A detailed understanding of the age and depositional history of the sediment column overlying basement is essential for achieving the principal objectives of Leg 192, which are understanding the eruptive history of the Ontong Java Plateau and its environmental consequences. The Maastrichtian to Aptian sections at Sites 1183 and 1186 were characterized by poor core recovery. In addition, difficulties in application of planktonic foraminiferal and nannofossil zonal markers, resulting from poor core recovery and fossil preservation, complicated correlation of the sections to other sequences from this interval, placement of stage boundaries, and estimates of absolute ages. Strontium isotope stratigraphy provides an independent source of age information that can help refine stratigraphic interpretations (e.g., Jenkyns et al., 1995; MacLeod et al., 2003).
The seawater Sr isotope curve (87Sr/86Sr ratios) provides an independent chemostratigraphic means of correlation in marine sequences (e.g., McArthur et al., 2001). Marine carbonates, in particular, have been shown to faithfully record seawater Sr isotope ratios at deposition (Burke et al., 1982; DePaolo and Ingram, 1985). Moreover, because the ocean is well mixed with respect to Sr (the residence time of Sr in the ocean is on the order of 106 yr, whereas the mixing time is on the order of 103 yr), stratigraphic fluctuations in the Sr isotope composition of marine carbonate strata are globally synchronous (e.g., Veizer and Compston, 1974; Brass, 1976; Burke et al., 1982; Koepnick et al., 1985; Hess et al., 1986). For the parts of the timescale where the seawater Sr isotope curve is well established, Sr isotope stratigraphy can be used to provide age control in sediments that are difficult to date by other means, such as shallow-water carbonates (e.g., Ludwig et al., 1988; Jenkyns et al., 1995; Lehmann et al., 1999).
The seawater Sr isotope stratigraphy of the Cretaceous has been the subject of a number of detailed investigations. The similarity of stratigraphic changes recorded in different sections and on a variety of fossil groups and mineral types strongly suggests that the seawater Sr isotope curve for this period is robust and reproducible (e.g., McArthur et al., 1993, 1994, 2001; Jones et al., 1994; Huber et al., 1995; Sugarman et al., 1995; Bralower et al., 1997; Mearon et al., 2003). Much of the data collected has been integrated in the LOWESS curve, which provides a reference (McArthur et al., 2001) for estimating ages using Sr isotope measurements. In parts of the Cretaceous where seawater 87Sr/86Sr values changed rapidly, Sr isotopes can be used to provide important stratigraphic information in sections that are difficult to date because of poor core recovery and those that contain multiple unconformities.
Cretaceous sections recovered during Leg 192 present stratigraphic challenges that Sr isotope stratigraphy is well suited to address. The sections are composed of chalk and limestone containing microfossils that range from well to poorly preserved. In addition, recovery was low throughout most of the holes drilled. The combination of poor preservation in places and poor recovery made application of traditional microfossil biostratigraphies problematic, and these problems were compounded in the lower part of the Site 1183 section where nannofossil data suggest the presence of at least three unconformities (Sikora and Bergen, submitted [N1]).