Age modelling of late Quaternary marine sequences in the Adriatic: Towards improved precision and accuracy using volcanic event stratigraphy

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Abstract

The first part of this paper presents a review of the problems that constrain the reliability of radiocarbon-based age models with particular focus on those used to underpin marine records. The reasons why radiocarbon data-sets need to be much more comprehensive than has been the norm hitherto, and why age models should be based on calibrated data only, are outlined. The complexity of the probability structure of calibrated radiocarbon data and the advantages of a Bayesian statistical approach for constructing calibrated age models are illustrated. The second part of the paper tests the potential for reducing the uncertainties that constrain radiocarbon-based age models using tephrostratigraphy. Fine (distal) ash layers of Holocene age preserved in Adriatic prodelta sediments are analysed geochemically and compared to tephras preserved in the Lago Grande di Monticchio site in southern Italy. The Monticchio tephras have been dated both by radiocarbon and varve chronology. The importance of basing such comparisons on standardised geochemical and robust statistical procedures is stressed. In this instance, both the Adriatic and Monticchio geochemical measurements are based on wavelength dispersive spectrometry, while discriminant function analysis is employed for statistical comparisons. Using this approach, the ages of some of the Adriatic marine ash layers could be estimated in Monticchio varve years, circumventing some of the uncertainty of radiocarbon-based age models introduced by marine reservoir effects. Fine (distal) ash layers are more widespread and better preserved in Mediterranean marine sequences than realised hitherto and may offer much wider potential for refining the dating and correlation of Mediterranean marine sequences as well as marine-land correlations.

Introduction

Accurate dating and correlation are essential for the construction of reliable models of sediment architecture and influx rates in marine basins, as well as for establishing the links between changes in sedimentation and palaeoenvironmental events. In recent years, the survey and analytical tools employed in the investigation of marine sequences have become more sophisticated, increasing the precision with which palaeoenvironmental records can be assessed. At the same time, analysis of Greenland ice cores (e.g Alley et al., 1993; Dansgaard et al., 1993; North Atlantic Ice Core Project Members, 2004; Rasmussen et al., 2005) has shown that environmental changes during the late Quaternary were much more abrupt than previously understood, with some marked changes in climate possible within a few decades. These developments have fuelled a demand for greater geochronological precision in the analysis of marine sedimentary sequences, to support better resolved palaeoenvironmental records. The question arises, however, as to whether available geochronological methods can deliver this increased precision.

The method most routinely employed to date marine fossils and sediments spanning the last ca. 50,000 years is radiocarbon dating. It is now evident that this method cannot normally provide age estimates for Holocene events at greater than a centennial precision, while true uncertainties of the order of millennia will commonly constrain dates for pre-Holocene events (see below). In this paper we explain why conventional procedures for the construction of radiocarbon-based age models are less secure than has been commonly assumed, and why the wider application of tephrostratigraphy, the basis of a regional Event Stratigraphy scheme, can yield more reliable results.

The discussions focus on records from the Adriatic Sea, as the views expressed here have emerged from workshop activities conducted within the EU-funded EURODELTA and EUROSTRATAFORM projects. The general bathymetric and geographical context of the study area, as well as locations of all cores and sites referred to in the text, are shown in Fig. 1. One of the key aims of these collaborative initiatives was the development of more robust age models to underpin precise correlations of marine sediment sequences in various sectors of the Mediterranean, including the Adriatic Sea. However, the recommendations developed below have much wider application to the dating of marine sequences in general.

Section snippets

Limitations of radiocarbon dating

Four general sources of uncertainty constrain the precision and accuracy of radiocarbon dates obtained from marine samples: (1) analytical (laboratory) precision; (2) factors affecting the geological (stratigraphical) integrity of dated materials; (3) marine reservoir errors; and (4) calibration procedures. Each of these is discussed in turn.

Scientific context

Tephra layers are widely encountered within late Quaternary sediment sequences in the Mediterranean, particularly in the central basins (e.g. Keller et al., 1978; Paterne et al., 1988, Paterne et al., 1990). They are routinely employed to date and correlate marine records (e.g. Calanchi et al., 1996, Calanchi et al., 1998; Siani et al., 2004). Some of them can also be traced to terrestrial sites, including lake basins, where they can be reasonably well dated. Of particular note in this respect

Conclusions

The key conclusions to emerge from the arguments presented in this paper are as follows.

  • (1)

    Radiocarbon-based age models for Holocene stratigraphical records cannot normally deliver better than centennial precision, unless a minimum of 24 radiocarbon dates is available to support the models. Age models for pre-Holocene periods are even less precise, with uncertainties commonly millennial in scale, mainly because of the limitations of currently available calibration models. Radiocarbon dating of

Acknowledgements

We gratefully acknowledge the following two main sources of funding support for the work reported in this paper:

  • (1)

    EURODELTA Concerted Action, a project dedicated to European Co-ordination on Mediterranean Prodeltas funded within the European Union's 5th Framework research programme entitled Energy, Environment and Sustainable Development (Contract No. EVK3-CT-2001-20001).

  • (2)

    EUROSTRATAFORM, a scientific research programme exploring the fate of sediment particles from their source in rivers through to

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    1

    Present address: BLOCKLEY & POLLARD: Research Laboratory for Archaeology, University of Oxford, Dyson Perrins Building, South Parks Road, Oxford OX1 3QY, UK.

    2

    Present address: IFREMER, DRO/GM BP 70, 29280 Plouzané, France [email protected] 33-298-224572.

    3

    Present address: Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, PO Box 7456, Austin, TX 78713, USA.

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