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Braun et al., 2021

A sequential record of the Llandovery δ13Ccarb excursions paired with time-specific facies: Anticosti Island, eastern Canada

Braun, M. G., Daoust, P., Desrochers, A.
AjakiriPalaeogeography, Palaeoclimatology, Palaeoecology
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The Silurian climate was volatile and punctuated by multiple short-lived climate events, each associated with biotic crisis and perturbations to the global carbon cycle. In total, seven positive carbon isotopic excursions are globally recognized in the Silurian with the large excursions of the Wenlock-Ludlow interval in the Baltic succession garnering the most research; these excursions are recorded in association with cyclic facies changes and the enigmatic resurgence of microbial carbonates. Climate models, hypothesizing cycles of alternating humid and arid climate states, have been developed to explain these paired lithologic-isotopic fluctuations; however, little work has been done to test these models on lower Silurian strata. To test for the presence of δ13Ccarb excursions and the stratigraphic motif of proposed climate events, we developed a high-resolution integrated stratigraphic framework for the upper Hirnantian to mid-Telychian (Upper Ordovician to Llandovery) carbonate-dominated succession of Anticosti Island, by examining ~450 m of strata from a recent drill core, supplemented by ~120 m of outcrop. Four facies assemblages and three time-specific facies are identified and organized into three orders of superimposed transgressive-regressive cycles. High-resolution isotopic curves reveal the presence of four global positive carbon isotope excursions all sequentially recorded in the study interval; the HICE (peak of +5‰), Early Aeronian (peak of +2‰), Late Aeronian (peak of +6‰), and Valgu (peak of +3.5‰) excursions. Common cyclic stratigraphic trends are recognized with these δ13Ccarb excursions, where each excursion is associated with a shift from shale-rich to shale-poor facies, an intermediate-order transgression, and a minor δ18O excursion. These repetitive lithologic-isotopic patterns in the Llandovery succession of Anticosti Island are linked to cyclic fluctuations between low-latitude humid and arid climate states. In addition to these patterns, microbial-rich carbonates are associated with the Late Aeronian and Valgu δ13Ccarb excursions on Anticosti Island; this suggests a genetic link between the elevated δ13C values and the proliferation of calcified cyanobacteria and associated microfossils. This integrated stratigraphic model linking carbon excursions with climate and carbonate factory shifts provides an excellent framework for future studies, using a wide variety of geochemical systems, to better understand the processes that drove environmental and biodiversity changes through the Llandovery.

Viimati muudetud: 26.11.2022
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