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Geological Society, London, Special Publications; 2002; v. 195; p. 445-461;
DOI: 10.1144/GSL.SP.2002.195.01.25
© 2002 Geological Society of London

Discovery of the Toba Ash (c. 70 ka) in a high-resolution core recovering millennial monsoonal variability off Pakistan

Ulrich Von Rad1, Klaus-Peter Burgath1, Muhammad Pervaz2 & Hartmut Schulz3

1 Bundesanstalt für Geowissenschaften und Rohstoffe, D-30631 Hannover, Germany u.vonrad{at}t-online.de
2 Hydrocarbon Development Institute of Pakistan, Islamabad, Pakistan
3 Institut für Ostseeforschung, PF 301161, D-16112 Rostock-Warnemünde, Germany

A discrete Toba Ash layer in the northeastern Arabian Sea was detected near the base of a 20.2 m long piston core (289KL) recovered from the oxygen minimum zone off the Indus delta. In addition to the Toba Ash, we discovered two highly disseminated, vitreous, rhyolitic ‘ash layers’ in two annually laminated box cores: a ‘Younger Ash’ (about AD 1885–1900), and an ‘Older Ash’ (about AD 1815–1830). The glass shards were probably derived from eruptions of Indonesian volcanoes, although it was not possible to correlate these two ashes with well-known historical eruptions. We discuss source, transport and deposition of distal ash-fall layers in the Arabian Sea, which are derived from violent ultra-Plinian eruptions on the Indonesian volcanic archipelago, as well as their use for palaeoclimatic correlation. Core 289KL has a complete, high-resolution stratigraphic record of the past 75 ka with 21 interstadials (IS) or Dansgaard-Oeschger (D-O) cycles and equivalents of Heinrich events H1–H6. The high-frequency record of this core shows rapid climate oscillations with periods around 1.5 ka and can be tuned precisely to the {delta}18O record of a Bay of Bengal core and to the GISP-2 ice core from Greenland. The Toba event (70 ± 4 ka BP), which is well documented in the Arabian Sea and Bay of Bengal records at the end of IS-20, as well as in the Greenland ice, is an excellent stratigraphic marker horizon to validate this correlation. The apparent synchronous appearance of the various D-O oscillations and Heinrich events, which has been documented for many northern hemisphere localities, can be explained only by fairly rapid atmospheric circulation changes. Changes in the intensity of the Indian summer monsoon are tightly coupled with suborbital climate oscillations in the northern hemisphere via atmospheric moisture and heat circulation.