Shatsky Rise is a large igneous province in the northwest Pacific that rises from surrounding abyssal plains to a present-day water depth of 2000 m. This feature originally formed in the central equatorial Pacific at ~148 Ma and subsequently moved south and then northwest to its present-day position (Sager et al., 1999; Shipboard Scientific Party, 2002). During the Paleocene and Eocene, Shatsky Rise lay at ~20°N (Larson et al., 1992; Shipboard Scientific Party, 2002). Thermal subsidence models (Ito and Clift, 1998) and rudist fossils dredged from the south edge of the Southern High (Sager et al., 1999) indicate that Shatsky Rise was subaerially exposed during and immediately after eruption. During the Paleogene, however, Shatsky Rise submerged to water depths between 1500 and 4000 m (Ito and Clift, 1998; Sager et al., 1999). Importantly, Shatsky Rise received little siliceous material during the Paleocene and Eocene and has always been distant from a continental margin and associated terrigenous inputs (Bralower, Premoli Silva, Malone, et al., 2002). These factors simplify interpretations of the sedimentary record.
Eight sites were drilled on Shatsky Rise (Shipboard Scientific Party, 2002). Boreholes at two of these sites were chosen for this study. Site 1209 is located close to the top of the Southern High at 32°39.10´N, 158°30.36´E (2387 m water depth), and Site 1211 is located 100 km to the southwest on the southern flank of the Southern High at 32°00.13´N, 157°51.00´E (2907 m water depth) (Fig. F2). The two sites thus comprise a depth transect of 520 m. Site 1211 was drilled at the location of Deep Sea Drilling Project (DSDP) Site 305 (Larson, Moberly, et al., 1975). According to thermal subsidence curves (Ito and Clift, 1998), Site 305 (and, hence, Site 1211) lay beneath ~2.5 km of water during the Paleocene and Eocene. Site 1209 was under ~2.0 km of water during this time, assuming a similar depth offset between Sites 1209 and 1211 throughout the Cenozoic. The CCD in the equatorial Pacific was probably between 3 and 4 km for good portions of the Paleocene and Eocene (van Andel, 1975; Lyle, Wilson, Janacek, et al., 2002). Sites 1209 and 1211 should, therefore, be ideal locations to monitor fluctuations in the early Paleogene lysocline.
Three holes were cored at each of Sites 1209 and 1211. These holes extend to ~320 meters below seafloor (mbsf) at Site 1209, and ~180 mbsf at Site 1211 (Shipboard Scientific Party, 2002). By splicing together sections from the holes, composite logs were constructed for both sites (Shipboard Scientific Party, 2002). Depths on these logs and in this study are thus reported in meters composite depth (mcd). Apparently continuous Paleocene and Eocene sections occur from ~140 to 255 mcd at Site 1209 and from ~85 to 150 mcd at Site 1211 (Shipboard Scientific Party, 2002) (Fig. F3). Paleocene and Eocene sediments at both sites are primarily very pale orange nannofossil and moderate yellowish brown clayey nannofossil ooze, commonly showing decimeter- to meter-scale cyclicity. Interestingly, this cyclicity is more pronounced at the shallower Site 1209. Paleocene and Eocene sediments at Sites 1209 and 1211 were dated using planktonic foraminiferal assemblages (M.R. Petrizzo, pers. comm., 2004), using age datums provided in Berggren et al. (1995). This biostratigraphy is fairly similar to that suggested on the ship (Bralower, Premoli Silva, Malone, et al., 2002) and renders average Paleocene–Eocene sedimentation rates of 1.5 m/m.y. in Hole 1211A and 3 m/m.y. in Hole 1209A (Shipboard Scientific Party, 2002, p. 121). Values are typical for plateaus in open-ocean, deepwater settings (Kennett, 1982).
Sediments deposited during several critical episodes of Paleogene climate evolution (Fig. F1) were recovered in both Holes 1209A and 1211A (Fig. F2). These episodes include the Eocene/Oligocene transition, the EECO and the IETM, as well as a mid-Paleocene biotic event characterized by the first occurrence (FO) of the nannolith Heliolithus kleinpellii (58.4 Ma) and primitive discoasters, both of which are important, and often dominant, components of late Paleocene and younger nannoplankton assemblages. Sediment deposited during this biotic event also contains a low-diversity planktonic foraminiferal assemblage dominated by Igorina tadjikastanensis (Shipboard Scientific Party, 2002; Petrizzo, this volume).
An increase in carbonate content and a lack of color cycles highlights the Eocene/Oligocene transition. Although no obvious lithologic changes characterize the EECO, a thin claystone marks the IETM and a phillipsite-rich, carbonate-poor horizon containing abundant fish teeth and manganese-coated foraminifers marks the mid-Paleocene biotic event (Shipboard Scientific Party, 2002). Additionally, intervals of low sedimentation rate are found in middle to upper Eocene sediment between 33.7 and 45 Ma in both holes (M.R. Petrizzo, pers. comm., 2004).
For this study, we collected 134 sediment samples of 10 cm3 from 137.56 to 251.38 mcd in Hole 1209A and 99 sediment samples of 10 cm3 from 84.60 to 147.46 mcd in Hole 1211A (Fig. F3). Samples were initially taken onboard ship at a resolution of three per core. In order to examine a potential relationship between sediment cycles and dissolution, one sample was also taken from obvious light and dark layers. Additional samples were collected every 10 to 20 cm over suspected dissolution episodes. On average and from each hole, there are about four samples for every 1 m.y. of sediment deposition.