Susan Brantley on Earth’s Geological Thermostat
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At the core of Earth’s geological thermostat is the dissolution of silicate minerals in the presence of atmospheric carbon dioxide and liquid water. But at large scales, the effectiveness and temperature sensitivity of this reaction depends on geomorphological, climatic, and tectonic factors that vary greatly from place to place. As described in the podcast, to predict watershed-scale or global temperature sensitivity, Susan Brantley characterizes these factors using the standard formula for the temperature dependence of chemical reaction rates using an empirically-determined activation energy for each process. Overall, her results suggest a doubling of the weathering rate for each 10-degree rise in temperature, but this value changes with the spatial scale of the analysis.
Susan Brantley is a Professor in the Department of Geosciences at Pennsylvania State University.
Podcast Illustrations
Images courtesy of Susan Brantley unless otherwise indicated.
The weathering process depends on minerals contained within each rock type. Here, a basalt canyon wall in Hawaii has been heavily weathered. The principal minerals involved are olivine, pyroxene, and plagioclase feldspar.
A heavily weathered quartz diorite (a form of granitic rock) in Puerto Rico. The unweathered granite contains potassium feldspar, plagioclase, and smaller amounts of biotite, hornblende, and muscovite, which weather in the presence of water to form clays and dissolved positively-charged ions such as Na+, K+, Ca++, and Mg++.
Models for soils and watersheds on silicate rocks in the humid half of Earth’s land surface. The reaction fronts (gray hatched layers) indicate where the most CO2 is dissolved during silicate weathering. For a bedrock mineral such as CaSiO3, the weathering reaction 2CO2 + H2O + CaSiO3 —> Ca2+ + 2HCO3- + SiO2 produces dissolved inorganic carbon (derived from the atmosphere) that is transported, precipitated, and partially sequestered as buried carbonate minerals at the seafloor. In the podcast, Susan Brantley describes two simplified weathering regimes: kinetic-limited (KL) regimes and erosive-transport-limited (ETL) regimes. In the former, illustrated in (A), kinetic limitation of silicate weathering is indicated by soil profiles or catchment areas where the silicate minerals and weathering reaction surfaces are exposed everywhere at the land surface. In ETL, illustrated in (B), weathering of soil and watersheds is characterized by buried reaction fronts, and concentrations of weathering products increase until they cause dissolution to stop. The weathering rate from such watersheds is determined by the erosion rate of the soil. (C) shows how in watersheds of increasing size (shown as white teardrop shapes), the dominance of long flow paths drive the weathering rate towards the ETL regime. As watersheds become larger, they show a transition regime (TR) behavior between KL and ETL landscapes.
Brantley, S.L. et al. (2023), Science 379, 382
Scaling the weathering reaction to larger spatial scales. Graph (A) shows how the effective activation energy (Ea), which is a measure of temperature sensitivity, increases for kinetic-limited weathering as the spatial scale increases from lab up to watersheds. This reflects the additional temperature-dependent weathering mechanisms that come into play as the scale increases from 100-micron mineral grains to 10-cm rock fragments to 1-m soils to 100-100,000-m drainage basins. (B) shows the fractional land areas of weathering regimes. The global-scale activation energy was based on the fractional contributions of the weathering fluxes shown in (C). First, a temperature response of 11.7% per degree K was estimated using the relative proportions of global carbon dioxide consumption fluxes from the two main rock types, granite (74%) and basalt (26%). This was then used to upscale the activation energy to the globe based on the proportions from each weathering regime (C), assuming the contribution of runoff-limited watersheds to be negligible.
Brantley, S.L. et sl. (2023), Science 379, 382