ISEA reversed event in the Cretaceous Normal Super-chron （CNS）： ^40Ar／^39Ar dating and paleomagnetic results

ThegeomagneticfieldwasfoundtobeofnormalpolarityforalongtimeduringtheCretaceousbyHelsleyandSteiner[1]andthiswaslaternamedasCretaceousNormalSuperchron(CNS)[2].TheCNSlastedalmost37Ma(120—83Ma).Severalabnormalgeologicalevents,suchasanoceanicanoxicevent,alargenumberofvol-canismandglobalclimaticwarmingduringtheCNShavebeenreported[3—11].Thishasledtomanyquestionsbeingraised.IsittruethattheEarthsmagneticfielddidnotreverseatallintheCNS?Ifitdidreverse,howmanytimesandwhendidthishappen?Istherea…

ISEA reversed event in the Cretaceous Normal Superchron (CNS):40Ar/39Ar dating and paleomagnetic results

In order to provide the age for the ISEA (one reversed event in the CNS) a combined rock magnetic, paleomagnetic and geochronologic study has been conducted on two Cretaceous andesitic basalt lava flows from Sanbaoying, Liaoning Province of northeastern China. Rock magnetic investigations show that pseudo-single domain (PSD) titanomagnetite with poor-Ti content is the dominant magnetic mineral in Sanbaoying lava flows. Detailed systematic thermal demagnetization allowed us to isolate two remanence components after removing a low temperature component at 100°C/150°C; an intermediate temperature component with normal polarity at 150°C/200°C–380°C/400°C and the characteristic remanence component (ChRM) with reversed polarity above 400°C/450°C.⁴⁰Ar/³⁹Ar dating results show that Sanbaoying rocks were erupted at 116.0 ± 0.3 Ma. Combination of paleomagnetic results and⁴⁰Ar/³⁹Ar dating indicates that the reversed event recorded in Sanbaoying lavas corresponds to the ISEA event in the CNS. Our age determination for ISEA reversed event provides chronological evidence to study the relevance between global geological events that occurred in the CNS and geodynamo processes.