Pore morphology of polar firn around closure revealed by X-ray tomography

Abstract

Abstract. Understanding the slow densification process of polar firn into ice is essential in order to constrain the age difference between the ice matrix and entrapped gases. The progressive microstructure evolution of the firn column with depth leads to pore closure and gas entrapment. Air transport models in the firn usually include a closed porosity profile based on available data. Pycnometry or melting-refreezing techniques have been used to obtain the ratio of closed to total porosity and air content in closed pores, respectively. Xray-computed tomography is complementary to these methods, as it enables one to obtain the full pore network in 3-D. This study takes advantage of this nondestructive technique to discuss the morphological evolution of pores on four different Antarctic sites. The computation of refined geometrical parameters for the very cold polar sites Dome C and Lock In (the two Antarctic plateau sites studied here) provides new information that could be used in further studies. The comparison of these two sites shows a more tortuous pore network at Lock In than at Dome C, which should result in older gas ages in deep firn at Lock In. A comprehensive estimation of the different errors related to X-ray tomography and to the sample variability has been performed. The procedure described here may be used as a guideline for further experimental characterization of firn samples. We show that the closed-to-total porosity ratio, which is classically used for the detection of pore closure, is strongly affected by the sample size, the image reconstruction, and spatial heterogeneities. In this work, we introduce an alternative parameter, the connectivity index, which is practically independent of sample size and image acquisition conditions, and that accurately predicts the close-off depth and density. Its strength also lies in its simple computation, without any assumption of the pore status (open or close). The close-off prediction is obtained for Dome C and Lock In, without any further numerical simulations on images (e.g., by permeability or diffusivity calculations).

Multi-tracer study of gas trapping in an East Antarctic ice core

“…During the drilling operation, about 100 m of mature ice was retrieved and analyzed using gas continuous-flow analysis (gas CFA, first developed by Stowasser et al, 2012;Chappellaz et al, 2013). Methane concentration in enclosed bubbles was measured using the gas CFA system of IGE coupled with a laser spectrometer SARA based on optical-feedback cavity-enhanced absorption spectroscopy (OF-CEAS; Morville et al, 2005).…”

Multi-tracer study of gas trapping in an East Antarctic ice core

“…Computed X-ray tomography is a technique gaining traction for the study of firn densification and pore closure (Barnola et al, 2004;Freitag et al, 2004;Gregory et al, 2014;Schaller et al, 2017;Burr et al, 2018). Briefly, several 2D X-ray scans of the samples are performed under different angles.…”

“…Notably, pore closure is quantified through the evolution of the closed porosity ratio CP , defined as the ratio between the closed pores volume and the total pore volume (Mitchell et al, 2015;Burr et al, 2018 volumes. On the other hand, thanks to the explicit representation of the pore network, tomography data can be used to access variations at scales finer than the sample size.…”

“…The values of closed porosity obtained using both pycnometry and tomography are displayed Figure 2-a against relative density. Burr et al (2018) proposed to use the connectivity index as an indicator of pore closure. This index is defined as the ratio between the volume of the largest pore and the volume of all pores in a given sample, and thus equals one for a fully open sample and sharply drops near zero for partially closed samples.…”

“…While several studies focus on the cold and arid sites of this region, none of them to our knowledge attempts to encompass a detailed description of stratification, closed porosity and gas trapping within the same firn. As particular examples, the study of Fujita et al (2016) describes the link between density variability and ionic content in three firns core near Dome Fuji without addressing pore closure, while the studies of Schaller et al (2017) and Burr et al (2018) propose detailed descriptions of the pore network closure but without discussing stratification.…”