OCEAN REDOX CONDITIONS IN A LESS OXYGENATED ORDOVICIAN WORLD: A CASE STUDY ON THE NON-EUXINIC KATIAN ORGANIC-RICH MUDROCKS OF SOUTHERN ONTARIO
The Collingwood Member (ca. 450 Ma) is a calcareous ORM (Ca > 10%). These rocks have higher average TOC in the St. Joseph (SJ, 6.8 wt.%) and Little Current (LC, 7.4 wt.%) cores compared with the Collingwood (CW, 3.8 wt.%) and Mount Forest (MF, 1.6 wt.%) cores. The Mo/U EF ratios (EF = Al-normalized enrichment factor) from SJ and LC are mostly close to the modern seawater ratio (Mo/USW) without pronounced Mo (EFs <30) and U (EFs <10) enrichments, suggesting a suboxic to anoxic/non-sulfidic environment with no clear evidence of water column euxinia. The Mo/USW ratios from CW and MF are 0.1-1 and <0.3, respectively, pointing to predominately oxic-suboxic and sporadic anoxic/non-sulfidic bottom waters. The overlying Rouge River Member (ca. 449 Ma) is a siliciclastic-dominated ORM (Ca < 6%) with an average TOC content of ~2 wt.% from six drillcores. The Mo/USW ratios (0.1-1) from the Port Stanley, Pickering, SJ, and CW cores suggest fluctuating bottom water redox conditions from oxic to anoxic (non-sulfidic) whereas a narrow range of Mo/USW ratios (0.3-0.7) suggests pervasive suboxic conditions at the Chatham and MF localities. These interpretations are consistent with Mo/Re EF ratios.
Overall, the RSTM data suggest regional spatiotemporal variations in redox conditions for the Collingwood Member and Rouge River Member. These units represent a case study of the less oxygenated Ordovician world, during which a larger extent of suboxic-anoxic seafloor was likely compared with today. Future work will examine ocean redox conditions from the perspective of metal isotopes.