Toward a better understanding of the Cretaceous geomagnetic polarity superchron via paleosecular variation studies at the Earth's equator
Final Report Abstract
Insights into past geodynamo regimes can be learned from paleomagnetic studies of secular variation, i.e., tracking the time dependent changes in field direction and intensity. For the former, we collected 534 samples from a 1400 m-thick, paleontologically well-described section in northern Peru. Thermal demagnetization isolates stable magnetization directions carried by greigite. We explore the ramifications on the S value, which paleomagnetists routinely use to quantify paleosecular variation, that arise from directional analysis, sun compass correction, bedding correction, sampling frequency, outlying directions and different recording media. The sum of these affects can readily raise S values by more than 20%. S values from northern Peru are indistinguishable from other S values for the Cretaceous normal superchron, as well as those for the last 5 Ma. Summing over all the potential uncertainties, we come to the pessimistic conclusion that the S value is an unsuitable parameter to constrain geodynamo models. Alternatively, no statistical difference in paleosecular variation exists during much of the Cretaceous normal superchron and during the last 5 Ma. We then sought to understand why the S value is latitude dependent [S(l)]. The origin of this latitude dependency is widely attributed to a combination of time-varying dipole and non-dipole components. The slope and magnitude of S(l) are taken as a basis to understand the geomagnetic field and its evolution. We found that S(l) stems from a mathematical aberration of the conversion from directions to poles. Directional populations thus better quantify local estimates of paleosecular variation. Of the various options, the precision parameter, k, is likely the best choice. We also carried out a paleointensity study using two paleointensity methods on 128 samples from volcanic rocks in Peru and Ecuador. Oxidation of the remanence carriers was a serious problem. Only one site gave reliable results, with relatively good agreement from both methods. The paleointensity value from the volcanic rocks erupted near the end of the superchron (88.8±1.6 Ma) yields a moment that is quite similar to the present magnetic moment of the Earth. The sum of our observations leads to the conclusion that the energy state of the geodynamo during the Cretaceous normal superchron is likely similar to that during actively reversing times. In other words, superchrons are part and parcel of a single geodynamo regime that has acted since at least 167 Ma and probably much longer. B.F. Johnson, Pinning down a magnetic mystery, Earth, February 2009
Publications
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Quantifying paleosecular variation during the Cretaceous Normal Superchron. 2009 EGU Vienna:
Linder, J.; Gilder, S. A.
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Vagaries of calculating paleosecular variation (S- value): an example from the geomagnetic equator. 2010 EGU Vienna
Linder, J.; Gilder S. A.
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Variation of paleosecular variation: calculating a S-value from the geomagnetic equator. 2010 AGU Fall Meeting, San Francisco. Outstanding Student Paper Award from the Geomagnetism and Paleomagnetism section of AGU
Linder, J.; Gilder, S. A.
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(2011), Geomagnetic secular variation recorded by sediments deposited during the Cretaceous normal superchron at low latitude, Physics of the Earth and Planetary Interiors, 187, 245–260
Linder, J., and S. A. Gilder
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The geomagnetic secular variation S parameter: A mathematical artifact. 2011 AGU Fall Meeting, San Francisco
Linder, J.; Gilder S. A.
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(2012), Latitude dependency of the geomagnetic secular variation S parameter: A mathematical artifact, Geophysical Research Letters, 39, L02308
Linder, J.M., and S. A. Gilder
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Energy regime of the geodynamo during the Cretaceous Normal Superchron via paleosecular variation and paleointensity, PhD Dissertation, Ludwig Maximilians Universität, Munich, Germany, 2012
Linder, J.M.
