Early Oligocene geomagnetic field behavior from Deep Sea Drilling Project site 522

Abstract

Hydraulic piston coring operations at Deep Sea Drilling Project site 522 in the South Atlantic retrieved an unusually continuous section of late Eocene to late Oligocene pelagic sediments, which we sampled at 3–4 cm intervals (∼3–5 kyr). Natural remanent magnetization demagnetization studies indicate a well‐behaved remanence. Various rock magnetic procedures strongly suggest the magnetic carrier is dominated by pseudo‐single domain magnetite appropriate for recording relative intensity variations of the paleomagnetic field. Nine zones of unusually low relative paleointensity were identified within the 2 my Chron C12R interval. Seven can be typified by a ∼20–40 kyr interval of low field intensity accompanied by apparently random, low‐amplitude, short‐duration directional fluctuations. The other two are of approximately equal duration and intensity but exhibit an orderly progression of directional changes that result in well‐defined virtual geomagnetic pole (VGP) paths confined along a preferred meridian of ∼70–90°W longitude. We propose that both styles occur when the main dipole term diminishes significantly but that the former result when undiminished “normal” secular variation is continuous during the period of low axial dipole moment. We propose that the other two lows in relative paleointensity, along with one reversal record, reflect a field structure of low axial dipole moment dominated by a low‐degree nonzonal spherical harmonic term. Alternatively, the confined VGP paths could be an artifact of heavy remanence smoothing between nonantipodal, semistable transitional geomagnetic pole positions. Geographical control of VGP paths, particularly along ∼70–90° W longitude, has recently been noted for much younger reversals. The site 522 record may indicate that the underlying cause of this phenomenon was present at 32 Ma. We compare our C12R record of paleointensity lows with C12R marine magnetic anomaly “tiny wiggles”. These data appear to indicate that C12R tiny wiggles resulted from periods of low geomagnetic field intensity that were sometimes accompanied by directional excursions.