Phase relationships between orbital forcing and the composition of air trapped in Antarctic ice cores

Abstract

Abstract. Orbital tuning is central for ice core chronologies beyond annual layer counting, available back to 60 ka (i.e. thousands of years before 1950) for Greenland ice cores. While several complementary orbital tuning tools have recently been developed using δ 18 O atm , δO 2 /N 2 and air content with different orbital targets, quantifying their uncertainties remains a challenge. Indeed, the exact processes linking variations of these parameters, measured in the air trapped in ice, to their orbital targets are not yet fully understood. Here, we provide new series of δO 2 /N 2 and δ 18 O atm data encompassing Marine Isotopic Stage (MIS) 5 (between 100 and 160 ka) and the oldest part (340-800 ka) of the East Antarctic EPICA Dome C (EDC) ice core. For the first time, the measurements over MIS 5 allow an inter-comparison of δO 2 /N 2 and δ 18 O atm records from three East Antarctic ice core sites (EDC, Vostok and Dome F). This comparison highlights some site-specific δO 2 /N 2 variations. Such an observation, the evidence of a 100 ka periodicity in the δO 2 /N 2 signal and the difficulty to identify extrema and mid-slopes in δO 2 /N 2 increase the uncertainty associated with the use of δO 2 /N 2 as an orbital tuning tool, now calculated to be 3-4 ka. When combining records of δ 18 O atm and δO 2 /N 2 from Vostok and EDC, we find a loss of orbital signature for these two parameters during periods of minimum eccentricity (∼ 400 ka, ∼ 720-800 ka). Our data set reveals a time-varying offset between δO 2 /N 2 and δ 18 O atm records over the last 800 ka that we interpret as variations in the lagged response of δ 18 O atm to precession. The largest offsets are identified during Terminations II, MIS 8 and MIS 16, corresponding to periods of destabilization of the Northern polar ice sheets. We therefore suggest that the occurrence of Heinrich-like events influences the response of δ 18 O atm to precession.

On the use of δ18Oatm for ice core dating

“…Our dataset complements previously available EDC d 18 O atm series between 0 and 40 ka, 98e152 ka and 300e800 ka (Dreyfus et al, 2007(Dreyfus et al, , 2008Landais et al, 2013;Bazin et al, 2016) with an overlap between 300 and 374 ka. To display a continuous d 18 O atm record over the last 800 ka, the d 18 O atm record presented on Fig.…”

“…The samples are then melted and left at room temperature for at least 1h30 in order to extract the air trapped in ice samples. The air samples are transferred through water vapor and CO 2 traps one by one and are cryogenically trapped into a manifold immersed in liquid helium (Capron et al, 2010;Bazin et al, 2016). Two exterior air samples are analyzed for each daily batch of measurements.…”

“…2). While Vostok and Dome C are both drawn on the AICC2012 chronology, the Dome F chronology DFO2006 has been transferred to AICC2012 by Bazin et al (2016) using the volcanic synchronization of EDC and Dome F over the last 216 ka (Fujita et al, 2015). The resolution of Vostok and Dome F records is lower than the EDC record (~1.27 ka and~1.15 ka compared to~0.55 ka resolution respectively for the last 400 ka).…”

“…Another ice core parameter, the dO 2 /N 2 , was proposed as an orbital tuning tool (Bender, 2002). Indeed, the dO 2 /N 2 outlines variations at orbital scale that are in phase with the local summer insolation (Bender, 2002;Kawamura et al, 2007;Suwa and Bender, 2008;Landais et al, 2012;Bazin et al, 2016). The relationship between dO 2 /N 2 and the local summer insolation is likely established through the near-surface snow metamorphism (Hutterli et al, 2010) that influences snow density down to the pore close-off depth during firnification (Fujita et al, 2009).…”

“…In this paper we first present new high-resolution d 18 O atm and dO 2 /N 2 reference records from EDC ice core over the last 400 ka and hence complement the study of Bazin et al (2016) focused on the 300e800 ka period. We use these composite records to decipher the orbital and millennial components of the d 18 O atm signal and explore mechanisms driving the d 18 O atm variations in comparison with East Asian d 18 O calcite .…”