Astronomically calibrated 40 Ar/ 39 Ar age for the Toba supereruption and global synchronization of late Quaternary records Michael Storey a,1 , Richard G. Roberts b , and Mokhtar Saidin c a Quaternary Dating Laboratory, Department of Environmental, Social and Spatial Change, Roskilde University, DK-4000 Roskilde, Denmark; b Centre for Archaeological Science, School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW 2522, Australia; and c Centre 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, was the largest terrestrial volcanic event of the Quaternary. Ash and sulfate aerosols were deposited in both hemispheres, forming a time-marker horizon that can be used to synchronize late Qua- ternary records globally. A precise numerical age for this event has proved elusive, with dating uncertainties larger than the millen- nial-scale climate cycles that characterized this period. We report an astronomically calibrated 40 Ar/ 39 Ar age of 73.88 ± 0.32 ka (1σ, full external errors) for sanidine crystals extracted from Toba deposits in the Lenggong Valley, Malaysia, 350 km from the erup- tion source and 6 km from an archaeological site with stone arti- facts buried by ash. If these artifacts were made by Homo sapiens, as has been suggested, then our age indicates that modern humans had reached Southeast Asia by ∼74 ka ago. Our 40 Ar/ 39 Ar age is an order-of-magnitude more precise than previous estimates, resolving the timing of the eruption to the middle of the cold interval be- tween Dansgaard–Oeschger events 20 and 19, when a peak in sul- fate concentration occurred as registered by Greenland ice cores. This peak is followed by a ∼10 °C drop in the Greenland surface temperature over ∼150 y, revealing the possible climatic impact of the eruption. Our 40 Ar/ 39 Ar age also provides a high-precision cali- bration point for other ice, marine, and terrestrial archives contain- ing Toba sulfates and ash, facilitating their global synchronization at unprecedented resolution for a critical period in Earth and hu- man history beyond the range of 14 C dating. geochronology | ice core timescale | paleoclimate | volcanic ash | human dispersal T he late Quaternary is a period highlighted by recurrent cli- matic variations on millennial to decadal timescales [known as Dansgaard–Oeschger (D-O) events in the Greenland ice core record (1)], the evolution of anatomically modern humans in Africa and their subsequent dispersal worldwide, and a range of biotic extinctions and extirpations. The largest volcanic event of the last 2 million years—the Toba supereruption in Sumatra— also occurred during this period (2, 3), about 74 thousand years (ka) ago. The impact of this event on climate, ecosystems, and human evolution remains the subject of ongoing debate (3–8), partly because the precise age of the Toba eruption is poorly constrained (Table 1). Layers of volcanic ash and codeposited sulfate aerosols in pri- mary context represent widespread, geologically instantaneous time markers that can be used to synchronize sedimentary and ice-core records separated by hundreds to thousands of kilo- meters. The immense magnitude of the 74-ka Toba eruption and the location of the caldera vent close to the equator makes cor- relation of volcanic ash (Youngest Toba tuff, YTT) and aerosols feasible on a global scale. YTT ash has been recorded in the South China Sea, the Arabian Gulf, and southern Indian Ocean (Fig. 1), and the distribution of sulfate aerosols injected into the atmosphere by this eruption may have been global. A prominent sulfate spike, attributed to the Toba eruption but not accompanied by YTT ash, has been recorded in Greenland ice cores between the D-O 20 and 19 interstadial warming events (9, 10). This sulfate record recently has been found in the Antarctic European Project for Ice Coring in Antarctica (EPICA) Dronning Maud Land (EDML) ice core (11), enabling bipolar synchroniza- tion of D-O events 20 and 19 and Antarctic Isotope Maxima (AIM) 20 and 19. In the high-resolution North Greenland Ice Core Project (NGRIP) ice core, the maximum sulfate anomaly is located at 2,548-m depth (10, 11) and is followed by a rapid 3.5‰ negative δ 18 O isotopic excursion in the depth interval between 2,548 and 2,547 m, corresponding to a ∼10 °C drop in the Greenland mean annual surface temperature in as little as 150 y. The cooling led to a particularly cold stadial between D-O events 20 and 19 that has been linked to the effects of the Toba erup- tion. However, the reported ages for YTT ash (Table 1) have uncertainties of several thousand years and lack the resolution to differentiate between these millennial-scale climate cycles, lim- iting progress on synchronizing late Quaternary records and assessing the possible impact of the eruption on climate and eco- system response in different regions. We carried out high-precision 40 Ar/ 39 Ar dating of sanidine crystals separated from a widespread volcanic ash in the Leng- gong Valley, Malaysia, which has been correlated to the eruption from the Toba caldera, located 350 km to the west (12). Com- pared with previous K-Ar, 40 Ar/ 39 Ar, and fission-track age esti- mates (13, 14), our newly determined, astronomically calibrated 40 Ar/ 39 Ar 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). This large improvement in precision allows tight correlation of the timing of the Toba eruption with the sulfate peaks in the Greenland and Antarctic ice cores and their relation to D-O and AIM events 20 and 19. Also, because the precision on this age is comparable to those obtained from high-resolution uranium- thorium (U-Th) dating of speleothems (15), the Toba eruption now can be placed on a precise radioisotopic timescale alongside climatic records from lower latitudes, facilitating global syn- chronization of regional records. Results and Discussion Volcanic ash is widespread in the Lenggong Valley and has been correlated to the 74-ka ignimbrite erupted from the Toba Author contributions: M. Storey, R.G.R., and M. Saidin designed research; M. Storey per- formed research; M. Storey and R.G.R. analyzed data; and M. Storey and R.G.R. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. Freely available online through the PNAS open access option. 1 To whom correspondence should be addressed. E-mail: [email protected]. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. 1073/pnas.1208178109/-/DCSupplemental. 18684–18688 | PNAS | November 13, 2012 | vol. 109 | no. 46 www.pnas.org/cgi/doi/10.1073/pnas.1208178109 Downloaded by guest on March 2, 2021
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Astronomically calibrated 40Ar/39Ar age for theToba supereruption and global synchronizationof late Quaternary recordsMichael 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 forArchaeological Science, School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW 2522, Australia; and cCentre forGlobal 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,was the largest terrestrial volcanic event of the Quaternary. Ashand sulfate aerosols were deposited in both hemispheres, forminga time-marker horizon that can be used to synchronize late Qua-ternary records globally. A precise numerical age for this event hasproved elusive, with dating uncertainties larger than the millen-nial-scale climate cycles that characterized this period. We reportan astronomically calibrated 40Ar/39Ar age of 73.88 ± 0.32 ka (1σ,full external errors) for sanidine crystals extracted from Tobadeposits in the Lenggong Valley, Malaysia, 350 km from the erup-tion source and 6 km from an archaeological site with stone arti-facts buried by ash. If these artifacts were made by Homo sapiens,as has been suggested, then our age indicates that modern humanshad reached Southeast Asia by ∼74 ka ago. Our 40Ar/39Ar age is anorder-of-magnitude more precise than previous estimates, resolvingthe timing of the eruption to the middle of the cold interval be-tween Dansgaard–Oeschger events 20 and 19, when a peak in sul-fate concentration occurred as registered by Greenland ice cores.This peak is followed by a ∼10 °C drop in the Greenland surfacetemperature over ∼150 y, revealing the possible climatic impact ofthe eruption. Our 40Ar/39Ar age also provides a high-precision cali-bration point for other ice, marine, and terrestrial archives contain-ing Toba sulfates and ash, facilitating their global synchronizationat unprecedented resolution for a critical period in Earth and hu-man history beyond the range of 14C dating.
The late Quaternary is a period highlighted by recurrent cli-matic variations on millennial to decadal timescales [known
as Dansgaard–Oeschger (D-O) events in the Greenland ice corerecord (1)], the evolution of anatomically modern humans inAfrica and their subsequent dispersal worldwide, and a range ofbiotic extinctions and extirpations. The largest volcanic event ofthe last 2 million years—the Toba supereruption in Sumatra—also occurred during this period (2, 3), about 74 thousand years(ka) ago. The impact of this event on climate, ecosystems, andhuman evolution remains the subject of ongoing debate (3–8),partly because the precise age of the Toba eruption is poorlyconstrained (Table 1).Layers of volcanic ash and codeposited sulfate aerosols in pri-
mary context represent widespread, geologically instantaneoustime markers that can be used to synchronize sedimentary andice-core records separated by hundreds to thousands of kilo-meters. The immense magnitude of the 74-ka Toba eruption andthe location of the caldera vent close to the equator makes cor-relation of volcanic ash (Youngest Toba tuff, YTT) and aerosolsfeasible on a global scale. YTT ash has been recorded in theSouth China Sea, the Arabian Gulf, and southern Indian Ocean(Fig. 1), and the distribution of sulfate aerosols injected into theatmosphere by this eruption may have been global.
A prominent sulfate spike, attributed to the Toba eruption butnot accompanied by YTT ash, has been recorded in Greenlandice cores between the D-O 20 and 19 interstadial warming events(9, 10). This sulfate record recently has been found in the AntarcticEuropean Project for Ice Coring in Antarctica (EPICA) DronningMaud Land (EDML) ice core (11), enabling bipolar synchroniza-tion of D-O events 20 and 19 and Antarctic Isotope Maxima(AIM) 20 and 19. In the high-resolution North Greenland IceCore Project (NGRIP) ice core, the maximum sulfate anomaly islocated at 2,548-m depth (10, 11) and is followed by a rapid 3.5‰negative δ18O isotopic excursion in the depth interval between2,548 and 2,547 m, corresponding to a ∼10 °C drop in theGreenland mean annual surface temperature in as little as 150 y.The cooling led to a particularly cold stadial between D-O events20 and 19 that has been linked to the effects of the Toba erup-tion. However, the reported ages for YTT ash (Table 1) haveuncertainties of several thousand years and lack the resolution todifferentiate between these millennial-scale climate cycles, lim-iting progress on synchronizing late Quaternary records andassessing the possible impact of the eruption on climate and eco-system response in different regions.We carried out high-precision 40Ar/39Ar dating of sanidine
crystals separated from a widespread volcanic ash in the Leng-gong Valley, Malaysia, which has been correlated to the eruptionfrom the Toba caldera, located 350 km to the west (12). Com-pared with previous K-Ar, 40Ar/39Ar, and fission-track age esti-mates (13, 14), our newly determined, astronomically calibrated40Ar/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).This large improvement in precision allows tight correlation ofthe timing of the Toba eruption with the sulfate peaks in theGreenland and Antarctic ice cores and their relation to D-O andAIM events 20 and 19. Also, because the precision on this age iscomparable to those obtained from high-resolution uranium-thorium (U-Th) dating of speleothems (15), the Toba eruptionnow can be placed on a precise radioisotopic timescale alongsideclimatic records from lower latitudes, facilitating global syn-chronization of regional records.
Results and DiscussionVolcanic ash is widespread in the Lenggong Valley and has beencorrelated to the 74-ka ignimbrite erupted from the Toba
Author contributions: M. Storey, R.G.R., and M. Saidin designed research; M. Storey per-formed research; M. Storey and R.G.R. analyzed data; and M. Storey and R.G.R. wrotethe paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Freely available online through the PNAS open access option.1To whom correspondence should be addressed. E-mail: [email protected].
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1208178109/-/DCSupplemental.
caldera, based on its biotite composition (12). At the archaeo-logical site of Kota Tampan (16), located on an ancient terraceof the Perak River, in situ ash occurs among and above stoneartifacts that may have been manufactured by anatomicallymodern humans (17). The ash is up to 5 m thick, irregularlydistributed, mostly very fine grained [<100 μm (12)], and notsuitable for 40Ar/39Ar dating. In 2011, however, drilling of a borehole 6 km north of Kota Tampan revealed a crystal-rich, coarserfacies to the ash, about 1.3 m thick and 5 m above the meta-morphic basement rocks (Fig. 1). Sanidine crystals up to 2 mm inlength were handpicked from a sample of this ash for 40Ar/39Ardating experiments and were analyzed as 45 separate single- andmultiple-grain aliquots using an NU Instruments Noblesse multi-collector noble-gas mass spectrometer. The enhanced performancecharacteristics of this instrument and the high signal-to-noise ratioof the ion-counting detectors lend themselves to increased accu-racy and precision of 40Ar/39Ar ages for late Quaternary samples.Details are given in Materials and Methods and in SI Text.The 45 aliquots gave model 40Ar/39Ar ages of between 67.0 and
167 ka, all but four of which fall in a narrow age range (71.1–78.7 ka), forming a Gaussian distribution in a probability plot(Fig. 2). The weighted mean of this main population is 74.0 ± 0.3 ka[1σ analytical errors, including neutron fluence monitor; meansquare of the weighted deviates (mswd) = 1.92, probability offit (prob.) = 0.0004, n = 41]. We applied an outlier-rejectionscheme to the main population to discard ages with normal-ized median absolute deviations of >1.5 (Dataset S1) (18),resulting in a weighted mean age of 73.88 ± 0.32 ka (1σ; mswd =0.95, prob. = 0.56, n = 36). An inverse isochron plot gives a sta-tistically identical age of 73.6 ± 0.5 ka (1σ; mswd = 1, prob. =
0.12, n = 39) (Fig. S1). The 40Ar/36Ar intercept of 300 ± 3 isstatistically indistinguishable from the atmospheric ratio of 298.6 ±0.3 (19), thus indicating limited influence from excess argon orxenocrysts in the aliquots and further supporting the weightedmean age result.These 40Ar/39Ar ages have been cross-calibrated against the
astronomically dated A1 tephra (A1T) from Crete [6.943 ±0.0025 (1σ) Ma (20)] using R-values previously determined onthe Roskilde Noblesse (Table S1), where R is 40Ar*/39ArK(sample)/40Ar*/39ArK (standard) (21, 22). Because the A1T ageis known independently of the K-Ar system, the 40Ar/39Ar age ofthe Toba sanidine crystals calculated relative to the A1T requiresknowledge only of the 40K total decay constant (equation 5 ofref. 22). As highlighted in ref. 23, this approach avoids incorpo-rating the uncertainty associated with the 40K branching ratio andalso is relatively insensitive to the chosen 40K total decay con-stant and its uncertainty of (5.464 ± 0.107) × 10−10 y−1 (24). In-cluding the latter, as well as the uncertainty on the astronomicalage of A1T [0.036% at 1σ (20)], does not increase the ± 0.32-kauncertainty on the weighted mean age of the Toba sanidinecrystals at two decimal places (Fig. 2).The high-resolution NGRIP ice core records ∼6 y of high
sulfate concentration with a spike at 2,548-m depth, near thestart of the prolonged coldest part of the stadial between D-O 20and 19 (Fig. 3). This sulfate anomaly has been attributed to theToba eruption (9–11), but the proposed correlation cannot beconfirmed because of the ± 5-ka uncertainty associated with theoriginal Greenland Ice Sheet Project 2 (GISP2) ice core modelage estimate of 71 ka (9) and the absence of microtephra (10).Some support for the Toba origin of the sulfate peak in the
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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 ofthe Lenggong borehole is the archaeological site at Kota Tampan (KT) located on an ancient terrace of the Perak River, where YTT ash occurs between andabove stone tools. (Inset) Star on the world map marks the location of the Toba caldera, and the dashed line encompasses presently known occurrences of YTTash. 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 the4.7–5.95 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.
Storey et al. PNAS | November 13, 2012 | vol. 109 | no. 46 | 18685
Greenland ice cores is given by the recovery of glass shards fromArabian Sea sediments; these shards have been geochemicallyassociated with the Toba eruption and were deposited close tothe Marine Isotope Stage 4/5 boundary (25).
High-precision U-Th ages of 75.92 ± 0.15 ka and 71.75 ± 0.11ka (1σ) for the D-O 20 and 19 warming events, respectively, havebeen obtained for speleothems from the northern rim of the Alps(NALPS), a region that shares a dominant Atlantic influence andcommon oxygen isotopic signal with Greenland (15). These agesare systematically younger by about 400 to 600 y than thosederived 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 sulfatepeak at 2,548-m depth, relative to the D-O 20 and 19 warmingevents, is resolved to ± 50 y using the GICC05modelext time-scale. When combined with the NALPS ages for D-O 20 and 19,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 agefor the Toba eruption, so these events cannot have been sepa-rated by more than a few centuries. This correlation affirms theoriginal hypothesis that the large sulfate spikes in the Greenlandice cores between D-O 20 and 19 are most likely related to theToba eruption (9).Recent identification of an identical pattern of sulfate spikes
in the Antarctic EDML core (11) now permits comparison of ice-core records from the two poles to elucidate leads and lags in theclimate system and to test the hypothesis of a bipolar see-saw(27) contemporaneous with D-O and AIM events 20 and 19. Ourprecise 40Ar/39Ar age for the Toba eruption provides a well-constrained, radioisotopic-based calibration point for geologicalarchives that lie beyond the range of 14C dating and containtraces of Toba ash or sulfates in primary depositional context,thus facilitating the synchronization of ice, marine, and terres-trial records of past environments.Our 40Ar/39Ar age also has implications for the evolution and
dispersal of Homo sapiens. The 74-ka Toba eruption has beenvariously implicated or exonerated in causing human populationbottlenecks in Africa, mammal extirpations in Southeast Asia, andenvironmental changes in India (4–8), where stone tools attributedto anatomically modern humans are buried by YTT ash (4).Stone tools also are buried within and below YTT ash at KotaTampan (16, 17) and are considered the handiwork of H. sapiens(17). If confirmed, then this evidence would support genetic esti-mates (28) for the first wave of dispersals of modern humans outof Africa and into Asia before 74 ka and argue against the initial
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Fig. 2. Plot of laser fusion 40Ar/39Ar ages for the YTT sanidine crystals; thevertical scale is a relative probability measure of a given age occurring in thesample (31). Outliers 2308–20 and 2308–36 are off scale (full data are listedin Dataset S1), and other outliers (as defined in the text) are shown as opencircles. ACs was used as the neutron fluence monitor and as an intermediateinternal standard. The Toba results can be cross-calibrated against the as-tronomically dated A1 tephra (A1T) from Crete (20) using R-values based onRoskilde Noblesse data: RFCs A1Ts = 4.0813 ± 0.0013 (20) and RACs FCs = 0.04182± 0.00007 [Roskilde aliquot of 2008 EARTHTIME intercalibration experiment(32)]. The latter value is numerically identical to but slightly more precisethan the recently published value of 0.04182 ± 0.00009 (33), which also useda Noblesse mass spectrometer. The identical R-values from these two studiesyield an age for ACs of 1.1869 Ma relative to A1Ts. This ACs age is compa-rable to a recently published result (34) but is younger than the precise ageestimate reported in ref. 35. A weighted mean of the filtered YTT sanidinedata (n = 36/45) gives RYTT
A1Ts = 0.010621 ± 0.000046 (1σ), which translatesto an astronomically calibrated 40Ar/39Ar age of 73.88 ± 0.32 ka for the Tobaeruption using the algorithms of ref. 23. Including the uncertainties on theastronomical age of A1T and the 40K total decay constant does not furtherincrease the YTT age uncertainty at two decimal places.
Table 1. Age estimates for the Toba eruption, D-O warming events 19 and 20, and the sulfate peak at 2,548-mdepth in the Greenland NGRIP ice core
73.76 ± 0.16jj U-Th (D-O 19 warming) + ΔT** This study73.61 ± 0.20jj 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.jjAssuming 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.
18686 | www.pnas.org/cgi/doi/10.1073/pnas.1208178109 Storey et al.
exit occurring only after the eruption. As in India, the absence ofassociated human fossils at Kota Tampan precludes a definitiveverdict (7), and it also is possible that the Kota Tampan artifactswere manufactured by Denisovans or modern humans who hadexchanged genes with Denisovans in Southeast Asia (28, 29). Ifthe descendants of these toolmakers survived the Toba super-eruption, they could have spread eastwards during the following2 millennia of cooler climate and lowered sea level, taking ad-vantage of the newly exposed continental shelf and land bridges.
Materials and MethodsAngular but occasionally idiomorphic sanidine crystals of up to ∼2 mm inlength, identified using a Bruker Tornado micro-XRF spectrometer, wereextracted from the >500-μm fraction of the P3/D4 ash sample from bore holeBH1-2011 (Fig. 1). The sanidine crystals were hand picked and then ultra-sonically leached in cold 10% (vol/vol) hydrofluoric acid for ∼1 min to
remove adhering volcanic glass, followed by ultrasonic rinsing in deionizedwater. Approximately 80 mg of glass-clear sanidine crystals were loaded intoa single well of a seven-well, 18-mm–diameter aluminum sample disk for40Ar/39Ar dating (Fig. S2). Alder Creek sanidine (ACs), acting as the neutronfluence monitor and as an intermediate internal standard, was loaded intofour evenly spaced wells on the sample disk, which then was wrapped inaluminum foil and encapsulated in a heat-sealed quartz tube. Fast neutronirradiation was carried out in the General Atomics TRIGA reactor using theCadmium-Lined In-Core Irradiation Tube (CLICIT) facility at Oregon StateUniversity for 0.25 h on December 5, 2011. Argon isotopic analyses of thegas released by laser fusion of single- and multiple-grain sanidine aliquots(Datasets S1 and S2) were made on a fully automated, high-resolution NuInstruments Noblesse multi-collector noble-gas mass spectrometer (Nu Instru-ments) at Roskilde University, using previously documented instrumentationand procedures (20, 30) that are summarized here.
Before fusion, crystals were gently degassed of loosely adhering argon byheating 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-ped with a −130 °C cold trap, to remove H2O, and two water-cooled SAESGetters GP-50 pumps to absorb reactive gases. Analyses of unknowns,blanks, and monitor minerals were carried out in identical fashion during afixed period of 400 s in 14 data-acquisition cycles, in which 40Ar and 39Arwere measured on the high-mass ion counter (HiIC), 38Ar and 37Ar on theaxial ion counter (AxIC), and 36Ar on the low-mass ion counter (LoIC), withbaselines measured every third cycle. Measurement of the 40Ar, 38Ar, and36Ar ion beams was carried out simultaneously, followed by sequentialmeasurement of 39Ar and 37Ar. Beam switching was achieved by varying thefield of the mass spectrometer magnet and with minor adjustment of thequad lenses. All signals incorporate a small to insignificant correction fordetector deadtime using the following deadtime constants: HiIC, 27 ns; AxIC,37 ns; LoIC, 24 ns. The data collection and reduction were carried out usingthe program “Mass Spec” (by A. Deino, Berkeley Geochronology Center,Berkeley, CA). Individual fusion analyses were bracketed by one or multipleblank analyses. The precision on the blank measurement for the low-abundance isotope 36Ar is better than ±0.5% (1σ). In comparison with sin-gle-collector peak-switching measurements, multicollection allows moredata to be gathered in a fixed time, but for accurate and reproducible agedeterminations the method requires that the relative efficiencies of thedifferent detectors be well known. As previously described for the RoskildeNoblesse (20, 30), by reference to the atmospheric argon isotopic composi-tion (19), correction factors that combine detector efficiencies (detectorintercalibration) and mass fractionation into single terms are based on themeasurement of a time series of measured atmospheric argon aliquotsdelivered from a calibrated air pipette using the following detector con-figurations: (40Ar/36Ar)HiIC/LoIC, (
40Ar/38Ar)HiIC/AxIC, and (40Ar/36Ar)HiIC/AxIC (Fig.S3). Decay and other constants, including correction factors for interferenceisotopes produced by nucleogenic reactions, are given in Table S1.
ACKNOWLEDGMENTS.We thank Saiful Shahidan and Shyeh Sahibul Karamahfor help during fieldwork; Paul Renne for supplying monitor mineral ACs-2;Matt Heizler for organizing the 2008 EARTHTIME intercalibration experi-ment; and Kim Mogensen for assisting in the drafting of Fig. 1. We thankTiffany Rivera and other members of the GTSnext Marie Curie Initial Train-ing Network (funded by the European Community’s Seventh FrameworkProgramme) for valuable input. The Quaternary Dating Laboratory at Ros-kilde University is funded by the Villum Foundation. R.G.R. is supported bythe Australian Research Council and M. Saidin by Apex University grants,Universiti Sains Malaysia.
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a)
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D-O 20
-35-40-45
NGRIP
NALPS (U-Th)
sulfate peak
sulfate peak
(GICC05modelext )
2548 m
2535 m
2578 m
Fig. 3. (Left) NALPS U-Th ages for the D-O 20 and 19 warming events (15).The estimated age of the 2,548-m sulfate spike is based on the U-Th agesand the GICC05modelext timescale ages of the sulfate spike relative to theD-O 20 and D-O 19 events (Table 1). The uncertainty on this age is estimatedat ±50 y based on the close correspondence in the U-Th and GICC05modelextestimates of the duration of time between the D-O 20 and 19 events. Notethe correspondence between the age of the sulfate spike at 2,548-m depthand the 40Ar/39Ar YTT age, which is shown with full external uncertainties at1σ. (Right) NGRIP δ18O data for the D-O 20 and 19 interval on the GICC05-modelext timescale (26); note the systematic offset to higher ages com-pared with the more precise U-Th ages for these warming events. The sulfateanomaly at 2,548-m depth is followed by a negative shift in δ18O of upto ∼3.5‰ in about 150 years. Based on a combination of two independentpaleothermometry methods, this interval corresponds to a ∼10 °C drop inthe Greenland mean annual surface temperature (36). Three key depth–agereference horizons are labeled on the right-hand axis.
Storey et al. PNAS | November 13, 2012 | vol. 109 | no. 46 | 18687
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18688 | www.pnas.org/cgi/doi/10.1073/pnas.1208178109 Storey et al.
