Palaeoenvironmental response to the ∼74 ka Toba ash-fall in the Jurreru and Middle Son valleys in southern and north-central India

Abstract

Distal deposits of rhyolitic volcanic ash from the ∼74 ka "supervolcanic" eruption of Toba, in northern Sumatra, are preserved in numerous river valleys across peninsular India. The Toba eruption is hypothesized to have resulted in climate change and the devastation of ecosystems and hominin populations. This study reports the results of the analysis of sediments and stratigraphical sequences from sites in the Jurreru and Middle Son valleys in southern and north-central India. The aim of the study is to determine the extent of palaeoenvironmental change in both valleys as a result of the ash-fall. Inferences based on evidence from the Jurreru valley are more detailed, where pre- and post-Toba palaeoenvironmental changes are divided into seven phases. The results indicate that ash-fall deposits in both valleys underwent several phases of reworking that possibly lasted for several years, indicating that ash was mobile in the landscape for a considerable period of time prior to burial. This could have enhanced and lengthened the detrimental effects of the ash on vegetation and water sources, as well as animal and hominin populations.

A high-precision 40Ar/39Ar age for the Young Toba Tuff and dating of ultra-distal tephra: Forcing of Quaternary climate and implications for hominin occupation of India

“…These sites were chosen because they preserve primary ash-fall layers (4e5 cm thick) in the basal deposits (Petraglia et al, 2007;Jones, 2010) with black flecks suspected of being small biotite shards. The identification of air-fall deposits was based upon a number of field observational criteria: the unit forms a homogenous, uniformly-bedded deposit which blankets preexisting topography with only minor variation in thickness locally; the unit shows sharp contrast with underlying sediments and overlying ash deposits; the unit shows no cross bedding features, but may have weakly developed planar bedding; the unit is characterised dominantly by glass shards and the absence of nonvolcanic detrital material (Gatti et al, 2011;Blinkhorn et al, 2012;Lewis et al, 2012;Matthews et al, 2012).…”

“…4) there are several metres of reworked tephra, concentrated as a consequence of alluvial processes (Jones, 2010;Lewis et al, 2012;Petraglia et al, 2012). Chronometric ages (OSL) have been obtained for the alluvial deposits within the valley (Williams et al, 2006), although studies disagree on the stratigraphic position of the ash-fall in the sequence (Jones and Pal, 2009;Jones, 2010). Despite the fact that there is an air-fall tephra at Ghoghara, IRSL dating of this section has produced young ages below the ash (Neudorf et al, 2012).…”

“…The tephra varies between 1 and 2.5 m thick and extends over several kilometres. Notable is the presence of a thin (approximately 4 cm) basal primary air-fall tephra unit found above palaeosols (Petraglia et al, 2007;Jones, 2010;Blinkhorn et al, 2012;Matthews et al, 2012). Above the thin primary air-fall tephra layer ( Fig.…”

“…Previous analysis of sediments and ash deposits in the Son and Jurreru River valleys strongly suggested an association of the ash with the YTT event (e.g., Petraglia et al, 2007;Jones, 2010). The current 40 Ar/ 39 Ar study of the ash is the first robust chronometric study of the ash deposits in the Son and Jurreru River valleys that confirm the previous data and inferences about the age of the deposits, thus allaying criticisms that the ash could represent the OTT event (e.g., Westaway et al, 2011).…”

“…Scattered throughout India there are more exposures of air fall ash in river valley cuttings and at excavated archaeological sites (Jones, 2010, Fig. 1).…”