A 1000-year high precision record of δ¹³C 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.