Paleomagnetic Analyses of Short Normal Polarity Magnetic Anomalies in the Matuyama Chron

Abstract

We document three short normal intervals in the natural remanent magnetization of sediments within the Matuyama Chron. These three anomalous zones of magnetization between the Jaramillo and Olduvai subchrons were identified from continuous measurements of archive halves from Hole 803 A using the pass-through 2G cryogenic magnetometer at Scripps. The U-channel samples were taken from the three intervals, analyzed using the pass-through system, and then cut into discrete 1 -cm-thick samples. Measurements on discrete samples confirmed the presence of the upper normal polarity zone. Based on sedimentation rate calculations, this zone is confidently correlated with the Cobb Mountain Subchron. For the two other anomalous zones, complete thermal demagnetization revealed a high-stability component (250°-575°C) of reversed polarity. The intensity of the low-stability normal polarity component, normalized by susceptibilit y, remains roughly constant throughout the entire interval sampled, whereas the intensity of the high-stability reversed component is much lower within the normal zone than outside. We interpret these two normal zones, then, as periods of low (reversed polarity) geomagnetic field intensity resulting in low magnetization of the sediments; the periods of these low magnetization reversed polarity zones are completely masked by the component acquired viscously in a normal polarity field.