An EARTHTIME chronology for the Matuyama-Brunhes geomagnetic field reversal

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Author(s): Singer, B. S.; Jicha, B. R.; Coe, R. S.; Mochizuki, N.
Author Affiliation(s): Primary:
University of Wisconsin-Madison, Geoscience, Madison, WI, United States
University of California Santa Cruz, United States
Kumamoto University, Japan
Volume Title: AGU 2012 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2012; American Geophysical Union 2012 fall meeting, San Francisco, CA, Dec. 3-7, 2012. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: The Matuyama-Brunhes (MB) reversal is a global marker horizon observed in sedimentary and volcanic rocks and polar ice cores. Advances in rock magnetic measurement techniques in both sediments and lava flows, astrochronologic methods applied to sediment cores, and radioisotopic dating of lava flows during the past two decades has produced a large number of paleomagnetic records of the MB reversal. A sufficiently large set of paleomagnetic data from around the globe exists that the surface field during the MB reversal has been modeled using spherical harmonics. These paleomagnetic observations and statistical models in turn can be used to explore, validate, and refine numerical dynamos that produce field reversals of varied complexity and duration. The most precise age for the MB reversal, 773±1 ka (2σ), is determined using O isotope-based astrochronology on high-deposition rate sediments in 5 North Atlantic ODP cores. Despite these advances, we have only a limited set of 40Ar/39Ar age determinations from fewer than two dozen lava flows that record the MB reversal and the precursory magnetic field behavior during the 20 kyr prior to the reversal. Of these, only 7 lava flows associated with the MB reversal itself have been 40Ar/39Ar dated. The sparseness of this data set--coupled with the limited precision and accuracy that reflect 40Ar/39Ar experimental techniques of more than a decade ago--now severely hamper our ability to construct an accurate time line for the MB reversal and thus to calibrate the 40Ar/39Ar, U-Pb, and astrochronologic clocks using Quaternary strata. Forensic investigation of the Ar isotope data collected from the 7 MB lavas more than a decade ago (long prior to the EARTHTIME initiative) reveals that system blanks were an order of magnitude larger than today. Moreover, the 40Ar/36Ar of these blanks were below the atmospheric ratio, albeit with significant scatter and large uncertainties. Twelve years of pumping on this analytical system with only a single filament change in the MAP 215-50 spectrometer has purged the system such that today blanks are exceptionally low, atmospheric in composition, and far more reproducible than when the published MB data were collected. These findings lead to the conclusion that there may be a +1% bias in the ages published for most of the MB lava flows relative to an age of 28.02 Ma for the Fish Canyon sanidine (FCs). Accordingly, we have undertaken new 40Ar/39Ar incremental heating experiments on lava flows from Haleakala caldera and Tahiti that record the MB reversal and its precursor. In parallel we have obtained new 40Ar/39Ar laser fusion data from sanidine in the normally-magnetized Bishop Tuff for comparison to the U-Pb zircon age obtained using EARTHTIME protocols. Relative to an age of 28.201 Ma for the FCs standard, we obtain a mean isochron age from two of the Haleakala MB lavas of 772±11 ka, identical to the MB age of 773 ka determined via astrochronology. The age of a lava associated with the MB precursor on Tahiti is 808±32 ka. The weighted mean of 27 Bishop Tuff sanidine fusions is 767±3 ka, identical to the U-Pb age of 17 zircons of 767±1 ka. These initial results indicate that for the Quaternary period the astrochronologic, U-Pb, and K-Ar clocks are most tightly intercalibrated using the FCs standard age and decay constants of Kuiper et al. (2008).
Year of Publication: 2012
Research Program: ODP Ocean Drilling Program
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Brunhes Chron; Cenozoic; Magnetic field; Marine sediments; Matuyama Chron; Ocean Drilling Program; Paleomagnetism; Quaternary; Reversals; Sediments; Upper Cenozoic; Upper Quaternary
Record ID: 2014070753
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by, and/or abstract, Copyright, American Geophysical Union, Washington, DC, United States

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