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A volcanic marker (92 ka) for dating deep east Antarctic ice cores

https://doi.org/10.1016/j.quascirev.2006.07.009Get rights and content

Abstract

Tephra layers recorded in East Antarctic ice enable reliable linking over long distances and, when correlated with well-dated eruptions in the source area, provide absolute ages for improving the accuracy of model-based ice chronology. We used chronostratigraphic information and grain-specific geochemical data (major elements by electron microprobe and trace elements by LA-ICP-MS) to suggest that a tephra layer from the EPICA-Dome C and Dome Fuji ice cores is the distal counterpart of the Mt. Berlin (Marie Byrd Land province, West Antarctica) pyroclastic unit 40Ar/39Ar dated to 92.5±2.0 and 92.2±0.9 ka. Such one-to-one correlation, which is proposed here for the first time for the East Antarctic deep climatic archives, provides independent age constraints for glaciological modelling of core timescales.

Introduction

Deep ice cores from polar caps provide a wealth of detailed information on the evolution of the climate, environment, and atmosphere over the last glacial–interglacial cycles (EPICA community members, 2004; North Greenland Ice Core Project members, 2004).

Interpretation of paleoclimate records in deep ice cores requires accurate timescales, which can be developed in various ways (Parrenin et al., 2001; Rasmussen et al., 2006). At low snow accumulation sites, dating is based in large part on glaciological models that aim to calculate the initial thickness (accumulation rate) and the resulting thinning of ice layers along the core. For deep ice cores extracted from the East Antarctic plateau (Fig. 1), the complex parameters of recently developed models were best adjusted by means of indirect age control windows (e.g. comparison with dated continental time series, correlation with marine isotopic records, orbital tuning) (Parrenin et al., 2001; EPICA community members, 2004). However, due to the poor direct chronological information, the current ice core timescales are not entirely satisfactory: dating uncertainties increase with depth, and the duration of easily recognizable climatic events (e.g. the last interglacial period) may differ by a few millennia from one record to another (Watanabe et al., 2003).

Volcanic ash (tephra) layers preserved in the Antarctic ice series (Smellie, 1999) represent particularly useful chronostratigraphic and dating tools. The elemental composition and geochemical signature of glass shards can be used to identify volcanic sources and establish precise links between records (Dunbar et al., 2003; Narcisi et al., 2005). Correlation of tephra to historical eruptions allows a refinement of the dating of shallow ice sections (Palais et al., 1990). The mineral and glass phases of Quaternary pyroclastic material are also potentially suitable for absolute dating (Ton-That et al., 2001; Fattahi and Stokes, 2003); however, the distal ash layers in the East Antarctic plateau series are too faint (a few mg) and too fine-grained (mostly 10–20 μm in size) (e.g. Basile et al., 2001) to be dated directly by current radiometric analytical techniques. Alternatively, the correlation between a given distal tephra layer and its radiometrically dated proximal counterpart in the source volcano allows the transfer of absolute ages to Antarctic ice cores (Taylor et al., 2004).

40Ar/39Ar dating techniques can be successfully applied to the alkalic rocks erupted from West Antarctica volcanoes (e.g. Harpel et al., 2004). A new set of accurate radioisotope ages has been recently obtained for several Late Quaternary pyroclastic deposits produced by powerful explosive eruptions in Marie Byrd Land (hereafter MBL) volcanoes (Wilch et al., 1999; McIntosh and Dunbar, 2004) (Fig. 1). Since the distal facies of the dated tephra deposits may be widely dispersed over the Antarctic ice sheets, the recent age determinations have opened the very attractive prospect of absolute dating of deep cores recovered in the East Antarctic plateau.

By comparing chronostratigraphic information and geochemical (major and trace elements) data on several individual glass shards, we propose that a tephra layer occurring in the European Project for Ice Coring in Antarctica (EPICA)-Dome C and Dome Fuji (DF) long ice records (East Antarctica) is correlated with a 40Ar/39Ar-dated pyroclastic deposit erupted from the Mt. Berlin volcano, West Antarctica. This correlation may help refine the timescale of the climatic record in East Antarctic ice.

Section snippets

The distal tephra marker in East Antarctic ice records

The deep ice cores drilled in the EPICA-Dome C (EDC), DF, and Vostok sites have provided detailed climatic records of past glacial cycles (Petit et al., 1999; Watanabe et al., 2003; EPICA community members, 2004). The ice core record of the last 200 ka contains several (10–30) discrete air-fall tephra layers (Fig. 2) that have been characterized through the geochemical composition of glass shards (Basile et al., 2001; Kohno et al., 2004; Narcisi et al., 2005). A few ash layers are common to the

Correlation to a dated Marie Byrd Land eruption and implications for ice-core dating models

The MBL volcanic province, consisting of 18 large central vents and numerous parasitic cones, is tectonically related to the West Antarctic rift system and started its activity in the Middle Miocene (LeMasurier, 1990; LeMasurier and Rex, 1991). Late Quaternary explosive volcanic activity in MBL was reconstructed in detail through field work and through geochemical and radiometric analysis of mapped pyroclastic deposits (Wilch et al., 1999; McIntosh and Dunbar, 2004). About 20 eruptions, which

Acknowledgements

We thank N.W. Dunbar, W.E. LeMasurier, F. Parrenin and T.I. Wilch for helpful discussions and comments on this manuscript.

Research was carried out in the framework of the PNRA-MIUR Italian Project on Glaciology and was financially supported by PNRA Consortium through collaboration with ENEA, Roma.

This work is a contribution to the “European Project for Ice Coring in Antarctica” (EPICA), a joint European Science Foundation/European Commission scientific programme, funded by the EU (EPICA-MIS)

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