15. Oxygen and Carbon Isotope Studies of Carbonate Sediments from Site 167, Magellan Rise, Leg 17

At Site 167 a nearly complete sedimentary section ranging in age from Pleistocene to Tithonian and composed almost entirely of calcareous ooze, chalk, cherty chalk, cherty limestone, and argillaceous limestone was drilled to a depth of approximately 1170 meters where the basaltic basement was reached. The relative continuity and age of the section makes it useful for studies of deep-sea diagenesis and paleotemperatures. This chapter presents the stable oxygen and carbon isotope analyses made originally with such studies in mind. Further, these analyses added to and compared with previous JOIDES and other results indicate long-range O 1 8 / O 1 6 trends in marine sediments (Figure 1). This chapter should be read in conjunction with Chapters 13 and 20 of this volume (Schlanger et al.; Douglas and Savin) and the 167 Site Report. Anderson and Schneidermann (in press) stated that the isotopic composition of upper Cretaceous limestone from Sites 146 and 153 (Leg 15, Caribbean) indicates a decrease in both δ θ 1 8 and δC*3 values with depth. This observed "oxygen shift" is postulated to have been caused by "cementation and recrystallization" of the limestone (Anderson and Schneidermann, in press). The cementation and recrystallization is then postulated to have been caused by high temperatures induced in the pelagic sediments by volcanic activity during late Cretaceous time in the region. This chapter will attempt to show that the high negative δ θ 1 8 spike recorded by Anderson and Schneidermann in the Coniacian-Santonian interval may be a world-wide event and may not necessarily be caused entirely by volcanic activity.