Isotopic Composition of the Ocean during Geologic History

8 Isotopic Composition of the Ocean during Geologic History 

One of the most sensitive tracers recording the composition of ancient sea water is the isotopic composition of chemical sediments precipitated from sea water. The following discussion concentrates on the stable isotope composition of oxygen, carbon, and sulfur, but in recent years other isotope systems have been included such as Ca (De La Rocha and De Paolo 2000 Schmitt et al. 2003 Fantle and de Paolo 2005 Farkas et al. 2007) and B (Lemarchand et al. 2000, 2002 Joachimski et al. 2005) and Li (Hoefs and Sywall 1997). One of the fundamental questions in all these approaches is which kind of sample provides the necessary information, in the sense that it represents the ocean water composition at its time of formation and has not been modified subsequently by diagenetic reactions. 

It is generally agreed that continental glaciation and deglaciation induce changes in the 5 0-value of the ocean on short time scales. There is, however, considerable debate about long-term changes. 

The present ocean is depleted in by at least 6%c relative to the total reservoir of oxygen in the crust and mantle. Muehlenbachs and Clayton (1976) presented a model in which the isotopic composition of ocean water is held constant by two different processes (1) low temperature weathering of oceanic crust which depletes 

8 C-values of limestones vary mostly within a band of 0 3%c since at least 3.5 Ga (Veizer and Hoefs 1976). The longer term C-isotope trend for carbonates has be punctuated by sudden shifts over short time intervals named carbon isotope events , which are considered to represent characteristic features, and have been used as time markers for stratigraphic correlations. 

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