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On the gas-ice depth difference (Delta depth) along the EPICA Dome C ice core

Abstract : We compare a variety of methods for estimating the gas/ice depth offset (Delta depth) at EPICA Dome C (EDC, East Antarctica). (1) Purely based on modelling efforts, Delta depth 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 Delta depth 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 Delta depth 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 Delta depth 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.
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F. Parrenin, S. Barker, T. Blunier, J. Chappellaz, Jean Jouzel, et al.. On the gas-ice depth difference (Delta depth) along the EPICA Dome C ice core. Climate of the Past, European Geosciences Union (EGU), 2013, 8, pp.1239- 1255. ⟨10.5194/cp-8-1239-2012⟩. ⟨insu-00843989⟩

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