Stable isotope ratios in swale sequences of Lake Superior as indicators of climate and lake level fluctuations during the Late Holocene

Beach ridges along the coastline of Lake Superior provide a long-term and detailed record of lake level fluctuations for the past 4000 cat BP. Although climate change has been invoked to explain these fluctuations, its role is still in debate. Here, we reconstruct water balance by employing peat samples collected from swale deposits present between beach ridge sequences at two locations along the coastline of Lake Superior. Carbon isotope ratios for Sphagnum remains from these peat deposits are used as a proxy for water balance because the presence or absence of water films on Sphagnum controls the overall isotope discrimination effects. Consequently, increased average water content in Sphagnum produces elevated delta C-13 values. Two maxima of Sphagnum delta C-13 values interpreted to reflect wetter conditions prevailed from 3400 to 2400 cat BP and from about 1900 to 1400 cat BP. There are two relatively short drier periods as inferred from low Sphagnum delta C-13 values: one is centered at about 2300 cat BP, and one begins at 1400 cat BP. A good covariance was found between Sphagnum delta C-13 values and reconstructed lake-levels for Lake Michigan in which elevated carbon isotope values correlate well with higher lake levels. Based on this covariance, we conclude that climate exerts a strong influence on lake levels in Lake Superior for the past 4000 cat BP. (c) 2005 Elsevier Ltd. All rights reserved.