A major glacial-interglacial change in aeolian dust composition inferred from Rare Earth Elements in Antarctic ice

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Abstract

We present the first Rare Earth Elements (REE) concentration record determined in 294 sections of an Antarctic ice core (EPICA Dome C), covering a period from 2.9 to 33.7 kyr BP. REE allow a detailed quantitative evaluation of aeolian dust composition because of the large number of variables (i.e. 14 elements). REE concentrations match the particulate dust concentration profile over this period and show a homogeneous crustal-like composition during the last glacial stage (LGS), with only a slight enrichment in medium REE. This signature is consistent with the persistent fallout of a mixture of dust from heterogeneous sources located in different areas or within the same region (e.g. South America). Starting at ∼15 kyr BP, there was a major change in dust composition, the variable character of which persisted throughout the Holocene. This varying signature may highlight the alternation of single dust contributions from different sources during the Holocene. We observe that the frequent changes in REE composition at the onset of the Holocene (10–13.5 kyr BP) are linked to dust size and in turn to wind strength and/or the path of the atmospheric trajectory. This may indicate that atmospheric circulation dictated the composition of the dust fallout to East Antarctica at that time. Although the dust concentrations remained fairly low, a notable return towards more glacial dust characteristics is recorded between 7.5 and 8.3 kyr BP. This happened concomitantly with a widespread cold event around 8 kyr BP that was 400–600 years long and suggests a moderate reactivation of the dust emission from the same potential source areas of the LGS.

Introduction

Ice cores provide compelling evidence that airborne dust from the austral continents reached Antarctica during the past climatic cycles (e.g. Wolff et al., 2006, Fischer et al., 2007, Delmonte et al., 2008). However, dust concentrations in Antarctic ice are extremely low, ranging from ∼15 ng g−1 during interglacials up to ∼800 ng g−1 during glacial stages. As snow accumulation rates were reduced during cold periods, a glacial/interglacial flux ratio of ∼25 was deduced by Lambert et al. (2008). The dust trapped in Antarctic ice is composed of detrital minerals such as clays, quartz and feldspars (Gaudichet et al., 1988). In particular, illite, chlorite, smectite and kaolinite are present at any time but a significantly smaller amount of kaolinite was observed in Last Glacial Maximum (LGM) samples (Gaudichet et al., 1992).

One important question of Antarctic glaciology concerns the provenance of dust trapped in the ice. The answer would provide important information on the past atmospheric circulation, serving to validate global circulation models and offering clues on the ancient environmental conditions of the surrounding continents. Sr and Nd isotope studies in East Antarctic ice cores (Delmonte et al., 2008), along with atmospheric circulation modelling (Lunt and Valdes, 2001) have identified South America as a major dust contributor during glacial stages.

During cold periods, weathering and glacial erosion likely played an important role in the dust production in southern South America (Gaiero et al., 2007, Sugden et al., 2009) and a persistent westerly circulation might have allowed the transfer of dust towards the interior of Antarctica (Krinner and Genthon, 2003). However, there is a lack of knowledge concerning the characteristics and the source of the dust transported to Antarctica during interglacial stages. Some preliminary studies suggested that glacial/interglacial changes in dust composition may have occurred (Gabrielli et al., 2005a, Winckler and Fischer, 2006, Siggaard-Andersen et al., 2007, Delmonte et al., 2007, Lanci et al., 2008, Marino et al., 2008). It was also suggested that Australia could be an important contributor of dust for Antarctica during the Holocene (Revel-Rolland et al., 2006) and at present (Li et al., 2008).

Rare earth elements (REE) can provide a robust, specific and versatile tool for the geochemical characterization of the aeolian dust in Antarctic ice. The 14 REE (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) range in atomic number from 57 (La) to 71 (Lu). From here on, we differentiate between light REE (LREE; La, Ce, Pr, Nd), medium REE (MREE; Sm, Eu, Gd, Tb, Dy, Ho) and heavy REE (HREE; Er, Tm, Yb, Lu). As their atomic mass increases, their radius decreases but they keep the same external electronic configuration. Therefore, their chemical properties remain essentially identical, allowing REE to behave like isotopes. REE are lithophilic refractory elements that, due to their low solubility, are mostly transported in the environment in the particulate phase. All these characteristics prevent REE from being strongly fractionated by weathering and diagenetic processes (except Ce and Eu that might be fractionated by redox processes) and thus they are ideal as geochemical tracers (Henderson, 1984).

The main advantage of using REE is that, as they are 14 elements, they are potentially more suitable for delineating changes in the aeolian dust composition. In addition, REE in polar ice are rarely influenced by sources (e.g. volcanic ash fallout as found by Wei et al., 2008) other than continental rocks and soil dust (Gabrielli et al., 2009).

Here, we show the first record of REE concentrations determined in the EPICA ice core from Dome C in Antarctica (hereafter EDC) from 2.9 to 33.7 kyr BP, which includes the Holocene, the last transition, the LGM and part of the last glacial epoch. Based on these novel dust tracers we provide strong evidence of a large glacial/interglacial change in dust composition that started at ∼15 kyr BP and continued through the Holocene, except for an oscillation that occurred ∼8 kyr BP which indicates a short return towards more glacial-like dust. In addition, we present a preliminary evaluation of the dust provenance and the linkage between the dust composition and the atmospheric transport.

Section snippets

Samples

The samples originate from a deep ice core drilled in Dome C on the East Antarctic Plateau (75°06' S; 123°21' E; altitude 3233 m; mean annual temperature −54 °C) within the framework of the European Project for Ice Coring in Antarctica (EPICA) (EPICA community members 2004). A total of 294 samples were extracted from between 112 and 656 m depth which, according to the EDC3 timescale (Parrenin et al., 2007) spans a period between 2.9 and 33.7 kyr BP. Temporal spacing of the samples is ∼60 and ∼140

REE concentrations and enrichment factors

REE concentrations show large variations during the last climatic cycle with lower values during the Holocene (e.g. average La = 0.39 pg g−1; Lu = 0.003 pg g−1) and higher values (20–30 times) during the LGS (e.g. La = 7.6 pg g−1; Lu = 0.078 pg g−1) (Table 2). REE are highly correlated with the dust concentrations determined in the same samples (R ∼ 0.90–0.95) and their plotted time series are very similar (Fig. 2a, b).

The REE enrichment factors [Ef(REE)Ce] calculated as (REEice/Ceice)/(REEcrust/Cecrust),

REE as a tracer of aeolian dust provenance

REE physical characteristics prevent them from being strongly chemically fractionated by environmental processes. However, the literature highlights how REE can be mobilized under certain conditions. A classic experimental work (Balashov and Girin, 1969) shows that 20–95% of the REE in clays are readily leachable and therefore may be available for migration, with the MREE being most susceptible and the LREE least. In particular, REE can be mobilized during both humid and temperate weathering (

Conclusions

REE prove to be sensitive indicators of changes in aeolian dust composition trapped in Antarctic ice during the last climatic cycle. The dust shows a persistent crustal-like REE composition during the LGS that was possibly produced either by the atmospheric mixing of the abundant emissions derived from multiple source areas or by the averaging of individual heterogeneous sources within one single region (e.g. South America). If confirmed by further developments in the REE analysis, the LREE

Acknowledgments

This work was supported in Italy by the Consorzio per l'Attuazione del Programma Nazionale delle Ricerche in Antartide, under projects on Environmental Contamination and Glaciology. In France it was supported by the Institut Universitaire de France, the Agence de l'Environnement et de la Maîtrise de l'Energie, the Institut National des Sciences de l'Univers and the Université Joseph Fourier of Grenoble. This work is a contribution to the European Project for Ice Coring in Antarctica (EPICA), a

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