M7 Solid Earth Couplings Surface System

Topic

Couplings between the solid Earth and the surface system—comprising the atmosphere, hydrosphere, cryosphere, and biosphere—operate on timescales ranging from thousands to millions of years and serve as major long-term regulators of the climate system. Volcanism injects carbon dioxide and sulfur dioxide into the atmosphere; chemical weathering of silicates consumes that same carbon dioxide at a rate dependent on both the temperature and precipitation of the region where it occurs; glacial erosion alters the terrain's topography and provides feedback for the isostatic uplift of that same terrain; and deep geological degassing sustains the carbon cycle on a geological timescale.

The silicate-carbonate thermostat is the long-term negative feedback mechanism that stabilizes Earth's climate: if atmospheric carbon dioxide rises and temperatures increase as a result, silicate weathering accelerates; this accelerated weathering consumes more carbon dioxide than before, and the additional consumption causes carbon dioxide levels and temperatures to drop back toward equilibrium, thereby closing a self-correcting feedback loop. The time carbon dioxide remains within this cycle before being consumed ranges from three hundred thousand to five hundred thousand years.

Without this silicate-carbonate thermostat, Earth would likely have experienced—irreversibly—either a runaway greenhouse effect like the one dominating Venus today, or a complete global freeze like that which occurred during the episodes known as "Snowball Earth."

Post-glacial isostatic rebound, meanwhile, alters regional seismicity: when the pressure exerted on the crust by the weight of ice is reduced as the ice retreats, the effective stress on pre-existing faults in the region changes, and this change can trigger earthquakes even in areas that typically exhibit very low seismicity, such as Scandinavia or Canada. Glacial erosion accelerates significantly during a glaciation, and this accelerated erosion produces rapid exhumation that is recorded in the rock's own thermochronological history; furthermore, the millions of cubic kilometers of sediment that this erosion ultimately deposits on continental margins load the lithosphere differentially from place to place, thereby—along with the previously described silicate-carbonate thermostat and isostatic rebound—completing the set of couplings between the solid Earth and the surface system with which this description began.

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