Extended Data Figure 7: Stability in the course of the measurement series, characterization of the amount dependency of the measurement system, and tests with bubble-free ice.

a, NOAA-3 standard gas measurements performed at the end of each measurement day. No significant drifts were observed in the course of the measurement series, and the standard deviations for δ¹⁵N and δ¹⁸O were respectively 0.14‰ and 0.32‰ (n = 31), as indicated by the grey areas (±1σ) around the means (dashed lines). b, NOAA-1 standard gas measurements resulting in similar peak areas to the preceding ice-sample measurement routinely performed throughout the measurement series. These measurements covering the full range of peak areas from ice samples did not reveal any significant amount dependency. The mean and standard deviation (dashed lines and grey areas) for δ¹⁵N were 6.24‰ ± 0.22‰ and those for δ¹⁸O were 44.18‰ ± 0.59‰ (±1σ, n = 58), in agreement with the expected values (solid lines). c, Measurements of different amounts of NOAA-1 standard gas which was stored in the extraction pots while pieces of bubble-free ice were grated. Dashed lines and grey areas indicate the means and standard deviations (±1σ, n = 10), and solid lines indicate the expected values. On the basis of these measurements with bubble-free ice and the results shown in b, N₂O, δ¹⁵N and δ¹⁸O were not corrected for amount dependency. However, during the extraction of standard gas over ice, a −1.38‰ offset was introduced in δ¹⁸O. Accordingly, all Taylor Glacier δ¹⁸O values were corrected by +1.38‰.