Assessing environmental controls on the distribution of long-chain alkenones in the Canadian Prairies

Long-chain alkenones (LCAs) have been used for decades to reconstruct quantitative sea-surface temperature records, but they also have a great potential for paleotemperature reconstructions in lacustrine settings. Here, we investigated how the presence and abundance of LCAs in surface sediments from 106 lakes varied with environmental conditions in lakes of the northern Great Plains (Canadian Prairies) in southern Saskatchewan, Canada. Consistent with preliminary research, we found LCAs in 55% of surveyed lakes, with mean concentrations of 143 μg/g dry sediment, but very high concentrations (up to 2.3 mg/g dry sediment) in 7% of lakes. Statistical analyses indicate that salinity and stratification play key roles in determining LCA presence and abundance supporting previous research in Spain and the northern Great Plains, USA. Overall, the alkenone unsaturation index (U37K) was not correlated significantly with average summer water temperature, probably because the timing of maximum LCA production occurs during the spring season. We conclude that improved seasonal sampling is required within the study lakes to better identify the timing and habitat of haptophyte production, and allow development of environmental temperature reconstruction tools

Climate and environmental variability during the late Holocene in the Canadian Great Plains: a multi–proxy biomarker approach

According to the IPCC Fifth Assessment Report (IPCC, 2014), climate change is projected to reduce surface and groundwater resources and significantly undermine water quality and availability. Modelling studies have demonstrated that for each degree of global warming, approximately 7% of the global population is expected to be exposed to a decrease of replenishable water resources of at least 20% (IPCC, 2014). The Canadian Great Plains, encompassing large portions of the provinces of Alberta, Saskatchewan and Manitoba, is one of the regions where IPCC hydrological and air temperature models show the lowest agreement (<60%); future drought scenarios appear to be particularly complex as, although dry spells are projected to last longer, winter rainfall is also predicted to increase. Climate modelling for future scenarios cannot be achieved by modern observations alone, the potential changes are too vast and complex, and the timescale of occurrence is not yet fully understood. Understanding modern climate change is therefore reliant on our ability to characterise the Earth’s climate system at multiple timescales and over a variety of time periods. Particular focus should be given to atmospheric circulation, such as El Niño Southern Oscillation (ENSO), since simulations of droughts at global scale during the 20th century have shown strong correlations between ENSO and hydrological droughts. A multi-proxy approach is presented in this study, combining biomarkers (Long Chain Alkenones – LCAs, Long Chain Diols – LCDs, and Glycerol Dialkyl Glycerol tetraethers – GDGTs), X-ray fluorescence (XRF) and sedimentary organic analysis (Total Organic Carbon –TOC, carbon/nitrogen ratio – C/N). Biomarkers were evaluated for quantitative paleo-climate and -environmental reconstruction with focus on GDGTs. Two types of GDGTs (branched – br, and isoprenoidal – iso) were studied in 96 lakes from the Saskatchewan province, within the Canadian Great Plains. It was observed that the iso-GDGT based TEX86 index could not be applied to the studied lakes due to the large relative proportion of soil derived and methanogenic components. Similarly, br-GDGT derived indices, such as MBT/CBT, were not applicable as the predicted temperatures were considerably lower or higher than those measured. Statistical analysis conducted on the GDGT dataset showed that temperature and salinity accounted for a large and statistically independent fraction of the variation in the dataset. Based on these results, two novel regression models were proposed for salinity using br-GDGTs: Brsalinity (R2 = 0.76, RMSE = 4.9 g/L) and Brsalinity.b (R2 = 0.84, RMSE = 3.47 g/L). This is the first study that presents a GDGT-based calibration for salinity. This thesis presents the first study of lacustrine downcore application of biomarkers in the Canadian Great Plains to determine past climatic and hydrologic variability in this region over the past 2000 years. Two long lacustrine sediment cores from the northern (Humboldt Lake) and southern (Lac Pelletier) Great Plains, were retrieved to investigate shifts in air and water temperature, hydrology and lake productivity over the past 2000 years within the two lakes and their catchment. For the Humboldt Lake sediment core all three biomarkers were analysed for air and water temperature reconstruction. LCA proxies including R3b, UK37 and UK'37 were successfully applied downcore and they provided the first quantitative May Water Temperature (WT), MAAT and spring WT records specific to the Saskatchewan prairies. Additionally, LCA results, suggest that RIK37 and fGII can be qualitatively applied to infer past salinity but also might have the potential to be applied quantitatively as paleo-salinity proxy. For Humboldt Lake, GDGTs had to be excluded due to sample contamination. Finally, statistical analysis between LCDs and recorded MAAT and WT did not produce reliable models and were therefore not used for paleoclimate reconstruction. For the Lac Pelletier sediment core, GDGTs and LCD were successfully applied as palaeo-thermometers for MAAT and autumn WT, respectively. GDGT-based Brsalinity.b calibration developed in this thesis was also applied downcore. However, as instrumental record for salinity was not available, Brsalinity.b was qualitatively used to infer salinity changes. By combining the data generated from XRF, biomarkers, TOC and C/N, this study provided novel quantitative information on the climate and hydrological history of the Canadian Great Plains. Specifically, this thesis suggests that the Great Plains are not an uniform hydrological region, and distinct factors affect water availability and quality in different part of the plains. It was also inferred that the north to south divide seen today as a result of the mid-tropospheric pattern has been present consistently in the past 2000 years. It is evident from the results presented in this thesis that solar forcing and large-scale climate events are key drivers of shifts of air temperature in the Canadian prairies. However, and perhaps more importantly, negative phase of ENSO (La Niña) and Pacific Decadal Oscillation in conjunction with increased wind activity and glacier fluctuations were identified as the fundamental drivers of the extent and severity of wet and dry periods in the Canadian Great Plains