Freshwater Ostracodes in Quaternary Permafrost Deposits in the Siberian Arctic

Ostracode analysis was carried out on samples from ice-rich permafrost deposits obtained on the Bykovsky Peninsula (Laptev Sea). A composite profile was investigated that covers most of a 38-m thick permafrost sequence and corresponds to the last ca. 60 kyr of the Late Quaternary. The ostracode assemblages are similar to those known from European Quaternary lake deposits during cold stages. The ostracode habitats were small, shallow, cold, oligotrophic pools located in low centred ice wedge polygons or in small thermokarst depressions. In total, 15 taxa, representing 7 genera, were identified from 65 samples. The studied section is subdivided into six ostracode zones that correspond to Late Quaternary climatic and environmental stadial-interstadial variations established by other paleoenvironmental proxies: (1) cold and dry Zyrianian stadial (58–53 kyr BP); (2) warm and dry Karginian interstadial (48–34 kyr BP); (3) transition from the Karginian interstadial to the cold and dry Sartanian stadial (34–21 kyr BP); (4) transition from the Sartanian stadial to the warm and dry Late Pleistocene period, the Allerød (21–14 kyr BP); (5) transition from the Allerød to the warm and wet Middle Holocene (14–7 kyr BP); and (6) cool and wet Late Holocene (ca. 3 kyr BP). The abundance and diversity of the ostracodes will be used as an additional bioindicator for paleoenvironmental reconstructions of the Siberian Arctic.     

Mineral magnetic and geochemical records of late Glacial climatic change from two northwest European carbonate lakes

We present the results of mineral magnetic measurements and geochemical analyses of late Glacial sediments from two marl-precipitating lakes in the Northwest of England. Mineral magnetic assemblages dominated by detrital and/or authigenic ferrimagnetic minerals, and enhanced delivery of metal elements, characterise a lower (Oldest Dryas) and an upper (Younger Dryas) phase of catchment instability, with detrital clay and silt sedimentation. Magnetic mineral assemblages with lower concentrations of finer ferrimagnetic grains characterise the authigenic carbonate sediments (marls). The marls indicate both enhanced lake productivity and catchment stability in response to prevailing warm conditions during the Bølling - Allerød Interstadial. The Bølling - Allerød marl phase contains two short-term, low amplitude shifts characterised by changes in the concentration and the size of ferrimagnetic grains. These shifts may represent the Older Dryas and the Amphi-Atlantic Oscillation, short-lived Northern hemisphere climatic deteriorations. Overall, the results suggest that marl lakes are sensitive indicators of Lateglacial climatic change, and that these changes are readily identifiable through the use of mineral magnetic measurements.     

Diatom responses to late-glacial and early-Holocene environmental changes at Kråkenes, western Norway

A stratigraphic diatom sequence is presented for the period from 13,870-9,170 cal BP from Kråkenes Lake, western Norway. Changes in species assemblages are discussed with reference to the changing environmental conditions in the Allerød, Younger Dryas, and the early Holocene and to the development of the aquatic ecosystem. The site is sensitive to acidification, and diatom-based transfer functions are applied to estimate the past pH status. The combination of rapid sediment accumulation together with an excellent calibrated radiocarbon chronology means that the rate of inferred pH change and associated increase in [H⁺] can be assessed and compared with recent, anthropogenically acidified situations.The Allerød diatom assemblages are dominated by benthic taxa particularly Fragilaria species, indicating an unproductive, alkaline, turbid, and immature system. Diatoms are absent in the early part of the Younger Dryas, but subsequently a sparse planktonic flora develops reflecting decreased turbidity and/or increased nutrient supply. A clear sequence of diatom assemblages is seen in the early Holocene. A short-lived peak of Stephanodiscus species indicating a period of increased nutrient availability occurred at ca. 11,500 cal BP. Throughout the early Holocene, acid-tolerant species increasingly replaced less acidophilous, circumneutral taxa.The lake became slightly more acid during the Allerød, but this was statistically insignificant in a trend test involving regression of pH or [H⁺] in relation to age. Diatom-inferred pH declined rapidly during the early Holocene period investigated (9,175-11,525 cal BP) with a statistically significant overall rate of 0.024 pH units per 100 yrs. This consisted of an older (ca. 11,525-10,255 cal BP) phase, where pH fell more rapidly at up to 0.095 pH units per 100 yrs; and a younger phase after ca. 10,500 cal BP where the pH fall was extremely slow (0.008 pH units per 100 yrs) and was not statistically significant.In the Allerød a combination of low catchment productivity together with disturbance, weathering, and minerogenic inwash ensured that the base-cation status remained relatively high. In the Holocene the catchment soils stabilised and base cations were sequestered by terrestrial vegetation and soil. This resulted in reduced base-cation leaching and this, together with the production of organic acids caused the lake to acidify, reaching an equilibrium by ca. 10,000 cal BP.     

The postglacial environmental development of Raffles Sø, East Greenland: inferences from a 10,000 year diatom record

A 341 cm long sediment sequence was recovered from the unofficially named Raffles Sø on Raffles Ø, outer Scoresby Sund region, East Greenland. The sediment sequence consists in the upper part (0–230 cm) of a stratified gyttja enriched in organic carbon and biogenic silica whereas the lower core part (235–341 cm) is composed of terrigenous, consolidated glacio-limnic sediments. ¹⁴C-AMS measurements indicate that the sediment sequence represents the entire Holocene lake history from 10,030 calibrated radiocarbon years.The geochemical parameters (opal, total organic carbon (TOC), total nitrogen (TN)) and the total diatom concentration show similar developments during the Holocene, and reflect changes in biological production and nutrient input into the lake. These records clearly reveal a broad Holocene TOC-opal-maximum interval between 5200 and 1800 cal. yrs BP.The diatom flora consisted of 66 taxa representing 20 genera but only seven taxa were abundant and, sometimes, these were monospecifically dominant during the Holocene. In the sediment core from Raffles Sø four successive stratigraphical zones can be distinguished. Accumulation of diatom valves began at 9900 cal. yrs BP with a Stephanodiscus minutulus (Kütz.) Cleve and Möller dominated assemblage (stratigraphic zone 1) followed by a diatom flora dominated by Cyclotella pseudostelligera Hustedt and, less frequently, by Diatoma tenuis Agardh (9400 until 5900 cal. yrs BP, zone 2). Cyclotella sp. A, a taxon which belongs to the Cyclotella rossii-comensis-tripartita-complex, was the dominant floral element between 5200 and 1800 cal. yrs BP (zone 3). From 1800 cal. yrs BP, the periphytic taxa Fragilaria capucina var. gracilis (Østr.) Hustedt and F. capucina var. rumpens (Kütz.) Lange-Bertalot attained highest relative abundances, also almost monospecifically (zone 4).The distribution and composition of the diatom assemblages in the sediment record from Raffles Sø probably reflect past variations in the extent of the lake-ice cover during the growing season. More or less ice-free conditions during summer may have prevailed during the early Holocene until ca. 1800 cal. yrs BP, which allowed growth of planktonic diatoms (Cyclotella taxa) in the pelagic lake region. From 1800 cal. yrs BP, colder conditions lead to a perennial lake-ice cover with a small ice-free moat in summer which favored the growth of periphytic, littoral species (Fragilaria capucina varieties).     

Chironomid-inferred late-glacial and early-Holocene mean July air temperatures for Kråkenes Lake, western Norway

A chironomid data-set calibrated to July air temperatures, based on 44 lakes in western Norway, is used to reconstruct mean July air temperatures from late-glacial and early-Holocene fossil chironomid assemblages at Kråkenes Lake. The calibration function is based on Weighted Averaging Partial Least Squares regression and has a root mean square error of prediction (RMSEP) of 1.13 °C, a r² of 0.69, and a maximum bias of 2.66 °C. All these statistics are based on leave-one-out cross-validation. A calibration function based on summer surface-water temperatures has a poorer performance (RMSEP = 2.22 °C, r² = 0.30, maximum bias = 5.29 °C). The reconstructed July air temperatures at Kråkenes rise to 10.5 °C soon after deglaciation, are about 11.5 °C in the Allerød, decrease to 9.5-10 °C in the Younger Dryas, and rise rapidly within 15 yrs to 11.5 °C at the onset of the Holocene. There is a two-step rise to 13 °C or more in the early-Holocene. The likely over-estimation of Younger Dryas temperatures and under-estimation of early-Holocene temperatures probably result from the limited temperature range represented by the existing calibration set. The data set is currently being expanded to include lakes with warmer air temperatures (> 14 °C) and with colder air temperatures (< 8 °C).     

Early-Holocene geochemical evolution of saline Medicine Lake,South Dakota

Medicine Lake is a highly saline, meromictic, magnesium sulfate, closed-basin lake in northeastern South Dakota. The geochemical, mineralogical, and magnetic stratigraphies of sediments deposited from about 10.8 to 4.5 ka B.P. document the evolution of the saline brine in response to climatic change in the early to mid-Holocene. During the spruce occupation of the Medicine Lake catchment (10.8–10.0 ka B.P.), dark-grey massive basal sediments with low total-sulfur and carbonate content, upwardly increasing organic-carbon content, and high magnetic susceptibility were deposited in a deep freshwater lake. As the vegetation in the area changed from spruce to birch to oak and elm and finally to prairie between 10.0 and 9.2 ka B.P., and as the lake became shallow and salinity increased from <2 to >10%, light-and dark-grey calcareous and organic-carbon-rich banded sediments with low total-sulfur content and low magnetic susceptibility were deposited. Previous studies have shown that during the forest/prairie transition the lake then changed abruptly from fresh to saline as it lost a substantial portion of its volume. During the early prairie period (9.2–5.5 ka B.P.), alternating sections of aragonite-rich laminae and grey massive sediments with high total-sulfur content and multiple gypsum layers were deposited in a meromictic environment under conditions of fluctuating lake levels and salinity. Continued aridity during the mid-Holocene (5.5–4.5 ka B.P.) probably maintained the lake at relatively low levels and high salinity as dark-grey generally massive sediments with moderate total-sulfur, carbonate, and organic-carbon content and no measurable magnetic susceptibility were deposited.     

Late Holocene changes in ultraviolet radiation penetration recorded in an East Antarctic lake

Late Holocene changes in the ultraviolet radiation (UVR) penetration in a lake in the Larsemann Hills (East Antarctica) were reconstructed using sediment core proxies based on fossil pigments (scytonemins and its derivatives) and siliceous microfossils. The influence of changes in lake depth on the UVR proxy was excluded by applying a correction, based on the non-linear relation between modern scytonemin concentrations and lake depth in a regional reference data set, and the record of past lake depths inferred using a diatom based transfer function in the sediment core. Results showed four well-defined maxima in the UVR proxy during the last 1600–1800 years, centred around 1820–1780, 1580–1490, 790–580 and 680–440 AD. Several mechanisms may account for these observed changes in UVR penetration, including past variability in cloud cover, atmospheric turbidity, ozone column depth, snow cover on the lake ice, DOM concentrations and lake-ice thickness and transparency resulting from temperature fluctuations. Although some gaps remain in our knowledge of scytonemin production in relation to the limnology of Antarctic lakes, the results highlight the importance and potential of the sediments in these highly transparent water bodies as archives of changes in past UVR receipt at the Earth’s surface.     

Oxygen isotope analysis of diatom silica and authigenic calcite from Lake Pinarbasi, Turkey

There is increasing interest in the ¹⁸O/¹⁶O ratio of diatom silica, particularly for lakes where carbonates are absent. Here we compare the ¹⁸O/¹⁶O ratios preserved in diatom silica and authigenic calcite from an open, spring-fed, freshwater lake core from Turkey which spans marine oxygen isotope stage 3. The two sets of isotope data show contrasting trends in spite of their mutual dependence on the water ¹⁸O/¹⁶O ratio and lake-water temperature. The most likely explanation for this divergence is difference in seasonality of biological productivity mediated by the strongly continental climate of the Anatolian plateau. Diatom silica and authigenic calcite are precipitated from solutes in the lake-water at different times of the year. Diatom productivity follows a well-defined seasonal cycle, peaking first and most importantly in the spring and then in the autumn. The precipitation of calcite follows productivity by all forms of photosynthetic organisms that deplete CO₂ but in most lakes this occurs during the summer months. The ¹⁸O_calcite curve shows mean summer temperature maxima at ca. 30–35 and ca. 58 ka BP while the intervening data represent a period of relatively cool summers. The ¹⁸O_diatom curve shows bipolar results (15–20 and 29–33), which suggests that at least two discrete sources or processes contributed to the oxygen composition of the diatoms but probably involved a dilution mechanism to shift the isotopic values. The most likely source of depleted water is from snow entering the lake during the spring thaw. We infer that many authigenic calcite curves from regions with markedly seasonal climates may be temporally limited to a few summer months and that diatom silica provides complementary data on seasonally-specific water isotopic composition rather than a substitute for analyses based on carbonate.     

Solar variability and the levels of Lake Victoria,East Africa,during the last millenium

A new diatom series with 1–6 year resolution from Lake Victoria, East Africa, shows that lake level minima occurred ca. 820–760, 680–660, 640–620, 370–340, and 220–150 calendar years BP. Inferred lake levels were exceptionally high during most of the Little Ice Age (ca. 600–200 calendar years BP). Synchrony between East African high lake levels and prolonged sunspot minima during much of the last millenium may reflect solar variabilitys effects on tropical rainfall, but those relationships reversed sign ca. 200 years ago. Historical records also show that Victoria lake levels rose during every peak of the ca. 11-year sunspot cycle since the late 19th century. These findings suggest that, if these apparent tropical sun–climate associations during the last millenium were real, then they were subject to abrupt sign reversals.Electronic Supplementary Material to this article is available at .     

Late-glacial and early-Holocene Trichoptera (Insecta) from Kråkenes Lake, western Norway

Fossil Trichoptera (caddisfly) remains have been identified and quantitatively recorded in the late-glacial and early-Holocene sediments from Kråkenes Lake, western Norway. The sediment sequence was deposited between 12,300 and 8850 ¹⁴C BP, covering the Allerød, Younger Dryas, and early-Holocene periods. The first Trichoptera were recorded at 12,000 ¹⁴C BP, and during the Allerod a diverse assemblage of Limnephilidae taxa developed in the lake. By about 11,400 ¹⁴C BP the relatively thermophilous Polycentropus flavomaculatus and Limnephilus rhombicus were present, suggesting that the summer water temperature was at least 17 °C. This temperature fell by 5-8 °C at the start of the Younger Dryas, and the thermophilous taxa were replaced within 20-40 ¹⁴C yrs by Apatania spp., including the arctic-alpine A. zonella, suggesting a maximum summer water temperature of 10-12 °C. The Trichoptera assemblage was impoverished in numbers and in diversity over the next 200 yrs as the severe conditions of the Younger Dryas developed. As soon as temperatures rose and glacial meltwater and silt input ended about 700 ¹⁴C yrs later, the resident Apatania assemblage expanded immediately, within 10 yrs. About 130 yrs later, thermophilous taxa replaced Apatania, and a much more diverse assemblage than in the Allerod occupied the varied habitats made available by the development of the Holocene lake ecosystem. The 130 yr delay may have been caused by a gradual temperature increase crossing a critical threshold, or by the time taken for thermophilous taxa to migrate from their Younger Dryas refugia.     

Quantification of Holocene lake-level changes in Finnish Lapland using a cladocera – lake depth transfer model

During recent years, numerous studies dealing with Holocene lake level fluctuations have been conducted in Finnish Lapland. However, no quantification of lake level variations exists to date. Here, we applied a recently developed modern cladocera – lake depth transfer model to subfossil cladocerans analysed from three small and shallow (< 6 m) kettle-hole lakes in northwestern Finnish Lapland to provide estimates of the amplitudes of long-term lake-level changes in the region. The quantitative inferences were compared to pollen, charcoal and geochemical records from one of the study sites. The lake levels were inferred to be high during the early Holocene; they faced marked reduction up to 4–6 m in the mid-Holocene (≈7000–4000 cal yr BP), and rose again during the latter part of the Holocene. There is some indication of lowered lake levels around 1500 cal yr BP, but interpretation of such small-scale changes is hazardous due to large prediction errors in the initial cladoceran model. The overall pattern of the Holocene lake level variation generally followed the regional changes in climate humidity as reconstructed in previous studies by means of other sedimentary proxy indicators, such as pollen and oxygen isotopic compositions. We postulate that changes in winter precipitation may have had a greater influence on lake-levels than variations in summer precipitation or evaporation.     

The environmental history of a mountain lake (Lago Paione Superiore, Central Alps, Italy) for the last c. 100 years: a multidisciplinary, palaeolimnological study

A palaeoecological study of an oligotrophic alpine lake, Paione Superiore (Italy), provided a record of historical changes in water quality. Historical trends in lake acidification were reconstructed by means of calibration and regression equations from diatoms, chrysophycean scales and pigment ratios. The historical pH was inferred by using two different diatom calibration data sets, one specific to the alpine region. These pH trends, together with the record of sedimentary carbonaceous particles and chironomid remains, indicate a recent acidification of this low alkalinity lake.Concentration of total organic matter, organic carbon, nitrogen, biogenic silica (BSiO₂), chlorophyll derivatives (CD), fucoxanthin, diatom cell concentration and number of chironomid head capsules increased during the last 2–3 decades. When expressed as accumulation rates, most of these parameters tended to decrease from the past century to c. 1950, then all except P increased to the present day. A marked increase in sedimentary nitrogen may be related to atmospheric pollution and to the general increases in output of N in Europe. High C/N ratios indicate a prevailing allochthonous source of organic matter.Finally, the increase in measured air temperature from the mid-1800's appeared to be related to lake water pH before industrialization: cold periods generally led to lower pH and vice-versa. The more recent phenomenon of anthropogenic acidification has apparently decoupled this climatic-water chemistry relationship.     

Aquatic macrophyte vegetation development in Kråkenes Lake, western Norway, during the late-glacial and early-Holocene

Macrofossil analyses were carried out on the late-glacial and early-Holocene sediments of the radiocarbon-dated master core at Kråkenes Lake, western Norway, to investigate the aquatic vegetation changes. Ranunculus sect. Batrachium and Nitella were the earliest pioneers after deglaciation ca. 12,300 ¹⁴C yr BP. The Allerød vegetational succession was very slow during ca. 1000 ¹⁴C yrs in a cool climate and conditions that were similar to those above tree-line in Norway today. The rapid cooling at the start of the Younger Dryas stadial caused extensive disturbance, and with the development of an active cirque glacier in the catchment, plants and animals were almost exterminated from the lake by inflow of permanently cold and turbid water. Rising temperatures caused the glacier to melt at the end of the Younger Dryas. The biotic response to the rapid warming was immediate, with pioneer Ranunculus sect. Batrachium and Nitella expanding within 1-3 decades, closely followed by other elodeids. The lake witnessed a remarkable isoetid succession, with phases dominated by Limosella aquatica, Subularia aquatica, Elatine hydropiper, Isoetes echinospora, and, later, I. lacustris. About 800 yrs into the Holocene most of the macrophytes declined. The short-lived isoetids became extinct, but other taxa probably survived vegetatively. The reasons for this decline are unknown, but are probably related to nutrient depletion in combination with other factors. About 550 yrs later, I. lacustris and Nymphaea colonised, and a stable flora and vegetation developed. This study illustrates the large and rapid changes that occurred over the first 1400 yrs of the Holocene in the macrophyte flora and vegetation in Kråkenes Lake before stability was attained, pointing to the value of a palaeoecological study in tracing aquatic successions over time, and highlighting our lack of knowledge of the underlying ecological factors responsible for such rapid and marked changes.     

Recognition of the 1811–1812 New Madrid earthquakes in Reelfoot Lake, Tennessee sediments using pollen data

Reelfoot Lake is located within the New Madrid Seismic Zone, a region characterized by ongoing seismic activity and the locus of a series of large earthquakes (m _b >7) during 1811–1812. Coseismic uplift and subsidence from the 1811–1812 events formed the lake basin from a partially inundated alluvial bottomland forest. Lithologic, chronologic, and palynologic data from a vibracore are used here to characterize the 1811–1812 earthquake record in lacustrine sediments. The stratigraphic record consists of a poorly consolidated upper silt, an intervening 10-cm sand layer, overlying a compact lower silt. Calibrated radiocarbon age estimates on wood samples from both silt units indicate deposition during historical time (1490–1890 AD).Better age estimates were obtained by correlating pollen assemblage data from the upper and lower silt with the historical record of land-use change in the Reelfoot Lake region. Two factors resulted in changing plant distributions (and hence pollen assemblages) in Reelfoot Lake sediments: 1) altered drainage patterns of Reelfoot Creek and Bayou de Chien resulting from 1811–1812 uplift and subsidence, and 2) deforestation and subsequent cultivation beginning approximately 1850 AD. The upper silt is characterized by a oak/cedar arboreal pollen (AP) assemblage, showing a mixture of upland and alluvial bottomland AP influx from the region to the open lake basin. Non-arboreal pollen (NAP) in the upper silt shows increasing abundance of Composites, particularly ragweed pollen indicating cultivation. This unit was deposited after the 1811–1812 earthquakes. The intervening sand layer was apparently emplaced by earthquake activity, or represents colluvium derived from most recent (1811–1812) coseismic uplift of Reelfoot scarp, which forms the western margin of the lake. The lower silt is characterized by a baldcypress/cedar AP assemblage with minor percentages of other flood-tolerant AP genera, interpreted as a baldcypress-dominated bottomland forest. Pollen influx in this environment is dominated by gravity-component deposition from local sources. The NAP in the lower silt shows that ragweed is rare or absent, suggesting pre-settlement conditions and deposition prior to 1811–1812.This is the 15th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.     

Biogeochemistry of silica in Devils Lake: implications for diatom preservation

Diatom-salinity records from sediment cores have been used to construct climate records of saline-lake basins. In many cases, this has been done without thorough understanding of the preservation potential of the diatoms in the sediments through time. The purpose of this study was to determine the biogeochemistry of silica in Devils Lake and evaluate the potential effects of silica cycling on diatom preservation. During the period of record, 1867–1999, lake levels have fluctuated from 427 m above sea level in 1940 to 441.1 m above sea level in 1999. The biogeochemistry of silica in Devils Lake is dominated by internal cycling. During the early 1990s when lake levels were relatively high, about 94% of the biogenic silica (BSi) produced in Devils Lake was recycled in the water column before burial. About 42% of the BSi that was incorporated in bottom sediments was dissolved and diffused back into the lake, and the remaining 58% was buried. Therefore, the BSi accumulation rate was about 3% of the BSi assimilation rate. Generally, the results obtained from this study are similar to those obtained from studies of the biogeochemistry of silica in large oligotrophic lakes and the open ocean where most of the BSi produced is recycled in surface water. During the mid 1960s when lake levels were relatively low, BSi assimilation and water-column dissolution rates were much higher than when lake levels were high. The BSi assimilation rate was as much as three times higher during low lake levels. Even with the much higher BSi assimilation rate, the BSi accumulation rate was about three times lower because the BSi water-column dissolution rate was more than 99% of the BSi assimilation rate compared to 94% during high lake levels. Variations in the biogeochemistry of silica with lake level have important implications for paleolimnologic studies. Increased BSi water-column dissolution during decreasing lake levels may alter the diatom-salinity record by selectively removing the less resistant diatoms. Also, BSi accumulation may be proportional to the amount of silica input from tributary sources. Therefore, BSi accumulation chronologies from sediment cores may be effective records of tributary inflow.     

Palaeolimnology of the Upper Lerma Basin,Central Mexico: a record of climatic change and anthropogenic disturbance since 11 600 yr BP

The Upper Río Lerma valley, Estado de México, is a high-altitude (2575 m a.s.l.) basin floored by Quaternary alluvial, lacustrine and pyroclastic deposits. Two pits were dug in the swampy bed of the recently drained L. Chiconahuapan. Ten ¹⁴C dates have been obtained from these profiles, which consist of diatomaceous organic lake muds and peats with intercalated tephras. The oldest unit is the Upper Toluca Pumice (Tripartite Ash), dated 11 580±70 yr BP. Analyses of sediment chemistry, loss-onignition, mineral-magnetic variations and subfossil diatom assemblages provide evidence of environmental changes since this date. Alkaline ponds or freshwater lakes developed during the intervals 9000–6000, 6000–5500, 3600–1400 and 800–0 yr BP, and acidic marshes or bogs during the intervening dry episodes. An important phase of accelerated erosion, beginning around 3100 yr BP and culminating around 1400–700 yr BP, appears to have been associated with human disturbance of the basin soils.     

Wind-determined sediment distribution and Holocene sediment yield in a small,Danish, kettle lake

Sediment distribution was mapped by multiple corings in a small oligotrophic lake in northwestern Denmark. Sediment cores along a representative West-East transect were dated by ¹⁴C and correlated using pollen, mineral magnetics and general lithology. Estimates of whole-lake Holocene sediment accumulation were used to calculate sediment yield (terrestrial erosion). Results indicate that: 1) sediment yield was low 10000–5000 BP and increased strongly 5000–2500 BP and again 2500–1200 BP, 2) sediment focusing and waves and currents induced by strong winds were the major processes controlling sediment distribution throughout the Holocene; 3) the dominant wind direction of strong winds has been westerly throughout the Holocene; and 4) the lake was probably more productive in the last 5000 years than in the period from 10000 to 5000 BP.     

Rock varnish microlamination dating of late Quaternary geomorphic features in the drylands of western USA

Varnish microlamination (VML) dating is a correlative age determination technique that can be used to date and correlate various geomorphic features in deserts. In this study, we establish a generalized late Quaternary (i.e., 0–300 ka) varnish layering sequence for the drylands of western USA and tentatively correlate it with the SPECMAP oxygen isotope record. We then use this climatically correlated varnish layering sequence as a correlative dating tool to determine surface exposure ages for late Quaternary geomorphic features in the study region. VML dating of alluvial fan deposits in Death Valley of eastern California indicates that, during the mid to late Pleistocene, 5–15 ky long aggradation events occurred during either wet or dry climatic periods and that major climate shifts between glacial and interglacial conditions may be the pacemaker for alteration of major episodes of fan aggradation. During the Holocene interglacial time, however, 0.5–1 ky long brief episodes of fan deposition may be linked to short periods of relatively wet climate. VML dating of alluvial desert pavements in Death Valley and the Mojave Desert reveals that pavements can be developed rapidly (< 10 ky) during the Holocene (and probably late Pleistocene) in the arid lowlands (< 800 m msl) of these regions; but once formed, they may survive for 74–85 ky or even longer without being significantly disturbed by geomorphic processes operative at the pavement surface. Data from this study also support the currently accepted, “being born at the surface” model of desert pavement formation. VML dating of colluvial boulder deposits on the west slope of Yucca Mountain, southern Nevada, yields a minimum age of 46 ka for the emplacement of these deposits on the slope, suggesting that they were probably formed during the early phase of the last glaciation or before. These results, combined with those from our previous studies, demonstrate that VML dating has great potential to yield numerical age estimates for various late Quaternary geomorphic features in the western USA drylands.     

Late Quaternary palaeosols in the Yangtze Delta, China, and their palaeoenvironmental implications

The stiff clays beneath Holocene strata in the Yangtze Delta are interpreted as palaeosols, based on pedogenic features including illuvial argillans, voids, cracks, iron–manganese concretions and nodules, and carbonate materials. Variations in clay content with depth, the occurrence of foraminifera, and the environmental magnetism characteristics of the palaeosols suggest that their parent materials are floodplain deposits. Anisotropy of magnetic susceptibility (AMS) parameters of the sediments correlate well both with sedimentary dynamics and depositional processes in different sedimentary environments, and AMS analysis may be a useful tool for inferring sedimentary environments. Based on their considerable thickness (> 3.0 m), vertical changes of palaeosol maturity, and occurrence of ageing argillans throughout the palaeosols, the palaeosols are inferred to be compound ones (pedocomplexes) resulting from alternating deposition and pedogenesis on the palaeointerfluve of the Yangtze River. Phytoliths in the palaeosols indicate that climates turned generally from warm and wet to cold and dry with marked fluctuations during development of the palaeosols. This suggests that the palaeosols developed mainly during the marine regression prior to the last glacial maximum. The vertical distribution of manganese materials (e.g., concretions, nodules, speckles and mottles) in the palaeosols suggests that the groundwater tables were about 2.0–3.0 m beneath the upper boundary of the palaeosols during their development. Yellow–brown streaks and speckles are abundant from the middle to lower parts of the palaeosols, and layers with high CaCO₃ content occur in the strata just beneath the palaeosols, but which lack caliche, suggesting that the groundwater table fluctuated markedly during their development. Voids and cracks filled with clays, and carbonate nodules (less than 0.1 mm in diameter) with thin iron–manganese rims are abundant in the palaeosols, indicating that wet and dry seasons were marked during the palaeosol development. It is suggested that the monsoonal influence was marked in the Yangtze Delta, with both winter and summer monsoons strongly developed during the development of the late Quaternary palaeosols.     

Reconstructing air temperature at eleven remote alpine and arctic lakes in Europe from 1781 to 1997 AD

Pristine and sensitive environments, such as remote alpine and arctic lakes, are particularly susceptible to the effects of climate change. However, these remote environments do not have sufficiently long instrumental climate records to support studies on contemporary climate change. The issue of the scarcity of instrumental climate data at remote regions is addressed by reconstructing monthly mean air temperatures from 1781 to 1997 AD at eleven remote alpine and arctic lakes in Europe, as part of the MOuntain LAke Research (MOLAR) project. Stepwise multiple regression is applied to establish linear transfer functions of temperatures between each of eleven upland records and twenty homogenised long lowland records. Twelve monthly transfer functions are obtained for each lake. The skill of these transfer functions is found to range typically between 60 and 99%. The lower skill values generally correspond to winter months. The temperature reconstructions obtained using the transfer functions need to be corrected with vertical temperature gradients. Air-temperature lapse rates were obtained for each lake region by spatial interpolation of radiosonde air-temperature data (1990–1997). The resulting reconstructions at each lake were checked using air-temperature data (1996–1997) from automatic weather stations installed at the lakes during the MOLAR project. We estimate the typical reconstruction errors to be about 1.3 °C for low-sun months and about 0.98 °C for high-sun months. Trend analyses on the reconstructed annual mean air temperatures at the lakes show two distinct types of trends for the 19th and 20th centuries. During the period 1801–1900, the western European lakes show no significant trend whereas annual mean air temperatures at the eastern European lakes decrease significantly. The period 1901–1997 presents a warming trend at all but the Fennoscandian lakes. Our results are in good agreement with previous studies on the spatial distribution and magnitude of temperature change in Europe. Principal component analysis performed on the reconstructed annual mean air temperature reveals two different regimes of trends for the past two centuries. It also allows a regional clustering of the inter-annual variability of air temperature at the lakes to be identified.