We present a magnetostratigraphic record from the western Philippine Sea that is tied to a marine δ 18 O record for the past 2.14 million years. The ages of geomagnetic reversals were astronomically calibrated by tuning the oxygen isotopic stratigraphy, yielding a chronology for the following subchrons: Matuyama/Brunhes boundary, 781 ± 3 ka (slightly above δ

Little has been known about the earliest Toba eruptive episodes that created the largestknown caldera complex of Quaternary age. Here we report evidence for the eastward dispersal of the oldest Toba tuff in South China Sea sediments to 2500 km away from the source. The tephra deposits occur below the Brunhes-Matuyama geomagnetic boundary (778 ka) and slightly above the Australasian microtektite layer (793 ka). Calibrated by astronomically tuned oxygen isotope stratigraphy, the middle Pleistocene Toba eruption occurred during the deglaciation at 788 ؎ 2.2 ka, according to the tephra occurrence between marine isotope stages 20 and 19. This refined age is in good agreement with the 40 Ar/ 39 Ar date of 800 ؎ 20 ka for the Toba tephra (layer D) from Ocean Drilling Program (ODP) Site 758, but significantly younger than the commonly cited Ar/Ar age of 840 ؎ 30 ka. The eruption expelled at least 800-1000 km 3 dense-rock-equivalent of rhyolitic magma on the basis of the widespread tephra-fall deposit in the basins of the Indian Ocean and the South China Sea. In spite of its exceptional magnitude, the timing of this major eruption does not indicate a causal linkage between this event and a long-term global climatic deterioration.

Abstract-Microtektites from two deep-sea cores in the South China Sea and the West Philippine Sea are identified as belonging to the Australasian tektite strewn field based on the morphology, chronostratigraphic occurrence, and geographical location of these microtektites. The higher concentrations of microtektites (> 1000/cm2) in the marginal seas of the western Pacific, with the peak concentration in the South China Sea, support the hypothesis of a large impact crater in Indochina. These two new occurrences lead to a more precise dating of the impact event at 793 ka, whereas the size of the Australasian source crater on the Indochina Peninsula is estimated to be 90-1 16 km.

A detailed high-resolution oxygen isotope stratigraphy over the last two glacial-interglacial cycles in the southern South China Sea is presented, based upon analysis of Globigerinoides sacculifer (planktic foraminifera) in Core MD972151. Major characteristic isotopic events were identified and correlated to the standard SPECMAP chronology. In addition, radiocar bon dating and a few biostratigraphic, geomagnetic and tephrochronologic markers were employed to enhance the reliability and resolution of the age model. The tephra layer of the Toba Eruption at 71 ka helps to anchor the boundary of Stages 5/4, and provides a yardstick correlating to the cold period between interstadials 19 and 20 identified in the GISP2 Greenland ice core. The last appearance of pink Globigerinoides ruber and the Blake magnetic polarity event in the lower part of the core helped us to locate the transition of Stage 6/Se. Our high-resolution oxygen isotope stratigraphy, however, indicates that the age of the Blake magnetic reversal occurred during the Stage 6/Se transition instead after the warmth peak of the isoto pic event Se. The resulting age model shows that the 26.54 m sequence from Core MD972151 provides a continuous record from 153 to 0.94 ka, with sedi mentation rates varying between 5.6 and 78 cm/kyr. The temporal resolu tion of the isotopic record is about 60 -150 years per sample for the upper part of the core and 250 years for the lower part, respectively.

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