Biosphere changes during the Carnian-Norian in Tasmania: A new perspective from PAH analysis from the Paleo Antarctic Circle.

 

The Carnian–Norian, Late Triassic was a period of major global climatic change, often associated with a transition from arid to more humid conditions that began with the Carnian Pluvial Episode (CPE). This transition to wetter conditions is often linked to the emplacement of the Wrangellia Large Igneous Province (LIP), ~233 Mya in northern Panthalassa. Here, we present new organic geochemical data (molecular fossils) combined with organic carbon isotopes and elemental data from sediments of the Bicheno 3A core. These sediments represent fluvial deposits of the Upper Parmeener Supergroup, a freshwater sequence from eastern Tasmania. They offer a unique opportunity to examine the impact of this major climate transition at high latitudes in the paleo-southern polar circle. During the Late Triassic Tasmania was located at a latitude of -69°S, a position today occupied by the frozen continent of Antarctica.

 

Sediment samples from the BIC 3A core were processed and then analyzed in a gas chromatography (GC: Agilent 7890A System) for n-alkane and Polycyclic Aromatic Hydrocarbon (PAH) compounds. They were detected using a GC/mass spectrometry (Agilent 5975C Triple-Axis Detector.) Preliminary results from this study show an increase in soil erosion through the core based on higher concentrations of dibenzofuran and dibenzothiophene. A rise in retene concentrations during a similar time interval suggests an increased influx of terrestrial inputs further supporting enhanced hydrological conditions through the Late Triassic. Concentrations of an aggregate of PAHs spanning 3 to 7 rings associated with biomass burning also increase during a similar interval, suggesting increased wildfire activity, possibly driven by a shift to more humid conditions. When comparing the higher molecular weight PAHs (5- to 7-ringed PAHs) to the lower molecular weight PAHs (3-ring PAHs), a higher burning intensity and temperature towards the upper section of the core is observed since higher temperature wildfires produce PAHs with higher molecular weights. This upper section also shows indications of localized fire. Parameters including that of fluoranthene/ (fluoranthene + pyrene) and 1- and 2-methylphenanthrene/phenanthrene used to determine the origin of PAHs support that the PAHs from this sediment are of pyrogenic origin. This PAH profile from Tasmania represents a unique continuous record of climate change in high latitude Southern Hemisphere through the Carnian Pluvial Episode into the Norian, highlighting the complex interplay between fire and intensified rainfall in the Triassic paleo-Antarctic, as well as the major climatic and ecosystem changes experienced during this period.