BIOGEOCHEMICAL EVIDENCES OF OSCILLATING REDOX CONDITIONS DURING DEPOSITION OF ORGANIC-RICH INTERVALS IN THE MIDDLE DEVONIAN MARCELLUS SHALE

The fundamental controls on burial and preservation of organic matter in Middle Devonian organic-rich (OR) black shales in the Appalachian Basin are still debated. Geochemical, isotopic, petrographic and biomarker analyses were performed on samples from 3 well cores obtained from Marcellus Shale to understand dominant biogeochemical controls on deposition of OR intervals. The ratios of C_org/N_org and C_org/P_tot were significantly higher in the OR interval. The higher C_org/N_org and C_org/P_tot ratios of the preserved organic matter may reflect decompositional losses of N and P due to microbial processes, indicating recycling of nutrients in this interval. In addition, our data shows positive correlations between C_org/N_org, C_org/P_tot ratios and TOC content indicating that the recycled nutrients might have promoted primary productivity resulting in higher accumulation of organic matter in the OR intervals. Multiple proxies like highly variable trace metal concentrations (e.g., U, Mo, V), δ¹⁵N, δ³⁴S_spy, Pr/Py, Ts/Tm hopane ratios, and Pr/n-17 vs Py/n-18 cross plots also provide supporting evidence of oscillating plaeoredox conditions during the deposition of OR intervals. In addition, sedimentary features and agglutinated benthic foraminifera in the OR interval indicate that there were short-term shifts between oxic/anoxic boundaries near the sediment-water interface. Multiple lines of evidence suggest that conditions were not permanently anoxic during formation of organic rich zones and nutrient recycling under oscillating redox conditions might have played a key role in sustaining high levels of primary productivity, resulting in deposition of the OR intervals.