Abstract
Abstract. We compare a variety of methods for estimating the gas/ice depth offset (Δdepth) at EPICA Dome C (EDC, East Antarctica). (1) Purely based on modelling efforts, Δdepth can be estimated combining a firn densification with an ice flow model. (2) The diffusive column height can be estimated from δ15N and converted to Δdepth using an ice flow model and assumptions about past average firn density and thickness of the convective zone. (3) Ice and gas synchronisation of the EDC ice core to the GRIP, EDML and TALDICE ice cores shifts the ice/gas offset problem into higher accumulation ice cores where it can be more accurately evaluated. (4) Finally, the bipolar seesaw hypothesis allows us to synchronise the ice isotopic record with the gas CH4 record, the later being taken as a proxy of Greenland temperature. The general agreement of method 4 with methods 2 and 3 confirms that the bipolar seesaw antiphase happened during the last 140 kyr. Applying method 4 to the deeper section of the EDC core confirms that the ice flow is complex and can help to improve our reconstruction of the thinning function and thus, of the EDC age scale. We confirm that method 1 overestimates the glacial Δdepth at EDC and we suggest that it is due to an overestimation of the glacial lock-in depth (LID) by the firn densification model. In contrast, we find that method 1 very likely underestimates Δdepth during Termination II, due either to an underestimated thinning function or to an underestimated LID. Finally, method 2 gives estimates within a few metres of methods 3 and 4 during the last deglacial warming, suggesting that the convective zone at Dome C cannot have been very large at this time, if it existed at all.
Highlights
Ice cores provide a wealth of information on past climatic variations (Jouzel et al, 2007; Pol et al, 2011) and on past greenhouse gases concentrations (Luthi et al, 2008; Loulergue et al, 2008) at time scales ranging from centennial to orbital (Earth orbit variations, 104–105 yr).To interpret the records from ice cores, it is essential to derive accurate chronologies (e.g., Parrenin et al, 2007b)
What is important for paleoclimatic studies is the lock-in depth (LID) where gas diffusion becomes negligible, and which is slightly smaller than the close-off depth (COD) where it is Published by Copernicus Publications on behalf of the European Geosciences Union
We have shown that the bipolar seesaw antiphase relationship is generally supported by the ice–gas cross synchronisation of EDC to the GRIP, EDML and TALDICE ice cores
Summary
Ice cores provide a wealth of information on past climatic variations (Jouzel et al, 2007; Pol et al, 2011) and on past greenhouse gases concentrations (Luthi et al, 2008; Loulergue et al, 2008) at time scales ranging from centennial to orbital (Earth orbit variations, 104–105 yr). Parrenin et al.: On the gas-ice depth difference ( depth) not possible to pump air (Witrant et al, 2011) The determination of this ice/gas offset is essential, to derive the phase relationship between proxies recorded in the ice phase and in the gas bubbles. Depth is independent of the reference age scale used It is only weakly dependent on accumulation rate since both the LID and the thinning function are only weakly affected by changes in accumulation rate. In this paper we focus on the evaluation of the depth along the EDC (EPICA Dome C) ice core using different approaches They fall into two broad categories: (1) estimation of the initial LID of gas bubbles and estimation of the thinning of snow/ice layers; and (2) determination of synchronous events in gas and ice proxy records. We use the same depth–depth relationship for both gas and ice datasets, i.e. we assume that age as a function of age is the same for both cores
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