Showing papers on "Varve published in 1992"

Spatial and temporal distribution of diatoms in sediments of Lake Malawi, Central Africa, and ecological implications

Abstract: Lake Malawi lies in a major rift valley in Central Africa that is some 700 m deep and 550 km long. A total of 242 cores and 111 grab samples were recovered between 1984 and 1989 and form the data base for a study of diatom distribution in this lake basin. The sediments consist of homogeneous diatomites, pelagic diatomaceous muds, varved diatomite-mud couplets, turbidites, littoral sand sheets and extensive deposits of ferro-manganous nodules. Fossil diatoms show major temporal and regional contrasts between the southern, central and northern areas of the lake. A wide variety of Aulacoseira species occur in the southern lake sediments. To the north, the Nkhotakota region is generally characterised by Stephanodiscus and Aulacoseira, with occasional diatomite laminae composed of Aulacoseira or Nitzschia. The central parts of the lake show the greatest variation, with Stephanodiscus, Nitzschia and Aulacoseira all being prominent. The northern region is dominated by Aulacoseira nyassensis throughout most core sequences. Variability in these assemblages appears to be controlled by Si:P ratios, Si concentrations, turbulence and light penetration. These factors themselves are influenced by differences in the depth and duration of lake mixing due to variations in wind strength, seiches and bottom topography among different regions of the lake and from lakewide circulation patterns.

"Varve" counting vs. tephrochronology and 137Cs and 210Pb dating: A comparative test at Skilak Lake, Alaska

Abstract: The age of recently deposited sediments in Skilak Lake has previously been estimated only by counting [open quotes]varves[close quotes]. We measured the sedimentation rate of Skilak Lake using two radionuclides, [sup 137]Cs and [sup 210]Pb, and found it to be about an order of magnitude lower than previous estimates based on varve counts. We also identified several tephras through a combination of visual inspection, core X-radiographs, observation of variations of the magnetic susceptibility, intensity of magnetization of the unconsolidated sediments, and microprobe analyses of volcanic glass shards. Tephrochronologic dates using matches with the Katmai 1912 tephra and an Augustine tephra from 500-550 yr B.P. are in good agreement with the sedimentation rate estimates based on the radioisotope data, and indicate that the rhythmite layers at Skilak Lake are not annual varves. These new estimates of sedimentation rate reaffirm that care is needed in varve dating and require that earlier work on sunspot and climate changes thought to have been recorded in the Skilak Lake sediments be reevaluated. 16 refs., 6 figs., 1 tab.

Climatic change evidence and lacustrine varves from maar lakes, Germany

Abstract: Annually laminated, non-glacial lake sediments from Lake Holzmaar (Eifel, western Germany) were investigated using large “Merkt” thin sections. The absolute age of varve intervals with variations in thickness and composition were correlated to climatic changes recorded by glacier fluctuations in the Alps. Back to 8800 years VT (varve time = varve years before 1950) glacier advances coincide with sedimentation rate minima; prior to 8800 years VT they coincide with sedimentation rate maxima. The early and middle Holocene sediments suggest a periodicity of about 1000 years for cold/warm cycles. A sequence of 512 varve-thickness measurements was subjected to spectral analysis. These provide apparent evidence for a 11-year sun-spot cycle.

Dust transport and palaeoclimate during the Oldest Dryas in Central Europe―implications from varves (Lake Constance)

Abstract: This paper evaluates evidence for seasonal loess deposits in peri-Alpine Lake Constance at the end of the last Glacial (Oldest Dryas chronozone). The usefulness of such lacustrine deposits for palaeoclimatic interpretations is discussed. The sedimentology of laminated couplets comprising yellow and grey silts has been investigated to test the hypothesis that the couplets are varves (i.e. annual rhythmites) comprising alternations of loess and glacial silt and clay. The laminae are mostly less than 1 mm thick and include from bottom to top: (1) a matrix of well-sorted, non-graded fine yellow silt with sand-size intraclasts, (2) coarsening-upward grey silt with a cap of fining-upward silt to clay. The latter is typical for varves in modern proglacial lakes and reflects summer and winter deposits (silt and clay, respectively). We propose that the lack of grading and the matrix supported fabric is indicative of aeolian transport and therefore interpret the yellow laminae as loess deposits. Volcanic glass intraclasts in the loess layers are probably derived from volcanic terrain to the west of the lake, indicating an easterly palaeowind direction. Deposition of loess in the lake occurred regularly at the beginning of each annual cycle, suggesting that the palaeowinds were associated with winter and/or spring conditions. Two transport scenarios are suggested to explain the sand grains scattered in this deep-water lacustrine record. 1. The grains may have been transported as bedload over the annual winter ice-cover of the lake under moderate wind strengths, frozen into the ice, and released for deposition during spring melt. 2. The sand grains were blown directly out onto the lake water by very strong winds during spring. The first scenario is contrary to the general view that loess was transported during summer, and that loess deposits thus reflect summer conditions only. Loess input to the lake shows a transitional decrease after ca. 14.3 kyr BP and cessation at ca. 14 kyr BP, probably as a result of a change of wind behaviour, increased humidity and/or vegetational changes during the Oldest Dryas in central Europe.

Possibilities for Reconstructing Radiocarbon Level Changes During the Late Glacial by Using a Laminated Sequence of Lake

Abstract: Laminated sediments of Gościąz Lake can be used as an independent source of material for calibrating the radiocarbon time scale. The varve chronology is based on three long cores from the deepest part of the lake, with one additional core from the second deepest part. From pollen and Cladocera spectra and stable isotope and chemical content sequences, we have determined the Allerod(AL)/Younger Dryas(YD) and Younger Dryas/Preboreal(PB) boundaries in the three long cores with relatively good accuracy, and have tentatively defined the AL/YD boundary in the fourth core. The Younger Dryas period contains at least 1520 varves, with 980 varves in fragments well replicated in all four cores. The duration of the Younger Dryas as recorded in sediments of Gościąz Lake corresponds well to the duration derived from 230Th/234U and 14C dates on Barbados corals, but disagrees with estimates from Soppensee, Lake Holzmaar and Swedish varves. Two AMS dates of terrestrial macrofossils from the PB and YD periods seem to fit both the data obtained for Swiss lake sediments and Barbados corals.

Climatic change evidence and lacustrine varves from maar lakes, Germany*

Abstract: Annually laminated, non-glacial lake sedi- ments from Lake Holzmaar (Eifel, western Germany) were investigated using large "Merkt" thin sections. The absolute age of varve intervals with variations in thickness and composition were correlated to climatic changes recorded by glacier fluctuations in the Alps. Back to 8800 years VT (varve time =varve years before 1950) glacier advances coincide with sedimentation rate minima; prior to 8800 years VT they coincide with sedi- mentation rate maxima. The early and middle Holo- cene sediments suggest a periodicity of about 1000 years for cold/warm cycles. A sequence of 512 varve- thickness measurements was subjected to spectral anal- ysis. These provide apparent evidence for a l 1-year sun-spot cycle. four sites (Lake Gosciaz, Elk Lake, Lake Holzmaar, Lake Meerfelder Maar) with organic, nonglacial varves, extending from the present back to the termination of the Pleistocene, when the abrupt change from glacial to interglacial climatic conditions occurred (Dean et al. 1984; Goslar et al. 1989; Zolitschka 1989, 1990, 1991a, c). Varved sediment sequences record complex paleo- climatic signals which have been regarded as imperfect meteorological time series (Anderson and Koopmans 1963). This paper reports qualitative and quantitative evidence of climatic changes during the last 12900 years using data on sedimentation rate and varve com- position in combination with proxy-data of non-lacus- trine origin.