A flexible approach to assessing synchroneity of past events using Bayesian reconstructions of sedimentation history

The dating of depths in two or more cores is frequently followed by a study of the synchroneity or otherwise of events reflected in the cores. The difficulties most frequently encountered are: (a) determining precisely the depths associated with the events; and (b) determining the ages associated with the depths. There has been much progress in recent years in developing tools for the study of uncertainties in establishing chronologies. This has not yet been matched by similar progress in modelling event/depth relationships. This paper proposes a simple and flexible approach, showing how uncertain events can be married to uncertain chronologies. Difficulties in studying event/depth/age relationships typically involve a confounding of two different problems. First, what exactly do we mean by an ‘event’ – a point in history, a single depth in the core corresponding to a single time, or a depth/time range? Sometimes ‘event’ is in fact a shorthand for a space-time process. Do the data reflect more than one type of event/process? This can reflect vagueness in definition. Second, what are the sources and implications of the uncertainties? Here we illustrate the issues involved by examination of several features seen in north European Holocene pollen records. The Alnus rise is regarded as a diachronous early Holocene event; in contrast the Ulmus decline is widely seen as a near synchronous event in the mid-Holocene. The third feature we examine is the interval between the Ulmus decline and the first occurrence of Cerealia-type pollen. The evidence for these events lies in cores of lake sediment from which are determined: (a) the proportions of pollen at many depths; and (b) radiocarbon age estimates from, usually, fewer depths. For this illustration we focus on six sites. We draw attention to a new and flexible method (implemented in the free R software package Bchron; [Haslett, J., Parnell, A., 2008. A simple monotone process with application to radiocarbon dated depth chronologies. Journal of the Royal Statistical Society: Series C 57 (4), 399–418]) for the establishment of the uncertainties surrounding the dating of samples in such cores. We illustrate its flexibility by assessing the synchroneity of past events