New MIS 19 EPICA Dome C high resolution deuterium data: Hints for a problematic preservation of climate variability at sub-millennial scale in the “oldest ice”

Abstract

Marine Isotope Stage 19 (MIS 19) is the oldest interglacial period archived in the EPICA Dome C ice core (~ 780 ky BP) and the closest “orbital analogue” to the Holocene — albeit with a different obliquity amplitude and phase with precession. New detailed deuterium measurements have been conducted with a depth resolution of 11 cm (corresponding time resolution of ~ 130 years). They confirm our earlier low resolution profile (55 cm), showing a relatively smooth shape over the MIS 20 to MIS 18 time period with a lack of sub-millennial climate variability, first thought to be due to this low resolution. The MIS 19 high resolution profile actually reveals a strong isotopic diffusion process leading to a diffusion length of at least ~ 40 cm erasing sub-millennial climate variability. We suggest that this diffusion is caused by water-veins associated with large ice crystals at temperatures above −10 °C, temperature conditions in which the MIS 19 ice has spent more than 200 ky. This result has implications for the selection of the future “oldest ice” drilling site.

Introduction

The EPICA Dome C ice core providing more than 800 thousands of years (ky) of climate variability is the longest available climatic record obtained from ice cores. Thanks to different proxies such as stable water isotopes (Jouzel et al., 2007), aerosols (Lambert et al., 2008, Wolff et al., 2010a), CH₄ (Loulergue et al., 2008, Spahni et al., 2005) or CO₂ (Lüthi et al., 2008, Siengenthaler et al., 2005), local, regional and global variations of climate and environment have been characterized over eight glacial–interglacial cycles (Fig. 1). If long term climate variability is considered to be driven by variations of incoming insolation due to slow changes of orbital parameters (Berger and Loutre, 1991, Laskar et al., 1993), the precise role of seasonal and latitudinal insolation distribution remains disputed. The climate system response involves threshold effects and internal feedbacks, making the understanding of the Earth climate complex (Paillard, 1998).

Interglacial periods offer the possibility to compare the climate system response under different orbital configurations and without the major influence of northern hemisphere glacial ice sheets. Thanks to EPICA Dome C ice core strong differences have been revealed between the nine past and current interglacial periods regarding their duration, trend or intensity (Fig. 1, Fig. 2) (EPICA-community-members, 2004, Jouzel et al., 2007, Masson et al., 2000). The current interglacial period has been compared to past interglacials with a specific focus on Marine Isotope Stage (MIS) 5 and 11 (Loutre and Berger, 2003, Loutre, 2003, Masson-Delmotte et al., 2006, Ruddimann, 2005), with implications for the future long term influence of Earth's orbit and the timing of the next Ice Age.

In this context, the following study focuses on the oldest interglacial archived in the EPICA Dome C (EDC) ice core: MIS 19, occurring around 780 thousands of years (ky) before present (B.P.). The full sequence of events, from the end of the previous glacial period (MIS 20) to the beginning of the following one (MIS 18), is detected between depths of 3147 and 3190 m. Corresponding ages range between ~ 749 and ~ 801 ky with an absolute uncertainty of 6 ky (2σ) (Fig. 3) as given by the current EDC3 chronology (Parrenin et al., 2007a). This dating is relatively well constrained for MIS 19, thanks to the Matuyama–Bruhnes geomagnetic reversal identification from Beryllium 10 measurements (Dreyfus et al., 2008, Raisbeck et al., 2006). The integrity of the climate records in EDC ice is confirmed down to ~ 801 ky (Jouzel et al., 2007), despite indications of ice flow perturbation in the bottom part of the core (Dreyfus et al., 2007).

MIS 19 is interesting for two reasons: (i) it occurred during an orbital configuration quite similar to the present one; and (ii) the EPICA Dome C low resolution proxy records exhibit comparable magnitudes for MIS 19 and the Holocene (Fig. 1, Fig. 2). Section 2 is thus dedicated to a detailed comparison of orbital contexts and EPICA records for these two periods.

During the glacial inception from MIS 19 to MIS 18, the low resolution EPICA Dome C water stable isotope record (Jouzel et al., 2007) has revealed millennial variability principally marked by the occurrence of three consecutive warm events (hereafter called Antarctic Isotope Maxima — AIM, following EPICA-community-members, 2006, and noted A, B, C on Fig. 2). These AIM events were also identified in the CH₄ and CO₂ signals (Loulergue et al., 2008, Lüthi et al., 2008). To improve the temporal resolution and to document the sub-millennial climate variability during MIS 19, we have conducted new high resolution measurements of water stable isotope ratios (δD of HDO molecules, hereafter called deuterium) on the EDC ice core fine cuts (Section 3). This new sampling, allowing to increase the temporal resolution from ~ 650 years (low resolution published data) to ~ 130 years, confirms the robustness of the previous low resolution isotopic signal (Section 4.1) but reveals at the same time a strong smoothing of the deuterium signal (Section 4.2), which erases potential new information on climate variability. This result leads to an analysis of the isotopic diffusion in ice, a process commonly studied for its implication in the impairment of climatic signals (Section 4.3). Section 4.4 is then dedicated to the comparison between modelled and data-derived diffusion calculation. This comparison suggests that the deepest EDC ice is affected by physical processes that alter the preservation of the full stable isotope variability. We finally investigate the possible mechanisms at play.