THE JOSÉ EXCURSION: A GLOBAL CARBON CYCLE RESPONSE TO THE ARENIG TRANSGRESSION IN THE EARLY ORDOVICIAN

Carbonate carbon stratigraphic profiles recovered from five contemporaneous sections of the basal El Paso Group in western Texas and southern New Mexico reveal the occurrence of a rapid and short-lived negative excursion of more than 2‰ in conjunction with deposition of the distinctive, oolitic José Member of the Hitt Canyon Formation. Correlation to other carbonate carbon stratigraphic profiles spanning the Late Cambrian to Middle Ordovician indicate the excursion—here termed the José Excursion—is virtually coincident with the first appearance of the conodont Paroistodus proteus, a commonly used proxy for the base of the ‘Arenig Series’.

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 excursions—the Sunwaptan HERB and Skullrockian BRIE events—are 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.