An example of the Middle to Late Devonian marine nitrogen cycle from mudstones of the Horn River Group, Northwest Territories, Canada

Abstract

The Middle to Late Devonian was characterized by the widespread deposition of organic-rich mudstone units and successive biotic crises and anoxic events in the marine realm, the cause of which remains debated and requires constraints from associated marine conditions. This study provides an example of the marine nitrogen cycle throughout the late Eifelian to middle Frasnian anoxic pulses. We present new and previously published organic whole-rock N (δ15Nbulk) and carbon (δ13Corg) isotopic datasets from organic-rich mudstone units of the Horn River Group (Canol and Hare Indian Formations) and overlying Imperial Formation in the Central Mackenzie Valley, Northwest Territories, Canada. In the ConocoPhillips Mirror Lake N-20 core, δ13Corg ranges from −31.0 ‰ to −24.3 ‰ with δ15Nbulk from −3.8 ‰ to +1.9 ‰, whereas the Husky Little Bear N-09 core is characterized by δ13Corg from −31.0 ‰ to −27.2 ‰ and δ15Nbulk from −2.0 ‰ to +5.9 ‰. The N isotopic signatures near 0 ‰ and a lack of δ15Nbulk – δ13Corg relationship are characteristic of N2 fixation by primary producers. Regular oscillations in δ15Nbulk are interpreted as the product of episodic, mild oxygenation events. Together, our δ13Corg and δ15Nbulk results suggest that locally, N2 fixation was the dominant source of N for primary producers in the late Eifelian to middle Frasnian, despite fluctuations in δ13Corg and global marine paleoredox. These findings contribute to our understanding of the nitrogen speciation and bioavailability associated with anoxic events, biotic crises, and widespread organic carbon burial in the Eifelian to Frasnian oceans.