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Abstract
AbstractEarlier gas measurements of firn air (atmosphere in open pore channels) at polar sites have revealed the occurrence of gas fractionation phenomena during bubble close-off, in addition to well-known thermal and gravitational gas separation. Nevertheless, because of difficulties posed by measurement, little is known about the distribution of air constituents in already closed pores (bubbles) in firn. Herein, we describe the application of high-sensitivity pure rotational Raman spectroscopy, combined with sample immersion in the fluorocarbon-based inert fluid for removing the optical disturbance by diffused reflection. That application efficiently elicits information about nitrogen and oxygen composition ratios (N2/O2 or O2/N2) for each air bubble in firn. The developed methodology presents important implications for elucidating how gas records are formed and modified in the course of pore close-off in polar firn.
Highlights
When polar firn transforms into ice, i.e. loss of permeability, the surrounding atmospheric gases are trapped in the ice matrix as air bubbles (e.g. Cuffey and Paterson, 2010)
The air constituents preserved in the air bubbles and hydrates provide unique archives to reconstruct past atmospheric compositions (e.g. Petit and others, 1999)
Little is known about how the N2/O2 climate signal forms, it is believed to originate from changes in surface-snow texture caused by solar radiation: the snow texture might determine the duration of firn-to-ice transition and might determine the degree of gas fractionation during bubble close-off (Fujita and others, 2009; Lipenkov and others, 2011)
Summary
When polar firn transforms into ice, i.e. loss of permeability, the surrounding atmospheric gases are trapped in the ice matrix as air bubbles (e.g. Cuffey and Paterson, 2010). Because of the low overburden pressure, the density of gases inside bubbles is lower in firn than in deep ice, considerably reducing the Raman scattering intensity.
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