THE LOCK-STEP CORRESPONDENCE BETWEEN THE RISE OF SILURIAN PINNACLE REEFS AND δ13CCARB EXCURSIONS SUGGESTS REEF EVOLUTION WAS TRIGGERED BY PERTURBATION OF THE GLOBAL CARBON CYCLE

The first appearance of pinnacle reef tracts, extending tens to hundreds of meters above the Silurian sea floor, marked a step change in marine biosphere evolution. The advent of a greatly steepened sea floor topography opened new ecological niches that served as “evolutionary cradles” for organism diversification in the delayed rebound from the Late Ordovician mass extinction. However, the timing and drivers of this event has remained poorly understood. To unravel these complex temporal relationships, we used carbonate carbon isotope (δ¹³C_carb) stratigraphy, in combination with biostratigraphy and facies analysis, to build a sequence stratigraphic model for the western margin of the Michigan Basin. The results revealed that the first appearance of Silurian pinnacle reef tracts was associated with the Manitowoc carbon isotope excursion near the middle of the Telychian Stage, several million years earlier than previously proposed.

In addition, our work identifies a striking relationship between pulses of reef development and changes in global δ¹³C_carb values and sea level. Expanding our investigation to encompass the Cambrian through Devonian strengthens the observed interrelationships between these phenomena worldwide. From Cambrian microbial reefs to metazoan colonization in the Middle Ordovician, and through the Late Ordovician mass extinction, reef size and abundance is loosely tied to δ¹³C_carb excursions. A fundamental shift occurred with the first appearance of Telychian pinnacle reef tracts. From this point onward into the Devonian and beyond, the proliferation of pinnacle reefs shows a lock-step coincidence with δ¹³C_carb excursions. Viewed through this new perspective, the correlation between prolific periods of reef development with short-lived carbon isotope (δ¹³C_carb) excursions and eustatic sea level change suggests that Paleozoic reef evolution was the product of environmental forcing by perturbations of the global carbon cycle.