Tephrochronology of the Toba tuffs: four primary glass populations define the 75‐ka Youngest Toba Tuff, northern Sumatra, Indonesia (original) (raw)
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Geochemical variability in distal and proximal glass from the Youngest Toba Tuff eruption
The Youngest Toba Tuff (YTT, erupted at c. 74 ka) is a distinctive and widespread tephra marker across South and Southeast Asia. The climatic, human and environmental consequences of the YTT eruption are widely debated.Although a considerable body of geochemical data is available for this unit, there has not been a systematic study of the variability of the ash geochemistry. Intrinsic (magmatic) and extrinsic (post-depositional) chemical variations bring fundamental information regarding the petrogenesis of the magma, the distribution of the tephra and the interaction between the ash and the receiving environment. Considering the importance of the geochemistry of the YTT for stratigraphic correlations and eruptive models, it is central to the YTT debate to quantify and interpret such variations. Here, we collate all published geochemical data on the YTT glass, including analyses from 67 sites described in the literature and three new samples. Two principal sources of chemical variation are investigated: (i) compositional zonation of the magma reservoir and (ii) post-depositional alteration. Post-depositional leaching is responsible for up to ca. 11 % differences in Na2O/K2O and ca. 1 % differences in SiO2/Al2O3 ratios in YTT glass from marine sites. Continental tephra are 2 % higher in Na2O/K2O and 3 % higher in SiO2/Al2O3 with respect to the marine tephra. We interpret such post-depositional glass alteration as related to seawater-induced alkali migration in marine environments. Crystal fractionation and consequential magmatic differentiation, which produced order-of-magnitude variations in trace element concentrations reported in the literature, also produced major element differences in the YTT glass. FeO/Al2O3 ratios vary by about 50 %, which is analytically significant. These variations represent magmatic fractionation involving Fe-bearing phases. We also compared major element concentrations in YTT and Oldest Toba Tuff (OTT) ash samples, to identify potential compositional differences that could constrain the stratigraphic identity of the Morgaon ash (western India); no differences between the OTT and YTT samples were observed.
Uncertainty over the identity and age of Toba tephras across peninsular India persists, with radiometric age dates contradicting earlier compositional data, which have been used to identify this important stratigraphic marker as the Youngest Toba Tuff (YTT). To address this issue, new single glass shard analyses have been performed for samples from Morgaon and Bori (north-western India), which have recently been dated at c. 800 ka. These, and indeed all Toba tephra samples thus far analysed from India, show the presence of four populations of glass shards (defined by their Ba/Y ratio), which uniquely identifies them as products of the c. 75-ka Youngest Toba eruption. Confirmation that the YTT fingerprint is characteristic comes from new analyses of Oldest Toba Tuff (OTT) glass shards from five sites in the Indian Ocean. These are compositionally identical to Layer D from the ODP site 758 sediment core (c. 800 ka), and belong to a single, low-Ba population, clearly different from YTT. These analyses show that there is essentially no reworked OTT material in the YTT eruption, and indicate unequivocally that all known Toba tephra occurrences in India belong to the c. 75-ka Youngest Toba eruption.
Journal of Quaternary Science
The Youngest Toba Tuff contains five distinct glass populations, identified from Ba, Sr and Y compositions, termed PI (lowest Ba)-PV (highest Ba), representing five compositionally distinct pre-eruptive magma batches that fed the eruption. The PI-PV compositions display systematic changes, with higher FeO, CaO, MgO, TiO 2 and lower incompatible element concentrations in the low-SiO 2 PIV/PV, than the high-SiO 2 PI-PIII compositions. Glass shard abundances indicate PIV and PV were the least voluminous magma batches, and PI and PIII the most voluminous. Pressure estimates using rhyolite-MELTS indicate PV magma equilibrated at~6 km, and PI magma at~3.8 km. Glass population proportions in distal tephra and proximal (caldera-wall) material describe an eruption which commenced by emptying the deepest PIV and PV reservoirs, this being preferentially deposited in a narrow band across southern India (possibly due to jet-stream and/or plinian eruption transport), and as abundant pumice clasts in the lowermost proximal ignimbrites. Later, shallower magma reservoirs erupted, with PI being the most abundant as the eruption ended, sourcing the majority of distal ash from co-ignimbrite clouds (PI-and PIII-dominant), where associated ignimbrites isolated earlier (PIV-and PV-rich) deposits. This study shows how analysis of tephra glass compositional data can yield pre-eruption magma volume estimates, and enable aspects of magma storage conditions and eruption dynamics to be described.
Determination of the area density of spontaneous fission tracks (ρs) in glass shards of Toba tephra is a reliableway to distinguish between the Youngest Toba Tuff (YTT) and the Oldest Toba Tuff (OTT). The ρs values for YTT, uncorrected for partial track fading, range from 70 to 181 tracks/cm2 with aweightedmean of 108±5 tracks/cm2, based on 15 samples. Corrected ρs values for YTT are in the range of 77–140 tracks/cm2 with a weighted mean of 113 ± 8 tracks/cm2, within the range of uncorrected ρs values. No significant difference in ρs exists between YTT samples collected from marine and continental depositional settings. The uncorrected ρs for OTT is 1567 tracks/cm2 so that confusion with YTT is unlikely. The ρs values of the Toba tephra at Bori,Morgaon, and Gandhigramin northwestern India indicate a YTT identity, in agreementwith geochemical data on their glass shards, the presence of multiple glass populations, and a glass fission-track age determination. Therefore, the view of others that OTT is present at these sites – and thereby indicates a Lower Pleistocene age for the associated Acheulean artifacts – is incorrect.
Correlation of the oldest Toba Tuff to sediments in the central Indian Ocean Basin
Journal of Earth System Science, 2010
We have identified an ash layer in association with Australasian microtektites of ∼ 0.77 Ma old in two sediment cores which are ∼ 450 km apart in the central Indian Ocean Basin (CIOB). Morphology and chemical composition of glass shards and associated microtektites have been used to trace their provenance. In ODP site 758 from Ninetyeast Ridge, ash layer-D (13 cm thick, 0.73-0.75 Ma) and layer-E (5 cm thick, 0.77-0.78 Ma) were previously correlated to the oldest Toba Tuff (OTT) eruptions of the Toba caldera, Sumatra. In this investigation, we found tephra ∼ 3100 km to the southwest of Toba caldera that is chemically identical to layer D of ODP site 758 and ash in the South China Sea correlated to the OTT. Layer E is not present in the CIOB or other ocean basins. The occurrence of tephra correlating to layer D suggests a widespread distribution of OTT tephra (∼ 3.6 × 10 7 km 2), an ash volume of at least ∼ 1800 km 3 , a total OTT volume of 2300 km 3 , and classification of the OTT eruption as a super-eruption.
Earth and Planetary Science Letters, 2004
Paroxysmal pyroclastic flow deposits sourced from Maninjau caldera in west-central Sumatra are dated at 50±3 ka (n=3) using the isothermal plateau and diameter corrected fission-track (ITPFT and DCFT, respectively) techniques on glass shard constituents. In addition, charcoal obtained from tall trees in position of growth within the paroxysmal flow deposit on the upper flanks for the caldera are also dated at 52.3±2 14C ka (n=2) and 51.1±3.2 14C ka (n=1) using an acid–base, wet oxidation, stepped combustion (ABOX-SC) and standard acid–base–acid (ABA) 14C techniques, respectively. The close correspondence in 14C ages of charcoal sample splits analysed at two laboratories (Australian National University, Australia and Waikato University, New Zealand) verifies the reliability of these 14C techniques up to at least 50 ka.Based on concordant ages derived from glass-FT and 14C techniques, an age of 52±3 ka is assigned to the latest silicic eruptive activity at Maninjau caldera. This chronology is further confirmed by the occurrence of a silicic tephra bed that closely underlies paroxysmal Maninjau deposits at two sections and is correlated with Youngest (75 ka) Toba Tephra (YTT) erupted from Toba caldera in north-central Sumatra. This study not only provides a much needed regional chronological reference point for Quaternary deposits in west-central Sumatra but also extends the minimum age range of the glass-FT technique from 75 ka down to c. 50 ka that is now for the first time within the extended maximum age range of the 14C technique.
Journal of Shoulder and Elbow Surgery, 2006
An ash layer newly discovered in Core MD01-2393 from the southwestern South China Sea has been studied in order to characterize its major element features. The layer, 4.0-cm thick, light grayish, and silt size, occurs right at the Marine Isotope Stage 4–5 transition, ca. 74 kyr ago. The morphology and geochemistry of glass shards, combined with oxygen isotope and carbonate stratigraphy, confirm the youngest Toba eruption in northern Sumatra as the origin of the ash layer. Major element data on mineral crystals (i.e. biotite, plagioclase, and hornblende) from the ash layer suggest that biotite is phenocrystic while hornblende and some plagioclase are xenocrystic, implying that these xenocrysts were incorporated into the youngest Toba magma before the eruption.
New occurrence of Youngest Toba Tuff in abyssal sediments of the Central Indian Basin
Marine Geology, 1999
Volcanic glass and pumice found in siliceous abyssal sediments of the Central Indian Basin, south of the Equator, have previously been assigned various origins including intra-basin volcanism, Indonesian arc, and Krakatau. Rhyolitic glass shards dispersed in sediments from 8 cores that we have examined are compositionally identical to those of fallout deposits of the Youngest Toba Tuff erupted at 74 ka from northern Sumatra. The correlation extends the distribution of coarse (>63 µm) glass shards some 1500 km south of the previously known fallout zone, and into the Southern Hemisphere (reaching latitudes of ca. 14ºS). This provides evidence for bi-hemispheric dispersal of the ash cloud and supports global dispersal of gas and aerosols from the eruption. Such dispersal could have facilitated the global impact of the eruption.
Marine records reveal multiple phases of Toba's last volcanic activity
Scientific Reports, 2023
The Indonesian Young Toba Tuff (YTT), classically dated around 74 ka BP, is considered as a shortlived explosive cataclysmic super-eruption. The huge amounts of ash and SO 2 emitted are likely to have triggered a volcanic winter which accelerated the transition to the last glaciation, and may have induced a human genetic bottleneck. However, the global climatic impact of the YTT or its duration are hotly debated. The present work offers a new interpretation of the Toba volcanic complex eruptive history. Analysing the BAR94-25 marine core proximal to the Toba volcanic center and combining it with high-resolution tephrostratigraphy and δ 18 O stratigraphy, we show that the Toba complex produced a volcanic succession that consists of at least 17 distinct layers of tephra and cryptotephra. Textural and geochemical analyses show that the tephra layers can be divided in 3 main successive volcanic activity phases (VAP1 to VAP3) over a period of ~ 50 kyr. The main volcanic activity phase, VAP2, including the YTT, is likely composed of 6 eruptive events in an interval whose total duration is ~ 10 ka. Thus, we suggest that the eruptive model of the Toba volcano must be revised as the duration of the Toba volcanic activity was much longer than suggested by previous studies. The implications of re-estimating the emission rate and the dispersion of ashes and SO 2 include global environmental reconstitutions, climate change modelling and possibly human migration and evolution.