Variations of Oxygen-18/Oxygen-16 and Carbon-13/Carbon-12 in the Calcium Carbonate Pleistocene Varved Clays from Toronto, Canada

Abstract

EXTENSION of isotope techniques for the determination of environmental conditions of ancient limestone deposition to fresh-water carbonates requires some knowledge of seasonal variation of ¹³C/¹²C and ¹⁸O/¹⁶O ratios. If the limestone unit under investigation is varved, seasonal change in isotopic composition will be superimposed on compositional trends extending over longer periods of time. In an examination of varved sediments deposited in Rita Blanca Lake in Blancan time near Channing, Texas, the lighter-coloured, summer portion of the varves was shown to be relatively enriched in carbon-13 and oxygen-18 (ref. 1). The isotopic composition of both summer and winter layers was that of fresh-water carbonate and not marine carbonate². In this sequence of sediments, time trends of δ¹⁸O and δ¹³C were not masked by seasonal variations, and hence were considered to indicate significant climatic changes which took place over about 1,400 years spanned by varved deposition. Since Rita Blanca Lake was located very much south of the Pleistocene ice sheets, a 20.4-in. sequence of varved clays from the proglacial lake sediments of the Don Beds of probable Sangamon age, now exposed in central Toronto, Canada, was collected for comparison.