Relative Paleointensity Variations in Icelandic Lavas from Chron C5N

Near-bottom magnetic anomaly data from the northeast Pacific provide a record of geomagnetic field variations within anomaly 5 (Chron C5N). We sampled approximately 100 flows in eastern Iceland to assess whether the intensity fluctuations inferred from the near-bottom anomalies could be recognized in this magnetostratigraphic section. Establishing a time series of absolute paleointensity variations requires both that most sampling sites yield reliable paleointensity data and that high precision ages are available to facilitate correlation with features in the anomaly data. Because these requirements proved difficult, we have determined relative paleointensity for these lavas using the pseudothellier technique (Tauxe et al., 1995). The Icelandic lavas meet commonly accepted criteria for relative intensity determination. Most (90%) have maximum unblocking temperatures consistent with magnetite as the remanence carrying phase. Paleointensities were determined only for samples with median destructive fields > 20 mT. For these samples, susceptibility and ARM intensity are well correlated (R=0.82), suggesting that the effective magnetic grain size is relatively constant. Comparison of Thellier double heating and pseudothellier results indicates a ratio of about 3:1 for ARM:TRM (R = 0.79), allowing absolute calibration of our relative intensity data. With this calibration, the distribution of virtual axial dipole moments from the Icelandic samples is virtually identical to that of absolute paleointensities older than 0.3 Ma (Selkin and Tauxe, 2000) with an average value approximately half of the present dipole moment. The general within site consistency, serial correlation, and systematic relationship to directional variations of the resulting paleointensity record all suggest that the pseudothellier technique has provided a reliable estimate of relative intensity fluctuations in these volcanic rocks. Tauxe, L., Pick, T. and Y.S.Kok, 1995, Geo. Res. Lett., 22: 2885-2888. Selkin, P.A. and L. Tauxe, 2000, Phil. Trans. R. Soc., 358: 1065-1088.