Retrieving the paleoclimatic signal from the deeper part of the EPICA Dome C ice core

An important share of paleoclimatic information is buried within the lowermost layers of deep ice cores. Because improving our records further back in time is one of the main challenges in the near future, it is essential to judge how deep these records remain unaltered, since the proximity of the bedrock is likely to interfere both with the recorded temporal sequence and the ice properties. In this paper, we present a multiparametric study (δD-δOice, δ Oatm, total air content, CO2, CH4, N2O, dust, high-resolution chemistry, ice texture) of the bottom 60 m of the EPICA (European Project for Ice Coring in Antarctica) Dome C ice core from central Antarctica. These bottom layers were subdivided into two distinct facies: the lower 12 m showing visible solid inclusions (basal dispersed ice facies) and the upper 48 m, which we will refer to as the “basal clean ice facies”. Some of the data are consistent with a pristine paleoclimatic signal, others show clear anomalies. It is demonstrated that neither large-scale bottom refreezing of subglacial water, nor mixing (be it internal or with a local basal end term from a previous/initial ice sheet configuration) can explain the observed bottom-ice properties. We focus on the high-resolution chemical profiles and on the available remote sensing data on the subglacial topography of the site to propose a mechanism by which relative stretching of the bottom-ice sheet layers is made possible, due to the progressively confining effect of subglacial valley sides. This stress Published by Copernicus Publications on behalf of the European Geosciences Union. 1634 J.-L. Tison et al.: Retrieving the paleoclimatic signal from the deeper part of the EPICA Dome C ice core field change, combined with bottom-ice temperature close to the pressure melting point, induces accelerated migration recrystallization, which results in spatial chemical sorting of the impurities, depending on their state (dissolved vs. solid) and if they are involved or not in salt formation. This chemical sorting effect is responsible for the progressive build-up of the visible solid aggregates that therefore mainly originate “from within”, and not from incorporation processes of debris from the ice sheet’s substrate. We further discuss how the proposed mechanism is compatible with the other ice properties described. We conclude that the paleoclimatic signal is only marginally affected in terms of global ice properties at the bottom of EPICA Dome C, but that the timescale was considerably distorted by mechanical stretching of MIS20 due to the increasing influence of the subglacial topography, a process that might have started well above the bottom ice. A clear paleoclimatic signal can therefore not be inferred from the deeper part of the EPICA Dome C ice core. Our work suggests that the existence of a flat monotonic ice–bedrock interface, extending for several times the ice thickness, would be a crucial factor in choosing a future “oldest ice” drilling location in Antarctica. 1 Paleoclimatic signals in basal layers of deep ice cores Deep ice cores retrieved from the two present-day major ice sheets on Earth, Greenland in the north and Antarctica in the south, delivered a wealth of unique paleoclimatic archives over the last decades. These allowed reconstruction of global climatic and environmental conditions over the last 800 000 years, including unprecedented records of cyclic changes in the composition of greenhouse gases (CO2, CH4, N2O). An important share of that paleoclimatic information is buried within the lowermost sections of those deep ice cores, due to the mechanical thinning of annual accumulation layers with depth. Improving the records further back in time is therefore one of the main challenges of ice core science in the near future (IPICS, 2009). A major concern in this regard is to judge how far down we can trust the paleoclimatic signals stored within the ice, since the proximity of the bedrock is likely to interfere both with the recorded temporal sequence and with the ice properties. This in turn is closely linked to the thermal and hydrological regime at the bottom of the ice sheet, as shown previously in the literature describing basal layers of deep ice cores (e.g. Goodwin, 1993; Gow et al., 1979; Gow and Meese, 1996; Herron and Langway, 1979; Jouzel et al., 1999; Koerner and Fisher, 1979; Souchez, 1997; Souchez et al., 1993, 1995a, b, 2000a, 2002b, 2003, 2006, 1994, 1998; Tison et al., 1994, 1998, Weis et al., 1997). In some cases, where the ice–bedrock interface is clearly below the pressure-melting point (pmp) as, for example, at the GRIP (−9 C) or the Dye-3 (−12 C) ice coring sites in Greenland, single or multiple mixing events between the present-day ice sheet ice and local ice remnants of previous (or even initial) ice sheet configurations are encountered (Souchez, 1997; Souchez et al., 1994, 1998, 2000b; Verbeke et al., 2002). Where the ice–bedrock interface is at the pmp, the meteoric ice has the potential to melt at a rate that would depend on the heat budget at the ice–bedrock interface (geothermal heat flux, internal friction and conduction through the overlying ice). In some cases, where the subglacial topography allows it, like at the Antarctic Vostok site, a subglacial lake will exist. Again, depending on the heat budget but also on the subglacial lake water circulation pattern, lake ice will form at the ice–water interface in substantial amounts (e.g. Jouzel et al., 1999; Souchez et al., 2000a 2002a, 2003). This ice, evidently, does not carry paleoclimatic information. Furthermore, in the case of large subglacial lakes (such as Lake Vostok) where the ice column above can be considered in full hydrostatic equilibrium buoyancy, re-grounding of the ice sheet on the lee side of the lake will induce dynamical perturbations (such as folds), even in the meteoric ice above, as demonstrated for MIS11 (Raynaud et al., 2005) and for the ice just above the accreted lake ice (Souchez et al., 2002a, b, 2003). A less well-documented case, however, is the one where no significant water body exists at the ice–bedrock interface. If only melting occurs at the interface, with no water accumulation and no refreezing (as, for example at the NGRIP site in Greenland), can we then rely on the paleoclimatic information gathered in the basal layers? The EPICA (European Project for Ice Coring in Antarctica) Dome C ice core potentially provides us with an opportunity to investigate that specific case. In this paper, we are using a multiparametric approach, combining new and existing low-resolution (50 cm) data for the bottom 60 m of ice from the EDC (EPICA dome C) ice core with a new high-resolution (1.5 to 8 cm) chemical data set in order to better understand the processes at work and evaluate how these might have altered the environmental archive. 2 The EPICA Dome C ice core The Dome C deep ice core (EDC) is one of the two ice cores drilled in the framework of the European Project for Ice Coring in Antarctica (EPICA). It is located at Concordia Station (Dome C – 750604 S; 1232052 E), about 1200 km south of the French coastal station, Dumont d’Urville, and 720 km north-east of the Russian Vostok Station. Detailed GPS surface topography and airborne radar surveys were conducted in 1994–1995 in order to optimize the choice for the drilling location (Remy and Tabacco, 2000; Tabacco et al., 1998). These provided clear features of the bedrock and surface topography, showing a set of north–south-trending parallel valleys around 20 km wide and 200–400 m deep in the bedrock, corresponding to smooth elongated undulations a few metres high at the surface. The Cryosphere, 9, 1633–1648, 2015 www.the-cryosphere.net/9/1633/2015/ J.-L. Tison et al.: Retrieving the paleoclimatic signal from the deeper part of the EPICA Dome C ice core 1635 A final drilling depth of 3259.72 m was reached in December 2004, about 15 m above the ice–bedrock interface (to prevent from eventually making contact with subglacial meltwaters). The ice temperature was −3 C at 3235 m and a simple extrapolation to the bottom indicates that the melting point should be reached at the interface (Lefebvre et al., 2008). The top ca. 3200 m of the EDC ice core have already been extensively studied and provided a full suite of climatic and environmental data over the last 8 climatic cycles (e.g. Delmonte et al., 2008; Durand et al., 2008; EPICA Community members, 2004; Jouzel et al., 2007; Lambert et al., 2008; Loulergue et al., 2008; Lüthi et al., 2008; Wolff et al., 2006). Raisbeck et al. (2006) confirmed the old age of the deep EDC ice by presenting evidence for enhanced Be deposition in the ice at 3160–3170 m (corresponding to the 775–786 kyr interval in the EDC2 timescale) consistent with the age and duration of the Matuyama–Brunhes geomagnetic reversal. A coherent interpretation of CO2 and CH4 profiles (Lüthi et al., 2008; Loulergue et al., 2008) also established the presence of Marine Ice Stages (MIS) 18 (ca. 739–767 kyr BP) and 19 (ca. 767–790 kyr BP). However, a detailed study of the isotopic composition of O2 and its relationship to daily Northern Hemisphere summer insolation and comparison to marine sediment records showed potentially anomalous flow in the lowermost 500 m of the core with associated distortion of the EDC2 timescale by a factor of up to 2. This led to the construction of the new, currently used, EDC3 timescale (Parrenin et al., 2007). Note that efforts are still ongoing to refine this timescale, combining multi-site data sets and using δOatm and O2 /N2 as proxies for orbital tuning (Landais et al., 2012; Bazin et al., 2013). As described below, the bottom 60 m of the available core acquired distinctive properties, as a result of processes driven by the proximity of the ice–bedrock interface. We will therefore, in accordance with the previous literature (e.g. Knight, 1997; Hubbard et al., 2009) refer to it as “basal ice”. The last 12 m of the available core show visible solid inclusions (Fig. 1a), which are traditionally interpreted as a sign of interactions with the bedrock. These inclusions are spherical in shape, brownish to reddish in colour, and generally increase both in size and density with increasing depth. They however remain evenly distributed within the ice, therefore qualifying as a “basal dispersed facies” in existing classifications (e.g. Hubbard et al., 2009). Between 3248.30 m (first occurrence of inclusion visible by eye) and 3252.15 m the inclusions are only sparse (0 to 10 inclusions per 55 cm ice core length) and less than 1 mm in diameter. In the lower 8 m, inclusions get bigger (up to 3 mm in the last 50 cm sample) and reach more than 20 individual inclusions per 50 cm ice core length. In several cases, especially for the bigger inclusions, these are “enclosed” in a whitish ovoid bubble-like feature (e.g. upper left corner of Fig. 1a). Careful visual examination of the texture of each individual inclusion suggests that these generally consist of a large number of smaller aggregates although individual particles also occur. In most cases, these d