Glacial–interglacial dynamics of Antarctic firn columns: comparison between simulations and ice core air-δ<sup>15</sup>N measurements

Abstract

Abstract. Correct estimation of the firn lock-in depth is essential for correctly linking gas and ice chronologies in ice core studies. Here, two approaches to constrain the firn depth evolution in Antarctica are presented over the last deglaciation: outputs of a firn densification model, and measurements of δ15N of N2 in air trapped in ice core, assuming that δ15N is only affected by gravitational fractionation in the firn column. Since the firn densification process is largely governed by surface temperature and accumulation rate, we have investigated four ice cores drilled in coastal (Berkner Island, BI, and James Ross Island, JRI) and semi-coastal (TALDICE and EPICA Dronning Maud Land, EDML) Antarctic regions. Combined with available ice core air-δ15N measurements from the EPICA Dome C (EDC) site, the studied regions encompass a large range of surface accumulation rates and temperature conditions. Our δ15N profiles reveal a heterogeneous response of the firn structure to glacial–interglacial climatic changes. While firn densification simulations correctly predict TALDICE δ15N variations, they systematically fail to capture the large millennial-scale δ15N variations measured at BI and the δ15N glacial levels measured at JRI and EDML – a mismatch previously reported for central East Antarctic ice cores. New constraints of the EDML gas–ice depth offset during the Laschamp event (~41 ka) and the last deglaciation do not favour the hypothesis of a large convective zone within the firn as the explanation of the glacial firn model–δ15N data mismatch for this site. While we could not conduct an in-depth study of the influence of impurities in snow for firnification from the existing datasets, our detailed comparison between the δ15N profiles and firn model simulations under different temperature and accumulation rate scenarios suggests that the role of accumulation rate may have been underestimated in the current description of firnification models.

Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities

“…This comparison serves as a prerequisite for the comparison with outputs of the revised model over the same period for the same polar sites. The comparison between the old LGGE model and δ 15 N data over the last deglaciation shows the same patterns already discussed in Capron et al (2013). At Greenland sites, there is an excellent agreement between model and data showing both the decrease in the mean δ 15 N level between the LGM and the Holocene and the ∼ 0.1 ‰ peaks in δ 15 N associated with the abrupt temperature changes (end of the Younger Dryas, Bølling-Allerød, Dansgaard-Oeschger 2, 3, and 4; Figs.…”

“…We have used this estimate of asymmetric uncertainty on the amplitude of temperature change during deglaciation in our study. Recent studies have also suggested that the relationships between water isotopes and temperature and between water isotopes and accumulation rate can be applied with confidence in Antarctica for glacial temperature reconstruction (Cauquoin et al, 2015), while one should be cautious for interglacial temperature reconstruction with warmer conditions than today (Sime et al, 2009). Finally, a recent estimate of the deglacial temperature increase based on δ 15 N measurements at WAIS led to a 11.3 • C temperature increase over the last deglaciation (1 • C warming to be attributed to change in elevation).…”

“…In particular, it has been suggested by Capron et al (2013) that the firn densification rate is underestimated at very low temperatures. We also examine the possible influence of impurity concentration in the LGGE model following the approach from Freitag et al (2013) and Hörhold et al (2012).…”

Critical porosity of gas enclosure in polar firn independent of climate

“…As indicated by δ 15 N measurements as a proxy for firn height (Sowers et al, 1992), up-to-date firn air models seem to have difficulties to estimate past lock-in depths for the East Antarctic plateau (Landais et al, 2006;Capron et al, 2013) and to synchronize age dating of individual ice cores (Parrenin et al, 2012). We suggest that incorporation of our results will help to overcome these problems, as current approaches are based on temperature-dependent lockin (Martinerie et al, 1992) and the Barnola model (Goujon et al, 2003).…”

“…These modeled chronologies are based on the current knowledge of bubble trapping in polar firn and are particularly sensitive to the critical porosity via the assumed temperature dependence. Deviations from the simple relationships used to reconstruct past temperatures and accumulation rates from the water isotopic composition have been suggested as a possible explanation (Landais et al, 2006), while the hypothesis of a large glacial convective zone as an important factor has been ruled out (Capron et al, 2013). Recently, the inclusion of impurity effects has reduced the mismatch for East Antarctic sites; however, it reduces the agreement between modeled and measured δ 15 N for high-accumulation sites (Breant et al, 2017).…”