SITE 1150

EPMA data for 23 individual tephras (layers and pods) were obtained from Site 1150 (Tables T1, AT1), spanning an age interval, suggested by original shipboard age estimations, of ~24 to 665 ka. On the basis of major element geochemical composition it is possible to suggest two correlations between Site 1150 tephras and previously described isochrons.

First, tephra Sample 186-1150A-2H-5,136-138 cm, appears to correlate (Fig. F7) with the ~40-ka SpFa-1 tephra erupted from the Shikotsu caldera in Hokkaido (Machida, 1999). This tephra has been reported in the Pacific Ocean, offshore from Hokkaido and Northeast Honshu. Confirmation of this correlation would extend the known range of this deposit farther to the south. Correlation with the 20-ka Aira-Tn tephra, supported by TiO2/MgO ratios, can be rejected because of significant differences in K2O. Acceptance of the correlation with tephra SpFa-1 would indicate that the original ship-based age model (74 ka) had overestimated the age for this depth (15.06 mbsf) by ~30 k.y.

Second, tephra Sample 186-1150A-4H-1, 10-12 cm (and duplicated in core catcher Sample 3H-CC, 22-25 cm), appears to correlate with the Aso-4 tephra, erupted at ~84-89 ka from the Aso caldera (Machida and Arai, 1992; Machida et al., 1985). The Aso-4 tephra represents the youngest and largest of the Aso eruptions and one of the largest Japanese eruptions of the middle to late Quaternary. Geochemically, it has a distinct bimodality, and despite the many caveats expressed in this paper concerning the potential for geochemical equifinality and for miscorrelation, we can be reasonably confident in this correlation, supported by TiO2/K2O, FeO/MgO, and SiO2/CaO ratios (Fig. F8). Acceptance of the correlation with the Aso-4 tephra would indicate that the original ship-based age model (~130 ka) had overestimated the age for this depth (26.8 mbsf) by ~45 k.y. It is of interest to those with specific interest in high-resolution stratigraphic investigations from ODP cores to note the evidence of core overlap that this tephra provides. Both samples (186-1150A-4H-1, 10-12 cm, and 3H-CC, 22-25 cm) are clearly from the same real depth, as indicated by the tephra layer. The core catcher (186-1150A-3H-CC) tephra sample is assigned a depth of 27.27 mbsf by coring protocol and the tephra from the lower core (186-1150A-4H) a depth of 26.8 mbsf. As the geochemical evidence is not readily contradicted, one is forced to assume that Cores 186-1150A-3H and 4H overlapped by 47 cm. Whereas this may not be of significance in terms of long-term shifts, those investigating high-resolution paleoceanographic and paleoclimatic shifts from ODP piston cores should be mindful of this issue, which will only be revealed in the rare instances where marker horizons are present and confirmed as such (see also Robinson, 1990).

Although these are the only widespread terrestrial and marine tephras that we have suggested may potentially correlate with Site 1150 tephras, there may be further correlative potential with additional tephras in the Japan region, including both Holocene and Pleistocene deposits. Finally, for Site 1150, one tephra layer (Sample 186-1150A-9H-3, 2-4 cm) is, in comparison with the dominantly rhyolitic tephras from Sites 1151 and 1150 and the Quaternary terrestrial record, uncharacteristically andesitic to dacitic (Fig. F9). This tephra, dated by the original shipboard age models to ~660 ka, appears to correlate (Fig. F9) with dacitic and andesitic tephras defined by Pouclet et al. (1986) from Holes 582B, 583A, and 583D (their samples 9, 10, 27, and 29) from Leg 87. These are of undetermined Quaternary age. Whether tephra Sample 186-1150A-9H-3, 2-4 cm, is correlative with one of these is unclear, albeit a distinct possibility.

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