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 recent sedimentation history of Aqualate Mere (central England): assessing the potential for lake restoration

As part of English Natures Lakes Flagship Project to address adverse environmental impacts on selected, important lakes, a proposal has been made to dredge Aqualate Mere. The site has experienced rapid, recent sedimentation thought to be derived from a nearby canal. The aim of this study has been to determine the recent sedimentation history of the site in order to assess the possibility of the disposal to land of its sediments and the efficacy of this form of lake restoration. A predominantly clayey silt layer was found across the lake beneath which darker, organic-rich sediments were noted. This transition may represent the input of canal-derived sediments, although it may reflect other environmental changes at this time. The radiometric dating technique employed was unable to date this sediment boundary. A further change in the characteristics of the upper part of the clayey silt layer may represent an additional influence of the canal. Heavy metal levels were modest, whereas nutrient levels were relatively high and some pesticides were detected. Topsoil erosion supplying nutrients and other compounds associated with agriculture have been an important source of the lower layers of the clayey silt sediments in particular. The highest levels of most pollutants were found in the finer sediments in the uppermost (post-1950s) part of the sediment profile. These sediments appear to reflect a change in the characteristics of the sediments of the canal, which was associated with a change in the nature of its water supply. The key geochemical properties of the sediments should not preclude the land-based disposal of dredged materials under current UK regulations for waste management. Accurate estimation of sediment quantities was limited, as the interface between the recent and underlying sediments was not positively identified at all sample points.     

Recent environmental changes inferred from the sediments of small lakes in Yellowstone's northern range (Engstromet al., 1991)

Sedimentation rate data of Engstromet al. (1991) were analyzed in relation to the elevation of the eight ponds studied. The ... asynchroneity of the stratigraphic changes among the lakes ... that they noted was transformed into a pattern of time-transgressive change of sedimentation rate with elevation. Sediment units representing increasing (and decreasing) sedimentation rate and accelerating (and decelerating) sediment accumulation were correlated from pond to pond, suggesting that the sediment-producing condition somehow moved upslope over time at a rate of 3.4 to >5 m yr^–1. Climatic data appear not to explain this monotonic pattern, but dendrochronological data and elk population numbers suggest that ungulate foraging patterns might. Engstromet al. (1991) concluded that ...the lake-sediment records in the northern range do not convincingly show systematic direct or indirect effects of ungulate grazing during the history of the Park. Reinterpretation of these sediment data suggests that detailed reanalysis might lead to a different conclusion.     

Hydrologic control on the oxygen-isotope relation between sediment cellulose and lake water, western Taimyr Peninsula, Russia: Implications for the use of surface-sediment calibrations in paleolimnology

Systematic variability occurs between the oxygen isotopic composition of lake water sampled in mid-summer 1993 and cellulose extracted from surficial sediments of a suite of lakes spanning the forest-tundra transition near Noril'sk, Russia. Some tundra and all forest-tundra lakes show greater deviation from expected cellulose-water isotopic separation than forest lakes, apparently because of greater sensitivity to ¹⁸O-depleted snowmelt contributions. Cellulose derived from aquatic plants naturally integrates fluctuations in lake water ¹⁸O, providing a signal that is inherently more representative of average thaw season lake water ¹⁸O than the measure of instantaneous ¹⁸O obtained from an individual sample of lake water. Thus, indiscriminate use of empirical cellulose-water relations derived from calibration samples could lead to erroneous assessment of paleohydrology from the oxygen-isotope stratigraphy of sediment cores from arctic lakes. However, deviation from the expected cellulose-water fractionation is a source of lake-specific hydrologic information useful for qualifying paleoenvironmental interpretations and possibly constraining non-isotopic methods that rely on surface-sediment calibrations.     

Variation in the composition of Lake Bonneville marl: a potential key to lake-level fluctuations and paleoclimate

Lake Bonneville marl provides a stratigraphic record of lake history preserved in its carbonate minerals and stable isotopes. We have analyzed the marl in shallow cores taken at three localities in the Bonneville basin. Chronology for the cores is provided by dated volcanic ashes, ostracode biostratigraphy, and a distinctive lithologic unit believed to have been deposited during and immediately after the Bonneville Flood.A core taken at Monument Point at the north shore of Great Salt Lake encompasses virtually the entire Bonneville lake cycle, including the 26.5 ka Thiokol basaltic ash at the base and deposits representing the overflowing stage at the Provo shoreline at the top of the core. Two cores from the Old River Bed area near the threshold between the Sevier basin and the Great Salt Lake basin (the main body of Lake Bonneville) represent deposition from the end of the Stansbury oscillation ( 20 ka) to post-Provo time ( 13 ka), and one core from near Sunstone Knoll in the Sevier basin provides a nearly complete record of the period when Lake Bonneville flooded the Sevier basin (20–13 ka).In all cores, percent calcium carbonate, the aragonite to calcite ratio, and percent sand were measured at approximately 2-cm intervals, and ¹⁸O and ¹³C were determined in one core from the Old River Bed area. The transgressive period from about 20 ka to 15 ka is represented in all cores, but the general trends and the details of the records are different, probably as a result of water chemistry and water balance differences between the main body and the Sevier basin because they were fed by different rivers and had different hypsometries. The Old River Bed marl sections are intermediate in position and composition between the Monument Point and Sunstone Knoll sections. Variations in marl composition at the Old River Bed, which are correlated with lake-level changes, were probably caused by changes in the relative proportions of water from the two basins, which were caused by shifts in water balance in the lake.This is the second paper in a series of papers published in this issue on Climatic and Tectonic Rhythms in Lake Deposits.     

Climatic and anthropogenic influence on the stable isotope record from bulk carbonates and ostracodes in Lake Neuchâ, Switzerland, during the last two millennia

Lake Neuchåtel is a medium sized, hard-water lake, lacking varved sediments, situated in the western Swiss Lowlands at the foot of the Jura Mountains. Stable isotope data (18O and 13C) from both bulk carbonate and ostracode calcite in an 81 cm long, radiocarbon-dated sediment core represent the last 1500 years of Lake Neuchåtel's environmental history. Comparison between this isotopic and other palaeolimnologic data (mineralogical, geochemical, palynological, etc.) helps to differentiate between anthropogenic and natural factors most recently affecting the lake. An increase in lacustrine productivity (450–650AD ca), inferred from the positive trend in 13C values of bulk carbonate, is related to medieval forest clearances and the associated nutrient budget changes. A negative trend in both the bulk carbonate and ostracode calcite 18O values between approximately 1300 and 1500AD, is tentatively interpreted as due to a cooling in mean air temperature at the transition from the Medieval Warm Period to the Little Ice Age. Negative trends in bulk carbonate 18O and 13C values through the uppermost sediments, which have no equivalent in ostracode calcite isotopic values, are concomitant with the recent onset of eutrophication in the lake. Isotopic disequilibrium during calcite precipitation, probably due to kinetic factors in periods of high productivity is postulated as the mechanism to explain the associated negative isotopic trends, although the effect of a shift of the calcite precipitation towards the warmer months cannot be excluded.     

The sedimentary record of environmental lead contamination at St. Louis,Missouri (USA) area smelters

Southeastern Missouri has been a major Pb mining region since 1720 AD. Missouri mines produce a Pb ore that has a distinctive elevated ²⁰⁶Pb/²⁰⁷Pb isotopic ratio (>1.30) that is easily recognized in Pb-contaminated sediments. Five ²¹⁰Pb dated sediment cores from Horseshoe Lake, Madison County, Illinois were examined to reconstruct Pb-contamination of the site from southeastern Missouri mines and from a local Pb smelter located adjacent to the lake. Pb concentrations increased in the cores from 5 ppm in the early 1800s to approximately 350 ppm in the late 1940s and 1950s. Pb concentrations in recently deposited sediment range from 100 to 300 ppm depending on the location within the lake. Throughout the 1800s and early 1900s the ²⁰⁶Pb/²⁰⁷Pb ratios in the sediment cores increased indicating contamination from southeastern Missouri (mean = 1.243). After the local smelter began recycling lead-acid storage batteries in the 1950s, the ²⁰⁶Pb/²⁰⁷Pb ratio declined (mean = 1.224) suggesting contamination of Horseshoe Lake with Pb from sources elsewhere around the world. The results of this study demonstrate how isotopic ratios of Pb can be used to reconstruct historical anthropogenic contamination.     

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

Mn-carbonates are documented in the late-glacial varved sediments from Big Watab Lake, Minnesota, USA. The Mn-carbonate is authigenic and forms rims around contemporaneous epilimnetic calcite. Although such carbonates are found in minor amounts throughout the entire late-glacial sequence, significant quantities of Mn-carbonate are associated mainly with laminated intervals.Because of the suspected difference in isotopic fractionation between different carbonate minerals, the stable-isotopic compositions of bulk carbonate samples are used as a proxy for relative amounts of the Mn-carbonate in the sediment. High ¹⁸O and low ¹³C values are associated with abundant Mn-carbonates. Low ¹⁸O and high ¹³C values are associated with only minor concentrations of Mn-carbonates.The oxygen-18 record is correlated with fluctuations in the vegetation assemblage based on pollen spectra using a multiple regression model with backward elimination. The proposed link between the sedimentary archive and local vegetation is the mediation of advective mixing in the lake by forest composition. In this model, periods of forest closure resulted in a well-stratified water column that was anoxic at the sediment/water interface, permitting the formation of authigenic Mn-carbonates. Openings of Artemisia in the forest allowed wind shear to mix oxygen to depth, causing bioturbation of the laminations and preventing the formation of Mn-carbonate.     

Oxygen isotope ratios of organic matter in arctic lakes as a paleoclimate proxy: field and laboratory investigations

Paleoclimate research based on the stable isotopic composition of lake sediments is often hampered by the lack of preservation of suitable material for isotopic analysis. We examined organic material as a proxy for past water isotopic composition in a series of experiments. First, we cultured aquatic moss under constant illumination, temperature, and water ¹⁸O, and show that new cellulose records source water ¹⁸O precisely (r² = 0.9997). Second, we analyzed paired lakewater and vegetation samples collected from sites spanning strong climatic gradients. In field conditions, the relationship between organic ¹⁸O and water ¹⁸O is more variable, though it is still controlled by environmental water isotopic composition. However, terrestrial mosses in the arctic are often significantly enriched in ¹⁸O relative to aquatic mosses in nearby lakes due to their use of different water sources. Third, we measured ¹⁸O of cellulose extracted from disseminated sedimentary organic material. In the majority of the middle- to high-arctic lakes in this study, the ¹⁸O of disseminated sediment cellulose is greatly enriched relative to the expected values based on lakewater ¹⁸O, suggesting a significant component of terrestrial cellulose. This interpretation is supported by radiocarbon dates from a Holocene sediment core in which ¹⁴C ages of sediment cellulose are 700-5000 yrs older than the enclosing sediments. We conclude that aquatic cellulose can be used as a reliable tracer of lakewater isotope ratios, but terrestrial cellulose often dominates the sedimentary cellulose pool in places such as Baffin Island where sedimentation rates are low enough to allow the degradation of aquatic cellulose. Care must be taken when interpreting sediment cellulose ¹⁸O records where diagenesis has played a role, because terrestrial cellulose is more resistant to degradation, and therefore can predominate in environments with low organic carbon burial.     

Sub-sampling unconsolidated sediments: A solution for the preparation of undistrubed thin-sections from clay‐rich sediments

Using the principles of electroosmosis, a subsampling technique for clay-rich sediments is introduced that produces undisturbed thinsections from sediment cores. The fragile sample remains in the same disposable box throughout the process, preventing collapse of its structure during subsampling, manipulation and impregnation, as well as other potential problems.     

Lake levels in the southern Bolivian Altiplano (19°–21°S.) during the Late Glacial based on diatom studies

This study is focused on the endorheic Uyni-Coipasa Basin located in the southern Bolivian Altiplano. Stratigraphical and fossil diatom studies based on a detailed radiocarbon chrnology revealed six phases in water-level changes and paleosalinity variations. At 15,430±80 yr B.P., lacustrine conditions settled in the southern Bolivian Altiplano. A saline lake, characterized by benthic meso-metasaline species, reached +4 m altitude above the present bottom of the basin. After 15,430±80 yr B.P., the level rapidly rose to +27 m, as suggested by a tychoplanktonic mesosaline flora. Between 14,500 years and 13,000 years, finely lanminated sediments at +32 m contained successively a dominance of epiphytic mesosaline to hypersaline species and tychoplanktonic oligosaline diatoms, indicating weak fluctuations in water-level and salinity. At 13,000 years, strong changes in the diatom flora occurred; epiphytic oligo-hypersaline diatoms were replaced by planktonic meso-polysaline species. They indicate a deep salt lake (the lake reached +100 m). After 12,000 years, the lake level abruptly dropped, as suggested by fluviatile sediments with a benthic mesopolysaline diatom flora. The main lake was replaced by shallow saline ponds. A wet pulse occurred at 11,400 years, characterized by low water level (+7 m) and high salinity. This lacustrine phase remained until 10,400 yr B.P. These data indicate changes in Precipitation minus Evaporation (P-E). Our regional interpretations are based on a comparison with teh available data on the northern (Lake Titicaca) and southern (Lipez are) Bolivian Altiplano and on the northern Chilean Altiplano (Atacama Desert).     

Late Quaternary palaeohydrology of the Konya Basin, Turkey, based on isotope studies of modern hydrology and lacustrine carbonates

The Konya plain in south central Anatolia, Turkey, which is now largely dry, was occupied around the time of the Last Glacial Maximum by a fresh-oligosaline lake covering more than 4000 km². Sediment cores from three residual water bodies (Pinarbai, Akgöl and Süleymanhaci) within the larger Pleistocene lake basin, have been analysed using a multidisciplinary approach. The sediment sequences are dated as spanning the last 50 Ka years, although breaks in sedimentation mean that there is only partial chronological overlap between them. Carbon and oxygen isotope analyses on lacustrine carbonate from the three cores give contrasting isotope profiles which reflect the different ages and independent hydrological behaviour of different sub-basins through the late Quaternary. Distinguishing changes that are regional from local effects is aided by modern isotope hydrology studies and by comparing the carbonate ¹³C and ¹⁸O values to diatom and other analyses undertaken on the same cores.     

A reconnaissance study of oxygen, hydrogen and strontium isotopes in geochemically diverse lakes, Western Nebraska, USA

Reconnaissance ¹⁸O,, D, and ⁸⁷Sr data for fifteen lakes in the Western Lakes Region of the Sand Hills of Nebraska indicate dynamic hydrologic systems. The rather narrow range of ⁸⁷Sr from lake water (1.1 to 2.1) and groundwater (0.9 to 1.7) indicates that the groundwater is generally unradiogenic. Groundwater residence times and relatively unradiogenic volcanic ash within the dune sediments control the ⁸⁷Sr values. Based on the mutual variations of ¹⁸O and D, the lakes can be divided into three groups. In Group 1, both ¹⁸O and D values increase from spring to fall. The ¹⁸O and D values in Group 2 decreased from spring to fall. Group 3 are ephemeral lakes that went dry some time during 1992. The data and isotopic modeling show that variations in the ratio of evaporation relative to groundwater inflow, local humidity conditions, and the _a has substantial influence on the isotopic composition. In addition, isotopic behavior in ephemeral lakes can be rather unusual because of the changing activities of water and mineral precipitation and redissolution. The annual and interannual isotopic variability of these lakes which is reflected in the paleonvironmental indicators may be the rule rather than the exception in these types of systems.     

Environmental changes in an alpine lake (Gossenköllesee, austria) over the last two centuries – the influence of air temperature on biological parameters

Changes in microfossils (diatoms, chrysophytes, chironomids and cladocera remains), geochemistry and deposition of atmospheric pollutants have been investigated in the sediment records of the alpine lake Gossenköllesee (Tyrol, Austria) spanning the last two centuries. The sediment records were compared with seasonal and annual air temperature trends calculated for the elevation (2417 m a.s.l.) and the geographical position (47° 1346N, 11° 0051E) of the lake, and with precipitation records available since 1866 from Innsbruck. Temperature trends followed a 20–30 year oscillation between cold and warm periods. Regarding long-term changes, temperature trends showed a U-shaped trend between 1780 and 1950, followed by a steep increase since 1975.Physical, geochemical, and organic parameters were not controlled by air temperature. Among the biological records only diatoms and chrysophytes reacted to air temperature changes: the relative abundance of planktonic diatoms increased during warm periods and changes in mean annual alpine air temperature explained 36.5% of their variation. The relation between abundance of seasonal stomatocyst types and air temperature varied on two different time scales: while summer stomatocysts were influenced by short term temperature fluctuations, the autumn stomatocysts were affected only by the long term changes. Other biological parameters exhibited a constant species composition (chironomids, pigments) or changes were small and independent of temperature (cladocera). Spheroidal carbonaceous fly-ash particles, and trends in Pb and Cr indicated increasing deposition of atmospheric pollutants but had no detectable effects on the biological parameters either. In respect to temperature variations over the last 200 years, this alpine lake is much less sensitive than expected and has thus to be regarded as a well buffered site. However, temperature alone is not sufficient to understand changes in species composition and other biogeochemical processes with unknown historical patterns might have affected species composition more strongly.     

Palynology of Lagoa dos Patos sediments,Rio Grande do Sul,Brazil

With the purpose of studying the vegetation and climatic changes in the last millenia of the Coastal Plain of Rio Grande do Sul, Brazil, a palynological study was made of the sediments of the northern part of Lagoa dos Patos lagoon. Twenty-four samples from a 2.26 m core taken at a depth of 7.70 m (30° 50 50 S and 50° 59 05 W) were collected.The analyses revealed marine transgression at 5170±120 years B.P., giving rise to local vegetation consisting chiefly of xerophytes and halophytes. Vegetation characteristic of a humid environment was present along adjacent portions of the Coastal Plain at this time. Transgression increased at about 4080±110 years B.P., when the greatest level of tidewater was reached. This coincided with the beginning of forest vegetation development along the inner portions of the Coastal Plain. These data suggest that marine transgression may have been a consequence of higher temperatures and more humidity. After 4000 years B.P., regression occurred, resulting in fresh waters characteristics in the northern portion of the lagoon. The development of forest vegetation began at this time.This is the fourth in a series of papers published in this issue on Paleolimnology in Southern South America. Dr. C. A. Fernández served as guest editor for these papers.     

Geochemical and organic petrological characterization of the organic matter of lacustrine Eocene oil shales (Prinz von Hessen,Germany): reconstruction of the depositional environment

Optical and geochemical techniques were applied to sedimentary organic matter from the profundal area of the Eocene Lake Prinz von Hessen, which formed in a pull-apart basin on the Sprendlinger Horst, near Darmstadt, Germany. Variations in total sulphur content (S tot) and total organic carbon content (TOC), hydrogen index (HI), oxygen index (OI) and ¹³C values of the organic matter were used to reconstruct the lakes filling history. Following an initial rapid deepening phase, open lake conditions developed with HI reaching more than 500 mg HC/g TOC and TOC values up to 40%. The productivity of the lake was probably high and organic matter preservation was enhanced by a stratified water column. As the lake began to fill with sediment and became shallower, TOC and HI values declined, as the lake water was better oxygenated and preservation conditions declined. ¹³C values between –31 and –27 are controlled by the mixing of aquatic (algae and microbial mats) and terrigenous organic matter (wood, spores, pollen and cuticles). Following a rapid drop in lake level, shallow lake conditions alternated with swamp deposits (lignites) in the basin center. The organic matter preserved during this stage is strictly terrigenous in nature and experienced oxic degradation (HI 100 mg HC/g TOC). ¹³C values between –26 and –24 are typical for Eocene terrigenous matter. The inferred lake level fluctuations are interpreted to have been controlled by tectonic as well as climatic processes.     

The Taconite Inlet Lakes Project: a systems approach to paleoclimatic reconstruction

A comprehensive study of meteorological, hydrological, limnological and sedimentological conditions in the watersheds of density-stratified (meromictic) lakes around Taconite Inlet, Northern Ellesmere Island, N.W.T., Canada was carried out from 1990–1992. Lakes C1 and C2 contain seawater trapped by isostatic uplift as the former embayments became isolated from the sea. These lakes, and Lake C3, contain varved sediments which provide an annually resolvable paleoclimatic record. By studing the major systems influencing sedimentation in one of these lakes (Lake C2) a better understanding of the climatic controls on varve formation, and hence on the paleoclimatic signal in the varved sediment record, was obtained. The varves of Lake C2 provide a proxy record of summer temperature for the region.This is the first in a series of papers published in this issue on the Taconite Inlet Lakes Project. These papers were collected by Dr R. S. Bradley.     

Field calibration and performance of sediment traps in a eutrophic holomictic lake

In wind-stressed and unstratified Lough Neagh low aspect ratio sediment traps, which allow for internal resuspension, collected sediment at a rate close to the natural sediment accumulation rate inferred from dated sediment cores. High aspect ratio traps grossly overestimated the natural rate of sediment accumulation as did, but to a lesser extent, burial rate measurements on an artificial stratigraphic marker. The former type of trap deployed over long exposure periods is therefore recommended as providing the best estimate of net downward particle flux in lakes such as Lough Neagh. However, it is emphasized that all sediment trap types, when operated in shallow turbulent lakes, will to a greater or lesser extent be contaminated by secondary or redeposited material and so will not provide a direct measure of primary sedimentation. Use of tube traps in lakes such as Lough Neagh should not, however, be discounted since they can provide a record of the quality of sedimenting material through time.     

AMS radiocarbon dating of organic fractions from varved lake sediments: an empirical test of reliability

Replicate cores of annually laminated (varved) sediments have been used to test the accuracy and precision of chronologies of lake sediment accumulation over the last 1000 years, based on AMS 14C measurements. The internally consistent results show that in the case of the soft-water lake sediments studied (Kassjön, northern Sweden), all the organic fractions that include aquatic components give significantly older dates than expected. Only the single terrestrial macrofossil and the fine, unidentified residual particulate fraction provide dates close to the true age of the sediments. The results also show that age discrepancies for some fractions are not constant over time. The age discrepancies may arise from some carbon reservoir effect within the aquatic ecosystem, from resuspension and focusing of older marginal organic material into the deepest part of the lake or from some combination of these processes. More work is needed on the 14C age of organic fractions in varve-dated sediments from a range of lake types.     

Paleohydrology and paleochemistry of Lake Manitoba,Canada: the isotope and ostracode records

Lake Manitoba, the largest lake in the Prairie region of North America, contains a fine-grained sequence of late Pleistocene and Holocene sediment that documents a complex postglacial history. This record indicates that differential isostatic rebound and changing climate have interacted with varying drainage basin size and hydrologic budget to create significant variations in lake level and limnological conditions. During the initial depositional period in the basin, the Lake Agassiz phase (12–9 ka), ¹⁸O of ostracodes ranged from –16 to –5 (PDB), implying the lake was variously dominated by cold, dilute glacial meltwater and warm to cold, slightly saline water.Candona subtriangulata, which prefers cold, dilute water, dominates the most negative ¹⁸O intervals, when the basin was part of proglacial Lake Agassiz. At times during this early phase, the ¹⁸O of the lake abruptly shifted to higher values; euryhaline taxa such asC. rawsoni orLimnocythere ceriotuberosa, and halobiont taxa such asL. staplini orL. sappaensis are dominant in these intervals. This positive covariance of isotope and ostracode records implies that the lake level episodically fell, isolating the Lake Manitoba basin from the main glacial lake.¹⁸O values from inorganic endogenic Mg-calcite in the post-Agassiz phase of Lake Manitoba trend from –4 at 8 ka to –11 at 4.5 ka. We interpret that this trend indicates a gradually increasing influence of isotopically low (–20 SMOW) Paleozoic groundwater inflow, although periods of increased evaporation during this time may account for zones of less negative isotopic values. The ¹⁸O of this inorganic calcite abruptly shifts to higher values (–6) after 4.5 ka due to the combined effects of increased evaporative enrichment in a closed basin lake and the increased contribution of isotopically high surface water inflow on the hydrologic budget. After 2 ka, the ¹⁸O of the Mg-calcite fluctuates between –13 and –7, implying short-term variability in the lake's hydrologic budget, with values indicating the lake varied from outflow-dominated to evaporation-dominated. The ¹³C values of Mg-calcite remain nearly constant from 8 to 4.5 ka and then trend to higher values upward in the section. This pattern suggests primary productivity in the lake was initially constant but gradually increased after 4.5 ka.This is the sixth in a series of papers published in this issue on the paleolimnology of arid regions. These papers were presented at the Sixth International Palaeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. R. Chivas served as guest editor for these papers.