[A-CC27] Ice cores and paleoenvironmental modeling

[ACC27-04] Unveiling the influence of impurities on recrystallization and crystal orientation in the deepest layers of Antarctic ice cores

*Shuji Fujita^1,2, Tomotaka Saruya¹, Motohiro Hirabayashi¹, Kumiko Goto-Azuma^1,2, Jun Ogata¹, kaori Fukuda¹, kyotaro Kitamura¹, Hiroshi Ohno³, Iizuka Yoshinori⁴ (1.National Institute of Polar Research, Research Organization of Information and Systems(ROIS), 2.Graduate Institute for Advanced Studies, SOKENDAI, Department of Advanced Studies,, 3.Kitami Insititute of Technology, Japan, 4.ILTS, Hokkaido University Japan)

Keywords:ice sheet, ice core, recrystallization, impurities, deformation, dust

In the dynamics of polar ice sheets, the deepest few hundred meters are the least understood in terms of structure and properties. This is due to the challenges of accessing and collecting ice core samples from the interior and deep parts of the Antarctic continent. However, in the context of polar ice sheet flow, the deformation of ice at these depths is crucial to the overall movement of ice. Moreover, recent research (Saruya et al. 2024) has shown that the presence of impurities closely relates to the frequency of recrystallization and the formation of crystal c-axis orientation clusters. To investigate the impact of these impurities on c-axis clustering and dynamic recrystallization, we utilized a 3035m long ice core drilled at Dome Fuji, a summit in East Antarctica. Specifically, we selected eight depths within the deepest 20% of the total depth (approximately 2,400m to about 3,000m). These 7-meter sections of the ice core were shaped into prismatic samples with cross-sections of 32mm x 32mm, and the strength of crystal c-axis clusters was continuously examined with a high resolution of about 15mm. Additionally, using the same prismatic samples, we conducted continuous flow analysis (CFA) to analyze impurity concentrations (particularly Si, Ca, Al, Fe, Na, Mg, SO₄) with a high resolution of about 10mm. We then examined the relationship between the strength of crystal c-axis clusters and these impurity concentrations. The main results are as follows: Continuous analysis revealed that recrystallization appears as numerous intermittent spikes when viewed in the depth direction. Furthermore, the occurrence of these "recrystallization spikes" is primarily in ice with concentrations of dusty impurities below a certain threshold, with virtually no occurrence in ice with concentrations above this threshold. Specifically, the threshold relationships with the number concentration of dust and elements such as Si, Ca, Al, Fe were clearly identified. In contrast, the relationships with Na, Mg, SO₄ were inconsistent and unclear. Moreover, in the depth zone corresponding to the deepest 10% of the ice sheet, ice with impurity concentrations that would not normally lead to recrystallization at shallower depths showed a weakening of c-axis cluster strength and the occurrence of recrystallization. This study provides data for unraveling the effects of depth, ice age, impurity concentration, and temperature on the frequency of recrystallization within ice sheet ice.

References: Saruya, T., Miyamoto, A., Fujita, S., Goto-Azuma, K., Hirabayashi, M., Hori, A., Igarashi, M., Iizuka, Y., Kameda, T., Ohno, H., Shigeyama, W., and Tsutaki, S.: Development of deformational regimes and microstructures in the deep sections and overall layered structures of the Dome Fuji ice core, Antarctica, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-3146, 2024.