USING CARBONATE ISOTOPES TO UNDERSTAND HOLOCENE ENVIRONMENTAL VARIABILITY IN THE HURON BASIN, ONTARIO, CANADA

Southern Ontario has experienced major hydrological and climatic changes since the time of glacial Lake Algonquin. Here, we use the oxygen and carbon isotopic compositions of shells and pedogenic carbonate from post-glacial Holocene fluvial and shoreline sediments preserved in the Huron basin to better understand environmental conditions during their formation. Thus far, shells have been sampled from: (i) the Transitional phase, during which isostatic rebound redirected outlet channels and, in conjunction with warming trends, caused substantial lowering of Huron basin lake levels before they slowly increased again, and (ii) the later Nipissing phase from which lake levels have since declined by ~10 m. These data are compared with results reported by Godwin (1985) from a range of Algonquin- to Nipissing-age localities. Pedogenic carbonate-bearing sandy sediments have also been collected from Michael's Bay, Manitoulin Island; sediments along the sampled transect record shoreline positions since the Nipissing phase.

Two species of the aquatic gastropod Valvata were selected for isotopic analysis because they utilize only DIC during shell formation, isotopic vital effects are limited, and growth occurs over a single-season. These species are also abundant in samples from the time range of interest. Compared to the Algonquin phase (oxygen, mean +20±3‰; carbon, mean -6.2±2‰; Godwin, 1985), the Transitional phase shows the largest range in carbon (–10.6 to –1.9‰, mean –6.4‰) and oxygen (+14.4 to +24.6‰, mean +18.7‰) isotopic compositions. Large ranges are also observed for the isotopic compositions of the Nipissing shell samples: +14.9 to +23.1‰, mean +19.7‰ for oxygen; –9.4 to –1.8‰, mean –5.7‰ for carbon. The large variation in oxygen isotopic compositions of the Transitional and Nipissing phase samples reflects the more dynamic range of fluvial environments, and the isotopic composition of precipitation that fed them at these times. The large range in carbon isotopic compositions is indicative of the greater diversity and productivity of the habitats that had become available as climatic conditions improved.

Godwin, A.G. (1985) Stable isotope analyses on postglacial fluvial and terrestrial molluscs from the Kincardine area of southern Ontario. MSc thesis, University of Waterloo.