Continuous measurements of methane mixing ratios from ice cores

Abstract

Abstract. This work presents a new, field-deployable technique for continuous, high-resolution measurements of methane mixing ratios from ice cores. The technique is based on a continuous flow analysis system, where ice core samples cut along the long axis of an ice core are melted continuously. The past atmospheric air contained in the ice is separated from the melt water stream via a system for continuous gas extraction. The extracted gas is dehumidified and then analyzed by a Wavelength Scanned-Cavity Ring Down Spectrometer for methane mixing ratios. We assess the performance of the new measurement technique in terms of precision (±0.8 ppbv, 1σ ), accuracy (±8 ppbv), temporal (ca. 100 s), and spatial resolution (ca. 5 cm). Using a firn air transport model, we compare the resolution of the measurement technique to the resolution of the atmospheric methane signal as preserved in ice cores in Greenland. We conclude that our measurement technique can resolve all climatically relevant variations as preserved in the ice down to an ice depth of at least 1980 m (66 000 yr before present) in the North Greenland Eemian Ice Drilling ice core. Furthermore, we describe the modifications, which are necessary to make a commercially available spectrometer suitable for continuous methane mixing ratio measurements from ice cores.

Where to find 1.5 million yr old ice for the IPICS "Oldest-Ice" ice core

“…However, this number is approximately in line with model based reconstructions (Fig. 9) (van Liefferinge and Pattyn, 2013;Pattyn, 2010), who combined the geological information on the geothermal heat flux for certain rock types occurring at the base of Antarctica with the localized information of subglacial lakes and an ice sheet model to constrain bottom melting and the age of the ice at the bottom. because of the severe lack of information on bedrock geology and heat production by radio decay in this region.…”

“…(3) assumes no horizontal advection of heat, i.e., we assume a perfect dome position. In case of EDC, where horizontal surface velocities are on the order of a few centimeters per year (Vittuari et al, 2004), this criterion seems to be reasonably fulfilled. We will also use m = 0.5 for all our later calculations, essentially assuming that any potential OldestIce coring site will also be located on a dome and will have flow conditions similar to Dome C. Note that other sites with more complex flow conditions (for instance in outflow regions) would require different thinning functions, thus different exponents.…”

Analytical constraints on layered gas trapping and smoothing of atmospheric variability in ice under low-accumulation conditions

“…The extent of the CFA-based damping was determined by performing a step test (left panel of Fig. S2), i.e., a switch between two synthetic mixtures of degassed DI water and synthetic air standards of different methane concentrations, following the method of Stowasser et al (2012). It shows that the CFA system can resolve signals down to the centimeter scale.…”

“…S2). The smoothing characteristics of our measurement system were determined experimentally as in Stowasser et al (2012). The CFA smoothing induces a damping of about 18 % of the modeled artifacts.…”

“…Detailed descriptions of this method have been reported before (Stowasser et al, 2012;Chappellaz et al, 2013;Rhodes et al, 2013). Ice core sticks of 34 by 34 mm were melted at IGE at a mean rate of 3.8 cm min −1 using a melt head as described by Bigler et al (2011), and the water and gas bubble mixture was pumped toward a low-volume T-shaped glass debubbler.…”