In this study a multiproxy approach (including sedimentary techniques, diatoms, molluscs, foraminifera, sedimentary pigments, isotopes, pollen and plant macrofossils) has been adopted to assess environmental change over the last ~9,000 years at three Danish coastal sites (Kilen, Norsminde Fjord and Korup Sø). Particular focus has been placed on periods of intense human coastal occupation, identifiable in Denmark's rich coastal archaeological record (i.e. shell midden accumulation periods), to test critically, hypotheses that changes in the marine environment were contemporary with major cultural and societal changes over the last ~9,000 years. For example, it has been proposed that a decrease in salinity was responsible for the widespread oyster decline, apparent in the Neolithic layers of a number of Danish shell middens. This hypothesis, however, remains speculative to date, lacking any high-resolution and quantitative salinity data covering the Mesolithic-Neolithic transition. Inside the agricultural era, two more phases of shell midden accumulation occur (i.e. during the Pitted Ware/Single Grave cultural period and the Iron Age), suggesting that people must have returned to the sea at these times for increased exploitation of its resources. A diatom-based salinity transfer function (WAPLS-C3 model, r2 boot = 0.923, RMSEP= 0.36 square root salinity units) based on a trans-Baltic training set has been applied to fossil diatom datasets from each site for quantitative assessment of salinity change over the study period. The multiproxy results presented in this study demonstrate a close connection between environmental change and human exploitation of marine resources over the Holocene. This relationship, however, is complex, with the individual fjord systems often exhibiting spatially different responses (i.e. variations in the sedimentary regime, salinity, productivity and nutrient status) to changes in key forcing mechanisms such as sea level change, climate change and human impact upon the catchment (following the introduction of agriculture). Environmental hypotheses for cultural change are reviewed on the basis of the evidence presented in this study. Diatomenvironmental relationships have also been modelled (using multivariate techniques) at Korup Sø and Norsminde Fjord using proxy data as ‘predictor' variables for changes in the terrestrial and marine environment. These results suggest that a variety of marine, climatic, human and catchment related processes are important in explaining a proportion of the variation in the fossil diatom datasets, but these influences tend to vary temporally throughout the profile (e.g. human impact becomes important after ~3,900 BC).

This thesis uses palaeolimnological records to reconstruct Holocene ontogeny trends from four lakes in south western Greenland. The research addresses four hypotheses investigating how Holocene lake ontogeny trends vary under different climatic settings, how long-term changes in ontogeny relate to periods of established climatic change in the region, the similarities between proxies within the lakes and between the lakes, and the role of vegetation in lake ontogeny. The study region occupies the widest ice-free area of south western Greenland and is characterised by a climatic gradient. The area inland and nearer to the ice-margin is arid, receives less precipitation and is warmer relative to the coastal areas. A paired lake approach, using lake records from two inland lakes and two coastal lakes, was adopted to examine the role of climatic setting upon lake development trajectories. Specifically, diatoms were used to reconstruct DI-alkalinity from the lakes using a DI-alkalinity model created from existing training sets in the region (WA Cla model, r2boot = 0.76, RMSEP = 0.28 log alkalinity units), sedimentary pigments to investigate trends in production and sedimentary parameters to reconstruct organic and minerogenic accumulation rates. All four lakes experienced comparable Holocene long-term ontogeny trajectories; maximum alkalinity in the first ~ 1000 cal. year BP of deglaciation followed by maximum production during the peak of Holocene Thermal Maximum (HTM) warming (~7000 -6000 cal. years BP). Following the HTM, all lakes demonstrated oligotrophication and a decline in pH. Vegetation development and catchment stabilisation at the end of the HTM may be important in determining the onset of oligotrophication in vegetated catchments. However, the impact of vegetation development on lake ontogeny cannot be isolated from the changes in the lakes associated with the colder and wetter climate which occurred at the end of the peak HTM warming. The timings of the large transitions in the ontogeny trajectories are comparable with established periods of Holocene climatic variability in the region; climate forcing drives ontogeny in these lakes. However, there are short-term differences between the lakes indicating that lakes have different thresholds of ecological change and may respond differently to the same climate forcing. It is concluded that ontogeny is driven by climate but lakes may respond differently to forcing depending on catchment specific characteristics which can filter out the climate signal or cause climate to influence the lake in a more direct way.

The Chukchi and Beaufort Sea shelves host diverse and productive seafloor ecosystems important for carbon and nitrogen cycling for the Arctic Ocean. The benthic food web transfers energy from primary producers to high trophic level organisms (e.g., birds, fish, and mammals), which are important for cultural practices and subsistence hunting by Native Alaskans. This work focuses on the trophic ecology of arctic food webs through use of several different approaches. First, variation in the natural abundance of stable carbon and nitrogen isotopes facilitated the identification of trophic pathways and, subsequently, allowed the comparison of trophic guilds and food webs from the Chukchi and Beaufort Seas. Compared to water column and sedimentary organic matter end-members, second trophic level grazers and suspension feeders were conspicuously ¹³C-enriched throughout the Chukchi Sea, which supports the hypothesis that microbial degradation of organic matter occurred prior to metazoan assimilation. Second, food web recovery from disturbances caused by exploratory oil drilling at the seafloor that had occurred approximately 20 years prior were assessed in both the Chukchi and Beaufort Seas. Based on isotopic trophic niche overlap between organisms common to drilled and reference sites in the Chukchi and Beaufort Seas, the oil drilling sites had similar food web structure, indicating recovery from the activity associated with the drilling process. Third, photosynthetic pigment biomarkers were used to better understand the diagenetic process, specifically focusing on how both microbial and metazoan grazing pathways degrade organic matter in relation to seasonal sea ice retreat in the Chukchi Sea. The benthic macrofaunal and microbial food web caused rapid degradation of organic matter upon the initial pulse of microalgal food sources to the seafloor. These diagenetic pathways are linked to the ¹³C-enrichment of residual organic matter, which corresponds to the stable isotope values measured in the benthic macrofauna. Lastly, high-precision liquid chromatography and spectrophotometry were compared for estimating sedimentary pigments in the marine environment. Substantial differences in pheopigment (chlorophyll degradation products) concentrations were observed between the two techniques, suggesting the need for revisions to the monochromatic spectrophotometric equation that relates absorbance to pigment concentrations. One pheopigment, pheophorbide, was found to interfere with the accuracy of the spectrophotometric equation and caused the overestimation of pheopigments. / text