Very old firn air linked to strong density layering at Styx Glacier, coastal Victoria Land, East Antarctica

Youngjoon Jang,Jinho Ahn,Seong-Joong Kim,Seong Joon Jun,Soon Do Hur,Taejin Choi,Sang Bum Hong,Yoshinori Iizuka,Ji-Woong Yang,Christo Buizert,Akira Hori,Yeongcheol Han,Sang-Young Han,Hun-Gyu Lee,Pieter Tans

doi:10.5194/tc-13-2407-2019

Abstract

Abstract. Firn air provides plenty of old air from the near past, and can therefore be useful for understanding human impact on the recent history of the atmospheric composition. Most of the existing firn air records cover only the last several decades (typically 40 to 55 years) and are insufficient to understand the early part of anthropogenic impacts on the atmosphere. In contrast, a few firn air records from inland sites, where temperatures and snow accumulation rates are very low, go back in time about a century. In this study, we report an unusually old firn air effective CO2 age of 93 years from Styx Glacier, near the Ross Sea coast in Antarctica. This is the first report of such an old firn air age (>55 years) from a warm coastal site. The lock-in zone thickness of 12.4 m is larger than at other sites where snow accumulation rates and air temperature are similar. High-resolution X-ray density measurements demonstrate a high variability of the vertical snow density at Styx Glacier. The CH4 mole fraction and total air content of the closed pores also indicate large variations in centimeter-scale depth intervals, indicative of layering. We hypothesize that the large density variations in the firn increase the thickness of the lock-in zone and, consequently, increase the firn air ages because the age of firn air increases more rapidly with depth in the lock-in zone than in the diffusive zone. Our study demonstrates that all else being equal, sites where weather conditions are favorable for the formation of large density variations at the lock-in zone preserve older air within their open porosity, making them ideal places for firn air sampling.

Highlights

Bubbles trapped in ice cores preserve ancient air and allow direct measurements of the atmospheric composition in the past (e.g., Petit et al, 1999)
We examined a snow pit, located 10 m away from the main ice core borehole, 2 years after drilling to understand the physical properties such as layers, density, and ice grain size of the upper firn at the Styx site
We suggest that sites with higher density variations at the lock-in zone (LIZ) have a high possibility of a thick LIZ and old firn air, even in warm, relatively highprecipitation coastal climates

Summary

Introduction

Bubbles trapped in ice cores preserve ancient air and allow direct measurements of the atmospheric composition in the past (e.g., Petit et al, 1999). It is difficult to obtain air samples over the past several decades from ice cores since the more recent air has not yet been completely captured into bubbles closed off from the atmosphere. Reported firn air ages date back only several decades at the sites where snow accumulation rates are relatively high (Table 1). Old firn air (> 55 years) was observed only at sites where surface temperatures and snow accumulation rates are low such as the South Pole (Battle et al, 1996) and inland Antarctic Megadunes (Severinghaus et al, 2010)

Methods

Results

Conclusion