MATERIALS AND METHODS

This paper presents stable isotopic data for bulk samples from the western Pacific with an emphasis on the Campanian and Maastrichtian record in Ocean Drilling Program (ODP) Holes 1183A and 1186A. These sites are on OJP and were recovered during ODP Leg 192. In addition, five other holes from OJP (Deep Sea Drilling Project [DSDP] Holes 288A and 289 and ODP Holes 807C, 1184A, and 1185A) and one hole from Manihiki Plateau (DSDP Hole 317A) were examined. The sections are dominated by chalk and limestone with chert interbeds (Andrews, Packham, et al., 1975; Schlanger, Jackson, et al., 1976; Kroenke, Berger, Janacek, et al., 1991; Mahoney, Fitton, Wallace, et al., 2001). Calcareous nannofossils, foraminifers, and radiolarians are the dominant sediment sources at all sites. Terrigenous material is a minor component, and the limestones are consistently >90 wt% and generally >95 wt% calcium carbonate. Estimated Late Cretaceous paleolatitudes range from ~15° to ~30°S (Fig. F1); burial depths range from <600 m in Hole 317A and the upper Maastrichtian in Hole 288A to >1300 m for the upper Campanian in Hole 807C (Andrews, Packham, et al., 1975; Schlanger, Jackson, et al., 1976; Kroenke, Berger, Janacek, et al., 1991; Mahoney, Fitton, Wallace, et al., 2001). Backtracking current bathymetry, taking into consideration expected subsidence related to the cooling of the igneous pile and to the weight and compaction of the sedimentary cover, suggests Maastrichtian water depths ranged from ~2000 to >3000 m (J. Ogg et al., unpubl. data).

Bulk samples were collected from the Maastrichtian to Campanian from six DSDP/ODP Holes (288A, 289, 317, 807C, 1183A, and 1186A). Sampling density is greatest in Holes 807C, 1183A, and 1186A, where a relatively even distribution of samples across the Campanian–Maastrichtian interval is available. Stratigraphic coverage was enhanced by analyzing carbonate-bearing residues of Cretaceous and Tertiary samples from Leg 192 shipboard X-ray diffraction (XRD) studies. Comparable results spanning the Neogene–Albian are available for Hole 807C (Corfield and Cartlidge, 1993). An ~1-g portion of each bulk sample was cleaned ultrasonically under distilled deionized water, dried at <50°C, and powdered with an agate mortar and pestle. Preparation of samples for Leg 192 shipboard XRD analysis was similar (Mahoney, Fitton, Wallace, et al., 2001).

A 50- to 100-µg separate of each powder was loaded into an individual reaction vial, and its ¹³C and ¹⁸O values were measured in the Biogeochemistry Isotope Laboratory at the University of Missouri. Samples were reacted at 70°C in 100% H₃PO₄ on a Kiel III carbonate device. The evolved CO₂ was cryogenically distilled, and its isotopic composition was measured online using a Finnigan MAT DeltaPlus isotope ratio mass spectrometer. Results are reported in standard delta notation relative to the Vienna Peedee Belemnite (VPDB) standard. The instrument is calibrated for each run based on the average of multiple analyses of NBS-19 standard. External precision of the instrument through the duration of this study was <0.03 for ¹³C and <0.05 for ¹⁸O (1 , standard deviation).

Age estimates are based on nannofossils and Sr isotope stratigraphy. For Holes 1183A and 1186A, ages estimated from ⁸⁷Sr/⁸⁶Sr ratios measured on a subset of the bulk samples are in good agreement with Campanian–Maastrichtian biostratigraphy (Bralower et al., this volume). To construct age models for these holes, ⁸⁷Sr/⁸⁶Sr ratios were converted to age using tables from McArthur et al. (2001) after adjusting the OJP data by –0.0000175. This adjustment was proposed as an empirical correction for interlaboratory differences (MacLeod et al., 2003) but may reflect age differences in the datums used to project the stratigraphy onto the geologic timescale (Bralower et al., this volume). The Sr age model for each site is a linear fit to the resulting age/depth plot, and these fits are statistically excellent (Fig. F2). Nannofossil datums are used to tie Leg 192 holes to the other four holes studied (Table T1). Ages for the intervals between the biostratigraphic tie points in the other four holes are interpolated.