In-situ observation of bubble trapping in polar firn

• Main Authors: Schaller, Christoph Florian, Freitag, Johannes, Sowers, Todd, Vinther, Bo, Weinhart, Alexander, Eisen, Olaf
• Format: Conference Object
• Language: unknown
• Published: 2017
• Subjects: Antarc*; Antarctica; East Antarctica; Greenland
• Online Access: https://epic.awi.de/id/eprint/49628/; https://epic.awi.de/id/eprint/49628/1/EGU2017-14721-1.pdf; https://hdl.handle.net/10013/epic.05b61388-a40d-4e9d-968e-b76257565c78

The air trapped in polar ice cores is not a direct record of past atmospheric composition but is strongly influenced by the process of firnification as bubbles are only sealed at a certain point, when the respective horizontal layer reaches a so called “critical” porosity. In order to investigate this process, we performed high-resolution (approximately 25μm) 3D-XCT measurements of the complete lock-in zone for two polar ice cores representing opposite extremes of the temperature and accumulation rate range: B53, close to Dome Fuji, East Antarctica and RECAP_S2, Renland, Greenland. For every 1m core segment, we scanned a minimum number of five sections of approximately 3.5cm height of the full core diameter with a focus on homogenous layers. This allows us to non-destructively deduce detailed profiles of open and closed porosity on a solid statistical basis. For each of the cores individually, we find that the trapping of bubbles in a single layer is solely determined by its total porosity and thereby independent of depth. We can confirm the existence of a distinct Schwander-type relation of closed and total porosity. Even though the two cores deviate from each other significantly in critical porosity, 0.0907 for B53 compared to 0.1025 for RECAP_S2, we observe many similarities. We hypothesize, that the determining factors of bubbletrapping are the average size and variability of pore space structures. This could potentially allow the reconstruction of past close-off porosities from the remaining pore structures in deep ice, e.g. from bubble number densities.