THE JOSÉ EXCURSION: A GLOBAL CARBON CYCLE RESPONSE TO THE ARENIG TRANSGRESSION IN THE EARLY ORDOVICIAN
High resolution stratigraphic correlation of El Paso Group sections suggest the José Excursion was a response to the globally-recognized sea level transgression at the beginning of the Arenig. Contemporaneous microbiolite buildups from Texas to the Argentine Precordillera were abruptly terminated in coincidence with the occurrence of more negative carbon isotopic values, suggesting a direct link between initiation of the excursion and sea level rise. A unique sub-meter thick micritic unit is found in conjunction with minimum δ13C values in the El Paso Group, and can be correlated with higher precision than any other interval in the succession. The environmental implications of this unit are currently unclear.
Significant negative excursions in marine carbonate carbon isotopic ratios were not uncommon in the Cambrian and Early Ordovician, but other excursionsthe Sunwaptan HERB and Skullrockian BRIE eventsare found in conjunction with sea level lowstand events and sequence boundaries. It is relatively easy to develop model scenarios that link these negative δ13C excursions to sea level fall through enhanced oxidation of organic compounds in shallowing environments. This mechanism is difficult to employ when linking negative δ13C excursions to sea level rise, especially since there is no evidence of a significant Early Ordovician terrestrial biosphere. One possibility is that sea level rise and a brief release of volatile sedimentary carbon were both induced by surface ocean warming, but alternative explanations are possible and will be discussed in the context of key lithostratigraphic units.