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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, ; 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 (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 and sulfate aerosols were deposited in both hemispheres, forming cores between the D-O 20 and 19 interstadial warming events a -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 (EPICA) Dronning proved elusive, with dating uncertainties larger than the millen- Maud Land (EDML) ice core (11), enabling bipolar synchroniza- nial-scale cycles that characterized this period. We report tion of D-O events 20 and 19 and Antarctic 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 , 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 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 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- | 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 (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 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 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 (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. The enhanced performance ref. 22). As highlighted in ref. 23, this approach avoids incorpo- characteristics of this instrument and the high signal-to-noise ratio rating the uncertainty associated with the 40K branching ratio and of the -counting detectors lend themselves to increased accu- also is relatively insensitive to the chosen 40K total decay con- − − racy and precision of 40Ar/39Ar ages for late Quaternary samples. stant and its uncertainty of (5.464 ± 0.107) × 10 10 y 1 (24). In- Details are given in Materials and Methods and in SI Text. cluding the latter, as well as the uncertainty on the astronomical The 45 aliquots gave model 40Ar/39Ar ages of between 67.0 and age of A1T [0.036% at 1σ (20)], does not increase the ± 0.32-ka 167 ka, all but four of which fall in a narrow age range (71.1– uncertainty on the weighted mean age of the Toba sanidine 78.7 ka), forming a Gaussian distribution in a probability plot crystals at two decimal places (Fig. 2). (Fig. 2). The weighted mean of this main population is 74.0 ± 0.3 ka The high-resolution NGRIP ice core records ∼6 y of high [1σ analytical errors, including neutron fluence monitor; mean sulfate concentration with a spike at 2,548-m depth, near the square of the weighted deviates (mswd) = 1.92, probability of start of the prolonged coldest part of the stadial between D-O 20 fit(prob.)= 0.0004, n = 41]. We applied an outlier-rejection and 19 (Fig. 3). This sulfate anomaly has been attributed to the scheme to the main population to discard ages with normal- Toba eruption (9–11), but the proposed correlation cannot be ized median absolute deviations of >1.5 (Dataset S1)(18), confirmed because of the ± 5-ka uncertainty associated with the resulting in a weighted mean age of 73.88 ± 0.32 ka (1σ; mswd = original Greenland Project 2 (GISP2) ice core model 0.95, prob. = 0.56, n = 36). An inverse isochron plot gives a sta- age estimate of 71 ka (9) and the absence of microtephra (10). tistically identical age of 73.6 ± 0.5 ka (1σ; mswd = 1, prob. = Some support for the Toba origin of the sulfate peak in the

2of5 | www.pnas.org/cgi/doi/10.1073/pnas.1208178109 Storey et al. Downloaded by guest on October 6, 2021 High-precision U-Th ages of 75.92 ± 0.15 ka and 71.75 ± 0.11 10 ka (1σ) for the D-O 20 and 19 warming events, respectively, have been obtained for speleothems from the northern rim of the fl 8 (NALPS), a region that shares a dominant in uence and common isotopic signal with Greenland (15). These ages are systematically younger by about 400 to 600 y than those 6 derived for these D-O events using the NGRIP GICC05mode- lext timescale (26), but the latter age estimates are an order-of- magnitude less precise (10). The timing of the NGRIP sulfate 4 peak at 2,548-m depth, relative to the D-O 20 and 19 warming events, is resolved to ± 50 y using the GICC05modelext time- scale. When combined with the NALPS ages for D-O 20 and 19, Relative probability 2 the age of the sulfate peak in the NGRIP core can be con- strained to 73.7 ka, with a 1σ uncertainty ± 0.2 ka or better (Table 1). This age is within the uncertainty of our 40Ar/39Ar age 66 70 74 78 82 for the Toba eruption, so these events cannot have been sepa- fi Age (ka) rated by more than a few centuries. This correlation af rms the original hypothesis that the large sulfate spikes in the Greenland Fig. 2. Plot of laser fusion 40Ar/39Ar ages for the YTT sanidine crystals; the ice cores between D-O 20 and 19 are most likely related to the vertical scale is a relative probability measure of a given age occurring in the Toba eruption (9). sample (31). Outliers 2308–20 and 2308–36 are off scale (full data are listed Recent identification of an identical pattern of sulfate spikes in Dataset S1), and other outliers (as defined in the text) are shown as open in the Antarctic EDML core (11) now permits comparison of ice- fl circles. ACs was used as the neutron uence monitor and as an intermediate core records from the two poles to elucidate and lags in the internal standard. The Toba results can be cross-calibrated against the as- tronomically dated A1 tephra (A1T) from Crete (20) using R-values based on climate system and to test the hypothesis of a bipolar see-saw Roskilde Noblesse data: RFCs = 4.0813 ± 0.0013 (20) and RACs = 0.04182 (27) contemporaneous with D-O and AIM events 20 and 19. Our A1Ts FCs 40 39 ± 0.00007 [Roskilde aliquot of 2008 EARTHTIME intercalibration experiment precise Ar/ Ar age for the Toba eruption provides a well- (32)]. The latter value is numerically identical to but slightly more precise constrained, radioisotopic-based calibration point for geological ± 14 than the recently published value of 0.04182 0.00009 (33), which also used archives that lie beyond the range of C dating and contain EARTH, ATMOSPHERIC, a Noblesse mass spectrometer. The identical R-values from these two studies traces of Toba ash or sulfates in primary depositional context, AND PLANETARY SCIENCES yield an age for ACs of 1.1869 Ma relative to A1Ts. This ACs age is compa- thus facilitating the synchronization of ice, marine, and terres- rable to a recently published result (34) but is younger than the precise age trial records of past environments. estimate reported in ref. 35. A weighted mean of the filtered YTT sanidine 40 39 = YTT = ± σ Our Ar/ Ar age also has implications for the evolution and data (n 36/45) gives R A1Ts 0.010621 0.000046 (1 ), which translates Homo sapiens to an astronomically calibrated 40Ar/39Ar age of 73.88 ± 0.32 ka for the Toba dispersal of . The 74-ka Toba eruption has been eruption using the algorithms of ref. 23. Including the uncertainties on the variously implicated or exonerated in causing human population astronomical age of A1T and the 40K total decay constant does not further bottlenecks in Africa, mammal extirpations in Southeast Asia, and increase the YTT age uncertainty at two decimal places. environmental changes in India (4–8), where stone tools attributed ANTHROPOLOGY to anatomically modern humans are buried by YTT ash (4). Stone tools also are buried within and below YTT ash at Kota Greenland ice cores is given by the recovery of glass shards from Tampan (16, 17) and are considered the handiwork of H. sapiens Arabian Sea sediments; these shards have been geochemically (17). If confirmed, then this evidence would support genetic esti- associated with the Toba eruption and were deposited close to mates (28) for the first wave of dispersals of modern humans out the Marine Isotope 4/5 boundary (25). of Africa and into Asia before 74 ka and argue against the initial

Table 1. Age estimates for the Toba eruption, D-O warming events 19 and 20, and the sulfate peak at 2,548-m depth in the Greenland NGRIP ice core Event Age ± 1σ (ka) Dating method Source

Toba eruption 75 ± 12 K-Ar (biotite) (14) 74 ± 3 K-Ar (sanidine) (14) 73 ± 4 40Ar/39Ar (sanidine) (13) 68 ± 7 Fission track (glass) (13) 73.88 ± 0.32 40Ar/39Ar (sanidine) This study D-O 19 warming 72.1 ± 1.7* Age read from GICC05modelext timescale (26) This study 71.74 ± 0.11†,‡ U-Th (stalagmite) northern Alps (NALPS) (15) D-O 20 warming 76.5 ± 1.7§ Age read from GICC05modelext timescale (26) This study ‡ { 75.91 ± 0.15 , U-Th (stalagmite) NALPS (15) NGRIP 2,548.0 m sulfate peak 74.2 ± 1.7 GICC05modelext timescale (10) 73.76 ± 0.16jj U-Th (D-O 19 warming) + ΔT** This study jj †† 73.61 ± 0.20 U-Th (D-O 20 warming) - ΔT This study

*D-O 19 defined here by δ18O isotopic maximum at 2,533.22-m depth in NGRIP ice core to allow direct comparison with NALPS data. †δ18O isotopic maximum, NALPS. ‡ U-Th ages from ref. 15 are adjusted here from b1950 to b2k values to align with the GICC05modelext timescale (26). §Onset of rapid warming marked by δ18O shift at 2,579.2-m depth, NGRIP. { Onset of rapid warming, NALPS. jj Assuming an error of ± 50 y on ΔT. **ΔT = GICC05modelext sulfate peak age − GICC05modelext DO-19 age. ††ΔT = GICC05modelext DO-20 age − GICC05modelext sulfate peak age.

Storey et al. PNAS Early Edition | 3of5 Downloaded by guest on October 6, 2021 NALPS (U-Th) remove adhering volcanic glass, followed by ultrasonic rinsing in deionized water. Approximately 80 mg of glass-clear sanidine crystals were loaded into NGRIP (GICC05modelext ) D-O 19 a single well of a seven-well, 18-mm–diameter aluminum sample disk for 72 D-O 19 40 39 2535 m Ar/ Ar dating (Fig. S2). Alder Creek sanidine (ACs), acting as the neutron fluence monitor and as an intermediate internal standard, was loaded into four evenly spaced wells on the sample disk, which then was wrapped in 73 aluminum foil and encapsulated in a heat-sealed quartz tube. Fast neutron sulfate peak irradiation was carried out in the General Atomics TRIGA reactor using the -Lined In-Core Irradiation Tube (CLICIT) facility at Oregon State sulfate peak University for 0.25 h on December 5, 2011. Argon isotopic analyses of the 74 2548 m gas released by laser fusion of single- and multiple-grain sanidine aliquots Age (ka) Toba 40Ar/ 39Ar age (Datasets S1 and S2) were made on a fully automated, high-resolution Nu Instruments Noblesse multi-collector noble-gas mass spectrometer (Nu Instru- 75 ments) at Roskilde University, using previously documented instrumentation and procedures (20, 30) that are summarized here. Before fusion, crystals were gently degassed of loosely adhering argon by D-O 20 76 heating with a defocused low-power beam (0.3 W) from a 50-W Synrad CO2 laser. Sample gas cleanup was through an all-metal extraction line, equip- D-O 20 2578 m ped with a −130 °C cold trap, to remove H2O, and two water-cooled SAES -45 -40 -35 Getters GP-50 pumps to absorb reactive gases. Analyses of unknowns, blanks, and monitor minerals were carried out in identical fashion during a fixed period of 400 s in 14 data-acquisition cycles, in which 40Ar and 39Ar 38 37 Fig. 3. (Left) NALPS U-Th ages for the D-O 20 and 19 warming events (15). were measured on the high-mass ion counter (HiIC), Ar and Ar on the 36 The estimated age of the 2,548-m sulfate spike is based on the U-Th ages axial ion counter (AxIC), and Ar on the low-mass ion counter (LoIC), with 40 38 and the GICC05modelext timescale ages of the sulfate spike relative to the baselines measured every third cycle. Measurement of the Ar, Ar, and 36 D-O 20 and D-O 19 events (Table 1). The uncertainty on this age is estimated Ar ion beams was carried out simultaneously, followed by sequential 39 37 at ±50 y based on the close correspondence in the U-Th and GICC05modelext measurement of Ar and Ar. Beam switching was achieved by varying the estimates of the duration of time between the D-O 20 and 19 events. Note field of the mass spectrometer magnet and with minor adjustment of the the correspondence between the age of the sulfate spike at 2,548-m depth quad lenses. All signals incorporate a small to insignificant correction for and the 40Ar/39Ar YTT age, which is shown with full external uncertainties at detector deadtime using the following deadtime constants: HiIC, 27 ns; AxIC, 1σ.(Right) NGRIP δ18O data for the D-O 20 and 19 interval on the GICC05- 37 ns; LoIC, 24 ns. The data collection and reduction were carried out using modelext timescale (26); note the systematic offset to higher ages com- the program “Mass Spec” (by A. Deino, Berkeley Geochronology Center, pared with the more precise U-Th ages for these warming events. The sulfate Berkeley, CA). Individual fusion analyses were bracketed by one or multiple anomaly at 2,548-m depth is followed by a negative shift in δ18Oofup blank analyses. The precision on the blank measurement for the low- to ∼3.5‰ in about 150 years. Based on a combination of two independent abundance isotope 36Ar is better than ±0.5% (1σ). In comparison with sin- paleothermometry methods, this interval corresponds to a ∼10 °C drop in gle-collector peak-switching measurements, multicollection allows more the Greenland mean annual surface temperature (36). Three key depth–age data to be gathered in a fixed time, but for accurate and reproducible age reference horizons are labeled on the right-hand axis. determinations the method requires that the relative efficiencies of the different detectors be well known. As previously described for the Roskilde Noblesse (20, 30), by reference to the atmospheric argon isotopic composi- exit occurring only after the eruption. As in India, the absence of tion (19), correction factors that combine detector efficiencies (detector associated human fossils at Kota Tampan precludes a definitive intercalibration) and mass fractionation into single terms are based on the verdict (7), and it also is possible that the Kota Tampan artifacts measurement of a time series of measured atmospheric argon aliquots were manufactured by Denisovans or modern humans who had delivered from a calibrated air pipette using the following detector con- fi 40 36 40 38 40 36 exchanged genes with Denisovans in Southeast Asia (28, 29). If gurations: ( Ar/ Ar)HiIC/LoIC,( Ar/ Ar)HiIC/AxIC, and ( Ar/ Ar)HiIC/AxIC (Fig. S3). Decay and other constants, including correction factors for interference the descendants of these toolmakers survived the Toba super- produced by nucleogenic reactions, are given in Table S1. eruption, they could have spread eastwards during the following 2 millennia of cooler climate and lowered sea level, taking ad- ACKNOWLEDGMENTS. We thank Saiful Shahidan and Shyeh Sahibul Karamah vantage of the newly exposed continental shelf and land bridges. for help during fieldwork; Paul Renne for supplying monitor mineral ACs-2; Matt Heizler for organizing the 2008 EARTHTIME intercalibration experi- Materials and Methods ment; and Kim Mogensen for assisting in the drafting of Fig. 1. We thank ∼ Tiffany Rivera and other members of the GTSnext Marie Curie Initial Train- Angular but occasionally idiomorphic sanidine crystals of up to 2mmin ’ fi ing Network (funded by the European Community s Seventh Framework length, identi ed using a Bruker Tornado micro-XRF spectrometer, were Programme) for valuable input. The Quaternary Dating Laboratory at Ros- > μ extracted from the 500- m fraction of the P3/D4 ash sample from bore hole kilde University is funded by the Villum Foundation. R.G.R. is supported by BH1-2011 (Fig. 1). The sanidine crystals were hand picked and then ultra- the Australian Research Council and M. Saidin by Apex University grants, sonically leached in cold 10% (vol/vol) hydrofluoric acid for ∼1 min to Universiti Sains Malaysia.

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