Interpreting the hydrological history of a temporary pond from chemical and microscopic characterization of siliceous microfossils

The hydrological history of a temporary pond in South Carolina was inferred from a 5500-year record of siliceous microfossils, including diatoms, freshwater sponge spicules, chrysophyte cysts, plates of testate amoebae and plant phytoliths. Microfossil abundance was estimated by microscopic quantification of siliceous particles and by chemical extractions of silica. Diatom, sponge and mineral particle volumes were correlated with silica concentrations attributable to these fractions. Both techniques suggested a sequence of four distinct community types. Basal sediments (4630–5520 ¹⁴C YBP) containing phytoliths and sponge spicules indicative of a wetland community were covered by sediments dominated by the remains of planktonic protists (3750–4630 ¹⁴C YBP) suggesting a transition from a vegetated marsh to an open-water, permanently flooded pond. Microfossil assemblages above this zone indicate the return of a wetland community ca. 3750 YBP that persisted until recently, when pond water levels stabilized as a result of seepage from a reservoir constructed nearby in 1985. This study suggests that the suite of siliceous microfossils commonly found in pond sediments can be used to infer historical alternations between macrophyte and plankton-dominated states in shallow basins. Regional climate inferences from this record include a mid-Holocene hydrological maximum and the onset of the modern climate ca. 3500 YBP.     

Holocone paleovegetation and paleohydrology of a prairie pothold in southern Saskatchewan, Canada

The Andrews site represents one of countless prairie potholes found in areas of hummocky moraine on the northern Great Plains. Sediments from a depth of 5.8 to 3.1 m at this 'kettle-fill' site in the Missouri Coteau upland of southern Saskatchewan, Canada, provides a record of vegetation, climate, and hydrologic changes within a small, ca 30 m diameter, closed-drainage basin from ca 10.2 to 5.8 ka. Plant macrofossil analyses of 67 samples, 6 ¹⁴C ages, and stratigraphy were used to identify 5 zones, representing the paleohydrological changes that followed deglaciation in southern Saskatchewan.Results of this study indicate that with the melting of residual stagnant ice a pond (>2 m deep) with abundant aquatic, emergent, and shoreline plants developed in the basin at ca 10.2 ka and persisted until at least ca 8.8 ka. During this time there was a shift in upland vegetation from a white spruce forest (Zone II) to a deciduous parkland at ca 10 ka (Zone III). As climate warmed, brackish and alkaline conditions developed coincident with shallowing of the pond at the end of Zone III. The perennial water phase ended at ca 8.8 ka and was followed by a low-water stand lasting ca 1100 years. Prairie fires and slopewash from unstable slopes were dominant sedimentological processes until ca 7.7 ka (Zone IV). Water levels began to rise and between ca 7.7 and 5.8 ka a semi-permanent pond was established in a grassland setting (Zone V). After ca 5.8 ka this prairie pothole wetland became ephemeral, to the point that plant macrofossils could not be preserved, a situation continuing today. Interactions between climate change, variability in local groundwater supply, and sedimentological processes likely account for the paleohydrologic events reconstructed at the Andrews site.     

Late Holocene Environmental Reconstructions from Lake Solai,Kenya

Playa lake systems tend to be overlooked archives of paleoenvironmental change due to the likelihood of a short and intermittent record of deposition. Groundwater-fed wetlands associated with these climate-sensitive playas, however, preserve changes in hydrologic budget and are thus valuable archives for semiarid regions. This study examines the paleoecological record of a groundwater-fed wetland from Lake Solai, Kenya. Biological proxies are used to reconstruct paleoenvironmental change and climate impacts over the past millennium. Dry conditions persisted between CE 1115 and 1490, followed by wetter conditions during the Little Ice Age. Near surface sediments indicate increasing anthropogenic impact through pastoralism.     

Spatially discontinuous modern sedimentation in Georgian Bay, Huron Basin, Great Lakes

High-resolution seismic reflection profile data show that the modern sediment cover (over the last 150 years) in Georgian Bay is thin and spatially discontinuous. Sediments rich in ragweed pollen, largely derived from siltation linked to land clearing and European settlement, form a thin, discontinuous veneer on the lakebed. Much of the lakebed consists of exposed sediments deposited during the late glacial or early postglacial. Accumulation rates of modern sediments range from < 0 mm/year (net erosion) to ∼3.2 mm/year, often within a few hundred metres spatially. These rates are much lower than those reported for the main basin of Lake Huron and the other Great Lakes, and are attributed to the low sediment supply. Only a few small rivers flow into Georgian Bay, and most of the basin is surrounded by bedrock of Precambrian gneiss and granite to the east, and Silurian dolostone, limestone and shale to the west. Thick deposits of Pleistocene drift, found on the Georgian Bay shoreline only between Meaford and Port Severn, are the main sediment source for the entire basin at present. Holocene to modern sediments are even absent from some deep basins of Georgian Bay. These findings have implications for the ultimate fate of anthropogenic contaminants in Georgian Bay. While microfossil assemblages in the ragweed-rich sediments record increased eutrophication over the last 150 years, most pollutants generated in the Georgian Bay catchment are not accumulating on the lakebed and are probably exported from the Bay.     

Postglacial history of a cedar swamp in southeastern Connecticut

Stratigraphic and palynologic analyses of sediment cores from a large mire, combined with geologic and hydrologic studies of its watershed, provide a late Quaternary record of environmental change at Cedar Swamp in southeastern Connecticut. Since deglaciation of the area, the basin has evolved from an open lake characterized by the rapid accumulation of allochthonous inorganic sediments to an ombrotrophic mire with peat accumulation keeping pace with the gradual rise in the water table. Lithostratigraphic, pollen, and chronologic evidence suggest that the long-term trend of basin infilling and paludification was interrupted by two intervals (14 000–13 000 and 8000–4700 yr B.P.) when the water table elevation dropped at least 1 m.     

Coastal Massachusetts pond development: edaphic,climatic, and sea level impacts since deglaciation

Kettle ponds in the Cape Cod National Seashore in southeastern Massachusetts differ in their evolution due to depth of the original ice block, the clay content of outwash in their drainage basins, and their siting in relation to geomorphic changes caused by sea-level rise, barrier beach formation, and saltmarsh development. Stratigraphic records of microfossil, carbon isotope, and sediment changes also document late-glacial and Holocene climatic changes.The ponds are separated into 3 groups, each of which follow different development scenarios. Group I ponds date from the late-glacial. They formed in clay-rich outwash, have perched aquifers and continuous lake sediment deposition. The earliest pollen and macrofossil assemblages in Group I pond sediments suggest tundra and spruce-willow parklands before 12 000 yr B.P., boreal forest between 12 000 and 10 500 yr B.P., bog/heath initiation and expansion during the Younger Dryas between 11 000 and 10 000 yr B.P., northern conifer forest between 10 500 and 9500 yr B.P., and establishment of the Cape oak and pitch pine barrens vegetation after 9500 yr B.P. Sedimentation rate changes suggest lowered freshwater levels between 9000 and 5000 yr B.P. caused by decreased precipitation on the Atlantic Coastal Plain. Lake sediment deposition began in the middle Holocene in Group II ponds which formed in clay-poor outwash. These ponds date from about 6000-5000 yr B.P. In these ponds sediment deposition began as sea level rose and the freshwater lens intersected the dry basins. The basal radiocarbon dates of these ponds and stable carbon isotope analyses of the pond sediments suggest a sea-level curve for Cape Cod Bay. Holocene topographic changes in upland and the landscape surrounding the ponds is reconstructed for this coastal area.Group III ponds in the late Holocene landscape of the Provincelands dunes originated as interdunal bogs about 1000 yr B.P. and became ponds more recently as water-levels increased. Peat formation in the Provincelands reflects climatic changes evident on both sides of the Atlantic region.This is the 8th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.     

Early Holocene brackish closed basin conditions in Georgian Bay, Ontario, Canada: microfossil (thecamoebian and pollen) evidence

Microfossils have been critical in unravelling the complex postglacial history of Georgian Bay. Thecamoebians (testate amoebae/rhizopods) record paleolimnological conditions, and pollen stratigraphy allows correlation across the basin, where sedimentation has been spatially and temporally discontinuous. Because parts of Georgian Bay have been non-depositional or erosional since the end of the Nipissing transgression (~5,000 (5,800 cal) BP), early Holocene features are exposed on the lakebed. Among these are shoreline features, such as submerged beaches and relict channels, associated with low-level Lake Hough that was driven far below the level of basin overflow. Cores taken throughout Georgian Bay record the existence of closed basin conditions that persisted several centuries around 7,500 (8,300 cal) BP, corresponding to the late Lake Hough lowstand. Evidence for hydrologic closure includes a low-diversity centropyxid-dominated thecamoebian fauna around the boundary between pollen subzones 2a and 2b in the Flowerpot Beach core, Flowerpot and Killarney basins, and in Severn Sound. This low-diversity centropyxid-dominated fauna is interpreted as recording the development of slightly brackish conditions as a result of a hydrologic deficit associated with relatively arid conditions in the Great Lakes basin during the early Holocene pine zone (~8,800–7,200 (9,900–8,050 cal) BP). The rest of the Holocene record in Georgian Bay (where it is preserved) is more diverse and dominated by difflugiid thecamoebians: predominantly Difflugia oblonga prior to human settlement, and Cucurbitella tricuspis since high-density human occupation and agriculture (and resulting eutrophication) began with the Wendat First Nations people around Severn Sound about 750 years ago. The implication that water budget fluctuations leading to discernible variations in lake level and water chemistry occurred in the relatively recent geologic past is significant to studies of global climate change and resource management in the Great Lakes, one of the world’s largest freshwater resources.     

Holocene Lake Evolution in the Elmali Basin,Southwest Turkey

The spatial coverage of paleoecological research from southwestern Turkey is expanded by reporting on a ca. 12,690 ¹⁴C yr BP (14,935 cal yr BP) proxy record recovered from the Elmali basin. Four AMS radiocarbon age determinations, the litho-stratigraphic analysis of a lake bed core, and the analysis of subsurface sediment samples from 15 shallow auger holes across the basin document sedimentation patterns during the Holocene. Based on the widespread occurrence of Chara gyrogonite, and several species of ostracoda and gastropoda, the Elmali basin was dominated by lacustrine and palustrine environments but was continually influenced by alluvial fan sedimentation. Contrasting stratigraphy in the Kara Göl and Avlan Gölü sub-basins is a result of basin morphology, and possibly hydrologic control by karst features, and sub-basin isolation due to alluvial fan development. The cyclical deposition of marl/lime mud, gyttja, and peat in the Kara Göl core is indicative of periodic fluctuations in water level across a broad shallow basin, whereas the continuous clay record observed at Avlan Gölü implies deep-water sedimentation within a plugged former karst collapse feature. Calcareous clay deposited between 14,935 and 11,180 cal yr BP signals the growth and expansion of paleo Lake Elmali, which at its peak during the late Pleistocene, may have inundated over half of the of the 180 km² Elmali basin.     

Reconstructing fluctuations of a shallow East African lake during the past 1800 yrs from sediment stratigraphy in a submerged crater basin

The sedimentology of an 8.22-m long core of late-Holocene deposits in the submerged Crescent Island Crater basin of Lake Naivasha, Kenya, is used to reconstruct decade-scale fluctuations in lake-surface elevation during the past 1800 yrs. Lake-depth inference for the past 1000 yrs is semi-quantitative, based on (1) relationships between lake level and bottom dynamics predicted by wave theory, and (2) historical validation of the effects of lake-level fluctuation and hydrologic closure on sediment composition in Crescent Island Crater and nearby Lake Oloidien. In these shallow fluctuating lakes, organic-carbon variation in a lithological sequence from clayey mud to algal gyttja is positively correlated with lake depth at the time of deposition, because the focusing of oxidized littoral sediments which dilute autochthonous organic matter before burial is reduced during highstands. The lake-level reconstruction for Lake Naivasha agrees with other adequately dated lake-level records from equatorial East Africa in its implication of dry climatic conditions during the Mediaeval Warm Period and generally wet conditions during the Little Ice Age. Crescent Island Crater survived widespread aridity in the early-19th century as a fresh weedy pond, while the main basin of Lake Naivasha and many other shallow East African lakes fell dry and truncated their sediment archive of Little Ice Age climatic variability.     

Early Holocene drought in the Laurentian Great Lakes basin caused hydrologic closure of Georgian Bay

Multiple proxies record aridity in the northern Great Lakes basin ~8,800–8,000 cal (8,000–7,200) BP when water levels fell below outlets in the Michigan, Huron and Georgian Bay basins. Pollen-climate transfer function calculations on radiocarbon-dated pollen profiles from small lakes from Minnesota to eastern Ontario show that a drier climate was sufficient to lower the Great Lakes, in particular Georgian Bay, to closed basins. The best modern climate analog for the early Holocene late Lake Hough stage in the Georgian Bay basin is Black Bass Lake near Brainerd MN. Modern annual precipitation at Brainerd is ~35% lower than at Huntsville ON, in the Georgian Bay catchment; warmer summers and colder, less snowy winters make Brainerd drier than the Georgian Bay snow belt. These values parallel transfer function reconstructions for the early Holocene from pollen records at five small lakes in the Georgian Bay drainage basin. Higher evaporation and evapotranspiration due to greater seasonality during the early Holocene produced a deficit in effective moisture in Georgian Bay that is recorded by the jack/red pine pollen zone that spanned ~8,800–8,200 cal (8,000–7,500) BP. This deficit drove late Lake Hough ~5 m below Lake Stanley in the Huron basin, following diversion of Laurentide Ice sheet meltwater from the Great Lakes basin. The level of Georgian Bay largely depends not on fluvial input from its own drainage basin, but rather from Lake Superior, where the early Holocene moisture deficit was greater. Reconstruction of paleoclimates in Minnesota, northwestern Ontario and Wisconsin produced a closed lake in the Superior basin, which removed the main water input to Georgian Bay. Once the inflow through the St. Marys River was reduced and inflow from other tributary streams was adjusted for isostatic and climatic differences, input was <5% of modern values. Consequent high evaporation rates produced a significant fall in lake level in the Georgian Bay basin and a negative water budget. This reduction in basin supply, together with the high conductivity of stagnant water in late Lake Hough inferred from microfossils in lowstand sediments, peaked at the end of the jack/red pine zone, ~8,300–8,200 (7,450 ± 90) BP. These major hydrologic changes resulting from climate change in the recent geologic past draw attention to possible declines of the Great Lakes under future climates.     

Ecological influences of Thule Inuit whalers on high Arctic pond ecosystems: a comparative paleolimnological study from Bathurst Island (Nunavut,Canada)

Until recently, major anthropogenic impacts on freshwater ecosystems were believed to be rare in North America prior to the period of European colonization. However, recent paleolimnological and archaeological data collected from the Canadian Arctic suggest that the whaling activities of Thule Inuit, who lived in small, nomadic communities, altered freshwater ecosystems centuries earlier. Using a comparative paleolimnological approach from two ponds situated adjacent to a former Thule winter settlement on south-eastern Bathurst Island (Nunavut, Arctic Canada), we record marked ecological changes in pond ecology due to eutrophication from the Thule’s activities. The geography of our study site provided an interesting and rare opportunity for a comparative paleolimnological study of long-term Thule impacts on polar limnology, because our two study ponds (only ~50 m apart) were nearly identical in size and in geological and climatic settings, but differed markedly in the magnitude of Thule influence. Here, we recorded striking changes in diatom species assemblages, spectrally-inferred primary production, and nutrient geochemistry, indicating eutrophication in a small pond draining 18 Thule whale houses. Input of marine-derived nutrients from sea mammal carcasses used by the Thule for both sustenance and the construction of winter settlements, as well as other anthropogenic activities, coincided with a notable increase in the eutrophic diatom taxon Stephanodiscus minutulus, whereas no comparable changes were recorded in the nearby control pond for the duration of the sedimentary record. Although the diatom changes recorded in the affected site persisted after the period of Thule occupation, the most recent sediments and water chemistry suggest that the pond has largely recovered to near pre-impact conditions.     

The influence of Asian summer monsoon variability on the water balance of a Tibetan lake

Past water-balance changes in Tibetan lakes are generally attributed to changes in the strength of the summer monsoon. However, the water balance of a lake reflects many different water fluxes, which are controlled by many climatic and hydrologic processes. In this research, weather data and evaporation models are used to determine the climatic cause of a recent water-balance change in Ahung Co, a lake in central Tibet. Between 1995 and 2001, lake level rose at least 20 cm and the lake began to overflow. Results indicate that an increase in summer monsoon precipitation over the lake and drainage basin is responsible for the rise in lake level. Stronger monsoon conditions between 1995 and 2001 also led to decreased lake evaporation and basin evapotranspiration due to increased clouds and humidity. This contributed to the rise in lake level, but to a much smaller extent than the increase in monsoon precipitation. Lake evaporation during the spring and fall was also reduced between 1995 and 2001 due to longer lasting ice cover. Variations in ice cover play a small role in the overall water balance of Ahung Co, however, because the lake area is small compared to the drainage basin area. If these results hold true for the past, water-balance fluctuations inferred from the geochemistry of sediments from Ahung Co provide a record of variations in monsoon precipitation during the Holocene.     

Holocene environmental changes in the Whitefish Dunes Area, Door Peninsula, Northern Lake Michigan Basin, USA

A two meter sediment core taken from an inter-dune pond in Whitefish Dunes State Park, Door Peninsula, Wisconsin, provides a record of paleoenvironmental changes in the area from approximately 3600 RCYBP to 5100 RCYBP. The hydrology of the Whitefish Dunes pond is now apparently dependent on ground water recharge from local precipitation that infiltrates into the dunes and from Clark Lane, making it very sensitive to low magnitude, short term climate fluctuations.Changes in lithology, molluscan species diversity, oxygen isotopes from gastropod shells, and sediment organic content permit division of the core into several intervals representing different environmental regimes. The basal sediments are older than 5130 RCYBP and resemble those of the Liberty Grove Member of the Pleistocene Horicon Formation. The oldest Holocene sediments recorded in the core are an organic rich layer dated at 5130 RCYBP, which accumulated in a depression on the surface of a sand bar. Continued rise in the local water table during the Nipissing Transgression produced paludal conditions in the area, which were followed by the onset of lacustrine conditions. During this interval, the area supported a diverse molluscan fauna. Minor, short term climate fluctuations are reflected in sediment and oxygen isotope changes which are absent from the record in deep water cores from Lake Michigan. This phase in the history of the area ended with the eutrophication of the pond and the development of dunes. Information from Whitefish Dunes pond and other onshore sites within the Lake Michigan basin are an important source of data for reconstructing the history of water levels and climate changes for this area of the Great Lakes drainage.     

A ca. 800-Year Lithologic Record of Drought from Sub-annually Laminated Lake Sediment, East Java

Lithostratigraphic analyses of a sub-annually laminated core from Ranu Lamongan, a maar lake on the island of Java, document considerable changes in the lake’s chemistry and water balance over the past ca. 800 calendar years. Composition of the dark (clastics) and light (diatoms and/or calcium carbonate minerals) couplets suggests that these laminations form in response to seasonal changes in rainfall and water-column overturn in the lake. Calcium carbonate is not continuous in the core, and when it occurs it varies, sometimes abruptly, in carbonate phase and elemental composition (low Mg-calcite Mg-calcite, and aragonite). A significant correlation between Mg/Ca changes and δ¹⁸O variations in authigenic calcium carbonate suggest the basin is highly sensitive to hydrologic variation. Lithologic data suggest calcium carbonate precipitates and thus records hydrologic conditions during the dry season – a season in which rainfall anomalies are highly correlated with the phase of ENSO. Our carbonate-based record of Mg/Ca shows variability in evaporative concentration on a quasi-seasonal frequency for the past ca. 800 years. Our record shows two multi-decadal periods of drought – ca. 1275–1325 and ca. 1450–1650 CE – the latter of which was especially strong and/or prolonged. Our record also shows a possible change in drought frequency at around 1650 CE, in which periods of calcium carbonate precipitation and Mg/Ca change shifted from multi-decadal to interannual variability. Given the strong correlations between modern-day drought in East Java and ENSO variability, our drought record may indicate a regime shift in the behavior of the ENSO system about 350 years ago. Finally, comparisons between our record and others suggest that variation in ENSO on centennial and sub-centennial scales is not strongly associated with changes in the global mean climate state.     

Reconstructing lake and drainage basin history using terrestrial sediment layers: analysis of cores from a post-glacial lake in New England, USA

Four sediment cores and twenty-five ¹⁴C ages from Ritterbush Pond in northern Vermont provide a detailed and continuous temporal record of Holocene lake and watershed dynamics. Using visual logs, carbon content, magnetic susceptibility, stable isotope signatures, and X-radiography, all measured at 1-cm scale, we identify and date discrete layers of terrestrially-derived sediment in the organic-rich, lacustrine gyttja. These inorganic layers range in thickness from <1 mm to >10 cm and range in grain size and sorting from homogeneous silt to graded sand. AMS radiocarbon ages both from macrofossils within the thickest layers, and gyttja bracketing these layers, provide the basis for correlation among the cores, the dating of 52 basin-wide sedimentation events, and the development of a detailed sedimentation chronology for the Holocene.Physical, chemical, and isotopic analyses suggest the inorganic layers are terrestrially derived and result from hydrologic events large enough to erode and transport sediment from the watershed into the pond. The temporal and spatial distribution of the inorganic layers suggests changing basin-wide sedimentation and thus erosion dynamics since deglaciation over 12,000 years ago. Specifically, for intervals lasting 400 to 1000 years, during the early (>8600 cal yBP), middle (6400 to 6800 cal yBP) and late Holocene (1800 to 2600 cal yBP), the Ritterbush Pond watershed eroded more rapidly than at other times and terrestrially derived material poured into the pond. Analysis of Ritterbush Pond sediments demonstrates the potential for North American lakes to preserve a record of drainage basin dynamics.     

Sources of Sediment in Lake Pepin on the Upper Mississippi River in Response to Holocene Climatic Changes

Sediments from Lake Pepin on the Mississippi River, southeastern Minnesota, are used as provenance tracers to assess variations in hydrology and sediment-transport during the middle Holocene. Three rivers contribute sediment to Lake Pepin, and each catchment is characterized by a distinctly different geologic terrain. The geochemical fingerprint for each drainage basin was determined from the elemental composition of heavy minerals in the silt-sized fraction of modern sediment samples. Down-core elemental abundances were compared with these fingerprints by use of a chemical-mass-balance model that apportions sediment to the source areas. We observed a decreased contribution from the Minnesota River during the interval ~6700–5500 ¹⁴C yr BP, which we attribute to decreased discharge of the Minnesota River, likely controlled by a combination of precipitation, snow melt, and groundwater input to the river. This hydrologic condition coincides with the mid-Holocene prairie period recorded by fossil pollen data. The occurrence of this feature in a proxy record for hydrologic variations supports the hypothesis that the mid-Holocene prairie period reflects drier conditions than before or after in midwestern North America.     

High-resolution paleoecological analysis of recent disturbance in a southern Chilean Nothofagus forest

A 1600 year paleoecological record of environmental change is developed from a small lake in the Nothofagus forest of southern Chile (45.5°S, 72°W). High resolution fossil pollen, charcoal, sedimentological, and chrysophycean stomatocyst analyses are used to investigate the impacts of natural and anthropogenic disturbances on terrestrial and lacustrine environments. Chronological control is based on a combination of ²¹⁰Pb and ¹⁴C dating. Temporal resolution during the past 150–200 yr is ca. 8 yr/sample. The macroscopic charcoal record correlates very closely with historical and dendroecological records of 20th century anthropogenic burning in this region. The chrysophyte stratigraphy indicates that this burning had immediate impacts on the lake itself, while the pollen record provides evidence for a succession of vegetation changes lagging slightly behind the disturbance. These palynological changes are very similar to the pollen signal of European disturbance in northeastern North America. Pre-European shifts in chrysophyte assemblages may be due to the influence of earthquake activity on the lake, though there is no corroborative evidence in the pollen or charcoal records. This study demonstrates that high resolution paleoecological methods can be used to help bridge the temporal gap between traditional ecological and paleoecological studies of environmental change in the temperate forests of southern South America.     

Distribution of diatoms among intermittent ponds on the Atlantic Coastal Plain: development of a model to preduct drought periodicity from surface-sediment assemblages

Inferences of past climate from the fossil record in lakes rely on the accurate quantification of a relationship of fossilizing organisms to their environment. Whereas the relationship of diatoms to water chemistry parameters has been modeled in many systems, few studies adequately address the relationship of diatoms to physical properties, such as water depth or hydrology, that may be more directly tied to climate. We examined the composition of modern diatoms in surface sediments of 75 isolated ponds (mostly Carolina bays) of the Atlantic Coastal Plain to: (1) assess the influence of physical and chemical variables on the distribution of diatoms among ponds of the region, and (2) develop a model that predicts hydroperiod (a measure of pond permanence) from diatom assemblages. We constructed two hydroperiod calibration models: the first infers hydroperiod from the weighted-average optima and tolerances of taxa along the hydroperiod gradient, the second bases inferences on the hydroperiod estimates of compositionally similar samples. Both approaches incorporate a-priori and post-hoc tests of assumptions often inherent in the construction of transfer functions. Diatom assemblage composition had strong, approximately linear relationships to hydroperiod, water depth, and calcium concentration in non-metric multidimensional ordination space; effects of other variables, including pH, were non-linear or ambiguous. Overall, the assemblages reflected the dilute, acidic chemical characteristics of bays. The assemblages contained differing abundances of euterrestrial, benthic and planktonic taxa, depending on a pond's susceptibility to drying. A weighted-averaging regression model based on taxon-specific hydroperiod optima generated adequate, unbiased hydroperiod inferences from diatom species composition (r² = 0.81). This model may be used to infer past drought episodes from fossil diatom assemblages at appropriate sites on the Atlantic Coastal Plain.     

Late Holocene environmental changes inferred from diatoms in a lake on the western Taimyr Peninsula, northern Russia

Changes in the diatom assemblages preserved in a sediment core taken from a small lake located north of arctic treeline on the western Taimyr Peninsula, Russia, were examined in order to investigate late Holocene (i.e., ca 5000 cal yr BP to present) climatic and environmental changes within the region. Early diatom assemblages were dominated by benthic Fragilaria taxa and indicate a transitional phase in the lake history, most likely reflecting lake development and environmental change associated with treeline retreat to the south of the study site. Concurrent with pollen and macrofossil evidence of a vegetation shift to shrub tundra in the catchment basin at ca 4200 cal yr BP, an increase in cold-water taxa, followed by little change in diatom assemblages until ca 2800 cal yr BP, suggests that conditions were relatively cool and stable at this time. The last 2000 years of the Middendorf Lake record have been marked by fluctuating limnological conditions, characterized by striking successional shifts between Fragilaria pinnata and Aulacoseira distans var. humilis. Recent conditions in Middendorf Lake indicate an increase in diatom taxa previously rare in the record, possibly associated with twentieth-century climatic warming. The Middendorf Lake record indicates that significant limnological change may occur in the absence of catchment vegetation shifts, suggesting late-Holocene decoupling of aquatic and terrestrial responses to climatic and hydrological change. Our study results represent one of the few paleoecological records currently available from northern Russia, and highlight the need for further development of calibration data sets from this region.     

Paleoenvironmental Changes in the Lake Baringo Basin,Kenya, East Africa Since AD 1650: Evidence from the Paleorecord∗

This article presents some of the findings of a multi-indicator investigation of the history of vegetational changes and land degradation in the Lake Baringo basin, Kenya, East Africa, during the Late Holocene. ¹⁴C- and ²¹⁰Pb-dated record of the lithostratigraphy is used to reconstruct the paleoenvironment in the region of the lake. The stratigraphic record from Lake Baringo reveals the presence of two abrupt dry episodes at ca. AD 1650 and AD 1720 in east Africa that led to drying up of the lake. The record also shows evidence of a third period of desiccation at ca. AD 1880, which resulted in lowering of the lake level and is corroborated by oral tradition from the area. This article shows the potential of how the paleoenvironmental record can be combined with the historical record to understand East Africa's paleoenvironment.