Comparing Devonian Upper Ocean Redox Trends Across Laurentia Using I/Ca Ratios of Marine Carbonates

Abstract:

The Devonian Period (~ 419 to 359 Ma) witnessed a significant pO₂ growth in atmosphere, which is thought to be related with enhanced organic matter burial related to the expansion of land plants and the associated increased flux of nutrients to oceans. On the other hand, a range of relatively moderate bio-crises to well-known extinctions and anoxic events also occurred in the Devonian marine system under the high pO₂ background. This study reports new I/Ca data spanning the Early to Late Devonian from the southern Illinois Basin (east Laurentia) and Great Basin (west Laurentia), and compares the I/Ca ratios with those reported from Appalachian Basin. Our preliminary results show that during Pragian to Emsian stage, the Illinois Basin records low I/Ca ratios (< 0.5 μmol/mol) that are comparable to baselines seen in Proterozoic and early Paleozoic, while the Great Basin records higher values (~ 1-3 μmol/mol). During the Eifelian stage, the Illinois and Great Basin have consistently low I/Ca ratios (mostly < 1 μmol/mol), which are much lower than those reported from the coeval Appalachian Basin (as high as ~ 6-8 μmol/mol). From Givetian Stage to the Late Devonian, I/Ca ratios start to increase to as high as ~ 2 μmol/mol both in the Illinois and Great Basin. Considering I/Ca as an upper water proxy which reflects local redox conditions and the different locations of three sections across Laurentia, the heterogenous Devonian I/Ca profiles are likely driven by variations in local seawater redox and/or diagenesis decreasing original I/Ca values. Overall, our data shows large spatial and temporal variations of I/Ca ratios during the Devonian Period. The inferred upper ocean redox changes will be examined for potential relationships with previously reported marine anoxia and bio-crises under the background of high pO₂ level.