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).

Geochemistry of bed and suspended sediment in the Mississippi river system: provenance versus weathering and winnowing.

Stream-bed sediment for the size fraction less than 150 microm, examined in 14,000 samples collected mostly from minor tributaries to the major rivers throughout the Mississippi River drainage system, is composed of 5 mineral fractions identified by factor analysis-Al-silicate minerals, quartz, calcite and dolomite, heavy minerals, and an Fe-Mn fraction. The Al-silicate fraction parallels its distribution in the regolith, emphasizing the local sediment source as a primary control to its distribution. Quartz and the heavy-mineral fraction, and associated trace elements, exhibit a complementary distribution to that of the Al-silicate fraction, with a level of enrichment in the bed sediment that is achieved through winnowing and sorting. The carbonate fraction has a distribution suggesting its dissolution during transport. Trace elements partitioned onto the Fe-Mn, possibly amorphous oxyhydride, fraction are introduced to the streams, in part, through human activity. Except for the heavy-mineral fraction, these fractions are identified in suspended sediment from the Mississippi River itself. Although comparison of the tributary bed sediment with the riverine suspended sediment is problematic, the geochemistry of the suspended sediment seems to corroborate the interpretation of the geochemistry of the bed sediment.