Diatom analysis of three Norwegian cores

The purpose of this project is to supply diatom data to the Norwegian Institute of Water Research for cores from three Norwegian lakes: Steinsfjorden, Lyseren and Gjersjøen. The data will feed into a palaeolimnological project which also includes analysis of algal pigments and radiometric dating of the cores. The study aims to assess shifts in the diatom assemblages and to determine the nature of the baseline assemblages. Additionally the project aims to apply an existing diatom-phosphorus (P) transfer function to the diatom data in order to infer the trophic histories of the lakes

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

An oxygen isotope record of lacustrine opal from a European Maar indicates climatic stability during the Last Interglacial

The penultimate temperate period, 127–110 ka before present (BP), bracketed by abrupt shifts of the global climate system initiating and terminating it, is considered as an analogue of the Holocene because of a similar low global ice‐volume. Ice core records as well as continental and marine records exhibit conflicting evidence concerning the climate variability within this period, the Last Interglacial. We present, for the first time, a high‐resolution record of oxygen isotopes in diatom opal of the Last Interglacial obtained from the Ribains Maar in France (44°50′09″N 3°49′16″E). Our results indicate that the Last Interglacial in southwestern Europe was generally a period of climatic stability. The record shows that the temperate period was initiated by an abrupt warm event followed midway by a minor climatic transition to a colder climate. An abrupt isotopic depletion that occurs simultaneously with abrupt changes in pollen and diatom assemblages marks the end of the temperate period, and is correlative with the Melisey I stadial. Variations in the isotopic composition of lake‐water related to the isotopic composition of precipitation and evaporation dominate the biogenic opal oxygen isotope record

Identifying sediment discontinuities and solving dating puzzles using monitoring and palaeolimnological records

Palaeolimnological studies should ideally be based upon continuous, undisturbed sediment sequences with reliable chronologies. However for some lake cores, these conditions are not met and palaeolimnologists are often faced with dating puzzles caused by sediment disturbances in the past. This study chooses Esthwaite Water from England to illustrate how to identify sedimentation discontinuities in lake cores and how chronologies can be established for imperfect cores by correlation of key sediment signatures in parallel core records and with long-term monitoring data (1945–2003). Replicated short cores (ESTH1, ESTH7, and ESTH8) were collected and subjected to loss-on-ignition, radiometric dating (210Pb, 137Cs, and 14C), particle size, trace metal, and fossil diatom analysis. Both a slumping and a hiatus event were detected in ESTH7 based on comparisons made between the cores and the long-term diatom data. Ordination analysis suggested that the slumped material in ESTH7 originated from sediment deposited around 1805–1880 AD. Further, it was inferred that the hiatus resulted in a loss of sediment deposited from 1870 to 1970 AD. Given the existence of three superior 14C dates in ESTH7, ESTH1 and ESTH7 were temporally correlated by multiple palaeolimnological proxies for age-depth model development. High variability in sedimentation rates was evident, but good agreement across the various palaeolimnological proxies indicated coherence in sediment processes within the coring area. Differences in sedimentation rates most likely resulted from the natural morphology of the lake basin. Our study suggests that caution is required in selecting suitable coring sites for palaeolimnological studies of small, relatively deep lakes and that proximity to steep slopes should be avoided wherever possible. Nevertheless, in some cases, comparisons between a range of contemporary and palaeolimnological records can be employed to diagnose sediment disturbances and establish a chronology

Using palaeolimnological and limnological data to reconstruct the recent history of European lake ecosystems: Introduction

Summary 1. As future climate change is expected to have a major impact on freshwater lake ecosystems, it is important to assess the extent to which changes taking place in freshwater lakes can be attributed to the degree of climate change that has already taken place. 2. To address this issue, it is necessary to examine evidence spanning many decades by combining long-term observational data sets and palaeolimnological records. 3. Here, we introduce a series of case studies of seven European lakes for which both long-term data sets and sediment records are available. Most of the sites have been affected by eutrophication and are now in recovery. 4. The studies attempt to disentangle the effects of climate change from those of nutrient pollution and conclude that nutrient pollution is still the dominant factor controlling the trophic state of lakes. 5. At most sites, however, there is also evidence of climate influence related in some cases to natural variability in the climate system, and in others to the trend to higher temperatures over recent decades attributed to anthropogenic warming. 6. More generally and despite some problems, the studies indicate the value of combining limnological and palaeolimnological records in reconstructing lake history and in disentangling the changing role of different pressures on lake ecosystems

Injection of thermal and suprathermal seed particles into coronal shocks of varying obliquity

Context. Diffusive shock acceleration in the solar corona can accelerate solar energetic particles to very high energies. Acceleration efficiency is increased by entrapment through self-generated waves, which is highly dependent on the amount of accelerated particles. This, in turn, is determined by the efficiency of particle injection into the acceleration process. Aims. We present an analysis of the injection efficiency at coronal shocks of varying obliquity.We assessed injection through reflection and downstream scattering, including the effect of a cross-shock potential. Both quasi-thermal and suprathermal seed populations were analysed. We present results on the effect of cross-field diffusion downstream of the shock on the injection efficiency. Methods. Using analytical methods, we present applicable injection speed thresholds that were compared with both semi-analytical flux integration and Monte Carlo simulations, which do not resort to binary thresholds. Shock-normal angle θBn and shock-normal velocity Vs were varied to assess the injection efficiency with respect to these parameters. Results. We present evidence of a significant bias of thermal seed particle injection at small shock-normal angles. We show that downstream isotropisation methods affect the θBn-dependence of this result. We show a non-negligible effect caused by the crossshock potential, and that the effect of downstream cross-field diffusion is highly dependent on boundary definitions. Conclusions. Our results show that for Monte Carlo simulations of coronal shock acceleration a full distribution function assessment with downstream isotropisation through scatterings is necessary to realistically model particle injection. Based on our results, seed particle injection at quasi-parallel coronal shocks can result in significant acceleration efficiency, especially when combined with varying field-line geometry

Injection of thermal and suprathermal seed particles into coronal shocks of varying obliquity

Context. Diffusive shock acceleration in the solar corona can accelerate solar energetic particles to very high energies. Acceleration efficiency is increased by entrapment through self-generated waves, which is highly dependent on the amount of accelerated particles. This, in turn, is determined by the efficiency of particle injection into the acceleration process. Aims. We present an analysis of the injection efficiency at coronal shocks of varying obliquity. We assessed injection through reflection and downstream scattering, including the effect of a cross-shock potential. Both quasi-thermal and suprathermal seed populations were analysed. We present results on the effect of cross-field diffusion downstream of the shock on the injection efficiency. Methods. Using analytical methods, we present applicable injection speed thresholds that were compared with both semi-analytical flux integration and Monte Carlo simulations, which do not resort to binary thresholds. Shock-normal angle &theta; and shock-normal velocity V were varied to assess the injection efficiency with respect to these parameters. Results. We present evidence of a significant bias of thermal seed particle injection at small shock-normal angles. We show that downstream isotropisation methods affect the &theta;-dependence of this result. We show a non-negligible effect caused by the cross-shock potential, and that the effect of downstream cross-field diffusion is highly dependent on boundary definitions. Conclusions. Our results show that for Monte Carlo simulations of coronal shock acceleration a full distribution function assessment with downstream isotropisation through scatterings is necessary to realistically model particle injection. Based on our results, seed particle injection at quasi-parallel coronal shocks can result in significant acceleration efficiency, especially when combined with varying field-line geometry. &copy; 2013 ESO

Combining limnological and palaeolimnological data to disentangle the effects of nutrient pollution and climate change on lake ecosystems: Problems and potential

Summary 1. As long-term observational lake records continue to lengthen, the historical overlap with lake sediment records grows, providing increasing opportunities for placing the contemporary ecological status of lakes in a temporal perspective. 2. Comparisons between long-term data sets and sediment records, however, require lake sediments to be accurately dated and for sediment accumulation rates to be sufficiently rapid to allow precise matching with observational data. 3. The critical role of the sediment record in this context is its value in tracking the changing impact of human activity on a lake from a pre-disturbance reference through to the present day. 4. Here, we use data from a range of lakes across Europe presented as case studies in this Special Section. The seven sites considered all possess both long-term observational records and high-quality sediment records. Our objective is to assess whether recent climate change is having an impact on their trophic status and in particular whether that impact can be disentangled from the changes associated with nutrient pollution. 5. The palaeo-data show clear evidence for the beginning of nutrient pollution varying from the mid-nineteenth century at Loch Leven to the early and middle twentieth century at other sites. The monitoring data show different degrees of recovery when judged against the palaeo-reference. 6. The reason for limited recovery is attributed to continuing high nutrient concentrations related to an increase in diffuse nutrient loading or to internal P recycling, but there is some evidence that climate change may be playing a role in offsetting recovery at some sites. If this is the case, then lake ecosystems suffering from eutrophication may not necessarily return to their pre-eutrophication reference status despite the measures that have been taken to reduce external nutrient loading. 7. The extent to which future warming might further limit such recovery can be evaluated only by continued monitoring combined with the use of palaeo-records that set the pre-eutrophication reference

Diatoms and acid lakes. Proceedings of a workshop of the 8th International Diatom Symposium, Paris, September 1984

Diatom analysis of lake sedimentshasbecome one of the most important techniques used in the contemporary debate on lake acidification. In recent years the relationship between diatom assemblages and pH has been quantified allowing the pH history of individual lakes to be reconstructed. Much reliance is placed on these reconstructions yet we have little understanding of the causal mechanisms that underlie the impressive statistics. There has been little research on the ecology and physiology of diatom taxa in acid and acidifying waters and we have little information on the role of either planktonic or non-planktonic diatoms in acid lake ecosystems. This Workshop, hence, was not only devoted to the exchange of information on current research projects but also to a discussion of some of the ecological questions that require resolution to improve our understanding of the diatom:pH relationship