ECCENTRICITY FORCING OF PLIOCENE EARLY PLEISTOCENE CLIMATE REVEALED IN A MARINE OXYGEN-ISOTOPE RECORD

Milankovitch theory-that climate is controlled by variations in the Ea rth's orbital parameters-has gained wide acceptance for its ability to account for two climate cycles: a 23-kyr cycle that is phase-locked t o the precession-driven insolation cycle, and a 41-kyr cycle that is p hase-locked to the obliquity-driven insolation cycle(1-6). But, explai ning the observed similar to 100-kyr climate cycle in terms of Milanko vitch theory-especially for the Late Pleistocene ice-age cycle-remains controversial in spite of a strong correlation with the similar to 10 0-kyr cycle in the Earth's orbital eccentricity(5). One problem is tha t eccentricity affects insolation mainly by modulating the precession cycle; its direct contribution to radiation change is too small (<0.1% ) to effect the observed climate change directly(5,7). Another is the absence of a Late Pleistocene ice-volume cycle in oxygen-isotope recor ds to match the similar to 404-kyr component of the eccentricity cycle (5,8). Here we examine an oxygen-isotope record spanning the interval 1.2 to 5.2 million years ago, before the Late Pleistocene ice-age regi me. We find 404-kyr and similar to 100-kyr climate cycles which are co herent with eccentricity and which have amplitudes that are similar to the coexisting 23-kyr cycle. Analysis of these low-frequency cycles s uggests that they originate through an asymmetrical response mechanism that preferentially introduces variance into the climate system from the warmer portions of the eccentricity-modulated precession cycle. Ou r data thus support eccentricity's role in the origin of low-frequency oxygen-isotope cycles before the Late Pleistocene ice age.