Pollen analysis of sediment cores from lakes in the Rouyn-Noranda region, Quebec

Abstract

Ambrosia pollen was used as a marker for the relative dating of lake-sediment cores from eight lakes in the vicinity of Rouyn-Noranda. The profiles of Ambrosia counts versus depth in the cores of these lakes have a characteristic shape, and the first sharp increase from the bottom is taken as a marker horizon. The marker occurs at about 7.5 cm depth at Lake Marlon (4 km east of Rouyn-Noranda) and is found progressively deeper in the cores from lakes farther east, being about 13 cm deep in Lake Vert, 25 km north of Val-d'Or and 100 km east of Rouyn-Noranda. Lead isotope (210Pb) dating of the core from 'Gravel Pit' Lake (9 km east of Rouyn-Noranda) gives an approximate age of the Ambrosia marker that is consistent with the age of the development of the Noranda mining community (ca. 1926), and an average sedimentation rate over the whole core of about 0.11 cm/a. The somewhat earlier development of the region farther east, around Val-d'Or and the communities between Val-d'Or and Rouyn-Noranda, may account in part for the deeper position of the Ambrosia marker in these cores. However, the sedimentation rate in Lake Vert is between 0.15 and 0.17 cm/a (the boundaries corresponding to the ages of the Val-d'Or and Rouyn-Noranda communities, respectively), and is therefore greater than the sedimentation rate in 'Gravel Pit' Lake.