Fluctuations in epeiric sea carbonate production during Silurian positive carbon isotope excursions: A review of proposed paleoceanographic models

Positive excursions in the carbon isotopic ratio of marine carbonates (δ13Ccarb) are commonly attributed to an increase in the burial ratio of organic versus carbonate carbon in the global oceans. Regardless of whether this increased deposition of organic carbon was oceanographically or biologically induced, the fact that high carbon isotope values are most often attributed to the sequestration and burial of organic carbon remains. The notion that high δ13Ccarb values should be the result of organic rich deposition in epeiric sea settings is not surprising considering that the overwhelming majority of organic carbon burial in the modern oceans occurs in continental shelf and margin settings. What is surprising is that major positive carbon isotope excursions during the Silurian are recorded in clean carbonate successions with little to no appreciable black shale deposition in epeiric seas. In fact, Silurian δ13Ccarb excursions occurred during intervals of prolific reef development and an overall expansion of carbonate platform environments globally.This discrepancy has sparked considerable debate among Silurian carbon isotope specialists but has offered a valuable opportunity to re-examine the intricate connections among nutrients, primary productivity, carbonate sedimentology, and the global carbon cycle in the lower Paleozoic. Here, we consider several sedimentological and geochemical features of one of these Silurian positive carbon isotope (δ13Ccarb) excursions and discuss the broader implications of current Silurian δ13Ccarb models to the interpretation of lower Paleozoic carbonate sequences.