Robert J. Mott

Allerod--younger dryas lake temperatures from midge fossils in atlantic Canada.

Remains of freshwater midges are abundant in lake sediments, and their species distributions are closely related to the surface-water temperature of lakes; their distributions thus provide a powerful tool for paleoclimatology. The distribution of species in a core from Splan Pond in Atlantic Canada indicates that there were abrupt transitions in late-glacial temperatures between warm and cold states. The transitions are correlative with the well-known warm Aller�d and cold Younger Dryas events in Europe. These data thus confirm the inference from palynological data that these events affected regions on both sides of the Atlantic.

Climatic and limnological changes associated with the Younger Dryas in Atlantic Canada

Pollen, diatom and chironomid fossils from the sediments of a core from Brier Island Bog Lake, Nova Scotia were studied in an attempt to relate changes in microfossil composition to a climatic cooling in Atlantic Canada correlative with the European Younger Dryas ca. 10 to 11 ka. Our paleolimnological data were then compared to similar types of data from Splan Pond, New Brunswick to determine if there were any significant differences between a coastal and a more inland site. Nonarboreal pollen was dominant throughout the Brier Island core and the interval 10.0–11.0 ka did not show the typical decline in Picea and increases in tundra-like vegetation characteristic of many sites in Atlantic Canada. However, the limnological indicators did undergo marked changes in taxon composition. The chironomid assemblage was initially dominated by shallow-water, warm-adapted chironomid taxa followed by abundant Sergentia (a cold stenotherm) during 10–11 ka. Sergentia disappeared in the “post Younger Dryas” interval and the warm-adapted genera resumed dominance. Chironomid-inferred paleotemperature reconstructions revealed that at both Brier Island Bog Lake and Splan Pond, summer surface-water temperatures dropped abruptly to between 13 and 17°C during the 10–11 ka interval, suggesting that a cooler climate was present in Atlantic Canada correlative with the European Younger Dryas. Diatom assemblage changes during the same period corroborate the occurrence of limnological fluctuations.

Late Quaternary diatom and chemical profiles from a meromictic lake in Quebec, Canada☆

Abstract An investigation of the chemistry and diatoms in the sediments of a small hardwater meromictic lake in Quebec indicated remains of both freshwater and brackish (halophilic) water diatoms in the organic sediments laid down after the lake was isolated by isostatic rebound from the Champlain Sea, ∼11,000 yr. ago. Water in deeper strata of the lake evidently was not completely fresh, giving rise to ectogenic meromixis. The loss of marine salts from the lake is believed to have taken 3000 yr. or so and that as this happened, salts of biological origin gradually accumulated in the monimolimnion so that maintenance of meromixis became biogenic. The sediment chemistry reflects primary successional changes in the vegetation of the area rather than a decline of ectogenic meromixis. As forest cover developed and soils stabilised on the rebounding land, erosional processes gave way to leaching processes as the source of nutrients to the lake. The appearance of eutrophic indicator diatoms, and an increased flux of nutrients to the sediments, suggest that a cycle of eutrophication occurred in the lake following the decline of hemlock, ∼4800 yr. ago. The recent sediments show an increase in concentrations of Cu, Zn and Pb as a result of anthropogenic activities.