LATE MIDDLE ORDOVICIAN ENVIRONMENTAL-CHANGE AND EXTINCTION - HARBINGER OF THE LATE ORDOVICIAN OR CONTINUATION OF CAMBRIAN PATTERNS

Positive excursions in carbon isotope compositions of carbonate (simil ar to 3 parts per thousand) and organic carbon (similar to 4 parts per thousand-6 parts per thousand) from the late Middle Ordovician (middl e Caradocian) of the midcontinent and the eastern United States indica te widespread increases in productivity and rates of organic carbon bu rial that may have drawn down atmospheric pCO(2), precipitating global cooling, although not. necessarily ice-sheet formation. These climati c changes were associated with regional orogenic uplift, a relative ri se in sea level, changes in epeiric sea circulation patterns, and carb onate platform destruction that led to regional extinction of marine b enthos, The combination of sea-level rise, changing ocean circulation, and extinction in the middle Caradocian is similar to the suite of en vironmental changes described for Cambrian biomere boundaries, suggest ing shared causes for these events, In contrast, middle Caradocian env ironmental changes are markedly different from the environmental patte rns associated with the Late Ordovician mass extinction, despite the e vidence for long-term cooling from the Middle to the Late Ordovician.