Microfacies of detrital event layers deposited in Quaternary varved lake sediments of the Piànico-Sèllere Basin (northern Italy)

A Quaternary interglacial lake sediment record from the Piànico-Sèllere Basin (northern Italy) consists of biochemical calcite varves with intercalated detrital layers. At the end of the Piànico Interglacial, continuous varve formation was replaced by predominantly detrital sedimentation. However, 427 varve-years before this shift, an abrupt increase in the frequency and thickness of detrital layers occurred. Microfacies analyses reveal a total of 152 detrital layers, ranging from 0·2 to 20·15 mm in thickness, deposited during the last 896 years of the Piànico Interglacial. Three microfacies types are distinguished: (i) graded layers, (ii) non-graded silt layers, and (iii) matrix-supported layers. The position of detrital layers within an individual varve provides additional information on the season in which they have been deposited. Microfacies analyses in combination with varve counting further enabled precise varve-to-varve correlation of the detrital layers for two sediment sections cropping out ca 130 m apart. The detailed intra-basin correlation allows the source regions of detrital layers to be inferred. Moreover, micro-erosion at sub-millimetre scale has been established. Of the described facies types, only the accumulation of summer and spring graded and non-graded silt layers abruptly increased before the end of interglacial varve formation whereas non-graded winter silt and matrix-supported layers are randomly distributed over the entire study period. Heavy rainfalls are assumed to have triggered spring and summer graded layers, so that the occurrence of these layers is thought to be a proxy for extreme precipitation events in the past.     

上新世湖相纹泥及其环境信息与时间标尺记录

纹泥反映了湖相沉积物中特有的沉积韵律,是气候的季节变化和年际变化的产物。山西榆社张村地区上新世晚期的湖心相沉积中发育了极为完好的纹泥剖面。纹泥深浅相间形成清晰的纹理,分别代表夏秋和冬春。一深一浅组成一对,代表一年。纹泥层的单层厚度平均为0．12～0．25mm／条,薄者＜0．1mm／条,厚者可达0．3mm／条。暗色纹泥层的厚度大于浅色纹泥层。暗色纹泥的有机质含量丰富,木本植物孢粉占优势,同时CaO的含量低;浅色纹泥恰好相反。湖相纹泥具有建立高分辨率时间标尺和环境演化序列的特殊优越条件。     

A local clay-varve chronology and proglacial sedimentary environment in glacial Lake Peipsi, eastern Estonia

Ten cores consisting of varved clay from the northern part of Lake Peipsi in eastern Estonia have been correlated using varve thickness variations and specific marker varves into a 375-year floating varve chronology. Continuous sedimentation during gradual ice recession is concluded from a clear transition from proximal to distal varves. Cyclic variations in varve thickness are caused mainly by thickness changes of clayey winter layers. This is interpreted to indicate increased influx of finer material due to faster melting of the glacier. The cyclic pattern of thickness change is explained by alternating periods of increased and decreased melting of the ice. Simultaneous accumulation of varved clay in glacial Lake Peipsi and in the Luga and Neva basins of Russia is concluded from the good visual correlation between the mean varve thickness diagrams for the three chronologies. Because the varve chronologies from northwestern Russia have been tentatively correlated to the Swedish varve chronology, the timing of the clay accumulation in glacial Lake Peipsi is placed between c . 13 500 and 13 100 varve years BP.     

Evidence of the final drainage of the Baltic Ice Lake and the brackish phase of the Yoldia Sea in glacial varves from the Baltic Sea

A clay-varve chronology based on 14 cross-correlated varve graphs from the Baltic Sea and a mean varve thickness curve has been constructed. This chronology is correlated with the Swedish Time Scale and covers the time span 11530 to 10250 varve years BP. Two cores have been analysed for grain size, chemistry, content of diatoms and changes in colour by digital colour analysis. The final drainage of the Baltic Ice Lake is dated to c . 10800 varve years BP and registered in the cores analysed as a decrease in the content of clay. This event can be correlated with atmospheric Δ14 C content and might have resulted in an increase in these values recorded between 11565 and 11545 years BP. The results of the correlation between the varve chronology from the Baltic Sea, the Greenland GRIP ice core and the atmospheric Δ14 C record indicate that c . 760 years are missing in the Swedish Time Scale in the part younger than c. 10250 varve years BP. A change in colour from a brownish to grey varved glacial clay recorded c . 10770 varve years BP is found to be the result of oxygen deficiency due to an increase in the rate of sedimentation in the early Preboreal. The first brackish influence is recorded c . 10540 varve years BP in the northwestern Baltic Sea and some 90 years later in the eastern Gotland Basin.     

Correlation of stadial and interstadial events in the south Swedish glacial varves with the GRIP oxygen isotope record

A number of correlated varve sequences from the local varve chronology in southeastern Sweden have been selected to make a 1040 varve years long mean varve thickness curve. Pollen analyses were carried out over an interval of 373 varve years in the northern part of the study area. The pollen stratigraphical data have been divided into local pollen assemblage zones which have been correlated with the radiocarbon-dated regional pollen assemblage zones. Based on variations in herb and tree pollen content of the analysed varve sequences, it has been possible to identify well-documented lateglacial pollen zones for southern Sweden, i.e. the Bölling interstadial (GI-1e), the Older Dryas cold event (GI-1d) and the early part of the Alleröd interstadial (GI-1c). The event stratigraphy in this study, based on varying varve thicknesses and the composition of the pollen flora in the varves, has been correlated with the oxygen isotope stratigraphy of the GRIP ice-core on Greenland between 13600 and 14400 GRIP ice-core years BP. It is concluded that five decadal warm events and one centennial warm event (15–60 and 100 varve years long, respectively) occur in the clay varve record along with one centennial cold event (150 varve years long), the Older Dryas (GI-1d).     

A new varve thickness record from Allt Bhraic Achaidh Fan,middle Glen Roy,Lochaber: implications for understanding the Loch Lomond Stadial glaciolacustrine varve sedimentation trends

Glen Roy, Lochaber is a key UK site for understanding Late Devensian environmental change, as it contains an annually-resolved glaciolacustrine varve record. This paper develops our understanding of varve sedimentation within Glen Roy through the examination of a new varve sequence located in a more proximal position on the Allt Bhraic Achaidh Fan, one of a series of major fans within the valley. This new varve record consists of c. 203 annual layers, much fewer years than at other sites in the Lochaber area probably due to five significant hiatuses within the record. Varve sediment characteristics and thickness are comparable to, but not statistically correlated with, other varve series that were used to construct a consolidated varve record for the area, the Lochaber Master Varve Chronology. Sedimentological characteristics, analysed by thin section micromorphology, suggest that varve thickness changes within the basin are controlled mainly by distance from the valley sides rather than the position of the ice margin during the Loch Lomond Readvance, as previously proposed.     

Sedimentological proof and dating of the Early Holocene volcanic eruption of Ulmener Maar (Vulkaneifel,Germany)

A minerogeniclayer occurs in early postglacial organic sediments from five maar lakes (West Eifel Volcanic Field, Germany). The mineralogy and stratigraphic position of this tephra suggests that it is related to the youngest German volcano, Ulmener Maar, nearby. Radiocarbon dating of wood from the base of the Ulmener Maar Tephra at two locations provide ages in agreement with an accelerator mass spectrometer ¹⁴C date for the minerogenic layer from sediments of Lake Holzmaar situated 13 km south-west of Ulmener Maar. The mean radiocarbon age is 9 560 years BP. Dating by varve chronology provides an age of 10017 years VT (varve time in years before 1950) or 10 895 years corrected VT. Based on palynology the Ulmener Maar Tephra was deposited at the end of the Preboreal. High values of natural remnant magnetization intensity, typical of pyroclastic material, confirm that this minerogenic layer differs in composition from other clastic deposits of the sedimentary record. Geochemical analyses reveal increased values of total trace elements for the Laacher See Tephra and Ulmener Maar Tephra. An isopach map based on thickness variations of the Ulmener Maar Tephra at five investigated maar lakes indicates that the tephra was mainly transported to the south west.     

Amplitude of ENSO cycles in the Santa Barbara Basin,off California,during the past 15 000 years

Varve thickness time series from ODP Site 893 in the Santa Barbara Basin (off California) were analysed to determine variation in the strength of El Niño/Southern Oscillation (ENSO) cycles during the past 15 000 yr. Mean varve thickness and variance changed over time, with thicker varves before ～8000 yr BP indicative of wetter than modern climates. A 100‐yr running standardisation was applied to correct for non‐stationarity. The contribution of ENSO‐scale variability was then estimated as the amplitude of 3–8 year bandpassed data. Results show multidecadal‐ to centennial‐scale modulation of the amplitude. On average, however, the amplitude of ENSO scale variability remained constant throughout the past 15 000 yr. We therefore conclude that, although the expression of ENSO cycles may have changed during the Holocene, there is no indication for a significant change in amplitude of interannual variability. Copyright © 2005 John Wiley & Sons, Ltd.     

Autumn snowfall and hydroclimatic variability during the past millennium inferred from the varved sediments of meromictic Lake A, northern Ellesmere Island, Canada

We examined the hydroclimatic signal in a record of annual lamina (varve) thickness from High Arctic Lake A, Ellesmere Island (83°00.00′N, 75°30.00′W). In this unglacierized catchment, nival melt is the dominant source for meltwater and transport of sediment to the lake, and autumn snowfall is highly influential on varve thickness through the amount of snow available for melt in the following year. For the period during which climatic data are available, varve thickness in Lake A was significantly correlated (r = 0.50, p < 0.01) with the cumulative snowfall from August to October (ASO) during the previous year and, to a lesser extent, ASO mean daily temperature (r = 0.39, p < 0.01) at Alert, Nunavut (175 km east). The varve thickness record, interpreted as a proxy record of ASO snowfall and, by extension, ASO temperature, indicated above-mean conditions during five periods of the past millennium, including most of the 20th century. These results corresponded well to other available high-resolution proxy climate records from the region, with some discrepancies prior to AD 1500 and during the period AD 1700-1900.     

A 750-yr record of autumn snowfall and temperature variability and winter storminess recorded in the varved sediments of Bear Lake, Devon Island, Arctic Canada

The varve record from High Arctic, proglacial Bear Lake reveals a regionally coherent hydroclimatic signal as well as complexities due to changing hydroclimatic and limnologic conditions. Varve formation is strongly dependent on underflows that exhibit variability in strength during the past 750 yr. Periods with reduced underflow sedimentation and accumulation rates fail to produce varves in the distal part of the lake. Isolated coarse silt and sand grains occur in 80% of the varves and are interpreted to be niveo-aeolian in origin. Coarse (>500 μm) sand grains deposited on the lake ice by strong winter winds are notably less common since A.D. 1850, likely due to reduced storminess. Regression of the varve thickness record with meteorological records indicates high correlations with autumn (September and October) temperatures and total monthly snowfall. These correlations are best at times when underflow activity is sufficiently strong to produce varves throughout the lake. The close association with warmer temperatures and snow-bearing synoptic systems moving north in Baffin Bay suggests that the primary climate signal in the varves is varying autumn snow pack that controls nival discharge in the following year. The similarity between the other records of melt season temperature and sea-ice cover and the Bear Lake record suggests that summer and autumn conditions were generally similar across the Baffin Bay region through much of the last millennium.     

Differential North Atlantic control of winter hydroclimate in late Holocene varved sediments of Lake Kortejärvi,eastern Finland

Sediment microfacies, geochemical μ‐XRF and X‐ray density analyses were conducted on varved sediments from Lake Kortejärvi (eastern Finland) covering the last 2700 years. The varves comprise couplets of detrital and organic sub‐layers throughout the complete time‐span. Based on their microfacies and stratigraphical position within a varve as well as comparisons with local discharge and meteorological data, thicker detrital layers are interpreted to reflect intensified snow‐melt floods following more humid winters. Detailed comparisons with monthly to annually resolved North Atlantic Oscillation (NAO) indices back to AD 1049 (901 a BP) suggest that multidecadal increases in snow‐melt layer thickness tend to be connected with a more positive phase of the NAO and, consequently, warmer winters. In contrast, distinct centennial intervals of thicker snow‐melt layers from −40 to 170, 280 to 460 and 1900 to 2300 a BP as well as around 2600 a BP do not consistently correspond to a particular NAO phase, but coincide with extended sea‐ice margins and a colder North Atlantic climate, causing intensified and southward shifted westerly cyclones. Our results point to a differential modification of North Atlantic winter hydroclimate working on varying time scales.     

Correlation of Alaskan varve thickness with climatic parameters, and use in paleoclimatic reconstruction

The thickness of varves in the sediments of Skilak Lake, Alaska, are correlated with the mean annual temperature (r = 0.574), inversely correlated with the mean annual cumulative snowfall (r = −0.794), and not correlated with the mean annual precipitation (r = 0.202) of the southern Alaska climatological division for the years 1907–1934 A.D. Varve thickness in Skilak Lake is sensitive to annual temperature and snowfall because Skilak Glacier, the dominant source of sediment for Skilak Lake, is sensitive to these climatic parameters. Trends of varve thickness are well correlated with trends of mean annual cumulative snowfall ( ) of the southern Alaska climatological division and with trends of mean annual temperature of the southern ( ) and northern ( ) Alaska climatological divisions. Trends of varve thickness also correlate with trends of annual temperature in Seattle and North Head, Washington ( , respectively). Comparisons of trends of varve thickness with trends of annual temperature in California, Oregon, and Washington suggest no widespread regional correlation. Trends of annual snowfall in the southern Alaska climatological division and trends of annual temperature in the southern and northern Alaska climatological divisions are reconstructed for the years 1700–1906 A.D. Climatic reconstructions on the basis of varve thickness in Skilak Lake utilize equations derived from the regression of series of smoothed climatological data on series of smoothed varve thickness. Reconstruction of trends of mean annual cunulative snowfall in the southern Alaska climatological division suggests that snowfall during the 1700s and 1800s was much greater than that during the early and mid-1900s. The periods 1770–1790 and 1890–1906 show marked decreases in the mean annual snowfall. Reconstructed trends of the annual temperature of the northern and southern Alaska climatological divisions suggest that annual temperatures during the 1700s and 1800s were lower than those of the early and mid-1900s. Two periods of relatively high annual temperatures coincide with the periods of low annual snowfall thus determined.     

Varve chronology of the glacial sediments in Blekinge and northeastern Skåne, southeastern Sweden

In sections and cores from an area of the Baltic Ice Lake in Blekinge complete varve series of fine-grained glacial sediments have been found. It is possible to divide the series, from bottom to top, into four varve types. A core from Karlshamn in Blekinge shows most varves of the investigated localities, in all 355 varves. Antevs' (1915) local chronology has been used, as the most recent revision of the Swedish time scale has not yet been completed. The chronology in this investigation ranges from - 325 to + 315, or 640 years. The varve chronology and the velocity of the ice recession, c. 90 m/year in northeastern Skåne, shows good agreement with the work of Antevs, whose unpublished diagrams have been re-worked and used in this investigation.     

The timing of regional Lateglacial events and post-glacial sedimentation rates from Lake Superior

We analyze both new and previously published paleomagnetic records of secular variation (PSV) from Lake Superior sediment cores and compare these records to correlated rhythmite (varve) thickness records to determine post-glacial sedimentation rates and to reassess the termination of glaciolacustrine varves in the basin. The results suggest that offshore sedimentation rates have exhibited considerable spatial variation over the past 8000 years, particularly during the mid-Holocene. We attribute offshore, mid-Holocene sedimentation changes to alterations in whole basin circulation, perhaps precipitated by a greater dominance of the Gulf of Mexico air mass during the summer season. Nearshore bays are characterized by high sedimentation rates for at least 1000 years after varve cessation and during a period between around 4500 and 2000 cal. BP. After 2000 cal. BP, sedimentation rates subsided to earlier rates. The increases between 4500 and 2000 cal. BP are probably due to lake level fall after the Nipissing II highstand.The older glaciolacustrine varve thickness records suggest that the influx of glacially derived sediment ended abruptly everywhere in the lake, except near the Lake Nipigon inlets. Multiple sediment cores reveal 36 anomalously thick varves, previously ascribed to the formation of the Nakina moraine, which were deposited just prior to varve cessation in the open lake. The PSV records support the observation that the cessation of these thick varves is a temporally correlative event, occurring at 9035±170 cal. BP (calibrated years before 1950, ca 7950–8250 ¹⁴C BP). This date would correlate to the eastern diversion of Lake Agassiz and glacial meltwater into Lake Ojibway.     

Climate variability and glacial processes in eastern Iceland during the past 700 years based on varved lake sediments

Striberger, J., Björck, S., Ingólfsson, Ó., Kjær, K. H., Snowball, I. & Uvo, C. B. 2010: Climate variability and glacial processes in eastern Iceland during the past 700 years based on varved lake sediments. Boreas, 10.1111/j.1502‐3885.2010.00153.x. ISSN 0300‐9483. Properties of varved sediments from Lake Lögurinn in eastern Iceland and their link to climate and glacial processes of Eyjabakkajökull, an outlet glacier of the Vatnajökull icecap, were examined. A varve chronology, which covers the period AD 1262–2005, was constructed from visual observations, high‐resolution images, X‐ray density and geochemical properties determined from X‐radiography and X‐ray fluorescence scanning. Independent dating provided by ¹³⁷Cs analysis and eight historical tephras verify the varve chronology. The thickness of dark‐coloured seasonal laminae, formed mainly of coarser suspended matter from the non‐glacial river Grímsá, is positively correlated (r=0.70) with winter precipitation, and our 743‐year‐long varve series indicates that precipitation was higher and more varied during the later part of the Little Ice Age. Light‐coloured laminae thickness, controlled mainly by the amount of finer suspended matter from the glacial river Jökulsáí Fljótsdal, increased significantly during the AD 1972 surge of Eyjabakkajökull. As a consequence of the surge, the ice‐dammed Lake Háöldulón formed and recurrently drained and delivered significant amounts of rock flour to Lake Lögurinn. Based on these observations, and the recurring cyclic pattern of periods of thicker light‐coloured laminae in the sediment record, we suggest that Eyjabakkajökull has surged repeatedly during the past 743 years, but with an increased frequency during the later part of the Little Ice Age.     

Advances in understanding calcite varve formation: new insights from a dual lake monitoring approach in the southern Baltic lowlands

We revise the conceptual model of calcite varves and present, for the first time, a dual lake monitoring study in two alkaline lakes providing new insights into the seasonal sedimentation processes forming these varves. The study lakes, Tiefer See in NE Germany and Czechowskie in N Poland, have distinct morphology and bathymetry, and therefore, they are ideal to decipher local effects on seasonal deposition. The monitoring setup in both lakes is largely identical and includes instrumental observation of (i) meteorological parameters, (ii) chemical profiling of the lake water column including water sampling, and (iii) sediment trapping at both bi-weekly and monthly intervals. We then compare our monitoring data with varve micro-facies in the sediment record. One main finding is that calcite varves form complex laminae triplets rather than simple couplets as commonly thought. Sedimentation of varve sub-layers in both lakes is largely dependent on the lake mixing dynamics and results from the same seasonality, commencing with diatom blooms in spring turning into a pulse of calcite precipitation in summer and terminating with a re-suspension layer in autumn and winter, composed of calcite patches, plant fragments and benthic diatoms. Despite the common seasonal cycle, the share of each of these depositional phases in the total annual sediment yield is different between the lakes. In Lake Tiefer See calcite sedimentation has the highest yields, whereas in Lake Czechowskie, the so far underestimated re-suspension sub-layer dominates the sediment accumulation. Even in undisturbed varved sediments, re-suspended material becomes integrated in the sediment fabric and makes up an important share of calcite varves. Thus, while the biogeochemical lake cycle defines the varves’ autochthonous components and micro-facies, the physical setting plays an important role in determining the varve sub-layers’ proportion.     

A new middle Holocene varve diagram from the river Ångermanalven, northern Sweden: indications for a possible error in the Holocene varve chronology

A new varve diagram from the river Ångermanälven could be correlated to the postglacial varve chronology to between 4903 and 4415 varve years BP. An AMS ¹⁴C measurement on terrestrial macrofossils obtained between 4715 and 4706 varve years BP gave a calibrated age of between 5730 and 5040 calendar years BP. The discrepancy between varve and calender-year age indicates that an error or part of an error in the Swedish varve chronology may be situated between 2000 and 5000 varve years BP.     

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

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 Laacher See Tephra. Fifteen pollen subzones grouped in four biozones are defined by cluster analysis. After a period disturbed by microturbidites, only a part of the Bølling 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 Allerød (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 rate-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.     

Summer cooling of the Arabian Sea—a review

The summer cooling of the Arabian Sea has been reviewed in relation to the dynamic and thermodynamic processes. The differences in the quantum of cooling in the coastal regions have been attributed to the variations in the strength of coastal upwelling along the respective coasts. In the central portions of the Arabian Sea, the thickness of the surface layer and the thermal structure appear to be predominantly governed by the turbulent mixing of cold waters entrained into the surface layer in association with the deepening of current shear zone. The surface heat losses during the passage of a cyclone over the east central Arabian Sea account for only 40% of the total heat change in the surface layer while the rest is lost into the interior. Also presented are climatological mean patterns of the forcing parameters and their variations during certain years.     

A 2000-year record of solar forcing on varved lake sediment in eastern Finland

A high-resolution study was performed on varved sediments from Lake Lehmilampi in eastern Finland. Varve data was collected by digital image analysis using standard 1.8 mm thick samples impregnated in epoxy and X-rayed. Climatic variability is imprinted on varve properties (varve thickness and accumulation of mineral and organic matter) during the last 2000 years. The cumulative counting error of the varve record is estimated as 2.3%. Qualitative comparison of varve parameters and residual Δ¹⁴C constructed from tree-rings revealed close correspondence between the two records, suggesting solar forcing on lake sedimentation. Classical climatic periods of the last millennia, Medieval Climate Anomaly (1060–1280 in the varve record) and Little Ice Age (cooler phases culminating in 1340, 1465, 1545, 1680, 1850 and also in 1930 in the varve record) are clearly evident in the varve record. At present the physical link between solar activity levels and lake sedimentation has not been established.