The nature of a cryptochron from a paleomagnetic study of chron C4r.2r recorded in sediments off the Antarctic Peninsula

The magnetostratigraphy from Ocean Drilling Program (ODP) Site 1095, off the Pacific margin of the Antarctic Peninsula, contains an extra normal polarity event that occurs near the base of Chron 4r.2r (8.072-8.699 Ma), which we interpret to be cryptochron C4r.2r-1. Owing to the relatively high sedimentation rates (about 90 m/m.y.), this event is particularly well recorded at the site, spanning 4.99 m of the stratigraphic section. This allows the characteristics of the cryptochron to be investigated in greater detail than possible from marine magnetic anomalies, where it was originally identified, or from other sedimentary sections in which it has been recorded at much lower resolution. Our observations suggest that the cryptochron is a full geomagnetic reversal, in which both the direction and paleointensity attain levels similar to that of other normal polarity chrons at the site. Based on its position within Chron 4r.2r, the cryptochron started at 8.622 Ma and terminated 56 k.y. later at 8.566 Ma. At the transition zones bounding the cryptochron, the paleointensity collapses to near zero, but recovers within a few thousand years. Our results, as well as paleomagnetic observations from other thick sedimentary units, indicate that cryptochrons are not always purely paleointensity variations. Instead they are a record of short-term geomagnetic variability that includes short geomagnetic reversals, excursions, intervals of high paleosecular variation, and paleointensity lows, all of which are part of a vector field that varies in both strength and direction over time.