Four thousand years of atmospheric lead pollution in northern Europe: a summary from Swedish lake sediments

This paper presents a large palaeolimnological study of the pre-industrial and industrial history of atmospheric lead pollution deposition in Sweden. Both lead concentrations and ²⁰⁶Pb/²⁰⁷Pb ratios have been analysed in 31 lakes covering most of Sweden, plus one lake in north-west Russia. Four of the lakes have varved (annually-laminated) sediments. Isotope analysis is a sensitive and effective method to distinguish pollution lead from natural catchment lead and to detect early pollution influence, because the ²⁰⁶Pb/²⁰⁷Pb ratio in unpolluted background sediments in Sweden was > 1.3, while that of lead from pollution, derived from ores and coal, was < 1.2. The sediments show a consistent picture of past temporal changes in atmospheric lead pollution. These changes include: the first traces of pollution 3,500-3,000 yrs ago; a pollution peak in Greek-Roman Times (about 0 AD); lower lead fall-out between 400 and 900 AD; a significant and permanent increase in atmospheric lead fall-out from about 1000 AD; an increase with the Industrial revolution; a major increase following World War II; the maximum peak in the 1970s; and decreasing fall-out over the last decades. The four varved sediments provide high-resolution records of atmospheric pollution. They reveal pollution peaks about 1200 and 1530 AD which match the history of metal production in Europe. According to the varve records the lead pollution level in the late 1990s had decreased beneath the level of the 1530s. The pollution level 1200 AD was about 35% of the 1980s, when lead pollution was still near its all time high. About 50% of the total accumulated atmospheric lead pollution deposition through time was deposited in the pre-industrial period. The sediments also show a consistent picture of the geographic distribution of atmospheric lead deposition over time, with higher deposition in south Sweden and declining levels to the north, which supports the hypothesis that the main sources of pre-industrial atmospheric lead pollution in Sweden were cultural areas in mainland Europe and Great Britain.     

Lithological and geochemical record of anthropogenic changes in recent sediments of a small and shallow lake (Lake Pusty Staw,northern Poland)

The article presents the results of lithological and geochemical investigations of recent sediments of Lake Pusty Staw. The analysed sediments document about 300 years of the history of this lake. Historical materials indicate that significant changes in the catchment of the lake took place from the beginning of the 18th century (deforestation and afforestation) followed by 19th century tourist development, and 20th century industrialisation. The sediments were dated using the ²¹⁰Pb and ¹³⁷Cs method and core lithology. These made it possible to establish a reliable chronology to the year 1730. The calculated sedimentation rates ranged between 0.17 and 0.83 cm year^–1. There was a period of intensified erosion caused by land clearance between 1734 and 1810, which resulted in an acceleration in sedimentation rate (0.36 cm year^–1) and a change of lithological type of sediment from detritus gyttja to clayey gyttja. On the basis of Cu, Zn, Cd and Pb content in the sediments, it was determined that the beginning of pollution of the lake dates back to the 19th century and was caused probably by the existence of a health resort. A systematic increase in pollution occurred in the 20th century as a result of industrial plants. Normalised with respect to Al, the content of heavy metals in polluted sediments was from several to twenty times higher than in sediments of the preindustrial period, and a comparison of historical materials with changes of sediments in most cases made it possible to identify the direct causes of the increase in pollution.     

Correspondence of historic salinity fluctuations in Florida Bay, USA, to atmospheric variability and anthropogenic changes

Florida Bay is a highly dynamic estuary that exhibits wide natural fluctuations in salinity due to changes in the balance of precipitation, evaporation and freshwater runoff from the mainland. Rapid and large-scale modification of freshwater flow and construction of transportation conduits throughout the Florida Keys during the late nineteenth and twentieth centuries reshaped water circulation and salinity patterns across the ecosystem. In order to determine long-term patterns in salinity variation across the Florida Bay estuary, we used a diatom-based salinity transfer function to infer salinity within 3.27 ppt root mean square error of prediction from diatom assemblages from four ~130 year old sediment records. Sites were distributed along a gradient of exposure to anthropogenic shifts in the watershed and salinity. Precipitation was found to be the primary driver influencing salinity fluctuations over the entire record, but watershed modifications on the mainland and in the Florida Keys during the late-1800s and 1900s were the most likely cause of significant shifts in baseline salinity. The timing of these shifts in the salinity baseline varies across the Bay: that of the northeastern coring location coincides with the construction of the Florida Overseas Railway (AD 1906–1916), while that of the east-central coring location coincides with the drainage of Lake Okeechobee (AD 1881–1894). Subsequent decreases occurring after the 1960s (east-central region) and early 1980s (southwestern region) correspond to increases in freshwater delivered through water control structures in the 1950s–1970s and again in the 1980s. Concomitant increases in salinity in the northeastern and south-central regions of the Bay in the mid-1960s correspond to an extensive drought period and the occurrence of three major hurricanes, while the drop in the early 1970s could not be related to any natural event. This paper provides information about major factors influencing salinity conditions in Florida Bay in the past and quantitative estimates of the pre- and post-South Florida watershed modification salinity levels in different regions of the Bay. This information should be useful for environmental managers in setting restoration goals for the marine ecosystems in South Florida, especially for Florida Bay.     

A high resolution record of storm-induced erosion from lake sediments,New Zealand

The sustainable management of erodible pastoral hill country is a major focus of land use research in New Zealand. A multi-disciplinary study, using a high resolution lake sedimentation record, is being conducted to determine the role that cyclonic storms and natural and human-induced vegetation changes play in the erosion history of a landslide-prone hill country watershed.Sediment cores from Lakes Tutira and Waikopiro in northern Hawke's Bay were analysed to construct the magnitude-frequency history of storm-induced erosion since European settlement. Pulses of sediment representing individual storms can be clearly identified and are correlated to a storm history derived from analysis of a 93 year daily rainfall record. Correlation and dating are confirmed by pollen and diatom analysis,¹³⁷Cs distribution, tephrochronology and reference to a well documented land use history. Annually laminated, organic rich deposits, which occur in the uppermost sediments and represent the annual decomposition of biogenic material associated with eutrophication, are also used to confirm the chronology.A high correlation was found between storm sediment thickness and total storm rainfall (R²=0.8). Although sediment producing storms (>150 mm) occur on a near annual basis, the two largest storms (>600 mm) contributed 54% of the total sediment thickness.The presence of well defined storm sediment pulses has enabled the lake storage component of a sediment budget to be calculated for Cyclone Bola (1988), the most recent and largest rainstorm on record. The integration of this budget with the storm-magnitude-frequency history will be used to develop watershed-based models to predict the impacts of land use changes and the erosion response to climate scenarios.This is the third paper in a series of papers published in this issue on high-resolution paleolimnology. These papers were presented at the Sixth International Palaeolimnological Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. F. Lotter and Dr. M. Sturm served as guest editors for these papers.     

Arsenic contamination of lake sediments in Florida: evidence of herbicide mobility from watershed soils

Organic arsenical herbicides, which include monosodium methylarsonate (MSMA), have been applied to golf courses and lawns throughout Florida, USA, since the 1950s. These products convert rapidly to inorganic forms of arsenic (As) in soils and are mobilized readily. Leachates have been known to contaminate groundwater and surface waters, although past studies have not examined whether use of these products has led to significant As accumulation in lake sediments. We used paleolimnological methods to document the depositional history and inventories of total As in sediments and porewaters of Little Lake Jackson in Florida, which is adjacent to three golf courses. Six sediment cores, four of which were ²¹⁰Pb dated, showed porewater total As concentrations as high as 435 μg l⁻¹, and dry-sediment total As concentrations as high as 148 mg kg⁻¹. Approximately 537 kg of total As is present in >19,000 metric tons of sediment (dry mass), and an additional 18 kg of As is dissolved in 10.8 × 10⁴ m³ of porewaters. Total As content in surface sediments (mean = 47.3 mg kg⁻¹) exceeds the consensus-based sedimentary concentration for probable toxicity effects in freshwater benthic fauna. Surface and subsurface waters flow to the lake from topographically higher areas to the west, where golf courses and residential areas are located. Total As concentrations were elevated highly in monitoring wells and in a stream that flows between the golf courses and lake, but As was below detection limits in wells that were located at the distal perimeter of the golf courses. Subsurface and surface waters exit the lake towards topographically lower areas to the east. Nearly all As in sediments remains bound in the solid phase, indicating that As sedimentary profiles largely reflect depositional history. Sedimentary As concentrations are correlated strongly with aluminum and iron, which suggests that As was scavenged from lake waters during the past. Sedimentary As concentrations increased until the 1980s, then declined somewhat to the present time. Dissolved As was scavenged efficiently from the water column when hypolimnetic waters were oxygenated persistently, but after eutrophication led to a seasonally anoxic hypolimnion in the 1980s, apparently less As was co-precipitated, and more was lost to hydrological outflow. Arsenic accumulation in sediments might be common in areas where As derived from organic arsenical herbicide applications is directed by shallow water tables towards adjacent lakes.     

Redox remobilization and the heavy metal record in lake sediments: a modelling approach

The issue of whether heavy metal concentration profiles in lake sediments are vulnerable to redox mediated post-burial alteration stubbornly refuses to go away. Such behaviour is well-known in iron and manganese, due to great contrasts in the solubility of the reduced and oxidized chemical forms. A numbers of trace elements that also exhibit more than one oxidation state in lake sediments have also been shown to undergo diffusive migration in response to redox gradients (e.g., As, Co, Cr, V). What remains unclear is whether elements like Cd, Cu, Pb and Zn, that have only one stable oxidation state in sediments, are also subject to alteration. This paper reviews existing evidence and presents a model, based on current knowledge, to constrain the likely effects. It is concluded that some migration is likely, but that this is only significant at extremely low sediment mass accumulation rates. The formulation of the model highlights the need for further information about the physical form of iron and manganese oxyhydroxides in lake sediments.     

Evaluation of conservative lithogenic elements (Ti,Zr, Al,and Rb) to study anthropogenic element enrichments in lake sediments

In lake sediment investigations of heavy metal pollution history, it has become a common approach to calculate enrichment factors (EFs) by normalizing elemental distributions to a reference lithogenic element. However, this approach requires that the reference element remains stable once it has been deposited to the sediment (it is not affected by diagenetic processes). This is rarely studied in well-controlled field experiments. Here, we test the commonly used reference elements titanium (Ti), zirconium (Zr), aluminum (Al), and rubidium (Rb). We use a unique series of freeze cores collected in different years since 1979 in Lake Nylandssjön in northern Sweden. This lake has sediment with distinct varves (annually laminated deposit). Element concentrations in individual varves were analyzed using X-ray fluorescence (XRF) spectroscopy. By tracking the newly formed surface varve from different cores across this core series, i.e., following the element concentration in a specific varve as it becomes progressively aged, it was possible to assess the potential impact of diagenetic processes on geochemical signatures. Results confirm the conservative character of the studied elements; there was neither an increasing nor a decreasing concentration trend with time during sediment ageing for any of these elements. Secondly, we addressed the question ‘which of them is the most appropriate for EFs estimates with the aim of distinguishing anthropogenic from geogenic inputs, for example in pollution studies’. To assess the reliability of the EFs we used lead (Pb) as an example, because anthropogenic Pb in the sediment could be independently calculated using stable Pb isotopes. When anthropogenic Pb concentrations calculated with Pb-EFs were compared to the anthropogenic Pb concentrations derived from stable Pb isotopes, the differences found were 20% for Ti, 10% for Zr, 11% for Al, and 27% for Rb when upper continental crust concentrations were used for the background ratio. Based on the results from Nylandssjön our suggestions are that (1) when using EFs on a single core, which is the normal case in paleolimnology, multiple reference elements should be used together and (2) the results from those should be critically evaluated.     

Diatom-based paleolimnological reconstruction of regional climate and local land-use change from a protected sinkhole lake in southern Florida, USA

Despite their sensitivity to climate variability, few of the abundant sinkhole lakes of Florida have been the subject of paleolimnological studies to discern patterns of change in aquatic communities and link them to climate drivers. However, deep sinkhole lakes can contain highly resolved paleolimnological records that can be used to track long-term climate variability and its interaction with effects of land-use change. In order to understand how limnological changes were regulated by regional climate variability and further modified by local land-use change in south Florida, we explored diatom assemblage variability over centennial and semi-decadal time scales in an ~11,000-yr and a ~150-yr sediment core extracted from a 21-m deep sinkhole lake, Lake Annie, on the protected property of Archbold Biological Station. We linked variance in diatom assemblage structure to changes in water total phosphorus, color, and pH using diatom-based transfer functions. Reconstructions suggest the sinkhole depression contained a small, acidic, oligotrophic pond ~11000–7000 cal yr BP that gradually deepened to form a humic lake by ~4000 cal yr BP, coinciding with the onset of modern precipitation regimes and the stabilization of sea-level indicated by corresponding palynological records. The lake then contained stable, acidophilous planktonic and benthic algal communities for several thousand years. In the early AD 1900s, that community shifted to one diagnostic of an even lower pH (~5.6), likely resulting from acid precipitation. Further transitions over the past 25 yr reflect recovery from acidification and intensified sensitivity to climate variability caused by enhanced watershed runoff from small drainage ditches dug during the mid-twentieth Century on the surrounding property.     

A high resolution proxy-climate record from an arctic lake with annually-laminated sediments on Devon Island, Nunavut, Canada

Sediments from a 3 ha lake (75 °34.34N, 89 °18.55W) from the coastal region of northern Devon Island, Nunavut, Canada, contain discrete laminations in the deepest part of the basin. The laminations are varves as indicated by the correspondence between counts and thickness measurements of the couplets and ²¹⁰Pb dating. A 14 cm core representing 150 years of sedimentation contained laminated couplets consisting of a lighter inorganic layer with a higher percentage of calcium and magnesium, alternating with fine darker bands, typically more cohesive, and comprising higher proportions of silica and carbon. A reddish oxidation zone with higher iron and aluminum frequently separates the laminations. The dark layer represents a biogenic component deposited in summer and is made cohesive by bacterial filaments among the other particles. The light inorganic layer represents clastic deposition from allochthonous sources. Deposition rates were relatively consistent through the core with an increase in varve thickness in the 1950s. Diatom concentrations in the sediments increased by two orders of magnitude in this century, with major increases in the 1920s and 1950s. The increase in varve thickness and diatom abundance coincides with an increase in summer melt percentage in an ice core from the Devon Island Ice Cap (Koerner, 1977). The relatively high sedimentation rate (0.15 cm yr^-1) coupled with the consistency of deposition makes this lake a significant indicator for recent climate changes of the Canadian Arctic Archipelago.     

Late Quaternary lake response to climate change and anthropogenic impact: biomarker evidence from Lake Constance sediments

This study used organic matter in oligotrophic Lake Constance (southern Germany) to reconstruct lake environment and to disentangle the multiple factors, such as climate change and human impacts, which influence sedimentation in large lakes. A sediment core from Upper Lake Constance, which represents 16,000 years of Late Glacial and Holocene lake history, was analysed for organic biomarkers, hydrogen index and elements calcium, strontium, and magnesium. Magnetic susceptibility was measured to establish a high-resolution stratigraphic framework for the core and to obtain further information about changes with respect to relative allochthonous versus autochthonous sedimentation. Dinosterol—a biomarker for dinoflagellates—and calcium have low concentrations in Younger Dryas sediments and consistently high concentrations between 10,500 and 7,000 cal. years BP. These variations are attributed to changes in lake productivity, but are not reflected in the proportion of total organic carbon within the sediment. During the Younger Dryas and between 6,000 and 2,800 cal. years BP, concentrations and accumulation rates of land-plant-derived C₂₉-steroids (β-sitosterol, stigmastanol and stigmasterol), in combination with a relatively low HI, indicate periods of enhanced terrigenous input to the lake. For the Younger Dryas, higher runoff can be attributed to a cold climate, leading to decreased vegetation cover and increased erosion. After 6,000 cal. years BP, high terrestrial input may be explained by enhanced precipitation. Biomarker and HI results, in combination with archaeological studies, raise the question as to whether lakeshore settlements affected sedimentation in Upper Lake Constance between 6,000 and 2,800 cal. years BP.     

Stable isotopes and sediments from Pickerel Lake, South Dakota, USA: a 12ky record of environmental changes

Sedimentological parameters and stable O- and C-isotopic composition of marl and ostracode calcite selected from a 17.7-m-long core from the 8-m-deep center of Pickerel Lake, northeastern South Dakota, provide one of the longest (ca. 12ky) paleoenvironmental records from the northern Great Plains. The late Glacial to early Holocene climate in the northern Great Plains was characterized by changes from cold and wet to cold and dry, and back to cold and wet conditions. These climatic changes were controlled by fluctuations in the positions of the Laurentide ice sheet and the extent of glacial Lake Agassiz. We speculate that the cold and dry phase may correspond to the Younger Dryas event. A salinity maximum was reached between 10.3 and 9.5 ka, after which Pickerel Lake shifted from a system controlled by atmospheric changes to a system controlled by groundwater seepage that might have been initiated by the final withdrawal of Glacial Lake Agassiz. A prairie lake was established at approximately 8.7 ka, and lasted until about 2.2 ka. During this mid-Holocene prairie period, drier conditions than today prevailed, interrupted by periods of increased moisture at about 8, 4, and 2.2 ka. Prairie conditions were more likely dry and cool rather than dry and warm. The last 2.2 ka are characterized by higher climatic variability with 400-yr aridity cycles including the Medieval Warm Period and the Little Ice Age.Although the signal of changing atmospheric circulation is overprinted by fluctuations in the positions of the ice sheet and glacial Lake Agassiz during the late Glacial-Holocene transition, a combination of strong zonal circulation and strong monsoons induced by the presence of the ice sheet and high insolation may have provided mechanisms for increased precipitation. Zonal flow introducing dry Pacific air became more important during the prairie period but seems to have been interrupted by short periods of stronger meridional circulation with intrusions of moist air from the Gulf of Mexico. More frequent switching between periods of zonal and meridional circulation seem to be responsible for increased climatic variability during the last 2.2 ka.     

A record of late Holocene precipitation on the Central Tibetan Plateau inferred from varved lake sediments

Journal of Paleolimnology - Lake sediments on the Tibetan Plateau are important natural archives for studying past climate and environment changes. A precise sediment core chronology is a...     

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 between C:N ratios of lake sediments, organic matter sources, and historical deforestation in Lake Pleasant, Massachusetts, USA

The C:N ratios of lake sediments may reflect proportions of terrestrial and algal carbon contributing to accumulation of sediment. This possibility was tested in Lake Pleasant, Massachussetts, USA which underwent watershed deforestation in about 1780 A.D. The C:N profile of a 70-cm sediment core clearly reflected deforestation through a rise in C:N ratio caused by an increase in watershed contributions to sedimentary carbon. Spatial variability of C:N in modern surficial sediments is small compared to the change caused by deforestation.     

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.     

Paleoenvironmental change in wetlands of the Florida Everglades, southeast USA

In this special issue, we report on efforts to reconstruct paleoclimate/paleolimnology of the Florida Everglades, applying a wide range of techniques including sedimentological, micropaleontological and biogeochemical approaches. The papers included here describe results obtained by studies conducted in Everglades National Park and the greater South Florida Everglades by Florida Coastal Everglades Long Term Ecological Research Program (FCE LTER) collaborators. This multi-investigator project contrasts nutrient dynamics in two inland-to-marine transects aligned along separate drainages in southern Florida that differ in their susceptibility to coastal pressures and in volume of freshwater delivery. This effort focuses on the paleoecological aspects of FCE LTER research that address scales of ecosystem transformations driven by climate variability and change and human activities. The central question addressed by this body of work is “How is the shape of the freshwater-to-marine gradient in the Florida coastal Everglades controlled by changes in climate, freshwater inflow (i.e. through human activities), and disturbance (i.e. sea level rise, hurricanes, fire)?”     

Paleoecological investigations of recent lake acidification in northern Florida

Thirty-two northern Florida lakes were analyzed to construct a transfer function relating surface sediment diatom assemblages to lakewater pH (R ² =0.89, s.e.=0.34). A paleoecological analysis of sediment cores from six of these lakes indicated that two have become more acidic in the last 50 years. The diatom inferred (DI) pH of L. Barco has declined between 0.56–0.82 in the 1900's and DI ANC (acid neutralizing capacity) by 28–46 eq l^-1. The DI pH of nearby L. Suggs has declined 0.91 pH units and its DI ANC by 19 eq l^-1. The timing of the inferred acidification is synchronous with known increases in emissions of sulfates and nitrates that are associated with acidic precipitation. Also, the increasing accumulation of substances related to emissions from the burning of fossil fuels (e.g., Pb, PAH) co-occurs with the lowering of DI pH in the sedimentary record. However, other processes may have accounted for or contributed to recent lake acidification. For instance, the drawdown of local water tables by human consumption may decrease the inseepage of ANC to seepage lakes. Such an effect would be synchronous with increasing depositions of sulfate. There is also clear evidence that Florida lakes are naturally acidic. Thus, paleoecological results indicate acidic deposition to be at certain contributor, but not necessarily the sole cause, of the recent further acidification of some naturally acidic Florida lakes.This is the tenth of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D. F. Charles and D. R. Whitehead are guest editors for this series.     

Magnetic remanence acquisition in Finnish lake sediments

Summary. Magnetic studies have been carried out on organic sediments from five Finnish lakes to determine the carrier(s) of the stable NRM and to find how the remanence is acquired. Single or pseudo-single domain 'magnetite' is thought to carry the NRM. It was found that drying and cooling sediment samples resulted in a loss of NRM which was attributed to the misalignment of small magnetic particles. Low-field experiments were carried out on sediment samples in different physical states and from the results of these investigations it was concluded that post-depositional processes are important in the acquisition of an NRM. Stabilization of magnetic grains is thought to be due to the growth of gels in the organic sediment rather than to dewatering.     

Quaternary evolution of ephemeral playa lakes on the Southern High Plains of Texas, USA: cyclic variation in lake level recorded in sediments

Ephemeral playa lakes on the Southern High Plains northeast of Amarillo, Texas, are underlain by more than 10 m of Quaternary lake deposits. Sediments beneath 12 lakes were examined in 76 hollow-stem auger cores and in excavations. Stacked depositional cycles recognized in lake sediments record repeated phases of (1) initial highstand, (2) ephemeral lake, and (3) lake shrinkage and prolonged exposure. Sedimentary and soil structures show that during all phases the lakes were ephemeral, but that the duration and frequency of flooding varied, which caused variation in the relative amounts of accumulation, deflation, and soil formation. The highstand phase is documented by wave-cut benches and lake sediments that extend beyond present lake margins. Mud transported as suspended load was deposited from ponded water. Desiccation resulted in mudcracks and allowed deposition of eolian sand, but exposure episodes were relatively short or infrequent, and vertic soil formation, oxidation of organic material, and deflation of sand were minimal. Decreased frequency and duration of flooding resulting in increased pedogenic modification under conditions similar to those under which modern playa lake sediments accumulate. Eolian silt deposited on dry lake beds and clays deposited in flooded lakes were mixed by vertic soil processes during repeated wetting and drying. Organic material was partly oxidized and partly translocated down roots and cracks, and interbedded upland facies were gleyed. Episodes of lake shrinkage and more frequent exposure are recorded by reddening and formation of calcic horizons within lake muds. Red-brown eolian loam prograded across lake sediments, and calcic soils developed on it. These grassland slope facies record decrease in the size of the playa lakes.