In the oceans, sediments have accumulated for millions of years. The sediments consist partly of terrigenous material, which is detritus derived from erosion of the surrounding land masses, and biogenic material consisting of calcareous and siliceous skeletal fragments from micro-organisms. The terrigenous material is brought to the ocean floor mainly by turbidity currents, bottom currents, wind and ice. In the mid- and high-latitude oceans, Ruddiman et al. (1989) found that terrigenous detritus was mainly deposited during glacial episodes,
Low Latitude Stack (MD900963 -I- Site 677)
Figure 3.21 The SPECMAP stack plotted against age using the time-scale developed by Imbrie et al. (1984) (left) and the low-latitude stack tuned to the ice volume prediction model of Imbrie and Imbrie (1980) with numbering of the isotopic events. (From Bassinot et al., 1994)
reflecting transport by means of ice rafting of glacially eroded debris. In ocean sediment records, the ice-rafted debris (IRD) reflects former glacial episodes. In the North Atlantic, IRD deposition has been the most important mechanism for terrigenous sediment transport into the oceans; at 45CN, cycles of IRD deposition during the last glacial period have been recognized. These have been termed Heinrich layers (Heinrich, 1988) and reflect episodes of IRD arrival from icebergs drifting eastwards from the Laurentide ice sheet margins (Alley and MacAyeal, 1994).
In the deep oceans, the sediments are normally finer grained and dominated by biogenic material of carbonaceous and siliceous particles from marine micro-organisms (marine oozes). The detailed data for environmental change in the oceans come from the chemical and isotopic composition of these marine organisms. Variations in aluminium, barium, calcium and cadmium, and the isotopes of carbon, oxygen and uranium, reflect changes in circulation, nutrient supply and water temperature in the oceans. The application of oxygen isotope analysis to deep-ocean sediments has revolutionized Quaternary science (Figs 3.1 and 3.21). Oxygen exists in three isotopes (160, 17O and lsO), and 160 and lsO are used for oxygen isotope analysis of marine deposits. Ratios between lsO and 160 vary between 1:495 and 1:515 (average of about 1:500 or 0.2 per cent lsO). Ratios between the two oxygen isotopes are measured in relative deviations (<5180%o) from a laboratory standard value. The common standards used are PDB (belemnite shell) for carbonate analysis, and SMOW (Standard Mean Ocean Water) for analysis of water, snow and ice. PDB is +0.2%o in relation to SMOW.
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