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

Abstract

Abstract. An important constraint on mechanisms of past carbon cycle variability is provided by the stable isotopic composition of carbon in atmospheric carbon dioxide (δ 13 C-CO 2 ) trapped in polar ice cores, but obtaining very precise measurements has proven to be a significant analytical challenge. Here we describe a new technique to determine the δ 13 C of CO 2 at very high precision, as well as measuring the CO 2 and N 2 O mixing ratios. In this method, ancient air is extracted from relatively large ice samples (∼ 400 g) with a dryextraction "ice grater" device. The liberated air is cryogenically purified to a CO 2 and N 2 O mixture and analyzed with a microvolume-equipped dual-inlet IRMS (Thermo MAT 253). The reproducibility of the method, based on replicate analysis of ice core samples, is 0.02 ‰ for δ 13 C-CO 2 and 2 ppm and 4 ppb for the CO 2 and N 2 O mixing ratios, respectively (1σ pooled standard deviation). Our experiments show that minimizing water vapor pressure in the extraction vessel by housing the grating apparatus in a ultralow-temperature freezer (−60 • C) improves the precision and decreases the experimental blank of the method to −0.07 ± 0.04 ‰. We describe techniques for accurate calibration of small samples and the application of a mass-spectrometric method based on source fragmentation for reconstructing the N 2 O history of the atmosphere. The oxygen isotopic composition of CO 2 is also investigated, confirming previous observations of oxygen exchange between gaseous CO 2 and solid H 2 O within the ice archive. These data offer a possible constraint on oxygen isotopic fractionation during H 2 O and CO 2 exchange below the H 2 O bulk melting temperature.

A new set-up for simultaneous high-precision measurements of CO<sub>2</sub>, <i>δ</i><sup>13</sup>C-CO<sub>2</sub> and <i>δ</i><sup>18</sup>O-CO<sub>2</sub> on small ice core samples

“…This is beneficiary because the resulting lower water partial pressure in the extraction chamber (about 5-fold) reduces the risk of in situ CO 2 production by wet chemistry. This is supported by the findings of Bauska et al (2014) which indicated that operating at low temperatures improves precision and decreases the blank of the method. For our design this is reflected in a reduced system offset compared to the KUP NC (Sect.…”

“…In order to avoid the liquid phase of water, the air must either be extracted in a cooled vacuum chamber by dry mechanical techniques (e.g. Bauska et al, 2014) or by sublimation of the ice matrix (e.g. Schmitt et al, 2011).…”

“…Interference from drilling fluid contamination can potentially be a problem for certain samples. The rather large sample size allows measurement of N 2 O in addition (Bauska et al, 2014). OSU also operates a NC system for the analysis of CO 2 only (Ahn et al, 2009).…”

Spatial pattern of accumulation at Taylor Dome during Marine Isotope Stage 4: stratigraphic constraints from Taylor Glacier

“…1) across a synclineanticline pair containing ice spanning from ∼ 50 ka before present (BP) to the mid Holocene (7 ka) . Ice stratigraphy in the main transect dips approximately vertically so that it is possible to obtain large quantities of ice of the same age by drilling vertical or near-vertical ice cores (e.g., Baggenstos et al, 2017;Petrenko et al, 2017Petrenko et al, , 2016Schilt et al, 2014;Bauska et al, 2016Bauska et al, , 2018. Ice containing the full MIS 5-4 transition was formerly considered to be missing from the glacier (Baggenstos, 2015;Baggenstos et al, 2017), but we show here that a new ice core near the main transect contains an intact record with ice dating from 76.5-60.6 ka and air dating from 74.0-57.7 ka.…”

Spatial pattern of accumulation at Taylor Dome during the last glacial inception: stratigraphic constraints from Taylor Glacier

“…spectrometer (Maselli et al, 2013), and CH 4 using a Picarro laser spectrometer coupled to the melt stream similar to the system used in the field (Rhodes et al, 2013 was measured using an Agilent gas chromatograph equipped with a flame ionization detector coupled to a custom melt-refreeze extraction system (Mitchell et al, 2011), and CO 2 was measured on an Agilent gas chromatograph equipped with a Ni catalyst and a flame ionization detector coupled to a custom dry 5 extraction "cheese grater" system for carbon isotopic analyses (Bauska et al, 2014). Stable isotopes of N 2…”