Free‐shape 14C age–depth modelling of an intensively dated modern peat profile

Abstract

AbstractThe paper presents a new algorithm for finding the best‐fit age–depth model of a peat profile dated with 14C. This algorithm of free‐shape modelling is useful when the age–depth curve is wiggly and difficult to approximate by a simple mathematical function. The performance of the free‐shape algorithm is illustrated with the example of the MA2 peat profile from Mauntschas mire, using 29 radiocarbon dates spanning the last 1300 a, and where the age–depth curve revealed a relatively complicated shape.The best‐fit age–depth model searched for is a curve maximising the ‘Fit quality’, defined as a combination of fit of 14C dates to the radiocarbon calibration curve, general smoothness of the age–depth line, and similarity of relative changes in the modelled sediment accumulation rates to those suggested by independent data. The algorithm of free‐shape age–depth modelling searched for a reasonable equilibrium or compromise between the conditions above. Uncertainty of the free‐shape model was assessed with a Monte Carlo Markov chain built from large number of age–depth lines accepted with probabilities dependent on the ‘fit quality’ defined as above.Parallel fluctuations in concentrations of most pollen taxa in MA2 strongly suggest a complicated shape of the age–depth curve, which is supported in the upper part of the profile by the record of spheroidal carbonaceous particles, and in some levels also by the 14C dates. A modification of total pollen concentrations was therefore used as a quantitative indicator of relative changes of the peat accumulation rate. The use of pollen concentrations improved the fit of the 14C dates in several cases.Application of the final age–depth model to the pollen data resulted in a diagram of pollen accumulation which allows interpretation in ecological terms, whereas age–depth models that did not use pollen concentrations led to problematic and confusing results. The obtained age–depth model is therefore accepted as a good approximation of the actual conditions. Copyright © 2009 John Wiley & Sons, Ltd.