Oxygen Isotope Series Research Articles

The 100-kyr ice-age cycle: internal oscillation or inclinational forcing?

The ice-age hypothesis of Muller and MacDonald (1995, 1997a, 1997b) has two parts: (a) The 100-kyr cycle does not owe its existence to Milankovitch forcing; and (b) variations in inclination of Earth's orbit (i.e., the orbit's angle with the solar system invariable plane) provide the mechanism sought. In support of the first proposition, Muller and MacDonald point to the paradox that the spectrum of oxygen isotope series from deep-sea sediments contains no power for two prominent eccentricity cycles, 125 and 400 kyr. In support of the second proposition, they offer a match between the SPECMAP record (Imbrie et al. 1984) and a plot of the amplitude of orbital inclination, shifted by 33 kyr. The hypothesis of Muller and MacDonald is rejected in both parts, although an influence of inclination forcing is not precluded entirely. The paradox of the missing eccentricity cycles (125 and 400 kyr) is explained by suppression of the two longer cycles, and enhancement of the one near 96 kyr, as a result of internal oscillation. A Muller–MacDonald machine for making the 100-kyr ice-age cycles, however conceived, would have to have a memory near 30 kyr to provide for phase shift between input and output. Precisely this amount of memory is sufficient to produce the needed oscillation in Milankovitch machine here applied; thus, there is no advantage, from the point of view of either necessity or simplicity, in replacing Milankovitch forcing, with its precise phasing (despite the fuzzy physics), with inclination forcing, and with its severe problems in phasing (and, thus far, no physics at all).

Maximum Entropy Spectral Analysis of the geological time series of the oxygen isotope record from deep-sea cores

A Maximum Entropy Spectral Analysis of the geological time series of the oxygen isotope 18 for the last 782,000 years showed 98,000 years as the most prominent periodicity, followed by 40,000 years of less than half the amplitude and still smaller peaks at 24,000, 67,000, 84,000, 107,000 and 786,000 years. Among the astronomical parameters, only eccentricity showed a similar periodicity viz. 96,000 years. Obliquity had a prominent periodicity at 41,000 years which was reflected in the oxygen isotope series also. Corresponding to the bimodal precession peaks (23,000 and 19,000 years), the oxygen isotope series showed a peak at 24,000 years and smaller peaks at 22,000 and 19,000 years. Our results compare well with those obtained byThomson (1990) with a more sophisticated method.