The palaeomagnetic time–scale discussed in sections 2 and 3 is helpful in establishing the dates of a few important changes in climate and in marine and in marine faunas which occured more than 700 t.y. ago: it is of little help in dating more recent events. The most promising evidence for determining the ages of the later climatic changes comes from depp-sea sediments.
Arrhenius (1952) and Olausson (1960 A,b) found in cores taken by the Swedish Deep-Sea Expedition of 1947–8 a record of cyclic changes in the proportion of carbonate and in the abudance of foraminifera. These changes were interpreted as indications of broad climatic chnages. Emiliani (1955A) found somewhat similar cyclic changes in the proportions of the oxygen isotope 18O contained in foraminifera preserved in deep-sea sediments. Despites uncertainties in the exact interpretation of the variations of this isotope, it is agreed that they do reflect the major changes in climate. This deduction is supported by mire recent work, especially in the Caribbean, which has shown that the 18O variations are closely parallel to variariona in the amount of coarse material in the sediments and in the proportions of temperature-sensitive foraminifera.
The various parameters which give an indication of climate are usually presnted in graphs in which climate is plotted against depth below the top of the core. There are now sufficient cores with radiometric dates to make it possible to calibrate the scale of these graphs in terms of thousands of years. Figure 9 shows a climate–time curve
