Oxygen isotopic compositions margins

A meteoric origin is suggested for some of basin margin cements that have relatively low oxygen isotopic compositions yet could not have been exposed to high burial temperatures. These cements have relatively high interstitial volumes (> 30%) and are usually calcite, but in some cases are dolomite. Based on their occurrence in shallow... 

Multiple samples taken from concretions vary less than 2%o (and commonly less than l%o) in 8 C and 8 0 from core to margin, without any consistent trend. In contrast, variations in isotopic values between concretions in the same formation are greater oxygen isotopes commonly differ by 4%o (locally greater) within a single formation. A combination of variations in temperature and water composition seems to be the cause. In the Loiano Formation there is a significant difference between 8 0 values of bedding-parallel concretions and fault-parallel concretions, which reflects different times of cementation. 

Fig. 10. (A) Oxygen and carbon stable isotopic composition of calcite cements from the San Joaquin basin margins. Data sources from Table 1. (B) Oxygen and carbon stable isotopic composition of calcite cements from the San Joaquin basin centre. Data sources from Table 1. Note the restricted range of carbon isotopes compared with basin margin cements (cf. with (A)).

Fig. 9. Continued) and Ab O values (this study, and McCorkle et al 1997). The carbon isotopic offsets are calculated relative to bottom water DIC, and the oxygen isotopic offsets are calculated relative to calcite in equilibrium with bottom water using the expressions of McCorkle et al (1997) (solid diamonds) and Shackleton (1974) (open circles). The vertical line shows the trend in isotopic offsets predicted by a pore water b C effect only. The observed b C depletions of pore water DIC in the 0-0.5 cm depth interval range from — 1 to — 2.2%o at the nearby sites discussed by McCorkle et al (1997) (white hexagons, dotted line). Because pore water carbonate ion concentration is likely to be lower than bottom water values at these relatively shallow continental margin sites, a carbonate ion influence on shell composition would tend to cause Ab O values greater than zero (ellipse).

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