MY BIOGEOCHEMICAL ROMANCE: EXPANDED WATER COLUMN REDOX ARCHITECTURE EXPLAINS DECOUPLED BIOMARKER AND INORGANIC GEOCHEMICAL PROXY RECORDS THROUGH THE LATE ORDOVICIAN POINT PLEASANT AND UTICA SOURCE ROCK SHALES OF WEST CENTRAL OHIO, USA

The Point Pleasant (PP) and Utica Formations and other co-eval Late Ordovician Appalachian foreland basin source rock-reservoir shales have been extensively studied for their petroleum resource potential, yet biomarker and geochemical proxy studies remain relatively sparse and regional in nature. Here we present a new high-dimensional biogeochemical dataset from a core locality in present-day west central Ohio, USA.

Fe-speciation and redox-sensitive element (RSE) proxies record a predominantly anoxic, ferruginous signal for much of the section, tending toward more sulfidic conditions the Tr-PP and PP-U boundaries. Mo (ppm) and U (ppm) are only modestly enriched above avg. shale while V (ppm) and Zn (ppm) appear to reach hyper-enrichment values (> 500 ppm) at the PP-U boundary.

Biomarkers also broadly record signals of redox stratification, but may also be interpreted to record water column processes. Elevated Gammacerane Index (GI) values (> 5%) suggest an expansion of the chemocline to include a distinct anoxic marine zone (AMZ), inhabited by bacterial ciliates, that separates the oxic-sulfidic zones. We also report the presence of C40 aryl isoprenoids, which we tentatively identify as isorenieratane and the “orphan biomarker” paleorenieratane. These molecules are characteristic of phototrophic sulfide-oxidizing anaerobes (PSOAs) and are often invoked to identify photic zone euxinia (PZE), though our data does not preclude the possibility of synthesis under ferruginous conditions.

We quantitatively explore a number of hypotheses in an attempt to reconcile the seemingly decoupled Fe-speciation and biomarker redox signals, including physical transport processes and SL controls, diagenetic effects, organic matter sulfurization, the possibility of a dominant ‘ferro-metabolism’, among others. Despite its complexity, we invoke the best-fit depositional model for this data to represent periodic development of hyper-sulfidic conditions from otherwise low-moderate background sulfide concentrations in the water column. This would likely require a well-attenuated, efficient ‘cryptic’ S-cycle in the upper water column and could potentially allow for the contemporaneous preservation of a ferruginous Fe-speciation signal at the sediment-water interface.