ENVIRONMENTAL STRESS AS A CAUSE FOR PROLONGED RECOVERY FOLLOWING THE PERMIAN-TRIASSIC MASS EXTINCTION: A GEOCHEMICAL INVESTIGATION OF THE LOWER TRIASSIC UNION WASH FORMATION

The Permian – Triassic (P-T) mass extinction, which occurred approximately 251.9 million years ago, eliminated nearly 90 percent of all species on Earth. Recovery from this crisis was complex, with many regions demonstrating an unusually prolonged biotic rebound, while a few examples of rapid recovery have also been noted. Studies of the Lower Triassic rock record reveal persistent environmental stress in the form of widespread oceanic anoxia, as well as elevated sea surface temperatures that may have been lethal to many organisms; the distribution of harsh environmental conditions therefore likely played a strong role in determining recovery trends. Anoxic conditions have been hypothesized to have existed in deeper water settings along the western continental margin of Pangea during the Early Triassic based on a lack of bioturbation and benthic fossils in outer shelf to slope facies, but this interpretation has not been extensively tested using geochemical proxies. The Union Wash Formation of east-central California was deposited from the middle Early Triassic (Dienerian) to the early Middle Triassic (Anisian). The absence of macrofossils, only minor amounts of silt and sand, and the preservation of primary sedimentary features implies deposition in a deep, quiet, anoxic environment (Woods, 1998). The current study will analyze mudstone and micritic limestone samples from the lower portion of the middle member of the Union Wash Formation in order to test the hypothesis that the unit was indeed, deposited under reduced benthic oxygenation (Woods, 1998). Fifty powdered samples from the middle member will be prepared for geochemical analysis of trace elements indicative of benthic oxygenation (V, Mo and U), primary productivity (Ba, Cu, Ni and Zn) and detrital input (Al, Ti, and Zn). Variations in trace element content will not only determine if the unit was deposited under anoxic conditions, but also if those conditions were being driven by shifts in primary productivity or water mass chemistry. Comparison of biotic recovery trends to the distribution of environmental stresses will allow for better determination of the role of environment in shaping the recovery from the Permian – Triassic mass extinction.