Isotopic, geochemical and petrographic analysis of the Otuk Formation, Northern Alaska

Abstract

The Triassic-Jurassic boundary is associated with one of the big five Phanerozoic mass extinction events and is characterized by global negative δ13C excursions, indicating a major disruption in the carbon cycle. The end-Triassic extinction event was caused by Central Atlantic Magmatic Province volcanism, that initiated the breakup of the supercontinent Pangea. Abundant greenhouse gas emissions, including CO2 and CH4, from the volcanic activity, affected multiple environmental factors. Global warming, ocean acidification, deoxygenation, mass mortality, and lithological change are documented across the boundary. These environmental fluctuations are also observed due to human-induced global climate change, making understanding the end-Triassic extinction significant. Northern Alaska during the Late Triassic has evidence of being the location of an upwelling zone, influencing redox conditions on the sea floor. I hypothesize that isotopic, geochemical and petrographic analyses into Northern Alaska’s Late Triassic Otuk Formation will give a better understanding of the depositional environments in which it formed. Our research has documented the Triassic-Jurassic boundary using carbon and nitrogen isotopes in the rock record, where previously it was not well recorded in Alaska. Research into the Otuk Formation also helped identify the high petroleum potential of these organic-rich rocks. Late-Triassic Northern Alaska rocks were deposited in marine oxic to anoxic conditions, impacting organic burial and paleo¬ redox conditions. Paleo-redox conditions of euxinic environments were identified through the presence of pyrite framboids. A negative carbon isotope excursion was identified in the Otuk Formation, also corresponding with a change in lithofacies and fossil size and abundance. Sedimentary petrology and TOC values documented environmental changes from oxic bivalve¬ rich facies to dark, low-oxygen, organic shales across the Triassic-Jurassic boundary. The Triassic- Jurassic boundary is observed in Northern Alaska, and can act as a marker for multiple environmental changes at that time.