My Biogeochemical Romance: Correlation of inorganic geochemical proxy data and biomarker composition to reveal oceanic redox structure in the Late Ordovician Utica-Point Pleasant Formation

Samantha Ritzer, Stanford University, Department of Geological Sciences, Stanford, United States, Jochen J Brocks, Australian National University, Canberra, ACT, Australia and Erik A Sperling, Scripps Institution of Oceanography, Biological Oceanography, La Jolla, United States
The Middle to Late Ordovician was a period of significant restructuring and transition toward a ‘modern’ Earth system state as the geosphere and biosphere became more intimately linked. Here, we focus specifically on the middle Late Ordovician (Katian) for its unique juxtaposition as a relatively stable period – with respect to oceanic redox conditions – between a number of major environmental and biological transitions that may reveal clues about the overall state and dynamic nature of the Early Paleozoic Earth system.

This work aims to understand the development of Late Ordovician biogeochemical feedback mechanisms using a quantitative approach by correlating inorganic geochemical proxy data and biomarker composition through the Utica-Point Pleasant (U-PP) formation of the Appalachian Basin. The lithological development of the U-PP from carbonate platform-dominated facies to organic-rich calcareous black shales reflects a combination of both tectonically driven and glacioeustatic controls on sea level, providing the temporal and stratigraphic framework necessary for meaningful interpretation of any correlations in the geochemical data. Preliminary results from samples taken at roughly 0.5m resolution suggest that the U-PP can be characterized by 3-5 distinct facies, across which redox broadly transitions from an oxic to ferruginous (anoxic, Fe-replete) environment. Initial results from biomarker analyses provide evidence that euxinic conditions (anoxic, H2S-replete) reached the photic zone; placed in contrast with a ferruginous inorganic geochemical signal, this suggests an intriguing redox structure comprising a more marginal euxinic wedge overlying a deep ferruginous water column during this time. Using multivariate statistical methods and ensemble machine learning techniques, we attempt to elucidate the biogeochemical controls on both organic and inorganic geochemical proxy values.

Ultimately, the information that can be unlocked by the organic fraction may provide insight into both the evolution of the Late Ordovician Earth system as well as the mechanistic underpinnings responsible for Fe and trace elemental proxy values, such as the influence of terrestrialization on weathering, clay formation and diagenesis, and shelfal Fe cycling and transport to the deep basin.