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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.
