A flexible approach to assessing synchroneity of past events using Bayesian reconstructions of sedimentation history
Introduction
An issue of considerable importance to our understanding of Quaternary palaeoenvironmental history is that of establishing synchroneity in events recognised in two or more stratigraphic records. Where the event appears asynchronous, an important second issue is that of establishing the extent of the asynchrony (see e.g. Davis, 1983, Birks, 1989, Alley et al., 1997, Haas et al., 1998, Bennett and Fuller, 2002, Blaauw et al., 2007). A common challenge is the uncertainty in establishing the age of the event in each of those records, as well as the identification of the event itself. This paper proposes a new and flexible approach to modelling such uncertainties and thus to the drawing of appropriately qualified scientific conclusions.
We illustrate this new approach using three events apparent in palynological data in northern Europe from six sites at each of which there is partial 14C dating information. These events are: (1) the early- to mid-Holocene increase in abundance of Alnus (alder) pollen (‘the alder rise’); (2) the mid-Holocene decline in abundance of Ulmus (elm) pollen (‘the elm decline’); and (3) the first mid- to late-Holocene occurrence of Cerealia-type pollen. We assess the degree of synchroneity across a number of sites of the first two events, and also compare the intervals between the last two events at the same sites. We suggest, however, that the overall approach is of wide relevance.
Notwithstanding its central importance in many aspects of Quaternary science, the problems associated with making such assessments of synchroneity have received remarkably little attention. For example, while a web search readily finds dozens of papers using the term “degree of synchroneity/synchronicity of an event” in the context of the Holocene, neither the terms synchroneity nor event are typically defined. Yet precise formal definitions are vital for the discussion of uncertainty. As discussed in this paper, we define an ‘event’ to be a unique point in time at a precise location in space. We study the time differences between pairs of such events each measured with uncertainty. Technically these are measures of the degree of diachroneity, in the presence of statistical noise. Typically events, as so defined, reflect unobserved space-time processes, such as the Ulmus decline, and study focusses on spatial structure in the degree of diachroneity.
From a much wider spatio-temporal perspective, the Ulmus decline across NW Europe is itself an ‘event’; indeed this is the sense in which we use it in the previous paragraph. It would be pedantic to insist always on separate terms for both the ‘unique in time and space’ event and the ‘spatiotemporal process’ event. Thus in general discussion below we will sometimes use the term in both senses, leaving the context to make it clear to the reader. Nevertheless, in our discussion of synchroneity in the presence of uncertainty, events are as defined above and as elaborated and illustrated below.
We identify three general aspects of the problem. First, there are problems associated with characterising the event itself, and hence in determining the depth at which the event occurred in a given stratigraphic sequence. If we are to associate an event with a point in history, a single depth in the core, what are the implications? Closely related to this is the establishment of the uncertainty about this depth, given the data available. Finally, there are challenges in assigning an age to this depth, with an associated statement of uncertainty. The latter are issues of statistical inference.
The structure of the paper is as follows. In Section 2 we discuss approaches to event definition, chronology modelling and synchroneity. Section 3 presents the data and proposes depth intervals for the events. Section 4 presents an illustration of various approaches applied to six selected sites around north western Europe. Finally, in Section 5, we discuss the potential of the new approach with further illustrations.
Section snippets
Methods
We discuss here the identification of events in terms of depth and the subsequent estimation of their associated ages, with uncertainty on both. Our simplest proposals in respect of depth uncertainty are very easy to implement and can be regarded as typically adequate approximations to a formal statistical analysis. In respect of age estimation, especially for depths where 14C age information is not available, the implementation requires specialist software (Bchron), although the concepts are
Data used and event identification at six example sites
We illustrate the potential of this new approach by addressing the issue of the synchroneity of events as recorded in palynological data from six sites in northern Europe. The six sites selected lie along a broad west–east transect extending from the British Isles to Poland and lying between 50° and 60° N latitude (Fig. 3; Table 1). In addition to requiring that sites fell along this transect, we also required: (a) that they were located below 250 m a.s.l.; (b) that their stratigraphic record
Chronologies
Age–depth plots illustrating the chronologies obtained via Bchron for the six sites are shown in Fig. 5. These plots serve to highlight a number of features of the chronologies obtained using our new technique. First, because our method develops chronologies consistent with all of the radiocarbon determinations, the age uncertainties are in some circumstances much less than those associated with individual age estimates calibrated in isolation. Thus, for example, at Lake Solso (Fig. 5(d)) the
Discussion
We discuss the implications of the building of uncertain chronologies using Bchron and its use and potential in the analysis of the degree of event synchroneity.
Acknowledgements
We would like to thank the authors of our six sites (R. Watkins, K.D. Bennett, W.Dörfler, B.V. Odgaard, H. Almquist-Jacobson and K. Szczepanek) for the use of their data. JH would like to thank the Durham Institute of Advanced Study. JH and ACP would like to thank Science Foundation Ireland for their support.
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