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Lake biotic responses to natural climate variability, fire disturbances, and human impacts over the Holocene were studied at two proximate sites in southwestern Québec. Sediments from Lac Noir and Lac Brûlé had annually deposited laminations (varves), enabling for the precise dating of continuous time-series and high-resolution analysis of subfossil diatom assemblages. The Lac Noir (45°46’31”N, 75°8’23”W, 176 m a.s.l.) record spanned ~11000 years of the Holocene. Stratigraphic changes in diatom assemblages of the lake could be divided into early, mid-, and late periods, broadly paralleling Milankovitch-scale climate intervals and vegetation changes inferred from regional palynological records. The early Holocene (11.1-8.0 ka) climate was cooler and dry, vegetation in the region was comprised of Picea-dominated woodlands, and the lake diatom flora included primarily benthic taxa. Warming in the mid-Holocene (8.0-3.6 ka) allowed for stabilization of soils and forests in the catchment, stronger thermal stratification in the lake, and resultant increases in oligo-mesotrophic diatom taxa such as Discostella stelligera. During the late Holocene (3.6 ka to present), an increase in the abundance of deciduous trees (e.g. Betula and Alnus) in response to cooling led to nutrient-enrichment and higher overall lake productivity. The record from Lac Brûlé (45°43’09”N, 75°26’32”W, 270 m a.s.l.) encompassed the last ~1200 years of the late Holocene. Generalized additive models (GAM) revealed a tight coupling between diatoms and catchment-mediated processes (e.g. vegetation and disturbances), which were closely aligned with climate variations. During the Medieval Warm Period (800-1300 CE), pollen-based inferences of warmer summer temperatures were associated with high abundances of Cyclotella bodanica var. intermedia and Cyclotella rossii; this signalled oligotrophic lake conditions and prolonged thermal stratification. The onset of the Little Ice Age (1450-1850 CE) marked a cooling in the region, and a decline in Tabellaria flocculosa str. IIIp indicated increased nutrient loading from the catchment area. Situated less than 300m from Lac Brûlé are remnants of the Wallingford-Back Mine, which ran from 1924-1972 CE; activities at the mine resulted in local changes to nutrient availability and primary productivity at this site. In previous studies of both Lac Noir and Lac Brûlé, pollen records had indicated overall similarities in the vegetation histories in response to climate variability during the late Holocene. Diatom assemblages were influenced by individual lake conditions and were thus unique to each site; nevertheless, they were closely linked with local and regional patterns of vegetation composition. A main point of difference in the paleo-records from both lakes was attributed to a local fire in the Lac Brûlé catchment at 1345 CE, which caused an early decline in hemlock (Tsuga). The decrease in hemlock was seen at Lac Noir only centuries later, and diatoms in each lake responded according to vegetation changes within their own respective catchments. This research shows that high-resolution sampling of lake sediments is able to detect diatom responses to both long-term and abrupt changes in the environment. Individual sites show similarly timed responses of other proxy-indicators, such as pollen and cladocera, to climate and land-use changes. However, distinct differences in the aquatic biota of well-dated proximate sites can be used to identify influences of regional climate variations, which are sometimes masked by localized, non-climatic processes.

Previous research indicates that future reductions in Arctic sea ice cover (SIC) could alter storm tracks and precipitation patterns in western North America and negatively impact water resources in the American southwest. Other research suggests that multiple periods of increased precipitation and/or cooler temperatures in the American southwest during the Little Ice Age (LIA) were due to strong El Niño events; historical records also describe expanded Arctic SIC at this time. We use 16th-19th century Arctic SIC records from the ACSYS Historical Ice Chart Archive as a basis for expanding Arctic SIC from 1870 HadISST data to theoretical LIA extents. Then, in a suite of sensitivity studies, we investigate the relative influences of and interactions between El Niño-Southern Oscillation (ENSO) related sea surface temperature (SST) variation and varying Arctic SIC in controlling storm tracks, precipitation patterns, and overall climate conditions in the American southwest. We find that tropical Pacific SSTs greatly influence climate system response to variability in Arctic SIC, with ENSO-Neutral SSTs permitting the greatest response. Additionally, the degree of expansion and symmetry of Arctic SIC also influence precipitation regime response. These findings suggest that the climate response to future Arctic SIC retreat may not only be highly dependent on the spatial patterns and extent of SIC reductions, but also upon ENSO variability, such that El Nino events may reduce the potential climate impact of ice reductions as compared to Neutral or La Nina events. / Master of Science

Climatic change has been the subject of investigation and spirited debate for more than three centuries. One important element of this debate has been the search for and definition of unique, impermanent climatic regimes measurable by historic time. The Medieval Warm Period and the Little Ice Age are the two most commonly referenced and discussed of such regimes. This thesis examines the theories and debates that preceded and surrounded the formal definition of the Little Ice Age as an historic period of approximately 1550-1850 AD. This thesis begins by describing early attempts to measure and record climatic conditions during the late seventeenth and early eighteenth centuries while also demonstrating that climatic change and climatic influence were matters of concern for both the scientific and philosophical elite and the public. By the first decade of the nineteenth century, however, discussion of climatic change had begun to center on comparisons of the medieval past and the cooler present. Climatic change itself often intruded on debates about past climates during the early nineteenth century. By 1900, however, both scholars and laymen had begun to recognize that some form of climatic change had occurred in the sixteenth century. Early twentieth century scholars such as Otto Pettersson, Charles Rabot, and Ellsworth Huntington helped define the boundaries and significance of historical climatic regimes. When Francois Matthes wrote of a "little ice-age" in 1939, he was not creating a wholly new idea; he was instead engaging in a centuries-old debate over the climatic conditions of the last millennium.

Diatom analyses were carried out on sediment cores collected from two low-lying, closed-drainage basins (PAD 9 - 58??46. 46?N, 111??19. 48?W; PAD 12 - 58??57. 29?, 111??19. 74?) in the Peace sector of the Peace-Athabasca Delta (PAD), Alberta, Canada, to provide >1000 year long records of hydro-ecological change. Results from diatom analyses were compared with macrofossil and stable isotope records from the same cores and assessed within the framework of an Athabasca River headwater climate record inferred from isotope dendroclimate data. Results from PAD 9 and PAD 12 sediment cores indicated closed-drainage conditions during the Medieval Warm Period (MWP) and the post-Little Ice Age and high water conditions during the Little Ice Age (LIA). High water levels at PAD 9 and PAD 12 reflected high water conditions on Lake Athabasca and the Rivi??re des Rochers or possibly the Peace River during the LIA (~AD 1600-1900). High water conditions were also observed at low-lying sites in the central and southern regions of the PAD (PAD 31 and PAD 37), and corresponded with evidence of high streamflows on the North Saskatchewan River. In contrast, desiccation evident at PAD 5, a site largely isolated from river influence, reflected atmospherically dry conditions during the LIA. Consistent with changes observed at PAD 5, sediment records at PAD 15, an oxbow lake off the Revillion Coup??, demonstrated low flood frequency during the early to mid-1700s. Increased water levels evident at low-lying sites located in proximity to the central open-drainage network of lakes and rivers were likely due to higher flows on the Athabasca River and potentially on the Peace River. High flows on rivers of the PAD may be attributed to snowmelt-dominated runoff during the LIA relative to the rainfall-dominated runoff during MWP (prior to ~AD 1600) and the post-LIA period (~AD 1900 to present).

Lake sediment cores spanning the last 2000 years from four sites across the Canadian Arctic Archipelago (CAA) document the responses of terrestrial and freshwater ecosystems to regional climate variability. Biogenic silica (BSi) records in cores from Banks Island, NWT (Lake B503; 72.3245, -123.4036, 84 masl), Bathurst Island, Nunavut (PR01; 75.6497, -99.1144, 30 masl), Prince of Wales Island, Nunavut (SW08; 72.3177, -97.2678, 104 masl), and Ellesmere Island, Nunavut (CV03; 79.9211, -82.9348, 363 masl) were used to examine the relationship between diatom production and climate. A pollen record from Prince of Wales Island provided the first high-resolution July temperature reconstruction for the last 1000 years for the central CAA. Dissolution was evident in three out of the four lakes; core SW08 contained no BSi above detection and cores CV03 and PR01 only contained values above detection in the uppermost sediments, suggesting that the preservation of biogenic silica (BSi) in the sediment is likely influenced by sedimentary carbonates. A BSi sequence from core B503 showed that diatom production was affected by climate changes such as the Medieval Climate Anomaly and the Little Ice Age. The vegetation on southern Prince of Wales Island underwent marked transitions during the Little Ice Age and Medieval Climate Anomaly, which was mainly observed in the proportion of Cyperaceae and Poaceae. The mean July temperature reconstruction showed a long-term cooling from 1080-1915 CE with a sustained cold period from 1800-1915 CE prior to 20th-century warming. A synthesis of paleoclimate records from across the Arctic demonstrated that pollen-based reconstructions record both high and low frequency climate variability, when sampling resolution is sufficient, and can improve regional climate reconstructions.

A 147cm sediment core from Lake DV09, northern Devon Island, Nunavut, Canada (75° 34’34”N, 89° 18’55”W) contains annually-laminated (varved) sediments, providing a 1600-year record of climate variability. A minerogenic lamina deposited during the annual thaw period and a thin deposit of organic matter deposited during the summer and through the winter, together form a clastic-organic couplet each year. The thinnest varves occur from AD800-1050, and the thickest from AD1100-1300, during the Medieval Warm Period. The relative sediment density is also highest during this period suggesting increased sediment transport energy. The coldest period of the Little Ice Age appears to be during the AD1600s. Varve widths over the past century indicate climate warming in the region. / This research program was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Canadian Foundation for Climate and Atmospheric Sciences (CFCAS). A tuition bursary from Ultramar Inc. also helped in making this research possible. Logistical support was provided by the Polar Continental Shelf Project (PCSP Contribution number 04508).

A 147cm sediment core from Lake DV09, northern Devon Island, Nunavut, Canada (75° 34’34”N, 89° 18’55”W) contains annually-laminated (varved) sediments, providing a 1600-year record of climate variability. A minerogenic lamina deposited during the annual thaw period and a thin deposit of organic matter deposited during the summer and through the winter, together form a clastic-organic couplet each year. The thinnest varves occur from AD800-1050, and the thickest from AD1100-1300, during the Medieval Warm Period. The relative sediment density is also highest during this period suggesting increased sediment transport energy. The coldest period of the Little Ice Age appears to be during the AD1600s. Varve widths over the past century indicate climate warming in the region. / This research program was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Canadian Foundation for Climate and Atmospheric Sciences (CFCAS). A tuition bursary from Ultramar Inc. also helped in making this research possible. Logistical support was provided by the Polar Continental Shelf Project (PCSP Contribution number 04508).

Diatom analyses were carried out on sediment cores collected from two low-lying, closed-drainage basins (PAD 9 - 58º46. 46?N, 111º19. 48?W; PAD 12 - 58º57. 29?, 111º19. 74?) in the Peace sector of the Peace-Athabasca Delta (PAD), Alberta, Canada, to provide >1000 year long records of hydro-ecological change. Results from diatom analyses were compared with macrofossil and stable isotope records from the same cores and assessed within the framework of an Athabasca River headwater climate record inferred from isotope dendroclimate data. Results from PAD 9 and PAD 12 sediment cores indicated closed-drainage conditions during the Medieval Warm Period (MWP) and the post-Little Ice Age and high water conditions during the Little Ice Age (LIA). High water levels at PAD 9 and PAD 12 reflected high water conditions on Lake Athabasca and the Rivière des Rochers or possibly the Peace River during the LIA (~AD 1600-1900). High water conditions were also observed at low-lying sites in the central and southern regions of the PAD (PAD 31 and PAD 37), and corresponded with evidence of high streamflows on the North Saskatchewan River. In contrast, desiccation evident at PAD 5, a site largely isolated from river influence, reflected atmospherically dry conditions during the LIA. Consistent with changes observed at PAD 5, sediment records at PAD 15, an oxbow lake off the Revillion Coupé, demonstrated low flood frequency during the early to mid-1700s. Increased water levels evident at low-lying sites located in proximity to the central open-drainage network of lakes and rivers were likely due to higher flows on the Athabasca River and potentially on the Peace River. High flows on rivers of the PAD may be attributed to snowmelt-dominated runoff during the LIA relative to the rainfall-dominated runoff during MWP (prior to ~AD 1600) and the post-LIA period (~AD 1900 to present).

A continuous, decadal-scale resolution multi-proxy record of climate variability over the past 1400 years in the northern Gulf of Mexico (GOM) was constructed from a box core recovered in the Pigmy Basin. Proxies include paired analyses of Mg/Ca and oxygen isotopes in the white variety of the planktic foraminifer Globigerinoides ruber and relative abundance variations of G. sacculifer in the foraminifer assemblages. Two multi-decadal intervals of sustained high Mg/Ca values indicate GOM sea-surface temperatures (SSTs) were as warm or warmer than near-modern conditions between 1000 and 1400 yrs BP. Foraminiferal Mg/Ca values during the coolest interval of the Little Ice Age (ca. 250 yrs BP) indicate that SST was 2 - 2.5 degrees Celcius below modern SST. Four minima in the Mg/Ca record between 900 and 250 yrs BP correspond with the Maunder, Spörer, Wolf and Oort sunspot minima, suggesting a link between solar insolation and SST variability in the GOM. An abrupt shift recorded in both the oxygen isotopic ratio of calcite and relative abundance of G. sacculifer occurs ~600 yrs BP. The shift in the Pigmy Basin record corresponds with a shift in the sea-salt-sodium (ssNa) record from the GISP2 ice core, linking changes in high-latitude atmospheric circulation with the subtropical Atlantic Ocean.