δ 13C values for carbonate in francolite clearly distinguishes between authigenic varieties precipitated within anoxic sediments and diagenetic varieties produced by phosphatization of carbonates. δ18O values are characteristic of a deposit and, whilst apparently in equilibrium with pore-water in Recent deposits, are probably unreliable as palaeotemperature indicators generally. The isotopic composition of carbonate may therefore be used as a pointer to the mechanism of origin of francolite.

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