This thesis presents palaeoenvironmental data from equatorial Africa covering two important time intervals; i) the warming period forming the Pleistocene/Holocene transition and ii) the last millennium. The Empakaai Crater, in northern Tanzania contains a lake from where sediment cores, spanning two time-slices 14.8-9.3 ka and 800-2000 AD, have been studied. Palaeoecological and palaeohydrological reconstruction is based on a multitude of proxies from the sediments, representing both catchment environment and the lakes aquatic ecosystem response. Between 14.8 and 10 ka the catchment vegetation and lake hydrology responded to both regional climate changes and local environment, but with different amplitude and frequency, reflecting temporal and spatial lags between the two systems. However, at c 10 ka both lake conditions and catchment vegetation showed drastic changes towards drier conditions. The record covering the last millennium reveals environmental changes related to climate and human activities. The catchment’s vegetation was affected by frequent fires, most probably human induced, while near shore vegetation responded to lake level fluctuation associated with rainfall variability. About 15 km from Empakaai Crater is an extensive abandoned irrigation system, the Engaruka complex, which was in active use between c 1400 AD and 1840 AD. By comparing a number of social and environmental factors potentially influencing the societal development at Engaruka it is shown that wet climate conditions have had positive effects on the societal development but also that dry climate conditions were not always disastrous to the society. The resemblance of the pollen taxa present is strong between the two time slices and pollen representing catchment conditions respond in similar manner in both records. The lake conditions are however very different between the two periods Thus the lake responds to both long and short term changes of variable amplitude, while the catchment vegetation seems to responds to high amplitude, low frequency changes.

Lake Hayq is a closed, freshwater basin on the eastern margin of the north-central highlands, Ethiopia. Using a sediment core extracted from the northern basin, this thesis aims to provide a high-resolution, detailed palaeolimnological reconstruction of changes to the environment and climate in the region since the late Pleistocene. A multi-proxy approach was applied, utilising diatoms, photosynthetic pigments and X-ray fluorescence (XRF) spectrometry. Lithological and chronological analyses were also performed, as well as the development of a transfer function to model diatom-inferred conductivity, and other quantitative analyses. Between ~ 15.6 15.1 cal kyr BP, Lake Hayq experienced a lowstand, synchronous with the timing of Heinrich Event 1 and an intense drought across East Africa. At ~ 15.1 cal kyr BP a lake began to develop at the core site in response to wetter, more humid conditions, most likely caused by the reactivation of the African-Indian monsoonal circulation. This was abruptly ended however at ~ 14.7 cal kyr BP, as the climate shifted back towards aridity and Lake Hayq shallowed, in contrast to the majority of other East African lakes, which continued to refill. This most likely reflects changes to the Indian Ocean monsoon system caused by variability in the Atlantic Meridional Overturning Circulation at this time, in conjunction with site-specific mechanisms affecting the delivery of precipitation to Lake Hayq. At ~ 12.3 cal kyr BP the African Humid Period resumed over Lake Hayq and the lake refilled, reaching maximum water depth between ~ 12.0 10.0 cal kyr BP. The lake was dominated by planktonic diatom taxa and photosynthetic pigments indicate it was meromictic. Lake level gradually declined throughout the Holocene, culminating in the termination of the African Humid Period. A high-resolution study of the period tentatively suggests that climate flickering , in the form of oscillations between dominant diatom taxa, occurred in the build up to the major climatic shift. The termination spanned ~ 600 cal years between ~ 5.2 4.6 cal kyr BP. A lowstand occurred between ~ 3.9 2.2 cal kyr BP, during which the lake became occasionally subsaline. In the late Holocene, ~ 2.2 1.3 cal kyr BP, Lake Hayq became deep and fresh again, although there is evidence of lake level variability. The palaeo-record from Lake Hayq indicates that it broadly experienced the same high-latitude, glacial-interglacial dynamics and sub-millennial shifts in climate found in other palaeolimnological records from across East Africa. The precise timing and expression of these climatic events is not always synchronous between Lake Hayq and other East African waterbodies however, most likely caused by local, site-specific positive feedback mechanisms and variability in lake morphometry. This highlights the heterogeneous pattern of climate across the region and the significance of regional drivers. This palaeo-record, spanning the late Quaternary, will help bridge gaps in current knowledge and understanding of the underrepresented, climatically sensitive and vulnerable north Ethiopian highlands. This is vital for future climate change modelling and regional downscaling, and may also inform ethnographic-archaeological research in a region considered to be the cradle of humankind.

The Danubian Plain is nothern part of the Pannonian Plain and in terms of palaeobotanical research it is an entirely unexplored area. However, this region is assumed to contain cryptic glacial refugia and migration routes of thermophilous biota northward after the Last Glacial Maximum. The late-glacial and Holocene natural long-term succession and temporal changes in the composition of plant communities of lowland lakes in the Slovakia is unknown. Šúr, a National Nature Reserve is a former lake, presently overgrown by alder carr forest. Successional development since the late-glacial was studied along two profiles. Macrofossil analysis was carried out on lake sediments extending to the Middle Pleniglacial (OIS 3). Using knowledge of the modern ecological requirements of taxa, past environmental conditions were reconstructed. The historical biogeography of selected plant species in Europe during the late- glacial and Holocene is also discussed. During the Middle Pleniglacial Myriophyllum spicatum and Chara sp. grew in the lake. During the late-glacial shallow basin contained a calcareous, meso- to eutrophic lake with Potamogeton filiformis, P. praelongus, Ranunculus subgen. Batrachium etc. During the early Holocene these elements disappeared and thermophilous plants immigrated, such as the...

This thesis presents temporal perspectives of lake acidification in Sweden. Sediment records have been used to study timing, trends and causes of acidification, and two different techniques for assessing past lake-water acidity are presented. A new technique for pH prediction, based on near-infrared (NIR) spectroscopy of surface sediments, is developed. This study shows that there is a pH related fraction of lake sediments that can be recorded by NIR analysis. Relationships between NIR spectra of surface sediment samples and measured lake-water pH values, and between NIR spectra of sediment cores and historical pH values inferred by diatom analysis, are modelled by partial least squares regression. The prediction errors of the models are comparable to those obtained by modelling of diatom and lake-water pH data. By further development NIR spectroscopy can become useful for inferring past pH, as well as several other lake-water parameters, from sediment cores. Diatom-based predictive models, using multivariate calibration methods, are developed for inferences of lake-water pH, alkalinity and colour. These models are used to provide a regional assessment of recent lake acidification in the provinces of Västerbotten and Norrbotten, northern Sweden. The study shows that a pH decline has occurred in some southeastern lakes, but that most of the lakes have not faced significant changes in lake-water pH, alkalinity and colour. The inferred water chemistiy changes are discussed in relation to atmospheric deposition and land-use. In a study of eight acid-sensitive Swedish boreal-forest lakes a past-analogue approach is used to test whether contemporary expansion of conifers could cause lake acidification. Water chemistry changes associated with the natural pre-historic colonization and expansion of spruce in Sweden (≈3000 years B.P.), at times of background atmospheric acidity, are inferred to evaluate the acidification ability of spruce per se on surface waters. This study shows that under natural, unpolluted conditions spruce colonization and expansion did not cause lake acidification. In a synthesis of palaeolimnological acidification research in Sweden a general model for pH- development for acid clear-water lakes in southern Sweden is presented. The pH-development from the last deglaciation to present time can be divided into four different periods; (i) a natural long-term acidification period (12000 B.P. - 2300 B.P., or later), with a gradual decrease in pH resulting from declining fluxes of base cations from catchment soils; (ii) a human induced alkalization period (2300 B.P. - 1900 A.D.), with a pH increase due to human activities in the catchments; (iii) the recent acidification period (about 1900 A.D. - present), when pH decreased towards 4.5 due to acid deposition and possibly ceased land-use; and (iv) the liming period (1970s - present), when pH often increases to values above 7 following lime treatment to counteract acidification. The implications of these past pH changes for the concept of contemporary lake acidification and for liming policy are outlined. / digitalisering@umu

The Northern Isles are strongly influenced by changes in the North Atlantic Ocean atmosphere system and, as they project northwards from the British Isles, provide an ideal geographical opportunity to study changing climatic gradients during the last glacial/interglacial transition along with the detection of regime shifts. Three proxies, diatoms, pollen, and micro-XRF sediment chemistry, have been employed to explore the nature and timing of environmental changes within the water columns and the wider catchments of Loch of Sabiston, Orkney, and Loch of Clumlie and Loch of Grimsetter, Shetland to better understand the nature and timing of environmental change within and among the island groups. The records are constrained by radiocarbon dating, supported by tephrochronology, and the Greenland ice core chronology to enable the comparison of the records produced by this study with previous research in the North Atlantic region. The diatom and lithological results from Loch of Sabiston suggest early deglaciation at c. 23,000 cal BP followed by gradual warming (GI-1e) punctuated by the cooling events coeval with GS-1 and GI-1b. However, the pollen record reflects a lagged response in the development from colonising cold tolerant vegetation to more temperate shrub and woodland communities. The Oracadian signal is dominated by the switching on and off of the accumulation of marl which serves as a supporting indicator of warmer conditions. The Shetland landscape appears to have been deglaciated later at c. 16,400 cal BP, but also has clear representation of GI-1e and the cooling events of GI-1b and GS-1. Both the Shetland and Orkney records record the dramatic cooling of the Younger Dryas but also suggest a two stage change from colder and drier to colder and wetter conditions before the onset of the Holocene. Shetland appears to have experienced less extreme climatic changes in comparison to Orkney despite being in the same present phytogeographical region. This is likely due to the former persisting in the arctic domain and the latter being closer to the latitudinal shifts in the warmer ocean circulation of the North Atlantic during the LGIT. Comparison of the three proxies demonstrates that they may differ by several hundred years in their response to dramatic climatic changes and, therefore, highlights the strength of multi-proxy approaches to reconstructing Quaternary environments. Combining proxies such as diatom and μ-XRF scanning techniques will provide a greater understanding of the processes occurring during environmental change in this region.

Multiple proxies derived from the Lake Loitsana sediment sequence (NE Finland) are employed to determine the timing of deglaciation, characterise an early Holocene proglacial lake stage and reconstruct Holocene lake development. Local-scale processes causing shifts in biological assemblages are identified and the most likely Holocene mean July air temperature (Tjul) development is assessed. The study area was deglaciated shortly prior to 10 700 cal. a BP. The sediment record reflects four local events; the presence of a glacial lake, glacial lake drainage and formation of Lake Loitsana, changes in fluvial input due to progressive wetland expansion, and gradual lake infilling. The results suggest that local events have driven changes in biological assemblages through various processes, and that biotic proxies reflect changes in environmental parameters in a highly individual manner. Furthermore, biological assemblages can themselves act as important drivers, influencing the composition of other assemblages. It is suggested that future studies should consider macrophyte abundance and food-web interactions as equally important factors when assessing changes in biological assemblages. Quantitative Tjul reconstructions based on biotic proxies display contrasting trends. While Tjul reconstructions based on pollen found in the Loitsana sequence display relatively low early Holocene values, plant macrofossil and chironomid data reflect warm summer conditions also during the early Holocene, i.e. at the peak of summer insolation. The early Holocene Tjul recorded by terrestrial pollen are affected by local factors possibly combined with a delayed response of the terrestrial ecosystem compared to the aquatic one. This study emphasises the importance of using multiple proxies in palaeoenvironmental studies and shows that local factors have a potential to drive changes in biological assemblages that can affect transfer-function based temperature reconstructions. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Epub ahead of print. Paper 2: Accepted. Paper 3: Manuscript.</p>