High-Latitude Paleointensities During the Cretaceous Normal Superchron From the Okhotsk–Chukotka Volcanic Belt

Abstract

We conducted an absolute paleointensity survey on 74 lava flows from the Okhotsk–Chukotka Volcanic Belt (NE Russia), emplaced 90–83 Ma toward the end of the Cretaceous Normal Superchron (CNS, 121–84 Ma). Relying on preliminary results, we restricted our analysis to eight lava flows (140 Thellier–Coe experiments), two of which also yielded successful Wilson determinations. A detailed analysis of the magneto-mineralogy—based on X-ray structural analysis, reflected-light microscopy, and thermomagnetic curves—indicates that the determinations from solely two lava flows can be fully trusted, with remanence carriers (a) dominated by low-titanium titanomagnetite, (b) showing unambiguous traces of high-temperature oxidation, and (c) yielding partial thermoremanent magnetization (pTRM) tails representative of pseudo-single-domain grains. Recalculated in terms of geomagnetic dipole strength, our two successful flow-mean determinations yield virtual dipole moments of 4.76 ± 0.26 × 1022 Am2 (N = 7) and 9.07 ± 0.84 × 1022 Am2 (N = 8). Using an updated version of the paleointensity database for the Cretaceous epoch, we stress that determinations based on nonglassy whole rocks, submarine basaltic glasses, and single crystals are mutually inconsistent, suggesting a separate analysis of their distributions is more appropriate. Despite statistically indistinguishable estimates of dipole strength before and after the onset of the CNS—in accord with recent studies refuting the existence of a strict correlation between chron duration and dipole moment—we found that the distribution of dipole moments during the CNS is slightly bimodal with a leptokurtic dominant mode, thus more inclined to produce outliers and suggesting distinct geomagnetic field behavior during the CNS.