Trace elements in Vostok Antarctic ice during the last four climatic cycles
Introduction
Deep Antarctic ice cores have the potential to give unique information on past natural changes in the biogeochemical cycles of trace elements. Deciphering these frozen archives for trace elements has however proved to be exceedingly difficult because of the very high purity of Antarctic snow and ice, which makes it a challenge to obtain fully reliable data from the analysis of ice cores whose outside is heavily contaminated during drilling operations. The impetus in the field came from the pioneering work of Clair Patterson and his colleagues at the California Institute of Technology. They developed the first ultra-clean procedures to efficiently decontaminate deep polar ice cores by mechanically chiseling off successive veneers of ice in progression from the contaminated outside to the center, in order to obtain the uncontaminated inner part of the core [1].
Until now these methods were only used to analyze ice cores of intermediate depths (less than 1 km), because such cores can be obtained from drill holes not requiring a wall retaining fluid, which greatly reduces the contamination of the outside of the core [2]. The main drawback of this is that these cores only cover rather short time periods (∼ 40,000 years, for the 905 m core drilled at Dome C in 1978, for instance [3]).
As such these methods were never used for the comprehensive analysis of very deep Antarctic ice cores that cover several climatic cycles, such as the 3623 m Vostok core [4] or the newly drilled 3190 m EPICA Dome C core [5]. The huge pressure encountered at these depths makes it mandatory to fill the drilling hole with a wall retaining fluid, which results in an enormous contamination of the outside of the core sections. The few attempts which were made to analyze such very deep cores for heavy metals were largely unsuccessful: even the most central parts of the cores were found to be contaminated, probably because a significant fraction of the external contamination was transferred towards the center during the decontamination of the sections [6], [7], [8].
The extremely time consuming analytical efforts needed are recompensed by the fact that trace element studies in deep ice cores provide remarkable information about the changes in different biogeochemical cycles over the last climatic cycles. Heavy metals can in fact help us to distinguish between the different natural sources from which the particles and the aerosol transported over Polar Regions have originated. Rock and soil dust, marine aerosol and volcanic emissions were the three main contributors to the overall heavy metals input in Polar Regions during the pre-industrialized period. They constituted important components that interact with atmospheric processes that could produce positive and/or negative feedback mechanisms influencing climate. As an example, the considerable glacial dust input to East Antarctica can be explained by the synergy of several factors involving the atmosphere, the ocean, the biosphere and the lithosphere. In this context a key role is played by the increased aridity of the continents due to changes in soil moisture and/or vegetation cover, as well as the enlargement of the dust-source areas caused by a drop in sea-level. In addition, the reduction in the intensity of the hydrological cycle leads to less efficient scavenging by precipitation and consequently a more efficient transport of dust.
We present here comprehensive data on past changes in the occurrence of various trace elements (Li, V, Cr, Mn, Co, As, Rb, Sr, Ba, Bi and U) in Antarctic ice during the last four climatic cycles (the past ∼ 410,000 years). They were obtained by analyzing various sections of the upper 3285 m of the 3623 m Vostok ice core, using improved decontamination procedures and highly sensitive inductively coupled plasma sector field mass spectrometry (ICP-SFMS).
Section snippets
Core sections
The 3623 m Vostok ice core was drilled from a fluid (kerosene) filled hole at the Russian Vostok Station (78°28′S, 106°48′E, elevation 3488 m, mean annual temperature − 55 °C) in East Antarctica. Thirty-seven sections (length 35–45 cm, diameter 10 cm) were selected from the part of the core, which corresponds to the last four climatic cycles (the last ∼410,000 years). The detailed deuterium profile previously published by Petit et al. [9], Fig. 1, allowed us to select sections dated from the
Changes in concentrations and fluxes during the last four climatic cycles
Concentrations measured in the innermost part of the 37 sections are given in Table 1. They are the first data ever published on past variations of these different elements in Antarctic ice during the last four climatic cycles. Concentrations range from 0.03 pg/g for Bi in the 3284.95 m section to 1.3 ng/g for Mn in the 2870.0 m section.
Pronounced variations in concentrations are observed for all the elements over the ∼ 410,000 years time period covered by our samples. For instance the highest
Conclusion
Our work has documented large natural changes in the occurrence of various trace elements in Antarctic ice over the last four climatic cycles, highlighting important changes in the atmospheric cycles of trace elements in the remote polar areas of the Southern Hemisphere in parallel with climate.
It will now be important to extend this study to the preceding climatic cycles within the new EPICA Dome C ice core, and to include other trace elements such as Hg and Se, which could be valuable tracers
Acknowledgements
The Vostok ice cores were obtained as part of a joint program between Russia, France and the USA. We thank the Russian Antarctic Expeditions, the Institut Polaire Paul Emile Victor and the Division of Polar Programs (NSF) for logistical support, and the drilling team from St. Petersburg Mining Institute for fieldwork. In Italy this study was performed within the framework of Projects on “Environmental Contamination” and “Glaciology and Paleoclimatology” of the Italian Programma Nazionale di
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