QUATERNARY RESEARCH 37, 333-345 (1992) Carbon Isotope Composition of Lake Sediments in Relation to Lake Productivity and Radiocarbon Dating RAMON ARAVENA Department of Earth Sciences and Quaternary Sciences Institute, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada BARRY G. WARNER Department of Geography and Quaternary Sciences Institute, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada GLEN M. MACDONALD Department of Geography, McMaster University, Hamilton, Ontario L8.S 4KZ, Canada AND KAREN I. HANF Department of Earth Sciences and Quaternary Sciences Institute, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Received January 31, 1991 Carbon- 13 profiles and radiocarbon dates were obtained from two Canadian kettle basins having similar geological and hydrological characteristics to develop criteria for evaluating the validity of radiocarbon dates on lake sediment from basins in calcareous terrain. Radiocarbon dates from a site in Alberta show a variable hard-water effect related to local hydrological changes during postglacial history of the lake basin, whereas radiocarbon dates from the other site in Ontario show no noticeable influence of old carbon during its history. These differences are mainly related to lake water residence time, which has influenced carbon isotopic exchange between atmospheric CO, and dissolved inorganic carbon in lake water. 6r3C values for bulk organic sediment and terrestrial and aquatic macrofossils reveal that the main component of lake sediment at both sites is autoc- thonous in origin. Furthermore, each site supported different submerged aquatic plant communities that used different sources of carbon for photosynthesis, thereby imprinting the organic sediments with a characteristic “C composition. Both sites reflect a clear relationship between 13C values and paleoproductivity. This study shows the individualistic response of the developing lake system to the hydrology, lake biota, and local geology, and demonstrates the problem of using r3C in lake sediments as a single criterion to recognize the validity of radiocarbon dates of lake sediment without supporting paleoecological information. 0 1992 University of Washington. INTRODUCTION In many parts of North America lakes and wetlands are the most common repos- itories of paleoenvironmental information, especially for the last 18,000 yr. Although a variety of direct dating techniques have been used to date lake and wetland sedi- ments, radiocarbon continues to be the fa- vored dating technique largely because it is convenient, most of the established Quater- nary chronology is based on this method, and the development of accelerator mass spectrometry (AMS) offers improved dat- ing capabilities, in general. It has long been known that radiocarbon dates of lake and wetland sediments in calcareous terrain or in close proximity to pre-Quaternary car- bonaceous deposits can be affected by old carbon (hard water effect) which can pro- duce dates that are too old (e.g., Deevey et al., 1954; Shotton, 1972; Karrow et al., 333 0033-5894192 $5.00 Copyright Q 1992 by the University of Washington. All rights of reproduction in any form reserved.