Cretaceous geomagnetic paleointensities: Thellier experiments on Pillow lavas and Submarine basaltic glass from the Ontong Java Plateau



[1] We present new Thellier paleointensity results on pillow lavas and pillow-rim submarine basaltic glass (SBG) from 120 Ma basalts erupted on the Ontong Java Plateau. All pillow lavas have suffered low-temperature alteration and are unsuitable for Thellier paleointensity experiments. In contrast, pillow-rim SBG is virtually impermeable to seawater and therefore has suffered little or no low-temperature alteration. Out of 21 selected SBG samples with magnetic moment larger than 10−10 Am2, 18 samples (from 12 chilled SBG margins) yielded paleointensity estimates of good technical quality. Rock magnetic measurements show the primary carrier of remanence is low-Ti titanomagnetite occurring in a continuous range of compositions starting from nearly pure magnetite. As suggested by a recent study, magnetic hysteresis parameters were monitored during the entire paleointensity experiment and a drastic change of parameters was observed above 400–440°C, which is in accordance with the partial thermoremanent magnetization checks failing significantly above the same temperature. We suggest that paleointensity estimates obtained below this critical temperature represent true determination of the paleomagnetic field strength. For several pillow margins two to three samples were measured, and in such cases the data agree well. Altogether we obtain reliable estimates from 12 SBG pillow-rims, yielding a mean paleointensity of 27.1 ± 13.4 μT and a corresponding virtual axial dipole moment of 5.8 ± 2.8 · 1022 Am2. This value is slightly higher than whole rock data from similar aged subaerial lava flows. The paleosecular variation of the intensity estimates is slightly larger than previously reported for paleointensity determinations for the Cretaceous Normal Superchron.

1. Introduction

[2] Several factors that influence the magnetohydrodynamic processes within the Earth's fluid outer core, such as, for example, heat flow across the core-mantle boundary and size of the inner core, may have varied through geologic time leading, to speculations that the Earth's magnetic field may have been significantly different in the geological past. Particularly intriguing is the Cretaceous Normal Superchron (CNS), during which the reversal process apparently ceased to operate for ∼37 million years (120.6 to 83.0 Ma [Channell et al., 1995; Cande and Kent, 1995]). Several authors have speculated that the CNS relates to anomalous boundary conditions at the core-mantle boundary [e.g., Vogt, 1975; Courtillot and Besse, 1987; Larson and Olson, 1991], causing a state of the geodynamo that is different from other times [Pal and Roberts, 1988; Merrill and McFadden, 1994]. In such case it is natural to suspect that long-term strength of the geomagnetic field might also have been different during the CNS.

[3] Recent drilling of the Ontong Java Plateau (OJP) during Ocean Drilling Program (ODP) Leg 192 resulted in the recovery of more than 500 meters of Cretaceous basaltic pillow lavas, providing an opportunity to constrain the intensity of the Earth's magnetic field shortly after onset of the CNS (∼120 Ma). In this paper we report results of rock magnetic and Thellier paleointensity analyses of whole rock as well as submarine basaltic glass (SBG) samples from the OJP and discuss the Earth's dipole moment during the CNS.

2. The Ontong Java Plateau

2.1. Emplacement and Age

[4] With a surface area of 2.0 × 106 km2 (outlined in Figure 1) and an estimated volume of 4–5 × 107 km3 [Eldholm and Coffin, 2000], the OJP is the largest of the so-called large igneous provinces. Reliable age determinations of the OJP material recovered during ODP Leg 192 include: (1) 40Ar-39Ar age determinations on samples from Sites 1186 and 1187 that gives ages ranging from 105 to 122 Ma [Chambers et al., 2002, L. M. Chambers, M. S. Pringle, and J. G. Fitton, 40Ar/39Ar whole rock results from ODP Leg 192: Implications for the analysis and interpretation of argon ages from low-K basalts, manuscript submitted to Earth and Planetary Science Letters, 2003], (2) Re-Os isotopic data on basalt samples from Sites 1183, 1185, 1186 and 1187 that yields a single isochron with an age of 121.5 ± 1.7 Ma [Parkinson et al., 2001], and (3) biostratigraphic dating of sediment intercalated with lava flows at Sites 1183, 1185, 1186 and 1187 suggesting that magmatism extended from latest Early Aptian to latest Aptian [Bergen, 2003; Sikora and Bergen, 2003], i.e., from 118 to 112 Ma [Gradstein et al., 1995].

Figure 1.

Predicted bathymetry [after Sandwell and Smith, 1997] of the Ontong Java plateau showing the locations of ODP Sites (Sites 807, 1183, 1185, 1186, and 1187) for which submarine basaltic glass was recovered and used for Thellier paleointensity experiments. The Ontong Java plateau is outlined. The bathymetric contour interval is 1000 m.

[5] Because of the uncertainties in the various dating methods, it is difficult to estimate the duration of OJP magmatism and it remains unclear whether the plateau formed in a single, short-lived event as originally suggested by Tarduno et al. [1991] or over a slightly longer period. Important for the present study is the general agreement between the radiometric and paleontological data indicating an Early Aptian to latest Aptian age of the OJP and hence formation during the Cretaceous Normal polarity superchron, which is consistent with the finding of normal polarity in all drilled basalts.

2.2. Paleolatitude

[6] The paleolatitude of the OJP must be known in order to translate paleointensity data into dipole moments. A paleolatitude derived from reconstruction in the assumed fixed hot spot reference frame is unsuitable, as Pacific hot spots have not remained fixed relative to the Earth's spin axis [Tarduno and Cottrell, 1997; Riisager et al., 2003; Tarduno et al., 2003]. For the present study we use the OJP paleolatitude 23°S derived from five separate time-averaged site-mean inclination estimates, all of which are statistically indistinguishable. The excellent internal consistency also strongly suggests that the studied basalts have suffered little or no tectonic disturbance since their emplacement [Riisager et al., 2003].

2.3. Sampling

[7] During ODP Leg 192, submarine lava flows and pillows were recovered at four different sites (Sites 1183, 1185, 1186, and 1187; Figure 1). The presence of pillow basalts spread over such a wide geographical area confirms that much, if not all, of the main plateau was submarine during eruption. Low-temperature alteration by seawater has affected all lava flows and pillow interiors in amounts similar to that found in normal seafloor basalts [Banerjee et al., 2003] resulting in breakdown of olivine phenocrysts and groundmass, and the formation of clay minerals (Figure 2). In contrast, pillow-rim SBG is virtually impermeable to seawater and as a consequence it is possible to find apparently unaltered SBG fragments (Figure 3). Analysis of molecular and total H2O in visually unaltered ODP Leg 192 SBG samples [Roberge et al., 2003] suggest that most samples have escaped low-temperature hydration, i.e., having very low amounts of molecular H2O (mostly below detection). The SBG selected for the paleointensity experiments was picked by one of us (XZ) during post-cruise studies and only from visually unaltered pillow margins (Figure 3).

Figure 2.

Photomicrographs displaying common groundmass alteration patterns of typical dark gray basalt in which olivine phenocrysts and the groundmass are completely replaced by smectite. Plagioclase and clinopyroxene are unaltered. Field of view = 5.5 mm [from Banerjee et al., 2003].

Figure 3.

Photograph of pillow basalt from which sample 1183A-67R-3W, 52–54 cm was obtained. Samples are cut from the ∼5 mm thick glass rim. Shown are (a) the cut surface and (b) the pillow surface [from Mahoney et al., 2002].

3. Rock Magnetic Properties

[8] The water-rock interaction that produced clay minerals in the interiors of lavas and pillows (Figure 2) has resulted in low-temperature oxidation of the primary titanomagnetite, as titanomaghemite is observed in rock magnetic experiments on whole rock basalt samples [Riisager et al., 2003; Zhao et al., 2003]. In the following we will only discuss the rock magnetic properties of the SBG. Magnetic hysteresis measurements were performed on a small companion sample from all SBG pillow rims selected for the Thellier paleointensity experiments using a Princeton Measurements Corporation alternating gradient force magnetometer (AGFM) at the University of California at Santa Cruz. Although the AGFM is highly sensitive and P1-type sample probes were employed, magnetic hysteresis measurements of the SBG proved to be very difficult due to their low magnetizations. In fact, paramagnetism was dominant for the majority of samples and only for 5 samples was it possible to isolate a ferromagnetic component after paramagnetic slope correction (Figure 4a). The hysteresis loops are similar to those obtained in other studies of fresh SBG samples [Pick and Tauxe, 1993a; Smirnov and Tarduno, 2003]: slightly wasp-waisted, suggesting the presence of single domain (SD) and superparamagnetic (SP) grains (Figure 4b). We further performed low-temperature magnetic experiments on the SBG samples using a Quantum Design magnetic property measurement system (MPMS) at the Institute of Rock Magnetism (University of Minnesota). A more or less pronounced Verwey transition within the temperature range 110–120 K is observed (Figure 4c), indicating the presence of relatively well-preserved magnetite particles.

Figure 4.

Typical magnetic hysteresis loops for SBG samples (a) 1186A-32R-2W, 55–57 cm, and (c) 1186A-34R-3W, 128–130 cm before (1) and after (2) paramagnetic slope correction. (b,d) Enlargement of hysteresis loops after paramagnetic slope correction. (e) Thermal demagnetization curve of low-temperature saturation isothermal remanent magnetization imparted in sample 1187a-13r-06w, 98–100 cm at 20 K (1) and variation in room temperature SIRM during cooling (2).

4. Paleointensity Determinations

4.1. Thellier Experiments on Nonglassy Whole Rock Basalts

[9] On the basis of visual appearance we selected 8 whole rock samples from grey basalts that seemingly had suffered the least amount of low-temperature alteration. Typical examples of Thellier experiments are shown in Figure 5. For all samples we observe onset of alterations at temperatures below ∼300°C, which we interpret to be the result of maghemite destruction to form magnetite and/or hematite. Arai-diagrams similar to these are in fact often reported from subaerial lavas as well. Our experiments demonstrate that it is difficult to obtain reliable Thellier paleointensity results from nonglassy ocean-floor basalts [Prevot et al., 1983].

Figure 5.

Typical examples of NRM-TRM plots and orthogonal projections of the associated NRM demagnetization for nonglassy whole rock samples (a) 1187A-12R-3W, 126–128 cm and (b) 1187-6R-2W, 119–121 cm.

4.2. Thellier Experiments on Submarine Basaltic Glass

[10] Variably sized sub-samples of SBG chips weighing between ∼50 and 1000 milligram, were peeled from the outer part of pillow rims and cleaned in diluted HCl in order to remove surface contamination. The chips were then fixed to thin-section glass slides using a nonmagnetic ceramic glue (ceramabond 618) with a thermal expansion coefficient similar to that of thin-section glass so that the chips stay fixed and oriented during laboratory heating experiments and measurements. Remanence measurements were made in a field free room on a 2G cryogenic magnetometer with a noise level of 1–2 · 10−12 Am2, using a custom-made quartz sample holder. In order to test possible magnetic contamination, an empty thin-section slide with a large blob of ceramic glue was given a TRM by heating it to 600°C and cooling in a 60 μT field. On the basis of the resulting magnetic moment, 8 · 10−12 Am2, we choose to select only SBG chips with a magnetic moment larger than 10−10 Am2 (see Table 1). Roughly 70 percent of all studied SBG chips fulfill this criteria with some chips, in fact, being much more magnetic (up to 10−8 Am2; Table 1).

Table 1. Summary of the Thellier Paleointensity Resultsa
SiteCoreSectionInterval, cmDepth, mbsfSampleNRM, 10−9 Am2Ba ± σ(μT)NInterval, °CqF, %MAD, degreeVADM, 1022 Am2
  • a

    Summary of the Thellier paleointensity experiments for each discrete SBG specimen. Ba ± σ (μT) is the paleointensity with its standard error; N is the number of points used for paleointensity determination; Interval is the lower and upper temperatures for the linear segment; q is a quality factor and F is the NRM fraction used for estimating the paleofield intensity [Coe et al., 1978 ]; MAD is maximum angular deviation, which is a qualitative indication of scatter about the principal component [Kirschvink, 1980]; VADM is the virtual axial dipole moment calculated based on the OJP paleolatitude of 23°S.

1183A67R3W52–541208.36I2.1809.9 ± 0.48190–44013.860.81.6 
1183A67R3W52–541208.36II0.2906.2 ± 0.27190–4008.032.51.9 
1183A67R3W52–541208.36mean 8.1     1.72
1185B16R1W64–66425.54I0.69619.1 ± 3.37160–3601.532.613.9 
1185B16R1W64–66425.54II0.97316.9 ± 0.78160–4006.832.30.5 
1185B16R1W64–66426.54mean 18.0     3.82
1186A32R2W55–57978.22I11.0014.8 ± 0.68190–4407.736.60.3 
1186A32R2W55–57978.22II55.4010.8 ± 0.78190–4404.032.60.9 
1186A32R2W55–57978.22mean 12.8     2.71
1186A34R3W128–130989.97I26.2032.4 ± 2.26190–3603.832.91.0 
1186A34R3W128–130989.97II1.19033.3 ± 3.49160–4402.733.41.4 
1186A34R3W128–130989.97mean 32.9     6.97
1187A4R2W68–70386.37 0.21229.0 ± 3.66160–3202.743.11.46.15
1187A4R3W60–70387.79 0.50236.7 ± 5.46160–3201.834.81.37.78
1187A5R7W51–54402.91 0.08940.6 ± 9.46160–3201.
1187A6R1W9–12403.49I0.67820.5 ± 1.65160–2803.033.14.2 
1187A6R1W9–12403.49II0.24117.8 ± 4.27130–3201.752.17.2 
1187A6R1W9–12403.49III0.09818.8 ± 1.87130–2802.635.25.8 
1187A6R1W9–12403.49mean 19.0     4.03
1187A6R3W3–5406.16 1.11017.7 ± 2.35160–2801.424.71.03.75
1187A7R2W86–88415.33 0.91932.6 ± 3.95160–2801.626.40.56.91
1187A9R3W0–3435.09 0.34722.4 ± 1.25160–2804.430.83.94.75
1187A13R6W98–100478.15 0.22255.7 ± 6.26160–3201.827.21.011.81
Mean ODP Leg 192 (N = 12)      27.1 ± 13.4     5.8 ± 2.8

[11] The Thellier experiments were carried out in Ar-atmosphere in a paleointensity furnace built at University of California at Santa Cruz with temperature reproducibility between heatings to the same temperature better than 2°C. Heating to the assigned temperature takes ∼90 min and cooling ∼45 min. We used the Coe version of the Thellier method [Coe, 1967] with the first heating and cooling cycle at each temperature performed in zero field and the second in a laboratory field of 40 μT. The following temperature steps was used: 100, 130, 160, 190, 220, 250, 280, 320, 360, 400, 440, 480, 520, 540, and 560°C. At every other temperature step a pTRM check [Coe et al., 1978] was performed to test whether the specimen had begun to undergo chemical alteration that affects its ability to acquire a thermal remanence. Owing to the low magnetic moment of SBG samples no attempt was made to detect the small pTRM-tails indicative of nonideal MD grains [Riisager and Riisager, 2001].

[12] We used four criteria to evaluate the quality of our paleointensity data. The criteria are similar to those used in other SBG paleointensity studies [Selkin and Tauxe, 2000], although we have modified the details somewhat: (1) Principal component analysis [Kirschvink, 1980] of the zero-field steps for the temperature interval used for paleointensity determination must have a maximum angular deviation (MAD) less than 15°, and the angle between the direction of the vector average of the data (which are anchored to the origin) with the principal component (which are anchored to the “center of mass” of the data) must be less than 10°. (2) For the temperature interval used for paleointensity estimate, the pTRM check should agree with the previous measurement to the same temperature to within 10%. (3) The quality factor, q, [Coe et al., 1978] must be larger than unity. (4) The number of accepted pNRM-pTRM points must be at least 6.

[13] Out of 21 SBG specimens, 18 yielded results that pass these acceptance criteria. All accepted SBG Thellier paleointensity data are listed in Table 1 and typical examples are shown on Figures 6 and 7. A viscous component of magnetization is in some cases observed at low temperatures and data points below 130°C are therefore not included in the analysis (Table 1). Although the average q-value of 3.9 is rather low, the data are generally of good technical quality. In fact, the lowest q-values are always observed for specimens with the lowest NRM magnetic moment (Figure 6a), whereas samples with higher NRM magnetic moment yield higher q-values. The main factor lowering these q-values is therefore instrumental noise rather than rock magnetic problems.

Figure 6.

Typical examples of NRM-TRM plots and orthogonal projections of the associated NRM demagnetization for SBG samples (a) 1187A-5R-7W, 51–54 cm, (b) 1185B-16R-1W, 64–66 cm, and (c) 1186A-32R-2W, 55–57 cm. Solid (open) circles on the NRM-TRM plots are accepted (rejected) points.

Figure 7.

Typical examples of NRM-TRM plots and orthogonal projections of the associated NRM demagnetization for SBG samples (a) 1186A-34R-3W, 128-130 cm, sample I, (b) 1186A-34R-3W, 128–130 cm, sample II, and (c) 1187A-4R-3W, 60–70 cm. Same notation as Figure 6.

[14] Noteworthy for all samples is the alteration of the magnetic minerals after heating above ∼400°C, as evidenced by the nonlinearity of pNRM-pTRM points and significant failure of pTRM checks (Figures 6 and 7). This is similar to observations in other SBG studies [Smirnov and Tarduno, 2003], and we will discuss this further below. In order to make paleofield estimates by cooling unit, we take the arithmetic average of any multiple specimens from a given chilled margin (Table 1). We obtain reliable estimates from altogether 12 different chilled SBG margins, yielding a mean paleointensity of 27.1 ± 13.4 μT (Table 1). The fact that the cooling units stem from 4 different drill sites suggest that they represent a sufficiently long time interval to average out secular variation.

5. Discussion

5.1. Reliability of SBG Paleointensity Estimates

[15] The success of paleointensity experiments using SBG is supported by results on glass from recent submarine lava flows that have yielded the known geomagnetic field intensity at the site [Pick and Tauxe, 1993a; Mejia et al., 1996; Carlut and Kent, 2000]. On the basis of our rock magnetic and Thellier experiments we conclude that the magnetic mineralogy of SBG samples from the OJP is like SBG samples from other sample regions and with different ages, i.e., the NRM is carried by low-Ti single domain titanomagnetite [Pick and Tauxe, 1994; Zhou et al., 2000; Selkin and Tauxe, 2000; Juarez and Tauxe, 2000; Smirnov and Tarduno, 2003]. The fact that the NRM in SBG samples is carried by low-Ti titanomagnetite has been taken by some authors [Heller et al., 2002] as an indication of secondary magnetomineralogy, because they argue that thermodynamic considerations predict a primary magnetomineralogy of high-Ti titanomagnetite (TM70) from a basaltic melt. The extreme cooling rate of pillow rim SBG may, however, preclude thermodynamic equilibrium. In fact, rock magnetic and transmission electron microscopy observations [Zhou et al., 2000] suggest that titanomagnetite in pillow rim SBG and interstitial glass close to the pillow rim has a lower and more variable Ti-content than the TM70 produced in more slowly cooled basaltic melts.

[16] In Thellier paleointensity experiments it is important to recognize that pTRM checks provide a necessary but not sufficient test for the reliability of the data. It has been suggested that pTRM checks might not detect formation of new magnetic inclusions in SBG during Thellier experiments and that monitoring of magnetic hysteresis parameters can provide an additional reliability check [Smirnov and Tarduno, 2003]. During our entire paleointensity experiments additional small SBG chips (up to 15 mg) were heated together with the main set of samples and their hysteresis parameters were measured after each temperature step. In Figure 8 we show typical examples of magnetic hysteresis monitoring, suggesting that new magnetic minerals are formed when heating above ∼400°C, which is, in fact, in excellent agreement with the observations that above ∼400°C all pTRM checks fail (Figures 6 and 7). Similar magnetic hysteresis monitoring results was obtained by Smirnov and Tarduno [2003] in their analysis of SBG samples from previous OJP drill sites (ODP Sites 803 and 807). On the basis of direct transmission electron microscopy observations, they suggested that the formation of new magnetic minerals are caused by partial melting of the SBG already at ∼400°C. The observations of Smirnov and Tarduno [2003] therefore suggest that paleointensity estimates can only be obtained below the critical temperature when melting takes place. We believe that our SBG paleointensity estimates, obtained at temperature intervals below ∼400°C represent true estimates of the paleomagnetic field strength.

Figure 8.

Magnetic hysteresis parameters as a function of heating temperature on splits of SBG samples (a) 1186-32R-2W, 55–57 cm (Thellier results are shown in Figure 6c), and (b) 1187-4R-2W, 68–70 cm. Shown are saturation magnetization (Ms) after slope correction, saturation remanent magnetization (Mrs), and the Mrs/Ms ratio (open squares).

5.2. Previous Thellier Paleointensity Results From the Ontong Java Plateau

[17] The only other Thellier paleointensity study of SBG samples from OJP was performed by Pick and Tauxe [1993b] and reviewed by Selkin and Tauxe [2000] based on pillow rims retrieved at ODP Site 807. However, as discussed above (paragraph 2.2) the paleolatitude estimate used by Selkin and Tauxe [2000] derived from reconstruction in the assumed fixed hot spot reference frame is not correct, and we therefore recalculate their virtual axial dipole moment (VADM) value using new paleolatitude data [Riisager et al., 2003]. The new VADM (5.5 × 1022 Am2) is then 14% higher than the original and in excellent agreement with the mean VADM obtained in this study (5.8 × 1022 Am2).

5.3. Field Strength During the CNS

[18] Unfortunately, it has proven difficult to find unaltered volcanic rocks of CNS age suitable for Thellier type paleointensity experiments. Almost all CNS paleointensity data, which pass minimum criteria of reliability, stem from geologic materials that are particularly resistant to alteration, i.e., submarine basaltic glass [Selkin and Tauxe, 2000] and plagioclase crystals [Tarduno et al., 2001, 2002]. In Table 2, seven studies for the period 80 to 130 Ma are listed that pass the mild selection criteria proposed by Riisager et al. [2002] of a minimum of nine successful paleointensity determinations from at least three cooling units. The data are shown in Figure 9 and we note that the OJP SBG is in fact not lower than contemporaneous whole rock data, in disagreement with the suggestion of Heller et al. [2002] that SBG only gives low paleointensity values. It is also worth noting that our paleointensity results exhibit somewhat greater variation than any previous reported paleointensity determinations for the Cretaceous Normal Superchron. On the basis of the only five reliable VDM estimates for the CNS shown in Figure 9, it is clearly not possible to say whether or not the average strength of the geomagnetic field was significantly different from the present-day field during the CNS.

Figure 9.

Evolution of mean virtual dipole moments (VDMs) and virtual axial dipole moments (VADMs) for 80 to 130 Ma, bracketing the Cretaceous Normal Superchron (83–120.6 Ma). The VDM and VADM estimates are listed in Table 2. Also shown is the geomagnetic polarity timescale to 130 Ma [Cande and Kent, 1995; Channell et al., 1995]. Numbering of the studies is the same as in Table 2.

Table 2. Selected VDMs and VADMs for the Period 80 to 130 Ma, Bracketing the Cretaceous Normal Superchrona
Study areaAge, MaRock-typeN/nB, μTVDM or VADM, 1022 Am2Reference
  • a

    Selected mean paleointensities and corresponding VDMs and VADMs for the period 80 to 130 Ma, bracketing the Cretaceous Normal Superchron. Abbreviations for the rock-type are: WR, whole rock; Plag, plagioclase crystals; SBG, submarine basaltic glass. N/n is the number of cooling units/discrete samples used for mean calculation. Note that the VADM of ODP Site 807 (Study 5) is recalculated based on new paleolatitude data [Riisager et al., 2003] and the mean paleointensity for Sihetun (Study 7) is recalculated excluding transitional directions.

1. Inner Mongolia, China92 ± 1WR6/3219.8 ± 9.93.2 ± 1.6Tanaka and Kono [2002]; Zhao et al. [2003]
2. Arctic Canada95 ± 1Plag8/5193.8 ± 5.212.7 ± 0.7Tarduno et al. [2002]
3. Rajmahal traps, India114.5 ± 1.5Plag8/5677.6 ± 9.412.5 ± 1.4Tarduno et al. [2001]
4. Ontong Java, ODP Leg 192120 ± 2SBG12/1827.7 ± 14.05.8 ± 2.8This study
5. Ontong Java, ODP Hole 807C120 ± 2SBG39/?26.0 ± 3.45.5 ± 0.7Selkin and Tauxe [2000]
6. Liaoning, China121 ± 1WR11/4227.0 ± 7.23.9 ± 1.0Zhu et al. [2001]
7. Sihetun, China127.5 ± 3.5WR11/2920.2 ± 2.63.5 ± 0.4Zhu et al. [2003]

6. Conclusions

[19] Rock magnetic measurements show the primary carrier of remanence in SBG to be low-Ti titanomagnetite. Out of 21 selected SBG samples with magnetic moment larger than 10−10 Am2, 18 samples (from 12 chilled SBG margins) yielded paleointensity estimates of good technical quality. As suggested by a recent study [Smirnov and Tarduno, 2003], magnetic hysteresis parameters were monitored during the entire paleointensity experiment and a drastic change of parameters was observed above 400–440°C, which is in accordance with our observation that the pTRM checks failed above the same temperature. We suggest that our paleointensity estimates obtained below this critical temperature represent true determinations of the paleomagnetic field strength. Results from the same pillow margin agree well. In total we obtained reliable estimates from 12 submarine basaltic glass margins that yield a mean paleointensity of 27.1 ± 13.4 μT and a corresponding virtual axial dipole moment of 5.8 ± 2.8 · 1022 Am2. This time-averaged mean-VDM is only slightly higher than contemporaneous whole rock data, and its standard deviation (paleosecular variation of intensity) is also a little larger than previous reported paleointensity determinations for the Cretaceous Normal Superchron. A very large variation within the CNS is observed among the five estimates of paleointensity deemed reliable, their corresponding estimates of dipole moment varying from 3.2 to 12.7 · 1022 Am2. Curiously, their average yields a grand mean of 8.0 · 1022 Am2, essentially today's value, but the number is too few and the variability too high to place significance on this coincidence.


[20] This research used samples 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. We thank Phil Rumford of ODP for helping SBG sampling. Discussions with our ODP Leg192 colleagues have been most stimulating and helpful. We particularly wish to acknowledge the expertise, helpful advice, and continuing discussions generously given by Lynne Chambers and Neil Banerjee. We are grateful to Dennis Kent, Jeffrey Gee, and an anonymous reviewer for their constructive comments on the original manuscript. MPMS measurements were performed at the Institute for Rock Magnetism (University of Minnesota) during a Visiting Fellowship (to PR). Funds for the Institute for Rock Magnetism are provided by the Keck Foundation and the University of Minnesota. We are pleased to acknowledge the support of the US National Science Foundation grant EAR-0207389, the Danish National Research Foundation (PR), and the Carlsberg Foundation of Denmark (JR). In our analysis we used software packages ThellierTool and Pmag1.6 developed by Drs. Roman Leonhardt and Lisa Tauxe, respectively. This is contribution 469 of CSIDE of IGPP at UC Santa Cruz.