Antarctic ozone hole modifies iodine geochemistry on the Antarctic Plateau

Abstract

Polar stratospheric ozone has decreased since the 1970s due to anthropogenic emissions of chlorofluorocarbons and halons, resulting in the formation of an ozone hole over Antarctica. The effects of the ozone hole and the associated increase in incoming UV radiation on terrestrial and marine ecosystems are well established; however, the impact on geochemical cycles of ice photoactive elements, such as iodine, remains mostly unexplored. Here, we present the first iodine record from the inner Antarctic Plateau (Dome C) that covers approximately the last 212 years (1800-2012 CE). Our results show that the iodine concentration in ice remained constant during the pre-ozone hole period (1800-1974 CE) but has declined twofold since the onset of the ozone hole era (~1975 CE), closely tracking the total ozone evolution over Antarctica. Based on ice core observations, laboratory measurements and chemistry-climate model simulations, we propose that the iodine decrease since ~1975 is caused by enhanced iodine re-emission from snowpack due to the ozone hole-driven increase in UV radiation reaching the Antarctic Plateau. These findings suggest the potential for ice core iodine records from the inner Antarctic Plateau to be as an archive for past stratospheric ozone trends.

Fig. 1. Evolution of iodine concentration and stratospheric ozone over Dome C.

Iodine concentration (grey line, raw data; bold black line, 10 year smoothing) and [I] anomalies, expressed as ng g⁻¹ (blue, positive; red, negative; solid lines with respect to the mean iodine concentration for the period of 1800–2012), were compared with the mean total ozone column above Dome C during the sunlit (1 September to 28 February) period of each year (red dots, TOC^DC in Dobson units). The top panel shows the modelled mean sunlit Actinic Flux at 300 nm (purple square, AF³⁰⁰, quanta cm⁻² s⁻¹ nm⁻¹) at the Antarctic surface within the closest model grid point to Dome C. The decrease in iodine concentration coincides with the decrease in TOC^DC and mirrors the increase in modelled AF³⁰⁰. The pink vertical bar shows the tipping point determined for [I] (1975 CE), which is considered to be the starting date for the ozone hole period.

Fig. 2. Iodine photochemical enhancement during the ozone hole period.

Modelled actinic flux (AF) and iodine photoactivation efficiency (J-iodine) at the snow surface in Dome C. a Wavelength-dependent AF reaching the model surface every decade since the 1960s (colour dots, left Y-axis). The right Y-axis shows the absorbance spectra of a frozen iodide solution in equilibrium with oxygen (I^-(O₂)) and air (I^-(air)) environments. b The normalized J-iodine of snow-trapped iodine between 1950 and 2010 relative to the mean J-iodine for the pre-1975 period (dashed-grey horizontal line in (b)). J-iodine wavelength integration was performed within the 280–313 nm band (see dashed-grey vertical lines in (a)) and continuously increased after 1975 due to the ozone hole-driven enhancement in UV radiation during the sunlit period of each year (i.e., from 1 September to 28 February).

Fig. 3. Iodine and sodium merged record from Dome C.

The upper panel shows the merged 1-year smooth records (see S3.5 in the SM for details) using snow pits and ice core data (iodine merged in black— I_M - and sodium merged in red—Na_M). Light grey and red lines show the raw data, and red and bold curves show the 5-year smoothing. The lower panel shows the iodine enrichment (I_enr= I/(Na*0.00000568, where 0.00000568 is the I/Na mass ratio in seawater), where the solid green line shows the 5-year smoothing. The dashed vertical line indicates the tipping point discontinuity at 1975 CE.