doi:10.1016/j.pepi.2005.09.015
Copyright © 2006 Elsevier B.V. All rights reserved.
The nature of a cryptochron from a paleomagnetic study of chron C4r.2r recorded in sediments off the Antarctic Peninsula
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Gary Actona, 
, 
, Yohan Guyodob, 1, and Stefanie Brachfeldc, 2,
aOne Shields Avenue, Department of Geology, University of California, Davis, CA 95616, USA
bLaboratoire des Sciences du Climat et de l’Environnement (LSCE), Domaine du CNRS, 12 Avenue de la Terrasse, Gif-sur-Yvette, Cedex 91198, France
cDepartment of Earth & Environmental Studies, Montclair State University, Montclair, NJ 07043, USA
Received 2 May 2005;
revised 9 September 2005;
accepted 9 September 2005.
Available online 28 February 2006.
Abstract
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.
Keywords: Cryptochrons; Excursions; Geomagnetic polarity timescale; Magnetostratigraphy; Geomagnetism; Relative paleointensity; Ocean Drilling Program; Site 1095; Antarctic Peninsula
Fig. 1. Location of ODP Site 1095. The Mercator map was generated with Generic Mapping Tools (Wessel and Smith, 1998) using the ETOPO2 relief data from the National Geophysical Data Center.
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Fig. 2. The magnetostratigraphy for Site 1095 is shown with the paleomagnetic inclination data (left side) from split-core sections (after demagnetization of 20–30 mT), U-channel samples (PCA = principal component analyses), and discrete samples (PCA) from Acton et al. (2002). The magnetic logging data (right) obtained from the Geologic High-resolution Magnetic Tool (GHMT) (Williams et al., 2002) give magnetic anomalies related primarily to magnetic polarity. The interpreted magnetostratigraphy (middle) for the inclination and GHMT logging data are nearly identical, with only minor depth shifts related to differences between the meters composite depth scale and the wireline logging depths. The interpreted magnetostratigraphy is compared with the polarity zonation predicted from the geomagnetic polarity timescale (GPTS) of Cande and Kent (1995) for the case where the sedimentation rate was constant over long periods of time. Sed. Rate = sedimentation rate. * = excursions, cryptochrons, or anomalous polarity zones observed in the inclination data; # = chrons and subchrons not well-defined or not identified in the sedimentary section.
Fig. 3. Orthogonal demagnetization diagrams for three discrete samples from core 1095B-34X that were progressively AF demagnetized. After removing the overprint in fairly low alternating fields, the demagnetization paths are linear, revealing a characteristic remanent magnetization (ChRM) direction that is interpreted to be the ancient magnetization. The overprint is most apparent in the normally magnetized zone because the normal polarity directions point steeply upward and the overprint points steeply downward. Sample 1095B-34X-3, 74 cm (395.54 mcd; middle diagram) lies within cryptochronozone 4r.2r-1, whereas the other two samples are from Chronozone 4r.2r directly above (left diagram; sample 1095B-34X-1, 139 cm, 393.19 mcd) and below (right diagram; sample 1095B-34X-6, 31 cm, 399.61 mcd) the cryptochronozone.
Fig. 4. Intensity decay and orthogonal vector demagnetization diagrams for a sample from within cryptochronozone 4r.2r-1 that was thermally demagnetized in 25 °C steps starting at 100 °C.
Fig. 5. Intensity decay and orthogonal vector demagnetization diagrams for a sample from within polarity transition zone for the termination of cryptochronozone 4r.2r-1. Unlike the sample from within the cryptochronozone, which is shown in Fig. 4, virtually no remanent magnetization component exists after removing the drilling overprint.
Fig. 6. Low temperature rock-magnetic data for three discrete samples from Core 34X. Top: thermal demagnetization of a low temperature saturation isothermal remanent magnetization (LTSIRM) acquired at 10 K with a 2.5 T magnetic induction. Bottom: cooling of an isothermal remanent magnetization acquired at room temperature (RTSIRM) with a 2.5 T magnetic induction.
Fig. 7. Paleomagnetic results across the cryptochron. From left to right: volume magnetic susceptibility (K), intensity of the natural remanent magnetization after AF demagnetization in a peak field of 30 mT (NRM30), anhysteretic remanent magnetization (ARM30) after AF demagnetization in a peak field of 30 mT, isothermal remanent magnetization acquired with a 1 T magnetic induction and AF demagnetized in a peak field of 30 mT (IRM30), ratio of ARM30 and IRM30, inclination, and relative paleointensity (squares = normalization by K, circles = normalization by ARM30, triangles = normalization by IRM30). The orangish red curve with small circles corresponds to split-core data. Squares, large circles, and triangles are for discrete samples. The two boxes with backslashes in them are inclination results from samples that were thermally demagnetized.
Fig. 8. Comparison of observed (a) and modeled (b) marine magnetic anomalies. The observed profile is a stack from the East Pacific Rise, which is taken from Fig. 11 of Cande and Kent, 1992a and Cande and Kent, 1992b. The model profile was generated assuming cryptochron 4r.2r-1 is 56 k.y. in duration, has an age of 8.566–8.622 Ma, and is a full polarity reversal, with the same magnetization intensity (1 A/m) as the rest of the 0.5-km-thick source layer. All other chron ages are based on the timescale of Cande and Kent (1995). A transition width of 0.5 km was used. A seafloor spreading rate of 40 km/m.y. was used prior to 8 Ma and 41 km/m.y. after 8 Ma.
Table 1.
Age and duration of Cryptochron 4r.2r-1
Depths are given meters composite depth (mcd) when available. Otherwise they are in meters below seafloor (mbsf). References are: (1) Schneider (1995); (2) Helen Evans (personal comm.), Evans and Channell (2003); (3) This study; (4) Kanazawa et al. (2001). C4r.2r (t) is the termination of Chron 4r.2r, which occurs at 8.072 Ma in Cande and Kent (1995) timescale. C4r.2r (o) is the onset of Chron 4r.2r, which occurs at 8.699 Ma in Cande and Kent (1995) timescale. C4r.2r-1 (t) is the termination of Cryptochron 4r.2r-1, which is estimated from is relative position within Chron 4r.2r. C4r.2r-1 (o) is the onset of cryptochron 4r.2r-1, which is estimated from is relative position within Chron 4r.2r.
Corresponding author. Tel.: +1 530 752 1861; fax: +1 530 752 0951.
1 Tel.: +33 1 69 82 35 62; fax: +33 1 69 82 35 68.
2 Tel.: +1 973 655 5129; fax: +1 973 655 4072.
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