Land plant evolution decreased, rather than increased, weathering rates

Abstract

Abstract The repeated evolution of trees is widely thought to have enhanced the capacity of silicate weathering via the impact of deep rooting. However, land plants are also responsible for wetland assembly and organic carbon burial. The total burial output of carbon via both organic and inorganic deposition must balance input to the exogenic system from volcanic outgassing on million-year time scales. Increased partitioning of carbon burial toward organic carbon and away from inorganic carbon reduces the marine carbonate burial flux, necessitating a lowered total flux of alkalinity to the oceans to maintain mass balance in the Earth’s surface carbon cycle. This flux includes the nutrient delivery from the terrestrial vegetation implicated as a driver of marine evolution, extinction, and environmental change including anoxia and black shale formation. Here, the burial of terrestrial organic carbon, first substantially in the Devonian and continuing through to the present, is argued to require a reduction in silicate weathering rates when compared to earlier times, given the independence of volcanic outgassing from weathering on short time scales. Land plants still may cause reductions in steady-state atmospheric CO2 levels, but via increasing the silicate weathering feedback strength, not silicate weathering rates. The mass-balance constraints on the long-term carbon cycle provide a mechanism for linking how land plant evolution simultaneously increased nutrient recycling and weathering efficiency of the Earth’s surface.