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The nature of a cryptochron from a paleomagnetic study of chron C4r.2r recorded in sediments off the Antarctic Peninsula
Physics of the Earth and Planetary Interiors 156 (2006) 213–222
The nature of a cryptochron from a paleomagnetic study of chron C4r.2r recorded in sediments off the Antarctic Peninsula Gary Acton a,∗ , Yohan Guyodo b,1 , Stefanie Brachfeld c,2 b
a One Shields Avenue, Department of Geology, University of California, Davis, CA 95616, USA Laboratoire des Sciences du Climat et de l’Environnement (LSCE), Domaine du CNRS, 12 Avenue de la Terrasse, Gif-sur-Yvette, Cedex 91198, France c Department of Earth & Environmental Studies, Montclair State University, Montclair, NJ 07043, USA
Received 2 May 2005; received in revised form 9 September 2005; accepted 9 September 2005
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. © 2006 Elsevier B.V. All rights reserved. Keywords: Cryptochrons; Excursions; Geomagnetic polarity timescale; Magnetostratigraphy; Geomagnetism; Relative paleointensity; Ocean Drilling Program; Site 1095; Antarctic Peninsula
1. Introduction
∗
Corresponding author. Tel.: +1 530 752 1861; fax: +1 530 752 0951. E-mail addresses: [email protected] (G. Acton), [email protected] (Y. Guyodo), [email protected] (S. Brachfeld). 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. 0031-9201/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2005.09.015
Cryptochrons, a term coined by Cande and Kent (1992a), are geomagnetic events of short duration (roughly 30 k.y. or less) whose origins are debated and whose characteristics are uncertain owing to the few direct observations that have been obtained from them. Rather than being identified from paleomagnetic studies of rocks or sediments, cryptochrons have been identified as small magnetic anomalies (referred to as “tiny
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wiggles”) observed in marine magnetic anomaly profiles over oceanic lithosphere created by seafloor spreading (Blakely and Cox, 1972; Cande and Kent, 1992a,b; Cande and LaBrecque, 1974). From such indirect potential field observations, it is impossible to determine the geomagnetic origin of the anomalies uniquely or to estimate their timing and duration accurately. Several studies have suggested that cryptochrons are caused primarily or solely by geomagnetic intensity variations (Bowers et al., 2001; Bowles et al., 2003; Cande and Kent, 1992b; Gee et al., 2000). Alternatively, cryptochrons could represent full polarity reversals of short duration (Blakely, 1974; Blakely and Cox, 1972; Emilia and Heinrichs, 1972), large changes in the geomagnetic field direction, such as observed for excursions or large secular variation events, or a combination of these in association with paleointensity decreases (Evans and Channell, 2003; Lund et al., 2001; Roberts and Lewin-Harris, 2000). We investigate the nature of one cryptochron, referred to as C4r.2r-1 (Cande and Kent, 1992a), which is particularly well recorded in sediments cored off the West Antarctic Peninsula at Ocean Drilling Program (ODP) Site 1095, as are other short geomagnetic events. We will show that the duration of this cryptochron is longer than previously inferred and that, unlike other short geomagnetic events, the paleomagnetic field fully reversed and the paleointensity recovered to above the long-term mean during the event. Thus, the nature of this cryptochron is similar to other subchrons that are part of the geomagnetic polarity timescale and represents just one form of behavior in the dynamically varying geomagnetic field. 2. Background and sampling Site 1095 (66.9853◦ S, 78.4878◦ W; 3842 m water depth) is located on the distal portion of a hemipelagic sediment drift on the continental rise off the northwestern, Pacific margin of the Antarctic Peninsula (Fig. 1). Four holes were cored at the site during ODP Leg 178, recovering a 570-m thick sedimentary section that extends from the Holocene to the late Miocene (0–10 Ma) (Barker et al., 1999). Note that all core, interval, and sample depths given below are meters composite depths (mcd) based on the mcd scale constructed by Barker (2001), which is 5.5 m shallower than the meters below seafloor depth (mbsf) scale for the cores we are considering in this study. Core recovery was high throughout the section, which was cored with the Advanced Piston Corer (APC) and Extended Core Barrel (XCB) systems. This enabled us to establish a magnetostratigraphy, which is fairly straight-
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.
forward and complete all the way from the termination of Chron C4Ar.2n (9.580 Ma) at ∼515 mcd through the Brunhes Chron (0–0.78 Ma) (Fig. 2). One exception occurs near the base of Chron 4r.2r (350.54–406.22 mcd; 8.072–8.699 Ma) where there is an extra normal polarity event (Acton et al., 2002; Barker et al., 1999). This event is not shown in most geomagnetic polarity time scales, although Cande and Kent (1995) have placed a geomagnetic event in the lower part of Chron 4r.2r, which they refer to as cryptochron C4r.2r-1. We focus our study on the interval that includes this cryptochron. We are confident that the geomagnetic event recorded in Core 34X in Hole 1095B from 394.36 to 399.35 mcd corresponds to cryptochron C4r.2r-1 because its age is consistent with biostratigraphic constraints (Iwai et al., 2002), its location within the magnetostratigraphy for the site is unambiguous (Acton et al., 2002), and the paleomagnetically-determined magnetostratigraphy is mirrored in the downhole logging data collected with the Geologic High-resolution Magnetic Tool (GHMT) (Fig. 2), which illustrates that no polarity chrons were missed in core-recovery gaps (Williams et al., 2002). Within the interval of interest, the sediment was cored using the XCB system and recovery was nearly 100%. Small gaps are common between ODP cores, although this does not impact our analysis of cryptochronozone 4r.2r-1 in Hole 1095B because it occurs entirely within Core 34X (cryptochronozone refers to the stratigraphic interval in which the cryptochron occurs). Like nearly all XCB cores, Core 34X is comprised of drilling bis-
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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.
cuits, which are pieces of core (typically 4–20 cm thick) that are separated by a thin layer of drilling slurry (typically < 2 cm thick) containing broken and ground up core material. The lithology of Core 34X and adjacent cores consist of greenish gray diatom-bearing silty clay and dark greenish gray silty clay. Within Core 34X, the sediments are finely laminated except in two moderately bioturbated intervals that occur near the top of section 6 and
base of section 5 and near the top of section 2 and base of section 1 (Barker et al., 1999). Discrete paleomagnetic samples were collected only from the drill biscuits. Because the biscuits can rotate azimuthally relative to each other, paleomagnetic declinations differ between biscuits and are not evaluated further in this study. Fortunately, the declination contributes little to the total remanence because the paleomagnetic direction is steep at Site 1095 (e.g., the present-day geo-
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centric axial dipole field inclination is −78◦ ), and so there is no ambiguity about the geomagnetic polarity. 3. Methods Paleomagnetic results were obtained every 4 cm along the split-core sections (6.6-cm-diameter core sections split length-wise that are up to 1.5-m long) and from 35 discrete samples (∼6–7 cm3 oriented cubes) within the interval of interest. Measurements were made using the cryogenic magnetometer onboard the JOIDES Resolution, with additional measurements made at the paleomagnetism laboratory at the University of Florida and at the Institute for Rock Magnetism at University of Minnesota. Natural remanent magnetization (NRM) was measured along the split-core sections following progressive alternating field (AF) demagnetization up to 30 mT. Discrete samples were typically AF demagnetized up to 80 mT. Three discrete samples were thermally demagnetized in 25 ◦ C steps from 100 to 700 ◦ C. Mean directions for the discrete samples were determined by principal component analysis (PCA), whereas we took the 30 mT AF demagnetization data as the representative NRM direction for the split-cores. Magnetic susceptibility was determined for both whole core sections and discrete samples. Anhysteretic remanent magnetization (ARM), isothermal remanent magnetization (IRM), and hysteresis parameters were determined for most discrete samples. In addition, lowtemperature magnetometry was conducted on small sub-
samples using a Magnetic Property Measurement System (MPMS) from Quantum Design. The raw paleomagnetic data for split-core and discrete samples and the PCA results for the discrete samples are available on the web as part of the Leg 178 Initial Reports volume (Barker et al., 1999). Pertinent additional paleomagnetic and rock magnetic results are given in supplementary data tables for discrete samples (Table A1) and for split-core sections (Table A2). 4. Results The results from Core 34X fit within the larger context of the magnetostratigraphy, paleomagnetism, and rock magnetism of Site 1095 (Fig. 2), which are described in Acton et al. (2002). As they noted, the characteristic remanent magnetization (ChRM) directions are well resolved over the cored interval, after removal of a low-coercivity or low-to-medium unblocking temperature drilling overprint, as is also observed in Core 34X (Figs. 3–5). Of the 35 discrete samples, only one sample had a relatively poorly resolved PCA direction (sample 178-1095B-34X-2, 46 cm) as evidenced by its high maximum angular deviation (MAD) angle and anomalous direction relative to adjacent samples. Sample 1781095B-34X-2, 149 cm also has an anomalous direction as well as low magnetic concentration relative to adjacent samples. For these reasons, neither sample is used in the following analyses. ChRM directions determined either from PCA for the rest of the discrete samples or
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.
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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.
from the demagnetization of the split-core samples at 30 mT give similar results (Fig. 2). The rock magnetic results from Core 34X indicate that the remanence is carried mainly by coarse pseudosingle domain (PSD) magnetite or titanomagnetite. This is consistent with the findings of Acton et al. (2002) for the rest of the section at Site 1095 and with the rock magnetic study of Brachfeld et al. (2002) on sediments from Site 1096, which is located within the same sediment drift as Site 1095. Verwey transitions for titanomagnetite, which are crystallographic phase transitions that occur near 110 K, are well developed for samples taken above, below, and in the cryptochronozone (Fig. 6). The average coercivity (Hc ) is 8.4 ± 1.5 mT (standard deviation), the ratio of the coercivity of remanence to the coercivity (Hcr /Hc ) is 4.0 ± 0.7, and the ratio of the saturation remanent magnetization to the saturation magnetization
(Mrs /Ms ) is 0.08 ± 0.02. These hysteresis parameters indicate the grain size is PSD, but close to the PSD/multidomain threshold (Day et al., 1977). The ratio of ARM/IRM provides additional information about grain size variations of ferrimagnetic particles, with larger values indicating finer particle size. Ideally, the lower the variation in the ARM/IRM values and in the magnetic concentration parameters (susceptibility, ARM, and IRM) the more homogeneous the sediments and the more ideal they are for relative paleointensity studies. Generally, variations less than about an order of magnitude are considered desirable (Tauxe, 1993). The variation across the cryptochronozone is small, with extreme values only differing by a factor of 2.6 (Fig. 7). Interestingly, both polarity transition zones bounding the cryptochronozone are associated with decreases in particle size, possibly indicating climatic or environmental
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.
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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.
changes concurrent with the polarity transitions. Similarly, the concentration parameters do not vary by more than factor of 10, with volume susceptibility varying by 9.3, ARM30 (the ARM after 30 mT demagnetization) varying by 7.0, and IRM30 varying by 7.3. Thus, the sediments are suitable for relative paleointensity study. Progressive demagnetization results provide additional rock magnetic insights. Most importantly, the sediments have a magnetic mineralogy that is capable of carrying a stable ChRM, and this ChRM is easily resolved following the removal of the ubiquitous drilling overprint in peak alternating fields of 10–15 mT or peak temperatures of 250 ◦ C. Virtually all the magnetization is removed by 80 mT AF demagnetization (Fig. 3), which is a maximum coercivity consistent with the presence of coarse PSD titanomagnetite and magnetite. Thermal demagnetization reveals that about 80–90% of magnetization decays between 275 and 425 ◦ C, consistent with the presence of titanomagnetite (Fig. 4). Another 10–20% of the total magnetization is removed between 525 and 600 ◦ C, indicating the presence of magnetite.
After removing the overprint, the remaining magnetization decays linearly to the origin of the diagrams, except in the polarity transition zone, where virtually no magnetization remains (Fig. 5). The higher coercivity component is the ChRM that is interpreted to be the ancient magnetization acquired at or shortly after deposition. The lack of a ChRM in the samples from within the polarity transition zones is a clear indication that the paleointensity had collapsed to near zero. Given the linear nature of the demagnetization paths outside the polarity transition zones, it is no surprise that any demagnetization step from about 20 to 70 mT gives virtually the same direction. This property allows us to take advantage of the large number of NRM data collected along split-core section. The normal polarity interval interpreted to be cryptochron 4r.2r-1 and the surrounding reversely magnetized sediment of Chron 4r.2r are equally well resolved by split-core and discrete samples (Fig. 7). Mean inclinations computed using the method of McFadden and Reid (1982) are consistent with the field reversing fully within the cryptochron. The mean inclination within the cryptochronozone for the discrete samples is −75.3 ± 8.3◦ (N = 16; precision parameter, k = 23.0) and for the splitcore sample is −69.4 ± 6.2◦ (N = 107; k = 15.0). Uncertainties are given as one-dimensional 95% confidence limits. Within the reversely magnetized portion of Chron 4r.2r between 370 and 406 mcd, the mean inclination for the discrete samples is 72.7 ± 6.0◦ (N = 13; k = 49.7) and for the split-core samples is 75.0 ± 4.9◦ (N = 592; k = 22.7). The only marginally significant difference, that between the absolute values of the split-core inclinations inside and outside the cryptochronozone, most likely results from a small unremoved drilling overprint (possibly within the slurry only), which would make the normal polarity directions shallower and the reversely magnetized directions steeper. The relative paleointensity estimated for the discrete samples using the NRM after 30 mT AF demagnetization (NRM30) divided by volume susceptibility (K), ARM, or IRM give very similar results (Fig. 7 and Table A1). An exception to this pattern occurs in the basal transition zone of the cryptochronozone where susceptibilitynormalized results for two samples give higher relative values. Overall the discrete results agree very well with the more extensive split-core record estimated from NRM30/K (Table A2). The relative paleointensity is calculated such that the mean has a value of one within the interval where both discrete and split-core samples have been collected (392.07–400.62 mcd), which allows the different relative paleointensity estimates to be scaled independently.
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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.
Together the data illustrate a clear pattern in which the paleointensity collapses to <3% and <10% of the mean value within the upper and lower polarity transition zones, respectively, but recovers within a few thousand years. The field recovery after a transition is often marked by a relative high in paleointensity as seen above the cryptochronozone (392–394 mcd) and after many other polarity transitions at this site. Similar behavior has been noted by Meynadier et al. (1994), and is referred to as the “sawtooth” paleointensity pattern. The cryptochronozone is an exception to this pattern as the paleointensity recovers gradually at the start of the cryptochronozone and then decays gradually at the end of the cryptochronozone. The paleointensity within the cryptochronozone is about 20–40% higher than the long-term mean relative paleointensity calculated for the interval from 200 to 450 m, although it is lower than the surrounding reversely magnetized sediments of Chronozone 4r.2r. The width of the cryptochronozone is 4.99 m and the polarity transitions bounding it are each about 20 cm wide. The average sedimentation rate in Chronozone C4r.2r is 89 m/m.y. (=55.68 m/0.627 m.y.), which is similar to the 92 m/m.y. (=198.27 m/2.148 m.y.) rate over all of Chronozone 4. The duration of the cryptochron, assuming constant sedimentation rates during C4r.2r, is
56 k.y. and the transition zones span ∼2 k.y. each. The onset and termination of the cryptochronozone occurs 6.87 and 11.87 m, respectively, above the onset of the 55.68-m-thick Chronozone 4r.2r. This places the onset of the cryptochron at 8.622 Ma and termination at 8.566 Ma (Table 1). 5. Discussion and conclusions Although cryptochrons observed in marine magnetic anomalies have been argued to result from paleointensity anomalies alone (most recently by Bowles et al., 2003), clearly this is not the case. Both the inclination and paleointensity across cryptochron C4r.2r-1 indicate it is a full polarity reversal. Furthermore, its duration is comparable to that of many of the shorter subchrons, which is undoubtedly true of many other cryptochrons (e.g., Roberts and Lewin-Harris, 2000). In general, one would expect cryptochrons to result from a variety of behavior noted for the geomagnetic field when it is not in a stable polarity state, which appears to be relatively common (Lund et al., 1998, 2001). We suggest cryptochrons are associated with short polarity reversals (subchrons), excursions, intervals of anomalously high paleosecular variation (PSV),
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.
8.612 8.597 8.528 8.622 8.639 8.554 8.563 8.483 8.566 8.599 58 34 45 56 40 0.70 0.40 0.42 4.99 0.30 12.1 11.7 9.4 88.8 7.5 128.66 128.58 93.39 399.35 229.65 127.96 128.18 92.97 394.36 229.35 122.13 122.43 89.10 350.54 225.40 Hole 845A Hole 845B Site 1092 Splice Hole 1095B Hole 1179C
129.71 129.78 95.00 406.22 230.10
Age of C4r.2r-1 (o) (Ma) Age of C4r.2r-1 (t) (Ma) Cryptochron duration (k.y.) Cryptochron thickness (m) Depth of Sedimentation C4r.2r-1 (o) (m) rate (m/m.y.) Depth of C4r.2r-1 (t) (m) Depth of C4r.2r (o) (m) Depth of C4r.2r (t) (m) Location name
Table 1 Age and duration of Cryptochron 4r.2r-1
1 1 2 3 4
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and paleointensity lows. Because paleointensity lows are associated with polarity transitions, excursions, and high PSV (e.g., Valet, 2003), the “tiny wiggles” in marine magnetic anomalies are likewise highly correlated with paleointensity lows. Our study merely indicates that the association is not exclusive and that the paleomagnetic direction plays a role. Ascertaining the exact field behavior during a cryptochron is further complicated by geographic differences in the geomagnetic field. A geomagnetic event recorded simultaneously at different positions around the world may be a paleointensity low with little directional change at one location, while being an excursion or a complete geomagnetic reversal at other locations. A case in point are the many excursions that are recorded in sediments along the Blake-Bahama Outer Ridge in the North Atlantic (Lund et al., 2001). In comparison, only a few of these are apparent in sediments further north at the Gardar Drift (Channell, 1999), even though sedimentation rates are similar at the two locations. This appears to be the case even for some subchrons, such as the Cobb Mountain and Reunion, which are not always recorded in sediments and lavas of the appropriate age. For both excursions and subchrons, however, the failure to observe directional variability at different sites may be controlled more by the fidelity of the recording sediments or lavas than by real spatial differences in the geomagnetic field. Even with the variability in recording fidelity, cryptochron C4r.2r-1 appears to be a full polarity reversal as it is recorded at several sites with a wide spatial distribution. Schneider (1995) first observed this cryptochron in equatorial sediments collected on ODP Leg 138 at Site 845 in the Pacific Ocean. After being identified on ODP Leg 178 at Site 1095 (Acton et al., 2002; Barker et al., 1999), it has since also been recognized in sediments from the South Atlantic Ocean on ODP Leg 177 at Site 1092 (Evans and Channell, 2003; Evans et al., 2004) and on ODP Leg 191 at Site 1179 in the northwest Pacific Ocean (Kanazawa et al., 2001; B. Horner-Johnson personal comm.). The cryptochronozone is about an order of magnitude thicker at Site 1095 than at the other three sites where it is observed (Table 1). Because the cryptochronozone is only 30–70 cm thick at the other sites and the data were generally gathered at 5 cm spacing, the resolution is fairly low for these sites. In contrast, the cryptochronozone at Site 1095 is 4.99-m-thick, which allows direction and paleointensity variations and the age and duration of the cryptochron to be much better resolved. Our preferred age (8.566–8.622 Ma) and duration (56 k.y.) is based on the Site 1095 results and agrees well with
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duce comparable modeled anomalies. Ultimately, both paleointensity and direction are recorded in the magnetic source layer of the oceanic lithosphere and contribute to the shapes of the anomalies that are recorded in marine magnetic anomaly profiles. Acknowledgements 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,b). 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.
the range of values from the other sites (Table 1), with the average age (8.553–8.599 Ma) being about 17 k.y. younger and the average duration (47 k.y.) 9 k.y. shorter than our preferred values. Our preferred age is, however, 49 k.y. younger than that estimated by Cande and Kent (1995) from marine magnetic anomalies and the duration more than triple their 16 k.y. estimate. Because the previous estimates of the age and duration of the cryptochron were based on only two marine magnetic profiles from the east flank of the East Pacific Rise (Cande and Kent, 1992a,b), such differences could easily be attributed to changes in seafloor spreading rates, asymmetry in seafloor spreading, ridge jumps, or to factors that may have affected the recording fidelity of the ocean crust in the region. Interestingly, cryptochron 4r.2r-1 is recorded better at all four drill sites than is subchron 4r.1n, which has a duration of 32 k.y. in the timescale of Cande and Kent (1995). This provides support for cryptochron 4r.2r-1 having a duration longer than subchron 4r.1n and several times larger than was originally estimated from marine magnetic anomalies. Given the quality of the paleomagnetic results from Site 1095 and their general agreement with other drill sites, we suggest that cryptochron 4r.2r-1 should be elevated to subchron status. Following the convention of Cande and Kent (1992a), this would simply entail renaming it subchron 4r.2r-1n and updating its age and duration to be consistent with the observations from Site 1095. The polarity reversal we are advocating, when inserted into the geomagnetic polarity timescale and used to model marine magnetic anomalies, produces a tiny wiggle comparable to that in observed marine magnetic anomalies (Fig. 8). Undoubtedly, many combinations of paleointensity and direction changes could likewise pro-
This research used samples and/or data provided by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. Funding for this research was provided by JOI/USSSP grants to Acton, Guyodo, and Brachfeld. During completion of this project, Acton was supported by the ODP at Texas A&M and by NSF proposal EAR0136498 to Ken Verosub. The authors thank Mike Jackson for his assistance at the Institute for Rock Magnetism (IRM). The IRM is supported by grants from the Earth Sciences Instrumentation and Facilities program of the National Science Foundation and the W.M. Keck Foundation. We also thank Jim Channell for providing access and assistance to the University of Florida paleomagnetism laboratory. Finally, we thank the Editor (Will Sager), two anonymous reviewers, and Helen Evans for their suggestions and comments. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.pepi.2005. 09.015. References Acton, G.D., Guyodo, Y., Brachfeld, S.A., 2002. Magnetostratigraphy of sediment drifts on the continental rise of West Antarctica (ODP Leg 178, Sites 1095, 1096, and 1101). In: Barker, P.F., Camerlenghi, A., Acton, G.D., Ramsay, A.T.S. (Eds.), Proceedings of the ODP, Sci. Results, 178. Ocean Drilling Program. College Station, TX. Barker, P.F., 2001. Composite depths and spliced sections for Leg 178 Sites 1095 and 1096, Antarctic Peninsula continental rise. In: Barker, P.F., Camerlenghi, A., Acton, G.D., Ramsay, A.T.S. (Eds.), Proceedings of the ODP, Sci. Results, 178. Ocean Drilling Program. College Station, TX. Barker, P.F., Camerlenghi, A., Acton, G.D., the Shipboard Scientific Party, 1999. Proc. ODP, Init. Repts., 178 [Online]. Available from World Wide Web:. [Cited 2005-08-26], Ocean Drilling Program, Texas A&M University, College Station, TX. Blakely, R.J., 1974. Geomagnetic reversals and crustal spreading rates during the Miocene. J. Geophys. Res. 79, 2979–2985. Blakely, R.J., Cox, A., 1972. Evidence for short geomagnetic polarity intervals in the early Cenozoic. J. Geophys. Res. 77, 7065–7072.
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Bowers, N.E., Cande, S.C., Gee, J., Hildebrand, J.A., Parker, R.L., 2001. Fluctuations of the paleomagnetic field during chron C5 as recorded in near bottom marine magnetic anomaly data. J. Geophys. Res. 106, 26379–26396. Bowles, J., Tauxe, L., Gee, J., McMillan, D., Cande, S.C., 2003. Source of tiny wiggles in Chron C5: a comparison of sedimentary relative intensity and marine magnetic anomalies. Geochem. Geophys. Geosyst. 4, 1049, DOI: 10.1029/2002GC000489. Brachfeld, S.A., Guyodo, Y., Acton, G.D., 2002. The magnetic mineral assemblage of hemipelagic drifts, ODP Site 1096. In: Barker, P.F., Camerlenghi, A., Acton, G.D., Ramsay, A.T.S. (Eds.), Proceedings of the ODP, Sci. Results, 178. Ocean Drilling Program. College Station, TX. Cande, S.C., Kent, D.V., 1992a. A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic. J. Geophys. Res. 97, 13917–13951. Cande, S.C., Kent, D.V., 1992b. Ultrahigh resolution marine magnetic anomaly profiles: a record of continuous paleointensity variations? J. Geophys. Res. 97, 15075–15083. Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res. 100, 6093–6095. Cande, S.C., LaBrecque, J.L., 1974. Behavior of the Earth’s palaeomagnetic field from small scale marine magnetic anomalies. Nature 247, 26–28. Channell, J.E.T., 1999. Geomagnetic paleointensity and directional secular variation at Ocean Drilling Program (ODP) Site 984 (Bjorn Drift) since 500 ka: comparisons with ODP Site 983 (Gardar Drift). J. Geophys. Res. 104, 22937–22951. Day, R., Fuller, M., Schmidt, V.A., 1977. Hysteresis properties of titanomagnetites: grain-size and compositional dependence. Phys. Earth Planet. Inter. 13, 260–267. Emilia, D., Heinrichs, D., 1972. Paleomagnetic events in the Brunhes and Matuyama epochs identified from magnetic profiles reduced to the poles. Mar. Geophys. Res. 1, 436–444. Evans, H.F., Channell, J.E.T., 2003. Upper Miocene magnetic stratigraphy at ODP site 1092 (sub-Antarctic South Atlantic): recognition of cryptochrons in C5n.2n. Geophys. J. Inter. 153, 483–496. Evans, H.F., Westerhold, T., Channell, J.E.T., 2004. ODP Site 1092: revised composite depth section has implications for Upper Miocene cryptochrons. Geophys. J. Inter. 156, 195–199. Gee, J.S., Cande, S.C., Hildebrand, J.A., Donnelly, K., Parker, R.L., 2000. Geomagnetic intensity variations over the past 780 kyr obtained from near-seafloor magnetic anomalies. Nature 408, 827–832.
Iwai, M., Acton, G.D., Lazarus, D., Osterman, L.E., Williams, T., 2002. Magnetobiochronologic synthesis of ODP Leg 178 rise sediments from the Pacific sector of the Southern Ocean: sites 1095, 1096, and 1101. In: Barker, P.F., Camerlenghi, A., Acton, G.D., Ramsay, A.T.S. (Eds.), Proceedings of the ODP, Sci. Results, 178. Ocean Drilling Program. College Station, TX. Kanazawa, T., Sager, W.W., Escutia, C., The Shipboard Scientific Party, 2001. Proceedings of the ODP, Init. Repts., 191 [CD-ROM]. Ocean Drilling Program, Texas A&M University, College Station, TX. Lund, S.P., Acton, G., Clement, B., Hastedt, M., Okada, M., Williams, T., The ODP Leg 172 Scientific Party, 1998. Geomagnetic field excursions occurred often during the last million years. Eos 79, 178–179. Lund, S.P., Williams, T., Acton, G.D., Clement, B., Okada, M., 2001. Brunhes Chron magnetic field excursions recovered from ODP Leg 172 sediments. In: Keigwin, L.D., Rio, D., Acton, G.D., Arnold, E. (Eds.), Proceedings of the ODP, Sci. Results, 172. Ocean Drilling Program. College Station, TX, pp. 1–18. McFadden, P.L., Reid, A.B., 1982. Analysis of paleomagnetic inclination data. Geophys. J. R. Astron. Soc. 69, 307–319. Meynadier, L., Valet, J.-P., Bassinot, F., Shackleton, N.J., Guyodo, Y., 1994. Asymmetrical saw tooth pattern of the geomagnetic field intensity from equatorial sediments in the Pacific and Indian Ocean. Earth Planet. Sci. Lett. 126, 109–127. Roberts, A.P., Lewin-Harris, J.C., 2000. Marine magnetic anomalies: evidence that “tiny wiggles” represent short-period geomagnetic polarity intervals. Earth Planet. Sci. Lett. 183, 375– 388. Schneider, D.A., 1995. Paleomagnetism of some Leg 138 sediments: detailing Miocene magnetostratigraphy. In: Pisias, N.G., Mayer, L.A., Janecek, T.R., Palmer-Julson, A., van Andel, T.H. (Eds.), Proceedings of the ODP, Sci. Results, 138. Ocean Drilling Program. College Station, TX, pp. 59–72. Tauxe, L., 1993. Sedimentary records of relative paleointensity of the geomagnetic field: theory and practice. Rev. Geophys. 31, 319–354. Valet, J.-P., 2003. Time variations in geomagnetic intensity. Rev. Geophys. 41, 1004, DOI: 10.1029/2001RG000104. Wessel, P., Smith, W.H.F., 1998. New, improved version of the Generic Mapping Tools released. Eos 79, 579. Williams, T., Louvel, V., Lauer-Leredde, C., 2002. Magnetic polarity stratigraphy from downhole logs, West Antarctic Peninsula, ODP Leg 178. In: Barker, P.F., Camerlenghi, A., Acton, G.D., Ramsay, A.T.S. (Eds.), Proceedings of the ODP, Sci. Results, 178. Ocean Drilling Program. College Station, TX.
The nature of a cryptochron from a paleomagnetic study of chron C4r.2r recorded in sediments off the Antarctic Peninsula Gary Acton a,∗ , Yohan Guyodo b,1 , Stefanie Brachfeld c,2 b
a One Shields Avenue, Department of Geology, University of California, Davis, CA 95616, USA Laboratoire des Sciences du Climat et de l’Environnement (LSCE), Domaine du CNRS, 12 Avenue de la Terrasse, Gif-sur-Yvette, Cedex 91198, France c Department of Earth & Environmental Studies, Montclair State University, Montclair, NJ 07043, USA
Received 2 May 2005; received in revised form 9 September 2005; accepted 9 September 2005
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. © 2006 Elsevier B.V. All rights reserved. Keywords: Cryptochrons; Excursions; Geomagnetic polarity timescale; Magnetostratigraphy; Geomagnetism; Relative paleointensity; Ocean Drilling Program; Site 1095; Antarctic Peninsula
1. Introduction
∗
Corresponding author. Tel.: +1 530 752 1861; fax: +1 530 752 0951. E-mail addresses: [email protected] (G. Acton), [email protected] (Y. Guyodo), [email protected] (S. Brachfeld). 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. 0031-9201/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2005.09.015
Cryptochrons, a term coined by Cande and Kent (1992a), are geomagnetic events of short duration (roughly 30 k.y. or less) whose origins are debated and whose characteristics are uncertain owing to the few direct observations that have been obtained from them. Rather than being identified from paleomagnetic studies of rocks or sediments, cryptochrons have been identified as small magnetic anomalies (referred to as “tiny
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wiggles”) observed in marine magnetic anomaly profiles over oceanic lithosphere created by seafloor spreading (Blakely and Cox, 1972; Cande and Kent, 1992a,b; Cande and LaBrecque, 1974). From such indirect potential field observations, it is impossible to determine the geomagnetic origin of the anomalies uniquely or to estimate their timing and duration accurately. Several studies have suggested that cryptochrons are caused primarily or solely by geomagnetic intensity variations (Bowers et al., 2001; Bowles et al., 2003; Cande and Kent, 1992b; Gee et al., 2000). Alternatively, cryptochrons could represent full polarity reversals of short duration (Blakely, 1974; Blakely and Cox, 1972; Emilia and Heinrichs, 1972), large changes in the geomagnetic field direction, such as observed for excursions or large secular variation events, or a combination of these in association with paleointensity decreases (Evans and Channell, 2003; Lund et al., 2001; Roberts and Lewin-Harris, 2000). We investigate the nature of one cryptochron, referred to as C4r.2r-1 (Cande and Kent, 1992a), which is particularly well recorded in sediments cored off the West Antarctic Peninsula at Ocean Drilling Program (ODP) Site 1095, as are other short geomagnetic events. We will show that the duration of this cryptochron is longer than previously inferred and that, unlike other short geomagnetic events, the paleomagnetic field fully reversed and the paleointensity recovered to above the long-term mean during the event. Thus, the nature of this cryptochron is similar to other subchrons that are part of the geomagnetic polarity timescale and represents just one form of behavior in the dynamically varying geomagnetic field. 2. Background and sampling Site 1095 (66.9853◦ S, 78.4878◦ W; 3842 m water depth) is located on the distal portion of a hemipelagic sediment drift on the continental rise off the northwestern, Pacific margin of the Antarctic Peninsula (Fig. 1). Four holes were cored at the site during ODP Leg 178, recovering a 570-m thick sedimentary section that extends from the Holocene to the late Miocene (0–10 Ma) (Barker et al., 1999). Note that all core, interval, and sample depths given below are meters composite depths (mcd) based on the mcd scale constructed by Barker (2001), which is 5.5 m shallower than the meters below seafloor depth (mbsf) scale for the cores we are considering in this study. Core recovery was high throughout the section, which was cored with the Advanced Piston Corer (APC) and Extended Core Barrel (XCB) systems. This enabled us to establish a magnetostratigraphy, which is fairly straight-
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.
forward and complete all the way from the termination of Chron C4Ar.2n (9.580 Ma) at ∼515 mcd through the Brunhes Chron (0–0.78 Ma) (Fig. 2). One exception occurs near the base of Chron 4r.2r (350.54–406.22 mcd; 8.072–8.699 Ma) where there is an extra normal polarity event (Acton et al., 2002; Barker et al., 1999). This event is not shown in most geomagnetic polarity time scales, although Cande and Kent (1995) have placed a geomagnetic event in the lower part of Chron 4r.2r, which they refer to as cryptochron C4r.2r-1. We focus our study on the interval that includes this cryptochron. We are confident that the geomagnetic event recorded in Core 34X in Hole 1095B from 394.36 to 399.35 mcd corresponds to cryptochron C4r.2r-1 because its age is consistent with biostratigraphic constraints (Iwai et al., 2002), its location within the magnetostratigraphy for the site is unambiguous (Acton et al., 2002), and the paleomagnetically-determined magnetostratigraphy is mirrored in the downhole logging data collected with the Geologic High-resolution Magnetic Tool (GHMT) (Fig. 2), which illustrates that no polarity chrons were missed in core-recovery gaps (Williams et al., 2002). Within the interval of interest, the sediment was cored using the XCB system and recovery was nearly 100%. Small gaps are common between ODP cores, although this does not impact our analysis of cryptochronozone 4r.2r-1 in Hole 1095B because it occurs entirely within Core 34X (cryptochronozone refers to the stratigraphic interval in which the cryptochron occurs). Like nearly all XCB cores, Core 34X is comprised of drilling bis-
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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.
cuits, which are pieces of core (typically 4–20 cm thick) that are separated by a thin layer of drilling slurry (typically < 2 cm thick) containing broken and ground up core material. The lithology of Core 34X and adjacent cores consist of greenish gray diatom-bearing silty clay and dark greenish gray silty clay. Within Core 34X, the sediments are finely laminated except in two moderately bioturbated intervals that occur near the top of section 6 and
base of section 5 and near the top of section 2 and base of section 1 (Barker et al., 1999). Discrete paleomagnetic samples were collected only from the drill biscuits. Because the biscuits can rotate azimuthally relative to each other, paleomagnetic declinations differ between biscuits and are not evaluated further in this study. Fortunately, the declination contributes little to the total remanence because the paleomagnetic direction is steep at Site 1095 (e.g., the present-day geo-
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centric axial dipole field inclination is −78◦ ), and so there is no ambiguity about the geomagnetic polarity. 3. Methods Paleomagnetic results were obtained every 4 cm along the split-core sections (6.6-cm-diameter core sections split length-wise that are up to 1.5-m long) and from 35 discrete samples (∼6–7 cm3 oriented cubes) within the interval of interest. Measurements were made using the cryogenic magnetometer onboard the JOIDES Resolution, with additional measurements made at the paleomagnetism laboratory at the University of Florida and at the Institute for Rock Magnetism at University of Minnesota. Natural remanent magnetization (NRM) was measured along the split-core sections following progressive alternating field (AF) demagnetization up to 30 mT. Discrete samples were typically AF demagnetized up to 80 mT. Three discrete samples were thermally demagnetized in 25 ◦ C steps from 100 to 700 ◦ C. Mean directions for the discrete samples were determined by principal component analysis (PCA), whereas we took the 30 mT AF demagnetization data as the representative NRM direction for the split-cores. Magnetic susceptibility was determined for both whole core sections and discrete samples. Anhysteretic remanent magnetization (ARM), isothermal remanent magnetization (IRM), and hysteresis parameters were determined for most discrete samples. In addition, lowtemperature magnetometry was conducted on small sub-
samples using a Magnetic Property Measurement System (MPMS) from Quantum Design. The raw paleomagnetic data for split-core and discrete samples and the PCA results for the discrete samples are available on the web as part of the Leg 178 Initial Reports volume (Barker et al., 1999). Pertinent additional paleomagnetic and rock magnetic results are given in supplementary data tables for discrete samples (Table A1) and for split-core sections (Table A2). 4. Results The results from Core 34X fit within the larger context of the magnetostratigraphy, paleomagnetism, and rock magnetism of Site 1095 (Fig. 2), which are described in Acton et al. (2002). As they noted, the characteristic remanent magnetization (ChRM) directions are well resolved over the cored interval, after removal of a low-coercivity or low-to-medium unblocking temperature drilling overprint, as is also observed in Core 34X (Figs. 3–5). Of the 35 discrete samples, only one sample had a relatively poorly resolved PCA direction (sample 178-1095B-34X-2, 46 cm) as evidenced by its high maximum angular deviation (MAD) angle and anomalous direction relative to adjacent samples. Sample 1781095B-34X-2, 149 cm also has an anomalous direction as well as low magnetic concentration relative to adjacent samples. For these reasons, neither sample is used in the following analyses. ChRM directions determined either from PCA for the rest of the discrete samples or
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.
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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.
from the demagnetization of the split-core samples at 30 mT give similar results (Fig. 2). The rock magnetic results from Core 34X indicate that the remanence is carried mainly by coarse pseudosingle domain (PSD) magnetite or titanomagnetite. This is consistent with the findings of Acton et al. (2002) for the rest of the section at Site 1095 and with the rock magnetic study of Brachfeld et al. (2002) on sediments from Site 1096, which is located within the same sediment drift as Site 1095. Verwey transitions for titanomagnetite, which are crystallographic phase transitions that occur near 110 K, are well developed for samples taken above, below, and in the cryptochronozone (Fig. 6). The average coercivity (Hc ) is 8.4 ± 1.5 mT (standard deviation), the ratio of the coercivity of remanence to the coercivity (Hcr /Hc ) is 4.0 ± 0.7, and the ratio of the saturation remanent magnetization to the saturation magnetization
(Mrs /Ms ) is 0.08 ± 0.02. These hysteresis parameters indicate the grain size is PSD, but close to the PSD/multidomain threshold (Day et al., 1977). The ratio of ARM/IRM provides additional information about grain size variations of ferrimagnetic particles, with larger values indicating finer particle size. Ideally, the lower the variation in the ARM/IRM values and in the magnetic concentration parameters (susceptibility, ARM, and IRM) the more homogeneous the sediments and the more ideal they are for relative paleointensity studies. Generally, variations less than about an order of magnitude are considered desirable (Tauxe, 1993). The variation across the cryptochronozone is small, with extreme values only differing by a factor of 2.6 (Fig. 7). Interestingly, both polarity transition zones bounding the cryptochronozone are associated with decreases in particle size, possibly indicating climatic or environmental
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.
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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.
changes concurrent with the polarity transitions. Similarly, the concentration parameters do not vary by more than factor of 10, with volume susceptibility varying by 9.3, ARM30 (the ARM after 30 mT demagnetization) varying by 7.0, and IRM30 varying by 7.3. Thus, the sediments are suitable for relative paleointensity study. Progressive demagnetization results provide additional rock magnetic insights. Most importantly, the sediments have a magnetic mineralogy that is capable of carrying a stable ChRM, and this ChRM is easily resolved following the removal of the ubiquitous drilling overprint in peak alternating fields of 10–15 mT or peak temperatures of 250 ◦ C. Virtually all the magnetization is removed by 80 mT AF demagnetization (Fig. 3), which is a maximum coercivity consistent with the presence of coarse PSD titanomagnetite and magnetite. Thermal demagnetization reveals that about 80–90% of magnetization decays between 275 and 425 ◦ C, consistent with the presence of titanomagnetite (Fig. 4). Another 10–20% of the total magnetization is removed between 525 and 600 ◦ C, indicating the presence of magnetite.
After removing the overprint, the remaining magnetization decays linearly to the origin of the diagrams, except in the polarity transition zone, where virtually no magnetization remains (Fig. 5). The higher coercivity component is the ChRM that is interpreted to be the ancient magnetization acquired at or shortly after deposition. The lack of a ChRM in the samples from within the polarity transition zones is a clear indication that the paleointensity had collapsed to near zero. Given the linear nature of the demagnetization paths outside the polarity transition zones, it is no surprise that any demagnetization step from about 20 to 70 mT gives virtually the same direction. This property allows us to take advantage of the large number of NRM data collected along split-core section. The normal polarity interval interpreted to be cryptochron 4r.2r-1 and the surrounding reversely magnetized sediment of Chron 4r.2r are equally well resolved by split-core and discrete samples (Fig. 7). Mean inclinations computed using the method of McFadden and Reid (1982) are consistent with the field reversing fully within the cryptochron. The mean inclination within the cryptochronozone for the discrete samples is −75.3 ± 8.3◦ (N = 16; precision parameter, k = 23.0) and for the splitcore sample is −69.4 ± 6.2◦ (N = 107; k = 15.0). Uncertainties are given as one-dimensional 95% confidence limits. Within the reversely magnetized portion of Chron 4r.2r between 370 and 406 mcd, the mean inclination for the discrete samples is 72.7 ± 6.0◦ (N = 13; k = 49.7) and for the split-core samples is 75.0 ± 4.9◦ (N = 592; k = 22.7). The only marginally significant difference, that between the absolute values of the split-core inclinations inside and outside the cryptochronozone, most likely results from a small unremoved drilling overprint (possibly within the slurry only), which would make the normal polarity directions shallower and the reversely magnetized directions steeper. The relative paleointensity estimated for the discrete samples using the NRM after 30 mT AF demagnetization (NRM30) divided by volume susceptibility (K), ARM, or IRM give very similar results (Fig. 7 and Table A1). An exception to this pattern occurs in the basal transition zone of the cryptochronozone where susceptibilitynormalized results for two samples give higher relative values. Overall the discrete results agree very well with the more extensive split-core record estimated from NRM30/K (Table A2). The relative paleointensity is calculated such that the mean has a value of one within the interval where both discrete and split-core samples have been collected (392.07–400.62 mcd), which allows the different relative paleointensity estimates to be scaled independently.
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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.
Together the data illustrate a clear pattern in which the paleointensity collapses to <3% and <10% of the mean value within the upper and lower polarity transition zones, respectively, but recovers within a few thousand years. The field recovery after a transition is often marked by a relative high in paleointensity as seen above the cryptochronozone (392–394 mcd) and after many other polarity transitions at this site. Similar behavior has been noted by Meynadier et al. (1994), and is referred to as the “sawtooth” paleointensity pattern. The cryptochronozone is an exception to this pattern as the paleointensity recovers gradually at the start of the cryptochronozone and then decays gradually at the end of the cryptochronozone. The paleointensity within the cryptochronozone is about 20–40% higher than the long-term mean relative paleointensity calculated for the interval from 200 to 450 m, although it is lower than the surrounding reversely magnetized sediments of Chronozone 4r.2r. The width of the cryptochronozone is 4.99 m and the polarity transitions bounding it are each about 20 cm wide. The average sedimentation rate in Chronozone C4r.2r is 89 m/m.y. (=55.68 m/0.627 m.y.), which is similar to the 92 m/m.y. (=198.27 m/2.148 m.y.) rate over all of Chronozone 4. The duration of the cryptochron, assuming constant sedimentation rates during C4r.2r, is
56 k.y. and the transition zones span ∼2 k.y. each. The onset and termination of the cryptochronozone occurs 6.87 and 11.87 m, respectively, above the onset of the 55.68-m-thick Chronozone 4r.2r. This places the onset of the cryptochron at 8.622 Ma and termination at 8.566 Ma (Table 1). 5. Discussion and conclusions Although cryptochrons observed in marine magnetic anomalies have been argued to result from paleointensity anomalies alone (most recently by Bowles et al., 2003), clearly this is not the case. Both the inclination and paleointensity across cryptochron C4r.2r-1 indicate it is a full polarity reversal. Furthermore, its duration is comparable to that of many of the shorter subchrons, which is undoubtedly true of many other cryptochrons (e.g., Roberts and Lewin-Harris, 2000). In general, one would expect cryptochrons to result from a variety of behavior noted for the geomagnetic field when it is not in a stable polarity state, which appears to be relatively common (Lund et al., 1998, 2001). We suggest cryptochrons are associated with short polarity reversals (subchrons), excursions, intervals of anomalously high paleosecular variation (PSV),
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.
8.612 8.597 8.528 8.622 8.639 8.554 8.563 8.483 8.566 8.599 58 34 45 56 40 0.70 0.40 0.42 4.99 0.30 12.1 11.7 9.4 88.8 7.5 128.66 128.58 93.39 399.35 229.65 127.96 128.18 92.97 394.36 229.35 122.13 122.43 89.10 350.54 225.40 Hole 845A Hole 845B Site 1092 Splice Hole 1095B Hole 1179C
129.71 129.78 95.00 406.22 230.10
Age of C4r.2r-1 (o) (Ma) Age of C4r.2r-1 (t) (Ma) Cryptochron duration (k.y.) Cryptochron thickness (m) Depth of Sedimentation C4r.2r-1 (o) (m) rate (m/m.y.) Depth of C4r.2r-1 (t) (m) Depth of C4r.2r (o) (m) Depth of C4r.2r (t) (m) Location name
Table 1 Age and duration of Cryptochron 4r.2r-1
1 1 2 3 4
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and paleointensity lows. Because paleointensity lows are associated with polarity transitions, excursions, and high PSV (e.g., Valet, 2003), the “tiny wiggles” in marine magnetic anomalies are likewise highly correlated with paleointensity lows. Our study merely indicates that the association is not exclusive and that the paleomagnetic direction plays a role. Ascertaining the exact field behavior during a cryptochron is further complicated by geographic differences in the geomagnetic field. A geomagnetic event recorded simultaneously at different positions around the world may be a paleointensity low with little directional change at one location, while being an excursion or a complete geomagnetic reversal at other locations. A case in point are the many excursions that are recorded in sediments along the Blake-Bahama Outer Ridge in the North Atlantic (Lund et al., 2001). In comparison, only a few of these are apparent in sediments further north at the Gardar Drift (Channell, 1999), even though sedimentation rates are similar at the two locations. This appears to be the case even for some subchrons, such as the Cobb Mountain and Reunion, which are not always recorded in sediments and lavas of the appropriate age. For both excursions and subchrons, however, the failure to observe directional variability at different sites may be controlled more by the fidelity of the recording sediments or lavas than by real spatial differences in the geomagnetic field. Even with the variability in recording fidelity, cryptochron C4r.2r-1 appears to be a full polarity reversal as it is recorded at several sites with a wide spatial distribution. Schneider (1995) first observed this cryptochron in equatorial sediments collected on ODP Leg 138 at Site 845 in the Pacific Ocean. After being identified on ODP Leg 178 at Site 1095 (Acton et al., 2002; Barker et al., 1999), it has since also been recognized in sediments from the South Atlantic Ocean on ODP Leg 177 at Site 1092 (Evans and Channell, 2003; Evans et al., 2004) and on ODP Leg 191 at Site 1179 in the northwest Pacific Ocean (Kanazawa et al., 2001; B. Horner-Johnson personal comm.). The cryptochronozone is about an order of magnitude thicker at Site 1095 than at the other three sites where it is observed (Table 1). Because the cryptochronozone is only 30–70 cm thick at the other sites and the data were generally gathered at 5 cm spacing, the resolution is fairly low for these sites. In contrast, the cryptochronozone at Site 1095 is 4.99-m-thick, which allows direction and paleointensity variations and the age and duration of the cryptochron to be much better resolved. Our preferred age (8.566–8.622 Ma) and duration (56 k.y.) is based on the Site 1095 results and agrees well with
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duce comparable modeled anomalies. Ultimately, both paleointensity and direction are recorded in the magnetic source layer of the oceanic lithosphere and contribute to the shapes of the anomalies that are recorded in marine magnetic anomaly profiles. Acknowledgements 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,b). 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.
the range of values from the other sites (Table 1), with the average age (8.553–8.599 Ma) being about 17 k.y. younger and the average duration (47 k.y.) 9 k.y. shorter than our preferred values. Our preferred age is, however, 49 k.y. younger than that estimated by Cande and Kent (1995) from marine magnetic anomalies and the duration more than triple their 16 k.y. estimate. Because the previous estimates of the age and duration of the cryptochron were based on only two marine magnetic profiles from the east flank of the East Pacific Rise (Cande and Kent, 1992a,b), such differences could easily be attributed to changes in seafloor spreading rates, asymmetry in seafloor spreading, ridge jumps, or to factors that may have affected the recording fidelity of the ocean crust in the region. Interestingly, cryptochron 4r.2r-1 is recorded better at all four drill sites than is subchron 4r.1n, which has a duration of 32 k.y. in the timescale of Cande and Kent (1995). This provides support for cryptochron 4r.2r-1 having a duration longer than subchron 4r.1n and several times larger than was originally estimated from marine magnetic anomalies. Given the quality of the paleomagnetic results from Site 1095 and their general agreement with other drill sites, we suggest that cryptochron 4r.2r-1 should be elevated to subchron status. Following the convention of Cande and Kent (1992a), this would simply entail renaming it subchron 4r.2r-1n and updating its age and duration to be consistent with the observations from Site 1095. The polarity reversal we are advocating, when inserted into the geomagnetic polarity timescale and used to model marine magnetic anomalies, produces a tiny wiggle comparable to that in observed marine magnetic anomalies (Fig. 8). Undoubtedly, many combinations of paleointensity and direction changes could likewise pro-
This research used samples and/or data provided by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. Funding for this research was provided by JOI/USSSP grants to Acton, Guyodo, and Brachfeld. During completion of this project, Acton was supported by the ODP at Texas A&M and by NSF proposal EAR0136498 to Ken Verosub. The authors thank Mike Jackson for his assistance at the Institute for Rock Magnetism (IRM). The IRM is supported by grants from the Earth Sciences Instrumentation and Facilities program of the National Science Foundation and the W.M. Keck Foundation. We also thank Jim Channell for providing access and assistance to the University of Florida paleomagnetism laboratory. Finally, we thank the Editor (Will Sager), two anonymous reviewers, and Helen Evans for their suggestions and comments. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.pepi.2005. 09.015. References Acton, G.D., Guyodo, Y., Brachfeld, S.A., 2002. Magnetostratigraphy of sediment drifts on the continental rise of West Antarctica (ODP Leg 178, Sites 1095, 1096, and 1101). In: Barker, P.F., Camerlenghi, A., Acton, G.D., Ramsay, A.T.S. (Eds.), Proceedings of the ODP, Sci. Results, 178. Ocean Drilling Program. College Station, TX. Barker, P.F., 2001. Composite depths and spliced sections for Leg 178 Sites 1095 and 1096, Antarctic Peninsula continental rise. In: Barker, P.F., Camerlenghi, A., Acton, G.D., Ramsay, A.T.S. (Eds.), Proceedings of the ODP, Sci. Results, 178. Ocean Drilling Program. College Station, TX. Barker, P.F., Camerlenghi, A., Acton, G.D., the Shipboard Scientific Party, 1999. Proc. ODP, Init. Repts., 178 [Online]. Available from World Wide Web:
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