Rates of peat accumulation over the past 200 years in fiveSphagnum-dominated peatlands in the United States

Using²¹⁰Pb-dating of peat cores, corroborated by pollen and acid-insoluble ash approaches, rates of vertical height growth, dry mass accumulation, and organic matter accumulation were determined for fiveSphagnum-dominated peatland sites (one in Minnesota, one in Pennsylvania, one on the Maryland/West Virginia border, two in West Virginia), spanning a mean annual temperature range of 4.5 °C and differing in total annual precipitation by a factor of almost 2. Site differences in rates of vertical height growth and dry mass accumulation were documented, but both within-core and between-site differences in bulk density and ash concentrations of peat confound efforts to relate vertical height growth and dry mass accumulation to net organic matter accumulation. Taking bulk densities and ash concentrations into account, rates of net organic matter accumulation over the past 150–200 years were strikingly similar at four of the five sites, an unexpected result given the general trend that with decreasing latitude, peat deposits become older, thinner, and more highly decomposed. More comprehensive studies are needed in which net organic matter accumulation is determined at several locations within a single peatland, at several peatlands within a particular geographic/climatic region, and at peatland sites in different geographic/climatic regions. If additional studies confirm that recent (past 200 years) net organic matter accumulation is relatively insensitive to broad-scale regional climatic differences, boreal and subarctic peatlands may continue to function as a net sink for atmospheric CO₂ and a net source of atmospheric CH₄ with no change in rates of net organic matter accumulation, even under predicted scenarios of global climate change.This publication is the third paper in a series of papers presented at the session on Past Climatic Change and the Development of Peatlands at the ASLO and SWS Meetings in Edmonton, Canada, May 30–June 3, 1993. Dr P. Kuhry and Dr S. C. Zoltai are serving as Guest Editors.     

Application of210Pb in soils

The spectroscopic measurement of soil samples is described.²¹⁰Pb data from a soil survey in western-Europe are briefly reviewed. The average²¹⁰Pb flux from the atmosphere, as determined from²¹⁰Pb _exc soil inventories, is 90 Bq m^–2a^–1. A simple one-dimensional box-chain model is described. The model simulates the vertical, post depositional transport of natural and fall-out radionuclides in the soil. Simulation of measured²¹⁰Pb _exc ,¹³⁴Cs,¹³⁷Cs, and²⁴¹Am soil profiles shows that mixing (bioturbation) is a very efficient transport mechanism. Lead seems to be strongly fixed to organic and clay particles. It is transported by the displacement of the organic and clay carrier substances. The mean residence time of lead, caesium, plutonium and americium in organic rich forest soils is in the order of 250–1000 years. An applicability study in investigate the use of²¹⁰Pb in erosion problems showed erosion rates from 60–180 g m^–2a^–1 on organic rich forest and meadow sites with 10°–25° slopes.This is the eighth of a series of papers to be published by this journal following the 20 ^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.     

Vegetation and climate of the last glacial-interglacial cycle in Southern Illinois,USA

Raymond Basin and Bald Knob Basin, Montgomery County, Illinois, formed as kettles during Illinoian time. Fossil pollen from these basins provides information on vegetation and climate during the last glacial-interglacial cycle. The pollen profile at Raymond Basin contains an expanded Sangamonian section and an early Wisconsinan section, but both are missing the late Wisconsinan. The ages for the following pollen zones are estimated by correlation with the deep-sea ¹⁸O record.In the late Illinoian,Picea-Pinus pollen zone 1, dating from about 150–130 ka, represents an apparently closed boreal coniferous forest indicating a cold late-glacial climate. The Sangamonian includes three major pollen zones ranging from about 130–75 ka. The early Sangamonian is represented byQuercus-Ulmus-Carya-Fagus dominance in zone 2, indicating vegetation comparable to the modern deciduous forest and climate that was warm and moist. The middle Sangamonian in zone 3, which is characterized byAmbrosia-Poaceae-Cupressaceae-Quercus pollen, suggests a savanna vegetation and a warm, dry climate. The late Sangamonian is subdivided into aQuercus-Ulmus-Carya subzone (4a) that indicates a mesic forest and greater precipitation; aQuercus-Ambrosia subzone (4b) that suggests drier climate and savanna conditions; and aQuercus-Liquidambar-Carya subzone (4c), containing the southeastern forest element,Liquidambar, suggesting the peak in warmth and moisture. The early Wisconsinan is represented by a transitionalPicea-Chenopodiineae zone (5). This unusual assemblage suggests a cool prairie-like vegetation, perhaps with scatteredPicea trees at the end of that depositional interval. The Middle Wisconsinan is marked by the return of high percentages ofPicea andPinus pollen in zone 6. The latest pollen zone (7) is dominated by Chenopodiineae pollen and is absent at Raymond Basin. It is most likely Holocene in age, and probably represents prairie conditions and warm, dry climate.Apparent surface-temperature and apparent effective-moisture curves were derived from the first detrended correspondence analysis (DCA) axis of the pollen data. The first axis correlates well with the normalized deep-sea ¹⁸O curve of sea surface temperature, and the second is controlled mainly by precipitation. The interglacial vegetation differs from that predicted by models driven by orbitally-induced insolation curves.This is the 9th 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.     

Evaluating the mobility of137Cs,239+240Pu and210Pb from their distributions in laminated lake sediments

Post-depositional mobility of¹³⁷Cs,^239+240Pu and²¹⁰Pb was assessed in six small lake basins by comparing sedimentary nuclide profiles with their known fallout history. Laminae couplets, when present, were determined to be varves because the¹³⁷Cs and^239+240Pu 1963 fallout peaks are present in laminae couplets corresponding to years 1962–1964. There is no evidence of mobility of²¹⁰Pb, because 1) mass accumulation rates based on²¹⁰Pb agree with those based on¹³⁷Cs and^239+240Pu peak depths and with those based on varve counts, and 2)²¹⁰Pb ages agree with varve ages. Significant mobility of¹³⁷Cs is evident from the penetration of¹³⁷Cs to depths 15–20 cm deeper than^239+240Pu. Deep penetration of¹³⁷Cs in spite of a sharp gradient below the peak is interpreted by a numerical model to suggest that¹³⁷Cs is present in two distinct forms in these sediments, 67–82% as an immobile form and 18–33% reversibly adsorbed with a K _d of approximately 5000. The profiles can be interpreted equally well assuming a small portionof the total¹³⁷Cs was present as an extremely mobile phase (K _d 5000) in the months to years following peak fallout, slowly becoming more strongly adsorbed. High NH ₄ ⁺ concentrations in porewaters may enhance diffusion of the mobile form of¹³⁷Cs, but not of the immobile form of¹³⁷Cs that defines the sharp gradient. Mobility of¹³⁷Cs is likely also enhanced by the low clay content and the high porosity of these sediments. Thus the first detection of¹³⁷Cs in the sediments cannot automatically be assumed to correspond to a date of 1952 (initial testing of thermonuclear weapons), although the depth of the peak can be assumed to correspond to 1963 (the year of maximum fallout from testing of thermonuclear weapons).^239+240Pu is a more reliable sediment chronometer than¹³⁷Cs because it is significantly less mobile.This is the sixth of a series of papers to be published by this journal following the 20 ^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P.G. Appleby is guest editing this series.     

A postglacial pollen and nonsiliceous algae record from Gegoka Lake, Lake County, Minnesota

As part of a multidisciplinary investigation of the Misiano archaeological site, pollen and nonsiliceous algae were recovered from a 262 cm core from Gegoka Lake, Lake County, Minnesota. The palynomorph assemblage from Gegoka Lake records changes in local and regional vegetational and lake productivity over the past 10 000 years. Pollen spectra indicate that vegetation progressed from a shrub parkland/open conifer forest, to a spruce-pine forest, to a mixed conifer-hardwood forest. Pinus banksiana/resinosa is replaced by Pinus strobus about 7000 years ago. A small rise in Gramineae in the upper 17.5 cm of the core is attributed to the expansion of wild rice (Zizania aquatica) in Gegoka Lake. Four cycles of nutrient enrichment are indicated by the Pediastrum and Scenedesmus maxima in the nonsiliceous algae record. Oscillations in the nonsiliceous algae abundance probably result from changing environmental and/or limnologic conditions. The decline in nonsiliceous algae in the upper 57.5 cm of the core suggests that there has been an apparent shift to more nutrient poor conditions in Gegoka Lake in the recent past.This is the 17th 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.     

Multi-proxy data from Kaksoislammi Lake in Finland: dramatic changes in the late Holocene cladoceran assemblages

A small lake, Kaksoislammi (60° 3830N, 24° 4550E), in southern Finland was studied for Cladocera, diatoms and pollen from a core which covers the entire Holocene. The diatom remains indicate a steady development from alkaliphilous taxa towards the dominance of acidophilous forms and lowering pH in the late Holocene. About 1800–1700 BP, dramatic changes took place in the microfauna, mainly the planktonic Cladocera. Bosmina longirostris, the dominant species, suddenly disappeared, and Daphnia, Chydorus sphaericus and Chaoborus increased. The change is simultaneous with a decline of the diatom-inferred pH to 4.8. It is probable that there was a sudden, profound change in predator-prey relationships. The acidity of the lake water probably increased to such a low level that it led to the disappearance of even the most acid-tolerant fish. Consequently invertebrate predators increased and quickly altered the species composition in the lake. There is also pollen evidence of the onset of Iron Age cultivation and grazing almost simultaneously with the faunal change. Therefore, it cannot be ruled out that the sudden lowering of pH was indirectly caused by prehistoric human activity; possibly the acidic peatland surrounding the lake was disturbed.     

Post-glacial vegetation history of the Mission Mountains,Montana

A pollen record from Rock Lake in the Mission Mountains, northwestern Montana reveals a four-zone sequence reflecting Holocene vegetation change. Chronologic control is provided by two well-known tephras, Glacier Peak (11 200 yr B.P.) and Mazama (6800 yr B.P.). The presence of Glacier Peak tephra above the basal inorganic sediments indicates deglaciation prior to 11 200 yr B.P. Colonizing vegetation (Zone I) after the fall of Glacier Peak tephra was dominated byArtemisia (sage) andAlnus (alder). The presence ofAbies needles,Picea needles, and oneTaxus needle in the core demonstrates that these taxa were at Rock Lake at the time Zone II sediments were deposited. The increase inPinus,Picea, andAbies pollen in Zone II (10 850-4750 yr B.P.) suggests warmer and drier conditions prevailed, and may record the Hypsithermal. The pervasiveness ofPicea andAbies pollen in Pollen Zone III (4750-3350 yr B.P.) suggests the emergence of the modern subalpine forest. Pollen Zone IV (3350 yr B.P.-present) is characterized by relatively little change in the pollen assemblages. One noted change, however, is the increase of Cyperaceae (sedge), which may indicate an expansion of shore-line around the lake, possibly reflecting increased precipitation.This is the 5th 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.     

Wetland development at Elk Island National Park,Alberta, Canada

Elk Island National Park, located in the southern part of the boreal forest of western Canada contains a wide variety of wetland types. The relationship of extant wetland vegetation is strongest to ionic water chemistry gradients and physical parameters such as mean water level, hummock height, elevation of the site, and the number of inlet and outlet streams. Successional development of a range of wetlands including bog, fen, and marsh are examined through stratigraphy and macrofossils. Analysis of three peat deposits located in small morainic depressional basins reveals that the wetlands are younger than 5620 yBP, and have early aquatic seral stages. Two parallel successional sequences were identified in the park, an aquatic — rich fen — poor fen — bog sequence and an aquatic — marsh — swamp sequence. Digression of the wetlands into different extant wetland types is not related to the age of the formation, peat depth, nor climate, but appears to be related to local edaphic features such as site elevation and catchment hydrology.This publication is the second paper in a series of papers presented at the session on Past Climatic Change and the Development of Peatlands at the ASLO and SWS Meetings in Edmonton, Canada, May 30–June 3, 1993. Dr P. Kuhry and Dr. S. C. Zoltai are serving as Guest Editors.     

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.     

Stable carbon isotope ratios in lake and swamp sediments as a proxy for prehistoric forest clearance and crop cultivation in the Neotropics

Close correspondence between stable carbon isotope ratios ( ¹³ C), pollen, and charcoal profiles in sediment cores from Laguna Zoncho and Machita swamp, Costa Rica, shows that prehistoric forest clearance and crop cultivation can be detected in the stable carbon isotope ratios of total organic carbon ( ¹3C _TOC ). Analyses of δ 13C _TOC complement evidence from pollen, charcoal, and phytoliths and provide a proxy that is sensitive to the intensity and/or proximity to core sites of prehistoric forest clearance and agriculture in watersheds. Stable carbon isotope analyses are particularly useful in situations in which other evidence of forest clearance and agriculture is limited.     

The Fossil Record of Freshwater Micro-Algae <Emphasis Type="Italic">Pediastrum</Emphasis> Meyen (Chlorophyceae) in Southern South America

The structure and function of many Korean ecosystems have been rapidly modified since the 1960s when industrialization of the nation began. Ulsan City was the first in Korea to develop into a major industrial complex. To assess anthropogenic impacts on ecosystems surrounding Ulsan, sediment cores were collected from Mujechi-neup (bog) and Sanggae reservoir of Ulsan, and these cores were ²¹⁰Pb dated using the CRS model. Physical and chemical characteristics and pollen were analyzed, and the rates of sediment accumulation were calculated. Unsupported ²¹⁰Pb inventories in Mujechi-neup and the Sanggae reservoir were 18.04 and 16.53 pCi cm^–2, and the corresponding ²¹⁰Pb fluxes were 0.56 and 0.52 pCi cm^–2 yr^–1, respectively. The overall accumulation rate of dry matter was 0.26 kg m^–2 yr^–1 since 1852 (14 cm in depth) in Mujechi-neup. In the Sanggae reservoir, the accumulation rates of dry matter were increased from 2.1 in 1965 to 6.0 kg m^–2 yr^–1 in 1999. Pollen analysis revealed that three pollen zones existed in Mujechi-neup; a Pinus pollen-dominated zone from 0 to 5 cm in depth (1974 year), an Alnus pollen-dominated zone from 5 to 15 cm in depth (1827 year), and a Quercus pollen-dominated zone below 15 cm in depth. The shift from an Alnus dominated zone to a Pinus dominated zone was related to the Korean War between 1951 and 1953 and reforestation activities in the 1970s. In the Sanggae reservoir, there was an increase of Humulus pollen since 1996, an increase of Ambrosia pollen since the 1960s and the decrease of Graminae pollen since 1993 with the expansion of the industrial area. Similar to Mujechi-neup, the Sanggae reservoir also consists of three pollen zones: a Pinus and Typha pollen-dominated zone from 0 to 10 cm in depth (since 1993), a Graminae pollen-dominated zone from 10 to 22 cm in depth (between 1947 and 1993), and a Pinus pollen-dominated zone below 22 cm in depth (before 1947). The increase of Typha and Humulus in the 1990s indicates an increase in the inflow of nutrients into the wetlands. Also, pollen analysis revealed that Ambrosia was introduced in the 1960s during industrialization of the area. The sediment was composed of humic peat in Mujechi-neup and was composed of clay in the Sanggae reservoir. High LOI, P, and low C/N ratio between the depths of 5 to 7 cm in the Sanggae reservoir indicates an increased input of P and N. As a result, the growth of Typha was at its maximum. The total Pb content in Mujechi-neup has been increasing since the 1870s, and its accumulation in the Sanggae reservoir has been increasing since the 1960s. Thus, the history of local-scale disturbances and human activities in the watershed was reconstructed through paleoecological studies in Ulsan.     

Water requirements for germination and early seedling establishment in four African savanna woody plant species

The aim of this study was to determine the water requirements for germination and early seedling establishment of four African savanna tree species, namelyAcacia karroo, A. nilotica, A. tortilisandMundulea sericea. The acacias are characteristic of nutrient-rich, andM. sericeaof nutrient-poor savannas. Imbibition times of scarified seeds ofA. karroo(4 h),M. sericea(6 h) andA. tortilis(8 h) were rapid relative toA. nilotica(28 h) and were inversely correlated with seed size. Imbibed water is lost in about 2 h after drying at 25°C for all species exceptA. nilotica(only 70% moisture loss). Seeds ofA. karrooandM. sericeakept at 40°C dried to below their original moisture contents within 2 h. Seeds ofA. niloticaandA. tortilisdried at 40°C lost viability relative to (undried) control seeds, while there was no significant loss of viability forA. karrooandM. sericea. For fully imbibed seeds to germinate in sandy savanna soils, all four species required at least the equivalent of 3 mm rainfall every 2 days under mild greenhouse conditions. However, most ungerminated seeds were still viable despite drying-out from a fully imbibed state. Maintenance of soil at 50% field capacity (FC), or watering to FC every 9th day is the maintenance requirements for 2-week-old seedlings ofA. niloticaandM. sericeato continue growth for a further 5 weeks under greenhouse conditions, althoughA. niloticamostly survived 25% FC with one seedling surviving 12·5% FC. Root penetration was rapid in sandy soils, withA. niloticaandM. sericeaattaining a depth of 40 cm within 15 days. Frequent, but not necessarily high, rainfall appears to be essential for germination and seedling survival over the first 7 weeks.     

Sir Vivian Fuchs FRS 1908-1999

A palaeoecological investigation in the Bodø area (Salten, Nordland) records a shift from Betula-dominated forest to mostly treeless vegetation during the last 4000 years. Corylus avellana, now at its northern limits in this region, was previously more abundant within the area. Deforestation started c. 3500–3000 BP, most likely connected to human activity during the early Bronze Age. After the onset of anthropogenic disturbance, heathland was established, initially with a dominance of Calluna vulgaris and Empetrum. Later on, c. 900 BP, Calluna was dominant. The heathland was probably subject to management, including burning. It should be viewed as a part of a northern extension of the western European heathland vegetation of the coast of SW Norway.     

Recent pollen sedimentation in Los Padres pond,Buenos Aires Province,Argentina

This paper presents the results of a palynological study of surface sediments and two short cores from Los Padres pond, Province of Buenos Aires, Argentina. The surface samples were taken from the center of the pond and from various near shore sites. Herbaceous taxa were abundant in the pollen spectra, representative of the surrounding steppe vegetation. Arboreal taxa represent cultivated trees located near the pond. Using multivariate analysis techniques, surface samples were divided into three groups: two of them reflect local conditions (high sand and organic matter concentrations) and are represented by local vegetation (Cyperaceae andMyriophyllum) and from the surrounding areas (Pinus, Plantago, Brassicaceae). The third group better represents the regional vegetation; it is characterized by grass steppe with shrubby communities found in the surrounding hilly areas. Pollen assemblages of two short cores were interpreted by analogy with the recent samples. The data provide information about the vegetational changes that have occurred since the first settlement, ca. 200 years ago. The presence of pollen types related to anthropogenic activities (Cupressus andEucalyptus) allowed the identification of different use of soils in the area. Historical evidence supports the conclusion.This is the third 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.     

History of a presently slightly acidic lake in northeastern Alberta,Canada as determined through analysis of the diatom record

The Holocene sedimentary diatom record from Otasan Lake, Alberta, has been analyzed to determine the development of this presently slightly acidic lake. The changes in the lake have been linked to the development of the Sphagnum-dominated catchment. Analysis of the stratigraphic data revealed four distinct zones. The lake record began ca. 8200 yrs BP with a benthic and alkaline diatom assemblage dominated by Ellerbeckia arenaria (Moore) Crawford. At ca. 7300 yrs BP planktonic species began to increase and dominate indicating increased water levels, decreased turbidity, and increased nutrient levels. Throughout the Holocene the peatland in the catchment encroached toward the modern lake margin and by ca. 5000 yrs BP lake acidity had changed sufficiently such that acidic diatom species dominated. Tabellaria flocculosa (Roth) Kütz.v. flocculosa Strain IIIp sensu Koppen dominated the record from ca. 5000 to ca. 3100 yrs BP. The lowest lake water pH was inferred for this zone. From ca. 3100 yrs BP to the present Fragilaria species, primarily F. construens v. venter (Ehr.) Hustedt, dominated the diatom assemblage. Diatom productivity and inferred pH were interpreted as stable. From correspondence analysis of the fossil samples, and from species assemblages, underlying gradients of pH, nutrient level, and water depth were inferred. The change from alkaline to slightly acidic conditions took place between ca. 8200 and ca. 5000 yrs BP. From ca. 3000 yrs BP to the present, lake water pH has remained fairly constant. Nutrient levels and water depth were inferred to have altered together. After ca. 8200 yrs BP, nutrients and water level began to increase until ca. 6000 yrs BP. Then, there was a gradual decline in these variables over the most acidic zone until ca. 3000 yrs BP, after which they, too, have remained fairly constant. Dominant Boreal Upland Vegetation was established by ca. 7200 yrs BP, and it was inferred that dominant climate patterns had been established at that time, but small changes in climate have occurred and the landscape in northeastern Alberta has only been stable for the last 3000 years.     

A sedimentary record of human disturbance from Lake Miragoane,Haiti

Lake Miragoane, Haiti is one of the largest, natural freshwater lakes in the Caribbean (A=7.06 km², z_max=41.0 m, conductivity = 350 S cm^–1). Lake waters are dominated by calcium and bicarbonate ions. The lake was thermally stratified, and oxygen profiles were clinograde during summer visits in 1983 and 1985. A 72-cm mud-water interface core was taken near the center of the lake and dated with ²¹⁰Pb. The local ²¹⁰Pb fallout rate is low (0.09 pCi cm^–2 yr^–1), about 20% of the global average. Bulk sedimentation rates ranged from 0.008 to 0.030 g cm^–2 yr^–1 during the past 130 years (0–8 cm depth). Sediment geochemistry and pollen have been analyzed in the topmost 58 cm of the section. Tentative ages were assigned to the core by extrapolation of ²¹⁰Pb dates. According to this preliminary chronology, the bottom part of the core (58–30 cm) records pre-Columbian sedimentation (1000–500 B.P.) and contains pollen evidence of intact, dry and mesic forest. Pre-Columbian deposits are rich in organic matter (x = 30%) and relatively poor in carbonates (x = 15% as CO₂). The top 30 cm of the core preserve the record since European contact (500 B.P. to present). Pollen data reveal two episodes of deforestation following European arrival. Consequent soil erosion is documented by a decrease in organic matter content (x = 15%) and an increase in carbonates (x = 27% as CO₂). Surficial sediments reflect the widespread deforestation and soil loss that characterize the watershed today.     

The impact of the Laacher See Volcano (11 000 yr B.P.) on terrestrial vegetation and diatoms

Late-glacial lake sediments containing the Laacher See Tephra (LST, 11 000 yr B.P.) have been analyzed for their pollen and diatom content at three sites at varying distances from the volcano and on different bedrock geologies. The aim was to test the null hypothesis that this major volcanic eruption had no effect on terrestrial pollen or aquatic diatom assemblages. The pollen spectra at all sites show a short-lived increase in grass pollen following the LST. Partial redundancy analysis and associated Monte Carlo permutation tests suggest, however, that the LST had no statistically significant effect at two sites but it had a statistically significant impact on the pollen assemblages at the site nearest (60 km) to the volcano.The diatom assemblages at the three sites changed individualistically after the LST deposition, with increases inAchnanthes minutissima at one site, an expansion ofAulacoseira species at another, and an increase ofAsterionella formosa andFragilaria brevistriata at the third site. Partial redundancy analysis and associated permutation tests suggest a statistically significant change in diatoms in relation to the LST and associated changes in sediment lithology at the one site situated on acidic bedrock. No significant impacts were found at the sites on volcanic or calcareous rocks. Due to the interaction between tephra and sediment lithology, it is not possible to conclude if the statistically significant diatom changes were a direct result of the LST deposition or an indirect result of lithological changes following LST deposition.This is the first paper in a series of papers published in this issue on high-resolution paleolimnology. These papers were presented at the Sixth International Paeolimnology 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.     

How reliable is the210Pb dating method? Old and new results from Switzerland

We present a historical overview of applications of²¹⁰Pb dating in Switzerland with a special emphasis on the work performed at the University of Bern. It is demonstrated that the average specific activity of²¹⁰Pb in the lower atmosphere is very constant and does not show seasonal variations. We then concentrate on new results from Lobsigensee, a very small lake, and on published and new data from Lake Zurich. Several²¹⁰Pb profiles from these lakes show obvious disturbances and a disagreement of the resulting sedimentation rate when compared to that for the 23 years defined by¹³⁷Cs peaks of 1986 (Chernobyl) and 1963 (bomb fallout).A mean sedimentation rate of about 0.14 g cm^–2 y^–1 is found in the oxic and suboxic center part of Lake Zurich. In the oxic locations, the²¹⁰Pb flux to the sediments was close to the atmospheric input of about 1/60 Bq cm^–2 y^–1. In other parts of the lake a significant deficit in the inventory of²¹⁰Pb was found in the sediments. This could be due to a chemical redissolution of²¹⁰Pb together with Mn under reducing conditions. In contrast, in the suboxic part of the lake (135 m depth) the flux of²¹⁰Pb was about twice the atmospheric input. This excess is not caused by allochthonous contributions and is tentatively explained by the transport of sediment material resulting from small slides at the very steep lake shores or more probably by reprecipitation of²¹⁰Pb together with Mn when the conditions in the lake water become locally and seasonally more oxidizing. Dissolved²¹⁰Pb may migrate from locations with reducing conditions and reprecipitate under more oxic conditions. Indeed, a correlation of Mn and²¹⁰Pb in sediments of Lake Zurich was found.This is the first of a series of papers to be published by this journal following the 20^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.     

A high resolution Holocene pollen record from Lago do Pires,SE Brazil: vegetation,climate and fire history

Lago do Pires (17° 57 S, 42° 13 W) is situated at 390 m a.s.l. in the foothills of the Serra do Espinhaço, 250 km from the Brazilian Atlantic coast. The original vegetation of the study area has been almost destroyed by pastoral activity. Relicts of a dense 20–30 m tall tropical semidciduous forest are present only on a few hill tops. The dry season of the Lago do Pires region lasts for 4 months and the annual precipitation is 1250 mm. A high resolution pollen record from a 16 m long sediment core, composed of 77 samples, subdivided in 7 zones and 4 subzones, allows a reconstruction of Holocene paleoenvironments. For the early Holocene (9720-8810 B.P.), the results indicate that the region surrounding the lake was dominated by a herb savanna (campo cerrado) withCuratella americana (cerrado tree) and high fire frequency. Species ofCecropia, Urticales and a few others, form small gallery forests along the water courses. This vegetation pattern is consistent with a long dry season (perhaps 6 months) and a low annual precipitation. Between 8810 and 7500 years B.P. gallery forests expanded in the valleys and suggest a period of higher rainfall with shortened dry season (perhaps 5 months). Fire was less frequent. Reduction of gallery forests followed (7500-5530 B.P.), probably related to a return of drier climatic conditions (5–6 months dry season, lower precipitation). Fires were more frequent. Between 5530 and 2780 years B.P. in the vallyes were forests and on the hills still an open cerrado. The dry season probably was about 5 months and the rainfall was higher than in the previous period. Later (2780-970 B.P.) the more open cerrado on the hills changed to more closed cerrado. A dense and closed semideciduous forest existed in the region only in the latest Holocene period (since 970 B.P.) under the current climatic conditions. The vegetation was no longer influenced by fire. A very strong human impact by deforestation and use of fire occurred in the last decades. Today cerrado vegetation is generally restricted to central Brazil and exists in several small isolated Islands (Hueck, 1956) in the area of semideciduous forest in SE Brazil which were more widespread during the drier periods of the Holocene. The wettest period of Holocene occurs in the present millenium.This is the 4th 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.     

Ostracode carbonate δ18O- and δ13C-signature of hydrological and climatic changes affecting Lake Neuchâtel,Switzerland, since the latest Pleistocene

Floating and grounded peat plateaus were studied in fens in the Yukon Territory (Canada). The peat deposit may be over 4 m thick and consists of a lower bed of aquatic peat overlain by humic fen peat, mesic fen peat and woody peat. Permafrost in the grounded peat plateaus is older than the 1200 year old White River Ash, whereas permafrost in the floating peat plateau is younger.Peat accumulation rates since 1200 years B.P. were greater in the fens (85–100 cm) than on the surface of the peat plateaus (25–55 cm). Where the peat plateau is free-floating, it will persist until the climate changes, causing the icy core to thaw. Where the peat plateau is frozen to the mineral substrate, it slowly drowns since the fen peat accumulates faster than the woody peat. This drowning results in degradation of the landform independently of the climate. Only degradation of floating peat plateaus can be used to identify climatic changes.This publication is the first paper in a series of papers presented at the session on Past Climatic Change and the Development of Peatlands at the ASLO and SWS Meetings in Edmonton, Canada, May 30–June 3, 1993. Dr. P. Kuhry and Dr S. C. Zoltai are serving as Guest Editors.