Orbital tuning , eccentricity , and the frequency 1 modulation of climatic precession

5 To be submitted to Paleoceanography. The accuracy of geologic chronologies can, in principle, be improved through 6 orbital tuning, the systematic adjustment of a chronology to bring the as-7 sociated record into greater alignment with an orbitally derived signal. It would 8 be useful to have a general test for the success of orbital tuning, and one pro-9 posal has been that eccentricity ought to covary with the amplitude enve-10 lope associated with the precession variability recorded in tuned geologic records. A common procedure is to filter a tuned geologic record so as to pass pre-12 cession period variability and compare the amplitude modulation of the re-13 sulting signal against eccentricity. There is a reasonable expectation for such 14 a relationship to be found in paleoclimate records because the amplitude of 15 precession forcing depends upon eccentricity. However, there also exists a re-16 lationship between eccentricity and the frequency of precession such that or-17 bital tuning generates eccentricity-like amplitude modulation in filtered sig-18 nals, regardless of the accuracy of the chronology or the actual presence of 19 precession. This relationship results from the celestial mechanics governing 20 eccentricity and precession, and from the interaction between frequency mod-21 ulation and amplitude modulation caused by filtering. When the eccentric-22 ity of Earth's orbit is small, the frequency of climatic precession undergoes 23 large variations and less precession energy is passed through a narrow-band 24 filter. Furthermore, eccentricity-like amplitude modulation is routinely ob-25 tained from pure noise records that are orbitally tuned to precession and then 26 filtered. We conclude that the presence of eccentricity-like amplitude modulation in precession-filtered records does not support the accuracy of orbitally 28 tuned timescales.