A HIGH RESOLUTION LATE ORDOVICIAN - EARLY SILURIAN COMPOSITE CARBON ISOTOPE PROFILE FROM WESTERN LAURENTIA

New inorganic carbon isotope data from Late Ordovician and Early Silurian limestones and dolostones of Nevada and British Columbia reveal fluctuations in the carbon cycle in advance of the well-known Hirnantian positive carbon isotope excursion. The composite curve relies on published biostratigraphy and the presence of distinctive features in the data as the basis for correlation.

High resolution data from the lower Beaverfoot Formation (British Columbia) and the Hanson Creek Formation and Unnamed Limestone (Nevada) show cyclical variability superimposed on a long term rise in δ¹³C preceding the Hirnantian Stage. The variability has a magnitude of <1.2 permil over short stratigraphic distances (7 cycles) and 3-4 permil over longer intervals (2 cycles). Oscillations in δ¹³C during the Pleistocene are thought to be related to climatic variability due to Milankovitch forcing. If any of the cyclicity in the Late Ordovician record can be attributed to orbital forcing, then the Hirnantian ice age may have been the culmination of climatic deterioration that began in Maysville or Richmondian time.

After the Hirnantian positive excursion, the Middle and Late Llandovery carbon isotope record exhibits a prolonged period of stasis close to 0 permil. The carbon isotopic profile of this interval has not been previously well-documented in western North America, and it suggests a dramatic shift in carbon cycling at the termination of the Hirnantian stage.