Paleolimnology and recent environmental change in Lake Baikal: an introduction and overview of interrelated concurrent studies

Recent environmental change research in Lake Baikal is introduced together with an overview of several interrelated papers published concurrently in this issue of Journal of Paleolimnology. Five themes are tackled by analysis of recent Baikal sediment cores, dating, geochemistry, particulate pollutants, magnetism and diatoms. The concurrent papers focus on the first four themes in some detail and summary results of diatom analysis (from Mackay et al., 1998) are given here. Taken together these studies provide a time-space framework for recent environmental change in Lake Baikal not previously available.There are significant shifts in species composition of the endemic planktonic diatom assemblages in uppermost sediments collected from throughout the lake. However, these changes usually precede the sediment record of low level but widespread contamination by industrial products. The most clear sign of industrial contamination is the presence of particles from fossil fuel combustion in sediment post dating the 1930s.Although evidence for widespread biostratigraphic changes by pollution is lacking, radionuclide, diatom, lithostratigraphic and magnetic stratigraphies indicate two main features, (i) it is possible to make stratigraphic correlations within and between basins using recent sediment cores, (ii) that turbidite deposits, from several to tens of cm thick, are frequently encountered in recent sediments.Turbidite deposits occur in ²¹⁰Pb dated and pre-²¹⁰Pb sediment core sections and are undoubtedly a major macro-disturbance feature in many deep water locations in Lake Baikal. If profiles are to be used as direct proxy records of climate variability, then screening of cores for turbidites is a pre-requisite for quality assurance in future paleoenvironmental studies.On-going international research including Swiss, Russian and British joint paleoenvironmental studies on the distribution and biological formation of recent sediments will hopefully lead to better interpretation of Holocene and pre-Holocene sediment records in Lake Baikal.     

Within-basin profile variability and cross-correlation of lake sediment cores using the spheroidal carbonaceous particle record

Spheroidal carbonaceous particles (SCPs) from the high temperature combustion of fossil-fuels are stored in lake sediments and provide an unambiguous record of industrially derived atmospheric contamination. It has been assumed that a single sediment core provides an accurate representation of SCP accumulation but to date there have been no studies to determine within-lake variability of the SCP sediment record. This paper describes the SCP profiles of ten sediment cores taken from the deep water area of Loch Coire nan Arr, a remote lake in north-west Scotland, UK. Although each core shows the basic SCP profile used for sediment dating in the UK there is considerable variation between cores. The conversion of SCP concentrations to cumulative percentages resolves a great deal of this variation with the result that more accurate cross-correlation and hence date allocation is possible, especially in the post-1945 section of the cores. However, significant departures from the usual SCP profile still reduce the effectiveness of this approach. It is concluded that: (i) SCP profiles from single cores from the deepest areas of the lake usually provide an accurate representation of the historical record of atmospherically deposited pollutants and that inter-core variability or noise does not affect the temporal interpretation of that profile, and (ii) comparisons of total SCP inventories are a better way of comparing historical deposition between sites as the impact of temporary variability is considerably reduced.     

Environmental records of carbonaceous fly-ash particles from fossil-fuel combustion

Analysis of fly-ash particles in lake sediments has become increasingly important in studies of environmental pollution and lake acidification history. Most fly-ash studies have concerned black spheroidal carbonaceous particles (SCP)(>5 m) produced from oil and coal combustion. This review paper provides a summary of this technique and its application, and focusses on our investigations in Sweden between 1979 and 1993. It consists of five parts: i) preparation and analysis methods, ii) historical trends in atmospheric deposition, iii) geographical surveys of atmospheric deposition, iv) sediment dating, and v) studies of sedimentation processes in lakes. Methods for preparation and analyses of SCP have been developed and applied to investigations using sediment, soil and snow samples. Stratigraphic trends of SCP concentrations in lake-sediment cores reflect the consumption history of fossil fuels. A characteristic temporal SCP pattern, with a marked concentration increase beginning after the 1940's and a peak in the early 1970's, has been recognized in most Swedish lakes and elsewhere in Europe. A survey of SCP concentrations in surface sediments of >100 lakes covering Sweden demonstrated that polluted areas in southern Sweden had >100 times higher SCP concentrations than clean areas in the north. The spatial distribution of SCP over Sweden is similar to the deposition pattern of long-range transported airborne pollutants, such as excess sulphate monitored by network stations. SCP also accumulate in soils, and soil analyses can be used for determining the integrated historical deposition of SCP at the local or regional scale. Finally, SCP have been used for indirect dating of sediment cores and as a marker to assess sediment distribution patterns within lake basins.     

Historical changes in sediments of Pyramid Lake,Nevada, USA: consequences of changes in the water balance of a terminal desert lake

Sediment cores from the shallow and deep basins of Pyramid Lake, Nevada, revealed variations in composition with depth reflecting changes in lake level, river inflow, and lake productivity. Recent sediments from the period of historical record indicate: (1) CaCO₃ and organic content of sediment in the shallow basin decrease at lower lake level, (2) CaCO₃ content of deep basin sediments increases when lake level decreases rapidly, and (3) the inorganic P content of sediments increases with decreasing lake volume. Variations in sediment composition also indicate several periods for which productivity in Pyramid Lake may have been elevated over the past 1000 years. Our data provide strong evidence for increased productivity during the first half of the 20th Century, although the typical pattern for cultural eutrophication was not observed. The organic content of sediments also suggests periods of increased productivity in the lake prior to the discovery and development of the region by white settlers. Indeed, a broad peak in organic fractions during the 1800's originates as an increase starting around 1600. However, periods of changing organic content of sediments also correspond to periods when inflow to the lake was probably at extremes (e.g. drought or flood) indicating that fluctuations in river inflow may be an important factor affecting sediment composition in Pyramid Lake.     

A paleoclimate record for the past 250,000 years from Summer Lake, Oregon, USA. 1. chronology and magnetic proxies for lake level

This study presents the age control and environmental magnetism components of a new, late Pleistocene paleoclimate record for the Great Basin of western North America. Two new cores from the Summer Lake sub-basin of pluvial Lake Chewaucan, Oregon, USA are correlated to basin margin outcrops on the basis of tephrochronology, lithostratigraphy, sediment magnetism and paleomagnetic secular variation. Eleven tephra layers were found in the cores that correlate to tephra identified previously in the outcrop. The Olema ash was also found in one of the cores; its stratigraphic position, relative to 3 dated tephra layers, indicates that its age is 50-55 ka, somewhat younger than has been previously reported. The Summer Lake sediments are divided into deep and shallow lake lithosomes based on sedimentary features. The stratigraphic position of these lithosomes support the tephra-based correlations between the outcrop and the cores. These sediments contain a well resolved record of the Mono Lake Excursion (MLE) and an earlier paleomagnetic excursion as well as a high quality replication of the paleosecular variation immediately above the MLE.Relative sedimentation rates increased dramatically toward the depocenter during intervals of low-lake level. In contrast, during intervals of high-lake level, relative sedimentation rates were comparable along the basin axis from the basin margin to the depocenter. The magnetic mineralogy of the Summer Lake sediments is dominated by pseudo-single domain (titano)magnetite and intervals of high/low magnetite concentration coincide with lithosomes that indicate high/low lake levels. Magnetic grain size also varies in accord with bulk sediment grain size as indicated by the silt/clay ratio. To a first order, variations in magnetic parameters, especially those attributable to the concentration of magnetic minerals, correlate well with global glacial/interglacial oscillations as indicated by marine oxygen isotope stages. This relationship can be explained by increased dissolution of (titano)magnetite minerals as lake level dropped and the lake became more productive biologically. This inference is supported by a correspondence between lower concentrations of magnetite with higher levels of total organic carbon and vice-versa.     

A synthesis of post-glacial diatom records from Lake Baikal

The biostratigraphy of fossil diatoms contributes important chronologic, paleolimnologic, and paleoclimatic information from Lake Baikal in southeastern Siberia. Diatoms are the dominant and best preserved microfossils in the sediments, and distinctive assemblages and species provide inter-core correlations throughout the basin at millennial to centennial scales, in both high and low sedimentation-rate environments. Distributions of unique species, once dated by radiocarbon, allow diatoms to be used as dating tools for the Holocene history of the lake. Diatom, pollen, and organic geochemical records from site 305, at the foot of the Selenga Delta, provide a history of paleolimnologic and paleoclimatic changes from the late glacial (15 ka) through the Holocene. Before 14 ka diatoms were very rare, probably because excessive turbidity from glacial meltwater entering the lake impeded productivity. Between 14 and 12 ka, lake productivity increased, perhaps as strong winds promoted deep mixing and nutrient regeneration. Pollen evidence suggests a cold shrub — steppe landscape dominated the central Baikal depression at this time. As summer insolation increased, conifers replaced steppe taxa, but diatom productivity declined between 11 and 9 ka perhaps as a result of increased summer turbidity resulting from violent storm runoff entering the lake via short, steep drainages. After 8 ka, drier, but more continental climates prevailed, and the modern diatom flora of Lake Baikal came to prominence. On Academician Ridge, a site of slow sedimentation rates, Holocene diatom assemblages at the top of 10-m cores reappear at deeper levels suggesting that such cores record at least two previous interglacial (or interstadial?) periods. Nevertheless, distinctive species that developed prior to the last glacial period indicate that the dynamics of nutrient cycling in Baikal and the responsible regional climatic environments were not entirely analogous to Holocene conditions. During glacial periods, the deep basin sediments of Lake Baikal are dominated by rapidly deposited clastics entering from large rivers with possibly glaciated headwaters. On the sublacustrine Academician Ridge (depth = 300 m), however, detailed analysis of the diatom biostratigraphy indicates that diastems (hiatuses of minor duration) and (or) highly variable rates of accumulation complicate paleolimnologic and paleoclimatic reconstructions from these records.     

Identifying sedimentation patterns in Lake Baikal using whole core and surface scanning magnetic suceptibility

Forty seven ca. 1 m sediment cores were collected from Lake Baikal during a summer cruise in 1996 and analysed for whole-core susceptibility. Fifteen of these cores were further analysed using a new prototype surface scanning sensor on board the ship R.V. Vereshchagin. The main purpose of this paper is to show that the measurement of Lake Baikal short cores using two susceptibility sensors gives valuable field data and can be used as a tool for identifying undisturbed sediment sequences. Four coring transects were sampled to identify sedimentation patterns reaching from the shelves and sub-basins of the near lake shore and across mainly the northern basin of Lake Baikal (water depth ca. 1500 m). Also in the sub-basins and in the southern basin other groups of cores were taken. One of the main sediment features of interest is that of turbidite sedimentation. Whole core magnetic susceptibility traces are used to identify turbidite fingerprints and correlate them between cores along the transects. The results from the two magnetic susceptibility sensors the whole-core sensor and the new prototype surface scanning sensor, both giving volume Kappa values, are compared and are found to be significantly correlated given the difference in resolution. The whole-core sensor gives a smoothed equivalent to a moving average curve of magnetic susceptibility while the surface scanner can give fine resolution (ca. 2 mm) results picking out fine peaks with Kappa values of between 150 to 650.The results show that most turbidite sedimentation can be clearly identified; they give a specific magnetic susceptibility fingerprint with larger Kappa values (up to 120) at the base of the turbidite corresponding with the settling of coarser sandy sediments and a steady and gradual decline in values to about 15 at the top of the turbidite where the fines settle incorporating the normal diatomaceous sedimentation. The main control on the magnetic susceptibility of the turbidite sediments is the concentration of ferrimagnetic minerals in different particle size fractions. The turbidites can be correlated between many of the cores collected along the transects but it must be noted that these correlations are partly speculative and will be confirmed with future dating, diatom analysis and geochemistry. Other very fine peaks of less than 5 mm in width identified using the surface scanning sensor may indicate concentrations of ferrimagnetic minerals, namely greigite, formed during the reduction phase.     

The sedimentary record of spheroidal carbonaceous fly-ash particles in shallow Danish lakes

Spheroidal carbonaceous particles (SCP) from combustion of oil and coal have been quantified in cores from top sediments of Danish shallow lakes. Chronologies were provided by ²¹⁰Pb-dating supplemented by measurements of other radionuclides (¹³⁷Cs, ¹³⁴Cs, ²⁴¹Am). All cores show indications of sediment mixing but most still retain a characteristic SCP record. Deposition rates of SCP were low until the 1920s, increased strongly after World War II, reached maximum levels around 1970 and decreased thereafter. These results were used to infer a new chronology for a sediment core from a shallow lake, from which a ²¹⁰Pb chronology had previously been published.SCP inventories are positively correlated with ²¹⁰Pb inventories and dry matter accumulated since 1900, suggesting that absolute SCP accumulation rates may be more influenced by sedimentary processes than by atmospheric fluxes. The concentrations of SCP in surface sediments of the six lakes do, however, correlate with regional SO₂ emissions.     

Lake Winnipeg sediment physical properties: interpretation, composite stratigraphic sections and calibration of acoustic reflection profiles

High resolution sediment physical properties, measured on gravity and piston cores collected during cruises to Lake Winnipeg, include bulk density, acoustic velocity, magnetic susceptibility, shear strength and colour reflectance. The high resolution data are used here to construct complete stratigraphic (composite) sections of Lake Winnipeg sediments from a series of individual, discontinuous cores for the North and South Basins. These composite sections are used to evaluate basin-wide glacial and post-glacial depositional histories and to compare the northern and southern basin histories. In addition, these sections provide a baseline depth reference for interpretation of the biostratigraphy, paleomagnetic record and rock magnetic stratigraphy. Some of the data (density and shear strength) are also be used to estimate sediment stress history for the two major lithostratigraphic units and their variations across the basin.     

Holocene sedimentation in Lake Winnipeg, Manitoba, Canada: implications of compositional and textural variations

Two seismic facies were recognized in the sedimentary sequence overlying acoustic basement in Lake Winnipeg. The upper facies, which overlies a regional unconformity, is termed the Lake Winnipeg Sequence. Based on the seismostratigraphy, lithostratigraphy, and radiocarbon dates of approximately 4000 and 7000 yr BP from material collected directly over the unconformity in the southern and northern parts of the lake, respectively, this facies has been interpreted as representing Holocene sedimentation. Results of compositional and textural analyses of the Holocene sediment (Winnipeg sediment) from thirteen long (>2 m) cores indicate a transgressional sequence throughout the basin. In the South Basin, the generally fining upward sequence is characterized at the base by silt-sized detrital carbonate minerals, quartz and feldspar which decrease in concentration upward. In this basin, the high carbonate content and V/Al and Zn/Al ratios are indicative of a Paleozoic and Cretaceous provenance for sediment derived from glacial deposits through shoreline erosion and fluvial transport, via the Red River. Sedimentation in the central part of the lake and the North Basin is attributed to shoreline erosion of sand and gravel beaches. Consequently, the texture of these sediments is generally coarser than in the South Basin, and the composition primarily reflects a Paleozoic and Precambrian provenance. The basin-wide decrease in Ca, total carbonate minerals, dolomite and calcite concentrations upward in the cores is reflected by a decrease in the detrital carbonate component in all but the most northern cores. Other basin-wide trends show an upward increase in organic content in all cores. An increase in grain size near the top of most cores suggests a major, basin-wide change in sedimentation within the last, approximately 900 years in the South Basin.     

Southern hemispheric westerlies control the spatial distribution of modern sediments in Laguna Potrok Aike,Argentina

We studied the internal lake processes that control the spatial distribution and characteristics of modern sediments at the ICDP (International Continental Scientific Drilling Program) deep drilling site in Laguna Potrok Aike, southern Patagonia, Argentina. Sediment distribution patterns were investigated using a dense grid of 63 gravity cores taken throughout the lake basin and 40 additional shoreline samples. Analysis of the surficial sediment distribution points to distinct internal depositional dynamics induced by wind-driven lake internal currents. Distribution maps illustrate that the spatial characteristics of analysed variables are linked to high erosional wave activity. Persistent wave action and littoral erosion along all shores, especially the eastern shore, is caused by prevailing Southern Hemispheric Westerlies. Several sediment variables (grain size, benthic diatoms, total inorganic carbon and calcium) indicate re-suspension of littoral sediment followed by re-distribution to profundal accumulation areas near the eastern shore. Variations within the catchment influence sediment characteristics in the north-eastern bay. That area is characterized by different mineralogical and sedimentological conditions as well as greater accumulation of pollen, inorganic carbon and diatoms. These findings are related to the influence of episodic inflow into this bay. Spatial differences in stable isotope values throughout the lake suggest that ephemeral tributaries around the lake basin may also contribute to the detected spatial sediment variations.     

A lake sediment-based proxy of floods in the Rocky Mountain Front Ranges,Canada

We analyzed lake sediment deposits and local hydrometric records to assess the potential for developing a high-resolution record of sediment delivery from the Rock Lake catchment, situated in the non-glacierized Front Ranges of the Rocky Mountains, Canada. Rhythmic couplets of silt–clay characterized the clastic sediments recovered from the deep central basin of the lake. Contemporary sediment yield to Rock Lake (10.7 ± 1.8 Mg km⁻² year⁻¹) is comparable to other studied Canadian Cordillera lakes that have sedimentary lithologies and absence of glacier cover, but distinct rhythmic deposition is relatively unique to this basin. Spatial patterns of deposition within the lake were assessed by correlating rhythmites between multiple sediment cores and by sub-bottom, acoustic profiling. Bracketed dates for a spatially continuous sequence of eight thick rhythmites were established by correlating laminations between core samples collected more than 30 years apart. We identified a consistent pattern between the rhythmite and hydrometric data series between 1975 and 2006 and determined that specific flooding events caused by summer rainstorms are associated with each of the eight thick rhythmites. We observed a good relationship between rhythmite thickness and total flood volume that exceeded a threshold discharge. Acoustic profiling showed that the lake could be a good candidate for longer-term proxy development. We discuss how some of the methods used in this study could benefit ongoing paleoenvironmental assessments based on lacustrine rhythmite series.     

Ostracode stratigraphy and paleoecology from surficial sediments of Lake Tanganyika, Africa

We report here on the first detailed ostracode stratigraphic record to be obtained from late Holocene sediments of Lake Tanganyika. We analyzed four cores, three from the northern lake region and a fourth from a more southern lake locality, that collectively record ostracode assemblages under a variety of disturbance regimes. These cores provide a stratigraphic record of ostracode abundance and diversity, as well as depositional changes over time periods of decades to millennia. We have investigated the fossil ostracodes in these cores by looking at temporal changes of species diversity and population structure for the species present. All four cores provided distinct patterns of ostracode diversity and abundance. BUR-1, a northern lake core obtained close to the Ruisizi River delta, yielded a sparse ostracode record. Karonge #3, another northern core from a site that is closely adjacent to a river delta with high sediment loading, yielded almost no ostracodes. The third core 86-DG-14, taken from a somewhat less disturbed area of the lake, suggests that there have been recent changes in ostracode populations. Through most of the lower portion of this core, ostracode abundance is low and species richness is relatively constant. Above 7 cm there is a marked increase in ostracode abundance and a corresponding decrease in species richness, probably signaling the onset of a major community disturbance, perhaps due to human activities. The southernmost core, 86-DG-32, is from a site that is well removed from influent rivers. Ostracode abundance varies erratically throughout the core, whereas species richness is relatively constant and high throughout the core. The temporal variation evident in ostracode community makeup both within and between the studied cores may be a result of naturally patchy distributions among ostracodes, coupled with local extinctions and recolonizations, or it may reflect inadequate sampling of these high diversity assemblages. In either case, these cores illustrate the potential to obtain high resolution ostracode records from the rich, endemic fauna of Lake Tanganyika that can be used to address questions about the history of community structure and human impacts in this lake.     

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

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

A mineral magnetic and scaled-chrysophyte paleolimnological study of two northeastern Pennsylvania lakes: records of fly ash deposition, land-use change, and paleorainfall variation

A combined mineral magnetic and scaled chrysophyte study of lake sediments from Lake Lacawac and Lake Giles in northeastern Pennsylvania was conducted to determine the effects of land-use and sediment source changes on the variation of pH, conductivity, and alkalinity inferred from biotic changes. Ten 30–40 cm long gravity cores were collected from Lake Lacawac and three from Lake Giles. Isothermal remanent magnetizations (IRMs) were given to the lake sediments in a 1.3 T magnetic field to measure magnetic mineral concentration variations. IRM acquisition experiments were conducted to identify magnetic mineralogy. The bedrock, soils and a peat bog on the shores of Lake Lacawac were also sampled for magnetic analysis to determine possible lake sediment sources. The top 10 cm of sediment collected from Lakes Lacawac and Giles was two to four times more magnetic than deeper sediment. ²¹⁰Pb dating suggests that this intensity increase commenced circa 1900. SEM images of magnetic extracts from the highly magnetic sediments indicates the presence of magnetic fly ash microspheres from fossil fuel burning electric power generation plants. The similarity in magnetic coercivity in the top 8 cm lake sediments and in the peat bog supports an atmospheric source for some of the magnetic minerals in the youngest lake sediments. The highly magnetic sediments also contain an antiferromagnetic mineral in two cores closest to Lake Lacawacs southeastern shore. This magnetic mineral is only present deep in the soil profile and would suggest erosion and significant land-use changes in the Lacawac watershed as another cause for the high magnetic intensities (concentrations) in the top 10 cm of the lake sediments. The most significant changes in the scaled chrysophyte flora occurred immediately above the 10 cm level and were used to infer a doubling of the specific conductivity between circa 1910 and 1929. These variations also support land-use changes in the Lacawac catchment at this time. A similar shift in the scaled chrysophte flora was not observed in the top of Lake Giles, however, distinct changes were found in the deeper sections of the core coupled with a smaller peak in magnetic concentration. Fourier analysis of the ²¹⁰Pb-dated lake sediment magnetics indicates the presence of a 50 year period, low amplitude variation in the Lake Lacawac, Lake Giles, and Lake Waynewood (Lott et al., 1994) magnetic concentration records. After removal of the land-use/fly ash magnetic concentration peak by Gaussian filtering, the 50 year variation correlates strongly from lake to lake even though the lakes are in different watersheds separated by up to 30 km. When this magnetic variation is compared with Gaussian-filtered rainfall variations observed in New York City and Philadelphia over the past 120–250 years there is a strong correlation suggesting that magnetic concentration variations can record regional rainfall variations with an approximately 50 year period. This result indicates that magnetics could be used to document regional variations in climatic change.     

A whole-basin, mass-balance approach to paleolimnology

Lake sediments record the flux of materials (nutrients, pollutants, particulates) through a lake system both qualitatively, as changes in the composition of geochemical and biological tracers, as well as quantitatively, through changes in their rate of burial. Burial rates provide a direct link to contemporary (neo-) limnological studies as well as management efforts aimed at load reductions, but are difficult to reconstruct accurately from single cores owing to the spatial and temporal variability of sediment deposition in most lakes. The accurate determination of whole-lake burial rates from analysis of multiple cores, though requiring more effort per lake, can help resolve such problems and improve our understanding of sediment heterogeneity at multiple scales. Partial solutions to these problems also include focusing corrections based on ²¹⁰Pb flux, co-evaluation of concentration profiles, trend analysis using multiple lakes, and trend replication based on a small number of cores from the same lake. Recent multi-core studies demonstrate that no single core site faithfully records the whole-lake time-resolved input of materials, but that as few as five well-placed cores can provide a reliable record of whole-lake sediment flux for morphometrically simple basins. Lake-wide sediment fluxes can be coupled with reconstructed outflow losses to calculate historical changes in watershed and atmospheric loading of nutrients, metals, and other constituents. The ability of paleolimnology to accurately assess the sedimentary flux and extend the period of reference into the distant past represents an important contribution to the understanding of biogeochemical processes and their response to human and natural disturbance.     

Holocene environmental changes and development of Figurnoye Lake in the southern Bunger Hills, East Antarctica

The lithology, radiocarbon chronology, granulometry, geochemistry and distribution of diatoms were investigated in three sediment cores from fresh-water Figurnoye Lake in the southern Bunger Hills, East Antarctica. Our paleolimnological data provide a record of Holocene environmental changes for this region. In the early Holocene (prior to 9.0 ± 0.5 kyr BP), warm climate conditions caused intensive melting of either the floating glacier ice mass or glaciers in the immediate lake surroundings, leading to the accumulation of terrigenous clastic sediments and limiting biogenic production in the lake. From ca. 9.0 ± 0.5 to 5.5 ± 0.5 kyr BP, highly biogenic sediments dominated by benthic mosses formed, indicating more distal glaciers or snowfields. A relatively cold and dry climate during this period caused weaker lake-water circulation and, likely, occurrence of lake ice conditions were more severe than present. The distribution of marine diatoms in the cores shows that, sometime between 8 and 5 kyr BP, limited amounts of marine water episodically penetrated to the lake, requiring a relative sea-level rise exceeding 10–11 m. During the last ca. 5.5 ± 0.5 kyr BP, sedimentation of mainly biogenic matter with a dominance of laminated microbial mats occurred in the lake under warm climatic conditions, interrupted by relative coolings: the first one around 2 kyr BP and then shortly before recent time. Between ca. 5.5 and 4 kyr BP, the drainage of numerous ice-dammed lakes took place in the southern Bunger Hills and, as a result, drier landscapes have existed here from about 4 kyr BP.     

Magnetic properties of recent sediments in Lake Baikal, Siberia

Mineral magnetic measurements of six ²¹⁰Pb-dated surface cores from different basins of Lake Baikal, Siberia, show temporal records controlled by a range of internal and external processes. With the exception of sediments on the Academician Ridge, there is clear evidence for widespread reductive diagenesis effects on the ferrimagnetic component coupled with neo-formation of paramagnetic iron minerals. Greigite formation, bacterial magnetosome accumulation and turbidite layers may affect the properties of some sediment levels. Concentrations of canted antiferromagnetic minerals (eg. haematite) appear to increase from the 19th century onwards. These minerals are less affected by dissolution processes and probably represent detrital minerals delivered by catchment fluvial processes. The magnetic evidence for recent atmospheric pollution by fossil-fuel combustion processes is weak in all the cores, and supports the findings from studies of spherical carbonaceous particles (SCPs) and heavy metals that pollution is largely restricted to the southern basin. Correlations between recent sediments based on magnetic data may be insecure over long distances or between basins.     

Constraining Holocene lake levels and coastal dune activity in the Lake Michigan basin

Sediment cores collected from embayed lakes along the east-central coast of Lake Michigan are used to construct aeolian sand records of past coastal dune mobility, and to constrain former lake levels in the Lake Michigan basin. Time series analysis of sand cycles based on the weight-percent aeolian sand within lacustrine sediment, reveals statistically significant spectral peaks that coincide with established lake level cycles in Lake Michigan and the Gleissberg sunspot cycle of minima. Longer cycles of ~ 800 and ~ 2200 years were also identified that correspond to solar cycles. Shorter cycles between 80 and 220 years suggest a link between coastal dune mobility, climate, and lake levels in the Lake Michigan basin. Radiocarbon-dated sedimentary contacts of lacustrine sediment overlying wetland sediment record the Nipissing transgression in the Lake Michigan basin. Lake level rise closely mimics the predicted uplift of the North Bay outlet, with lake level rise slowing when outflow was transferred to the Port Huron/Sarnia outlet. The Nipissing highstand was reached after 5000 cal (4.4 ka) BP.     

Diatom-inferred lake level from near-shore cores in a drainage lake from the Experimental Lakes Area,northwestern Ontario,Canada

We inferred late Holocene lake-level changes from a suite of near-shore gravity cores collected in Lake 239 (Rawson Lake), a headwater lake in the Experimental Lakes Area, northwestern Ontario. Results were reproduced across all cores. A gravity core from the deep central basin was very similar to the near-shore cores with respect to trends in the percent abundance of the dominant diatom taxon, Cylcotella stelligera. The central basin, however, does not provide a sensitive site for reconstruction of lake-level changes because of the insensitivity of the diatom model at very high percentages of C. stelligera and other planktonic taxa. Quantitative estimates of lake level are based on a diatom-inferred depth model that was developed from surficial sediments collected along several depth transects in Lake 239. The lake-level reconstructions during the past ~3,000 years indicate that lake depth varied on average by ±2 m from present-day conditions, with maximum rises of ~3–4 m and maximum declines of ~3.5–5 m. The diatom-inferred depth record indicates several periods of persistent low levels during the nineteenth century, from ~900 to 1100 AD, and for extended periods prior to ~1,500 years ago. Periods of inferred high lake levels occurred from ~500 to 900 AD and ~1100 to 1650 AD. Our findings suggest that near-shore sediments from small drainage lakes in humid climates can be used to assess long-term fluctuations in lake level and water availability.