30 Myr of polarity stratigraphy and relative paleointensity from Equatorial Pacific sediments (ODP Sites 1218 and 1219, Leg 199)

ODP Sites 1218 and 1219 yielded an exceptional record of the direction and relative intensity of the geomagnetic field during most of the Miocene and Oligocene. The sedimentation rate in the Oligocene and the lower Miocene carbonate-rich pelagic sediments is > 10 m/Myr, and measurements on u-channel samples allowed a high-resolution magnetostratigraphy to be resolved. Moreover, the sediment has uniform magnetic properties that made it suitable for relative paleointensity estimates. Relative paleointensity was computed using the slope of thenatural remanent magnetization (NRM) versus anhysteretic (ARM) and isothermal remanent magnetizations (IRM) during AF demagnetization. ARM and IRM were AF demagnetized using the same alternating fields used for NRM demagnetization This high-resolution record of magnetic polarity and relative paleointensity has been used to determine whether small-scale magnetic anomalies on the ocean floor (cryptochrons) represent short polarity subchrons, intensity fluctuations of the geomagnetic field, or both.