Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events

Abstract

The Devonian Period was characterized by major changes in both the terrestrial biosphere, e.g. the evolution of trees and seed plants and the appearance of multi-storied forests, and in the marine biosphere, e.g. an extended biotic crisis that decimated tropical marine benthos, especially the stromatoporoid-tabulate coral reef community. Teleconnections between these terrestrial and marine events are poorly understood, but a key may lie in the role of soils as a geochemical interface between the lithosphere and atmosphere/ hydrosphere, and the role of land plants in mediating weathering processes at this interface. The e¡ective-ness of terrestrial £oras in weathering was signi¢cantly enhanced as a consequence of increases in the size and geographic extent of vascular land plants during the Devonian. In this regard, the most important palaeobotanical innovations were (1) arborescence (tree stature), which increased maximum depths of root penetration and rhizoturbation, and (2) the seed habit, which freed land plants from reproductive dependence on moist lowland habitats and allowed colonization of drier upland and primary successional areas. These developments resulted in a transient intensi¢cation of pedogenesis (soil formation) and to large increases in the thickness and areal extent of soils. Enhanced chemical weathering may have led to increased riverine nutrient £uxes that promoted development of eutrophic conditions in epicontinental seaways, resulting in algal blooms, widespread bottomwater anoxia, and high sedimentary organic carbon £uxes. Long-term e¡ects included drawdown of atmospheric pCO 2 and global cooling, leading to a brief Late Devonian glaciation, which set the stage for icehouse conditions during the Permo-Carboniferous. This model provides a framework for understanding links between early land plant evolution and coeval marine anoxic and biotic events, but further testing of Devonian terrestrial^marine teleconnections is needed.

Biophysical constraints on the origin of leaves inferred from the fossil record

“…The observed trends are entirely consistent with the relaxation of an energetic constraint mediated by CO 2 -driven stomatal evolution. However, we recognize that once flat leaves had evolved, in concert with root and vascular systems (22,23), they would have quickly propagated throughout terrestrial ecosystems by conferring significant selective benefits in competition for light and space. This competition is manifested in the fossil record by the well-documented evolution of plant height during the same interval (22).…”

Disrupted continental environments around the Devonian-Carboniferous Boundary: introduction of the tener event

“…The climatic changes had dramatic impact on marine environments leading to a major diversity crisis (Algeo & Scheckler, 1998). On continents however, though important modifications can be observed, there is no direct impact of the Hangenberg Biocrisis on the general diversity.…”

Evolution of leaf-form in land plants linked to atmospheric CO2 decline in the Late Palaeozoic era

“…2) would have been the selective pressure driving improved water and nutrient transport, crucial for plants as their heights rapidly increased from the Middle Devonian onwards 10 . However, by the Middle to Late Devonian, fossilized plant assemblages reveal that vascular systems were probably more ef®cient 20 , and water uptake from the soil improved by the evolution of deep rooting systems 21 .…”