High-resolution magnetic records in pelagic sediments: The Oligocene geomagnetic field, the Brunhes/Matuyama geomagnetic reversal, and rock magnetic changes at the Eocene/Oligocene boundary.

摘要

An 11 million year-long record of the Oligocene geomagnetic field has been obtained from remanent magnetism in pelagic sediments of Deep Sea Drilling Project (DSDP) Hole 522. Sample spacing of 4 cm yielded a high resolution of approximately one specimen per 4 to 8 kyr. This paleomagnetic record is in excellent agreement with marine magnetic anomalies, and includes many of the so-called "tiny wiggles". Other features include consistent major declines in paleointensity (DIPs) at reversal boundaries, occasional DIPs between reversal boundaries that correlate to marine magnetic anomaly tiny wiggles, and intra-chron paleointensity vs. depth profiles approximating symmetric arch-like shapes or "ragged plateaus". Few "saw-tooth" paleointensity profiles were observed over this interval.; "Rock magnetic" measurements of these Site 522 samples indicate a major difference between Eocene and Oligocene sediments that parallel changes in {dollar}sp{lcub}13{rcub}{dollar}C and {dollar}sp{lcub}18{rcub}{dollar}O isotope records as well as carbonate trends. Low temperature magnetic tests reveal a greater superparamagnetic (SP) contribution to the Eocene sediments. We hypothesize that dissolution in the Oligocene sediments preferentially removed the smaller SP grains and that this dissolution was brought about by increased labile carbon flux that resulted from increased productivity during the earliest Oligocene.; High resolution paleomagnetic data from five pelagic DSDP and Ocean Drilling Project sites indicate that the Brunhes/Matuyama polarity transition was characterized by a "double DIP", with DIP1 occurring 15 kyr before the polarity reversal. This double DIP structure was found even for low sedimentation rate cores. A viscous remanent magnetization contribution to NRM was identified in two of the records and, until removed by AF or thermal demagnetization, was found to blur the "double-DIP" nature of the paleointensity profiles into an apparent single-DIP, and also resulted in an apparent but erroneous "saw-tooth"-like post-transitional sudden increase in paleointensity. The confirmed global existence of this DIP so closely preceding a major reversal invites questions about its relation to the reversal itself.