SEAWATER CIRCULATION AND CHEMISTRY OF THE PERMIAN PHOSPHORIA SEA: SOUTHEAST IDAHO

The Late Permian Phosphoria Basin, an epicratonic basin centered in southeast Idaho, was the site of accumulation of a major sedimentary phosphate deposit. Phosphate (PO ₄ ^3-) production has come largely from the Meade Peak Member of the Phosphoria Formation. Although no single modern marine basin is an exact analog of the basin in which the deposit accumulated, the mass budget of PO₄^3- demonstrates that the hydrography and marine chemistry and biology of the Phosphoria Sea were similar to single aspects of different modern marine upwelling systems. The rain rate of PO₄^3-onto the seafloor, which settled out of the photic zone (0 to approximately 50-m depth) largely as algal debris with a given stoichiometry, and its precipitation from sediment pore water and eventual burial as carbonate fluorapatite were 5.5 g m ^-2 yr ^-1. The value represented approximately 25% of the 3.0 µg L ^-1 of PO ₄ ^-3 imported from the open ocean between 100-m and 250-m depth. The other 75% of this biologically essential and limiting nutrient were exported back into the open ocean in the Ekman layer (0 to about 100-m depth). The exchange rate of water between the Phosphoria Sea and the open ocean established an upwelling rate of the imported water into the photic zone of 78 m yr ^-1, primary productivity at 290 g m ^-2 yr ^-1 of carbon, and a residence time of water in the basin of 4.5 yr. The Cd, Cu, Mo, and Zn stoichiometry of planktonic debris and their accumulation rates support these values. The accumulation rates of Cr, U, V, and the rare-earth elements indicate that bacterial respiration in the bottom water was denitrifying, which further limited the O ₂ concentration of the imported seawater. An accumulation rate of the terrigenous sediment fraction of 2.5 to 10 g m ^-2 yr ^-1, based on major-element oxides and a third suite of trace elements (B, Ba, Co, Hf, Li, Ta, Th), confirms speculation that the Phosphoria Basin was sediment starved.