CONCLUSIONS

Oxygen isotope ¹⁸O values in the benthic foraminifer C. mundulus decrease from values of ~2.20 in upper Chron 9n (Oi2b of Miller et al., 1991) to values of ~1.90 in upper Chron 8r. The decrease (0.30–0.60) occurs within Chron 8r in three steps at ~26.7, 26.5, and 26.35 Ma (GTS 2004) and is here termed the "Chron 8r ¹⁸O shift." The final decrease at ~26.35 Ma marks the initiation of the late Oligocene climatic optimum. The initial decrease at ~26.7 Ma is closely associated with the last occurrence of P. opima (P21/P22 planktonic foraminifer zonal boundary) between 322.51 and 322.76 mcd in lowermost Chron 8r. This zonal boundary may represent a useful marker for the Chron 8r ¹⁸O shift. Overall, this study is a modest step toward an integrated magnetic polarity and astronomical chronostratigraphy for the late Oligocene based on Site 1237.