TRACE ELEMENT ACCUMULATION MODELING OF THE PRIMARY PRODUCTIVITY, WATER-COLUMN CHEMISTRY, AND HYDROGRAPHY OF DEPOSITION OF THE MEADE PEAK MEMBER OF THE PHOSPHORIA FORMATION
This same model is applied to the Meade Peak Member of the Phosphoria Formation, a Permian black shale and phosphate deposit in the northwestern United States. The rates of accumulation of marine Cd, Cu, Ni, V, Zn, and PO4-3 indicate a mean level of primary productivity of 500 g organic carbon m-2 yr-1, with the rates calculated for the different elements showing close agreement. Even allowing for large errors in assumed parameters (e.g., ± 70% in fraction of organic matter fluxing from the photic zone), the range of primary productivities calculated is within that observed in modern marine environments. The accumulation rates of the predominantly hydrogenic elements Cr and V identify bottom water redox conditions as denitrifying and the residence time of bottom water as being approximately 4 yr.
The source of bottom water imported into the Phosphoria Basin was an oxygen minimum layer of the open ocean, in which the oxygen concentration was approximately 5 to 20% of saturation. A lower oxygen concentration would have promoted sulfate reduction in the bottom water. The advance of bottom-water redox conditions to this level is precluded by the lack of accumulation of a hydrogenic fraction of sulfide forming metals (Cd, Cu, Mo, and Zn) from bottom water.