A high-resolution, continuous delta C-13 record spanning the Ordovician-Silurian boundary on Anticosti Island, eastern Canada

摘要

One of the best-exposed and most complete stratigraphic records from paleotropical areas spanning the Ordovician-Silurian (O-S) boundary is located on Anticosti Island, eastern Canada. Our study is the first one to sample strata superbly exposed at low tide along the west coast of Anticosti Island, thus providing a previously unexploited, nearly complete stratigraphic interval (similar to 300 m) at the O-S boundary for delta C-13 chemostratigraphy. A new high-resolution delta C-13 curve with more than 500 data points spaced at every similar to 0.5 mhas been produced, rectifying important pitfalls of previously published delta C-13 curves (i.e., low sampling resolution, variable sampling intervals, stratigraphic gaps). This new high-resolution delta C-13 curve displays a lower and an upper positive Hirnantian Isotope Carbon Excursion (HICE) recognized elsewhere around the globe. The ascending limb of the lower HICE starting from baseline values of + 0.5 parts per thousand corresponds to the upper 20 m of the late Katian Vaureal Formation, but delta C-13 peak values of + 2.5 parts per thousand occur in the lower part of the Hirnantian Ellis Bay Formation. In spite of a delta C-13 record segmented by a few stratigraphic hiatuses, the upper HICE, with its peak values of + 4.5 parts per thousand, is well recorded in the upper part of the Hirnantian Ellis Bay Formation. When compared with sections from around the globe, our delta C-13 curve displays a distinct long-term trend with a long sustained lower HICE followed abruptly by the upper HICE and return to baseline values of + 0.5 parts per thousand prior to the Rhuddanian. The continued descending isotopic trend well into the Becscie Formation suggests that the O-S boundary may occur at a higher stratigraphic level (up to 30 m) than previously interpreted. Active subsidence combined with moderate initial water depths prior to the Hirnantian were key factors controlling the deposition of a thick O-S sedimentary succession with a few hiatuses on Anticosti Island and capturing a comprehensive, reliable delta C-13 record across this interval.