Examination of Nitrogen Isotopes as a Proxy for Water Column Redox States during Deposition of Marine Shales: A Comparison of the Woodford and Springer Shales, Anadarko Basin, Western Oklahoma
Rivera, Keith; Quan, Tracy M.
In this study, we examine the use of sedimentary δ15N as a proxy for water column redox state during the deposition of productive marine shales. On geologic time scales, the most influential reactions in the nitrogen cycle are nitrogen fixation, nitrification, and denitirification, and their relative predominance is controlled by the redox state of the water column. Associated with each of these reactions is a characteristic nitrogen isotopic fractionation, which is reflected in the organic matter produced; this organic matter is subsequently transferred into the sediments. Both nitrogen fixation and nitrification have relatively low fractionation factors that result in depleted δ15N values for organic matter. Denitrification prefers to utilize the isotopically lighter 14N nitrate, therefore enriching the residual nitrate pool, which is then used to produce organic matter with enriched δ15N values. Incomplete denitrification is predominant under suboxic water column conditions, while nitrogen fixation and/or nitrification are dominant in anoxic and oxic environments. Therefore, bulk sediment samples with enriched δ15N values can be assumed to have been deposited under suboxic conditions, while intervals with depleted δ15N values would likely have been deposited under anoxic or oxic conditions, provided that post-depositional diagenesis does not occur.
In order to evaluate the use of sedimentary δ15N as a proxy for water column redox states, we compared the δ15N values from the anoxic Woodford Shale with those from the suboxic Springer Shale. Four cores were chosen in Grady and Custer counties in Oklahoma based on location within the Anadarko Basin and the relative locations of the Woodford and Springer cored intervals. In general, we found that anoxic depositional environments have relatively low sedimentary δ15N values, while relatively enriched sedimentary δ15N values correlate to suboxic depositional environments. These results confirm that sedimentary δ15N can be used as a tool to identify and evaluate anoxic intervals, which can enhance the identification of productive intervals within shales.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013