Deep Sea Cores

Since the development of the piston corer in 1947, it has been possible to take long cores of sediment from the ocean bottom. A wide variety of such sediment exists, but those most studied have been the oozes. Oozes form by accumulation of the dead skeletons of foraminifera and radiolaria, together with a variable amount of inorganic material. These sediments build up very slowly, from 10 mm per 1000 years up to 50 mm per 1000 years, but their sequence is uninterrupted. Some cores have provided an unbroken record for the whole of the Qua ternary. The main problem with such sequences is disturbance by animals living on the ocean bottom. Dating of the succession is accomplished by radiocarbon, which covers the last 70,000 years (the top 400 mm or so of most cores) and by palaeomagnetism, which provides fixed points of known date, between which further dates have to be interpolated. There are still problems with the detailed dating of cores. Two main types of investigation have been carried out: («) Faunal variation. Foraminifera can be identified from their skeletons. Changes occur in the relative proportions of different species, through the sequence shown in the cores. Such variation is compared mathematically with the known ecology of today’s foraminifera. This allows the reconstruction of variation in such factors as surface temperature and salinity throughout the time which the cores represent. (b) Oxygen isotope ratios. Sea water contains a number of isotopes of oxygen, the most common being 18O and 16O. During periods of growing ice-sheets, water rich in 16O is preferentially bound up in ice-sheets. Conversely, sea water becomes relatively rich in ,8O. Thus the 18O/16O ratio is an indicator of the global extent of ice-sheets. Both isotopes are incorporated into the skeletons of foraminifera. These skeletons then become deposited as ocean-bottom ooze, appear in cores and can be analysed for ,8O/16O content. Temperature may also affect the rate at which either isotope is taken up by the foraminifera. This effect can be eliminated by studying only species that live deep in the water, where temperature varies little. This technique has been combined most successfully with palaeomagnetism by Shackleton and Opdyke, on core number V28-238 from the south Pacific. Numbered stages are assigned to the fluctuations in isotope ratio up the core. At least eight major glacial episodes are shown to have occurred during the past 700,000 years. This is difficult to correlate with terrestrial sequences of Quaternary deposits.