From time to time in its history, the Earth has suffered an 'Ice Age', when large parts of its land surface have been covered by glaciers and its ocean surface by sea ice. This has happened at least during the late Precambrian, the Ordovician and the Carboniferous. The earth is again in an Ice Age. It began to develop during a phase of global cooling at the Eocene-Oligocene boundary (35-40 Ma) and has intensified during the late Tertiary and Quaternary.
Antarctica is the continent most susceptible to glaciation, and evidence of the first growth of the Antarctic ice sheet from the Middle Miocene (c. 14 Ma) is the first sign of severe cooling. Some stages in progressive cooling are:
2.4 Ma: the first icebergs dropped detritus in the North Atlantic; 2 Ma: the first Arctic marine microfaunas in the North Sea;
0.75-0.8 Ma: detritus dropped from icebergs and glacial tills in the Forth approaches, showing that Scottish-centred ice sheets extended into shallow waters;
0.55 Ma: the first time that a Scottish-centred ice sheet extended to the edge of the western continental shelf.
The cooling of global climate has not however been smooth and gradual. It has shown complex patterns of variation on all timescales. The best continuous evidence of global climate change through the Late Tertiary and the whole of the Quaternary comes from low sedimentation rate cores from the deep ocean that preserve a record of changing oceanic water temperature in changing microfaunal assemblages (Fig. 15.1a)