1 Introduction

[2] Attributing causes to past variations of atmospheric carbon dioxide (CO 2 ) concentration (defined as the mole fraction of CO 2 in dry air) will help to improve prediction of the future interplay between the climate and the carbon cycle. Studies of CO 2 and its13C/12C stable isotope ratio (referred to as δ13C from here on), involving overlapping Antarctic ice cores, firn and contemporary air records, remain key evidence for recent natural variations in the C cycle as well as the impact of human CO 2 emissions on the global atmosphere.

[3] The record of atmospheric CO 2 concentration extracted from the ice cores drilled at Law Dome (East Antarctica) covering the last 2000 years [Etheridge et al., 1996; MacFarling Meure et al., 2006] has been extensively used in modeling studies to constrain the global C cycle of the past [Joos and Bruno, 1998; Trudinger et al., 1999, 2005] and its interactions with the climate system [Ammann et al., 2007; Cox and Jones, 2008; Frank et al., 2010]. Models of the global C cycle that include the stable isotopes of C [Joos et al., 1999; Trudinger et al., 2002b; Gerber et al., 2003; Köhler et al., 2006] have found an additional constraint in the companion record of δ13C from the same site in Antarctica [Francey et al., 1999, hereinafter F99]. This is a high precision data set, due to meticulous examination of all aspects of the procedure, representing the state of the art in 1999 for measured δ13C changes in air trapped in ice cores. The Law Dome δ13C record has shown late preindustrial Holocene (LPIH) variations of atmospheric δ13C and has confirmed that the CO 2 released into the atmosphere during the Industrial Period, since 1750 A.D.) has a C source depleted in13C, providing key evidence for the attribution of atmospheric CO 2 variations during the Industrial Period to anthropogenic activities.

[4] Thanks to the characteristics of the drilling sites at Law Dome (high accumulation rate, low temperature, and small quantities of impurities in the ice) [Etheridge and Wookey, 1989; Morgan et al., 1997], no other ice core has, so far, provided such a precise and well‐resolved δ13C record of the last 1000 years. Even though another high‐resolution record of δ13C over the Industrial Period was produced by Kawamura et al. [2000] from the Antarctic H15 ice core, unexplained differences from the F99 data set of, on average, 0.2–0.3‰ in the mean level and up to 0.5‰ around 1800 A.D. were found. Records of δ13C covering the last glacial period [Machida et al., 1996; Smith et al., 1999] or the Holocene period [Indermühle et al., 1999; Elsig et al., 2009] do not both span the LPIH and overlap firn measurements and have used the F99 record to link their longer records to direct atmospheric measurements. No other paleoarchives (speleothems, corals, and sediments) have been found to be comparably useful, especially in the Industrial Period.

[5] More than 10 years after its publication, the Law Dome δ13C record remains one of the main sources of information for models inferring the C sources and sinks associated with past variations of atmospheric CO 2 concentration over the last 1000 years [Stocker et al., 2011]. Nevertheless, the sampling density of the Law Dome δ13C record needs to be increased, especially over the LPIH where significant changes of the atmospheric CO 2 concentration have been found (e.g., beginning of the seventeenth century) [MacFarling Meure et al., 2006]. This can improve our understanding of the mechanisms responsible for natural changes of the atmospheric CO 2 concentration in a period, like the LPIH, where the Earth's system was very close to current conditions, but with anthropogenic influences not yet a predominant part of the carbon cycle.

[6] Measurements of δ13C in air extracted from the firn have been used to match the F99 record with modern atmospheric measurements. The firn is the upper part of an ice sheet, where bubbles are not yet closed, and air with mean age up to several decades can be pumped directly out of the snow matrix and collected for analysis [Schwander et al., 1988]. Shallow firn air measurements can be compared to direct atmospheric measurements of δ13C [Sugawara et al., 2003; F99], which started in 1976 at Mauna Loa in the Northern Hemisphere [Keeling et al., 1989] and in 1978 at Cape Grim in the Southern Hemisphere [Allison and Francey, 2007]. Similarly, deep firn air measurements can be compared to ice core data. While recent (younger than 1950 A.D.) ice core air samples from Law Dome agreed closely with Law Dome firn air and direct atmospheric records, a difference with older South Pole firn samples became apparent [Trudinger, 2000, p. 108].