THE ROLE OF SOILS AS AN INTERFACE BETWEEN TERRESTRIAL AND MARINE ECOSYSTEMS: THE MIDDLE TO LATE DEVONIAN

The Middle to Late Devonian Period was characterized by major changes in the terrestrial biosphere, e.g., the evolution of trees and seed plants and the appearance of multi-storied forests, as well as 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 and the role of land plants in mediating weathering processes at this interface. The effectiveness of terrestrial floras in weathering was significantly enhanced as a consequence of increases in the size and geographic extent of vascular land plants during the Middle to Late Devonian. In this regard, the most important paleobotanical 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 may have resulted in transient episodes of intensified pedogenesis (soil formation) and to large, permanent increases in the thickness and areal extent of soils. Enhanced chemical weathering led to increased riverine nutrient fluxes that promoted development of eutrophic conditions in epicontinental seaways, resulting in algal blooms, widespread bottomwater anoxia, and high sedimentary organic carbon fluxes. Such disturbances were inherently transient in character owing to (1) reduced fluxes of riverine nutrients as soils matured and stabilized, and (2) negative climatic feedbacks, i.e., global cooling associated with drawdown of atmospheric CO₂, causing reduced rates of chemical weathering in pedogenic environments. The role of soils as a geochemical interface between the terrestrial and marine biospheres has received insufficient attention, yet pedogenic weathering rates are a fundamental control on bionutrient availability and, hence, an important influence on patterns of evolution and extinction in marine ecosystems.