Paleowildfire at the end-Triassic mass extinction: Smoke or fire?

Abstract

Polycyclic Aromatic Hydrocarbons (PAHs) are routinely used as proxies for wildfire in geological sediments associated with large igneous province (LIP) driven CO2 increases and mass extinction events. One example is the end-Triassic mass extinction event (ETE) driven by Earth's most laterally extensive LIP, the Central Atlantic Magmatic Province (CAMP). However, many PAH records often lack critical information including identifying specific source(s) of PAHs (e.g., pyrogenic vs. petrogenic), intensity of paleowildfire events, whether PAHs represent predominant smoke signals that can travel substantial distance from the burn origin, and if evidence of PAH as markers for soil erosion exists. To better understand ETE wildfire events, a detailed evaluation of PAH distributions from the Italcementi section in the Lombardy Basin, Italy covering the latest Rhaetian was undertaken. We report the best evidence of wildfire activity occurs above the initial carbon isotope excursion (CIE) which is routinely used to chemostratigraphically correlate between ETE sections, rather than within the initial CIE as evidenced at other sections. This wildfire event was intense, short-lived, and occurred during a calcification crisis and δ13Corg anomaly, thereby linking terrestrial and marine ecosystem stress. Evidence of a more prolonged but less intense wildfire event and/or evidence for smoke signals takes place above this interval before the onset of a second calcification crisis. By comparing PAH records from Italy, Greenland, Poland, the UK, and China, during the ETE, few sections show evidence for intense (i.e., higher-temperature) wildfire activity during the initial CIE. However, these investigated PAH records show prolonged increases in the low-molecular-weight (LMW) combustion-derived PAH phenanthrene. We interpret this to represent widespread (and possibly more intense) wildfire activity further from the deposition sites, since LMW combustion-derived PAHs are the major PAHs in smoke aerosols that can travel vast distances, and/or less intense wildfire activity that characteristically produce LMW combustion-derived PAHs. In comparing PAH data, we find widespread wildfire activity across multiple basins supporting wildfire activity was an important ecological stressor in the terrestrial realm during the ETE.