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Ar Age for the Toba Supereruption and Global Synchronization of Late Quaternary Records

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Ar Age for the Toba Supereruption and Global Synchronization of Late Quaternary Records Astronomically calibrated 40Ar/39Ar age for the Toba supereruption and global synchronization of late Quaternary records Michael Storeya,1, Richard G. Robertsb, and Mokhtar Saidinc aQuaternary Dating Laboratory, Department of Environmental, Social and Spatial Change, Roskilde University, DK-4000 Roskilde, Denmark; bCentre for Archaeological Science, School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW 2522, Australia; and cCentre for Global Archaeological Research, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia Edited by Thure E. Cerling, University of Utah, Salt Lake City, UT, and approved September 24, 2012 (received for review May 16, 2012) The Toba supereruption in Sumatra, ∼74 thousand years (ka) ago, A prominent sulfate spike, attributed to the Toba eruption but was the largest terrestrial volcanic event of the Quaternary. Ash not accompanied by YTT ash, has been recorded in Greenland and sulfate aerosols were deposited in both hemispheres, forming ice cores between the D-O 20 and 19 interstadial warming events a time-marker horizon that can be used to synchronize late Qua- (9, 10). This sulfate record recently has been found in the Antarctic ternary records globally. A precise numerical age for this event has European Project for Ice Coring in Antarctica (EPICA) Dronning proved elusive, with dating uncertainties larger than the millen- Maud Land (EDML) ice core (11), enabling bipolar synchroniza- nial-scale climate cycles that characterized this period. We report tion of D-O events 20 and 19 and Antarctic Isotope Maxima an astronomically calibrated 40Ar/39Ar age of 73.88 ± 0.32 ka (1σ, (AIM) 20 and 19. In the high-resolution North Greenland Ice full external errors) for sanidine crystals extracted from Toba Core Project (NGRIP) ice core, the maximum sulfate anomaly is deposits in the Lenggong Valley, Malaysia, 350 km from the erup- located at 2,548-m depth (10, 11) and is followed by a rapid 3.5‰ tion source and 6 km from an archaeological site with stone arti- negative δ18O isotopic excursion in the depth interval between facts buried by ash. If these artifacts were made by Homo sapiens, 2,548 and 2,547 m, corresponding to a ∼10 °C drop in the as has been suggested, then our age indicates that modern humans Greenland mean annual surface temperature in as little as 150 y. 40 39 had reached Southeast Asia by ∼74 ka ago. Our Ar/ Ar age is an EARTH, ATMOSPHERIC, The cooling led to a particularly cold stadial between D-O events AND PLANETARY SCIENCES order-of-magnitude more precise than previous estimates, resolving 20 and 19 that has been linked to the effects of the Toba erup- the timing of the eruption to the middle of the cold interval be- tion. However, the reported ages for YTT ash (Table 1) have – tween Dansgaard Oeschger events 20 and 19, when a peak in sul- uncertainties of several thousand years and lack the resolution to fate concentration occurred as registered by Greenland ice cores. differentiate between these millennial-scale climate cycles, lim- ∼ This peak is followed by a 10 °C drop in the Greenland surface iting progress on synchronizing late Quaternary records and ∼ temperature over 150 y, revealing the possible climatic impact of assessing the possible impact of the eruption on climate and eco- the eruption. Our 40Ar/39Ar age also provides a high-precision cali- system response in different regions. ANTHROPOLOGY bration point for other ice, marine, and terrestrial archives contain- We carried out high-precision 40Ar/39Ar dating of sanidine ing Toba sulfates and ash, facilitating their global synchronization crystals separated from a widespread volcanic ash in the Leng- at unprecedented resolution for a critical period in Earth and hu- gong Valley, Malaysia, which has been correlated to the eruption man history beyond the range of 14C dating. from the Toba caldera, located 350 km to the west (12). Com- pared with previous K-Ar, 40Ar/39Ar, and fission-track age esti- geochronology | ice core timescale | paleoclimate | volcanic ash | mates (13, 14), our newly determined, astronomically calibrated human dispersal 40Ar/39Ar age for the YTT ash of 73.88 ± 0.32 ka (1σ, full ex- ternal errors) is an order-of-magnitude more precise (Table 1). he late Quaternary is a period highlighted by recurrent cli- This large improvement in precision allows tight correlation of Tmatic variations on millennial to decadal timescales [known the timing of the Toba eruption with the sulfate peaks in the as Dansgaard–Oeschger (D-O) events in the Greenland ice core Greenland and Antarctic ice cores and their relation to D-O and record (1)], the evolution of anatomically modern humans in AIM events 20 and 19. Also, because the precision on this age is Africa and their subsequent dispersal worldwide, and a range of comparable to those obtained from high-resolution uranium- biotic extinctions and extirpations. The largest volcanic event of thorium (U-Th) dating of speleothems (15), the Toba eruption the last 2 million years—the Toba supereruption in Sumatra— now can be placed on a precise radioisotopic timescale alongside also occurred during this period (2, 3), about 74 thousand years climatic records from lower latitudes, facilitating global syn- (ka) ago. The impact of this event on climate, ecosystems, and chronization of regional records. – human evolution remains the subject of ongoing debate (3 8), Results and Discussion partly because the precise age of the Toba eruption is poorly constrained (Table 1). Volcanic ash is widespread in the Lenggong Valley and has been Layers of volcanic ash and codeposited sulfate aerosols in pri- correlated to the 74-ka ignimbrite erupted from the Toba mary context represent widespread, geologically instantaneous time markers that can be used to synchronize sedimentary and Author contributions: M. Storey, R.G.R., and M. Saidin designed research; M. Storey per- ice-core records separated by hundreds to thousands of kilo- formed research; M. Storey and R.G.R. analyzed data; and M. Storey and R.G.R. wrote meters. The immense magnitude of the 74-ka Toba eruption and the paper. the location of the caldera vent close to the equator makes cor- The authors declare no conflict of interest. relation of volcanic ash (Youngest Toba tuff, YTT) and aerosols This article is a PNAS Direct Submission. feasible on a global scale. YTT ash has been recorded in the Freely available online through the PNAS open access option. South China Sea, the Arabian Gulf, and southern Indian Ocean 1To whom correspondence should be addressed. E-mail: [email protected]. (Fig. 1), and the distribution of sulfate aerosols injected into the This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. atmosphere by this eruption may have been global. 1073/pnas.1208178109/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1208178109 PNAS Early Edition | 1of5 Downloaded by guest on October 6, 2021 BH1-2011 Lenggong NGG2 Sand 1 2 EDML 3 Sandy clay 4 LP 5 KT Toba P3/D4 Strait of Malacca ash Perak River 6 Depth (m) Silty 7 clay 8 Silty Lake Toba 9 gravel 10 11 km Hornfels 0 100 200 Fig. 1. (Left) Location of the Toba caldera, north Sumatra, and Lenggong, Perak (LP), Malaysia, site of the BH1-2011 borehole. Six kilometers to the south of the Lenggong borehole is the archaeological site at Kota Tampan (KT) located on an ancient terrace of the Perak River, where YTT ash occurs between and above stone tools. (Inset) Star on the world map marks the location of the Toba caldera, and the dashed line encompasses presently known occurrences of YTT ash. Sulfate aerosols correlated to the YTT eruption show a global distribution, occurring in both the Greenland NGRIP (NG) and GISP2 (G2) ice cores (9–11) and the Antarctic EDML ice core (11). (Right) Simplified log of the upper part of the BH1-2011 bore hole. Spot sampling (standard penetration test) in the 4.7–5.95 m depth interval recovered a crystal-rich, coarser facies to the ash 1.5 m above gravel sediments that, in turn, rest on metamorphic basement rocks. The star marks the position of the sample (P3/D4) dated in this study. caldera, based on its biotite composition (12). At the archaeo- 0.12, n = 39) (Fig. S1). The 40Ar/36Ar intercept of 300 ± 3is logical site of Kota Tampan (16), located on an ancient terrace statistically indistinguishable from the atmospheric ratio of 298.6 ± of the Perak River, in situ ash occurs among and above stone 0.3 (19), thus indicating limited influence from excess argon or artifacts that may have been manufactured by anatomically xenocrysts in the aliquots and further supporting the weighted modern humans (17). The ash is up to 5 m thick, irregularly mean age result. distributed, mostly very fine grained [<100 μm (12)], and not These 40Ar/39Ar ages have been cross-calibrated against the suitable for 40Ar/39Ar dating. In 2011, however, drilling of a bore astronomically dated A1 tephra (A1T) from Crete [6.943 ± hole 6 km north of Kota Tampan revealed a crystal-rich, coarser 0.0025 (1σ)Ma(20)]usingR-values previously determined on 40 39 facies to the ash, about 1.3 m thick and 5 m above the meta- the Roskilde Noblesse (Table S1), where R is Ar*/ ArK 40 39 morphic basement rocks (Fig. 1). Sanidine crystals up to 2 mm in (sample)/ Ar*/ ArK (standard) (21, 22). Because the A1T age length were handpicked from a sample of this ash for 40Ar/39Ar is known independently of the K-Ar system, the 40Ar/39Ar age of dating experiments and were analyzed as 45 separate single- and the Toba sanidine crystals calculated relative to the A1T requires multiple-grain aliquots using an NU Instruments Noblesse multi- knowledge only of the 40K total decay constant (equation 5 of collector noble-gas mass spectrometer.
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