ABSTRACT
Chapter 8 examined the profound and unidirectional change in ocean systems throughout the first 89% of the Earth's history. This chapter explores the main features of change in the remaining 11% (from Cambrian onwards, the Phanerozoic Eon), and reveals processes at several scales and due to several causes. The key aspect is that, although unidirectional change remains an element of Phanerozoic oceans, the ocean system was much more stable, with major element concentrations being constant (they have conservative behaviour, see Chapter 4). However, although salinity seems to have remained stable, there are examples of large-scale regional variation, such as in the Messinian salinity crisis of the Miocene Epoch, 6 Ma (Chapter 10). A persistent Phanerozoic feature is cyclic processes, which also affected ocean chemical change. The controlling factors operated at a variety of scales. Large-scale cyclicity relates principally to plate tectonic activity, with global or regional effects, and leaving sedimentary deposits several km thick in places where there were rapidly subsiding basins. At the other end of the spectrum, the smallest scale of cm-thick bedding can be related to local sedimentary dynamics. Paradoxically, however, thin beds deposited over large areas may be controlled by extraterrestrial forcing of variations in planetary motion, affecting climate; so, some of the smallest scale deposits may be caused by the largest scale processes affecting the oceans. Cyclicity scales form a hierarchy, referred to as first order, second order and so on; up to 10 orders have been recognised in some places. Although the concept of cycles has been around for years, the last 20 years have seen great leaps in our understanding of Phanerozoic change, much of which is palaeoceanographic. The information presented and discussed in this chapter therefore draws on understanding of modern ocean processes, and shows the contrasts between the modem and ancient oceans.
