THE CONTRIBUTION OF ORBITAL FORCING TO THE PROGRESSIVE INTENSIFICATION OF NORTHERN-HEMISPHERE GLACIATION

In this study, we reconstruct the timing of the onset of Northern Hemi sphere glaciation. This began in the late Miocene with a significant b uild-up of ice on Southern Greenland. However, progressive intensifica tion of glaciation did not begin until 3.5-3 Ma, when the Greenland ic e sheet expanded to include Northern Greenland. Following this stage w e suggest that the Eurasian Arctic and Northeast Asia were glaciated a t approximately 2.74 Ma, 40 ka before the glaciation of Alaska (2.70 M a) and about 200 ka before significant glaciation of the North East Am erican continent (2.54 Ma). We also review the suggested causes of Nor thern Hemisphere glaciation. Tectonic changes, such as the uplift of t he Himalayan and Tibetan Plateau, the deepening of the Bering Strait a nd the emergence of the Panama Isthmus, are too gradual to account ent irely for the speed of Northern Hemisphere glaciation. We, therefore, postulate that tectonic changes may have brought global climate to a c ritical threshold, but the relatively rapid variations in the Earth's orbital parameters and thus insolation, triggered the intensification of Northern Hemisphere glaciation. This theory is supported by compute r simulations, which despite the relative simplicity of the model and the approximation of some factors (e.g. using a linear carbon dioxide scenario, neglecting the geographical difference between the Pliocene and the present) suggest that it is possible to build-up Northern Hemi sphere ice sheets, between 2.75 and 2.55 Ma, by varying only the insol ation controlled by the orbital parameters. (C) 1998 Elsevier Science Ltd. All rights reserved.