The impact of land plant evolution on silicate weathering and the carbon cycle


The chemical weathering of silicate rocks is the primary sink of atmospheric CO2 over geologic timescales and is also the ultimate source of phosphorous and other key nutrients to the biosphere. Land plants play a key role in mediating weathering processes and their evolution has frequently been entertained as a driver of environmental and biotic evolution on a global scale. As such, land plants are widely expected to have enhanced silicate weathering. However, they 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 Earth system from volcanic outgassing on million-year time scales. Increased partitioning of carbon burial towards organic carbon and away from inorganic carbon reduces the marine carbonate burial flux, necessitating a lowered total flux of silicate weathering derived alkalinity to the oceans to maintain mass balance in the Earth’s surface carbon cycle. Here, the burial of terrestrial organic carbon, first in the Devonian and continuing to today, is argued to require a reduction in silicate weathering rates when compared to earlier times. Land plants still may cause reductions in steady-state atmospheric CO2 levels, but via increasing the silicate weathering feedback strength, not silicate weathering rates. Furthermore, land plants could have created shorter term perturbations to the carbon cycle that could have contributed to extinction events, however, the maximum impact should be expressed on the order of less than 105 years. In sum, 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.


Daniel Ibarra a geochemist and climate scientist working on the water and carbon cycles in terrestrial environments. His work includes studying the response of past and present terrestrial landscapes to changes in climate using modeling approaches, geochemical measurements, and field observations.  He is interested in the role that Earth's continents play in modulating habitable surface conditions over geologic time,  and in using sediments to reconstruct past changes in weathering and hydrologic fluxes over Plio-Pleistocene to Phanerozoic timescales. Currently he is a postdoc at UC Berkeley in Earth and Planetary Sciences supported by a Miller Institute Research Fellowship and UC President's Postdoctoral Fellowship.


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