Showing papers on "Varve published in 1985"
TL;DR: Two classes of field evidence firmly establish that late Wisconsin glacial Lake Missoula drained periodically as scores of colossal jokulhlaups (glacier-outburst floods) as mentioned in this paper.
Abstract: Two classes of field evidence firmly establish that late Wisconsin glacial Lake Missoula drained periodically as scores of colossal jokulhlaups (glacier-outburst floods). (1) More than 40 successive, flood-laid, sand-to-silt graded rhythmites accumulated in back-flooded valleys in southern Washington. Hiatuses are indicated between flood-laid rhythmites by loess and volcanic ash beds. Disconformities and nonflood sediment between rhythmites are generally scant because precipitation was modest, slopes gentle, and time between floods short. (2) In several newly analyzed deposits of Pleistocene glacial lakes in northern Idaho and Washington, lake beds comprising 20 to 55 varves (average = 30–40) overlie each successive bed of Missoula-flood sediment. These and many other lines of evidence are hostile to the notion that any two successive major rhythmites were deposited by one flood; they dispel the notion that the prodigious floods numbered only a few. The only outlet of the 2,500-km 3 glacial Lake Missoula was through its great ice dam, and so the dam became incipiently buoyant before the lake could rise enough to spill over or around it. Like Grimsvotn, Iceland, Lake Missoula remained sealed as long as any segment of the glacial dam remained grounded; when the lake rose to a critical level ∼600 m in depth, the glacier bed at the seal became buoyant, initiating underflow from the lake. Subglacial tunnels then grew exponentially, leading to catastrophic discharge. Calculations of the water budget for the lake basin (including input from the Cordilleran ice sheet) suggest that the lakes filled every three to seven decades. The hydrostatic prerequisites for a jokulhlaup were thus re-established scores of times during the 2,000- to 2,500-yr episode of last-glacial damming. J Harlen Bretz9s “Spokane flood” outraged geologists six decades ago, partly because it seemed to flaunt catastrophism. The concept that Lake Missoula discharged regularly as jokulhlaups now accords Bretz9s catastrophe with uniformitarian principles.
295 citations
TL;DR: In this article, the structure of the preserved siliceous phytoplankton assemblage is reconstructed from a box core retrieved from the varved sediments along the Guaymas slope of the Gulf of California.
62 citations
TL;DR: Investigations of lake sediments have helped not only to understand the modern lacustrine deposition, but also to develop an insight into the interrelations between climate, productivity, and sedimentation.
Abstract: Varves are sediments registering annual events. Carbonate varves register annual precipitation of calcium carbonate. Glacio-lacustrine varves record an annual freeze-over. The Swiss lakes provide field sites where annual changes of biologic productivity can be monitored, and where secular climatic variations can be recognized. Our investigations of lake sediments have helped not only to understand the modern lacustrine deposition, but also to develop an insight into the interrelations between climate, productivity, and sedimentation.
44 citations
TL;DR: In a small meromictic lake near Toronto, Canada, a mass mortality of photosynthetic bacteria followed the ventilation of the chemocline during fall (autumn) and resulted in 3-8 g m-2 day-1 of organic matter being deposited as a dark layer in sediment traps which were suspended in the permanently anaerobic zone.
Abstract: In a small meromictic lake near Toronto, Canada, a mass mortality of photosynthetic bacteria followed the ventilation of the chemocline during fall (autumn) and resulted in 3–8 g m-2 day-1 of organic matter being deposited as a dark layer in sediment traps which were suspended in the permanently anaerobic zone. This mass mortality of photosynthetic bacteria occurred in late autumn following the annual thermal destratification of the lake's mixolimnion. Wind mixing during this period of homeothermy resulted in the introduction of low levels of dissolved oxygen into the lake's chemocline. The ensuing mass mortality of photosynthetic bacteria resulted in the release of elemental sulphur as the sulphur-rich bacteria decomposed and sank to the bottom of the lake. The ferrous ions in the water below a depth of 15 m in Crawford Lake reacted with this sulphur to form black ferrous sulphides and pyrite which formed a dark microlamina on the lake floor. Each dark microlamina was overlain by a light coloured (calcite-rich) layer which was deposited each spring and summer during the 3 yr period of this study.
The mechanism of microlamina formation elucidated here has been based on the examination of bi-weekly sediment trap information. This approach has permitted an explanation of the mechanisms by which specific events such as calcite precipitation and phytoplankton seasonal succession are transcribed into the sediment record.
41 citations
TL;DR: In this paper, a sequence of 236 varve thickness measurements by Perkins and Sims from proglacial Skilak Lake in southern Alaska is shown to contain spectral features similar to those of the sunspot index.
Abstract: A sequence of 236 varve thickness measurements by Perkins and Sims from proglacial Skilak Lake in southern Alaska is shown to contain spectral features similar to those of the sunspot index. In general, the varve spectrum qualitatively mimics that of the sunspot index, with discrepancies perhaps arising from the misinterpretation of random summer layers as annual increments (varves). These discrepancies may be largely removed through maximization of the sunspot index-varve cross correlation whereby a time-scale factor increment is applied to the varve series measurements. Varve thickness is positively correlated with the sunspot index and with vanishing lag (insofar as can be resolved). As varve thickness and meteorological variables have previously been shown to be strongly correlated, a direct link between the sunspot index and varve thickness is suggested by way of climate response to solar variations.
38 citations
01 Mar 1985
TL;DR: In this article, the results of the 210Pb/210Po-measurements were used to date sediment cores from Lake Zurich (Switzerland) and revealed sediment accumulation rates of (0.055±0.015) g·cm−2·y−1.
Abstract: 210Pb- and137Cs-measurements, and varve counting have been used to date sediment cores from Lake Zurich (Switzerland). Two cores from different water depths were dated with210Pb/210Po and revealed sediment accumulation rates of (0.055±0.015) g·cm−2·y−1 and (0.09±0.03) g·cm−2·y−1, respectively. A comparable rate of (0.07±0.01) g·cm−2·y−1 has been obtained from137Cs measurements. These rates were confirmed by annual layer (varve)-counts which lead to rates of 0.07 g·cm−2·y−1. Constant210Pb activities were observed in the top 6 cm of the sediment cores. This constancy is generally explained in the literature by mixing processes caused by bioturbation and by distortion during coring operations of the uppermost water-rich fluffy sediments. However the distinct137Cs-maxima and the regular and undisturbed varve lamination of the top sediment observed in the cores of Lake Zurich contradict this assumption. In addition, measurements of7Be at the water/sediment interaces proved complete sediment core recovery and mechanically undisturbed sediments. Remobilization processes are assumed to cause the observed constant210Pb activities. Remobilization may also be the reason for an incomplete210Pb inventory in the sediments which contain only about 50% of the fallout from atmosphere. The results of the210Pb dating should therefore be considered with some care. If existent, varve counting represents the easiest and most reliable means for dating lake sediments.
38 citations
TL;DR: In this paper, a detailed sediment budget study was performed for a relatively shallow glacial lake in the Purcell Mountains of British Columbia, with it's relatively large water and sediment input.
Abstract: A detailed sediment budget study was undertaken for a relatively shallow glacial lake in the Purcell Mountains of British Columbia. The lake, with it's relatively large water and sediment input lev...
29 citations
TL;DR: This article applied the partial bleach (R-Γ) procedure of thermoluminescence (TL) dating of unheated sediments to the 2-4-μm sized feldspar-dominated grains from two components of a 10-11-k BP glaciolacustrine silt.
Abstract: The partial bleach (R–Γ) procedure of thermoluminescence (TL) dating of unheated sediments has been applied to the 2–4 μm sized feldspar-dominated grains from two components of a 10–11 ka BP glaciolacustrine silt. A previous TL study of the 4–11 μm sized feldspars from a ~12 cm thick summer layer of this varved deposit did not produce the correct age. Here it is shown that the 2–4 μm feldspar grains from a contiguous, 5 mm thick clay-rich winter varve give a satisfactory TL apparent age of 14.2 ± 2.3 ka. On the other hand, and consistent with the previous results, the 2–4 μm feldspar grains from the thicker summer layer yield an incorrect high apparent age of 55 ± 13 ka. These results have implications for general TL dating of waterlaid sediments.
22 citations
TL;DR: Sediment collected in traps in a newly formed lake in the blast-impact area at Mount St. Helens recorded a sediment yield that was about two orders of magnitude greater than for comparable basins with vegetation and similar precipitation as mentioned in this paper.
Abstract: Sediment collected in traps in a newly formed lake in the blast-impact area at Mount St. Helens recorded a sediment yield that is about two orders of magnitude greater than for comparable basins with vegetation and similar precipitation. Most sediment was mobilized by storms and runoff at the onset of the wet season. The sedimentation response to strongly seasonal precipitation, in the absence of vegetation, produced turbidites and graded annual couplets. The style of sedimentation suggests an alternate mechanism for the formation of long sequences of graded clastic varves.
20 citations
TL;DR: The Pichileufu-age glaciolacustrine sediments have anomalously low inclinations and show little movement of the magnetic vector over a 400 to 500 year period if rhythmites are interpreted to be varves.
Abstract: Glacial sediments and two volcanic flow units from northwestern Patagonia, Argentina were sampled for paleomagnetic measurements as part of an ongoing program to date and correlate the glacial stratigraphy. Samples taken from a large ball and pillow structure in the Anfiteatro Moraine are normal polarity and pass the fold test suggesting that the magnetization was locked in prior to the formation of the structure. Samples from Pichileufu-age glaciolacustrine sediments have anomalously low inclinations and show little movement of the magnetic vector over a 400 to 500 year period if rhythmites are interpreted to be varves. Tills in the Malleo valley, 120km north of Bariloche, that are tentatively correlated with the Pichileufu-age sediments are bracketed by normal polarity basalt flows from Pino Santo and Volcan Lanin. Anisotropy of magnetic susceptibility results from the Pichileufu and Anfiteatro Moraine sediments indicate that both units have a primary depositional fabric. A redeposition experiment of Pichileufu lake sediment suggests that it is capable of recording steep inclination geomagnetic fields. The Pichileufu sediments are interpreted to have recorded anomalous low inclination geomagnetic field behavior which is not characterized by the dramatic looping usually associated with geomagnetic excursions or polarity transitions. These observations may be explained by local dominance of a standing focus of the nondipole field for at least 400 to 500 years in the early- to mid-Brunhes normal polarity epoch.
TL;DR: The Elk Lake paleomagnetic record contributes significantly to our understanding of the recent history of the magnetic field in central North America, modifying several previous conclusions as mentioned in this paper, and provides a high-quality record of recent variations in the local magnetic field.
Abstract: Measurements of samples taken from three parallel cores from Elk Lake in northern Minnesota have been combined to produce a high-quality record of recent variations in the local magnetic field. Annual laminations in the sediment provide excellent time control for the record and evidence for a consistent depositional environment. The Elk Lake paleomagnetic record contributes significantly to our understanding of the recent history of the magnetic field in central North America, modifying several previous conclusions.
TL;DR: An analysis of the magnetic fabric and remanent magnetization of the Nottawasaga varves shows that the current directions are the same for the clay and silt layers and that the magnetic fabrics in the clay layer were not influenced by the Earth's magnetic field.
Abstract: An examination of the glacial varved clays that crop out in the banks of the Nottawasaga River, Ontario, shows that both the silt and clay layers are composed of sublaminae.An analysis of the magnetic fabric and remanent magnetization of the varves shows that the current directions are the same for the clay and silt layers and that the magnetic fabric in the clay layer was not influenced by the Earth's magnetic field. This indicates that bottom currents were active during the winter period and were of sufficient strength to align the particles of magnetite parallel to the current directions. This conclusion is not unique to the Nottawasaga varves but serves to illustrate that the conceptual model of glacial varve deposition requires modification.
TL;DR: A review of the deglaciation history of one of the large lake basins in Sweden, the Lake Siljan basin, is given in this paper, where a quantitative approach to the area is applied with respect to th...
Abstract: A review of the deglaciation history of one of the large lake basins in Sweden, the Lake Siljan basin is given. A quantitative approach to the deglaciation of the area is applied with respect to th...
TL;DR: In this article, the optical density field is used to enhance the varve signal, and the filtered series is then used to define a series of line segments representing varve boundaries.
Abstract: Varves from many environments have been used as paleoclimatic indicators. Measurement of varve thicknesses has typically been done manually, but an automated procedure would provide more accurate measurements with considerable savings in time. An automated method described herein involves digitizing an x ray of the core with a high-resolution optical densitometer. The two-dimensional optical density field is filtered to enhance the varve signal, and the filtered series is then used to define a series of line segments representing varve boundaries. The distance between adjacent boundaries is then averaged across the core to obtain the mean thickness of each layer. A section of thin laminae from near the bottom of a long core from the Gulf of California (Deep Sea Drilling Project Hole 480) is analyzed to illustrate the technique.