CLIMATIC IMPORTANCE OF THE MODULATION OF THE 100-KYR CYCLE INFERRED FROM 16-MY LONG MIOCENE RECORDS

The variations of the amplitude of the 100 kyr cycle are not yet clearly understood. Although the timing of the appearance of these cycles in the Pleistocene is well established, very little is known about the occurrence of such cycles and their evolution in most of the Neogene. Two detailed and continuous 16 m.y. long Miocene records of paleoceanographic significance are analyzed in order to study the dynamics of the 100 kyr cycle. One record results from counts of the relative abundance of the calcareous nannoplankton Coccolithus pelagicus in about 1000 samples taken in the Neogene section recovered from Ocean Drilling Program site 747 (southern Indian Ocean). This record, in which the chronologic resolution is similar to 17 kyr, is interpreted as reflecting migrations of the Antarctic polar front through time. The record of the wet-bulk density in the same section, which is interpreted as reflecting variations of paleoproductivity constitutes the second 16 m.y. record. It has a chronologic resolution of 3.5 kyr. The two series reveal the presence of 100 kyr cycle during the Miocene. This cycle is not stable through time and vary similarly in the two series. The amplitude of the 100 kyr cycle fluctuates quasiperiodically with primary periods close to 2.38, 1.14, and 0.8 m.y., equivalent to those of the modulation of the orbital parameters (obliquity and eccentricity). In addition, the amplitude of the 100 kyr cycle is stronger during cooler times. The interactions between climate and modulation of the eccentricity or the obliquity cycles are good candidates to explain that change in amplitude. It is concluded that the long term modulation of the orbital parameter has a significant influence on global climate.