Using paleolimnological techniques, I have studied surface-water acidification and the effects of liming in Swedish lakes on a decadal to millennial time-scale. The overall objective was to contribute to the discussion on the fulfilment of goals within the Swedish liming program. One of the main goals of this program is to restore lakes to natural or nearly natural conditions, i.e. to a reference condition as termed in the EU Water Framework Directive. In this context, a key issue is to define reference conditions. This is a central theme of my thesis, as lake sediments offer a unique way to study past lake conditions. Past lake-water acidity of 12 reference lakes in the Swedish liming program (ISELAW) was determined using diatom analysis of sediment cores. Pollen, lead, and flyash from coal/oil combustion were used as indicators of impact from land use and atmospheric pollution. A general trend in these lakes is an initial decline in pH after lake formation due to natural soil processes, which was then followed by rather low pH values (pH 5.3-6.5). In six of the lakes pH increased as a result of expansion of agriculture (burning, forest grazing) 2000 to 1000 years ago. Local mining and long-range airborne pollution have also impacted the lakes since medieval time. These results show that the conditions of the study lakes were not natural prior to industrialization and recent (20th century) acidification. The ISELAW lakes were selected on the basis of representing typical limed lakes, and they have been limed and monitored since at least the 1980s. A comparison of chemical/biological monitoring data and the paleolimnological data gives somewhat diverging results. Most of the monitoring data suggest that the lakes were subjected to acidification during the 20th century, but the paleolimnological data can only identify clear evidence of acidification in five of the 12 lakes, hence, all lakes were probably not recently acidified. According to conclusions from monitoring the lakes have recovered following liming. The paleolimnological data give a more complex picture and three different responses have been identified: 1) a return to a diatom composition found in the lake one hundred to several thousand years ago; 2) very small shifts in the diatom composition; or 3) a diatom composition previously not found in the lake. The latter response raised the question whether liming can cause an unnatural diatom community. A comparison of diatoms in surface sediment samples of 31 limed lakes with pre-industrial reference samples from 291 lakes showed that liming does not create an unnatural diatom composition. These results illustrate that the goals for liming were not reached in all of the limed lakes, and that paleolimnology can play an important role for assessments of acidification and liming. The comparative study also highlights the importance of designing monitoring programs that can produce reliable and long data series. Given the results of the paleolimnological investigations, it is obvious that we cannot assume that the 19th century represented a natural or near natural state, and thus is a realistic reference conditions. Natural long-term lake development and previous land-use impacts need to be considered in defining reference conditions. Neither can we disregard the fact that humans always will impact nature. Although paleolimnological studies are time consuming, I believe that they could be simplified to the extent that paleolimnology could become a routine method for environmental management.

Understanding the link between the Sun and climate is vital in the current incidence of global climate change, and 10Be in natural archives constitutes an excellent tracer for this purpose. As cosmic rays enter the atmosphere, cosmogenic isotopes like 10Be and 14C are formed. Variations in solar activity modulate the amount of incoming cosmic rays, and thereby cosmogenic isotope production. Atmospherically produced 10Be enters natural archives such as sediments and glaciers by wet and dry deposition within about a year of production. 10Be from natural archives therefore provides information on past solar activity, and because these archives also contain climate information, solar activity and climate can be linked. One remaining question is to what degree 10Be in natural archives reflects production, and to what extent the local and regional environment overprints the production signal. To explore this, 10Be was measured at annual resolution over the last 600 years in a Greenland ice core. Measurement potentials for these samples benefited from the development of a new laboratory method of co-precipitating 10Be with niobium. To diversify geographic location and archive media type, a pioneer study of measuring 10Be with annual resolution in varved lake sediments from Finland was conducted, with samples from the entire 20th century. Pathways of 10Be into lake sediments are more complex than into glacial ice, inferring that contemporary atmospheric conditions may not be recorded. Here, it is shown for the first time that tracing the 11-year solar cycle through lake sediment 10Be variations is possible. Results also show that on an annual basis, 10Be deposition in ice and sediment archives is affected by local environmental conditions. On a slightly longer timescale, however, diverse 10Be records exhibit similar trends and a negative correlation with solar activity. Cyclic variability of 10Be deposition persisted throughout past grand solar minima, when little or no sunspot activity was recorded. 10Be levels indicate that although solar activity has been high during the 20th century, levels are not unprecedented in the investigated 600 years. Aerosol 10Be/7Be values indicate possible influence of stratosphere-troposphere exchange on isotope abundance and the production signal.

Because of its toxic nature mercury is a threat to both wildlife and human health, and thus, it is an element of concern in the environment. Currently much of the mercury emitted to the atmosphere is derived from anthropogenic sources – both direct emissions and re-emission of already deposited anthropogenic mercury. Following deposition mercury is affected by a long array of processes, and this thesis has focused on trying to increase our knowledge on the net retention of mercury in lake sediments and peatlands. This information is vital in order to understand how mercury behaves in the environment and where mercury is at risk of becoming a problem. Knowledge about the retention of mercury is also important when using lake sediments and peat records as environmental archives over past mercury deposition. By using varved, annually laminated, sediments I have determined that lake sediments are reliable archives for inorganic mercury, but not for methylmercury. A study of the spatial distribution of mercury in a whole-lake basin shows that inorganic- and methylmercury are controlled by different sediment properties. Inorganic mercury is controlled by combination of fine-grained mineral matter and organic matter concentrations, whereas methylmercury is controlled by water depth and sulfur concentration. This study also shows that especially methylmercury have a very heterogeneous spatial distribution across the lake basin, something that might be of large importance when using lake sediments to calculate whole-lake burdens of mercury. In a study regarding the effects of vegetation on the net retention of mercury in a peatland I showed that there are considerable differences in both plant- and peat-mercury concentrations depending on vegetation type. This might have implications for the use of peat records as archives over atmospheric mercury deposition. Finally I have used a combination of a peat and a lake sediment record to study how past and recent climatic changes affects the stability of a peatland currently underlain by permafrost. Here we are able to show that destabilization of peatlands, as a result of permafrost melt, can cause a significant release of organically bound mercury from the mire to the surrounding aquatic environment. Considering the currently warming climate there is a risk of sub-arctic peatlands turning into mercury sources, which might be important to recognize when assessing current mercury pollution pathways.

A pollen profile from varved lake sediments sampled at 10-year intervals and spanning the past 1000 years is analyzed to understand the effects of climate change and anthropogenic activity on forests in southwestern Quebec. The forests responded rapidly to changes in temperature and precipitation during the Medieval Warm Period and Little Ice Age as well as to land-use changes associated with the European Settlement of the area. The transition into the Little Ice Age was abrupt and had significant impact on the pollen production within a couple of decades. A synthesis of this record with other high-resolution and well-dated pollen data from the conifer-hardwood forest of eastern North America shows consistent results across the whole area, indicating that very-high resolution pollen data can provide insight into multi-decadal climate variability and its impact on forest vegetation. Tree-ring records from the region show inter-annual fluctuations not always consistent between sites, while high-resolution pollen data record multi-decadal to century changes which enable us to interpret climatic effects on plant communities.

Allochthonous carbon can be a large proportion of the carbon budget in northern temperate and boreal lakes. This thesis uses stable carbon and oxygen isotopes to examine the fate of allochthonous dissolved organic matter (DOM) in northern temperate lakes, and to determine the importance of dissolved organic carbon (DOC) in lake carbon mass balances and in the δ¹³C of lake sediments. To use stable isotopes as a tool for studying DOC loss and sedimentation within lakes requires an understanding of processes that affect the δ¹³C and δ¹⁸O in aquatic systems. Photolysis is one mechanism that can account for the large allochthonous DOC loss within northern temperate lakes. There is, however, little research examining the effects of photolysis on stable isotopes (e.g. δ¹³C and δ¹⁸O) in aquatic systems, or how photodegradation of DOM affects the δ¹³C of lake sediments. To study the effects of DOM photodegradation on carbon and oxygen isotopes, stream waters from catchments with varying peatland coverage were incubated in Tedlar bags placed in water baths under natural sunlight. Results from three streams flowing into two oligotrophic headwater lakes (Harp and Dickie Lakes) indicate that O₂ consumption rates and dissolved inorganic carbon (DIC) production rates were an order of magnitude greater in light exposed treatments than in dark treatments, suggesting that light mediated processes control O₂ consumption and DIC production in incubations. The similarity between filtered, inoculated, and sterile treatments, indicate that photolysis was the dominant O₂ consuming and DIC producing process in the incubations, while the contribution of respiration to these processes was not detectable. Differences in both O₂ consumption rates and DIC production rates (normalized to DOC loss) among streams suggest that DOM photolability was an important factor in both O₂ loss and DIC production on a volumetric basis. A concomitant increase in δ¹⁸O-O₂ was observed with O₂ loss indicating that during the photo-oxidation of DOM, the lighter ¹⁶O isotopomer was preferentially consumed in the oxidation of DOC to CO₂. Fractionation factors for respiration, photolysis and other abiotic reactions were not a function of O₂ consumption rates and ranged between 0.988 and 0.995, which lies outside the range published for respiration (0.975-0.982). These are the first published photolytic fractionation factors. The δ¹³C-DIC produced collectively by photolysis, respiration, and other abiotic reactions in incubations exposed to natural sunlight ranged between –23‰ and –31‰, and were similar in the light incubations for each treatment, but different among streams. Together, the light and dark incubation data suggest that photolysis and other abiotic reactions were largely responsible for the DIC concentration and δ¹³C-DIC changes observed, while respiration is a relatively minor contributor. During the incubations, as DOC photodegraded to CO₂, the lighter ¹²C isotope was preferentially mineralized (or the moieties cleaved were depleted in ¹³C) leaving the residual δ¹³C-DOC 1‰ to 4‰ enriched, creating enrichment (ε) values up to ~–3‰. The change in final δ¹³C-DOC after DOM photodegradation was different for each inflow, ranging from ~1 ‰ to 8.0 ‰, and provides an average enrichment of –2.1‰ (Harp Inflows ε: –1.2‰; Dickie Inflows ε: –3.4‰). These ε values are in agreement with the average ε from previous incubations on 3 of the inflows and 3 published studies based on UV exposed bog water (Osburn et al., 2001), riverine waters (Opsahl and Zepp, 2001), and lyophilized Juncus leachate dissolved in lake water (Vähätalo and Wetzel, 2008) (average ε = –2.9‰). The structure of DOM changed during photolysis. Absorbance data indicated that the aromaticity, colour, UV absorption and the average molecular size of the DOC decreased. Additionally, after exposure to sunlight, C/N ratios of the DOC changed from high values (24-55), indicative of terrestrial inputs, to lower values (4-13) traditionally thought to be representative of algal or microbial inputs. This contradicts the conventional view that terrestrial DOC has C/N ratios >20, and shows that abiotic processes can alter allochthonous carbon structure and the residual allochthonous carbon can have C/N values similar to, or overlapping with, C/N ratios expected from algal or microbial carbon. With the loss of 61-90% of the DOC, the particulate organic carbon (POC) created accounted for 20-90% of the DOC lost. Values of δ¹³C-POC ranged from –25.7‰ to –27.7‰, with 80% of the samples within 1‰ of the initial δ¹³C-DOC indicating that the particulate carbon created from the photodegradation of DOM that settles to the lake sediments could be isotopically similar to the source DOC. Overall, these incubations indicate that the photodegradation of DOM can affect both concentrations and isotopes of O₂, DIC, DOC, and POC of the stream waters flowing into Harp and Dickie Lakes and are important to consider in lake dynamics of high DOC retention lakes. Two independent methods were used to examine the importance of allochthonous DOC to lake sediments. The first method used a two end-member mixing model to estimate the proportion of allochthonous and autochthonous carbon within the lake sediments. Inflow δ¹³C-POC data, δ¹³C-leaf litter measurements, and DOC photodegradation experiments were used to calculate average annual δ¹³C-POC values for the allochthonous end member. The average annual δ¹³C-POC values for the autochthonous end member were calculated using estimates of productivity, surface δ¹³C-CO₂ values and estimated average annual fractionation factors. Average annual δ¹³C-POC values from allochthonous and autochthonous sources for these lakes were distinct. Using the end members to calculate the relative contributions of allochthonous and autochthonous carbon to lake sediments revealed that the δ¹³C of the lake sediment can be significantly affected by the ratio of autochthonous and allochthonous contributions. Furthermore, peaks in the allochthonous contributions of carbon accompany the δ¹³C peaks in the sediment records to the lake sediments. This suggests that climate change and/or anthropogenic changes to the landscape, and the concomitant changes in DOC inputs to lakes, can be recorded in the sediment record indicating that sediment records are not just productivity signals, but also mass balance signals in high DOC retention lakes. In the second method carbon isotope budgets were completed to accompany the carbon mass budgets for Harp and Dickie Lakes. Mass-weighted average annual δ¹³C-DOC values from the inflows and outflows and δ¹³C-DIC values from the inflows varied by 0.2‰ to 1.3‰, suggesting the values are well constrained. Conversely, the range of weighted δ¹³C-DIC values from the outflows were larger (2.2‰) than those of the inflows. Calculated δ¹³C values of the lake sediment were not equal to the measured δ13C values of the lake sediments for either Harp or Dickie Lakes suggesting a problem lies within the mass balances, or the weighted average annual δ¹³C values used in the isotope budgets. To examine the sensitivity of the average annual weighted δ¹³C values for the carbon entering and exiting the lakes, and the mass of carbon entering the lakes δ¹³C of the lake sediments, a mass and isotope budget model was created. The model indicated that the δ¹³C of the lake sediments is sensitive to a number of parameters including the amount of DOC entering the lake, the δ13C-CO2 evaded from the lake, the areal water discharge rate (qs), the gas exchange coefficient (k), and pH. Many of these parameters required adjustments for the masses of carbon to match those presented in the mass balances suggesting that the mass balances averaged over 8 years have errors associated with them. However, changing the DOC load to the lakes in the model by the variability observed over all the years of the mass balances) indicates that the isotopic signature of the lake sediment could change by up to 2.5‰. This isotope change is large enough to account for the historical δ¹³C changes observed in the δ¹³C sediment record, suggesting that allochthonous DOC can drive the sediment record.

Allochthonous carbon can be a large proportion of the carbon budget in northern temperate and boreal lakes. This thesis uses stable carbon and oxygen isotopes to examine the fate of allochthonous dissolved organic matter (DOM) in northern temperate lakes, and to determine the importance of dissolved organic carbon (DOC) in lake carbon mass balances and in the δ¹³C of lake sediments. To use stable isotopes as a tool for studying DOC loss and sedimentation within lakes requires an understanding of processes that affect the δ¹³C and δ¹⁸O in aquatic systems. Photolysis is one mechanism that can account for the large allochthonous DOC loss within northern temperate lakes. There is, however, little research examining the effects of photolysis on stable isotopes (e.g. δ¹³C and δ¹⁸O) in aquatic systems, or how photodegradation of DOM affects the δ¹³C of lake sediments. To study the effects of DOM photodegradation on carbon and oxygen isotopes, stream waters from catchments with varying peatland coverage were incubated in Tedlar bags placed in water baths under natural sunlight. Results from three streams flowing into two oligotrophic headwater lakes (Harp and Dickie Lakes) indicate that O₂ consumption rates and dissolved inorganic carbon (DIC) production rates were an order of magnitude greater in light exposed treatments than in dark treatments, suggesting that light mediated processes control O₂ consumption and DIC production in incubations. The similarity between filtered, inoculated, and sterile treatments, indicate that photolysis was the dominant O₂ consuming and DIC producing process in the incubations, while the contribution of respiration to these processes was not detectable. Differences in both O₂ consumption rates and DIC production rates (normalized to DOC loss) among streams suggest that DOM photolability was an important factor in both O₂ loss and DIC production on a volumetric basis. A concomitant increase in δ¹⁸O-O₂ was observed with O₂ loss indicating that during the photo-oxidation of DOM, the lighter ¹⁶O isotopomer was preferentially consumed in the oxidation of DOC to CO₂. Fractionation factors for respiration, photolysis and other abiotic reactions were not a function of O₂ consumption rates and ranged between 0.988 and 0.995, which lies outside the range published for respiration (0.975-0.982). These are the first published photolytic fractionation factors. The δ¹³C-DIC produced collectively by photolysis, respiration, and other abiotic reactions in incubations exposed to natural sunlight ranged between –23‰ and –31‰, and were similar in the light incubations for each treatment, but different among streams. Together, the light and dark incubation data suggest that photolysis and other abiotic reactions were largely responsible for the DIC concentration and δ¹³C-DIC changes observed, while respiration is a relatively minor contributor. During the incubations, as DOC photodegraded to CO₂, the lighter ¹²C isotope was preferentially mineralized (or the moieties cleaved were depleted in ¹³C) leaving the residual δ¹³C-DOC 1‰ to 4‰ enriched, creating enrichment (ε) values up to ~–3‰. The change in final δ¹³C-DOC after DOM photodegradation was different for each inflow, ranging from ~1 ‰ to 8.0 ‰, and provides an average enrichment of –2.1‰ (Harp Inflows ε: –1.2‰; Dickie Inflows ε: –3.4‰). These ε values are in agreement with the average ε from previous incubations on 3 of the inflows and 3 published studies based on UV exposed bog water (Osburn et al., 2001), riverine waters (Opsahl and Zepp, 2001), and lyophilized Juncus leachate dissolved in lake water (Vähätalo and Wetzel, 2008) (average ε = –2.9‰). The structure of DOM changed during photolysis. Absorbance data indicated that the aromaticity, colour, UV absorption and the average molecular size of the DOC decreased. Additionally, after exposure to sunlight, C/N ratios of the DOC changed from high values (24-55), indicative of terrestrial inputs, to lower values (4-13) traditionally thought to be representative of algal or microbial inputs. This contradicts the conventional view that terrestrial DOC has C/N ratios >20, and shows that abiotic processes can alter allochthonous carbon structure and the residual allochthonous carbon can have C/N values similar to, or overlapping with, C/N ratios expected from algal or microbial carbon. With the loss of 61-90% of the DOC, the particulate organic carbon (POC) created accounted for 20-90% of the DOC lost. Values of δ¹³C-POC ranged from –25.7‰ to –27.7‰, with 80% of the samples within 1‰ of the initial δ¹³C-DOC indicating that the particulate carbon created from the photodegradation of DOM that settles to the lake sediments could be isotopically similar to the source DOC. Overall, these incubations indicate that the photodegradation of DOM can affect both concentrations and isotopes of O₂, DIC, DOC, and POC of the stream waters flowing into Harp and Dickie Lakes and are important to consider in lake dynamics of high DOC retention lakes. Two independent methods were used to examine the importance of allochthonous DOC to lake sediments. The first method used a two end-member mixing model to estimate the proportion of allochthonous and autochthonous carbon within the lake sediments. Inflow δ¹³C-POC data, δ¹³C-leaf litter measurements, and DOC photodegradation experiments were used to calculate average annual δ¹³C-POC values for the allochthonous end member. The average annual δ¹³C-POC values for the autochthonous end member were calculated using estimates of productivity, surface δ¹³C-CO₂ values and estimated average annual fractionation factors. Average annual δ¹³C-POC values from allochthonous and autochthonous sources for these lakes were distinct. Using the end members to calculate the relative contributions of allochthonous and autochthonous carbon to lake sediments revealed that the δ¹³C of the lake sediment can be significantly affected by the ratio of autochthonous and allochthonous contributions. Furthermore, peaks in the allochthonous contributions of carbon accompany the δ¹³C peaks in the sediment records to the lake sediments. This suggests that climate change and/or anthropogenic changes to the landscape, and the concomitant changes in DOC inputs to lakes, can be recorded in the sediment record indicating that sediment records are not just productivity signals, but also mass balance signals in high DOC retention lakes. In the second method carbon isotope budgets were completed to accompany the carbon mass budgets for Harp and Dickie Lakes. Mass-weighted average annual δ¹³C-DOC values from the inflows and outflows and δ¹³C-DIC values from the inflows varied by 0.2‰ to 1.3‰, suggesting the values are well constrained. Conversely, the range of weighted δ¹³C-DIC values from the outflows were larger (2.2‰) than those of the inflows. Calculated δ¹³C values of the lake sediment were not equal to the measured δ13C values of the lake sediments for either Harp or Dickie Lakes suggesting a problem lies within the mass balances, or the weighted average annual δ¹³C values used in the isotope budgets. To examine the sensitivity of the average annual weighted δ¹³C values for the carbon entering and exiting the lakes, and the mass of carbon entering the lakes δ¹³C of the lake sediments, a mass and isotope budget model was created. The model indicated that the δ¹³C of the lake sediments is sensitive to a number of parameters including the amount of DOC entering the lake, the δ13C-CO2 evaded from the lake, the areal water discharge rate (qs), the gas exchange coefficient (k), and pH. Many of these parameters required adjustments for the masses of carbon to match those presented in the mass balances suggesting that the mass balances averaged over 8 years have errors associated with them. However, changing the DOC load to the lakes in the model by the variability observed over all the years of the mass balances) indicates that the isotopic signature of the lake sediment could change by up to 2.5‰. This isotope change is large enough to account for the historical δ¹³C changes observed in the δ¹³C sediment record, suggesting that allochthonous DOC can drive the sediment record.

Thesis (Ph. D.)--University of California, Riverside, 2009. / Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed March 20, 2010). Includes bibliographical references. Also issued in print.

Thesis (Ph. D.)--University of Oregon, 2003. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 255-270). Also available for download via the World Wide Web; free to University of Oregon users.

Tämnaren är en slättsjö i östra Svealand där ammunition går att finna i bottensedimenten. Ammunitionen härstammar från övningar som Flygvapnet hade över sjön under 1950- och 1960-talen. Sjön är mycket grund och näringsrik och befinner sig i ett område som fortfarande påverkas starkt av landhöjningen orsakad av den senaste inlandsisens tillbakadragande. Detta innebär att Tämnaren växer igen i rask takt och beräknas vara i princip borta om ungefär 600 år. Sjön har ett skyddsvärde dels genom sin rikedom på olika fågelarter som håller till i närområdet men också ett rekreationsvärde för människor i området runt sjön. Detta gör att man försöker se vilka åtgärder som kan fungera för att hejda den snabba igenväxningen och muddring kan eventuellt bli aktuellt i sjön. Innan beslut om muddring tas bör halter och spridning av föroreningar från ammunitionen i sjöns sediment kartläggas.Muddring är en vanlig metod som används för att öka djupet i sjöar och vattendrag men den kan innebära vissa risker. Den här undersökningen har gjorts för att fastställa halter av tungmetallerna koppar, zink, kadmium och bly som finns i sjöns bottensediment. Vid eventuell muddring skulle dessa tungmetaller kunna spridas ut på ett större område i sjön och därför vill man veta metallhalterna och ifall sådana halter är skadliga. I den här undersökningen kunde både högre och lägre halter av tung-metaller detekteras, halterna för koppar och kadmium var de som var högst, i jämförelse med Natur-vårdsverkets, 2007, förväntade bakgrundshalter i svenska sediment. Det syntes också en tydlig trend i tungmetallshalterna då samtliga mätvärden visade upp högre halter i sydöstlig riktning i sjön. Även om inte alla halter var högre än väntat, med undantag för koppar och kadmium, så återstår risken att de kontaminerade sedimenten kan spridas vidare i sjön om man väljer att muddra den. Även om halterna är låga kan de fortfarande innebära risker att sprida dessa vidare till organismer och växtlighet i sjön.Miljöundersökningar är viktiga att göra innan man beslutar sig för att göra större ingrepp på naturen, såsom muddring, då det är viktigt att skydda den natur vi har. Tämnaren är en sjö som på många sätt är skyddad för att den har ett rikt djur- och naturliv som bör bevaras. Detta gör att det blir ännu mer aktuellt att göra ordentliga förundersökningar innan man tar sig för några större åtgärder. / Tämnaren is a lake in eastern Svealand, Sweden and it is a shallow, highly eutrophic lake that suffers from severe overgrowth and it will probably cease to exist in about 600 years due to land rise and sedimentation. The lake used to be the location for exercises for the Swedish Armed Forces during the years 1950 – 1960, and therefore old remnants of ammunition can be found on the lake bottom. There is a risk that these old remnants of ammunition could be leaking pollutants into the lake sediment and it is necessary to investigate the level of which these releases may be occurring. This thesis will focus on the four heavy metals copper, zink, cadmium and lead and analyses of these metals will be performed on the lake sediments. The goal is to determine the spatial distribution of the heavy metals and the content of them in the lake sediments since dredging might be necessary for the lake. The risks with dredging are many but in this thesis the dispersion of contaminated sediments are the primary consideration due to the conditions in the lake. The spatial distribution of heavy metals already dispersed is interesting to know because it is necessary to know if the ammunition is leaking pollutants through the lake.The results of the investigation showed some higher concentrations of heavy metals in the sediments, mainly for copper and cadmium. According to Naturvårdsverket, 2007, estimated background content of copper in sediment is 15 mg/kg and the highest measured value in Tämnaren is about ten times higher than that. For cadmium the estimated background value is 0,3 mg/kg and all measured values in this investigation were higher than that. For zink the estimated background value is 100 mg/kg and for lead 5 mg/kg and the measured values in this investigation showed lower values in general for both metals. It seems as some dispersion has already occurred, maybe due to wind and wave processes, causing resuspension of sediments. Overall the spatial distribution is showing a trend with higher heavy metal contents to south eastern direction in the investigated area which could be a consequence of wind and wave processes.Although the contents of heavy metals is a bit higher in comparison with estimated background contents in Swedish lake sediments only copper and cadmium are estimated to be higher than recom-mended for a safe environment. Due to the toxicity of copper and cadmium some caution is recommend-ed before the dredging process starts.

A lack of well-distributed, high-resolution records of Late Quaternary Asian palaeomonsoon variability remains an outstanding issue for palaeoclimatologists, and is especially marked in remote regions such as the mountains of southwestern China (Wang et al., 2005). Characterising the nature, timing and magnitude of climate variability in southwestern China is essential for understanding the regional climate as a whole, and the potential social, economic and environmental impacts that may result from Asian monsoon system changes. The NERC-funded research presented in this thesis focuses on a high altitude lake sediment record obtained from Lake Shudu, Yunnan Province, China. The lake is located on the southeastern edge of the Tibetan Plateau. The primary aims of this research were to identify and examine key environmental and climatic shifts which occurred in southwestern China during the Late Pleistocene (Dali) - Early Holocene Period; to examine the possible drivers of these changes; and to compare the findings with other regional proxy records in order to better understand climate dynamics in southwestern China. These aims were chosen in order to test the hypothesis that Late Quaternary millennial to centennial scale climatic and environmental changes in southwestern China were driven by changes in solar insolation and / or glacial climate boundary conditions, characterised by stepwise increases in palaeomonsoon intensity. AMS 14C radiocarbon dates obtained from bulk sediment samples and pollen concentrations indicated that the seven metre core (06SD) that forms the focus of this research spans the last c. 22.6 ka cal. yr BP, making it one of the longest high-resolution Late Quaternary records available for southwestern China. 06SD was examined using a multi-proxy approach incorporating physical, organic and palaeoecological analyses. The record captures the shift from colder, drier Pleistocene (Dali) conditions to warmer, wetter Holocene conditions and is punctuated by two events. The first event, centred at c. 17.3 ka cal. yr BP, possibly represents a phase of warmer and / or wetter conditions in response to rising solar insolation during the deglacial period. The second event, commencing at c. 11.7 ka cal. yr BP, possibly denotes the Pleistocene - Holocene Boundary. Overall, the findings of this research support the view that during the Late Pleistocene, Asian summer monsoon strengthening was non-linear and driven by changes in glacial dynamics and / or solar insolation.