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).     

A multi-proxy study of Holocene environmental change in the Maya Lowlands of Peten, Guatemala

We used multiple variables in a sediment core from Lake Peten-Itza, Peten, Guatemala, to infer Holocene climate change and human influence on the regional environment. Multiple proxies including pollen, stable isotope geochemistry, elemental composition, and magnetic susceptibility in samples from the same core allow differentiation of natural versus anthropogenic environmental changes. Core chronology is based on AMS ¹⁴C measurement of terrestrial wood and charcoal and thus avoids the vagaries of hard-water-lake error. During the earliest Holocene, prior to 9000 ¹⁴C yr BP, the coring site was not covered by water and all proxies suggest that climatic conditions were relatively dry. Water covered the coring site by 9000 ¹⁴C yr BP, coinciding with filling of other lakes in Peten and farther north on the Yucatan Peninsula. During the early Holocene (9000 to 6800 ¹⁴C yr BP), pollen data suggest moist conditions, but high ¹⁸O values are indicative of relatively high E/P. This apparent discrepancy may be due to a greater fractional loss of the lake's water budget to evaporation during the early stages of lake filling. Nonetheless, conditions were moist enough to support semi-deciduous lowland forest. Decrease in ¹⁸O values and associated change in ostracod species at 6800 ¹⁴C yr BP suggest a transition to even moister conditions. Decline in lowland forest taxa beginning 5780 ¹⁴C yr BP may indicate early human disturbance. By 2800 ¹⁴C yr BP, Maya impact on the environment is documented by accelerated forest clearance and associated soil erosion. Multiple proxies indicate forest recovery and soil stabilization beginning 1100 to 1000 ¹⁴C yr BP, following the collapse of Classic Maya civilization.     

Setting minimum head capsule abundance and taxa deletion criteria in chironomid-based inference models

Criteria for removing training set lakes and taxa in chironomidbased inference models, due to low abundances, have largely been ad hoc. We used an anoxia inference model and a hypolimnetic oxygen model from southcentral Ontario to determine what effect subfossil head capsule abundance and taxa deletion criteria have on fossil inference statistics. Results from six training set lakes suggest that a minimum abundance of 40–50 head capsules is sufficient for use in inference models, however more diverse samples likely require more than 50 head capsules. Taxa deletion criteria substantially improved the predictive ability of inference models (lowered the root mean squared error of prediction (RMSEP)). The common practice of including taxa with only 2% abundance in at least two lakes was one of the deletion criteria that much improved inference models. Similar deletion criteria, such as 2% in at least 3 lakes and 3% in at least 1 lake, produced comparable improvements (up to 18% reduction in RMSEP).     

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.     

Authigenic clays in East Africa: Regional trends and paleolimnology at the Plio–Pleistocene boundary, Olduvai Gorge, Tanzania

Ultrafine clays (<0.1 m) from six East African basins show wide chemical variability that can be conveniently described as the octahedral cation index (OCI), calculated as octahedral element ratio Mg/(Al+Fe). Values range from dioctahedral clays with OCI < 0.1 (upland soils/detrital sources) to trioctahedral clays with OCI > 4 (saline, alkaline lake deposits). The association of Mg-rich clays with saline, alkaline conditions provides a paleochemical proxy in sedimentary contexts. The octahedral chemistry of central-basin clays from paleolake Olduvai (1.79 Ma) records two lake expansions, with an intervening lake contraction, over 18,000 years. The initial lake expansion correlates well with the marine record of increased African humidity, but the subsequent drying episode does not, suggesting that some high-frequency environmental oscillations in East Africa may occur independent of obliquity and precession control. The clay record correlates well with the fossiliferous and archaeologically rich stratigraphy of the lake margin, and suggests that fossils and artifacts accumulated on the exposed lake margin over <4000 years. The chemistry of clays is a potentially powerful tool to add to multi-proxy studies in basins with alkaline waters, particularly in stratigraphic intervals that lack pollen, calcareous, or siliceous microfossils.     

Lacustrine oxygen isotope record of 20th-century climate change in central EuropeL evaluation of climatic controls on oxygen isotopes in precipitation

We report oxygen isotope data from a 108-yr (1885–1993) sequence with annual laminae of bio-induced authigenic calcite in a frozen core from Baldeggersee, a small lake in Central Switzerland. These isotope results provide proxy data on the isotopic composition of past precipitation in the Baldeggersee catchment region and are quantitatively compared with instrumental climate data (i.e. mean annual air temperature and atmospheric circulation pattern indices) to evaluate climatic controls on oxygen isotopes in precipitation.Monitoring the isotope hydrology of Baldeggersee demonstrates that the oxygen isotopic composition of the lake water is controlled by the isotopic composition of local atmospheric precipitation (¹⁸O_p) and that the isotopic signal of precipitation is preserved, albeit damped, in the lake calcite oxygen isotope record (¹⁸O_c). Comparison of the calcite oxygen isotope proxy for ¹⁸O_p in the catchment with historical mean annual air temperature measurements from Bern, Switzerland confirms that authigenic calcite reliably records past annual air temperature in the region. This ¹⁸O_c/temperature relationship is calculated to be 0.39/°C for the period 1900–1960, based on an isotope mass-balance model for Baldeggersee. An exception is a 0.8 anomalous negative shift in calcite ¹⁸O values since the 1960s. Possible explanations for this recent ¹⁸O_c shift, as it is not related to mean annual air temperature, include changes in ¹⁸O_p due to synoptic circulation patterns. In particular, the 0.8 negative shift coincides with a trend towards a more dominant North Atlantic Oscillation (NAO) index. This circulation pattern would tend to bring more isotopically more negative winter precipitation to the region and could account for the 0.8 offset in ¹⁸O_c data.     

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.     

Natural Geological Weighing Lysimeters: Calibration Tools for Satellite and Ground Surface Gravity Monitoring of Subsurface Water-Mass Change

Increased accuracy in measuring temporal variations in the Earth's gravity field allow inprinciple the use of gravity observations to deduce subsurface water-mass changes. This canbe with respect to a small area, or as a larger spatial average of water mass change usinggravity observations from low-altitude satellites, such as the forthcoming GRACE mission.At both scales, there is a need to validate gravity-based estimates against field recordings ofactual subsurface water-mass variations. In practice, this could prove difficult because thespatial integral of all water-storage change components can be subject to considerable fieldmeasurement error. An alternative approach to the validation process is proposed by whichsuitable geological formations are utilized as giant weighing devices to directly measure area-integratedwater-mass changes. The existence of such natural geological weighing lysimetersis demonstrated using observations from a replicated experimental site in New Zealand. Sitesof this type could be used to verify water-storage change estimates derived from sensitiveground surface gravity instrumentation. In addition, geological lysimeters could be used tomake local checks on the accuracy of any estimated regional water-mass time series, whichis proposed for satellite calibration. The land area weighed by a geological lysimeter increaseswith formation depth and it is speculated that recordings made at oil well depth may allowdirect monitoring of subsurface water mass changes at the regional scale.     

Chrysophyte reproduction and resting cysts: A paleolimnologist's primer

Chrysophyte algae produce siliceous resting cysts (stomatocysts) that are becoming an increasingly useful class of paleoecological indicator microfossils. This paper provides a review of the role that stomatocysts play in the life cycle and reproductive ecology of freshwater planktonic chrysophytes. Such information provides paleolimnologists with greater insight into the ecology of the vegetative, planktonic growth phase of species contributing stomatocysts to lacustrine microfossil assemblages. Specific chrysophyte reproductive characteristics discussed include: temporal dynamics of vegetative growth and encystment, cyst induction, cyst survivorship, germination requirements and recruitment strategies. This information serves as an introduction to a special issue of the Journal of Paleolimnology which is dedicated to the Application of Chrysophyte Stomatocysts in Paleolimnology.This is the first in a series of four papers published in this dedicated issue entitled Application of Chrysophyte Stomatocysts in Paleolimnology. Dr. C. D. Sandgren served as guest editor for these papers.     

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.     

Morphological variation of Stephanodiscus niagarae Ehrenb. (Bacillariophyta) in a Lake Ontario sediment core

Stephanodiscus niagarae populations in a core from Lake Ontario show systematic variation in radial puncta number and mean valve diameter. Puncta number is stable in specimens from samples deposited between 1720 and 1844. Puncta number gradually increases in samples deposited between 1860 and 1947, then increases rapidly to high numbers in samples deposited after 1964. Mean valve diameter is relatively high in specimens from samples deposited between 1720 and 1959. Diameter of specimens deposited during this period fluctuates around a mean of 63.2 m. Mean diameter of specimens deposited from 1959 to 1980 is only 42.5 m. We conclude that the trend in puncta number reflects the time course of eutrophication in Lake Ontario. We interpret the precipitous decline in mean diameter as an indication that conditions in the lake after the late 1950's did not permit sexual reproduction in this species.     

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.     

Development of paleolimnological inference models for pH, total nitrogen and specific conductivity based on planktonic diatoms

Canonical correspondence analysis (CCA was used to explore and identify statistically significant relationships between the distributions of planktonic diatoms and the physical and chemical properties of 50 Connecticut lakes. Six variables (pH, total nitrogen, calcium, sulfate, potassium and chlorophyll- a concentrations) were found to be significantly correlated with either or both of the first two extracted axes. The pH and calcium concentration, and to a lesser extent total nitrogen concentrations, were the most important variables controlling the distributions of planktonic diatoms in this suite of lakes. Paleolimnological inference models were developed for pH, total nitrogen (TN) and specific conductivity. Weighted averaging with (WAtol) and without (WA) tolerance downweighting, with and without bootstrap resampling techniques, and using either classical or inverse deshrinking methods were used to develop inference models for each variable. The pH and TN yielded sufficiently high 1/2 ratios and a highly significant first (constrained) axis when entered as single variables in both constrained and partially constrained CCA analyses, supporting the idea that reliable inference models could be developed for these variables. The r2 and RMSE of prediction values ranged from 0.73 to 0.86 and 0.37 to 0.6, respectively for pH, and from 0.4 to 0.64 and 59 g/l to 95 g/l, respectively for TN. Inference models for specific conductivity also yielded significant goodness-of-fit statistics. However, because specific conductivity was removed from the CCA analysis due to its high variance inflation factor and did not yield a significant relationship when entered as the sole variable in a partial constrained CCA, inference models for this variable will probably not yield any additional environmental information. The use of only planktonic diatoms in construction of inference models is discussed.     

Abrupt changes in climatic conditions across the late-glacial/Holocene transition on the N. E. Tibet-Qinghai Plateau: evidence from Lake Qinghai, China

A multi-proxy investigation of two sediment cores from the large closed-basin Lake Qinghai provides evidence of abrupt changes in paleolimnological conditions across the late-glacial/Holocene transition. The chronology of the lacustrine sediment sequence is framed by four AMS ¹⁴C ages for aquatic-plant macrofossil seeds. Four distinct stratigraphic units are identified on the basis of abrupt shifts in lithology, carbonate composition, ¹⁸O of authigenic carbonates, magnetic susceptibility characteristics, and total nitrogen content. These units represent four environmental stages that were each initiated by three abrupt changes in hydro-climatic regime at 11,600, 10,700, and 10,000 ¹⁴C yrs B.P. Each of the four environmental stages thus represents a characteristic precipitation-to-evaporation balance for the lake catchment. The paleoenvironmental evidence indicates that the lake before 11,600 ¹⁴C yrs B.P. was very shallow with carbonate production and organic productivity much lower than in the Holocene, suggesting a much colder and drier climate than in the Holocene. From 11,600 to 10,700 ¹⁴C yrs B.P., the presence of clastic laminations and Ruppia fossil seeds suggests an increased inflow of sediment-laden waters into the lake. Between 10,700 and 10,000 ¹⁴C yrs B.P., the development of a carbonate playa lake indicates that a negative water balance persisted. From 10,000 ¹⁴C yrs B.P. an abrupt increase in rainfall is suggested by a sudden termination of the playa lake environment and the diluted lake waters, as evidenced by negative shift in both total carbonate content and ¹⁸O values of mineral carbonate. However, the lake level during the early Holocene was about 20 m shallower than today, indicating that the effective moisture then was much lower than it is today. The multi-proxy record suggests a step-wise pattern of climatic change across the late-glacial/Holocene transition along with abrupt shifts in P-E balance on the N. E. Tibet-Qinghai Plateau. This pattern is characterized by reorganization of Asian monsoon circulation, which probably was determined by increasing summer insolation and changes in surface boundary conditions accompanying regional deglaciation. The arid event at 10,700–10,000 ¹⁴C yrs B.P. is interpreted as a Younger Dryas equivalent, although climatic cooling is not indicated by the evidence at hand.     

The sedimentary and geochemical record of Neogene-Quaternary water bodies in the Dead Sea Basin - inferences for the regional paleoclimatic history

Several waterbodies occupied the tectonic depressions along the Dead Sea transform during the NeogeneQuaternary. The earliest of these water bodies was the marine Sedom lagoon, which produced the SedomDead Sea brine. After the disconnection of the Sedom lagoon from the open sea several lakes were developed in the Dead Sea basinJordan Valley. Lake Amora (Samra) that existed from early to late Pleistocene, Lake Lisan (~ 70–15 kyr B.P.), and the Holocene Dead Sea. The lacustrine water bodies in the Dead Sea basin behave as amplifier lakes whose size and depth reflect the changing climatic conditions in the region. Lake level and limnological conditions of Lake Amora are not yet known, nevertheless, the lake probably extended over a large part of the Dead Sea basin-Jordan Valley. Lake Lisan level changed between ~ 330 and ~ 150 meters below sea level (m b.s.l.). Its maximum elevation was reached at ~ 27–23 kyr B.P. during marine isotope stage 2. Its minimum elevation was reached at ~ 47–43 kyr during marine isotope stage 3. Lake Lisan began to recede at ~ 17–15 kyr B.P. and at 12–11 kyr B.P. the postLisan water body declined to its minimum level. During most of the Holocene the lake (paleoDead Sea) stabilized at ~ 400 m b.s.l.The limnological evolution of water bodies in the Dead Sea basin reflects the climatic conditions in the region during the late Pleistocene, which fluctuated between wetter and drier periods. During Lisan time these fluctuations appear to be modulated by the cold and warm cycles, respectively in the northern Hemisphere. This relation is less obvious in the postLisan water body, where the strongest lake drop appears to occur during the Younger Dryas cold event.     

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.     

Organo-geochemical and stable isotope indicators ofenvironmental change in a marl lake, Malham Tarn, North Yorkshire,U.K.

Sediments of the marl lake Malham Tarn located in NW Englandpreserve an environmental record since 12 Ka. Eight Holocene pollen zones wereidentified, and the ¹³C of total organic carbon (TOC) showsthree stratigraphic divisions. The basal clay unit and overlayingsand/clay/marl unit have ¹³C of –24which decreases at the base of the principal marl unit to a mean value around–30, whilst the topmost black marl unit ¹³Cincreases to –28 at the surface. Representative samples of theseunits were selected for analysis of n-alkanes andn-fatty acids and their ¹³C.Samples of modern Chara and peat were analysed forcomparison. The clay unit has a minor contribution of redeposited matureorganic matter and autochthonous algae, the marl unit a high contribution ofChara, and the dark marl unit has a high contribution fromhigher plants. Compound-specific ¹³C revealssystematic differences between alkanes and fatty acids of different chainlength. The major shift in ¹³C in the short and medium chainfatty acids are probably due to the decreasing influence of carbonate rockflour as source of DIC. The major shift in ¹³C in the longchain n-fatty acids andn-alkanes could reflect the lower atmosphericCO₂ concentration at Last Glacial. The negative shift of short chainfatty acids in organic rich dark marls reflects introduction of detrital peatinto the lake. The ¹³C results show a dramatic change fromdominance of autochthonous plus eroded sources up to Pollen Zone IV, then slowcolonisation of the hinterland by higher plants, followed by constantChara contributions throughout the deposition of the marl,and a further increase of higher plant material after the rise in water levelin 1791.     

Paleolimnological investigations of anthropogenic environmental change in Lake Tanganyika: I. An introduction to the project

We investigated paleolimnological records from a series of river deltas around the northeastern rim of Lake Tanganyika, East Africa (Tanzania and Burundi) in order to understand the history of anthropogenic activity in the lakes catchment over the last several centuries, and to determine the impact of these activities on the biodiversity of littoral and sublittoral lake communities. Sediment pollution caused by increased rates of soil erosion in deforested watersheds has caused significant changes in aquatic communities along much of the lakes shoreline. We analyzed the effects of sediment discharge on biodiversity around six deltas or delta complexes on the east coast of Lake Tanganyika: the Lubulungu River delta, Kabesi River delta, Nyasanga/Kahama River deltas, and Mwamgongo River delta in Tanzania; and the Nyamuseni River delta and Karonge/Kirasa River deltas in Burundi. Collectively, these deltas and their associated rivers were chosen to represent a spectrum of drainage-basin sizes and disturbance levels. By comparing deltas that are similar in watershed attributes (other than disturbance levels), our goal was to explore a series of historical experiments at the watershed scale, with which we could more clearly evaluate hypotheses of land use or other effects on nearshore ecosystems. Here we discuss these deltas, their geologic and physiographic characteristics, and the field procedures used for coring and sampling the deltas, and various indicators of anthropogenic impact.     

Environmental magnetic and magnetic fabric studies in LakeWaynewood, northeastern Pennsylvania, USA: Evidence for changes inwatershed dynamics

An environmental magnetic and magnetic fabric study of sedimentscollected from Lake Waynewood, a post-glacial lake in the PoconoMountains of Pennsylvania, USA, provided a history of the lake's watersheddynamics over the past 3500 years. Two 5 m long, Mackereth coresof lake sediments and three watershed soil profiles were analyzed magnetically.Paleosecular variation and ¹⁴C measurements allowed timing ofchanges in the lake's watershed which are documented by changes in ARM,SIRM,, S-ratio, SIRM/, ARM/ and ARM/SIRMdowncore. Prior to 2900 years BP, there is little evidence for allogenicinflux. Dramatic changes in mineral magnetic properties and a strong AARmagnetic fabric appear approximately 2900 years BP, suggesting major changes inwatershed conditions, either in the hydrologic regime or in vegetative cover.Between 2900 and 1800 yrs BP, large variations in magnetic mineralogy areapparent, whereas about 1800 years BP, a single sediment source began todominate the magnetic mineralogy. About 100–200 years ago, conditionsagain changed, probably due to clearcutting and settlement of the watershed.Topsoil erosion appears to have dominated the magnetic signal. S and Mnconcentration downcore indicate that there is little evidence for reductiondiagenesis having caused the changes observed in magnetic mineral type andconcentration, except in the top 10 centimeters of the sediment column.