Ultra-low rare earth element content in accreted ice from sub-glacial Lake Vostok, Antarctica

Abstract

This paper reports the first rare earth element (REE) concentrations in accreted ice refrozen from sub-glacial Lake Vostok (East Antarctica). REE were determined in various sections of the Vostok ice core in order to geochemically characterize its impurities. Samples were obtained from accreted ice and, for comparison, from the upper glacier ice of atmospheric origin (undisturbed, disturbed and glacial flour ice). REE concentrations ranged between 0.8–56 pg g⁻¹ for Ce and 0.0035–0.24 pg g⁻¹ for Lu in glacier ice, and between <0.1–24 pg g⁻¹ for Ce and <0.0004–0.02 pg g⁻¹ for Lu in accreted ice. Interestingly, the REE concentrations in the upper accreted ice (AC₁; characterized by visible aggregates containing a mixture of very fine terrigenous particles) and in the deeper accreted ice (AC₂; characterized by transparent ice) are lower than those in fresh water and seawater, respectively. We suggest that such ultra-low concentrations are unlikely to be representative of the real REE content in Lake Vostok, but instead may reflect phase exclusion processes occurring at the ice/water interface during refreezing. In particular, the uneven spatial distribution (on the order of a few cm) and the large range of REE concentrations observed in AC₁ are consistent with the occurrence/absence of the aggregates in adjacent ice, and point to the presence of solid-phase concentration/exclusion processes occurring within separate pockets of frazil ice during AC₁ formation. Interestingly, if the LREE enrichment found in AC₁ was not produced by chemical fractionation occurring in Lake Vostok water, this may reflect a contribution of bedrock material, possibly in combination with aeolian dust released into the lake by melting of the glacier ice. Collectively, this valuable information provides new insight into the accreted ice formation processes, the bedrock geology of East Antarctica as well as the water chemistry and circulation of Lake Vostok.

Introduction

Lake Vostok is the largest of more than 145 sub-glacial lakes discovered beneath the East Antarctic ice sheet (Siegert et al., 2005). This large reservoir of freshwater (Kapitsa et al., 1996) is maintained in the liquid state by a positive heat balance at one end of the lake where the overlying ice is thicker and the pressure melting point consequently lower (Petit et al., 2005). Notably, Lake Vostok might constitute a long-standing isolated ecosystem, possibly supporting ancient microbial organisms (e.g. Christner et al., 2006, Lavire et al., 2006).

Lake Vostok measures approximately 275 by 65 km and has a total area and volume of 15,500 km² and ∼6100 km³, respectively (Masolov et al., 2008). The water depth reaches a maximum of 1650 m in the south and 150 m in the north while the overlying ice thickness varies between 3750 m in the south and 4150 m in the north. The ice ceiling is thus tilted and lies 250 and 750 m below sea level at the southern and northern ends of the lake, respectively (Masolov et al., 2001). Ice is actively accreted to this ceiling in the southern part of the lake whereas melting occurs in the northern portion (Siegert et al., 2001). Exactly how this accretion is accommodated remains unclear. Stable isotopes suggest that accreted ice could originate from a complex freezing process involving a slush of frazil ice, generated in super-cooled water, and its host water which is subsequently frozen in separate pockets (Jouzel et al., 1999). However, the stable isotopic values could also be explained by the glacier melting process (Souchez et al., 2003).

The age and the origin of Lake Vostok’s water remain uncertain. One possibility is that this large freshwater reservoir originates directly from melting of the overlying ice sheet (Siegert et al., 2000, Studinger et al., 2004). Recently, the suggestion that different Antarctic sub-glacial lakes are interconnected, with channels transferring their water from one reservoir to another (Wingham et al., 2006), has provided a dynamic view of their sub-glacial biotic and physical–chemical processes. However, as Lake Vostok is remote from the other Antarctic sub-glacial lakes, this basin could have been confined for millions of years, possibly since the onset of Antarctic glaciation.

An impetus for the study of Lake Vostok came from an international ice sheet deep drilling program conducted above the southern margin of Lake Vostok, where ice accretion occurs (Fig. 1). The ice core retrieved is the deepest ever obtained and its upper part, down to a depth of 3310 m, revealed climatic history and atmospheric composition over the last four glacial cycles (Petit et al., 1999). Below 3310 m depth, ice can be subdivided into three sections. Ice at depths between 3310 and 3450 m (disturbed ice) is of atmospheric origin but does not offer interpretable paleoclimate data due to strain by ice flow upstream of the drilling site. Ice between 3450 and 3538 m (glacial flour ice) is similar to disturbed ice except that it contains entrained basal material due to contact with the bedrock (Simoes et al., 2002, Souchez et al., 2002). In contrast, ice below 3538 m originates from the refreezing of Lake Vostok water (accreted ice) (Jouzel et al., 1999). Drilling operations stopped at 3659 m in the 2006–2007 season, just ∼100 m above the ice/water contact (Fig. 1).

The accreted ice can be further subdivided into two types. Accreted ice between 3538 and 3609 m, (hereafter Accreted Ice Type I; AC₁) is mainly characterized by a number of visible inclusions (up to few millimeters in size) suggested to be remnants of surface and/or hydrothermally-flushed out bedrock (deep vent) material (Jouzel et al., 1999, Bulat et al., 2004, Delmonte et al., 2004b). Interestingly, most of these inclusions vanish when the ice is melted (De Angelis et al., 2004) indicating that they are soft aggregates of very fine particles (De Angelis et al., 2005). However, two visible solid clasts of fine-grained lithic material were also found in AC₁ (Leitchenkov et al., 2007). In contrast with AC₁, the lower 50 m (3609–3659 m), (hereafter Accreted Ice Type II; AC₂) is characterized by the lack of visible inclusions. This difference is likely due to AC₁ and AC₂ originating from lake water that accreted in a shallow embayment (where the inclusions were entrapped) and over the deep water in the southwestern part of the lake, respectively (Bell et al., 2002, De Angelis et al., 2004) (Fig. 1).

Although Lake Vostok water has not been sampled directly, the deepest sections of the accreted ice have provided preliminary information on the physical, chemical and biological properties of this large sub-glacial reservoir. In addition to variation in the number of visible aggregates, the δD, gas content, crystal size, electrical conductivity (Jouzel et al., 1999) and the ionic content (De Angelis et al., 2004) all change abruptly at the glacier ice/accreted ice transition.

Very few investigations have been conducted in order to explain the origin of impurities entrapped in AC₁ (De Angelis et al., 2004, De Angelis et al., 2005, Leitchenkov et al., 2007). As a whole, scattered aggregates investigated by in situ X-ray fluorescence (De Angelis et al., 2005) appear to be composed of a mixture of fine aluminosilicate particles, carbonate-rich particles (5–10 μm) and larger structures where sulfate is linked to Mg (De Angelis et al., 2005). They also contain significant amounts of calcium sulfate (De Angelis et al., 2004), reduced sulfur species, Si (also in other minerals besides aluminosilicates) and to a lesser extent, O, P and Na. A different mineralogical analysis pointed out that these soft aggregates consist mainly of clay–mica minerals (possibly illite and chlorite; less than 0.5 μm in size; 30–60% of the total mineral content), subangular to angular quartz grains (10–40 μm; 30–60%) and a variety of accessory minerals (Leitchenkov et al., 2007).

In contrast with other species, Cl⁻ and Na⁺ were found to be homogeneously distributed as NaCl throughout large individual crystals of accreted ice (De Angelis et al., 2004). This finding was confirmed by the in situ observation of numerous fine (3–10 μm) diffused liquid brine micro-droplets coexisting with the sparser and larger aggregates. The X-ray signal from these droplets is dominated by Cl and significant amounts of Na (De Angelis et al., 2005). Altogether these observations were interpreted as evidence of haline water pulses carrying fine, solid debris from a deeper, evaporitic reservoir into the lake and of the presence of hydrothermal activity at the lake bottom (De Angelis et al., 2004, De Angelis et al., 2005). Hydrothermal input to the lake, related to tectonic activity, would also explain bacterial DNA fragments discovered in the accreted ice (Bulat et al., 2004, Lavire et al., 2006).

Rare earth elements (REE) have been widely adopted as proxies for several geochemical processes in cosmochemistry, igneous petrology, sedimentology and oceanography. This is because REE have a relative immobility in the terrestrial crust and a low solubility during weathering, but are readily fractionated in the environment because of their characteristic radius contraction across the lanthanide series (Henderson, 1984). As they are mostly transported in the particulate phase, the REE content of particulate matter is generally characteristic of the original source (Henderson, 1984). REE are thus a useful tool for the geochemical characterization of the impurities entrapped in the Vostok ice core.

Here we present REE concentrations determined in various sections of the Vostok ice core originating both from glacier and accreted ice. Only a few accreted ice sections were made available for this study and an additional contingent limitation is that, as Lake Vostok has not been sampled yet, we cannot directly compare the REE composition of the water and of the ice. Another difficulty inherent to our study involves the identification of the respective REE insoluble/soluble contributions to accreted ice from Lake Vostok (bedrock particles and dissolved ions) and melted glacier ice (aeolian particles). However, comparison of REE concentrations, Ce anomalies and normalized crustal REE patterns in glacier ice and accreted ice may provide the first indirect information regarding the insoluble particles/soluble species that are suspended/dissolved in Lake Vostok water.

Interestingly, REE determination in glacier ice also has the potential to provide information about the sources of aeolian dust reaching the Antarctic ice sheet during past climatic cycles, in similar fashion to using Sr and Nd isotopes (Delmonte et al., 2004a), and about detritus from the East Antarctic geologic basement entrapped in the glacial flour ice (Simoes et al., 2002).