δ13C and Sequence Stratigraphy of the Beekmantown Group, Quebec: Constraining the Carbon Cycle Perturbations During the Initiation of the GOBE

Carbon isotope (δ13C) trends in carbonate rocks are commonly interpreted in terms of changes in the global carbon cycle, with large fluctuations commonly linked to global environmental perturbations. However, the δ13C of seawater reflects a complex interaction between global, regional, and local processes, including carbonate deposition, changing carbon sources, organic carbon burial and oxidation, and local anoxia. Furthermore, the δ13C of precipitated carbonates can be strongly influenced by diagenesis. These processes can be directly and indirectly influenced by sea-level fluctuations since this controls the proximity to terrestrial sources, basin restriction, loci of carbonate production, and sources of meteoric fluids. Therefore, eustasy can either enhance or mute the signature of global seawater fluctuations in δ13C. A practical approach which can be used to distinguish between local controls and global signals on regional δ13C trends is to place the data within an age-calibrated, sequence stratigraphic framework. This study focuses on the Beekmantown Group, a succession of shallow-marine strata deposited on Southern Laurentia's passive margin during the Lower and Middle Ordovician (~ 485–450 Ma). During the first half of the Ordovician, Laurentia was characterized by tectonic quiescence, extensive epicontinental seas, and hot temperatures. Although the major pulse of the Great Ordovician Biodiversification Event (GOBE) occurs in the Upper Ordovician, biodiversity steadily increased beginning in Tremadocian, with a rise in planktonic diversity triggering major changes in food chains and ocean chemistry. Several studies have documented δ13C excursions in the Lower Ordovician, but most are not well characterized regionally, globally and temporally, so their exact relationship to global events is uncertain. Investigating the nature of these excursions and how they are expressed regionally provides key insights into the conditions of Lower Ordovician oceans and thus elucidating the background biogeochemical conditions during the initiation of the GOBE. Here, we present new carbon isotope data from the Tremadocian to Darriwilian from the Saint-Lawrence Platform (SLP), Quebec within a new sequence stratigraphic framework. To distinguish between local, regional and global controls on the C isotope signal, and to investigate the relationship between sea level and δ13C, we examine the record at multiple sequence scales. We correlate our data with other Lower-Middle Ordovician sections (Laurentia and Gondwana) and place those sections into a global geochronological framework based on biostratigraphy, radiometric dating and quantitative modelling. A precise understanding of the timing and the nature of the global and regional Carbon cycle perturbation during the Lower Ordovician will help elucidate the nature of the early expression of coupled changes in Earth's biosphere, paleogeography and climate that are characteristic of the Ordovician.