Rock Magnetic Cyclostratigraphy and Magnetostratigraphy of the Rainstorm Member of the Neoproterozoic Johnnie Formation indicate a 2.5 Myr Duration for the Negative 13C Isotopic Anomaly
Abstract
The Rainstorm Member of the Neoproterozoic Johnnie Formation from Death Valley, CA, contains a negative 13C isotopic anomaly that records the oxidation of the oceans with the rise of atmospheric oxygen just before the appearance of multi-cellular life. Previously, the only estimate for the duration of the globally observed 13C anomaly, 50 myr, came from thermal subsidence modeling of rocks in Oman. In the southern Nopah Range, CA, we collected rock magnetic samples from 6 to 45 m above the Johnnie oolite marker bed to test for cyclostratigraphy in mudstone carbonates that correlate to the lower third of the carbon anomaly. Our objective was to independently determine the duration of the oxidation event by looking for evidence of orbital cycles in the rock magnetic properties. We also collected 8 horizons of three oriented samples each between 10 m and 40 m above the oolite for a magnetostratigraphy to constrain our interpretation of the rock magnetic cyclostratigraphy. After thermal demagnetization treatments, the remanent magnetization showed 4 chrons (R-N-R-N) in the 30 m interval with E (reversed)-W(normal) declinations and shallow inclinations (mean: D=262.8°, I=1.3°), similar to previous paleomagnetic determinations for an equivalent part of the Rainstorm Member in the Desert Range, Nevada (Van Alstine and Gillett , 1979) . Our rock magnetic cyclostratigraphy, sampled at 25 cm intervals, shows a well-defined 5 m wavelength for a measure of the goethite-to-hematite ratio that is interpreted to indicate climate variability (precipitation to aridity) in the Johnnie Formation source area. In addition to the 5 m cycle, a smaller amplitude cycle is observed in the data series with an average wavelength of 0.75 m. Multi-taper method (MTM) spectral analysis shows significant power (> than the 95% confidence limits above the robust red noise) at these frequencies, but also at harmonics of the 5 m waveform. If the 5 m cycle is assumed to be short eccentricity with a period of ~109 kyr for this time, the 0.75 m cycle would have a period of 16.4 kyr, which is close to the 17.2 kyr precession period for the early Paleozoic. If the 5 m waveform is short eccentricity, the 40 m of section sampled in the Johnnie Formation represents 830,000 years, a period that could accommodate several geomagnetic polarity intervals as observed. Our estimate for the duration of the entire 13C isotopic anomaly would be approximately 2.5 myr, in contrast to the 50 myr duration previously determined, indicating a very rapid oxidation of the ocean before the explosion of multi-cellular life.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMGP53A..07K
- Keywords:
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- 1520 GEOMAGNETISM AND PALEOMAGNETISM / Magnetostratigraphy;
- 1527 GEOMAGNETISM AND PALEOMAGNETISM / Paleomagnetism applied to geologic processes;
- 1540 GEOMAGNETISM AND PALEOMAGNETISM / Rock and mineral magnetism