebensspuren) werden zur Rekonstruktion der Sedimentationsgeschichte und der Krustenbewegungen für das Kambrium, Ordovizium und Silur im Oslo-Gebiet herangezogen.
Zwei Beckenbildungs- und Krustensenkungszeiten können nachgewiesen werden. Sie sind durch eine Episode des Stillstandes oder der Aufwärtsbewegung der Kruste voneinander getrennt. Das zeigt sich durch Ablagerung flachmeerischer und z. T. konglomeratischer Sedimente am Ausgang des Ordoviziums deutlich an.All available bathymetric criteria (lithology, sedimentary structures, fossil orientation, current features benthos, and trace fossils) are used to reconstruct the sedimentary history and the crustal movements of the Oslo district during Cambrian, Ordovician, and Silurian times. Two phases of basin development and crustal subsidence are recognized, separated by an episode of crustal rest or uplift that resulted in the deposition of shallow marine and conglomeratic sediments at the close of Ordovician times.Tous les critères bathymétriques (lithologie, structures sédimentaires, orientation de fossiles, courants, faunes benthiques et traces fossiles) sont analysés pour reconstituer l'histoire sédimentaire et les mouvements de l'écorce terrestre pendant le Cambrien, l'Ordovicien et le Silurien dans la région d'Oslo.
On peut mettre en évidence deux phases d'affaissement qui sont séparées par un épisode d'arrêt ou de soulèvement du fond, caractérisé par lépôt de sédiments néritiques et de conglomérats à la fin de l'Ordovicien.Фациальный анализ по леозойских отложени й (кембрийская, ордови к ская и силурийская систем ы) в окрестностях Осл о (Норвегия) проводился по следующим критери ям: литология, структу ры осадков, ориентировка окаменелостей, напра вление, бентонные орг анизмы и следы их жизнедеятел ьности. В образовании геосин клинали отмечается д ва периода
Depth limits of Bermudan scleractinian corals: A submersible survey
No full textDepth distribution, zonation pattern and growth morphology of 17 hermatypic and 4 ahermatypic coral species were investigated at eight different locations along the Bermuda platform with the research submersible GEO and by SCUBA diving in August–September 1983. Hermatypic coral growth occurs to a depth of 50 to 70 m, with a single Montastrea cavernosa growing at 78 m. Dominant forms in shallow-water coral communities are Diploria sp. and Porites astreoides, while M. cavernosa, Agaricia fragilis and Scolymia cubensis occur in deep-water associations below 60 m. Vertical visibilities (up to 178 m) and distribution of the photosynthetically active radiation revealed good light penetration values (1% level at about 100 m depth), which should favour hermatypic coral growth to a much greater depth than it actually occurs. Nor should the prevailing temperatures limit the depth of coral growth. Most deep-water hermatypes observed grow on remnants of Pleistocene reefs down to about 60 m. The vast areas of large massed rhodolith nodules below 50 to 60 m are unsuitable bottom for coral colonisation. Macroalgae growth seems to be the strongest factor controlling coral growth in deep water. Bermuda stony corals have a low growth form diversity. Various intraspecific morphs may occur at the same as well as at different depths, with a general trend towards flatter shapes with depth. Comparison with a similar study on Red Sea corals suggests that annual distribution of radiant energy on the most northern Atlantic reefs of Bermuda may be responsible for the occurrence of flat and cuplike growth forms in relatively shallow water, and for the shallower depth limits of hermatypic growth
Quaternary and Pliocene turbidites in the Bahamas, Leg 101, Sites 628, 632, and 635
No full textTextural and compositional differences were found between gravity-flow sheets in an open-ocean environment on the northern slope of Little Bahama Bank (Site 628, Pliocene turbidite sequence) and in a closed-basin depositional setting (Site 632, Quaternary turbidite sequence). Mud-supported debris-flow sheets were cored at Site 628. Average mean grain size of the turbidite samples was lower, mud content was higher, and sorting was poorer than in comparable samples from Site 632. This reflects the deposition of proximal, low-energy turbidity currents and debris flows on a base-of- slope carbonate apron. No mud-supported debris-flow sheets were deposited in the investigated sediment sequence of Hole 632A. Many larger turbidity currents from around the margins of Exuma Sound may have reached this central ba- sin setting, depositing sediments that had been transported over longer distances. Planktonic components dominate in the grain-sized fraction (500-1000 fim) of turbidite samples from Hole 628A, while platform detritus is rare. We inter- preted this as resulting from the erosion and reworking of a large area of open-ocean slope sediments by gravity flows. In contrast, large amounts of benthic and platform components were found in the turbidite samples of Hole 632A. This may be explained by the fact that the slopes of the enclosed Exuma Sound are steep, and turbidity currents bypassed much of these slopes through pronounced channels, delivering more shallow-water detritus to the deep basin. Erosion of slope sediments, a possible source area of planktonic detritus, is assumed to be low. The small slope area in relation to the larger surrounding platform areas and lower production of planktonic components in the enclosed waters of Ex- uma Sound may also explain the observed low number of planktonic components at Hole 632A. Turbidite material from both open-ocean and enclosed-basin environments was deposited at Site 635
Three hundred eighty thousand year long stable isotope and faunal records from the Red Sea : influence of global sea level change on hydrography
No full textStable isotope and faunal records from the central Red Sea show high-amplitude oscillations for the past 380,000 years. Positive δ18O anomalies indicate periods of significant salt buildup during periods of lowered sea level when water mass exchange with the Arabian Sea was reduced due to a reduced geometry of the Bab el Mandeb Strait. Salinities as high as 53‰ and 55‰ are inferred from pteropod and benthic foraminifera δ18O, respectively, for the last glacial maximum. During this period all planktonic foraminifera vanished from this part of the Red Sea. Environmental conditions improved rapidly after 13 ka as salinities decreased due to rising sea level. The foraminiferal fauna started to reappear and was fully reestablished between 9 ka and 8 ka. Spectral analysis of the planktonic δ18O record documents highest variance in the orbital eccentricity, obliquity, and precession bands, indicating a dominant influence of climatically - driven sea level change on environmental conditions in the Red Sea. Variance in the precession band is enhanced compared to the global mean marine climate record (SPECMAP), suggesting an additional influence of the Indian monsoon system on Red Sea climates
Processes of modern sedimentation in the southern Weddell Sea, Antarctica – evidence from surface sediments
No full textThe influence of the NE winter monsoon on productivity changes in the Gulf of Aden, NW Arabian Sea, during the last 530ka as recorded by foraminifera
No full textBenthic and selected planktic foraminifera and stable isotope records were determined in a piston core from the Gulf of Aden, NW Arabian Sea that spans the last 530 ka. The benthic foraminifera were grouped into four principal assemblages using Q-mode Principal Component Analyses. Comparison of each of these assemblages with the fauna of the nearby regions enabled us to identify their specific environmental requirements as a function of variability in food supply and strength of the oxygen minimum zone and by that to use them as indicators of surface water productivity. The benthic foraminiferal productivity indicators coupled with the record of Globigerina bulloides, a planktic foraminifer known to be sensitive to productivity changes in the region, all indicate higher productivity during glacial intervals and productivity similar to present or even reduced during interglacial stages. This trend is opposite to the productivity pattern related to the SW summer monsoon of the Arabian Sea and indicates the role of the NE winter monsoon on the productivity of the Gulf of Aden. A period of exceptionally enhanced productivity is recognized in the Gulf of Aden region between similar to 60 and 13 kyr indicating the intensification of the NE winter monsoon to its maximal activity. Contemporaneous indication of increased productivity in other parts of the Arabian Sea, unexplained so far by the SW summer monsoon variability, might be related to the intensification of the NE winter monsoon. Another prominent event of high productivity, second in its extent to the last glacial productivity event is recognized between 430 and 460 kyr. These two events seem to correspond to periods of similar orbital positioning of rather low precession land eccentricity) amplitude for a relatively long period. Glacial boundary conditions seem to control to a large extent the NE winter monsoon variability as also indicated by the dominance of the 100 ka cycle in the investigated time series. Secondary in their importance are the 23 and 41 ka cycles which seem also to contribute to the NE monsoonal variability. Following the identification of productivity events related to the NE winter monsoon in the Gulf of Aden, it is possible now to extend this observation to other parts of the Arabian Sea and consider the contribution of this monsoonal system to the productivity fluctuations preserved in the sedimentary records. (C) 2000 Elsevier Science B.V. All rights reserved
Environmental changes during the last 9.000 years as reflected in a sediment core from Harrington Sound, Bermuda
No full textSea-level fluctuations during the last glacial cycle
No full textThe last glacial cycle was characterized by substantial millennial-scale climate fluctuations1, 2, 3, 4, 5, but the extent of any associated changes in global sea level (or, equivalently, ice volume) remains elusive. Highstands of sea level can be reconstructed from dated fossil coral reef terraces6, 7, and these data are complemented by a compilation of global sea-level estimates based on deep-sea oxygen isotope ratios at millennial-scale resolution8 or higher1. Records based on oxygen isotopes, however, contain uncertainties in the range of 30 m, or 1 °C in deep sea temperature9, 10. Here we analyse oxygen isotope records from Red Sea sediment cores to reconstruct the history of water residence times in the Red Sea. We then use a hydraulic model of the water exchange between the Red Sea and the world ocean to derive the sill depth—and hence global sea level—over the past 470,000 years (470 kyr). Our reconstruction is accurate to within 12 m, and gives a centennial-scale resolution from 70 to 25 kyr before present. We find that sea-level changes of up to 35 m, at rates of up to 2 cm yr-1, occurred, coincident with abrupt changes in climate
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