Dissertation Abstract
The effect of firn microstructure on firn air transport
Keegan, Kaitlin M 2014
Thayer School of Engineering, Dartmouth College (United States), 129 pp.
The processes affecting gas transport through the firn column are important to understand in order to accurately interpret climate records from ice cores. Historically, density measurements have been used to estimate open and closed porosity in the firn layers to inform firn densification models. These densification models directly affect the gas-age ice-age calculation that is used to determine the offset between ice and air bubble proxies of the ice core climate records. Through studies of firn microstructure, we hope to learn more about pore geometry and ultimately how gas transports through firn layers. Such studies will directly influence our understanding of the gas-age ice-age difference.
In this thesis, we find present three examples of firn microstructure affecting gas transport through the firn column, in ways that are inconsistent with the current understanding of the firn column. First, we examine the effect melt layers found in firn cores from dry snow region of Greenland. From permeability measurements, we find that the assumption that these melt layers are impermeable, or at least disrupt gas transport, to be incorrect. We compare the melt layers found at NEEM to other Greenlandic firn cores, to reveal that a widespread melt event occurred in 1889 and that warm temperatures combined with black carbon deposition due to forest fires were the cause of such event. We then examine the lock-in zone of the NEEM, Greenland firn column. We find that the lock-in depth of the NEEM firn to consist of 4.5 m of low permeability firn layers, unlike the single impermeable firn layer that is typically described. This extended lock-in depth of the NEEM lock-in zone helps in the understanding of a NEEM firn air campaign that requires a diffusion term within the non-diffusive lock-in zone, because the extended lock-in depth is capable of allowing a small amount of diffusion through the layers. Lastly, we examine the effect of impurities in firn on the layering and densification rates of these layers. We find that impurities of sulfuric acid, calcium carbonate, gypsum, hydroxyl herderite, and sodium chloride are present in the NEEM firn layers, and are more concentrated in the grain boundaries of layers. Despite the presence of these impurities within the layers, we find that the density of firn layers is more dependent on the microstructure of the snow at the time of deposition.
In this thesis, we find present three examples of firn microstructure affecting gas transport through the firn column, in ways that are inconsistent with the current understanding of the firn column. First, we examine the effect melt layers found in firn cores from dry snow region of Greenland. From permeability measurements, we find that the assumption that these melt layers are impermeable, or at least disrupt gas transport, to be incorrect. We compare the melt layers found at NEEM to other Greenlandic firn cores, to reveal that a widespread melt event occurred in 1889 and that warm temperatures combined with black carbon deposition due to forest fires were the cause of such event. We then examine the lock-in zone of the NEEM, Greenland firn column. We find that the lock-in depth of the NEEM firn to consist of 4.5 m of low permeability firn layers, unlike the single impermeable firn layer that is typically described. This extended lock-in depth of the NEEM lock-in zone helps in the understanding of a NEEM firn air campaign that requires a diffusion term within the non-diffusive lock-in zone, because the extended lock-in depth is capable of allowing a small amount of diffusion through the layers. Lastly, we examine the effect of impurities in firn on the layering and densification rates of these layers. We find that impurities of sulfuric acid, calcium carbonate, gypsum, hydroxyl herderite, and sodium chloride are present in the NEEM firn layers, and are more concentrated in the grain boundaries of layers. Despite the presence of these impurities within the layers, we find that the density of firn layers is more dependent on the microstructure of the snow at the time of deposition.