Firn Smoothing of Abrupt Methane Variations in the South Pole Ice Core

Trace gas records of abrupt Dansgaard-Oeschger (D-O) climate variability in polar ice cores are smoothed due to processes in the firn layer. Molecular diffusion and gradual bubble trapping are two specific mechanisms known to contribute to a broadened gas-age distribution, although these mechanisms remain poorly understood and attempts at modeling these processes fall short. The gas-age distribution in mature ice is commonly assumed to be roughly 10% of delta-age (Δage), or the gas-age, ice-age difference. In order to better understand the firn smoothing mechanism and ultimately improve interpretation of trace gas records, we examine an 88 m-long section (1,359-1,271 m or 32.9-27.8 ka BP gas-age) of the South Pole ice core. The section of interest encompasses D-O events 3, 4, and 5, corresponding to a series of abrupt CH₄ variations suitable for understanding the complex ways firn smoothing influences ice core records of abrupt atmospheric trace gas excursions. Using a melt-refreeze method to extract gases, a series of discrete methane measurements were obtained and processed. The South Pole data were compared to the continuous, ultra-high resolution WAIS-Divide Ice Core Methane Record, and found to be smooth relative to WAIS-Divide, although observed smoothing is less than both published model-based estimates and the 10% of Δage rule-of-thumb. By providing a well-resolved methane record and constraint on smoothing estimates, we hope to inform research that seeks to fill crucial gaps in our understanding of coupled trace gas excursion and climate dynamics over the past 40,000-50,000 years.