Extraction of non-continuous orbital frequencies from noisy insolation data and from palaeoproxy records of geomagnetic intensity using the phase of continuous wavelet transforms

Details Extraction of non-continuous orbital frequencies from noisy insolation data and from palaeoproxy records of geomagnetic intensity using the phase of continuous wavelet transforms Extraction of non-continuous orbital frequencies from noisy insolation data and from palaeoproxy records of geomagnetic intensity using the phase of continuous wavelet transforms

Mar 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009geoji.176..767s&link_type=abstract

Geophysical Journal International, Volume 176, Issue 4, pp. 767-781.

Astronomy and Astrophysics

Astronomy

2

Time Series Analysis, Wavelet Transform, Earth Rotation Variations, Geomagnetic Excursions, Palaeointensity

Scientific paper

The presence or absence of orbital frequencies in the palaeomagnetic field spectrum is the subject of intense discussions. While palaeoclimatic variations, i.e. glacial/interglacial alternation periods, are known to be governed by insolation variations via orbital frequencies, the link between such frequencies and the geomagnetic field maintenance processes (convection and rotation in the fluid metallic core) remains unclear. The study of frequency behaviour of long time-series proxy records requires a robust and linear time-frequency method with local and scale analysis properties. For these reasons, we use algorithms based on complex continuous wavelet theory and the stationary phase method, first developed in acoustics in the years 1988-1991, for extracting frequency and amplitude modulation laws of arbitrary signals from the phase of their wavelet transform. This method is robust in the presence of noise and does not require restrictive conditions on the energy of signals.
After introducing the basis of the method with some illustrations on periodic signals, we show the efficiency of our algorithms on a synthetic insolation signal (Milankovitch cycles), by extracting the main three orbital frequencies (precession, obliquity and eccentricity); then we show its robustness in the presence of a high level of noise. Finally, we apply this method to various proxy records of (i) past climate variations (δ18O), (ii) the past geomagnetic field intensity variations recorded by ocean sediments and (iii) along the deep-seafloor basalts and (iv) variations of the production of cosmogenic nuclide (10Be/9Be). Our results confirm the presence of orbital frequencies in all these palaeoproxy records and suggest an orbital forcing of geomagnetic field variations via geodynamo processes.

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