Shatsky Rise is a Mesozoic large igneous province in the west central Pacific Basin east of Japan, which is the approximate length (1300 km) of the state of California (Sliter and Brown, 1993). Three major topographic highs, the largest being Shatsky Plateau, compose Shatsky Rise (Fig. F1). Sediments found on Shatsky Rise range from Cretaceous to Pleistocene in age and are at relatively shallow burial depths (Sliter and Brown, 1993). Shatsky Rise has been the focus of eight Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drilling expeditions. Pleistocene to Miocene sediments were recovered at DSDP Sites 47, 48, 50, 305, 306, and 577 and ODP Site 810 on the southern plateau. Drilling during Leg 198 (Fig. F1) penetrated the Northern (Site 1207) and Central (Site 1208) Highs of Shatsky Rise in addition to the larger Southern High (Sites 1209–1214). The Cenozoic sections recovered at all of these sites except Site 1214 were ash bearing, and most sections contained rafted pumice fragments (Bralower, Premoli Silva, Malone, et al., 2002).
The ash record of Shatsky Rise is concentrated in the post-2.6-Ma section that accumulated during a period of global cooling. This increase in explosive volcanic activity at ~2.6 Ma has been noted around the globe and tentatively linked to the onset of Northern Hemisphere glaciation (e.g., Kennett, 1977; Kennett et al., 1977; Bray, 1974, 1979; Prueher and Rea, 1998). Furthermore, Natland (1993) suggested that ash frequency may be linked to changes in dispersal associated with jet stream migration.
The best ash record recovered during Leg 198 was at Site 1208, where there are a few ash layers in the upper Miocene section and a maximum at ~2.6 Ma (Cores 198-1208A-13H and 14H). This site contains more discrete ash layers (20 total) than the previous Shatsky record set at Site 810 (12). Site 1208 is also unique in that it contains a thicker than expected upper Cenozoic section that is attributable to the presence of sediment drift deposits (Bralower, Premoli Silva, Malone, et al., 2002). It is not known how sediment reworking on the Central High may have modified the ash record. Elsewhere on Shatsky Rise at other Leg 198 and previous DSDP sites, incomplete ash records may be attributed to the erosive effects of deep ocean currents. Prueher and Rea (1998) did not use Site 884 for their tephrochronolgy work because it is in the Meiji Drift. Therefore the ash record may provide some insight into drift sedimentation processes and dynamics across Shatsky Rise.
Natland (1993) analyzed ash beds and pumice fragments from Site 810 on Shatsky Rise located near Site 1212. Although Natland collected geochemical data, he did not attempt to correlate the ashes based on their geochemical signatures. Indeed, he used age models based on magnetostratigraphy to correlate the ashes from Site 810 on Shatsky Rise to DSDP Sites 578–580, which lie ~1000–1500 km to the east and northeast. Ash geochemistry, age relationships, and considerations of atmospheric and ocean currents led Natland (1993) to conclude that the source of the ash was most likely to the north, namely arc volcanoes along the Kurile and northern Japan margins. Natland (1993) described the vitric ash components as being colorless (rhyolitic) and tan/brown (andesitic). He did not attempt to correlate the Site 810 ashes to the few ash beds recovered at other DSDP sites on Shatsky Rise.
Shipboard smear slide analyses of the Leg 198 ashes indicated that they were compositionally variable, containing a variety of glass textures and mineral constituents (Bralower, Premoli Silva, Malone, et al., 2002). This study is designed to test whether detailed compositional modes can be used to "fingerprint" individual ash beds for correlation and serve as an alternative to geochemical analysis. Our hypothesis was that the proportions of various vitric (subdivided by morphology/texture) and mineral components can be used as a correlation tool to help correlate the ash record from north to south across Shatsky Rise.