INCREASING NITROGEN LIMITATION AT THE P-TR BOUNDARY: A PAN-OCEANIC PHENOMENON?

An abrupt, approximately 5 ‰ negative shift in the δ¹⁵N-AIR of sedimentary organic matter has been observed immediately preceding the main Permian-Triassic marine extinction event at Opal Creek, an outcrop in southwestern Alberta deposited in a deep-water outer shelf environment. While this nitrogen signal may be influenced by reworking, it is similar in magnitude and direction to shifts in δ¹⁵N observed by Luo et al. (2011) in many southern Chinese P-Tr sections where, like Opal Creek, the changes in nitrogen cycling coincide with the extinction of the dominant benthic fauna. This shift may represent a single major event of increasing nitrogen limitation in the Permian ocean while differences in absolute values may be indicative of different nutrient cycling regimes. At Opal Creek a substantial latest-Permian increase in continental erosion may have increased phosphate inventories at the same time that increased anoxia was favoring nitrogen loss through denitrification and coastal upwelling currents were weakening, leading to N:P ratios below Redfieldian values, and necessitating increased nitrogen fixation. In South China, the presence of ecologically complex carbonate platform environments suggest already oligotrophic conditions in the latest Permian that may have been pushed toward extreme N limitation by increased denitrification, as reflected in δ¹⁵N values of < 0 ‰. Here the δ¹⁵N record from Opal Creek is compared to new δ¹⁵N results generated from the unpublished Peck Creek section in northern British Columbia, which represents a similar facies succession to Opal Creek in a deeper, more basinal environment with continuous sedimentation. This allows evaluation of the geographic extent of end-Permian nitrogen cycle disturbances, and the degree to which they were limited to coastal environments. Nitrogen isotope analysis of abyssal Panthalassic sections will shed further light on how this phenomenon was related to changes in coastal productivity.