Marine phosphorite deposits and the nitrogen cycle.

We have attempted to demonstrate a possible relationship between phosphorite deposition and an increase in marine denitrification. The studies of others indicate that major phosphorite deposits are often associated with black shales and accumulated during only a few epochs of geologic history. Some of these epochs were also marked by mass extinctions of organisms. Such events are not as precisely nor as strongly correlated as we would like. Nevertheless, the correlations are strong enough to encourage further consideration of the effects of possible changes in the rate of denitrification within ancient oceans on the origin of phosphorite deposits, the extinctions of marine organisms, variations in the overall level of biological activity, and temporal fluctuations in the organic carbon content of sedimentary rocks (36).