4 - Geological evidence for orbital-forcing

Summary

The recognition of rhythmic patterns in sediments with the same periodicity as orbital variations is a key part of the acceptance of orbital variations as causes of climatic changes at the Earth's surface. The initial impetus to investigate orbital variations was the problem of the Quaternary ice-ages: what caused them, and why the ice came and went (Imbrie & Imbrie 1979). To this extent, investigation of orbital changes was initiated by a particular, apparently rhythmic, series of geological events. The situation has now shifted to serious consideration of the other parts of the geological record because of the perpetual nature of orbital variations. This chapter reviews, in reverse stratigraphical order, evidence for the existence of rhythmic patterns throughout the geological record (see Table 1.1 for an outline time-scale), the origin of ice-ages, and concludes with a discussion of the nature of the record itself.

The evidence is mostly derived from two different sedimentary situations. There are laminated sediments, from lake and marine basins, that are grouped into units that recur rhythmically. Periodicity of such sediments is determined either by assuming (or demonstrating) that the laminae are annual and counting the laminae in each unit, by estimating the age of the whole sequence from radiometric dating and assuming that each unit is an equal representation of the whole time-span, or by applying a time-series technique (Schwarzacher 1993). The second situation is deep-sea sediment that is still accumulating, and from which a variety of chemical and isotopic values indicate periodic climatic conditions.