CO-EVOLUTION OF EARTH SYSTEMS DURING THE DEVONIAN PERIOD (Invited Presentation)

The spread of higher land plants during the Devonian Period (419-359 million years ago) transformed continental surface environments and triggered major changes in Earth climate. A rapid increase in the number of land plant taxa occurred during the latter part of the Early Devonian (Pragian-Emsian stages), as the early euphyllophyte and lycophyte clades experienced major evolutionary diversifications. This event coincided with a strong cooling trend (as documented by conodont δ¹⁸O) and a major expansion of marine anoxia (as documented by carbonate δ²³⁸U), reflecting massive organic carbon burial and atmospheric CO₂ drawdown related to intensified chemical weathering and nutrient fluxes to the ocean. Although biodiversity is an imperfect proxy for biomass (which is the critical parameter in evaluating biotic effects on Earth-surface environments), it is likely to have scaled directly with biomass during the early stages of terrestrialization, when the appearance of new plant clades was related to the occupation of as-yet uncolonized land surfaces. A second milestone was the origination of trees and the appearance of the first forests during the late Middle Devonian (Givetian). Secular variation in the carbon-isotopic composition of vascular land plants, reflecting a transient rise in atmospheric pCO₂, coincided with climatic warming at that time. This ~5-million-year-long event interrupted the long-term trend of falling pCO₂ and global cooling linked to plant-induced enhancement of silicate weathering that characterized most of the Devonian. It was triggered by the rapid spread of closed-canopy forests, which reduced Earth-surface albedo and intensified evapotranspiration in the forest understory, leading to a transient surface warming and enhancement of soil organic matter recycling. Thus, the spread of vascular land plants during the Devonian led to both warming and cooling trends, depending on the nature of floral community changes and the specifics of vegetation-climate feedbacks. The uranium- and carbon-isotopic records to be presented in this talk provide some of the first high-resolution links between land-plant evolution and contemporaneous marine environmental changes during the Devonian.