Orbitally Tuned Stratigraphies From 2.5 to 8 Ma at Sites 1237 and 1241

The astronomical tuning technique is at present the most accurate absolute dating method for sediment records spanning the time interval of the last 35 Ma for which astronomers provide a valid and precise orbital solution for variations in Earth's orbital parameters (eccentricity, obliquity, precession). The orbitally tuned geological time scale already became the standard chronology for the last 5 Ma. The late Miocene magnetic Polarity Times Scale (PTS) was tuned by two groups. Hilgen et al. (1995, 2003) used land-based sediment sections and Shackleton et al. (1995) used marine sediment records from Leg 138 to calibrate the PTS. Both time scales, however, differ by about 180 kyr. More recently, Shackleton and Crowhurst (1997) and Shackleton et al. (1999) established a Miocene astronomical time scale at sediment records from Leg 154 (Ceara Rise). The lack of magnetostratigraphic information, however, complicates comparisons with the tuned magnetic polarity timescales. Our goal is to expand and to astronomically calibrate the 'Magnetic Polarity Time Scale', the oxygen isotope stratigraphy and biostratigraphy to 12 Ma at east Pacific Sites 1237 and 1241. So far, we developed an oxygen isotope stratigraphy from 2.5 to 6 Ma at Site 1241 that was based on tuning cyclic fluctuations in benthic d13C values and sand percentages of the carbonate fraction to orbital obliquity and precession, respectively. The isotope stratigraphy is nearly identical with that from Leg 154 (Sites 925/926). Site 1237 was orbitally tuned from 2 to 6.5 Ma. The tuning was mainly based on the GRAPE-density record although other proxy records were considered (magnetic susceptibility, color, carbonate sand fraction, isotopes). The astronomically calibrated ages for geomagnetic reversal boundaries are almost identical with those from the Cande and Kent timescale for the time interval from 2 to 5.2 Ma. Although geomagnetic reversal boundaries are not well defined between 5.2 and 6.5 Ma, our tuning suggests considerable differences to earlier APTS. Orbital tuning at Site 1237 suggests ages for late Miocene magnetic reversal boundaries to be about 150 kyr older than those from the Cande and Kent time scale and to be about 80 kyr older than those from Hilgen et al. (1995).