The earliest forest and associated wildfires linked to marine anoxia and mass extinctions during the late Devonian: geochemical and paleontological evidence

Abstract

The diversification and radiation of vascular plants during the Devonian is a critical life event in geological history. The overarching goal of this dissertation is to reconstruct the evolution patterns of early vascular plants through the Devonian and their impacts on terrestrial and marine environments. The dissertation includes three projects that address this goal from different aspects. Project I was motivated by the lack of detailed spatiotemporal records of forests and soils during the Devonian. I presented data from microscopic and geochemical analyses of the Upper Devonian Chattanooga Shale (Famennian Stage) in northeastern Alabama, USA. I found increases in plant residues (microfossils, vitrinite, and inertinite) and molecular biomarkers (long-chain normal alkanes, vascular plant and wildfire derived polycyclic aromatic hydrocarbons (PAHs)) throughout the section. These increases coincided with the intensification of continental weathering, as indicated by inorganic geochemical proxies (SiO2/Al2O3, Ti/Al, Zr/Al, and Chemical Index of Alteration (CIA)). These data suggest that the southern Appalachian Basin, a region representing the southernmost Euramerica, became increasingly forested during the Late Devonian. Furthermore, I performed a synthesis of vascular plant fossil records that were published to date, and the results show a more rapid southward progression of afforestation and pedogenesis than previously documented along the Acadian landmass during the Late Devonian. Project II was to evaluate the impacts of the global dispersal of forests and soils on marine anoxia during the Late Devonian mass extinction events. I established an ultra-high-resolution profile (centimeter-spaced intervals) of an Upper Kellwasser (UKW) extinction interval (uppermost Frasnian stage) from the Chattanooga Shale of Tennessee, USA. I applied multiple paleoenvironmental proxies to reconstruct changes in marine anoxia, marine primary productivity, terrestrial plant inputs, and sea-level changes. During UKW, the geochemical proxies for anoxia (aryl isoprenoids and MoEF) show frequent fluctuations, suggesting marine anoxia was periodic and short-lived. The fluctuation of anoxia coincided with those of plankton (short-chain normal alkanes and C27 steranes) and terrestrial plant biomarkers (long-chain normal alkanes and vascular plant-derived PAHs) and water-depth indicators (C29/C30 αβ hopane, Ti/Al, Zr/Al and CIA), suggesting the anoxic episodes were caused by pulsed inputs of organic matter from terrestrial plant and soil that were, in turn, regulated by sea-level variations. Results from time-series analysis of Ti/Al ratios profile through Late Frasnian–Early Famennian strata demonstrates that obliquity mediated the cycle of sea-level changes, providing the first evidence that recurring, episodic environmental stresses on marine organisms during the UKW mass extinction were paced by astronomic forcing. In Project III, I tested the hypothesis that the radiation of early forests and concurrent morphological evolution increased the frequency and spatial extent of wildfires. To date, the spatiotemporal evolutionary pattern of wildfires and underlying mechanisms during the Devonian remain poorly constrained. From works published to date, I synthesized global fire occurrences based on three paleo-wildfire proxies—fossil charcoals, inertinite maceral, and pyrogenic PAHs. Additionally, I performed a case study of reconstructing wildfire activities across the Frasnian–Famennian (F–F) boundary based on the contents of inertinite maceral and pyrogenic PAHs in the Upper Devonian Chattanooga Shale of Tennessee, USA. The results show that the wildfires increased dramatically across the F–F boundary and expanded rapidly across the Euramerica during the Famennian. I further analyzed the dispersal range, species, and key morphological features of vascular plants during the Devonian. I found the spatiotemporal expansion in wildfires through the Late Devonian were concurrent with the diversification and dispersal of the early trees, suggesting a rise in forest fires fueled by Archaeopteris. Axial diameter, leaf length, and leaf width also shows a rapid increasing trend through the Late Devonian, suggesting that forest fires favored the survival of tall trees with large leaves and eventually facilitated the expansion of the earliest forests in the Euramerica. This study demonstrates the effects of wildfires in shaping forest composition during the Late Devonian and highlights the long-term ecological significance of wildfires.