Comparison of Milankovitch periods between continental loess and deep sea records over the last 2.5 Ma

A high-resolution East Asian winter monsoon proxy record reconstructed from the Baoji loess section in China shows two major shifts in climate modes o ver the past 2.5 Ma, one occurring at about 1.7-1.6 Ma BP and the other at about 0.8-0.5 Ma BP. The 1.7-1.6 Ma shift is characterized by a rather abru pt transition of winter monsoon variability from various periodicities to d ominant 41-ka cycles, and accompanied by a substantial increase in intensit y of winter monsoon winds as manifested by an increase in average loess gra in size. The 0.8-0.5 Ma event shows a relatively gradual transition from co nstant 41-ka cycles to predominant 100-ka climatic oscillations with a sign ificant increase in amplitude. The 0.8-0.5 Ma shift matches that registered in deep-sea delta(18)O records, whereas the 1.7-1.6 Ma shift is absent in global ice volume changes. This comparison suggests that at about 1.6 Ma BP , the ice sheets in the Northern Hemisphere may have reached a critical siz e, sufficient to modulate changes in the global climate system. The discrep ancy of climate cyclicity between loess and deep-sea records over the 2.5-1 .6 Ma interval suggests that the older Matuyama climate evolution cannot be understood simply by a regular 41 ka cycle model on a global scale. More l ong proxy records derived from continental deposits are needed. (C) 1999 El sevier Science Ltd. All rights reserved.