Changes in climate and catchment processes over the middle and late Holocene in the boreal region of northeastern Ontario, Canada

Abstract

The climate of North America has varied substantially over the Holocene. Holocene Thermal Maximum (HTM) warmth occurred earlier in the west with associated aridity, compared to the east which was humid. Specific regions of North America, however, are underrepresented in continental reconstructions of Holocene climate including boreal northeastern Ontario (Canada). This thesis examines pollen, pigments, diatoms, and elemental geochemistry from three sediment cores retrieved from two small, headwater, boreal lakes in elevated watersheds to examine changes in climate, lake-production, lake-level, redox status, and terrestrial input over the middle and late Holocene of the region. Analysis of proxies suggest warm and wet climate conditions that were accompanied by high lake-production, low hypolimnetic oxygen concentrations, reducing sediment-water interface conditions, and a high degree of input from the watershed from ~6000–4000 cal yr BP. Lake-level may have been lower during this time as high rates of evapotranspiration likely counteracted high amounts of precipitation. These conditions were reversed past ~4000 cal yr BP due to declines in both temperature and precipitation. Cohesion among proxies within and between lake systems suggest trends are tied to regional changes in climate and that this region experienced a delayed and humid HTM compared to northwest Ontario. Intra-lake discrepancies in pigments and geochemistry, however, suggest basin-specific deposition of inorganic catchment material due to morphometry and sediment focusing. These discrepancies suggest that researchers should exercise caution when interpreting regional signals from single core studies. This thesis also examines the role of light attenuation in structuring modern diatom assemblages within lakes. Diatoms collected from surface sediments in depth-gradient transects suggest that light attenuation is a strong driver of benthic-planktonic (B:P) assemblage separation compared to other variables (e.g., the thermocline). Comparisons with previously studied boreal lakes suggest that DOC concentration can act as a predictor of B:P boundary depth. As new techniques are attempting to reconstruct DOC concentrations through time, past changes in light regime may soon be included in common paleolimnological interpretations. This thesis warns that ongoing anthropogenic warmth may result in eutrophication and alterations to light regimes within presently clear, oligo- and mesotrophic lakes in boreal northeastern Ontario.