Carbon isotope (δ 13C) stratigraphy across the Silurian–Devonian transition in North America: evidence for a perturbation of the global carbon cycle

Abstract

Carbon isotope (δ 13C) analyses of marine carbonates spanning the Silurian–Devonian transition are compared from three richly fossiliferous, well-dated sequences in North America. The three sections, in the central Appalachian Mountains (West Virginia), Great Basin (Nevada), and the southern Mid-continent (Oklahoma), reveal positive δ 13C shifts beginning in the late Pridoli and reaching peak values as heavy as +5.8‰ in the earliest Lochkovian following the first occurrence of the conodont species Icriodus woschmidti and the graptolite Monograptus uniformis. A positive shift in δ 13C is also recorded at this time in Gondwanan regions, including the global stratotype section and point for the Silurian–Devonian boundary at Klonk in the Czech Republic, as well as in sections in the Carnic Alps of Austria (Cellon), and Queensland, Australia. The available data from Euramerica and Gondwana are consistent with a scenario linking seawater δ 13C enrichment to a eustatic drop during the Silurian–Devonian transition. Seawater δ 13C likely increased as a result of enhanced carbonate weathering during exposure and erosion of older Silurian platform deposits. In addition, the fall in sea level appears to have enhanced nutrient delivery to the oceans and triggered an increase in organic carbon burial rates at or near the Silurian–Devonian boundary, as indicated by organic-rich deposition in Gondwanan basins. In terms of its magnitude, the Silurian–Devonian δ 13C excursion of ≥+5‰ appears to be among the largest well-documented events in the Paleozoic, comparable to the shifts in the Late Ordovician and Early Mississippian.