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The effect of microstructure on firn gas transport / by Kaitlin M. Keegan

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Author	Keegan, Kaitlin M.
Title	The effect of microstructure on firn gas transport / by Kaitlin M. Keegan
Date	2014
Abstract	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 present three examples of firn microstructure affecting the 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. The second example we examine is the lock-in zone of the NEEM 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 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 the firn layers and their ability to affect the densification rate of the firn. We find that impurities of sulfuric acid (H?SO?), calcium carbonate (CaCO?), gypsum (CaSO?*2H?O), hydroxyl herderite (CaBePO?(OH)), and sodium chloride (NaCl) in the NEEM firn layers, that 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 mircrostructure of the snow at the time of depostion.
Description	xii, 116 leaves : ill. (some col.), col. maps
Format	application/pdf
College	Thayer School of Engineering.
Issuing Body	Dartmouth Dissertations are made available through the Dartmouth Digital Library.
Degree	Thesis (Ph.D.)--Dartmouth College.
Copyright	All rights reserved. No part of this book may be reproduced in any form by any electronic or mechanical means without permission in writing from the author.
DOI	http://dx.doi.org/10.1349/ddlp.1207

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