A robust age model is essential to evaluate the changes in the hydrographic and sedimentary regime across the Oligocene/Miocene boundary (OMB) (Pfuhl et al., in press). Combined biostratigraphic and magnetostratigraphic age models for Ocean Drilling Program (ODP) Sites 1168 and 1170-1172, as of August 2002, were used in an initial comparison between sites. However, close study of records of percent CaCO3, weight percent sand (>63 µm), and stable oxygen and carbon isotopes (bulk sediment and benthic foraminifers) provides additional information that is crucial in recognizing hiatuses, especially the Marshall Paraconformity in the early Oligocene (~32-29 Ma) (Fulthorpe et al., 1996) and sedimentary sequences. A detailed lithologic description of Sites 1168 and 1170-1172 (Fig. F1) is given in the Leg 189 Initial Reports volume (Exon, Kennett, Malone, et al., 2001).
As a first step we attempt to correlate the benthic stable isotope records from Site 1170, with special focus on the Mi-1 and Mi-1a events (Miller et al., 1985) with those at Site 1090 (Billups et al., 2002) on the Agulhas Ridge south of Africa. A second step is to compare the results with the bio- and magnetostratigraphic data as of August 2002, but also see the study of Stickley et al. (this volume) and additional information on hiatuses/condensed sections or changes in sedimentation rate that are indicated by the measurements obtained during postcruise work (this study and Pfuhl et al., in press). The revised age model for Site 1170 incorporates the best fit between additional control points (this study) and the biomagnetostratigraphy (as of August 2002). This age model forms the basis for paleoceanographic correlation of Sites 1170-1172 and development of individual age models using the same overall approach as at Site 1170. In a final step we identify additional control points for Site 1168, which differs in its location on the continental margin from the open-ocean situation of Sites 1170-1172. A more recent version of the age models is presented by Pfuhl et al. (in press); however, the principles outlined here remain unchanged.