HIGH RESOLUTION CHEMOSTRATIGRAPHY OF THE SILURIAN SUCCESSION OF ANTICOSTI ISLAND: NEW GLOBAL δ13C CORRELATIONS OF LLANDOVERY STRATA

The Silurian Period was one of the most volatile intervals in Earth’s history as it was marked by a series of short-lived climatic events, strong sea level fluctuations, and oceanic turnovers, all of which are commonly associated with perturbations of the global carbon cycle. Seven major positive isotopic carbon excursions are recognized throughout the Silurian, often occurring in association with biotic turnovers. Historically, most Silurian research has focused on the large excursions present in the Wenlock and Ludlow epochs whereas the excursions of the Llandovery are comparatively understudied and more poorly known. The storm-dominated paleotropical carbonate succession superbly exposed on Anticosti Island in Eastern Canada represents one of the most complete, thickest, and well-preserved successions in the world of Upper Ordovician and lower Silurian strata. Recent work on Anticosti has produced a high resolution chemostratigraphic framework by sampling ~450 m of strata from a recent stratigraphic drill core, and ~120 m of strata along cliffs and river banks in the south-central portion of the island; in total over 600 data points (sampled at a 0.5-1.0m interval) for δ¹³C and δ¹⁸O were collected, corresponding to the entire exposed late Hirnantian to mid Telychian succession of Anticosti Island. Four distinct positive isotopic carbon excursions are recognized in the succession; the upper Hirnantian (+5‰), Lower Aeronian (+2‰), Upper Aeronian (+6‰) and Valgu (+3.5‰) excursions. When compared with coeval sections from around the world, Anticosti is the only section known to record these four excursions in sequence; additionally, with peak values of +6‰, this study documents the highest magnitude ever recorded for the Upper Aeronian Excursion across all palaeocontinents. Primary isotopic signal preservation is supported by the lack of significant covariance between δ¹³C and δ¹⁸O, and by little or no diagenetic resetting as suggested by the trace element geochemistry. Although unusual in deep geological time, this is possible when lithification occurs shortly after deposition in a closed diagenetic system.