Fine scale distribution of mtDNA haplotypes for the springtail Gomphiocephalus hodgsoni (Collembola) corresponds to an ancient shoreline in Taylor Valley, continental Antarctica

We examined the fine scale distribution of the endemic Antarctic collembolan Gomphiocephalus hodgsoni in Taylor Valley, southern Victoria Land using the mitochondrial DNA cytochrome c oxidase I gene. We found an area of sympatry in the mid-region of the valley between two common haplotype groups (2.4% sequence divergence). The area of sympatry coincided with the extent of proglacial Lake Washburn (approx. 8,000 ya). This lake existed as a result of the damming of lower Taylor Valley by the grounding of the Ross Ice Sheet and may have acted as an isolating barrier to dispersal/gene flow. We suggest that the phylogenetic break occurring in the vicinity of the ancient shoreline may be the result of previous isolation of refugial allopatric populations, followed by recolonisation into a secondary contact zone during the Holocene.     

The Lake Superior varve stratigraphy and implications for eastern Lake Agassiz outflow from 10,700 to 8900 cal ybp (9.5-8.0 C ka)

Glaciolacustrine rhythmites within sediment cores from Lake Superior record the regional recession of the Laurentide Ice Sheet (LIS) from 10,700 to 8900 cal ybp [ca. 9.5-8.0 ¹⁴C ka]. LIS retreat from Superior opened eastern Lake Agassiz outlets so that the rhythmites reflect the combined impacts of sediment-laden meltwater and Lake Agassiz discharge. Multiple rhythmite stratigraphies, a time series analysis of the thickness measurements, and high-resolution inorganic carbonate data demonstrate that this is an annual record (varved). The varve thickness records primarily document regional ice margin dynamics; correlative thick varve sequences at 9100 cal ybp [∼ 8.1 ¹⁴C ka] and 10,400-10,200 cal ybp [∼ 9.2-9.0 ¹⁴C ka] record two periods of enhanced glaciofluvial discharge, most likely moraine formation (the Nakina and Nipigon). General varve cessation is associated with the circumvention of Lake Agassiz and glacial meltwater into Lake Ojibway at 9040 cal ybp [∼ 8.1 ¹⁴C ka], although adjacent to the inlets from Lake Nipigon, rhythmic sedimentation persisted for 200 years.Positively identifying Lake Agassiz catastrophic discharge events remains speculative but seems feasible. Following retreat of Marquette ice that had re-advanced to fill the basin, the initial influx of Lake Agassiz water is expected at around 10,600 cal ybp [∼ 9.4 ¹⁴C ka], but at this time, most of northeastern Lake Superior was covered by ice. Three sets of thick-thin varves in western Lake Superior perhaps record influxes of Lake Agassiz at around 10,630, 10,600, and 10,570 cal ybp [∼ 9.4 ¹⁴C ka]. Varve formation in Superior coincides with high lake levels in Lake Huron, suggesting that high lake levels in Huron correspond to periods of high Agassiz and/or meltwater flow into Lake Superior.     

Particulate organic and dissolved inorganic carbon stable isotopic compositions in Taylor Valley lakes,Antarctica: the effect of legacy

In perennially ice-covered lakes of Taylor Valley, Antarctica, “legacy”, a carryover of past ecosystem events, has primarily been discussed in terms of nutrient and salinity concentrations and its effect on the current ecology of the lakes. In this study, we determine how residual pools of ancient carbon affect the modern carbon abundance and character in the water columns of Lakes Fryxell, Hoare, and Bonney. We measure the stable carbon isotopic compositions and concentrations of particulate organic carbon (POC) and dissolved inorganic carbon (DIC) in the water column of these lakes over four seasons (1999–2002). These data are presented and compared with all the previously published Taylor Valley lacustrine carbon stable isotopic data. Our results show that the carbon concentrations and isotopic compositions of the upper water columns of those lakes are controlled by modern processes, while the lower water columns are controlled to varying degrees by inherited carbon pools. The water column of the west lobe of Lake Bonney is dominated by exceptionally high concentrations of DIC (55,000–75,000 μmol l⁻¹) reflecting the long period of ice-cover on this lake. The east lobe of Lake Bonney has highly enriched δ¹³C_DIC values resulting from paleo-brine evaporation effects in its bottom waters, while its high DIC concentrations provide geochemical evidence that its middle depth waters are derived from West Lake Bonney during a hydrologically connected past. Although ancient carbon is present in both Lake Hoare and Lake Fryxell, the δ¹³C_DIC values in bottom waters suggest dominance by modern primary productivity-related processes. Anaerobic methanogenesis and methanotrophy are also taking place in the lower water column of Lake Fryxell with enough methane, oxidized anaerobically, to contribute to the DIC pool. We also show how stream proximity and high flood years are only a minor influence on the carbon isotopic values of both POC and DIC. The Taylor Valley lake system is remarkably stable in both inter-lake and intra-lake carbon dynamics. Handling editor: K. Martens     

Biostratigraphy and <Superscript>14</Superscript>C dating of a lake sediment sequence on the north-west Estonian carbonaceous plateau,interpreted in terms of human impact in the surroundings

The sediment stratigraphy of a medium-sized mixotrophic lake (Ruila) situated below the highest shoreline of the Baltic Ice Lake in the West-Estonian Lowland is described. The lake is without natural inlets our outlets. The reconstruction of vegetation and land-use history based on pollen data, combined with available archaeological data and detailed ¹⁴C dating allows us to give a provisional reconstruction of the temporal and spatial pattern of natural and human induced environmental changes in north-west Estonia during the Holocene. Both radiocarbon dates derived from terrestrial macrofossil dating by accelerator mass spectrometry (AMS) and conventional dating of bulk lake sediment are discussed. The isolation of the lake basin from the Yoldia Sea took place ca. 9700 cal B. C. The Ancylus Lake transgression at ca. 8400 cal B. C. did not reach the basin, but caused a ground water rise, seen in the sediment stratigraphy of the lake. The first signs of human impact on the pollen record appear ca. 5400 cal B. C. (Late Mesolithic). The history of arable farming has been divided into three periods: 1) introduction of crop cultivation and animal husbandry (1500 cal B. C. – A. D. 500); 2) establishment of animal husbandry A. D. 500–1000) and 3) establishment of crop cultivation and intensive cattle breeding (A. D. 1000–today). Due to unfavourable eda-phic conditions the introduction of arable farming was delayed for more than 1000 years compared with elsewhere on the north coast of Esotnia, and intensity of land-use never reached the same proportion as in these areas. Received August 15, 2001 / Accepted August 5, 2002 Correspondence to: Leili Saarse     

Isotopic evolution of Glacial Lake Agassiz: New insights from cellulose and porewater isotopic archives

Significantly differing estimates of the oxygen-isotope composition of Lake Agassiz have been obtained from two co-existing isotopic archives within a sediment core originating from Montcalm, Manitoba, in the southern basin of the ancient proglacial lake. Oxygen-isotope analysis of cellulose extracted from the sediments, which originated during the Lockhart phase ∼ 11,700-11,000 ¹⁴C yr BP, suggests that phytoplankton lived in surface waters having δ¹⁸O around − 18 ± 1‰ VSMOW, substantially enriched relative to connate porewaters in the same core, which indicate bottom waters had much lower values of around − 24.5 ± 0.5‰ VSMOW. This difference may be attributable to seasonal isotopic stratification of the upper part of the water column in the 250 m-deep lake. Modern observations from analogous environments in northern Canada suggest that inflow of evaporatively enriched runoff from the large area of deglaciated terrain contributing to Lake Agassiz, possibly enhanced by evaporation from the surface of the lake itself, could readily account for sufficient seasonal ¹⁸O enrichment in the epilimnion. Sediment porewaters, in contrast, have preserved the isotopic signature of hypolimnion waters supplied by a mixture of glacial meltwater and precipitation-derived runoff from the Laurentide Ice Sheet, and lack discernable isotopic alteration by evaporation. These new estimates are combined with inferred lake water compositions from other isotopic archives to develop a speculative framework for the isotopic evolution of the lake, providing improved constraints on the probable isotopic composition of Lake Agassiz outflow over time, which has important implications for efforts to trace and model its changing discharge.     

A late glacial to present diatom record from Lake Euramoo, wet tropics of Queensland, Australia

A new diatom record from Lake Euramoo on the Atherton Tableland, north Queensland, Australia is used to assess regional climate change and variability and their links to forcing at a local to global scale. The major factor driving diatom composition in the approximately fifteen thousand-year record appears to be regional moisture availability. Patterns of diatom preservation and other indicators, particularly sediment organic content, suggest that permanent deep water formed at the site from ca. 15,000 cal. yr BP. However, between 13,800 and 11,500 cal. yr BP, there was a notable phase of lower lake levels and effective precipitation. The timing and duration of this phase does not correspond to large-scale climate phenomena such as the Antarctic Cold Reversal or the Younger Dryas and supports emerging evidence for a variable climate regime in the south-west Pacific during the late glacial transition.The Early to Mid Holocene record is one of remarkable stability with 5000 years of sustained dominance by the planktonic diatom Aulacoseira ambigua. Conversely, the Mid to Late Holocene record is marked by distinct diatom variability superimposed on a series of sustained shifts in composition. Accentuated Late Holocene climate variability may aid in explaining intensified land use in indigenous populations and also suggests that Europeans may have arrived in the landscape at the time it was most vulnerable to perturbation.     

Annually laminated lake sediments and environmental changes in Bashang Plateau, North China

Angulinuo Lake is the biggest lake on the Bashang Plateau, North China, and is 47.6 km² in area and 2-6 m in depth. A core from the inner part of Angulinuo Lake was sliced and the sediment was observed by Scanning Electronic Microscope (SEM). Annual laminations characterized by variable color and grain size were found and interpreted as recording the cyclic deposition of lacustrine clay and aeolian dust. The results of chemical analysis of coarse grains conducted by SEM-EDAX, and grain size analysis of modern aeolian dust in the ice on Angulinuo Lake, support an aeolian origin for the light coarse layers. Image analysis technique was used to calculate the size and number of coarse grains in each layer. The coarse grains were fractionated into four classes: > 42 μm, 14-42 μm, 14-4.2 μm and 1.4-4.2 μm. In general, the abundance of the four classes shows similar temporal variation patterns. Around Angulinuo Lake, the winter monsoon is strong and transports aeolian dust into the lake. When the winter monsoon is strong, the size and amount of coarse grains are expected to increase. We infer that the winter monsoon was weaker during 8430-5440 year BP, and was unstable in the later part of this period. From 5440 year BP, the winter monsoon became stronger, and then weaker from 3250 to 2490 year BP. During 2490-1170 year BP, the winter monsoon was slightly stronger, but since 1170 year BP, it has become weaker again. The changes of the winter monsoon intensity recorded in the annual laminations in Angulinuo Lake sediments correspond well to environmental changes in North China and to changes in sea level during the same period. Periods of weaker winter monsoon correspond to times of higher sea levels while the periods of stronger winter monsoon correspond to the Neoglaciation stage in China and the periods of lower sea levels.     

Transition from shallow lake to a wetland: a multi-proxy case study in Zalavári Pond,Lake Balaton,Hungary

Lake Balaton, the largest shallow lake in Central Europe, has no natural outlet, therefore, underwent water level changes during its 15,000–17,000 years of history. The lake is very sensitive to both climate changes and human impacts. Surroundings have been inhabited since the Stone Age; however, heavy human impact can be recognized during the past 6000 years. In this study, we established three different stages for and reconstructed water level changes of Lake Balaton by geochemical data, subfossil Cladocera and diatom remains in the sediments of the Zalavári Pond, a part of the Kis-Balaton wetland. In 9900–8600 cal. year BP, climate was dry, water level was low, and there was a wetland in this area. Although organic matter content was low in the sediment, the ratio of Fe/Mn was high. Between 5600 and 5000 cal. year BP, water level increased, Fe/Mn ratio shows that oxygen conditions of sediments was improved in agreement with the relatively low number of diatom remains and dense chydorid remains. About 5000 cal. year BP, water level of Lake Balaton decreased as indicated by high organic content with low carbonate and high Fe/Mn ratio in the sediments (oxygen depletion). At the bottom of this section, high Fe and S concentrations showed accumulation of pyrite (FeS₂) that is common in wetlands with very low redox potential. Low abundance of Cladocera remains together with rich and diverse diatom flora confirm the low water level hypothesis. Our data support that the water level of Lake Balaton was higher between 8600 and 5000 cal. year BP than it is at present.     

The Origin and Evolution of Lake Vähä-Pitkusta, SW Finland – A Multi-Proxy Study of a Meromictic Lake

Lake Vähä-Pitkusta is a 35-m deep meromictic kettle-hole lake in SW Finland. The topmost 15cm of the sediment stratigraphy consists of black gyttja, representing the last ca. 600years. A sediment core from the deepest point was examined with multi-proxy methods to determine the origin and evolution of the basin, the onset of the meromictic conditions and the primary factors that initiated the meromictic processes. The results indicate that Lake Vähä-Pitkusta became isolated from the Baltic basin during the Yoldia Sea stage. After initial unstable sedimentary conditions the development continued steadily until ca. 2500–2300 cal.BP when there is indication of lake level rise and/or increased surface runoff, probably resulting from a cooler and moister climate. The diatom and cladoceran results indicate only a slight rise in the trophic state following the increased erosion. Pollen evidence suggests weak anthropogenic activity in the catchment during the last 1000 years, and this had only little effect on the trophic state, which remained rather low. Thus, it is most unlikely that the trophic state was the primary reason for the origin of meromixis, although it may have contributed to it. We tentatively suggest that the primary reason was the peculiar morphometric dimensions of Lake Vähä-Pitkusta (small, deep, sheltered from winds) which made it very sensitive and that even a very slight increase in soluble electrolytes may push it over the edge into meromixation. It is possible that this did not occur during the cold/moist period around ca. 2300 cal.BP but during the Medieval Climate Anomaly. Studies on sediment geochemistry are needed to verify this assumption.     

Mid to Late Holocene palaeoenvironment of Lake Eastern Juyanze (north-western China) based on ostracods and stable isotopes

An exposed section of nearly 13 m thick was investigated in the ancient depression of Lake Eastern Juyanze, north-eastern China. The succession of lacustrine and aeolian deposits covers a period from about 5400 to 2700 cal. a BP as indicated by four radiocarbon ages of macro-plant debris. Fifteen ostracod taxa were identified of which Limnocythere inopinata and Darwinula stevensoni are by far the most abundant species. Stable isotope (O, C) data of ostracod calcite as well as ostracod species abundances and X-ray diffraction results of bulk sediment samples were used to reconstruct the history of Lake Eastern Juyanze. Highest lake levels (water depth ∼10 m) occurred between 5100 and 4100 cal. a BP and were succeeded by intermediate and low levels until about 3150 cal. a BP. Three short-term events of complete desiccation were recorded between 3150 and 2900 cal. a BP. The lake experienced relatively high levels again for some decades at about 3000 cal. a BP and for at least two centuries after 2900 cal. a BP. According to the ostracod and gastropod record and the high abundance of a large diatom (Campylodiscus clypeus) in the lake sediments oligohaline to slightly mesohaline conditions were reconstructed for most of the lake periods (4-6 g/l). Geochemical and palaeontological records of Lake Eastern Juyanze reveal the retreat of the Asian monsoon after China’s Hypsithermal culminating in climate instability at about 3000 cal. a BP.     

Perennially ice-covered Lake Hoare,Antarctica: physical environment,biology and sedimentation

Lake Hoare (77° 38 S, 162° 53 E) is a perennially ice-covered lake at the eastern end of Taylor Valley in southern Victoria Land, Antarctica. The environment of this lake is controlled by the relatively thick ice cover (3–5 m) which eliminates wind generated currents, restricts gas exchange and sediment deposition, and reduces light penetration. The ice cover is in turn largely controlled by the extreme seasonality of Antarctica and local climate. Lake Hoare and other dry valley lakes may be sensitive indicators of short term (< 100 yr) climatic and/or anthropogenic changes in the dry valleys since the onset of intensive exploration over 30 years ago. The time constants for turnover of the water column and lake ice are 50 and 10 years, respectively. The turnover time for atmospheric gases in the lake is 30–60 years. Therefore, the lake environment responds to changes on a 10–100 year timescale. Because the ice cover has a controlling influence on the lake (e.g. light penetration, gas content of water, and sediment deposition), it is probable that small changes in ice ablation, sediment loading on the ice cover, or glacial meltwater (or groundwater) inflow will affect ice cover dynamics and will have a major impact on the lake environment and biota.     

Reconstruction of palaeoecological and palaeoclimatic conditions of the Holocene in the south of the Taimyr according to an analysis of lake sediments

A sediment core from Khatanga-12 Lake (Taimyr Peninsula, Krasnoyarsk krai) has been studied. The 131.5-cm-long core covers ca. 7100 years of sedimentation. Chironomid analysis, a qualitative reconstruction of the paleoenvironment in the region, and a quantitative reconstruction of variations of the mean July air temperature and in the water depth of the lake have been performed using Northern Russia chironomid-inferred mean July temperature models (Nazarova et al., 2008, 2011, 2015). Khatanga-12 Lake was formed during the Middle Holocene warming as a result of thermokarst processes. The development of the lake ecosystem at different stages of its development was influenced by climatic and cryolithogenic factors. The Middle Holocene warming, which occurred around 7100–6250 cal. years BP, activated thermokarst processes and resulted in the formation of the lake basin. Later, between 6250 and 4500 cal. years BP, a period of cooling took place, as is proved by chironomid analysis. The bottom sediments of the lake during this period were formed by erosion processes on the lake shores. The reconstructed conditions were close to the modern after 2500 cal. years BP.     

A catastrophic meltwater flood event and the formation of the Hudson Shelf Valley

The Hudson Shelf Valley (HSV) is the largest physiographic feature on the U.S. mid-Atlantic continental shelf. The 150-km long valley is the submerged extension of the ancestral Hudson River Valley that connects to the Hudson Canyon. Unlike other incised valleys on the mid-Atlantic shelf, it has not been infilled with sediment during the Holocene. Analyses of multibeam bathymetry, acoustic backscatter intensity, and high-resolution seismic reflection profiles reveal morphologic and stratigraphic evidence for a catastrophic meltwater flood event that formed the modern HSV. The valley and its distal deposits record a discrete flood event that carved 15-m high banks, formed a 120-km² field of 3- to 6-m high bedforms, and deposited a subaqueous delta on the outer shelf. The HSV is inferred to have been carved initially by precipitation and meltwater runoff during the advance of the Laurentide Ice Sheet, and later by the drainage of early proglacial lakes through stable spillways. A flood resulting from the failure of the terminal moraine dam at the Narrows between Staten Island and Long Island, New York, allowed glacial lakes in the Hudson and Ontario basins to drain across the continental shelf. Water level changes in the Hudson River basin associated with the catastrophic drainage of glacial lakes Iroquois, Vermont, and Albany around 11,450 ¹⁴C year BP (∼ 13,350 cal BP) may have precipitated dam failure at the Narrows. This 3200 km³ discharge of freshwater entered the North Atlantic proximal to the Gulf Stream and may have affected thermohaline circulation at the onset of the Intra-Allerød Cold Period. Based on bedform characteristics and fluvial morphology in the HSV, the maximum freshwater flux during the flood event is estimated to be ∼ 0.46 Sv for a duration of ∼ 80 days.     

Geochemical processes in the Lake Fryxell Basin (Victoria Land,Antarctica)

Major ion, nutrient, transition metal, and cadmium concentrations are presented for nine meltwater streams flowing into Lake Fryxell, a permanently stratified lake with an anoxic hypolimnion in Taylor Valley, Antarctica. For the major ions, stream compositions are considered in terms of dissolution of marine-derived salts and chemical weathering of local rocks. Although Lake Fryxell has undergone significant evaporative concentration, only calcite, of the simple salts, is predicted to precipitate. Geochemical budgets indicate, however, that large quantities of K, Mg, and SO₄ have also been removed from the lake. Reverse weathering may be an important sink for K and Mg, although magnesium removal with calcium carbonate phases is also likely. Assuming constancy of composition over recent geologic time, all of the salts in the Fryxell water column could have been delivered under present flows in about three thousand years (chloride age).Comparison of nutrient concentrations in these meltwater streams with other flowing waters in the world reveals that the Fryxell streams are strikingly deficient in NO₃-N but not PO₄-P. The apparent nitrogen deficiency in Lake Fryxell itself can be attributed to the low annual stream loadings of this nutrient.Stream concentrations and loadings are also presented for Mn, Fe, Co, Ni, Cu, and Cd. Dissolved metal concentrations correlate roughly with average crustal abundances, suggesting that chemical weathering is the major source for these elements. Vertical metal profiles within Lake Fryxell itself appear to be governed by the formation of insoluble sulfide phases, or, in the case of Mn, by MnHPO₄. However, dissolved nickel levels in sulfide-bearing waters are much higher than can be explained in terms of metal-sulfide equilibria, and we suspect that significant organic complexing of this metal is occurring in the deeper waters.     

Fungal diversity and functionality are driven by soil texture in Taylor Valley,Antarctica

The McMurdo Dry Valleys surface is mainly constituted from unconsolidated permafrost. Despite the combination of cold and dry conditions, transiently wetted soils close to lake edges are hotspots of intense biological activity, that can support the surrounding soil ecosystems in such extreme environments. These soils host simple microbial communities that allow easy characterization of the parameters determining microbial establishment and diversification. Soil samples were collected close to three different glacial lakes (Lake Fryxell, Lake Hoare and Lake Joyce) located along a longitudinal gradient from the lower to the upper Taylor Valley. Fungal diversity and functionality of sampled soils were studied through ITS1 metabarcoding sequencing. The correlation between the parameters describing fungal diversity (i.e. total richness, relative richness of dominant taxonomic and functional groups, and community composition) and the edaphic physicochemical parameters (i.e. pH, moisture, C, N, P, Na⁺, K⁺, Mg²⁺ and Ca²⁺, cation exchange capacity, and soil granulometry) was assessed. The fungal communities showed low richness (48 ± 32 OTUs per sample). Their composition was highly diversified even within different sites close to the same lake. The main parameters affecting the diversity and composition of fungal communities were soil texture, in turn influencing the retention of water and nutrients, and physicochemical properties. This is of particular concern for the survival of these communities, given the expected environmental changes due to global warming.     

Detecting human bacterial contamination in Antarctic soils

The presence of non-indigenous microbial contaminants resulting from human faecal contamination of old and currently occupied base and field camp sites in South Victoria Land, Antarctica, was assessed by PCR amplification of extracted soil DNA using species-specific PCR primers. Positive controls (samples recovered from the environs of Scott Base, including the sewage outfall) gave strong signals with Escherichia coli primers whereas Clostridium clostridiiforme primers yielded a signal only with the sewage outfall sample. A comparison was made of PCR amplification results from samples from the abandoned Canada Glacier camp site, the Lake Fryxell summer camp site, the Cape Bird Adelie penguin colony and pristine sites from relatively inaccessible regions of the Taylor Valley. Results indicated a possible residual level of E. coli contamination in the abandoned Canada Glacier camp site, but no significant contamination of the currently occupied Lake Fryxell camp site. These data may provide indirect evidence for improved awareness and standards of waste handling and disposal over the past two decades of Dry Valley field research. Accepted: 25 March 2000     

Sedimentological history of Bryant Canyon area, northwest Gulf of Mexico, during the last 135 kyr (Marine Isotope Stages 1-6): A proxy record of Mississippi River discharge

Bryant and Eastern Canyons are located in northwest Gulf of Mexico, and are characterized by a complex sedimentological history related to glacioeustatic cycles, river discharges, and interactions between depositional and halokinetic processes. This study is based on detailed sedimentological analysis from forty-eight long cores from these two canyons. This paper determines the evolutionary history of the canyons and assesses the response of sedimentary processes to morphological, climatic, hydrological, and sea-level changes.During the last glaciation, the upper and middle continental slope was supplied with sediments by low density turbidity currents derived from the depositional segregation (deposition of the coarsest material in the most proximal locations) of large turbidity currents initiated on the outer shelf. The lower continental slope was supplied with sediment by westward flowing bottom currents, originated from the entrainment of the most diluted wash-load and tails of turbidity currents from the Mississippi Fan.Bryant and Eastern Canyon systems were active during the penultimate glaciation, Marine Isotope Stage (MIS) 6, and were supplied with sediments by an ancestral shelf-margin Mississippi River delta. Gravity flows transported enormous amounts of sediment to the continental slope and abyssal plain of the northwest Gulf of Mexico. The sea-level rise at MIS 5 led to confinement of river-sourced sediments to the widespread continental shelf of the northwest Gulf of Mexico, and consequently to the cessation of gravity flows. During the first 40 kyr of MIS 5, salt diapirs transformed the canyons into a network of intraslope basins.The sea level dropped to the mid-shelf during MIS 3 and 4, but never reached the shelf-break, and therefore, river-sourced sediments remained largely confined to the shelf. However, seaward sediment transportation was achieved occasionally through turbidity currents related to sediment failures, storms, and high-river discharges. Four high river discharge events have been identified during this period. The first three were centred at 37, 45, and 53 cal ka BP. The last high river discharge occurred at the end of MIS 3 (29.4-33.2 cal ka BP), and resulted in the deposition of closely-spaced, mud turbidites over the entire continental slope. The Laurentide Ice Sheet (LIS) was restricted north of the upper Mississippi River valley during 60 to ∼ 30 cal ka BP and therefore, the high river discharge events are interpreted as melt-water events, related to brief southward advancements of the LIS, which resulted in the flooding of Mississippi River. The extensive lowering of sea level during the last glacial maximum (MIS 2) resulted in the almost direct discharge of Mississippi River sediments to the upper continental slope leading to the development of abundant turbidity currents. Eleven wet-dry cycles during this period are defined; they probably originated from episodic subglacial melt-water floods, released from southern parts of the LIS.The last deglaciation event is characterized by the development of a major melt water event at 16.5-13 cal ka BP that resulted in the deposition of distinct, organic-rich sediments. At about 13 cal ka BP, the melt water discharges of the LIS in North America switched from the Mississippi River to either the St. Lawrence or Mackenzie River valleys, causing the domination of hemipelagic sedimentation on the continental slope of the northwest Gulf of Mexico. Isotopic data indicate that melt-water discharges returned to the Mississippi River Valley at ∼ 11.4 cal ka BP. The absence of any sedimentological indication on the continental slope of the northwest Gulf of Mexico of the return of the melt-water discharges to the Mississippi River is attributed to the confinement of river-sourced sediments on the continental shelf due to the rise of the sea level.     

Palaeolimnology of the last crater lake in the Eastern Carpathian Mountains: a multiproxy study of Holocene hydrological changes

A multi-proxy investigation (loss-on-ignition, major and trace elements, pollen, plant macrofossil and siliceous algae) was carried out on the sediment of a crater lake (Lake Saint Ana, 950 m a.s.l.) from the Eastern Carpathian Mountains. Diatom-based transfer functions were applied to estimate the lake’s trophic status and pH, while reconstruction of the water-depth changes was based on the plant macrofossil and diatom records. The lowest Holocene water depths were found between 9000 and 7400 calibrated BP years, when the crater was occupied by Sphagnum-bog. Significant increases in water depth were found from 5350(1), 3300(2) and 2700 cal yr BP. Of these, the first two coincided with major terrestrial vegetation changes, namely (1) the establishment of Carpinus betulus on the crater slope and (2) the replacement of the lakeshore Picea abies forest by Fagus sylvatica. The chemical record indicated significant soil changes along with the canopy changes (from coniferous to deciduous) that led to increased in-lake productivity and pH. A further increase in water depth around 2700 cal yr BP resulted in stable thermal stratification and hypolimnetic anoxia that via P-release further increased in-lake productivity and eventually led to phytoplankton blooms with large populations of Scenedesmus. High productivity was depressed by anthropogenic lakeshore forest clearances from ca. 1000 cal yr BP that led to the re-establishment of P. abies on the lakeshore and consequent acidification of the lake water. On the whole, these data suggest that Lake Saint Ana is a vulnerable ecosystem: in-lake productivity is higher under deciduous canopy and litter, and considerably repressed by coniferous canopy and litter. The lake today subsists in a managed environment that is far from its natural state. This would be a dense F. sylvatica forest supplying more nutrients and keeping up a more productive in-lake flora and fauna. Guest editors: K. Buczkó, J. Korponai, J. Padisák & S. W. Starratt Palaeolimnological Proxies as Tools of Environmental Reconstruction in Fresh Water     

Palaeolimnological analyses as information source for large lake biomonitoring

A routinely applicable palaeolimnological sampling and analysis programme for large lake monitoring was developed as part of the Lake Saimaa Biomonitoring Project (1990–1993). The scope of palaeolimnological analyses is to gain background information of the recent past (ca. 0–200 yr) of the lake ecosystem that is being monitored. We analyzed short sediment cores from three contrasting basins of the complex lake Saimaa in eastern Finland (nutrient-poor clearwater Lake Puruvesi; oligotrophic mesohumic Lake Paasivesi, and eutrophied and polluted mesohumic Lake Haukivesi). Each basin shows a unique developmental history under the various forms of human influence on the ecosystem. The following research approaches are evaluated in the study: echosounding, dating (²¹⁰Pb varves, soot stratigraphy), element stratigraphies, biological remains (diatoms, cladocera, chironomids, sedimentary pigments).