Mark Pagani, Ken Caldeira, Robert Berner, David Beerling
The uplift and denudation of mountain ranges during the past 24 million years created conditions highly favorable to enhanced rates of atmospheric carbon dioxide drawdown by silicate chemical weathering. Proxy records indicate, however, that Earth’s atmospheric carbon dioxide concentrations did not fall below ~180 parts per million (ppm) during this time period. Stabilisation of atmospheric carbon dioxide concentrations near this minimum value suggests that strong negative feedback mechanisms inhibited further drawdown of atmospheric carbon dioxide by high rates of global silicate rock weathering. Here we investigate one possible negative feedback mechanism occurring under relatively low carbon dioxide concentrations and warm climate that is related to terrestrial plant productivity and its role in the decomposition of silicate minerals. We use simulations of terrestrial and geochemical carbon cycles and available experimental evidence to show that vegetation activity in upland regions of active orogens was severely limited by near-carbon dioxide starvation in combination with global warmth over this period. This effectively diminishes biotic-driven silicate rock weathering and thereby attenuates an important long-term carbon dioxide sink. Although our modelling results are semi-quantitative and do not capture the full range of biogeochemical feedbacks that could influence climate as carbon dioxide declines, our analysis indicates that the dynamic equilibrium between plants, climate and the geosphere likely buffered minimum atmospheric carbon dioxide concentrations during the past 24 million years.
David J. Beerling, Lyla L. Taylor, Catherine D. C. Bradshaw, Daniel J. Lunt, Paul J. Valdes, Steven A. Banwart, Mark Pagani, Jonathan R. Leake
1. The relative constancy of the lower limit on Earth’s atmospheric CO2 concentration ([CO2]a) during major tectonic episodes over the final 24 million years (Ma) of the Cenozoic is surprising because they are expected to draw-down [CO2] ]a by enhancing chemical weathering and carbonate deposition on the seafloor. That [CO2]a did not drop to extremely low values suggests the existence of feedback mechanisms that slow silicate weathering as [CO2]a declines. One proposed mechanism is a negative feedback mediated through CO2 starvation of land plants in active orogenic regions compromising the efficiency of the primary carboxylating enzyme in C3 plants (Rubisco) and diminishing productivity and terrestrial weathering.
2. The CO2 starvation hypothesis is developed further by identifying four key related mechanisms: decreasing net primary production leading to (i) decreasing below-ground C allocation, reducing the surface area of contact between minerals and roots and mycorrhizal fungi and (ii) reduced demand for soil nutrients decreasing the active exudation of protons and organic acids by fine roots and mycorrhizas; (iii) lower carbon cost-for-nutrient benefits of arbuscular mycorrhizas (AM) favouring AM over ectomycorrhizal root–fungal symbioses, which are less effective at mineral weathering, and (iv) conversion of forest to C3 and C4 grassland arresting Ca leaching from soils.
3. We evaluated the global importance of mechanisms 1 and 2 in silicate weathering under a changing late Miocene [CO2]a and climate using a process-based model describing the effects of plants and mycorrhizal fungi on the biological proton cycle and soil chemistry. The model captures what we believe are the key processes controlling the pH of the mycorrhizosphere and includes numerical routines for calculating weathering rates on basalt and granite using simple yet rigorous equilibrium chemistry and rate laws.
4. Our simulations indicate a reduction in the capacity of the terrestrial biosphere to weather continental silicate rocks by a factor of four in response to successively decreasing [CO2]a values (400, 280, 180 and 100 p.p.m.) and associated late Miocene (11.6–5.3 Ma) cooling. Marked reductions in terrestrial weathering could effectively limit biologically mediated long-term carbon sequestration in marine sediments.
5. These results support the idea of terrestrial vegetation acting as a negative feedback mechanism that counteracts substantial declines in [CO2] a linked to increased production of fresh weatherable minerals in warm, low-latitude, active orogenic regions.