Significance Cold and dry glacial-state climate conditions persisted in the Southern Hemisphere until approximately 17.7 ka, when paleoclimate records show a largely unexplained sharp, nearly synchronous acceleration in deglaciation. Detailed measurements in Antarctic ice cores document exactly at that time a unique, ∼192-y series of massive halogen-rich volcanic eruptions geochemically attributed to Mount Takahe in West Antarctica. Rather than a coincidence, we postulate that halogen-catalyzed stratospheric ozone depletion over Antarctica triggered large-scale atmospheric circulation and hydroclimate changes similar to the modern Antarctic ozone hole, explaining the synchronicity and abruptness of accelerated Southern Hemisphere deglaciation.

Abstract Glacial-state greenhouse gas concentrations and Southern Hemisphere climate conditions persisted until ∼17.7 ka, when a nearly synchronous acceleration in deglaciation was recorded in paleoclimate proxies in large parts of the Southern Hemisphere, with many changes ascribed to a sudden poleward shift in the Southern Hemisphere westerlies and subsequent climate impacts. We used high-resolution chemical measurements in the West Antarctic Ice Sheet Divide, Byrd, and other ice cores to document a unique, ∼192-y series of halogen-rich volcanic eruptions exactly at the start of accelerated deglaciation, with tephra identifying the nearby Mount Takahe volcano as the source. Extensive fallout from these massive eruptions has been found >2,800 km from Mount Takahe. Sulfur isotope anomalies and marked decreases in ice core bromine consistent with increased surface UV radiation indicate that the eruptions led to stratospheric ozone depletion. Rather than a highly improbable coincidence, circulation and climate changes extending from the Antarctic Peninsula to the subtropics—similar to those associated with modern stratospheric ozone depletion over Antarctica—plausibly link the Mount Takahe eruptions to the onset of accelerated Southern Hemisphere deglaciation ∼17.7 ka.

Long-term variations in global climate, such as the glacial/interglacial cycles recorded in paleoarchives, are linked to changes in Earth’s orbital parameters and insolation (1). Superimposed on this smooth, orbital-scale variability are abrupt changes in climate, resulting in substantial variations among glacial terminations (2) and suggesting that the evolution of each deglaciation may be influenced by climate drivers specific to that deglaciation (3). One such rapid change during the last termination began ∼17.7 ky before 1950 (17.7 ka), when paleoclimate records show sharp, nearly synchronous changes across the Southern Hemisphere (SH) such as a widespread retreat of glaciers in Patagonia (4, 5) and New Zealand (6), onset of rapid lake expansion in the Bolivian Andes (7), increases in summertime precipitation in subtropical Brazil (8), decreases in southern Australian aridity (9), and dust deposition recorded in ocean sediment cores (9, 10) (Fig. 1). At the same time, Antarctic ice cores record a sharp decrease in SH continental dust (11), a widespread decline in sea salt deposition, a marked upturn in water isotopic ratios indicating warming (12, 13), and a trend of increasing atmospheric methane (CH 4 ) (12) and carbon dioxide (CO 2 ) (14, 15) coincident with a drop in the stable carbon isotopic ratios in CO 2 (14) (Fig. 1). Although the causes are not certain, many of these rapid changes have been attributed to a sudden poleward shift in the westerly winds encircling Antarctica, with resulting changes in SH hydroclimate, sea ice extent, ocean circulation (6, 7, 9), and ventilation of the deep Southern Ocean (16).

Fig. 1. Changes in climate indicators during the last glacial termination relative to the 17.7 ka glaciochemical anomaly. Shading shows the ∼192-y glaciochemical anomaly. (A) Annually integrated (12) 65°S insolation and (B) WD δ18O (12); (C) WD CO 2 (15) and (D) Taylor Glacier δ13C of CO 2 synchronized to the WD CO 2 record (46); (E) WD CH 4 (60) and (F) WD mineral acidity; SH dust proxies (G) nssCa in the WD core, (H) Ca in the European Project for Ice Coring in Antarctica Dome C (EDC) (11) synchronized to WD using volcanic events, and (I) Fe in a South Australian ocean sediment core (9); (J) the surface UV indicator Br in the WD core; and (K) Botuvera speleothem δ18O that is a proxy for summertime precipitation in southeastern Brazil (8).

The new very high time resolution West Antarctic Ice Sheet (WAIS) Divide (WD) (12) ice core record from West Antarctica (Materials and Methods) shows that, following a long period of relative stability extending glacial-state climate conditions [>10 ky after the 65°S annually integrated insolation minimum marking the Last Glacial Maximum (LGM)], sea salt and SH continental dust aerosol concentrations, snowfall rates, water isotope ratios, and greenhouse gas concentrations [CO 2 (15), CH 4 ] changed at 17.7 ka or soon after, sharply at first and then more gradually (Fig. 1). Concentrations of the traditional continental dust tracer non-sea-salt calcium (nssCa) dropped 100-fold, from ∼7 ng⋅g−1 during the LGM to ∼0.07 ng⋅g−1 during the early Holocene, with nearly 50% of the total decrease occurring during the 400 y after 17.7 ka. Sea-salt sodium concentration (ssNa), thought to be a proxy for sea ice formation, decreased fivefold during the deglaciation, with ∼40% of the decline occurring in this same period (12). About 25% of the ∼8‰ δ18O and 10% of the 320 parts-per-billion CH 4 overall increases from LGM to the early Holocene values happened during these 400 y (12) (Fig. 1).

Measurements of nssCa and other SH dust proxies in the WD core (Figs. 1 and 2) (SI Appendix, Fig. S1)—complemented by new measurements (Dataset S1) in archived samples of the Byrd core (17) located 159 km from WD (Fig. 3) (SI Appendix, Fig. S2)—indicate that the abrupt climate change started in central West Antarctica ∼17.7 ka, with the sharp and sustained decline in SH continental dust deposition (Fig. 1) nearly synchronous (SI Appendix, Continuous Ice Core Measurements) with the start of marked increases in CH 4 and CO 2 (Fig. 2). Starting ∼60 (±18) y before the abrupt drop in dust and extending ∼132 y after was a unique, long-lived glaciochemical anomaly originally detected in limited discrete measurements of acidity, chloride, and fluoride in the Byrd core (17) (SI Appendix, Fig. S3). New continuous measurements of a broad range of elements and chemical species in the WD and Byrd cores (SI Appendix, Figs. S1 and S2) show that the glaciochemical anomaly consisted of nine pulses measured between 2,426.97 m and 2,420.04 m depth in the WD core, corresponding to a ∼192-y period from 17.748 ka to 17.556 ka (Fig. 2) on the WD2014 timescale (18). Evidence of this glaciochemical anomaly also has been traced throughout West Antarctica and parts of East Antarctica in ice cores (17) (Fig. 3) (SI Appendix, Fig. S3) and radar surveys (19).

Fig. 2. Selected high-resolution elemental and gas phase measurements through the ∼192-y glaciochemical anomaly in the WD ice core at 17.7 ka (gray shading) showing nine distinct pulses. Acidity, low-boiling-point heavy metals (e.g., Bi), and halogens (e.g., Cl) other than Br (Fig. 1) were highly elevated throughout the anomaly (SI Appendix, Fig. S1), with REE (e.g., Ce) enhanced only during the first ∼120 y. SH dust indicators (e.g., nssCa) were elevated only slightly, and slowly increasing greenhouse gas [CH 4 (60), CO 2 (15)] concentrations accelerated during the event (Fig. 1). Measurements in the Byrd core are similar (SI Appendix, Fig. S2). Calculated break points (1σ uncertainty) suggest that long-term changes in nssCa, CH 4 , and CO 2 concentrations in the WD core began during the 17.7 ka anomaly (Materials and Methods).

Fig. 3. Spatial extent of the glaciochemical anomaly. Evidence of the ∼192-y anomaly has been found >2,800 km from Mount Takahe in ice core (circles) chemical records (SI Appendix, Fig. S3) as well as radar surveys from much of West Antarctica. Also shown are area volcanoes (triangles). September/October horizontal wind vectors at 600 hPa based on 1981–2010 National Centers for Environmental Prediction reanalysis fields show transport patterns consistent with observations.

Evidence for Stratospheric Ozone Depletion Evidence for ejection of volcanic material from the 17.7 ka Mount Takahe Event into the stratosphere and/or enhanced tropospheric UV radiation was found in sulfur isotope anomalies in the WD and Byrd cores (Fig. 4). Sulfur concentrations during the ∼192-y event were 1.7 times higher than background, and δ34S values were lower, indicating a volcanic source of the elevated sulfur, since background marine and volcanic sources have δ34S signatures of 15 to 21‰ and 0 to 5‰, respectively (30). Furthermore, exposure to UV radiation, such as when volcanic sulfur is ejected into the stratosphere above the ozone layer, generates distinct changes in sulfur isotope mass-independent fractionation (MIF; expressed as nonzero values of Δ33S). Previous studies have shown that MIF from a single eruption into the stratosphere follows a distinct evolution from positive to negative Δ33S during the course of sulfate deposition (31), so low-resolution sampling and multiple overlapping explosive events may result in small values of Δ33S even for stratospheric eruptions. The Δ33S of sulfate from before the 17.7 ka event was within 2σ uncertainty of zero, as expected for nonstratospheric sulfate (31, 32). The nonzero Δ33S measured in the ice during the 17.7 ka event indicated that the volcanic sulfur was indeed bombarded by enhanced UV radiation, either in the stratosphere as a result of ejection above the ozone layer and/or in the troposphere after significant stratospheric ozone depletion. The resolution of the WD and Byrd samples, the multiple explosive phases of the Mount Takahe eruptions, and the low concentration of volcanic sulfur above background levels, however, meant that the magnitude of the MIF anomaly was significant outside of 2σ only for a few samples (Materials and Methods). Evidence for stratospheric ozone depletion and enhanced near-surface UV radiation also comes from changes in bromine concentration in the WD and Byrd ice core records. Photochemical reactions in near-surface snow cause rapid cycling between the snow and air for a broad range of reactive and volatile chemical species, including bromine and nitrate, and a net loss in the snow through time, known as reversible deposition (33, 34). The magnitude of this loss primarily depends on the duration and intensity of exposure of the snow to UV radiation, with duration determined by the burial rate (SI Appendix, Evidence for Reversible Bromine Deposition in Antarctic Snow), since light penetration below ∼0.3 m in the snowpack is much reduced and exposure intensity is determined by the level of impinging UV radiation. During 27 ka to 6 ka in the WD record, there were only two sustained declines in bromine concentration, and the longest and most pronounced decline exactly coincided with the 17.7 ka Mount Takahe Event (Fig. 1). Annual layering in fine insoluble dust particle concentrations in the WD record indicates that the snowfall rate did not change during this period, eliminating burial rate variations as the cause of the bromine decline and implicating increased surface UV radiation (SI Appendix, Snowpack Modeling). While bromine release from the snowpack is sensitive to changes in UV radiation, and that sensitivity increases with snow acidity (33), photochemical model simulations as well as examination of the WD record show that acidity alone is not sufficient to explain the observed bromine depletion. First, bromine concentrations in the WD and Byrd cores remained low even during periods between the nine volcanic pulses of the 17.7 ka Mount Takahe Event when concentrations of nearly all elements and chemical species, including acidity, returned to near-background concentrations (SI Appendix, Figs. S1 and S2). WD snow accumulation was relatively high (>100 kg⋅m−2⋅y−1) during this period (18), so the regions of high acidity in the core were well separated from regions of low acidity. Second, consistent with modern observations in which some but not all volcanic eruptions are associated with ozone depletion (26), evaluation of the 100 highest acidity events (annual average concentration >3.0 µeq⋅L−1) in the 10 ka to 25 ka WD record shows that 30% were not associated with bromine depletion and that depletion was not proportional to acidity (SI Appendix, Fig. S6). This clearly demonstrates that acidity alone did not lead to bromine depletion and suggests that both increased acidity and increased UV radiation were required, as predicted by modeling (SI Appendix, Snowpack Modeling). It is unclear whether modern ozone depletion and enhanced surface UV have resulted in bromine depletion in near-surface snow. First, modern chlorofluorocarbon-driven ozone depletion largely has been confined to spring, while volcanically driven Mount Takahe ozone depletion may have persisted throughout the austral summer when the impact on snowpack photochemistry was greater. Second, there was no recent increase in acidity in Antarctic snow, so the sensitivity of bromine reemission to enhanced UV radiation was low. Third, no suitable Antarctic ice core record of bromine was available to assess the modern period (SI Appendix, Comparisons to Modern Ozone Depletion).

Conclusion Although the climate system already was primed for the switch from a glacial to interglacial state by insolation changes (1) and NH land ice loss (42), the ∼192-y ozone hole resulting from the halogen-rich eruptions of Mount Takahe plausibly provided supplementary forcing during the last termination that drove the westerly wind belt poleward and altered SH hydroclimate, providing a straightforward explanation for the synchronicity and abruptness of the SH climatic changes and global greenhouse gases that occurred ∼17.7 ka.

Acknowledgments We acknowledge R. von Glasow for help with snowpack model simulations, and J. Stutz and R. Kreidberg for helpful discussions. The US National Science Foundation supported this work [Grants 0538427, 0839093, and 1142166 (to J.R.M.); 1043518 (to E.J.B.); 0538657 and 1043421 (to J.P. Severinghaus); 0538553 and 0839066 (to J.C.-D.); and 0944348, 0944191, 0440817, 0440819, and 0230396 (to K.C.T.)]. We thank the WAIS Divide Science Coordination Office and other support organizations. P.K. and G.K. were funded by Polar Regions and Coasts in a Changing Earth System-II, with additional support from the Helmholtz Climate Initiative.

Footnotes Author contributions: J.R.M. designed research; J.R.M., A.B., N.W.D., P.K., J.L.T., M.M.A., N.J.C., O.J.M., M.S., J.F.A., D.B., J.F.B., E.J.B., J.C.-D., T.J.F., G.K., M.M.G., N.I., K.C.M., R.M., G.P., R.H.R., E.S.S., J.P. Severinghaus, J.P. Steffensen, K.C.T., and G.W. performed research; J.R.M. contributed new reagents/analytic tools; J.R.M., A.B., N.W.D., P.K., J.L.T., M.S., E.J.B., C.B., J.C.-D., G.K., H.-F.G., N.I., K.C.M., and G.W. analyzed data; and J.R.M., A.B., N.W.D., P.K., J.L.T., E.J.B., C.B., H.-F.G., and G.W. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Data deposition: The data reported in this work have been deposited with the U.S. Antarctic Program Data Center, www.usap-dc.org/view/dataset/601008.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1705595114/-/DCSupplemental.