Lower Jaramillo polarity transition records from the equatorial Atlantic and Indian oceans

Gee, JS, Tauxe L, Barge E.  1991.  Lower Jaramillo polarity transition records from the equatorial Atlantic and Indian oceans. Proceedings of the Ocean Drilling Program Scientific Results. 121:377-394.


Two records of the geomagnetic polarity transition at the beginning of the Jaramillo Subchron (0.97 Ma) have been obtainedfrom sediments in the equatorial Atlantic (Leg 108, Site 665; 2.95°N, 340.33°E) and Indian (Leg 121, Site 758; 5.38°N, 90.35°E)oceans. Both cores yielded high-quality magnetostratigraphic results; however, the relatively low sedimentation rates, the weakmagnetizations, and complex demagnetization behavior of some transitional samples suggest that the record of the transitional fieldbehavior may be less reliable. In addition, variations in grain size preclude reliable paleointensity determinations although theremanence in both cores is apparently dominated by magnetite. Despite these possible complications, the two cores yield transitionalpaths that are neither far-sided nor near-sided. Together with published records that meet minimum reliability standards, the twoequatorial records presented here suggest that the lower Jaramillo transitional field morphology was significantly nonaxisymmetric.The mean normal and reversed inclinations from both cores deviate from the inclination expected from a geocentric axial dipole, asnoted in virtually all marine sediment cores. The observed inclinations provide further support for a polarity-dependent nondipolecontribution to the time-averaged field.