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Abstract

The clumped-isotope composition of oxygen, denoted by Δ₃₆, measures the proportional abundance of two heavy oxygen isotopes, ¹⁸O¹⁸O in oxygen, and is sensitive to photochemical and thermal properties of the atmosphere. Ozone photochemistry controls Δ₃₆ values via isotope exchange reactions, yielding a higher abundance of ¹⁸O¹⁸O than would be expected for a stochastic distribution. Colder temperatures increase this preference for ¹⁸O¹⁸O formation at equilibrium by 0.013‰/K. If tropospheric ozone and stratosphere-troposphere exchange fluxes remain unchanged, Δ₃₆ would record changes in free-tropospheric temperatures to provide a high-resolution record of upper tropospheric (UT) temperatures on glacial-interglacial timescales. We present an ice core record of Δ₃₆ values measured in the West Antarctic Ice Sheet Divide ice core spanning 15–8ka BP. Measured Δ₃₆ values decrease to Early Holocene (EH; 8-11ka BP)/Pre-industrial (PI; 1850CE) levels at the onset of the Bølling Allerød (BA; 14.7–12.9ka BP) warm period, and continues into the Younger Dryas (YD; 12.9-11.7ka BP). Δ₃₆ values reach PI/EH when global-mean surface temperatures are still colder than those of PI/EH. This indicates either a change in the lapse rate feedback, or a significant increase in tropospheric ozone burden. Our investigations reveal no evidence for increased O₃ and that the decrease in Δ₃₆ is coeval with rapid decreases in Northern Hemisphere ice sheet elevation. Previous work highlights the influence of changing ice sheet elevation on the thermal structure of the atmosphere. A plausible explanation for our observation is that during periods of abrupt climate change the cryosphere plays a predominant role in governing UT temperatures.