Changes in marine nitrogen fixation and denitrification rates during the end-Devonian mass extinction

Abstract

The end-Devonian was a critical interval in Earth history recording the transition from the Early Paleozoic greenhouse climate to the Late Paleozoic icehouse climate. A mass extinction at this time (the Hangenberg extinction) eliminated ~21% of genera and ~16% of families of marine invertebrates, although its causes remain poorly understood. The marine nitrogen cycle is intimately related to the nutrient status of seawater, microbial community composition, and the redox condition of the oceans, and, thus, it may provide insights into the mechanism(s) of the end-Devonian mass extinction and climate change. Here, we analyzed high-stratigraphic-resolution bulk-sediment nitrogen isotope variation (δ15Nbulk) in three sections from South China: (1) Long'an, an isolated carbonate platform section, (2) Qilinzhai, a shallow-water carbonate platform section between the Yangtze Oldland and the Youjiang Trough, and (3) Malanbian, a shallow-water carbonate platform section between the Yangtze and Cathaysia oldlands and distant from deep-water areas. Our δ15Nbulk records show a major perturbation to the marine nitrogen cycle in the Middle Siphonodella praesulcata Zone, which is marked by a decline in δ15Nbullk from +4‰ to 0‰ at Long'an and from +4.5‰ to +2.5‰ at Malanbian. This perturbation coincided with the main end-Devonian mass extinction as well as with a negative shift in carbonate carbon isotopes (δ13Ccarb). The shift to lower δ15N values suggests locally enhanced nitrogen fixation, possibly as a consequence of increased denitrification in response to globally expanded marine anoxia. Thus, these δ15Nbulk data are consistent with the hypothesis that intense anoxia was the immediate cause of the end-Devonian mass extinction. The δ15Nbulk records generated in this study suggest that there was significant spatial and temporal variation in the nitrogen isotope composition of seawater nitrate in the Yangtze marginal sea during the latest Devonian. This variation may have been related to the complex paleogeography of the South China Craton, which contained numerous isolated and continent-attached platforms and deep-water basins that influenced watermass circulation, nutrient inventories and primary productivity levels, and watermass ventilation and redox conditions.