TRACE ELEMENT ACCUMULATION MODELING OF THE PRIMARY PRODUCTIVITY, WATER-COLUMN CHEMISTRY, AND HYDROGRAPHY OF DEPOSITION OF THE MEADE PEAK MEMBER OF THE PHOSPHORIA FORMATION

The accumulation rates of trace elements (e.g., Cd, Cr, Cu, Mo, Ni, V, and Zn) in marine basins are determined by 1) the rate of terrigeneous sediment input, 2) the rate of organic matter and associated trace nutrients settling to the sea floor that results from primary productivity, and 3) the rate of removal from bottom water, as determined by bottom water redox conditions and residence time. A recent study of the Cariaco Basin (Piper and Dean, 2002) showed that the trace element accumulation rates of the two marine fractions - biogenic and hydrogenic - can be used to quantitatively model the biologic, chemical, and hydrographic properties of the water column, with close agreement to actual measured conditions.

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.