δ 13 C Chemostratigraphy of the Ordovician‐Silurian Boundary Interval

A review of the δ13C chemostratigraphy of five stratigraphically apparently continuous Ordovician-Silurian boundary sections in northern Europe, North America, and Asia suggests that the level of the systemic boundary falls in an interval with relatively uniform carbon isotope values. Hence, the systemic boundary cannot be defined in terms of δ13C chemostratigraphy. However, comparison between biostratigraphy and chemostratigraphy indicates that the graptolite-defined base of the Silurian is located at a stratigraphic level only a little higher than the end of the Hirnantian carbon isotopic excursion (HICE), which is the largest δ13C excursion known in the Ordovician. In the latest Ordovician, high-latitude Gondwana glaciations resulted in some very significant sea-level changes. One such regressive sea-level event occurred just below the HICE interval, and another during the time of falling post-peak HICE values. There seems to be a close correlation between the main glaciations, which tend to be marked by significant stratigraphic gaps at low to mid-latitudes, and major faunal extinction horizons. These extinctions, which are among the largest known in the Earth's Phanerozoic history, resulted in striking differences between the Late Ordovician and Early Silurian marine faunas.