SR AND ND ISOTOPES AT THE PERMIAN TRIASSIC BOUNDARY - A RECORD OF CLIMATE-CHANGE

We present a detailed curve of seawater Sr-87/Sr-86 for the Middle Permian to Triassic based on analyses of conodonts from overlapping sections in the U.S.A, and Pakistan, correlated using conodont biostratigraphy. The isotope ratio decreased in the Middle Permian at an average rate of 0.000062 Ma(-1), reached a minimum in the Capitanian (257-258 Ma), and increased in the Late Permian at an average rate of 0.000097 Ma(-1). The Late Permian rate of increase was roughly two and a half times greater than the average increase over the past 40 Ma, and approximately equal to the highest Cenozoic rates, which occurred over much shorter time intervals. Modeling results suggest that decreasing Middle Permian Sr-87/Sr-86 ratios were driven by changes in the riverine Sr flux to the oceans, while increasing ratios in the Late Permian/Triassic are attributed to both increased riverine Sr-87/Sr-86 and flux. The reduced Middle Permian riverine flux coincides with extreme continental aridity associated with the formation of Pangea and recorded by massive evaporite deposits. In addition, mountains in the equatorial region of Pangea may have created a rain shadow, thereby minimizing precipitation in regions that currently contribute the bulk of chemical weathering products to the ocean. Increasing riverine Sr-87/Sr-86 in the Late Permian is suggested by the observation that Nd-143/Nd-144 values decrease at the same time; however, the source of radiogenic Sr is not known. Frequently cited mechanisms for increasing Sr-87/Sr-86 in runoff, such as glaciations and continent-to-continent collisions, coincide instead with decreasing seawater Sr-87/Sr-86 in the Middle Permian. One possible source may have been deep erosion into older orogens, associated with a dramatic increase in chemical weathering in the Late Permian. The cause of enhanced weathering appears to have been increased levels of atmospheric CO2 with associated global warming and increased humidity. Proposed sources of CO2 include dissociation of gas hydrates and oxidation of organic matter during extreme sea level regression, as well as volcanic emissions from Siberian Traps eruptions. Continental floral and faunal distributions are consistent with this interpretation, as are oceanic delta(13)C patterns and variations in shallow-water sediment lithologies.