Strong evidence of high-frequency (sub-Milankovitch) orbital forcing by amplitude modulation of Milankovitch signals

Abstract

High-frequency (sub-Milankovitch) signals have occasionally been recognised in the sedimentary record but their interpretation is yet controversial. The transmission of most information involves modulating the original signal. Transmission in the geological sense is the process of changing the astronomical signal into insolation, climate and, finally, the observed sediment. Modulated signals may contain different frequencies from the original, and the former can sometimes be recognised more easily than the original ones. Thus, sedimentary cycles belonging to the high-frequency band could be interpreted as a possible modulation of the astronomical Milankovitch signals. To check this possibility, a detailed spectral analysis was carried out on a selected marl–limestone and limestone Jurassic succession, in which high-frequency signals in the sub-Milankovitch range, with a persistent peak at 13–14 kyr, were recognised. Relationships between frequencies of the registered peaks suggest that the sedimentation pattern is caused by the amplitude modulation (AM) of the Milankovitch forcing. In AM, the amplitude of a carrier frequency is modulated according to the value of a signal. The spectrum of the resulting AM signal is composed of three frequencies, the frequency of the carrier and two side tones, but the carrier and one side tone may be suppressed without destroying the information. The information remains in the other side tone, usually the one with the highest frequency. This version of AM is called single-sideband carrier-suppressed (SSBCS). For Milankovitch forcing, the implication is that the signals are not transmitted as added-up sinusoidal signals, but both the short eccentricity and the obliquity signals affect the sedimentation pattern by AM of the precession signal, which is the carrier frequency. The sub-Milankovitch peaks registered in the succession studied reveal the SSBCS AM of the original Milankovitch signals, with the SSBCS frequencies being highly significant in the estimated power spectra, after applying conservative tests assuming underlying red noise.