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Sedimentary phosphorite formation has occurred episodically over geologic time. Substantia] phosphorite deposits were formed during P-giant episodes of which the most widespread and long-lasting were the Precambrian-Cambrian and the Cretaceous-recent. The aim of this review is to reevaluate these two episodes in the light of sedimentary Sr, Nd, S, and C isotopic records. Some, although not all of these phases of phosphorite formation were accompanied by increases in seawater 87Sr/s6Sr unrelated to changes in seafloor spreading, which supports a possible link between orogeny and global weathering rates, P-input, and P-giant formation. Assessing the relationship between δ13C and phases of widespread phosphogenesis is more complicated due to the counterbalancing effects on seawater δ13C of productivity/organic matter deposition and subsequent phosphogenesis/early diagenetic carbon oxidation. It is recommended that proposed links between phosphogenesis and global changes of δ13C or P- input continue to be reexamined on a case-to-case basis. The application of secular trends in Nd isotopic ratios to unravelling the often vital role of paleocurrents in phosphorite formation is also discussed. In addition, recent isotopic research is outlined that has led to significant improvements in stratigraphic resolution around the Precambrian-Cambrian boundary. A temporal and causal connection is put forward between metazoan evolution, that is the introduction of bioturbation, fecal pellets, biomineralization, and filter feeders, which would have helped to concentrate mineral phosphate in sediments, and widespread phosphogenesis at this time.

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