Showing papers on "Varve published in 2000"

Recent warming in a 500-year palaeotemperature record from varved sediments, Upper Soper Lake, Baffin Island, Canada

Abstract: Laminated sediments from Upper Soper Lake on southern Baffin Island provide a new 500-year record of temperature change in the Arctic. Radiometric dating, using 210Pb and Pu, shows that the light- and dark-coloured laminae couplets are annually deposited varves. Dark laminae thickness is strongly correlated to average June temperature from Kimmirut (r = 0.82), reflecting the influence of temperature on snowmelt and fluxes of runoff and suspended sediment. This relationship allowed the construction of a palaeotemperature record that documents large-amplitude interannual to decadal variability superimposed on distinct century-scale trends, including 2°C average warming and maximum temperatures during the 1900s. Similar patterns of change are seen in individual and regionally averaged palaeotemperature records from around the circum-Arctic. Upper Soper Lake records temperatures, rates of change and variance during the twentieth century that are all anomal- ously high within the context of the last 500 years, and outside the observed range of natural variability. Comparisons of Upper Soper Lake and Arctic average palaeotemperature to proxy-records of hypothesized forcing mechanisms suggest that the recent warming trend is mostly due to anthropogenic emissions of atmos- pheric greenhouse gases. The magnitude of the warming and decade-scale variability throughout the records, however, indicate that natural forcing mechanisms such as changing solar irradiance and volcanic activity, as well as positive feedbacks within the Arctic environment, also play an important role.

Atmospheric radiocarbon calibration beyond 11,900 cal BP from Lake Suigetsu laminated sediments

Abstract: This paper presents an updated atmospheric radiocarbon calibration from annually laminated (varved) sedi- ments from Lake Suigetsu (LS), central Japan. As presented earlier, the LS varved sediments can be used to extend the radio- carbon time scale beyond the tree ring calibration range that reaches 11,900 cal BP. We have increased the density of 14C measurements for terrestrial macrofossils from the same core analyzed previously. The combined data set now consists of 333 measurements, and is compared with other calibration data.

Five centuries of interannual sediment yield and rainfall-induced erosion in the Canadian High Arctic recorded in lacustrine varves

Abstract: A 487-year sediment yield record based on the varves from Nicolay Lake, Nunavut, Canada, is used to investigate long-term yield variability. A general extreme value model, estimated with probability-weighted moments, was used to identify the magnitude and timing of the low-probability events. Exceptional sediment yields in 1951 and 1962 coincided with the two largest rainfalls on record. Smaller multiday rainfalls were also recorded in the varves as subannual rhythmites, although rhythmites were not generated by rainfall events that occurred during the nival flood or after prolonged warm weather. By accounting for probable soil moisture conditions and the timing of nival floods compared to major rainfall, rhythmites in all but 3 years of the sediment record can be explained by storms exceeding ∼13 mm total. Generally, more frequent extremes and increased variance in yield occurred during the 17th and 19th centuries, likely due to increased occurrences of cool, wet synoptic types during the coldest periods of the Little Ice Age.

Holocene regional and local vegetation history and lake-level changes in the Torneträsk area, northern Sweden

Abstract: A combination of pollen and macrofossil analyses from six lakes at altitudes between 370 and 999 m above sea level (a.s.l.) in the Tornetrask area reflect the Holocene vegetation history. The main field study area has been the Abisko valley at altitudes around 400 m a.s.l. The largest lake, Vuolep Njakajaure has annually laminated (varved) sediments. The chronology and sedimentation rates in the pollen-influx calculations are based on varve yrs in this lake and on radiocarbon dated terrestrial plant macrofossils in the other lakes. A strong increase of mountain birch (Betula pubescens ssp. tortuosa) during the early Holocene with a tree-line c. 300 m above the present, indicates that the summer temperature was c. 1.5 °C higher than today, assuming that the land uplift has been 100 m since then. Scattered stands of pine (Pinus sylvestris) may have been growing in the area immediately after the deglaciation but a forest consisting of pine and mountain birch expanded first at low elevations and reached the eastern parts of the Tornetrask area at c. 8300 cal BP and the western parts at c. 7600 cal BP. The highest pine-birch forest limit was not reached until 6300 cal BP (110 m above present pine limit). Warm and dry conditions during the pine forest maximum led to lowering of the water level documented in Lake Badsjon in the Abisko valley about 1-1.5 m lower than today. Pine and mountain birch were growing at the maximum altitude until c. 4500 cal BP. Assuming that land uplift has been in the range of 20-40 m since the mid-Holocene, this implies that the temperature was then c. 1.5-2 °C higher than today. Rising lake-levels and lowering limits of pine and mountain birch since c. 4500 cal BP indicate a more humid and cool climate during the late Holocene.

Preconditions for the formation of annually laminated lake sediments in southern and central Finland

Abstract: The geological and geomorphological settings of annually laminated sediments in Finnish boreal lakes in southern and central Finland were investigated. The purpose of the study was to identify the key sedimentary environmental factors of the morphometric features and catchment sediment types that control varve deposition in lakes. Altogether nearly 300 lakes were surveyed, of which 180 basins were cored. 35 of the cored lakes were found to contain partially laminated and 13 completely laminated sediments. Statistical analyses of the lake dataset, which also includes 15 previously located lakes composing varved sediments, show that two groups of lakes are more favourable for the formation of varves than others. The first group includes the majority of previously located varved lakes and is characterized by small-sized, relatively deep basins in glaciofluvial deposits. The second group is characterized by fairly equal proportions of till, bedrock and fine-grained sediment in the catchment, and it is more dependent on the shape of the catchment and basin itself than maximum depth and lake area. These lakes are located in the distal part of Younger Dryas end moraines or in the areas of ancient lake complexes or ice-lakes, and commonly comprise elastic-organic laminae couplets.

Land-ice teleconnections of cold climatic periods during the last Glacial/Interglacial transition

Abstract: Independent calendar year chronologies are a basic requirement for the establishment of high resolution land-ice teleconnections. The annually laminated Meerfelder Maar record provides both an independent chronology, established by varve counting, and high resolution lithological proxy data for the period of the last Glacial/Interglacial transition. These data reveal a series of four periods of climatic deterioration coinciding with negative isotopic deviations in the GRIP record signal, thus demonstrating the synchronicity of environment changes in Western Germany and temperature shifts in Greenland. The terrestrial data supports a further sub-division of the event stratigraphy based on the GRIP core, by introducing the cold event GI-1c2 between 13 500 and 13 400 calendar years BP. Multiproxy analyses reveal that the environmental response at Meerfelder Maar was not linear throughout the Lateglacial but was modified by local processes. A change in the response of the lake environment to climate deterioration was observed during sub-stage GI-1b (Gerzensee oscillation), the only event with gradual rather than abrupt transitions. The two-fold character of the Younger Dryas as seen in the GRIP record is more pronounced in the Meerfelder Maar record. This lithological signal occurred with a delay of 60 years to the GRIP signal, and has been linked to a shift in the catchment. It is proposed that the trigger for this shift was a trend towards a more humid second half of the Younger Dryas.

Sedimentation in arctic proglacial lakes: Mittivakkat Glacier, south‐east Greenland

Abstract: Several sediment cores were collected from two proglacial lakes in the vicinity of Mittivakkat Glacier, southeast Greenland, in order to determine sedimentation rates, estimate sediment yields and identify the dominant sources of the lacustrine sediment. The presence of varves in the ice-dammed Icefall Lake enabled sedimentation rates to be estimated using a combination of X-ray photography and down-core variations in 137 Cs activity. Sedimentation rates for individual cores ranged between 0.52 and 1.06 g cm ˇ2 year ˇ1 , and the average sedimentation rate was estimated to be 0.79 g cm ˇ2 year ˇ1 . Despite considerable down-core variability in annual sedimentation rates, there is no significant trend over the period 1970 to 1994. After correcting for autochthonous organic matter content and trap eAciency, the mean fine-grained minerogenic sediment yield from the 3. 8k m 2 basin contributing to the lake was estimated to be 327 t km ˇ2 year ˇ1 . Cores were also collected from the topset beds of two small deltas in Icefall Lake. The deposition of coarse-grained sediment on the delta surface was estimated to total in excess of 15 cm over the last c. 40 years. In the larger Lake Kuutuaq, which is located about 5 km from the glacier front and for which the glacier represents a smaller proportion of the contributing catchment, sedimentation rates determined for six cores collected from the centre of the lake, based on their 137 Cs depth profiles, were estimated to range between 0.05 and 0.11 g cm ˇ2 year ˇ1 , and the average was 0.08 g cm ˇ2 year ˇ1 . The longer-term (c. 100‐150 years) average sedimentation rate for one of the cores, estimated from its unsupported 210 Pb profile, was 0.10‐0.13 g cm ˇ2 year ˇ1 , suggesting that sedimentation rates in this lake have been essentially constant over the last c. 100‐150 years. The average fine-grained sediment yield from the 32. 4k m 2 catchment contributing to the lake was estimated to be 13 t km ˇ2 year ˇ1 . The 137 Cs depth profiles for cores collected from the topset beds of the delta of Lake Kuutuaq indicate that in excess of 27 cm of coarse-grained sediment had accumulated on the delta surface over the last approximately 40 years. Caesium-137 concentrations associated with the most recently deposited (uppermost) fine-grained sediment in both Icefall Lake and Lake Kuutuaq were similar to those measured in fine-grained sediment collected from steep slopes in the immediate proglacial zone, suggesting that this material, rather than contemporary glacial debris, is the most likely source of the sediment deposited in the lakes. This finding is confirmed by the 137 Cs concentrations associated with suspended sediment collected from the Mittivakkat stream, which are very similar to those for proglacial material. Copyright # 2000 John Wiley & Sons, Ltd.

Palynological analyses in the laminated sediment of Lake Holzmaar (Eifel, Germany): duration of Lateglacial and Preboreal biozones

Abstract: The laminated sediment of Lake Holzmaar (Germany) has provided a continuous varve chronology for the last 3500 varve years (vy) and beyond that a floating varve chronology back to more than 22500 vy BP. This chronology in calendar years, in combination with palynology, enables us to determine the timing. and the magnitude of Lateglacial and Early Holocene environmental changes on land (from 13838 to 10930 vy BP). The palynological diagram has a mean time resolution of 27 vy between samples. This paper establishes for the first time the biozonation for Lake Holzmaar below the Leacher see Tephra. Fifteen pollen subzones grouped in four biozones are defined by cluster analysis. After a period disturbed by microturbidites, only a parr of the Belling is present. Three cold periods have been evidenced by pollen analyses: the Older Dryas (96-vy-long), the Younger Dryas (654-vy-long) and the Rammelbeek phase (237-vy-long). The Allerod (883-vy-long) is bipartite with a first Betula-dominated period followed by a Pinus-dominated one. The Younger Dryas is also bipartite, with first a decrease of winter temperatures along with a change to a more continental climate. It is followed by a drier phase with a second decrease in temperatures. probably this time also affecting summer temperatures. The Preboreal is 702-yr-long. The duration of most phases corresponds to published records, except for that of the Younger Dryas. Cluster and rare-of-change analyses indicate a sharp change in the terrestrial vegetation assemblages that may be caused by a sedimentary hiatus of erosive origin during this cold and dry period. As a result the chronology of Holzmaar has to be revised most likely below the middle of the Younger Dryas. Comparison with the varve record of Meerfelder Maar, a neighbour maar lake, suggests adding 320 vy below 12025 vy.

AMS radiocarbon and varve chronology from the annually laminated sediment record of Lake Meerfelder Maar, Germany.

Abstract: The Holocene varve chronology of annually laminated sediment sequences from Lake Meerfelder Maar agree for most of the record with dendro-calibrated accelerator mass spectronomy radiocarbon dates from the same site. Only between 9710 and 9950 cal BP does an offset of 240 yr appear between both data sets. At this position, a micro-disturbance in the varve succession has been detected by thin section analyses and was quantified in terms of missing varves. A comparison with the nearby record from Lake Holzmaar, as well providing high resolution AMS (super 14) C and varve chronologies, revealed that such gaps (ca. 2% in time for the entire Holocene) are exceptional for these long-varved maar lake records. Moreover, since sections of missing years appear for both profiles at different stratigraphic positions, a combination of both the Meerfelder Maar and Holzmaar records enables us to bridge erroneous zones in varve chronologies. This confirms the high potential of two long-varved records in close vicinity to each other for the elimination of dating errors and for increasing chronological precision at a time resolution that is normally regarded as within the counting errors. Late Glacial varve and (super 14) C data beyond the dendro-calibration from Meerfelder Maar and their tentative tele-connections to other high resolution data sets reveal unexplained age discrepancies in the calendar year time scale of about 200 years.

A 160-k.y.-old record of El Niño–Southern Oscillation in marine production and coastal runoff from Santa Barbara Basin, California, USA

Abstract: A 160-k.y.-old laminated sediment record from the Santa Barbara Basin, California, analyzed using scanning electron microscope techniques, provides a history of interannual variability of marine production and coastal runoff. We used backscatter electron imagery to measure the components of the varve; these include a terrigenous lamina formed by seasonal runoff from winter rains together with a diatomaceous lamina that records marine production during the spring and early summer. Spectral analysis of terrigenous and diatomaceous laminae thickness reveals significant periodicities of 3.1 and 8.4 yr in the terrigenous series; these are indistinguishable, within the frequency resolution of the spectra, from significant periodicities of 3.5 and 7.6 yr in the diatomaceous series. The 3.1 and 3.5 yr periodicities record El Nino modulation of coastal runoff and marine production; the 8.4 and 7.6 yr periodicities are consistent with modulation by strong to very strong El Nino events. This is supported by the results of cross-spectral analysis of the terrigenous and diatomaceous records, which reveal inverse or antiphase relationships at 3.5 and 7.6 yr. Our work adds to a body of evidence that suggests that El Nino has been a persistent feature of late Quaternary climate variability.

Radiocarbon Calibration by Means of Varves Versus 14C Ages of Terrestrial Macrofossils from Lake Gościąż and Lake Perespilno, Poland

Abstract: This paper presents radiocarbon dates of terrestrial macrofossils from Lakes Gościąz and Perespilno, Poland. These data agree very well with most of the German pine calibration curve. In the Late Glacial, they generally agree with the data from Lake Suigetsu, Japan, and indicate constant or even increasing 14C age between 12.9 and 12.7 ka BP, rapid decline of 14C age around 12.6 ka BP, and a long plateau 10,400 14C BP around 12 ka BP. Correlation with corals and data from the Cariaco basin seems to support the concept of site-speficic, constant values of reservoir correction, in contradiction to those introduced in the INTCAL98 calibration. Around the Allerod/Younger Dryas boundary our data strongly disagree with those from the Cariaco basin, which reflects large discrepancy between calendar chronologies at that period. The older sequence from Lake Perespilno indicates two periods of rapid decline in 14C age, around 14.2 and 13.9 ka BP.

Relationship of Mn-carbonates in varved lake-sediments to catchment vegetation in Big Watab Lake, MN, USA

Abstract: Mn-carbonates are documented in the late-glacial varved sediments from Big Watab Lake, Minnesota, USA. The Mn-carbonate is authigenic and forms rims around contemporaneous epilimnetic calcite. Although such carbonates are found in minor amounts throughout the entire late-glacial sequence, significant quantities of Mn-carbonate are associated mainly with laminated intervals.Because of the suspected difference in isotopic fractionation between different carbonate minerals, the stable-isotopic compositions of bulk carbonate samples are used as a proxy for relative amounts of the Mn-carbonate in the sediment. High δ18O and low δ13C values are associated with abundant Mn-carbonates. Low δ18O and high δ13C values are associated with only minor concentrations of Mn-carbonates.The oxygen-18 record is correlated with fluctuations in the vegetation assemblage based on pollen spectra using a multiple regression model with backward elimination. The proposed link between the sedimentary archive and local vegetation is the mediation of advective mixing in the lake by forest composition. In this model, periods of forest closure resulted in a well-stratified water column that was anoxic at the sediment/water interface, permitting the formation of authigenic Mn-carbonates. Openings of Artemisia in the forest allowed wind shear to mix oxygen to depth, causing bioturbation of the laminations and preventing the formation of Mn-carbonate.

Late Saalian and Eemian deposits in the Amsterdam glacial basin

Abstract: During its maximum extension, the Saalian ice cap reached into the central Netherlands, where glacier tongues excavated over 100 m deep basins in the unconsolidated Middle and Early Pleistocene sediments. The basins are filled by relatively thick successions of Late Saalian, Eemian and Weichselian sediments. The fill of the Amsterdam glacial basin is among the best known and studied in the Netherlands. The Late Saalian sediments consist mainly of warves and ill-bedded clays and silts with, along its southern margin, influxes of sands from the surrounding ice-pushed ridges. During deposition of these sediments, the Amsterdam basin formed part of a large lake extending into the present North Sea. Draining of this lake at the end of the Late Saalian left small, shallow pools at the site of the glacial basins. Late Saalian and Eemian sediments are probably separated by a short break, although sedimentation may have been continuous in the deepest part of the basin. The Eemian deposits consist in main lines of a thin, diatom-rich sapropel at the base, overlain by an up to 30 m thick clay-rich sequence covered by a wedge of sand that measures more than 20 m in the northern part of the basin and that peters out southwards. As appears from the fauna, most of the clays were deposited in a lagoonal setting shielded behind a threshold and/or barrier. The rate of sediment supply was low so that lagoonal conditions were maintained over a long timespan. Sands derived from the surrounding ice-pushed ridges and transported by longshore drift and tidal currents formed a spit at the northern margin of the basin, which moved southward after eustatic sea-level rise stabilized and the lagoon was filled by clay. Loading of this clay-rich sequence by the spit and its washover fans induced subsidence, however, because of compaction, so that marine conditions were maintained until after the Eemian highstand. Fluvial and eolian sediments of Weichselian age, locally reaching a thickness of almost 10 m, eventually levelled the Amsterdam glacial basin.

Radiocarbon dating of varve chronologies; Soppensee and Holzmaar lakes after ten years.

Abstract: During the last decade, several radiocarbon-dated varve chronologies have been produced. The main goal at first was the extension of the 14C calibration curve beyond 10,000 BP. This paper aims to discuss varve chronologies of Soppensee and Holzmaar Lakes. Although both chronologies encountered problems, high-resolution 14C dating and relative varve time have been obtained for events during the Late Glacial.

AMS radiocarbon measurements from the Swedish varved clays.

Abstract: The Swedish varve chronology, or Swedish Time Scale, is an annual chronology based upon the successive correlation of more than 1000 varve-thickness diagrams. The Late Glacial-Early Holocene varved clays were deposited as glaciolacustrine sediments in the Baltic Sea during the recession of the Scandinavian ice sheet. Formation of varved clays continued throughout the Holocene and is still going on in the estuary of River Angermanalven in northern Sweden. Accelerator mass spectrometry (AMS) radiocarbon measurements, which have been performed on terrestrial plant macrofossils extracted from the varved clays, show--in comparison with other annual chronologies--that several hundreds of varve years are missing in the varve chronology. These findings are supported by, among others, pollen stratigraphic investigations on time-equivalent varve year intervals. If an effort were undertaken to evaluate the erroneous parts, the Swedish Time Scale would have the potential of becoming a continuous annual chronology.

Discrimination of two kinds of sedimentary laminae in maar lakes of China

Abstract: Varved lacustrine sediments have been found from high latitude to low latitude, even in tropical area. Paleoenvironmental and paleoclimatic research has increasingly relied on the use of varved sediments to provide high-resolution geochronologies and climatic reconstructions. However, a special depositional environment is necessary for varve formation and preservation. Generally, varves can be found in the sediments of maar lakes because of their special geological and hydrological environment. The preliminary research showed that a few carbonate laminations are developed in the Huguangyan maar lake, south China. They are poorly developed and not annual laminations, and may be related to periodically drought. Diatomaceous laminations are found in the maar lakes in northeast China. Diatomaceous laminations with cyclic nature corresponding with seasonal diatom blooming are the credible natural clock.

Radiocarbon in freshwater carbonates as tool of Late Quaternary studies

Abstract: Different methods of radiometric dating (C, U/Th, TL) and stable isotopes composition (C and O) in freshwater carbonates are used for analysis of sedimentologic process and reconstruction of paleoclimatic conditions as well as for stratigraphic purposes. Calcareous tufa and speleothems deposited in karst areas seem to be fightly significant indicators of the past changes because of their direct relation to the environment of deposition. Some important methodical and interpretation problems for Quaternary research in Poland were solved during the last ca. 20 years in Gliwice Radiocarbon Laboratory. Synthetic approach to C dating of calcareous tufa from Southern Poland, based on statistical analysis of correlation between lithologic type of tufaceous sediment, carbon isotope composition and apparent age was done. Results of measurements of δO and δC in tufa samples dated with C method are used to reconstruct Holocene climatic changes in southern and Eastern Poland. The stable isotope composition was used to estimate the mean annual temperatures in the interval 9500–2000 yr BP. On the basis of statistical analysis of several hundred C dates for speleothems from Krakow–Wielun Upland caves warm and cold climate periods were indicated during the last 50,000 years BP. Distribution of the dates indicate that growth of speleothems between 30 and 20 ka BP may be interpreted as reflecting changes of paleoclimatic conditions. Comparison of measured C activities in carbonate fractions of lake marl from Gościąz Lake with their varve ages allowed for accurate determination of secular changes of C dilution factor of total dissolved inorganic carbon. Basing on the geochemical model developed by Broecker and Walton it was possible to reconstruct mean lake level during the Late Glacial and Holocene. Basing on C dates of lithological boundaries in the cores from Gościąz lake basin the changes of lake level during the last 12 ka BP were reconstructed. Paleokarst forms known as “pipes” from Southwest England and South Wales were dated by C, U/Th and TL methods. C ages were obtained from carbonate cements within sandrock and on the pipe walls. Statistical analysis of dates and geochemical considerations indicate on periods of pipes pipes and pipes infillings.

Auto-correlation analysis in search of short-term patterns in varve data from sediments of Lake Gościā, Poland

Abstract: The annually laminated, or varved, sediments of Lake Gościź, Poland, cover the last c. 12900 years, from Late Allerod up to the present. We have analysed the thickness of 1912 varves in this time-series by means of auto-correlation analysis, in the hope of obtaining information on patterns in past climate, for example patterns of 11 years, which might indicate a relation with solar activity. We analysed the total varve thickness and the thickness of the summer and the winter layer. Two single-bootstrap experiments confirmed the validity of the methods in general, but they indicated that the moving-window technique with overlapping windows hampered the assessment of statistical significance. Three global significance assessment procedures, taking into account the total number of auto-correlation coefficients (ACs) that is tested, showed significance of the ACs only at a lag of one year and only for the winter layer and the total varve. This auto-correlation with the preceding year may be explained by a factor internal to the lake, for instance some depositional mechanism. We found no indication of a relation between varve formation and the sun-spot cycle.

Mineralogy of Lacustrine Sediments in the Schirmacher Range Area, Eastern Antarctica

Abstract: A mineralogical study of the lacustrine sediment cores from lakes and varve deposits of the lake shore in the Schirmacher Range of East Antarctica is presented. Fietd observations and laboratory analysis indicate quartz and feldspar as principal allogenic phases derived from local crystalline rocks surrounding the lake basin. Illite, chlorite and minor amount of mixed-layered minerals in the lacustrine sediments are interpreted to be products of weathering in the lake catchment. In general, the clay mineralogy of sediment cores and varve sediments is indicative of low-hydrolyzing depositional environment under high latitudes and subtle weathering.