Positive excursions in carbon isotope compositions of carbonate (∼ 3‰) and organic carbon (∼ 4‰–6‰) from the late Middle Ordovician (middle Caradocian) of the midcontinent and the eastern United States indicate widespread increases in productivity and rates of organic carbon burial that may have drawn down atmospheric pCO2, precipitating global cooling, although not necessarily ice-sheet formation. These climatic changes were associated with regional orogenic uplift, a relative rise in sea level, changes in epeiric sea circulation patterns, and carbonate platform destruction that led to regional extinction of marine benthos. The combination of sea-level rise, changing ocean circulation, and extinction in the middle Caradocian is similar to the suite of environmental changes described for Cambrian biomere boundaries, suggesting shared causes for these events. In contrast, middle Caradocian environmental changes are markedly different from the environmental patterns associated with the Late Ordovician mass extinction, despite the evidence for long-term cooling from the Middle to the Late Ordovician.

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