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Peatlands in northern Ontario, Canada, archive multiple biological indicators, including macrofossils, algae, testate amoebae, and pollen. These proxies can provide insights concerning the timing and nature of long-term climatic and environmental changes. The Hudson Bay Lowlands (HBL) of central Canada contain one of Earth’s largest continuous peatland complexes, and thus comprehensive spatial and temporal studies are needed to understand the implications of climate change on carbon cycling. Diatom assemblages were examined in three cores retrieved from ombrotrophic bogs across two Canadian terrestrial ecozones. Comparisons were made with testate amoebae and macrofossil data previously analyzed from these cores, as well as with regional pollen records from surrounding peatlands. From ~2000 to ~600 cal. BP, changes in diatom composition likely reflect hydrosere succession within the peatland, including fluctuations in connectivity to the water table and pH changes. From ~600 cal. BP to present, the synchronous timing of changes in diatoms and testate amoebae are tracking drying conditions and subsequent microhabitat variations that occur within bogs. It is possible that diatoms are tracking subtle changes in the stability of peat microforms including bog hollows and hummocks, highlighting their sensitivity to small chemical change, whereas testate amoebae are tracking the lowering of a peatland water table and subsequent drying of the peatland. The use of multiple proxies provides a more holistic approach to interpreting past ecological succession and responses to climate within peatlands. When present and well preserved, diatoms can be applied to track drying conditions in bogs, in terms of both hydrosere succession and present climatic change.

Sediments from South Lake, Brock Plateau (Melville Hills), Northwest Territories, provide one of the longest postglacial records from the mainland western Canadian Arctic, outside of eastern Beringia. Sedimentation commenced at least 13 900 cal. yr BP, and possibly as early as 16 000 cal. yr BP, in response to early deglaciation of the site. Pollen is present throughout the record, with an initial Artemisia-Salix assemblage indicative of very cold conditions, consistent with a locally severe Younger Dryas Stade or simply continued proximity of the Laurentide Ice Sheet margin ( c. 12 700 to 11 500 cal. yr BP). At c. 11 000 cal. yr BP, abiotic proxies signal a transition to warmer conditions, corroborated by a pollen assemblage dominated by Betula and Cyperaceae. Although South Lake was biologically productive during the early Holocene ( c. 11 000 to 7000 cal. yr BP), diatoms and other siliceous organisms are notably absent from the record, suggesting severe silica limitation. Rises in Alnus crispa and Picea mariana pollen at c. 7000 cal. yr BP suggest cooling and/or an increase in effective moisture. Wetter conditions and increased hydrological inputs and silica supply likely led to the establishment of a pioneering diatom community at c. 6500 cal. yr BP. Decreased organic sedimentation after c. 2000 cal. yr BP suggests cooler conditions. Additionally, changing niveo-eolian deposition of sand on lake ice varied with a c. 3000-year periodicity through the entire record. The South Lake multiproxy record supports the hypothesis that the Brock Plateau was one of the earliest deglaciated regions during the late Wisconsinan.

Changes in diatom assemblages in a sediment core taken from a tundra lake near the Lena River Delta, northeastern Siberia, reflect climatic and environmental changes throughout the Holocene. Significant limnological changes occurred at the same time as trees advanced into the region c. 9100 cal. yr BP (8500 radiocarbon yr BP) and subsequently retreated c. 3800 cal. yr BP (3500 radiocarbon yr BP). Prior to this tree-line shift, diatom assemblages were dominated by small benthic Fragilaria species, and diatom-inferred alka linity values were high, suggesting that climatic conditions were cool and relatively dry, and that lake pro ductivity was probably low. Coincident with tree-line advance into the region c. 9100 cal. yr BP, inferred alkalinity decreased and remained low throughout the forested period, reflecting soil development and increased organic runoff associated with the shift in catchment vegetation. The establishment of a larch forest in the catchment corresponded with an initial period of limnological change between 9100 and 7700 cal. yr BP, suggesting that soil and forest development took place over approximately 1400 years. Between c. 7700 cal. yr BP (c. 7000 radiocarbon yr BP) and 3800 cal. yr BP, a stable diatom assemblage dominated by Achnanthes species and low diatom-inferred alkalinity indicate the influence of organic runoff associated with a fully developed forest. A return to cooler conditions and reversion to shrub tundra c. 3800 cal. yr BP coincided with a shift back to a Fragilaria-dominated diatom assemblage and an increase in diatom-inferred alkalinity. Recent conditions have been marked by declining alkalinity estimates and a minor change in diatom assemblages, most likely reflecting an influx of humic substances from catchment peats. The close correlation between trends in reconstructed alkalinity and catchment vegetation suggests that diatom-inferred alkalinity may provide a sensitive record of soil and forest development in unpolluted circumneutral or alkaline tree-line lakes.