Modeling the consequences of land plant evolution on silicate weathering

Abstract

It has long been recognized that the advent of vascular plants in the Paleozoic must have changed silicate weathering and fundamentally altered the long-term carbon cycle. Carbon cycle models are frequently employed to quantify the effect of this state change in the Earth system. These models have suggested that plants likely played a key role in modulating atmospheric CO₂ in the past, with the largest plant-induced changes to the system in the late Paleozoic. These studies have, for the most part, focused on the effects of plants on weathering via their impacts on the soil environment. Yet, plants also modify the hydrological cycle, which may also have had implications for global weathering. Here, we evaluate the consequences of plant evolutionary innovation that have not been previously incorporated into carbon cycle models by coupling a one-dimensional vapor transport model to a reactive transport model of silicate weathering. Using this cascade of models, we investigate: 1) how evolutionary shifts in plant transpiration may have enhanced silicate weathering through increased downwind transport of water vapor to continental interiors; 2) the importance of deeply-rooted plants and their associated microbial communities in increasing soil CO₂ and weathering zone length scales; and, 3) the coupled effect of these two processes on weathering rates.

The hydrologic balance for our modeling approach is framed by energy/supply constraints (encapsulated through Budyko relationships) calibrated for minimally vegetated-, vascular plant forested-, and angiosperm-worlds. Using constraints for atmospheric vapor-transport over continents and terrestrial weathering fluxes, we find that the emergence of widespread transpiration and associated inland vapor recycling associated with the advent of deep-rooted vascular plants over the later Devonian increases weathering solute concentrations by a factor of 1.64 to 2.39 for a fixed atmospheric CO₂. The later domination of ecosystems by angiosperms in the late Cretaceous and Cenozoic, and the subsequent increase in transpiration fluxes, increased weathering solute concentrations by 7 to 55 percent leading to a cumulative total increase in weathering solute concentrations by a factor of 1.70 to 2.55. Partitioning of the vapor recycling effects and feedbacks on the weathering thermodynamics indicates that 55 percent of the increases in weathering concentrations are due to the thermodynamic effects of soil CO₂ with the remaining 45 percent attributable to hydrologic effects. Our estimates of the relative changes in weathering solute concentrations caused by land plant evolution are of a similar magnitude to relative flux scaling relationships implemented in existing carbon cycle models such as GEOCARBSULF (Berner, 2006), COPSE (Bergman and others, 2004), and GEOCLIM (Le Hir and others, 2011). As Phanerozoic plant evolution resulted in a more efficient generation of weathering solutes, a weaker dependence of silicate weathering on physical forcing factors such as temperature, pH of rainwater, tectonics, and prevailing lithology, must have resulted. Consequently, we postulate that terrestrial plant evolution likely contributed to a more stable climate system over the Phanerozoic.