Abstract
The geological record contains evidence for numerous pronounced perturbations in the global carbon cycle, some of which are associated with mass extinction. In the Carnian (Late Triassic), evidence from sedimentology and fossil pollen points to a significant change in climate, resulting in biotic turnover, during a time termed the ‘Carnian Pluvial Episode’ (CPE). Evidence from the marine realm suggests a causal relationship between the CPE, a global ‘wet’ period, and the injection of light carbon into the atmosphere. Here we provide the first evidence from a terrestrial stratigraphic succession of at least five significant negative C-isotope excursions (CIE)’s through the CPE recorded in both bulk organic carbon and compound specific plant leaf waxes. Furthermore, construction of a floating astronomical timescale for 1.09 Ma of the Late Triassic, based on the recognition of 405 ka eccentricity cycles in elemental abundance and gamma ray (GR) data, allows for the estimation of a duration for the isotope excursion(s). Source mixing calculations reveal that the observed substantial shift(s) in δ13C was most likely caused by a combination of volcanic emissions, subsequent warming and the dissociation of methane clathrates.
Highlights
The Late Triassic period represents a time of extreme aridity and relative environmental and climatic stability, interrupted only by a brief, but substantial switch to more humid conditions, followed by ecological crises in the middle Carnian, during the CPE1–5
Evidence from the marine realm of the existence of both a bulk (c. 2‰) and a compound specific (4‰) negative C-isotope excursions (CIE) at the onset of the Carnian Pluvial Episode’ (CPE) suggests the injection of 13C-depleted CO2 into the Earth system[3]
We interpret the lithological change at the base of the Dunscombe Mudstone Formation (DMF) to be concomitant with the environmental turnover found in marine realm during the CPE at the Julian 1–2 boundary[2]
Summary
We observe evidence for climatic warming during the CPE at sites from the eastern and northwestern Tethys[5, 17, 36] Such methane release would quickly be sequestered by both terrestrial and marine components of the global carbon cycle, and would contribute to the widespread deposition of the black shales evidenced during the CPE5. Compound specific and total organic carbon δ13C analysis of Late Triassic (Carnian) sediments from Devon (UK) provide the first evidence from non-marine strata of the negative CIE observed, coinciding with the CPE extinction event. By utilizing the persistent presence of strong 405 ka eccentricity cycles through the 20.4 m record of the DMF we have constructed an astronomical timescale for 1.09 Ma years of the Late Triassic, allowing us to estimate a likely duration for the C-isotope excursions associated with the CPE. Through source mixing analysis we calculate that a combination of volcanic emissions and subsequent methane release were the likely cause for the observed shift in δ13C and the associated extinction event
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