multiple charcoal profiles in a scottish lake taphonomy fire ecology human impact and inference abstract sedimentary microscopic charcoal in lacustrine environments has received much attention although this has been concerned largely with the deposition of modern charcoal or with distributions through time based on single profiles lake sediments are dynamic entities however and the phenomenon of sediment focusing for example will lead to the redistribution of pollen spores and charcoal the process may not always result in greatly altered fossil assemblage in percentage terms but values for individual indicators or their absolute values may be affected strongly this is investigated using multiple microscopic charcoal profiles covering many thousands of years from black loch fife eastern scotland relative and absolute measures of charcoal depiction are considered as a prelude to discussion of microscopic charcoal taphonomy in the context of a small lake fire ecology and human impacts the charcoal measures concentrations influx and charcoal to pollen ratios all follow similar patterns though with very different numerical values two cores have charcoal influx values of similar orders of magnitude for much of the holocene whereas quantities in two other cores vary by a factor of 16 after 1000 cal bp the differences between influx measures for total pollen and charcoal indicate that focusing effects between the two indicators are dissimilar correlations with pollen and spore taxa are explored and demonstrate the problems of discrepant data between profiles charcoal pollen patterns for specific cultural periods are examined for one profile and they provide data suggesting that a direct fire vegetation relationship for the site is unproven the need to adopt a multi causal approach to inference is emphasised

radiocarbon ages from two submerged strandline features in the western gulf of maine and a sea level curve for the northeastern massachusetts coastal region abstract new radiocarbon dates provide ages for two submerged strandline features on the massachusetts inner shelf these ages provide limited control on a relative sea level rsl curve for the late wisconsinan and holocene the curve indicates a late wisconsinan high stand of rsl of 33 m about 14 000 yr ago and a very short lived relative low stand of about 43 m at about 12 000 yr ago followed by a rise to present sea level rapid changes of rsl around 12 000 yr ago may be related to changes in global glacial meltwater discharge and eustatic sea level change shown by dated corals off barbados variations in the magnitude and timing of rsl change from south to north along the coast of the western gulf of maine are due to greater crustal depression and later deglaciation to the north

moisture and temperature changes associated with the mid holocene tsuga decline in the northeastern united states abstract a decline of hemlock tsuga populations at ca 5 5 ka thousands of calibrated radiocarbon years before 1950 ad stands out as the most abrupt vegetation change of the holocene in north america but remains poorly understood after decades of study recent analyses of fossil pollen have revealed a concurrent abrupt oak quercus decline and increases in the abundance of beech fagus and pine pinus on cape cod in eastern massachusetts but the replacement of drought tolerant oaks by moisture sensitive beeches appears inconsistent with low lake levels in the region at the same time the oak and beech changes are also limited to coastal areas and the coastal inland differences require an explanation here we develop a new lake level reconstruction from deep pond cape cod by using a transect of sediment cores and ground penetrating radar gpr profiles to constrain the past elevations of the sandy littoral zone of the pond the reconstruction shows that a series of multi century episodes of low water coincide with the abrupt hemlock and oak declines and interrupt subsequent phases of hemlock recovery the lake level variations equal precipitation deficits of 100 mm superimposed on a holocene long moisture increase of 400 mm however because moisture deficits do not easily explain the oak and beech changes we also evaluate how the climate preferences of the regional vegetation changed over time by matching the fossil pollen assemblages from deep pond with their modern equivalents reconstructions of the precipitation requirements of the vegetation correlate well even in detail with the lake level record r 0 88 at deep pond and indicate close tracking of effective moisture precipitation minus evapotranspiration by the vegetation despite the abrupt species declines which could have decoupled climate and vegetation trends reconstructions of the temperature preferences of the vegetation indicate that coastal sites may have cooled by 0 5 2 5 c after ca 5 5 ka while inland sites warmed by 0 5 1 c the change in coastal temperature preferences agrees with sea surface cooling in the western atlantic ocean of 1 c consequently the persistence of low hemlock abundance after 5 5 ka in the northeast u s may have resulted from oceanic changes that produced multi century droughts and thus delayed the post decline recovery of hemlock populations

holocene lake level trends in the rocky mountains u s a abstract the availability of water shapes life in the western united states and much of the water in the region originates in the rocky mountains few studies however have explicitly examined the history of water levels in the rocky mountains during the holocene here we examine the past levels of three lakes near the continental divide in montana and colorado to reconstruct holocene moisture trends using transects of sediment cores and sub surface geophysical profiles from each lake we find that mid holocene shorelines in the small lakes 4 110 ha were as much as 10 m below the modern lake surfaces our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000 2000 years ago we also discuss evidence for millennial scale moisture variation including an abruptly initiated and terminated wet period in colorado from 4400 to 3700 cal yr bp and find only limited evidence for low lake stands during the past millennium the extent of low water levels during the mid holocene which were most severe and widespread ca 7000 4500 cal yr bp is consistent with the extent of insolation induced aridity in previously published regional climate model simulations like the simulations the lake data provide no evidence for enhanced zonal flow during the mid holocene which has been invoked to explain enhanced mid continent aridity at the time the data including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium confirm the ability of large boundary condition changes to push western water supplies beyond the range of recent natural variability

surface sediments of transboundary lake peipsi composition dynamics and role in matter cycling to describe and analyse the role of sediments in the matter cycling in large shallow transboundary lake peipsi l peipsi in north eastern europe detailed surface sediment mapping was conducted on the basis of grain size the surface sediments fall into three groups coarse grained sediments prevailingly sands in the lake s southern part fine grained sediments mainly silts and silty sands both in the central deeper part within the 8 m depth contour the groups of deposits have a distinct spatial distribution determined mainly by the current system in the lake the main source of bottom sediments is the erosion of the lake floor and shores the role of the river input seems to be limited fine grained organic rich sediments are very cohesive playing the main role in the circulation of various inorganic and organic pollutants like nutrients and xenobiotics due to the cohesive character of the sediments their physical and chemical properties are extremely diverse and if the near bottom shear stress increases extreme meteorological events changes in the water level etc the lake floor may be subjected to episodic erosion and resuspension which may cause remobilisation of impurities in muddy sediments and their return to the food chain

a 14 000 year sediment yield record from western germany based on annually laminated lake sediments the reconstruction of sediment yield data for the catchment of lake holzmaar 205 8 ha in the eifel mountains western germany is based on high resolution geochemical and physical analyses of an absolutely dated sediment record this technique provides flux rates of allochthonous minerogenic deposition for the last 13 840 years the pleistocene holocene transition is characterized by instable soils and vegetation caused by adjustment of the catchment to interglacial climatic conditions these instabilities are the reason for elevated mean sediment yields in the catchment of 16 t km 2 yr 1 until 9800 cal bp during most of the holocene erosive processes are insignificant and sediment yields are very low 1 5 t km 2 yr 1 since humans cleared the forests for agriculture and early industrial purposes the natural environment has been disturbed and sediment yields increased to mean values of 14 t km 2 yr 1 during the last 2750 years comparisons of reconstructions from the eifel area with similar data from southern sweden and with measured data from lakes reservoirs and monitoring sites suggest that quantitative reconstructions based on lake holzmaar sediments may be regarded as reliable despite uncertainties related to the methods of reconstruction applied

late quaternary water level variations and vegetation history at crooked pond southeastern massachusetts abstract sediment cores collected along a transect in crooked pond southeastern massachusetts provide evidence of water level changes between 15 000 cal yr b p and present the extent of fine grained detrital organic accumulation in the basin inferred from sediment and pollen stratigraphies varied over time and indicates low water levels between 11 200 and 8000 cal yr b p and from ca 5300 to 3200 cal yr b p this history is consistent with the paleohydrology records from nearby makepeace cedar swamp and other sites from new england and eastern canada and with temporal patterns of regional changes in effective soil moisture inferred from pollen data the similarities among these records indicate that 1 regional conditions were drier than today when white pine pinus strobus grew abundantly in southern new england 11 200 to 9500 cal yr b p 2 higher moisture levels existed between 8000 and 5500 cal yr b p possibly caused by increased meridonal circulation as the influence of the laurentide ice sheet waned and 3 drier conditions possibly contributed to the regional decline in hemlock tsuga abundances at 5300 cal yr b p although sea level rise may have been an influence moist climatic conditions during the late holocene were the primary reason for a dramatic rise in water table elevations

strange attractor dimension as a new measure of seismotectonics around koyna reservoir india abstract the fractal dimension of a strange attractor based on 44 792 earthquakes around koyna reservoir in the period 1967 1981 has been determined for three periods during which the main shock of 1967 m 6 5 and the largest aftershocks of magnitude 5 0 in 1973 and 1980 occurred it was found that the fractal dimension remains almost the same and that at least five parameters are needed for earthquakes to be predictable in the region the non integral dimension obtained and the positive value of the largest lyapunov exponent show that the system is chaotic the phase space portrait of the koyna earthquake also shows that the trajectories lie on a strange attractor it is surmised that the fractal dimension is a new measure of seismotectonics around koyna region

shear margins in glaciers and ice sheets analytical and numerical techniques are used to examine the flow response of a sloped slab of power law fluid power n subjected to basal boundary conditions that vary spatially across the flow direction as for example near an ice stream margin with planar basal topography the primary assumption is that basal shear stress is proportional to the basal speed time a spatially variable slip resistance the ratio of mean basal speed to the speed originating from shearing through the thickness denoted as r gives a measure of how slippery the bed is the principal conclusion is that a localized disturbance in slip resistance affects the basal stress and speed in a zone spread over a greater width of the flow in units of ice thickness h the spatial scale of spreading is proportional to a single dimensionless number r n r n 1 1 n 1 derived from n and r the consequence for a shear zone above a sharp jump in slip resistance is that the shearing is spread out over a boundary layer with a width proportional to r n for an ice stream caused by a band of low slip resistance with a half width of wh the margins influence velocity and stress in the central part of the band depending on r n in comparison to w three regimes can be identified which for n 3 are quantified as follows low r defined as r 3 1w for which the boundary layers from both sides bridge across the full flow width and the driving stress in the center is supported almost entirely by side drag intermediate r for which the driving stress in the center is supported by a combination of basal and side drag shear zones that are narrower than predicted on the basis of this theory r 3 would require localized softening of the ice to explain the concentration of deformation at a shorter scale

the influence of lake morphometry on sediment focusing sediment focusing is a process whereby water turbulence moves sedimented material from shallower to deeper zones of a lake sediment focusing occurs in lakes in both the erosional coarse grained sediments and the transportational zone with the latter characterized by discontinuous sedimentation and resuspension the zone of accumulation is diefined as the zone where sedimentation is final i e no resuspension and where there is no further sediment focusing a geochemical tracer pb was used to trace sediment focusing patterns in 12 lakes of different morphomctries the area occupied by the zone of accumulation is predicted from the mean basin slope a with 86 of the variance explained only in large exposed lakes in which turbulence from waves is more severe are the resulting sediment distributions more erratic this study is the first to provide a general model of sc diment focusing patterns among lakes the study of lake bottom dynamics has received attention in the contexts of sediment contaminant and eutrophication studies the fact that sediments are major depositories for contaminants and nutrients has made them a focus for research in limnology also because sediments provide a record of past events in lakes an understanding of the manner in whllch sediments accumulate is essential for hindcasting interpretations of the historical record in sediments will be confounded if the record has been obscured by sediment resuspension the term sediment focusing was coined by likens and davis 1975 to describe the resuspension of sediments in shallower zones by waves and water currents with subsequent transport to and settling in the deeper zones of lakes much of the work

geochemistry of daihai lake sediments inner mongolia north china implications for provenance sedimentary sorting and catchment weathering abstract to advance the understanding of sediment distribution catchment weathering hydraulic sorting and sediment provenance in a tectonically stable basin the geochemistry of surface sediment samples from daihai lake in north china is presented mud bulk sediments were analyzed for 10 major and 30 trace elements organic carbon and nitrogen and for 87sr 86sr ratios in silicate fraction acid insoluble ai and carbonates acid soluble as enrichment in fe2o3 cao mno mgo k2o and p2o5 and their positive correlation with al2o3 rb sr ratios and bivariate discrimination diagrams of k vs rb and th u vs th suggest low chemical weathering intensity under semiarid conditions in the daihai catchment accumulation of clays in deeper water sites by sedimentary sorting results in an evident distribution of sediment composition with the lake water depth although similar chondrite normalized ree rare earth element patterns of lake sediment samples with lree light rare earth element enrichment negative eu anomalies and an almost flat hree heavy rare earth element pattern may suggest similar sedimentary source rocks original sediment provenance can be recognized on the basis of their distinctive 87sr 86sr ai ratios and immobile th zr ti and nb trace element signature separating from weathering and sedimentary processes as well as from grain size effect

integration of modern and past pollen accumulation rate par records across the arctic tree line a method for more precise vegetation reconstructions abstract in order to assess the potential role of the boreal forest in the early to mid holocene global dynamics it is critical that the tree line reconstructions are based on precise palaeorecords pollen accumulation rate par calculations provide a technique for estimating past plant population densities independently for each species when considering tree lines an important concept is the threshold par value for the absence presence and dominance of the most common tree species such threshold values are often based on modern lake sediment core top par values in northern fennoscandia records obtained through pollen monitoring now add an important element to these studies as they allow more precise definitions of the temporal and spatial limits of par values here we compare modern par values based on pollen monitoring data with sediment core top data to evaluate the robustness of the threshold values for the major tree species of finnish lapland we apply these threshold values to two fossil par records from this tree line region and as an example of the potential of the approach map the vegetation patterns in the area at 7000 cal yr bp the results indicate that the extent of the mid holocene tree line advance in northern fennoscandia was substantially less than climate vegetation model simulations have predicted despite their value and potential in vegetation reconstructions the use of par values requires careful validation of the data we stress the importance of basin size of precise and reliable sediment dating age depth modelling and assessment of the sedimentary processes in each study basin as prerequisites for par based studies

sediment focusing in mirror lake new hampshire1 analyses of multiple sediment cores from mirror lake new hampshire dated by 14c and pollen show that between 14 000 and 11 000 b p silts and sands were deposited over 85 of the basin after 11 000 b p organic rich sediments were focused into the deeper parts of the basin and no sediment accumulated in water 5 m deep late glacial particulate inorganic inputs were 33 times greater than modern losses from forested watersheds at hubbard brook after trees became established inorganic inputs fell sharply to modern levels late glacial organic inputs were a half the average during the last 11 000 years at four of the sites studied accumulation rates reached a maximum between 7000 and 4000 b p the precise date varies from site to site suggesting that it would be misleading to link peak accumulation in a single core to events in the watershed after 4000 b p organic sediment accumulation rates declined at all four sites parallel declines in accumulation rates of both pollen and inorganic sediments suggest that this decline is due to the process of sediment focusing rather than to a change in decomposition rates or in organic inputs related to autochthonous or allochthonous primary production

a simple model for the influence of push morainal banks on the calving and stability of glacial tidewater termini push morainal banks at the grounding lines of tidewater termini of temperate glaciers are the source of two types of restraining forces operating at the glacier terminus horizontal normal forces derive from the lateral support and transport of the bank of sediment at the terminus whereas a horizontal shear force operates along the base of a bank pushed in front of an advancing glacier the simple model we present suggests that bank related restraining forces are significantly larger than the restraining force derived from the hydrostatic pressure of water adjacent to the submerged terminus of a glacier during glacier advance restraining forces continually increase resulting in decreasing flow rates glacier thickening and the eventual cessation of advance during retreat restraining forces continually decrease resulting in increasing flow rates glacier thinning and the potential for unstable rapid sustained retreat the normal seasonal oscillatory advance retreat cycle of a glacier is moderated by restraining forces associated with push moraines unstable retreat is likely initiated when bank related restraining forces fall below some threshold value during the seasonal retreat cycle calving is not a primary cause of glacier retreat but is more likely a short term response to increased flow rates increased flow rates result in glacier thinning and an approach toward buoyancy both of which fluctuate seasonally in accordance with bank related restraining forces

holocene vegetation change at treeline north of yellowknife northwest territories canada abstract two radiocarbon dated cores from small lakes located approximately 25 km north of the mapped boundary between forest tundra and tundra provide records of postglacial vegetation change at the treeline near yellowknife nwt canada basal radiocarbon dates of 6180 and 7470 yr b p were obtained from the cores the fossil pollen evidence suggests that the initial vegetation was betula tundra with a peatland component alnus became an important constituent of the pollen assemblages between 6900 and 5500 yr b p both lakes record sharp increases in picea cf mariana pollen at approximately 5000 yr b p suggesting the establishment of forest tundra by 3500 yr b p picea mariana forest tundra had withdrawn the proportion of organic to inorganic sediment in the cores was at a maximum between 5000 and 3500 yr b p tundra has dominated the region since 3500 yr b p in northwestern canada the maximum northward advance of treeline occurred between 9000 and 5000 yr b p the asynchrony in treeline advance in central and northwestern canada may reflect that glacial ice persisted in the interior nwt longer than previously believed alternatively the asynchronous history of the treeline may be a result of the geometric properties of the long wave westerly disturbance that is manifest in the median summer position of the arctic front and ultimately controls the geographic location of the treeline

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

reservoir associated characteristics using deterministic chaos in aswan nurek and koyna reservoirs nurek aswan and koyna reservoirs were affected by moderate earthquakes with continuing seismic activity microearthquake data recorded through local networks have been used to determine the strange attractor dimensions using deterministic chaos which were found as 7 2 3 8 and 4 8 respectively this would imply that while 8 parameters are needed to model earthquakes near nurek reservoir only 4 to 5 parameters are needed for the aswan and koyna regions the differences in the strange attractor dimension suggest them to be a measure of seismotectonics around such reservoirs

hydrologic variability and the onset of modern el nino southern oscillation a 19 250 year record from lake elsinore southern california there are very few terrestrial palaeoclimate archives spanning the last glacial maximum through the holocene from coastal southern california yet knowledge of past climate dynamics is critical for assessing present and future constraints on the region s dwindling freshwater resources we present initial results from two drill cores extracted from the present day edge of lake elsinore southern california s largest natural lake using a multiproxy approach including lithologic description mass magnetic susceptibility loi 550 c total organic matter and loi 950 c total carbonate we infer first order long term climate change over the past 19 250 calendar years furthermore we suggest possible first order forcing mechanisms that drive this change which include presence absence of continental ice sheets insolation and complex ocean atmosphere interactions our results indicate four distinct millennial scale climate modes over the past 19 250 calendar years these modes include a wet last glacial maximum 19 250 17 120 cal yr bp a relatively dry late glacial holocene transition 17 120 9450 cal yr bp a wet early holocene 9450 7670 cal yr bp and a highly variable mid to late holocene climate i e alternating wet dry cycles 7670 cal yr bp present we attribute the mid to late holocene climate interval to the onset of el nino southern oscillation ca 7000 cal yr bp and a more vigorous hydrologic system these results are supported by a variety of regional terrestrial and marine palaeoclimate archives copyright 2005 john wiley sons ltd

a palaeolimnological record of diatom community dynamics and late holocene climatic changes from prescott island nunavut central canadian arctic palaeolimnological analyses from lake pw03 a small dilute lake on crystalline bedrock on prescott island nunavut canada 73 07 n 96 41 w 243 m a s l provide a sensitive record of the impacts of holocene climatic change on diatom communities a high resolution sequence from 3500 cal bp to present indicates that a significant change in diatom assemblages takes place at 2300 cal bp when alkaliphilous fragilarioids and planktonic taxa are replaced by benthic epiphytic and acidophilous diatoms these changes coincide with the onset of neoglacial cooling and likely reflect more persistent ice cover and associated acidification of lake water maximum diatom species richness occurs during this period of transition dense bryophyte communities in the littoral zone provide habitats to sustain diatom production even under the cooler conditions following 2300 cal bp the concentration of diatom valves declines at 500 cal bp coincident with cooling at the time of the little ice age and minor chang

three dimensional hydrodynamic and sediment transport modeling to test the sediment focusing hypothesis in upland lakes palaeolimnological studies rely on assumptions regarding the distribution and completeness of lake deposits that are not always fully supported by observations in particular the assumption that focusing of suspended sediments leads to preferential deposition in the deepest part of a lake is not always justified especially in upland lakes subject to energetic wind forcing few studies have investigated the hydrodynamic controls on lake sediment focusing especially the importance of wind driven currents in deep water we combine a three dimensional numerical hydrodynamic and suspended sediment model fvcom with a semi empirical wind wave model to investigate the potential mobility of bottom sediments in a small oligotrophic upland lake llyn conwy north wales uk exploratory simulations of wave and current generated bottom stress and suspended sediment dynamics confirm the expected importance of wave generated bottom stresses in shallower waters 3 m depth around the shore field survey shows that lake sediments are largely absent from this zone this is consistent with peripheral wave action as a sediment focusing mechanism in deeper water wind driven currents become the dominant contributor to bottom stress strong wind forcing events drive an energetic circulation with peak bottom stresses that intermittently exceed any realistic erosion threshold over a large proportion of the lake at depths far below those at which waves can be effective the spatial distribution of lake sediments and the completeness of the sediment record is thus determined by a complex interaction between wind driven circulation and bathymetry rather than by bathymetry alone although our sediment dynamics simulations are purely exploratory the results are consisted with survey results that show a patchy distribution of deep water accumulation some implications of these results for the selection of sediment coring locations and the interpretation of sediment records are considered

parallel climate and vegetation responses to the early holocene collapse of the laurentide ice sheet parallel changes in lake level and pollen data show that the rapid decline of the laurentide ice sheet lis between 10 000 and 8000 cal yr bp triggered a step like change in north american climates from an ice sheet and insolation dominated climate to a climate primarily controlled by insolation maps of the lake level data from across eastern north america show a reorganization of climate patterns that the pollen data independently match raised lake levels and expanded populations of moist tolerant southern pines pinus document that summer monsoons intensified in the southeastern united states between 9000 and 8000 cal yr bp simultaneously low lake levels and an eastward expansion of the prairie illustrate an increase in mid continental aridity after the hudson bay ice dome collapsed around 8200 cal yr bp lake levels rose in new england as populations of mesic plant taxa such as beech fagus and hemlock tsuga replaced those of dry tolerant northern pines pinus available moisture increased there after a related century scale period of colder than previous conditions around 8200 cal yr bp which is also recorded in the pollen data the comparison between pollen and lake level data confirms that vegetations dynamics reflect climatic patterns on the millennial scale

calving processes and the dynamics of calving glaciers abstract calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers in many parts of the world calving rates can increase dramatically in response to increases in velocity and or retreat of the glacier margin with important implications for sea level change despite their importance calving and related dynamic processes are poorly represented in the current generation of ice sheet models this is largely because understanding the calving problem involves several other long standing problems in glaciology combined with the difficulties and dangers of field data collection in this paper we systematically review different aspects of the calving problem and outline a new framework for representing calving processes in ice sheet models we define a hierarchy of calving processes to distinguish those that exert a fundamental control on the position of the ice margin from more localised processes responsible for individual calving events the first order control on calving is the strain rate arising from spatial variations in velocity particularly sliding speed which determines the location and depth of surface crevasses superimposed on this first order process are second order processes that can further erode the ice margin these include fracture propagation in response to local stress imbalances in the immediate vicinity of the glacier front undercutting of the glacier terminus by melting at or below the waterline and bending at the junction between grounded and buoyant parts of an ice tongue calving of projecting submerged ice feet can be regarded as a third order process because it is paced by first or second order calving above the waterline first order calving can be represented in glacier models using a calving criterion based on crevasse depth which is a function of longitudinal strain rate modelling changes in terminus position and calving rates thus reduces to the problem of determining the ice geometry and velocity distribution realistic solutions to the problem of modelling ice flow therefore depend critically on an appropriate choice of sliding law models that assume that basal velocities are controlled by basal drag can replicate much of the observed behaviour of calving glaciers with grounded termini but an important limitation is that they cannot be used to model floating glacier termini or ice shelves alternative sliding laws that parameterise drag from the glacier margins provide more flexible and robust ways of representing calving in ice sheet models such models can explain a remarkable range of observed phenomena within a simple unifying framework including downglacier increases in velocity and strain rates where basal and or lateral drag diminishes flow acceleration in response to thinning through time the tendency for glaciers to stabilise at pinning points in relatively shallow water or fjord narrowings the constraints on ice shelf stability and the contrasts in calving rates between tidewater and freshwater calving glaciers many unresolved issues remain however including the role played by the removal of backstress in the acceleration of retreating calving glaciers and the controls on melting at and below the waterline

short term flow dynamics of a retreating tidewater glacier leconte glacier alaska u s a an analysis of motion in the terminus region of leconte glacier alaska u s a delineates mechanisms that are important to tidewater glacier stability this glacier is undergoing rapid retreat since 1995 it has retreated 2 km and thinned 125 m at the present location of the terminus ice velocities in this region are quite high at the terminus they exceed 27 m d 1 our analysis reveals that fluctuations in speed and surface elevation are forced by ocean tides surface melt and precipitation the near terminus ice speed is 180 out of phase with the tide such that high tide corresponds to low speed smaller melt forced diurnal variations in speed are present throughout the lowermost 7 km speed ups in excess of 10 of the mean often occur after rainstorms as a result of increased basal water pressure and storage but the relation is not simple the velocity does not vary over the spring and summer seasons

nonequilibrium geomorphic processes and deterministic chaos abstract disequilibrium is a common if not dominant condition in geomorphology and other environmental sciences because disequilibrium in a system implies a time lag time steps and time delays in simple deterministic equations may reflect process and such equations have been shown to result in chaos after examining some models of hillslope and catchment development we present an oversimplified model of a hillslope in which we calculate the elevation of a point at time t 1 as a function we consider both linear and nonlinear terms of the difference in elevation between it and the next uphill and downhill points at time t we show how this simple hillslope model will not produce chaos under one set of limiting conditions that do not include nonlinearity but without proof we note that in most conditions the hillslope mode is so similar to other models in which chaos arises because of disequilibrium that chaotic behavior is a likely possibility a formulation wherein the time step is in a weathering and or erosion coefficient rather than the elevation is also proposed

three dimensional modeling of lacustrine diatom habitat areas improving paleolimnological interpretation of planktic benthic ratios three dimensional modeling of lake morphometry enables the calculation of lake volume planar surface area and basin surface area which are critical components of a conceptual model of how planktic and benthic habitat areas change with changing lake level we have applied three dimensional modeling to foy lake montana and compared model results of changing ratios of planktic and benthic habitat areas at all possible lake levels to sediment cores recovered from the lake the model allows us to produce semiquantitative depth reconstructions and greatly improves lake level reconstruction of this morphometrically complex basin the conceptual model also was modified to examine the influence of resuspension versus gravity redistribution of diatomaceous material on the sedimentary record comparison of the model results with both long and short sedimentary records from foy lake montana suggests that planktic benthic ratios may be a better predictor of mean lake depth than maximum lake depth the model developed here is simple enough to be easily implemented has few requirements can be adapted to suit any lake basin and can be augmented to examine a variety of different lake processes that affect habitat availability

pleistocene glacial and interglacial stratigraphy of new england long island and adjacent georges bank and gulf of maine provisional correlations of quaternary deposits of new england long island georges bank and the gulf of maine fig 1 chart 1 are based on 14c dates and amino acid racemization estimated aare ages borehole records and stratigraphic sections shallow seismic profiles and biostratigraphic analyses obtained since publication of summaries by schafer and hartshorn 1965 muller 1965 and pratt and schlee 1969 stratigraphic units record at least three and possibly four middle pleistocene and late pleistocene glaciations and intervening nonglacial episodes in the southern coastal zone fig 2 where continental glacial and marine nonglacial units are intercalated subsurface seismic units on georges bank may record two additional glaciations of middle pleistocene and early pleistocene ages most of the stratigraphic units shown in chart 1 are informal units and many of the informal stratigraphic names have not been adopted by all workers either for older units of the largely abandoned four glaciation chronology advocated by fuller 1914 woodworth and wigglesworth 1934 kay 1964a and flint 1971 or for local units of wisconsin age the stratigraphic units and limiting dates are grouped in chart 1 according to their positions in major lowland areas that controlled the configurations of ice lobes and other sectors of the ice sheet during late wisconsin deglaciation

the fracture of ice on scales large and small arctic leads and wing cracks from observations and calculations of crack patterns in ice it is suggested that a similar mechanism may account for cracking over a wide range of scales

interpretation of lake sediment accumulation rates the pattern of ancient sediment accumulation in lake basins is usually determined for the sole purpose of obtaining a chronology of the sequence we develop graphical representations of lake basins and how they fill with sediment in order to make generalisations about sediment patterns which can be used to distinguish those that relate to an aspect of changing environment from those that relate solely to the shape of the basin itself our goal is general observations that could lead to more robust interpretation of age depth models from lake basin sediments we show that in nearly all circumstances with constant sedimentation the overall pattern seen at a central core should be one of decreasing rate of sediment accumulation which tends to be constant towards the top in most situations the initial rate of sediment accumulation is particularly high because of the basin shape observed rates of sediment accumulation that increase up the core should normally indicate increasing sediment input either aut

a comparison of some rapid techniques for the measurement of density in soft sediments abstract four different methods of measuring density in soft sediments have been compared with the density bottle method measurement of the wet sediment volume was found to be the step which limited accuracy use of open calibrated rings during the slicing of a core overestimated the volume this problem could be avoided if a lid was placed on the ring during slicing but volume measurement could still be biased by the presence of gas bubbles measurement of the volume of sediment after dispersion in a liquid medium was found to overestimate the volume unless the bias introduced by difficulties in reading the final meniscus level was subtracted from each measured volume a cut off syringe was shown to be capable of estimating the volume of wet sediment with a high precision if reasonable precautions were taken

mid holocene drought and lake level change at elk lake clearwater county minnesota evidence from chirp seismic reflection data despite many studies of holocene paleoclimate records from small lakes in the mid continent of north america direct estimates of lake level changes associated with mid holocene aridity are rare varved sediments from elk lake clearwater county minnesota are among the best studied in terms of paleoenvironmental proxies yet the sedimentary architecture of those sediments has not been previously studied and the hydrological responses of the lake changes in level and volume are poorly known high resolution seismic reflection chirp profiles of elk lake reveal complex sedimentary basins in the lake a pattern of nearshore onlap of sediments onto older substrates and the focusing of sediments into several deep basins biogenic gas obscures sediments in the deepest parts of the basins but beneath the rest of the lake a three part holocene sequence is clear the transitions between these parts are correlated with lithological changes defined in earlier core studies sediments of the modern stage are

acoustic architecture of glaciolacustrine sediments deformed during zonal stagnation of the laurentide ice sheet mazinaw lake ontario canada abstract in north america the last laurentide ice sheet retreated from much of the canadian shield by zonal stagnation masses of dead ice severed from the main ice sheet by emerging bedrock highs downwasted in situ within valleys and lake basins and were commonly buried by sediment consequently the flat sediment floors of many valleys and lakes are now pitted by steep sided enclosed depressions kettle basins that record the melt of stagnant ice blocks and collapse of sediment at mazinaw lake in eastern ontario canada high resolution seismic reflection magnetic and bathymetric surveys integrated with onland outcrop and hammer seismic investigations were conducted to identify the types of structural disturbance associated with the formation of kettle basins in glaciolacustrine sediments basins formed as a result of ice blocks being trapped within a regionally extensive proglacial lake glacial lake iroquois 12 500 to 11 400 years bp that flooded eastern ontario during deglaciation kettles occur within a thick 30 m succession of parallel high frequency acoustic facies consisting of rhythmically laminated varved iroquois silty clays iroquois strata underlying and surrounding kettle basins show large scale normal faults fractures rotational failures and incoherent chaotically bedded sediment formed by slumping and collapse mazinaw lake lies along part of the ottawa graben and while neotectonic earthquake activity cannot be entirely dismissed deformation is most likely to have occurred as a result of the rapid melt of buried ice blocks seismic data do not fully penetrate the entire basin sediment fill but the structure and topography of bedrock can be inferred from magnetometer data the location and shape of buried ice masses was closely controlled by the graben like form of the underlying bedrock surface

magnetic studies of erosion in a scottish lake catchment 1 core chronology and correlation1 magnetic susceptibility opb 137cs 14c and paleomagnetic secular variation are used to establish the chronology of sedimentation in a set of cores from loch frisa in western scotland the 14c dates obtained are not compatible with the chronology derived from all the other techniques and this is ascribed to inwash of old particulate carbon from the watershed central cores show little evidence of changes in sedimentation rate over the last 150 years whereas marginal cores contain evidence of major increases arising from ploughing and from drainage associated with catchment afforestation since 1935 the results illustrate the value of a multiple core approach to sedimentation and erosion studies even where complex lake morphometry precludes calculation of sediment budgets

a time resolution methodology for assessing the quality of lake sediment cores that are dated by 137cs a methodology is proposed for grading the utility of lake sediment cores used to reconstruct pollution histories the observed distribution of sup 137 cs with depth in the core is compared to that expected from independent historic measurements of fallout deposition the width of the sup 137 cs peak is the core profile that corresponds to the fallout maximum of 1963 or the combined maxima of 1959 and 1963 is used to infer the inherent time resolution of the core i e the ability to distinguish events in the deposition history of the watershed the method is applied to a number of core profiles from various lakes in the us and appears to provide self consistent results

fayetteville green lake new york u s a viii mass balance for 137cs in water varved and non varved sediments abstract a mass balance for 137 cs was done in fayetteville green lake 43 of total 137 cs input was found in shallow water 0 18 m sediments 16 was found in deep water varved sediments 9 was found in the water column and 32 remained to be carried away by lake outflow shallow water sedimentary 137 cs must be resuspended and or dissolved to maintain current levels of 137 cs in the water column and current fluxes of 137 cs to deep water varved sediments 137 cs activities in individual varve year samples from 1946 to 1976 followed closely the pattern of published bomb fallout records but the varves contained only 28 34 of cumulative unit area fallout for this area in order to preserve the yearly record of 137 cs sedimentation the pore water diffusion coefficient for 137 cs must be 11 cm 2 s 1 bioturbation and physical mixing of these sediments does not occur some of the cores of shallow water sediments accumulated 137 cs in amounts greater than was supplied by fallout to the lake surface area this may be due to 1 horizontal transport of surface water 137 cs to shallow water sediments for direct absorption 2 bioconcentration by benthic algae and mosses or 3 localized funnelling of sediments

linking organic carbon sedimentation burial efficiency and long term accumulation in boreal lakes carbon c storage in lakes is now recognized as a significant sink of c at a global scale but the pathways that lead to this storage remain poorly understood in this study we attempt to reconstruct and connect the processes that lead to long term c accumulation in boreal lakes these include the rate of particulate organic c poc sedimentation in the water column and sediment metabolism operating at a temporal scale of weeks to months organic c accumulation in the top sediment layers integrated over scales of tens of years and long term organic c burial in lake sediment integrated over hundreds to thousands of years the sinking poc flux was tenfold higher than the short term sediment c accumulation rates in all systems and we found no direct relationship between this downward c flux and either the short term or long term c accumulation rates however the resulting c burial efficiency which ranged from 5 to 62 was strongly related to lake shape which ultimately constrains the time freshly deposited material that is exposed to oxygen and thereby regulates the fraction of the carbon sinking flux that is mineralized back to the atmosphere or permanently buried in the sediments small and deep lakes act as more efficient c sinks than large and flat lakes we also show that long term burial rates are nearly identical to current centennial scale accumulation rates and that therefore little degradation occurs after a few decades sediment c storage tends to be small 5 relative to lake c emissions but that this balance is also strongly related to lake morphometry

lateglacial nunataks in central scandinavia biostratigraphical evidence for ice thickness from lake flafattjonn tynset norway abstract this study reveals for the first time biostratigraphical evidence of nunataks emerging from the late weichselian ice sheet in central scandinavia almost 1 m of minerogenic basal layers from lake flafattjonn 1110 m a s l at tynset norway was deposited in the lateglacial possibly as early as 16 000 cal yr bp the sediments contain well preserved remains of plants diatoms and chironomids and indicate 1 a mosaic pioneer vegetation of dwarf shrub tundra snow bed vegetation and xerophytes on wind blown ridges 2 an ice free lake during summers with clear water at low ph due to little inwash 3 a continental climate with summers slightly warmer than today ca 10 c and most probably with very cold winters according to indications of permafrost and 4 a hiatus reduced sedimentation during at least the first half of the younger dryas when the lake was sealed by a more or less permanent ice cover the existence of lateglacial nunataks indicates a thin and multi domed lateglacial ice sheet in scandinavia early migration of arctic alpine plants to nunataks could explain their centric distribution in s norway furthermore vegetation establishment on the flafatet lateglacial nunatak indicates that s norway could have been vegetated by successions along the retreating ice from alpine nunataks and from coastal and southern areas the upland and lowland vegetation met at the final deglaciation

latitudinal differences in the influx of microscopic charred particles to lake sediments in finland forest fires produced charcoal throughout the holocene and charcoal was preserved in lake sediments this study compares differences in sedimentary charcoal between southern and northern finland y

sedimentation controls on the preservation and time resolution of climate proxy records from shallow fluctuating lakes lithological stratigraphies of 210pb dated sediment cores from four hydrologically interconnected fluctuating lake basins in kenya are used to investigate how differences in basin morphometry and physical limnology influence the preservation and time resolution of sedimentary climate proxy records the potential of lakes to accumulate an undisturbed sediment record is primarily determined by their relative depth the ratio between maximum depth and effective wind fetch chemically stratified crater lakes accumulate high quality climate proxy records because they complement great relative depth with topographical wind shelter resistance of density stratification to propagation of wind induced turbulence and absence of bioturbation in anoxic bottom waters the changes in water column circulation and bottom dynamics that accompany lake level fluctuations affect the time resolution of accumulating climate proxy signals and thus the apparent magnitude and frequency of inferred climatic events an episode of low lake level can be both over or underrepresented in the climate proxy record depending on the severity of the drawdown relative to the lake s critical depth of sediment accumulation decade scale hiatuses due to non deposition or erosion at low lake level may be difficult to recognize because mixing of unconsolidated muds deposited before and after the lowstand can obliterate evidence that the record has been truncated regional correlation of climate proxy records must consider both hydrology related differences in the climatic sensitivity of lakes and sedimentation related differences in the integrity of their climate history archives

modern pollen rain as a background for palaeoenvironmental studies in the serra dos carajas southeastern amazonia several alternative interpretations of the pollen records of lake sediments in the amazonia may arise due to a lack of understanding of the modern environments mainly related to pollen vegetation

a late wisconsin 32 10k cal a bp history of pluvials droughts and vegetation in the pacific south west united states lake elsinore ca continuous sub centennially resolved paleo terrestrial records are rare from arid environments such as the pacific south west united states here we present a multi decadal to centennial resolution sediment core lake elsinore ca to reconstruct late wisconsin pluvials droughts and vegetation in general the late wisconsin is characterized by a wetter and colder climate than during the holocene specifically conditions between 32 3 and 24 9k cal a bp are characterized by large amplitude hydrologic and ecologic variability highlighting this period is a 2000 year glacial mega drought 27 6 25 7k cal a bp during which the lake shallowed 3 2 4 5 m depth this period is approximately coeval with a lake manix regression and an increase in xeric vegetation in the san bernardino mountains baldwin lake the local last glacial maximum llgm is bracketed between 23 3 and 19 7k cal a bp a 3000 year interval characterized by reduced run off relative to the glacial colder conditions and vegetative stability maximum sustained wetness follows the llgm beginning at 19 7 and peaking by 14 4k cal a bp a two step decrease in runoff characterizes the lateglacial to holocene transition however the vegetation change is more complex particularly at the beginning of the younger dryas chronozone by 12 6 12 4k cal a bp the climate achieved near holocene conditions

climatic change in northern ethiopia during the past 17 000 years a diatom and stable isotope record from lake ashenge lake ashenge a closed basin lake near the northernmost penetration of summer monsoon rains is well placed to provide a continental record of past changes in the strength of the african monsoon system diatom and oxygen isotope analyses of the lake sediments confirm that the overall trend of climate change during the past 17 000 years was driven by precessional forcing punctuated by abrupt shifts that may be linked to changes in atlantic surface temperatures the lake level was low from at least 17 2 to 16 2 cal kyr bp and then rose between 16 2 and 15 2 cal kyr bp which may represent a temporary reactivation of the monsoonal circulation system following its reduced activity during the last glacial maximum the lake was significantly low between 13 6 and 11 8 cal kyr bp coinciding approximately with the younger dryas but beginning 900 years before its recognised onset in the greenland ice core record a major sedimentary hiatus covering the interval 11 8 to 7 6 cal kyr bp was probably caused by an early holocene lowstand the precise timing of which cannot be determined because pre lowstand sediments were eroded from the core site the lake filled to its overflow from 7 6 cal kyr bp until 5 6 cal kyr bp when the sediments record an abrupt lake response to the regional transition to arid conditions that mark the end of african humid period evidence is also presented for climate changes which may have been associated with the rise and fall of aksum ethiopia s first great civilisation

younger dryas climatic reversal in northeastern usa ams ages for an old problem abstract late glacial macrofossils from a 10 m core from alpine swamp new jersey were radiocarbon dated using accelerator mass spectrometry ams the arrival of the first trees to the area following deglaciation is indicated by maximum percentages of spruce pollen and a date of 12 290 440 yr b p on a single spruce needle subsequent spread of deciduous hardwoods was followed by the expansion of boreal taxa including spruce picea fir abies larch larix laricina paper birch betula papyrifera and alder alnus three ams dates on paper birch seeds and a spruce needle during this boreal expansion indicate that it took place between 11 000 and 10 000 yr b p the timing of this vegetational shift and its correlation with late glacial pollen stratigraphy from many sites in southern new england indicate that a climatic reversal correlative with the younger dryas characterized the north atlantic seaboard of the united states

a computational approach to quaternary lake level reconstruction applied in the central rocky mountains wyoming usa abstract sediment based reconstructions of late quaternary lake levels provide direct evidence of hydrologic responses to climate change but many studies only provide approximate lake elevation curves here we demonstrate a new method for producing quantitative time series of lake elevation based on the facies and elevations of multiple cores collected from a lake s margin the approach determines the facies represented in each core using diagnostic data such as sand content and then compares the results across cores to determine the elevation of the littoral zone over time by applying the approach computationally decisions are made systematically and iteratively using different facies classification schemes to evaluate the associated uncertainty after evaluating our assumptions using ground penetrating radar gpr we quantify past lake elevation changes precipitation minus evapotranspiration p et and uncertainty in both at lake of the woods and little windy hill pond wyoming the well correlated r 0 802 0 002 reconstructions indicate that water levels at both lakes fell at 11 300 8000 5500 and 4700 1600 cal yr bp when p et decreased to 50 to 250 mm yr differences between the reconstructions are typically small 10 24 mm yr since 7000 cal yr bp and the similarity indicates that our reconstruction method can produce statistically comparable paleohydrologic datasets across networks of sites

postglacial water level changes of a small lake in southern quebec canada sediment stratigraphy and palaeobotanical data from five cores were used to reconstruct the postglacial water level changes of a small lake in southern quebec the cores were taken along two transects from the centre of the lake to the margins lake level was relatively high before 11 000 cal bp when sedimen tation was characterized by the deposition of silty gyttja 13 000 cal bp and marl 13 000 11 000 cal bp from 11 000 to 7000 cal bp a sediment hiatus was indicated by the lack of two regional pollen zones between the marl and the overlying gyttja suggesting either nondeposition or erosion of the previously deposited sedi ments due to a low lake level the lake level was approximately 2 m lower than present on the basis of macrofossil assemblages a rise in the water level of about 1 m occurred around 7000 cal bp which was interrupted by a second lowering between 6100 and 4400 cal bp as indicated by a hiatus in the pollen stratigra phy during this low period the water level was at leas

theoretical calving rates from glaciers along ice walls grounded in water of variable depths calving has been studied for glaciers ranging from slow polar glaciers that calve on dry land such as on deception island 63 0 s 60 6 w in antarctica through temperate alaskan tide water glaciers to fast outlet glaciers that float in fiords and calve in deep water such as jakobshavns isbrae 69 2 n 49 9 w in greenland calving from grounded ice walls and floating ice shelves is the main ablation mechanism for the antarctic and greenland ice sheets as it was along marine and lacustrine margins of former pleistocene ice sheets and is for tide water and polar glaciers

a combined radiometric and mineral magnetic approach to recent geochronology in lakes affected by catchment disturbance and sediment redistribution abstract as a result of empirical testing of 210 pb dating models in a wide variety of lakes we conclude that there are many situations where despite major changes in sedimentation rates as a result of lake or catchment disturbance the crs constant flux model provides a sound basis for establishing recent chronology factors which may complicate or vitiate the assumptions of the model include short water residence time bioturbation for which we propose a modified dating model discontinuous or disturbed sedimentation sediment resuspension and shifts in sediment source in the case of the last three processes we illustrate ways in which the problems can be identified and evaluated using mineral magnetic measurements we propose an evaluative approach to 210 pb dating projects with emphasis on independent dating evidence on tests of internal consistency and on the use of data from several correlatable cores

the impact of cold north atlantic sea surface temperatures on climate implications for the younger dryas cooling 11 10 k the sensitivity of global climate to colder north atlantic sea surface temperatures is in vestigated with the use of the giss general circulation model north atlantic ocean temperatures 18 000 b p resembling those prevalent during the younger dryas were incorporated into the model of the present climate and also into an experiment using orbital parameters and land ice characteristic of 11 000 b p the results show that with both 11 000 b p and present conditions the colder ocean temperatures produce cooling over western and central europe in good agreement with younger dryas paleoclimatic evidence cooling also occurs over extreme eastern north america although the precise magnitude and location depends upon the specification of ocean temperature change in the western atlantic despite the presence of increased land ice and colder ocean temperatures the younger dryas summer air temperatures at northern hemisphere midlatitudes in the model are warmer than those of today due to changes in the orbital parameters chiefly precession and atmospheric subsidence at the perimeter of the ice sheets

measurement of temperature in a margin of ice stream b antarctica implications for margin migration and lateral drag ice temperature was measured in and around the chaotically crevassed south margin of ice stream b antarctica from 1992 to 1994 the temperatures at 30 m depth in the chaotic zone are about 12 k lower than in the adjacent uncrevassed ice due to the ponding of cold winter air at depths greater than 150 m there is clear evidence of internal heating of the ice due to the large shear deformation rate in the marginal zone analysis of the depth of cooling below the crevasses and of the internal heating gives two pieces of information first over the last half century the lateral shear stress averaged 2 0 x 10 5 pa in the top third of the margin and second the margin moved outward at an average rate of 7 3 m a these values do not involve any assumptions about the flow law of ice the uncertainties are roughly 20 the value of lateral shear stress indicates that the most of the drag on the ice stream is along its sides

source and distribution of pollen in the surface sediment of daihai lake inner mongolia daihai lake lies in the transitional area from semi humid to semi arid region and also in the north margin of the east asia monsoon the analysis of 65 pollen samples from the surface sediment of daihai lake and alluvium of its inflow rivers revealed the characteristics of pollen distribution and different transport vectors most pollen grains in daihai lake come from its inflow rivers although rain out and wind are also important activities of insects birds and humans may also carry pollen into the lake the pollen assemblages are consistent with the regional vegetation in general the variance of pollen assemblages in different parts of the lake reflects changes of local vegetation near the lakeshore pollen concentrations in the lake sediment surface are lower than that in fresh alluvium in its inflow rivers and the highest percentages and concentrations of major pollen types mainly appear in the margin part of the lake dca demonstrated that there are more similarities between lake sediments and the inflow rivers than there are differences

a 12 000 year history of vegetation and climate for cape cod massachusetts abstract pollen and charcoal analysis of radiocarbon dated sediment cores from duck pond in the cape cod national seashore provide a continuous 12 000 yr vegetation and climate history of outer cape cod a picea hudsonia parkland and then a picea pinus banksiana alnus crispa boreal forest association grew near the site between 12 000 and 10 000 yr b p this vegetation was replaced by a northern conifer forest of pinus strobus p banksiana and subsequently by a more mesophytic forest pinus strobus tsuga quercus fagus acer ulmus fraxinus ostrya as the climate became warmer and wetter by 9500 yr b p by 9000 yr b p a pinus rigida quercus association dominated the landscape high charcoal frequencies from this and subsequent levels suggest that the pine barrens association developed during a warmer and drier climate that lasted from 9000 to about 5000 yr b p increased percentages of pinus strobus pollen indicate a return to moister and cooler conditions by about 3500 yr b p a doubled sedimentation rate increased charcoal and increased herb pollen suggest land disturbance near the pond before european settlement these results suggest a rapid warming in the northeast in the early holocene and support a hypothesis of a rapid sea level rise at that time comparison of the pollen results from duck pond with those from rogers lake connecticut illustrates the importance of edaphic factors in determining the disturbance frequency and vegetation history of an area

palynology after y2k understanding the source area of pollen in sediments pollen grains preserved in lake and bog sediment provide a record of past vegetation that has been an important source of information about climate and land cover during the quaternary period yet from the beginning questions have been raised about the source area of pollen in sediment interpretation has been hampered by the lack of well developed theory treating the relationship between the spatial distribution of trees on the landscape and the percentages of pollen in sediment within the past decade however new theory models and empirical data show how heterogeneous vegetation is represented by pollen the distinction between local and regional pollen is explained by the prentice sugita dispersal deposition models which predict how the ratio of regional to local pollen changes with lake size sugita s model simulating a landscape with heterogeneous vegetation predicts the size of the relevant source area the area of vegetation reflected in between lake variations in pollen loading while demo

fayetteville green lake new york u s a abstract a varve chronology extending back to 1900 with dated stratigraphic markers has been established for deep water sediment in the northern part neck of meromictic fayetteville green lake and correlated to a similar chronology established earlier in the deeper southern end of the lake main basin precise estimates of net annual varve sedimentation rates are given for each year between 1938 and 1977 for the neck these rates are 27 higher than those previously reported in the main basin and range between 454 and 1090 g m 2 yr 1 varve sedimentation rates have significantly increased throughout the lake during the last 30 years with no changes in the relative ratio of varve sedimentation rates in the neck and the main basin although there is considerable year to year variation in this ratio higher varve sedimentation rates occur in shallower portions of the permanently stratified water probably because resuspension is unlikely in this low energy environment because of basin morphometry and meromixis sediment focusing creates higher varve sedimentation rates in relatively shallow water near the upper limit of the permanently stratified water rather than in the deepest water

a palaeoenvironmental assessment of human impact on wood lake okanagan valley british columbia canada abstract to ascertain the extent of recent impacts and to develop realistic goals for water quality improvement a palaeolimnological assessment of human impact on wood lake british columbia canada was initiated in 1992 geochemical and microfossil analyses on a series of short cores provided a detailed record of human impact since european settlement early post settlement sediments consisted of clastic materials rich in mg al fe p and associated metals an abrupt increase in marl deposition occurred about 1940 perhaps in response to diversion of inflowing water for irrigation and or an increase in algal productivity mo analyses provided a record of increasing hypolimnetic anoxia with eutrophication hg concentrations were highest in sediments deposited during the past decade microfossil analyses chironomids diatoms chrysophytes depicted the lake s progressive eutrophication and the disappearance of the profundal bottom fauna water quality improvements have been reported since 1975 but benthic invertebrate communities have recovered only slightly the diatoms may indicate a modest water quality improvement over the last 20 years wood lake was formerly an oligo mesotrophic lake with a well developed profundal fauna appropriate mitigative programmes could substantially improve water quality perhaps allowing re establishment of the natural oligo mesotrophic condition