A high-precision method for measurement of paleoatmospheric CO₂ in small polar ice samples

Abstract

We describe a high-precision method, now in use in our laboratory, for measuring the CO2 mixing ratio of ancient air trapped in polar ice cores. Occluded air in ice samples weighing ∼8–15 g is liberated by crushing with steel pins at −35°C and trapped at −263°C in a cryogenic cold trap. CO2 in the extracted air is analyzed using gas chromatography. Replicate measurements for several samples of high-quality ice from the Siple Dome and Taylor Dome Antarctic ice cores have pooled standard deviations of <0.9 ppm. This high-precision technique is directly applicable to high-temporal-resolution studies for detection of small CO2 variations, for example CO2 variations of a few parts per million on millennial to decadal scales.

A sublimation technique for high-precision measurements of δ<sup>13</sup>CO<sub>2</sub> and mixing ratios of CO<sub>2</sub> and N<sub>2</sub>O from air trapped in ice cores

“…1 ppmv for the air references and 2-3 ppmv for the different ice cores. This precision is comparable to the precision of mechanical extraction techniques and other studies using sublimation (Etheridge et al, 1996;Güllük et al, 1998;Siegenthaler et al, 2005b;Ahn et al, 2009;Lourantou et al, 2010a;Lüthi et al, 2010). For N 2 O we obtain a precision for reference air sample between 1.6 ppbv and 6.9 ppbv, and between 8.2 ppbv and 18.1 ppbv for the different ice cores.…”

“…Also the high solubility of CO 2 connected with a strong isotopic fractionation during gas-liquid transfer from the aqueous HCO − 3 /H 2 CO 3 system limits the applicability of wet extraction methods for CO 2 (Anklin et al, 1995;Zhang et al, 1995). The dry extraction methods, however, suffer from rather low extraction efficiencies, ranging from only 50 % for fully clathrated ice to up to 90 % for bubble ice (Etheridge et al, 1988;Ahn et al, 2009;Lüthi et al, 2010;Schaefer et al, 2011). Note within this paper the term extraction efficiency refers to the efficiency during the actual extraction process, i.e.…”

High-precision dual-inlet IRMS measurements of the stable isotopes of CO<sub>2</sub> and the N<sub>2</sub>O / CO<sub>2</sub> ratio from polar ice core samples

“…This air sample is warmed to room temperature and stored for a few hours before being expanded via valve 13 into a sample loop and analyzed with an Agilent 7890A GC to determine the CO 2 mixing ratio. The CO 2 measurement is similar to previous methodology at Oregon State University (Ahn et al, 2009), whereby CO 2 is separated with a Porapak Q 80/100 mesh column, reduced to CH 4 with a nickel catalyst, and measured with a flame ionization detector (FID). However, whereas the previous method uses a manometer in the sample loop to determine the total air injected into the GC, we employ a thermal conductivity detector (TCD) to measure amount of O 2 and N 2 in the sample.…”

A centrifugal ice microtome for measurements of atmospheric CO<sub>2</sub> on air trapped in polar ice cores

“…Existing dry extraction devices release only 60 %-90 % and 50 %-80 % of the enclosed air in pure bubble ice and in pure clathrate ice, respectively (Schaefer et al, 2011;Sowers and Jubenville, 2000;Lüthi et al, 2010;Ahn et al, 2009), whereas sublimation extracts 100 % of the enclosed air from all ice types. In the bubble to clathrate transition zone (BCTZ) of ice cores (depth zone in ice cores where bubbles and clathrates coexist and where a fractionation of CO 2 concentration in clathrates relative to bubbles occurs) this leads to offsets between different extraction techniques depending on their extraction efficiencies for the two ice types (Lüthi et al, 2010;Schaefer et al, 2011).…”