A 1000-year high precision record of delta13C in atmospheric CO2

Abstract

We present measurements of the stable carbon isotope ratio in air extracted from Antarctic ice core and firn samples. The same samples were previously used by Etheridge and co‐workers to construct a high precision 1000‐year record of atmospheric CO2 concentration, featuring a close link between the ice and modern records and high‐time resolution. Here, we start by confirming the trend in the Cape Grim in situ δ13C record from 1982 to 1996, and extend it back to 1978 using the Cape Grim Air Archive. The firn air δ13C agrees with the Cape Grim record, but only after correction for gravitational separation at depth, for diffusion effects associated with disequilibrium between the atmosphere and firm, and allowance for a latidudinal gradient in δ13C between Cape Grim and the Antarctic coast. Complex calibration strategies are required to cope with several additional systematic influences on the ice core δ13C record. Errors are assigned to each ice core value to reflect statistical and systematic biases (between ± 0.025‰ and ± 0.07‰); uncertainties (of up to ± 0.05‰) between core‐versus‐core, ice‐versus‐firn and firn‐versus‐troposphere are described separately. An almost continuous atmospheric history of δ13C over 1000 years results, exhibiting significant decadal‐to‐century scale variability unlike that from earlier proxy records. The decrease in δ13C from 1860 to 1960 involves a series of steps confirming enhanced sensitivity of δ13C to decadal timescale‐forcing, compared to the CO2 record. Synchronous with a ‘‘Little Ice Age’′ CO2 decrease, an enhancement of δ13C implies a terrestrial response to cooler temperatures. Between 1200 AD and 1600 AD, the atmospheric δ13C appear stable.

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

“…A novel method that employs sublimation to release the occluded air followed by GC-IRMS measurement techniques has made significant improvement to the precision obtained from small samples (0.05-0.09 ‰) . The highest precision measurements (0.025-0.05 ‰ ) to date were obtained from the Law Dome ice core spanning the last millennium using a large-volume ice grater and dual-inlet technique (Francey et al, 1999). However, this record was recently augmented and revised to account for a significant shift in the mean values of many of the measurements.…”

“…Dual-inlet mass spectrometry measurements thus require an accurate estimate of the N 2 O-to-CO 2 ratio in the ion beam. Previous methods to derive the N 2 O-to-CO 2 ratio include a lowprecision mass-spectrometric method that was susceptible to experimental in situ N 2 O production in the extraction apparatus , offline measurements on an aliquot of the same sample air (Francey et al, 1999), and an interpolation of separate data sets to the depths of the samples used for isotopic measurements . We utilize a method that measures the 14 N 16 O fragment produced in the mass spectrometer source to determine the abundance and ionization efficiency of N 2 O (Assonov and Brenninkmeijer, 2006).…”

20th century changes in carbon isotopes and water-use efficiency: tree-ring-based evaluation of the CLM4.5 and LPX-Bern models

“…unequivocal testimony to the input of isotopically light fossil and terrestrial carbon by human activities (Keeling et al, 1979;Francey et al, 1999). The δ 13 C data representing atmospheric air are used to quantify the global ocean and land carbon sources and sinks (Keeling et al, 1989;Joos and Bruno, 1998;Trudinger et al, 2002;Bauska et al, 2015).…”

Pleistocene to recent dietary shifts in California condors

“…Carbon isotope values were corrected for the global decrease in the 13 C content of atmospheric carbon dioxide, due largely to fossil fuel burning over the last 150 years (16)(17)(18). Based on ice core records (16), we applied a time-dependent correction of Ϫ0.005‰ per year between 1860 and 1960 and Ϫ0.022‰ per year since 1960, for a total correction of Ϫ1.2‰ for Pleistocene samples (17) and Ϫ1.5‰ for Holocene samples (18) (see Table 2, which is published as supporting information on the PNAS web site).…”