Showing papers on "Varve published in 1987"
TL;DR: The average sedimentation rate of Greifensee has been evaluated by using varve counting and 210Pb and fallout radionuclide dating from 4 different sites in the lake, to amount to 0.14 ± 0.02 g cm−2 yr.−1.
134 citations
TL;DR: The first known drainings of Ape Lake occurred in October 1984 and August 1986 as discussed by the authors, where about 54% of the volume of the lake was lost through a tunnel in the damming glacier.
Abstract: The glacier damming Ape Lake has withdrawn from its Neoglacial terminal moraines in the lake since early in this century at an average rate of 15 m/a. As a result, the first known drainings of Ape Lake occurred in October 1984 and August 1986. In each event, about 54% of the volume of the lake was lost through a tunnel in the damming glacier. Most of the remaining water was held in the deep basin of the lake behind partially breached Neoglacial terminal moraines. As the glaciers have withdrawn, the character of the sediments has changed. Sediments in the deep basin of the lake are varved, but the grain size, especially of the summer deposits, has decreased and rates of sedimentation have decreased from about 4 mm/a to less than 2 mm/a. In shallow water, deposition of varved sediments has given way to the deposition of massive sediments at rates of less than 1 mm/a. Ice-rafted debris is rare in deep water, despite the presence of calving bergs.During and following the first draining, significant subaerial ...
58 citations
TL;DR: The last floods from glacial Lake Missoula, Montana, probably ran into glacial lake Columbia, in northeastern Washington as discussed by the authors, and the upper Grand Coulee, Lake Columbia's late Fraser-age outlet, contains a section of 13 graded beds, most of them sandy and separated by varves.
52 citations
TL;DR: A 14C-dated magnetostratigraphy of absolute declination and inclination between 12500 and 10000 14C yr BP was recently developed for southern Sweden as discussed by the authors, and the correlation between the two independently dated time-scales suggest that at 10500-10200 14Cy r BP the varve chronology exceeds the 14C chronology by the order of 500-600 varve yr.
Abstract: A 14C-dated magnetostratigraphy of absolute declination and inclination between 12500 and 10000 14C yr BP was recently developed for southern Sweden. Recently also the Swedish geochronological time-scale, based on c. 11 500 annually deposited clay-varves, was connected with the present. It should therefore be possible to compare the two chronologies with a reliable magnetostratigraphic record in an appropriate clay-varve section. We have found such a site within the Middle Swedish end-moraine zone. Statistical correlations between the two independently dated time-scales suggest that at 10500–10200 14Cy r BP the varve chronology exceeds the 14C chronology by the order of 500-600 varve yr. Other correlations indicate that the difference between the two chronologies was less at 11000 14C yr BP, and further correlations between the time-scales at 12000 14C yr BP suggest that the difference between the chronologies increased steadily from 12000 to 10000 14C yr BP. If these correlations are correct they imply that the 14C production rate increased steadily during the Late Weichselian.
48 citations
TL;DR: The distribution of diatoms in varves shows that seasonal upwelling has persisted along the California coast and has remained strongly seasonal since the late Pleistocene to mid-Holocene as mentioned in this paper.
40 citations
TL;DR: For example, Lake Atitlan, a caldera lake in western Guatemala, was investigated for evidence of recent volcanic and tectonic activity as discussed by the authors, showing that no vents, faults, or folds are apparent on high-resolution seismic reflection profiles of lake sediment, representing at least 17,500 years and probably more than 35,000 years of deposition.
36 citations
TL;DR: Hazard Lake as mentioned in this paper is an ice-dammed lake formed in 1966 by the surge of the Steele Glacier and has been drained annually by means of a subglacial tunnel.
Abstract: "Hazard Lake," located in Kluane National Park, the Yukon, is an ice-dammed lake formed in 1966 by the surge of the Steele Glacier. Since 1975 it has drained annually by means of a subglacial tunnel. During July 1979, observations were made of lake processes before drainage and of sections described in the lake bottom after drainage. Three facies associations are believed to represent prelake, stable lake, and annual draining phases. Gravel commonly found at the base of sections represents deposition in a braided fluvial system prior to 1966. Lake sediments deposited between 1966 and 1975 are dominantly graded laminated silt deposited by turbid underflow from the major inflow stream. Lamination is probably caused by diurnal underflow events or daily variation in strength of underflow. No "winter" clay component of varved sediments is observed. During drainage a regressive sequence is deposited as the lake level drops, with sand-dominated sediments overlapping laminated silt where the main inflow stream en...
20 citations
TL;DR: In this paper, a sequence of 1337 laminae of cyclically varying thickness, interpreted as periglacial varves, obtained from drill cores of the late Precambrian Elatina Formation may record by proxy an ancient solar activity cycle similar to that of the present epoch Sun.
Abstract: A sequence of 1337 laminae of cyclically varying thickness, interpreted as periglacial varves, obtained from drill cores of the late Precambrian Elatina Formation may record by proxy an ancient solar activity cycle similar to that of the present epoch Sun. This inferred ancient activity cycle, represented by a sequence of 110 varve-cycles, has a variable period which approximates a normal distribution (chi square = 3.5), with mean and median values of 12 varve years, standard deviation 1.6 varve years, and maximum variability of ± 4 varve years. The time-sequence of varve-cycle periods consists of both deterministic and stochastic elements. The intrinsic period of the varve-cycle is modulated principally by a longer period of ∼ 160 varve years, during which the varve-cycle period displays superimposed stochastic variation. The power spectral density of the varve thickness sequence is endowed with a strong multiplet of spectral lines centered on the median period, a second weaker multiplet at ∼ 22–25 varve years, and several very long periods. The line spacing within the multiplets favors a separation of period mainly of about 160 varve years, while the superimposed random variability of the varve-cycle period is expressed in the amplitude of the sideband envelope. Running autocorrelation of the varve sequence supports the interpretation of frequency modulation (deviation ratio ∼ 1.5) of the late Precambrian solar activity cycle principally by a period of ∼ 160 varve years.
15 citations
TL;DR: In this paper, the authors present an approach, independent of previously used biostratigraphic techniques, for evaluating temporal variations in upwelling intensity in the Gulf of California using radiochemical analysis.
Abstract: The decrease in 14C/12C with depth in a varved sediment core raised from the eastern flank of Carmen Basin, Gulf of California, is greater than expected from varve and 210Pb chronology. The discrepancy is ascribed to a virtually monotonic decrease in upwelling intensity from several hundred years ago to the present. This radiochemical study presents an approach, independent of previously used biostratigraphic techniques, for evaluating temporal variations in upwelling intensity in the Gulf of California.
14 citations
TL;DR: In this paper, sediment from the Ruhuhu Basin (Ngaka Sub-basin, Sw Tanzania) is described and interpreted using surface and subsurface (42 boreholes) observations and analysed by sedimentological criteria.
Abstract: Sediments from the Paleozoic glaciation of Gondwana and overlying Lower Permian (Asselian) sediments from the Ruhuhu Basin (Ngaka Sub-basin, Sw Tanzania) are described and interpreted. All results were obtained from surface and subsurface (42 boreholes) observations and analysed by sedimentological criteria. Different facies development of tillites and varved shales with drop stones are compared, structural maps, i.e. palaeomorphology of the pre-Karoo basement, isopach configuration of the glacial sequence and depositional surface of the coal forming sequence are presented and discussed. Textural and sedimentological inventary evidence a tensional stress field of a slowly subsiding and extending basin. A conclusive plaeogeographic and depositional basin model is introduced with isolated glaciated mountainous areas surrounded by a peri-glacial lake which later was superseded by braided to meandering river type sedimentation and the formation of prolific coal swamp development.
8 citations
01 Jan 1987
TL;DR: In this paper, a novel mechanism of microlamina formation in oligomictic lakes is proposed, where during spring and summer, a light-coloured microlamamina is formed.
Abstract: SUMMARY 1. A novel mechanism of microlamina formation in oligomictic lakes is proposed. During spring and summer in Zaca Lake, an oligomictic lake in Santa Barbara, California, a light-coloured microlamina is formed. During winter, a mass mortality of sulphur-rich phototrophic bacteria occurs following the ventilation of the chemocline or the overturn of the lake. When this occurs, oxygen is introduced into the chemocline killing most of the anaerobic bacteria. When these sulphur-laden bacteria die and settle to the bottom of the lake, they form a black microlamina which is rich in ferrous monosulphides.
2. The varved stratigraphy of the Zaca Lake sediment core presents an opportunity to correlate stratigraphic events in the lake's varved sediments with events reported for its catchment over the last 200 years.
3. Colonization by Europeans dates back to the 1760s, making this one of the best-documented histories for a small lake in North America. A number of events have been correlated with stratigraphic information from the lake's varved cores.
4. Eutrophic diatom indicator species disappear downcore, where they are replaced by meso- and oligotrophic indicators. In addition, pollen from exotic trees planted in the lake's catchment appears at specific core depths associated with their pollen production stage.