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
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.