S18O records of planktonic foraminifera (Tiedemann et al. 1994; Shackleton et al. 1988; Morley and Dworetzky 1991) have shown that large northern Hemisphere glaciations began about 2.75 MaB.P.; from ~1 MaB.P. onwards, its intensity (ice volume) and the length of its glacial phases approximately doubled (Fig.4b).
Sometimes the closure of the Panamanian seaway and the resulting impact on North Atlantic deep water formation are mentioned in connection with the onset of the ice ages (Haug and Tiedemann 1998). The closure occurred 4.6 to 3.6 Ma ago. This is 1 Ma before the large Northern Hemipshere glaciations. The argument that increasing obliquity amplitudes between 3.1 and 2.5 MaB.P. caused the ice to build up, contradicts the fact that the decrease in obliquity amplitudes between 1 and 0.8 Ma does not coincide with ice retreat; on the contrary, it corresponds in time with an intensification of global glaciation (Fig.4a,b).
Fig. 4: (a) Orbital parameters of the earth and corresponding insolation values for 65°N for the last 6 million years according to Berger & Loutre (1991).(b) Benthic oxygen isotope records from Ocean Drilling Program Site 659 according to Tiedemann et al. (1994). The fluctuations in the S18O content of the foraminifera reflect the fluctuations of the global ice volume, with high values corresponding to the glacials and low values to the interglacials. Neither the beginning nor the intensification of the Quaternary glaciation period is correlated with the insolation (a).(c) Synopsis of the uplift and glaciation of the Tibetan plateau in their relation to other geoecological events. Comparison between (a) and (b) shows that an additional factor apart from orbital variations is required to explain both the start of the ice ages about 2.8 Ma and their increasing intensity from 1 Ma onwards. The closure of the Panama gateway occurred too early to be the terrestrial cause. The uplift of the Tibetan plateau, as far as it can be reconstructed from the onset of the summer- and winter monsoons, and, derived from this, the start of an autochthonous glaciation of Tibet from ~2.5 MaB.P. onwards, were synchronous with the onset of the global ice ages. Evidence that variations of the summer- and winter monsoon intensity documented by marine dust flux records and loess-palaeosol sequences on the Chinese loess plateau occurred in phase not with the insolation variation but with glacial-interglacial cycles (40 ky and ~100 ky periods), is a strong pointer to the existence of a Tibetan glaciation. Gradual uplift of the Tibetan Plateau towards the ELA (equilibrium line) level enabled an ice sheet of 2.4 million km2 to grow from ~1 Ma B.P. onwards. The resulting cooling effect supported a maximum expansion of the Nordic lowland inland-ice.
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