Rw Battarbee

210Pb dating by low background gamma counting

Lead-210 and radium-226 measurements by direct gamma assay can now provide a record of changing concentrations in lake sediments sufficiently reliable and precise to form a suitable basis for age/depth and dry-sedimentation-rate calculations. There are additional benefits in terms of non destructive sample preparation and simultaneous assay for other environmentally significant gamma-emitting radioisotopes (e.g. 137Cs and 241Am). Results from L. Fleet, S. W. Scotland illustrate the value of this approach especially in lakes with disturbed catchments where variable input of supported 210Pb has occurred.

The development of the aquatic ecosystem at Kråkenes Lake, western Norway, during the late glacial and early Holocene - a synthesis

This paper synthesises the palaeoecological reconstructions, including palaeoclimatic inferences, based on the available fossil record of plants (pollen, macrofossils, mosses, diatoms) and animals (chironomids, Cladocera, Coleoptera, Trichoptera, oribatid mites) in the late-glacial and early-Holocene sediments of Kråkenes Lake, western Norway, with special emphasis on changes in the aquatic ecosystem. New percentage and influx pollen diagrams for selected taxa provide insights into the terrestrial setting. The information from all the proxies is collated in a stratigraphical chart, and the inferred changes in the lake and its catchment are discussed. The individual fossil sequences are summarised by detrended correspondence analysis (DCA), and sample scores on the first DCA axes are plotted against an estimated calendar-year timescale for comparison of the timing and magnitude of changes in assemblage composition. The DCA plots show that the large late-glacial biotic changes were synchronous, and were driven by the overriding forcing factor of temperature. During the early Holocene, however, the changes in different groups were more gradual and were independent of each other, showing that other factors were important and interactive, such as the inwash of dissolved and particulate material from the catchment, the base and nutrient status of the lake-water, and the internal processes of ecosystem succession and sediment accumulation. This multi-disciplinary study, with proxies for changes in the lake and in the catchment, highlights the dependence of lake biota and processes not only on regional climatic changes but also on changes in the lake catchment and on internal processes within the lake. Rates of change for each group are also estimated and compared. The reaction times to the sharp temperature changes at the start and end of the Younger Dryas were very rapid and occurred within a decade of the temperature change. Aquatic organisms tracked the temperature and environmental changes very closely, and are probably the best recorders of late-glacial climatic change in the fossil record.

DATING OF RECENT LAKE-SEDIMENTS IN THE UNITED-KINGDOM AND IRELAND USING SPHEROIDAL CARBONACEOUS PARTICLE (SCP) CONCENTRATION PROFILES

Spheroidal carbonaceous particles (SCPs) produced from fossil-fuel combustion are found in lake sediments and provide a historical record of atmospheric pollutant deposition. The sediment record of these particles is remarkably consistent and the main features are now seen throughout Europe and beyond. Once the SCP record from a region has been established using accurately dated cores, SCP profiles can be used to determine dates at other sites in the region. This technique was developed in Sweden but until recently has had limited use in the UK and Ireland as the most useful dating feature, the subsurface peak, was not present in all cores. In recent cores this can now almost always be observed. Dating features for UK and Irish cores are found to differ slightly between regions. The potential exists for extra dates to be allocated to a profile by using characterisation of SCP surfaces to determine changes in fuel-type use. Dates could then be allocated to these changes using documentary sources to derive combustion histories. In future years, SCP dating will become increasingly valuable especially for mid- to late-nineteenth-century sediments as continual decay renders210 Pb progressively less useful for this time interval.

Experimental diatom dissolution and the quantification of microfossil preservation in sediments

Abstract Four laboratory experiments on fresh, modern diatoms collected from lakes in the Northern Great Plains of North America were carried out to assess the effects of dissolution on diatom abundance and composition. Marked differences in mean dissolution susceptibility exist between species, despite sometimes significant intra-specific variation between heterovalves. Twenty-four taxa were ranked according to susceptibility to dissolution using an exponential decay model of valve abundance. This dissolution ranking was used to derive two weighted indices of sample preservation. A third index (F) was based on a simple binary classification of valve morphology into dissolved and pristine categories, as distinguished by light microscopy (LM). When compared against rank indices and a measure of species diversity, this diatom dissolution index was found to be the best predictor of the progress of dissolution as estimated by total valve abundance or biogenic silica (BiSi) loss. Strong empirical relationships between F index values and diatom abundance (r2=0.84, n=32) and BiSi (r2=0.89, n=32) were developed and applied to a diatom sequence from a short core from Devils Lake, North Dakota, and compared to diatom-inferred and observed salinity at this site. The F index is a simple, effective diagnostic tool to assess important aspects of diatom preservation. The index can provide insight into Si cycling and record changes in conditions pertinent to diatom dissolution, and has a role in validation of transfer functions or other inferences derived from compositional data.

Alternative 210Pb dating: results from the New Guinea Highlands and Lough Erne

RECENT lake sediments can be dated using 210Pb and fall-out 137Cs. Pennington et al.1, and Robbins and Edgington2 have set out the assumptions used in calculating dates and estimating accumulation rates from the declining concentration of unsupported 210Pb in the near-surface sediments of Blelham Tarn and Lake Michigan respectively. In both cases, as in other papers3,4, an essential assumption is a constant initial concentration (c.i.c.) of unsupported 210Pb per unit dry weight in the sediment at each depth, whether or not any variations may have occurred in the rate of accumulation. This assumption requires that in undisturbed cores, unsupported 210Pb concentrations should always decline monotonically with depth. Figure 1 shows unsupported 210Pb concentrations in cores from Lough Erne, Northern Ireland and Lake Ipea, Papua New Guinea. The profiles are ‘kinked’ and show at one or more points, a marked increase in unsupported 210Pb concentration with depth. The levels at which this occurs in the cores range from 6 to 30cm. The increases cannot, therefore, be the result of the anomalously low surface concentrations noted elsewhere4. Associated biological, chemical and geophysical studies show that the profiles have not been significantly disturbed by physical or biological mixing. These profiles are not consistent with the c.i.c. deposition model and are regarded as evidence for the dilution of unsupported 210Pb by accelerated sediment accumulation. If such dilution has taken place without leading to a kink in 210Pb concentrations, the assumption of c.i.c. will lead to underestimation of the true age of the sediment below the onset of acceleration. In the case of the ‘kinked’ profiles, dates are not calculable using the c.i.c. deposition model alone. We have adopted an alternative approach to calculating 210Pb dates using as our main assumption a constant rate of supply (c.r.s.) of unsupported 210Pb to the sediment per unit time and deriving dates from the integrated activity of the radionuclide. Previous authors have referred to the possibility of calculating dates using this assumption5,6 but have not given a full account of the method or evaluated the results of its application. Details of the methods used here are set out elsewhere7. This paper compares and briefly evaluates the two alternative models as applied to the sediments of Lough Erne and Lakes Ipea and Egari. The c.r.s. based dates obtained have been compared with those derived either from c.i.c. based 210Pb dates or, in the case of the ‘kinked’ profiles, from a combination of 137Cs dating8 and c.i.c. based calculations. Figure 2 plots the resulting age against depth curves from Lough Erne, Fig. 3, some of the results from Lakes Ipea and Egari.

ACIDIFICATION OF LAKES IN GALLOWAY, SOUTH WEST SCOTLAND - A DIATOM AND POLLEN STUDY OF THE POST-GLACIAL HISTORY OF THE ROUND LOCH OF GLENHEAD

SUMMARY (1) Potential acidification by chemical weathering, soil leaching and organic matter accumulation in the catchment of the Round Loch of Glenhead is evaluated using diatom and pollen analysis of a radiocarbon dated sediment core. The Round Loch is situated on granite bedrock and is likely to have been sensitive to acidification throughout the postglacial period. (2) About 9000 years B.P. the open habitats of the late and early post-glacial were replaced by birch/hazel woodland, then oak/elm/pine woodland, and oak/hazel/alder woodland by about 5400 years B.P. After 5700 years B.P., blanket mires increasingly replaced woodland. The present vegetation on the blanket peat is a Molinia/Calluna heathland. (3) The lake was acid in the late-glacial period (pH 5 3-5 7). No evidence of long-term acidification in the early post-glacial period was found, and from 9000-4150 years B.P. the pH of the loch was probably between 5 3 and 5 6. In the mid post-glacial no indication of lake acidification that might be associated with peat development in the catchment was identified, and from 4100 years B.P. there is evidence that a slight increase in pH or nutrient availability or both occurred. (4) Despite a clear change from mineral to acid organic soils in the catchment in the mid post-glacial, feedback mechanisms operated to maintain a lake with pH stable at 5 and above for most of the post-glacial period. With the introduction of strong acid anions associated with acid deposition after A.D. 1800 the pH fell to its present value of 4 7.

Decline of the natterjack toad Bufo calamita in Britain: Palaeoecological, documentary and experimental evidence for breeding site acidification

Abstract Natterjack toads Bufo calamita have declined sharply at heathland sites in Britain during the 20th century. A significant feature of these habitats is the predominance of oligotrophic and dystrophic ponds on substrates with low buffering capacities. Acid ponds at one site, Woolmer Forest, were classified into two groups: shallow pools dominated by Sphagnum and high concentrations of organic solutes, and larger bodies of water in which pH was primarily influenced by inorganic anions (especially SO 4 ). Pond pHs responded transiently to episodes of very acid ( pH pH

Matching diatom assemblages in lake sediment cores and modern surface sediment samples: the implications for lake conservation and restoration with special reference to acidified systems

Restoration goals for damaged freshwater habitats can bedefinedaccording to ecological as well as to chemical criteria. Fordisturbed lakes, the sediment microfossil record can be usedtoselect potential modern analogue sites as possible restorationtarget ecosystems.Fossil diatom assemblages in two acidified lakes (Round LochofGlenhead and Loch Dee) in Galloway, Scotland, were comparedfloristically with modern surface sediment samples from ca. 200lakes in Britain, Ireland, Sweden and Norway using numericaltechniques. Mean squared Chi-squared dissimilarity (SCD)valuesbased on between sample Chi-square distance measures were usedtocompare samples.‘Space-for-time substitution’ using diatom assemblage matchingtechniques identified several modern analogue sites withHebrideanLoch Teanga and Irish Lough Claggan possessing modern diatomflorasmost similar to those which existed at the Round Loch ofGlenheadand Loch Dee before acidification.From the point of view of atmospheric pollution, the mostcloselymatching modern analogue sites were not necessarily in themostpristine regions. Some analogues occurred in UK regions ofmoderateor low acid deposition and modern diatom assemblages inatmospherically cleaner mid Norway were generally less similarfloristically.It is argued that identification of modern analogue sitesraisesthe possibility of using time-space substitution of closelymatchedmodern and fossil samples to infer whole lake ecosystems.Diatoms are however poor indicators of some water chemistryvariables and the two closest matched modern analogue siteshavetoo high calcium concentrations making faunistic comparisonsquestionable.Identification of good modern analogue lakes can be improvedbyusing selection criteria, other than diatoms, to pre-selectsites.Screening inappropriate sites according to water chemistry andbasin features combined with a larger biological database ofmodernand fossil samples offers a promising way of refining theselectionprocesses.Despite necessary refinements, modern analogue matching canpotentially identify whole lake ecosystems that can serve asbiological target communities for currently disturbed sites.Beingbased on biological rather than chemical criteria, thisapproachdoes not rely on species-water chemistry transfer functions.It istherefore directly relevant to lake conservation andrestorationobjectives and offers an alternative method for reconstructing lakepalaeo-environments.

Proxy records of climate change in the UK over the last two millennia: documented change and sedimentary records from lakes and bogs

This project of collaborative research (project Ha of the NERC TIGGER programme—Terrestrial Initiative in Global Geological Environmental Research) into the climatic history of the late Holocene used a variety of techniques, both tested and experimental, on carefully chosen sites in lowland and upland environments, to derive high quality proxy-records of climatic change over the last 2000 years. The methodology involved the use of high-resolution analyses (diatoms, cladocera, chironomids, ostracods, magnetics, pollen, macrofossils, humification, lipid biomarkers and stable-isotopes) of the stratigraphy of well-dated (AMS and conventional 14C, 210Pb, pollen, tephra, SCP spheroidal carbonaceous particles) cores from a remote montane lake and lowland lakes, and from a montane blanket and a lowland raised bog, linked to historical records of climate change. This paper reviews some of the major results obtained, both in the magnitude, synchroneity and periodicity of change, especially during the Little Ice Age, and in the evaluation of the various techniques used. The fact that such techniques can be validated and calibrated against a known climatic signal in the recent past, allows for better interpretation of changes in the more distant past.

Selective concentration and enumeration of tephra shards from lake sediment cores

This paper describes a technique for the enumeration of tephra shards from lake sediment cores whereby approximately 85% of unwanted sediment fractions (organic matter, biogenic silica, carbonates) are removed by selective stepwise chemical attack. The order of 100 samples can be prepared simultaneously in a modest-sized water-bath and the recovery rate of the technique is greater than 90%. The procedure is appli cable to all lake sediment types.