CONCLUSIONS

Here we present two consistent high-resolution orbitally tuned age models for Leg 202 Sites 1237 and 1241 as well as benthic isotope stratigraphies for Sites 1236, 1237, 1239, and 1241 that cover the time interval from 2.5 to 6 Ma. The age models for Sites 1237 and 1241 were generated by correlating the high-frequency variations in GRA density, percent sand of the carbonate fraction, and benthic ¹³C to the orbital solution of Laskar et al., (1993). Cross-spectral analyses revealed highly significant coherencies between their astronomically related frequency components and orbital obliquity and precession. The age models for Sites 1236 and 1239 were constructed by transferring ages from Site 1241 via correlation of isotope records. The establishment of astronomically calibrated isotope stratigraphies at Sites 1237 and 1241 with an average sampling interval of <3 k.y. along with the excellent paleomagnetic stratigraphy at Site 1237 with clear definitions of all Pliocene chrons allowed a direct comparison with previously published astronomical age models. The excellent correlation of benthic ¹⁸O and ¹³C stratigraphies between Pacific Site 1241 and Atlantic Site 925/926 (Ceara Rise) confirms the age model constructed at sediment sequences from Atlantic Leg 154. Furthermore, the most ages of the Pliocene polarity reversals at Site 1237 agree well with those of the generally accepted astronomical polarity timescale as summarized in Lourens et al. (2004; ATNTS2004). Hence, our work places the late Miocene to middle Pliocene sediment records from Leg 202 into a high-resolution, globally correlative, and astronomically calibrated stratigraphic framework and provides an excellent basis for further paleoceanographic studies.

The Atlantic-Pacific comparison of benthic ¹⁸O and ¹³C records indicates that the late Miocene and early Pliocene ¹³C signal is globally correlative and better structured than the ¹⁸O signal. Although the spectral variability of both parameters is dominated by 41-k.y. cycles with relatively constant phase relationships, the clarity and correlative nature of the 41-k.y. cycle is generally better developed in ¹³C records. This demonstrates that the benthic ¹³C records are not only a powerful medium for orbital tuning, but also a valuable tool for chronostratigraphic correlations that could assist the late Miocene and early Pliocene benthic oxygen isotope stratigraphy. This opportunity may primarily become important when extending the isotope stratigraphy further back in time, particularly for those intervals where the ¹³C response to cyclic changes in global climate is more clearly developed than in the oxygen isotope signal.

In summary, our results from Site 1237 and 1241 demonstrate the potential for expanding both the isotope stratigraphy and the astronomical polarity timescale to the base of the Neogene, as the composite depth of the APC-cored sequence at Site 1237 suggests a complete recovery of the last 30 m.y. along with an excellent magnetic stratigraphy.