The impact of climatic conditions on Pb and Sr isotopic ratios found in Greenland ice, 7–150 ky BP

Abstract

We present here a time series of Pb and Sr isotopes in Greenland ice over the last glacial cycle. The data were obtained by analysing the dust contained in small (less than 10 g) aliquots of sections of the GRIP (European Greenland ice core project) ice core. The concentrations of Pb, Ba and Sr vary by large factors of 70, 140 and 75 respectively over this time period and show a significant correlation with δ¹⁸O, used as a proxy for temperature. Elemental ratios of Pb, Ba and Sr indicate the provenance of dust is mainly crustal with possible volcanic influences but the contribution from seawater is negligible. The Pb and Sr isotopic ratios change over time and form two distinct groups above and below a temperature corresponding to a δ¹⁸O value of ∼ − 39‰. While changing elemental concentrations may be explained by changes in wind strengths, the variation in isotopic composition appears to reflect climate induced changes in the source areas of dust deposited in the snow in Greenland. Available source data suggest that dust may originate from the Gobi Desert in central Asia in cooler periods and from glacial sediments adjacent to Greenland in warmer periods.

Introduction

The polar ice sheets and glaciers have been shown to archive anthropogenic and natural emissions recording evidence of industrial activity and natural processes. They may therefore be useful for characterising atmospheric transport pathways for dust and contribute to an understanding of climatic phenomena. Lead and Sr isotopes are naturally occurring tracers in ice that are increasingly being used to elucidate the environmental impact of both natural and anthropogenic emission sources on Earth. For example, Pb isotopes have previously been used to study the origin of pollution in polar ice (Rosman et al., 1993, Rosman et al., 1997, Vallelonga et al., 2002a, Vallelonga et al., 2002b, Planchon et al., 2003) and other matrices (Rosman et al., 1998, Shotyk et al., 1998, Kober et al., 1999, Bollhofer and Rosman, 2000, Bollhofer and Rosman, 2001, Vallelonga et al., 2003). Sr isotopes have similarly been used to study the sources of transported dust (Basile et al., 1997, Krom et al., 1999, Sun, 2005).

Relatively few attempts have been made to interpret climatic effects from variations in the source of aerosol dust. The isotopic composition of Sr and Nd in dust in Greenland ice core samples was used to identify an origin in eastern Asia,. but no correlation with climate was detected (Biscaye et al., 1997, Svensson et al., 2000). Similar studies using Sr and Nd in Antarctica have identified Patagonia (South America) as the principal source of dust in ice, arriving in Antarctica during glacial periods (Basile et al., 1997). However, climate related correlations with Pb isotopic ratios have been clearly identified in ice core samples from Antarctica (Vallelonga et al., 2005, Rosman et al., 2005). Higher proportions of less radiogenic Pb were found to be present during glacial periods when temperatures were lower.

A correlation between temperature and isotopic composition (and subsequent identification of source regions) could provide information on the spatial extent of glaciation, changing vegetation cover, desert formation and other mechanisms thought to play a role in the character of dust reaching polar regions. Also for consideration are the exposure of continental shelves due to sea level fall, wind strength and trajectories, and possibly the “re-liberation” of dust produced during earlier periods of erosion.

In this paper we identify changes in Sr and Pb isotopic ratios in Greenland ice and seek to interpret these changes in terms of changing climatic conditions and differing source regions.