New occurrence of Youngest Toba Tuff in abyssal sediments of the Central Indian Basin (original) (raw)
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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.
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.
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.
Youngest Toba Tuff ashes in the Purna alluvial basin of central India have been encountered at various places and are represented by light-gray, yellowish-brown, grayish-orange-pink, loosely packed, fine-grained material. These are preserved as pockets, lenticles, and discontinuous beds of 0.5-3.0-m thickness in the arenoargillaceous sediments of fluvial environments. Based on color, thickness, and sedimentological features, this distal material is divisible into primary ash and reworked ash. Microscopically, the ashes consist predominantly of glass shards with bubble-wall morphologies, minor minerals, unidentifiable fine-grain admixture, and, rarely, channel sediments. The glass shards are colorless, transparent, angular, unfractured, and unaltered in nature. A total of six morphotypes have been identified: (1) uniradiate, or two-bubble-wall, shards, (2) Y-shaped, or triradiate, shards, (3) platy shards, (4) blocky shards, (5) shards with elongated vesicles, and (6) pumice shards. Facies architecture of various successions shows broadly low-energy fluviolacustrine depositional environments. Major-element analysis of the glass shards shows a high percentage of silica, followed by alumina, potassium , and sodium. In general, high percentages of major ions in the glass shards reflect rhyolitic magma composition. The minerals in the ash, though minor in quantity, are represented by quartz, feldspar, biotite, amphibole, pyroxene, olivine, allanites, and Fe-Ti oxides. Preservations of various paleoclimatic proxies are also recorded from pretephra successions and are represented by rhizoliths, rhizolith balls, rhizosphere, ant traces, and vertebrate remains.
Geochemical and Sedimentological Investigations of Youngest Toba Tuff ashfall deposits
PhD, 2013
The ~ 73 ka ‘super-eruption’ of the Toba caldera in Sumatra is the largest known eruption of the Quaternary. The products of this eruption, the Youngest Toba Tuff (YTT), have been implicated in global and regional climate deterioration with widespread ecological effects. In this thesis I study the YTT co-ignimbrite ashfall, in particular the mechanisms of transport, sedimentation and preservation of ash deposits. I use distal marine and terrestrial ash sediments: a) to estimate the volume of YTT ash fallout; b) to quantify variability in the geochemistry of YTT ash; c) to assess the reliability of YTT ash as a chronostratigraphic marker; and d) to determine local influences on the reworking of YTT ash deposits. Following the introductory chapters, I address topics a) and b) through detailed investigations of published physical and chemical evidence. Chapter three shows that particle size and sediment thickness do not decline exponentially with distance from the eruption vent, highlighting the limitations of current methods of volume estimation for co-ignimbrite super-eruptions. Chapter four analyses geochemical variation in 72 YTT samples, and reveals the signatures of magma chamber zonation and post-depositional alteration. I address topics c) and d) through fieldwork in six locations, and detailed analysis of ash samples from a wide variety of local depositional environments. Chapter five uses high-resolution stratigraphic analysis of the YTT layer in the Son Valley, India, to show that variable deposition and sediment reworking may compromise the reliability of the ash layer as an isochronous marker for interpreting archaeological sequences. Chapter six combines a new understanding of the mechanisms of reworking, using new data on microscopic characteristics of reworked ash at four sites in Malaysia to demonstrate the necessity of accounting for reworking in palaeoenvironmental reconstructions. I conclude that accurate analyses of distal ash deposits can reliably determine the chemical properties of the YTT eruption, and that a detailed understanding of deposition and reworking processes is essential to inference of the environmental impacts of super-eruptions.
Newly discovered eastern dispersal of the youngest Toba Tuff
Marine Geology, 2000
Volcanic glasses with minor mafic mineral fragments, such as biotite and hornblende, found in deep-sea sediments of the South China Sea Basin (SCSB) have been clearly identified as eruptive products of the Youngest Toba Tuff (YTT), northern Sumatra, Indonesia. The tephra layer occurs between marine oxygen isotopic event 5.1 (79.3 ka) and event 4.22 (64.1 ka), with an interpolated age of 74.0 ka, which is in good consistence with previous radiometric dating (73-75 ka) and ice-core dating (71^5 ka) of the YTT. The tephra consists predominantly of bubble-wall shards with minor elongated vesicles of pumice fragments. Geochemical characteristics of the tephra, such as high total alkali content, high 87 Sr/ 86 Sr ratio and uniformity of their compositions, all suggest that the recovered tephra is of the Youngest Toba Tuff. This finding supports an extended dispersal of coarse (Ͼ63 mm) glass shards over 1500 km northeast of the Toba caldera, a direction opposite to what previously conceived. While providing a better documentation of the distribution extent of the Toba ash, this report points to the need to reestimating the eruptive volume of the YTT and re-evaluating its environmental impact. ᭧
Environmental impact of the 73 ka Toba super-eruption in South Asia
Palaeogeography, …, 2009
The cooling effects of historic volcanic eruptions on world climate are well known but the impacts of even bigger prehistoric eruptions are still shrouded in mystery. The eruption of Toba volcano in northern Sumatra some 73,000 years ago was the largest explosive eruption of the past two million years, with a Volcanic Explosivity Index of magnitude 8, but its impact on climate has been controversial. In order to resolve this issue, we have analysed pollen from a marine core in the Bay of Bengal with stratified Toba ash, and the carbon isotopic composition of soil carbonates directly above and below the ash in three sites on a 400 km transect across central India. Pollen evidence shows that the eruption was followed by initial cooling and prolonged desiccation, reflected in a decline in tree cover in India and the adjacent region. Carbon isotopes show that C 3 forest was replaced by wooded to open C 4 grassland in central India. Our results demonstrate that the Toba eruption caused climatic cooling and prolonged deforestation in South Asia, and challenge claims of minimal impact on tropical ecosystems and human populations.
Youngest Toba Tuff in the Son Valley, India: a weak and discontinuous stratigraphic marker
Investigation of the climatic and environmental impacts of the Youngest Toba Tuff (YTT, ∼74 ka BP) eruption of Toba volcano, Sumatra, is crucial for understanding the consequences of the eruption for contemporaneous human populations. The Middle Son Valley, in India, was the first locality on the Indian subcontinent where the YTT was reported. The ash bed forms a discontinuous layer stretching for over 30 km along the river. Here we report on the stratigraphic contexts of YTT ash layers in alluvial deposits of the Middle Son Valley, in order to reconstruct the taphonomy of the ash deposits and the dynamic of their deposition. Although the distal ash has been studied since the 1980s, its stratigraphic integrity and the mechanisms and pathways involved in its transport and deposition have bit previously been assessed. We find that the YTT occurrences in the Middle Son Valley may not be reliable chronostratigraphical markers for millennial scale palaeoenvironmental reconstruction.