Abstract
Multi-proxy data such as pollen percentages, aquatic biota, and stable isotope ratios in lake sediments in conjunction with climate transfer functions can be used to reconstruct past climate. This has been the subject of some recent projects. Often, stable-isotope ratios of oxygen are used as an independent proxy for climate. Past climate so reconstructed can in turn be used to assess the response of past vegetation to climatic oscillations, for instance near the epoch boundaries. One important intermediate step is to establish the age of the stratigraphic sequence. Strong similarities between the δ 18O records from European lake sediments and the Greenland ice cores are of interest. The Greenland ice-core project (GRIP) provided δ 18O data that were dated using an ice-flow model. Although the physical laws behind the isotope series from ice and lake sediment are different, statistical methods can be used to match the two series. In this paper, a regression-based approach is suggested for series matching. The method is illustrated by analyzing a series of δ 18O records covering the Late-glacial interstadial (ca. 15,000–13,000 years b.p. [1950]) from Gerzensee, Switzerland. Regression methods for age-depth modelling have also been recommended by other authors. Such an approach leads to reproducible and statistically founded age estimates and can easily be updated to include new data and information as needed. In this paper, the modelling step is preceded by identifying comparable sub-sections in the two isotope series by empirically matching the local minima and maxima in the smoothed isotope values; regression models are then used locally for each sub-section. This accommodates for local differences in the parameters. Variations in the final age estimates caused by different choices of the smoothing (bandwidth) parameters used in the intermediate nonparametric smoothing step are also taken into account in this algorithm.
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Acknowledgements
This research was motivated by the encouragement of Brigitta Ammann (Bern) who suggested looking into the problem of statistical age estimation of a stratigraphic sequence in the context of her Rapid Warming Project.
J. Beran (Konstanz) kindly provided his max-min SPlus code and W. Tinner (Bern) drew attention to some important literature on age-depth modelling cited in this article. J. Schwander (Bern) read through an earlier draft of the manuscript and provided background information concerning the Greenland Ice Core Project. The author would like to thank all these colleagues for valuable discussions and H.J.B. Birks (Bergen) for detailed editorial remarks that significantly improved presentation. Helpful remarks by two referees are also acknowledged. The GRIP δ 18O values and the corresponding age estimates can be downloaded from http://www.glaciology.gfy.ku.dk/data/grip18o.txt maintained by the Glaciology Group of the University of Copenhagen.
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Communicated by Pim van der Knaap
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Ghosh, S. Regression-based age estimation of a stratigraphic isotope sequence in Switzerland. Veget Hist Archaeobot 15, 273–278 (2006). https://doi.org/10.1007/s00334-006-0059-5
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DOI: https://doi.org/10.1007/s00334-006-0059-5