Sunday, July 20, 2014

Life and the Formation of Continents

On Earth, the presence of life plays a major role in determining the chemistry of the atmosphere and oceans. A study by D. Höning et al. (2014) suggests that the presence of life may play an even deeper role in influencing the planet’s evolution. In particular, the presence of life can enhance continental weathering rates, thereby increasing the rates at which sediments wash into and settle on the bottom of the oceans. These sedimentary layers hold within them a significant amount of water and hydrated minerals. Along convergent plate boundaries, the oceanic crust gets subducted into the Earth’s mantle, bringing along the water-rich sedimentary layers. As the subducting oceanic crust dives deeper, the increasing lithostatic pressure squeezes free water out from the sedimentary layers in a process known as shallow dewatering.

The enhanced continental weathering rates due to the presence of life would lead to thicker sedimentary layers, and consequently, increase the amount of water being subducted. In addition, the enhanced continental weathering rates would reduce the amount of shallow dewatering due to a greater abundance of low-permeability deposits such as clay-rich deposits in the sedimentary layers. These low-permeability deposits effectively ‘seal off’ water entailed in the sedimentary layers from being squeezed out, and in doing so, reduces the amount of shallow dewatering, allowing more water to be transported by subduction deeper into the Earth’s mantle. Water that is not squeezed out becomes bound in stable hydrated minerals as it is dragged further down by the subducting oceanic crust.

Figure 1: Artist’s impression of an Earth-like planet hosting a system of rings.

Figure 2: Schematic cartoon depicting Earth’s global water cycle, where water is represented by large and small dots, its path by black arrows, and movement of the oceanic plate by white arrows. Initial water uptake occurs within the submarine oceanic crust and sediments. Water loss first occurs after the subduction trench through dewatering, followed by the formation of the water-rich partial melt. The partial melt drives arc volcanism and continental crust formation. However, a fraction of the water contained in the subducting plate is regassed into the mantle. The water leaves the convecting mantle at mid oceanic ridges (MOR) as free volatiles or becomes part of the newly formed oceanic crust. D. Höning et al. (2014).

At a depth of roughly 100 km, the hydrated minerals brought down by the subducting oceanic crust become unstable and releases water into the surrounding mantle. This lowers the melting temperature of the surrounding mantle and leads to partial melting. Buoyancy drives the partial melt towards the surface, causing surface volcanism and the formation of new continental crust. The amount of newly formed continental crust is directly proportional to the amount of water released to produce partial melting. A larger amount of water driven down by the subduing ocean crust and released to form partial melting would enhance the rate of production of continental crust.

Also, not all the water in the form of hydrated minerals is released to form partial melting. Some of it continues deeper into the Earth’s mantle where it dissolves, hydrating the mantle. This lowers the effective viscosity of the mantle (i.e. makes the mantle more fluid), which has the effect of stabilizing plate tectonics. Process such as subduction, volcanic activity and the formation of new continental crust depends a lot on plate tectonics. If the mantle was dry, plate tectonics might not occur due to the high mantle viscosity. In a way, plate tectonics needs water to operate.

On Earth, the oceans cover 70 percent of the planet’s surface, with land covering the remaining 30 percent. This study by D. Höning et al. (2014) show that the presence of life does play an important role in the formation of continents on Earth and should be applicable to the evolution of other Earth-like planets as well. In the study, a model depicting Earth with present day continental weathering and erosion rates show that after roughly 4 billion years, the planet reaches a steady state with continental area covering 40 percent of the planet’s surface and an upper mantle water concentration of 300 parts per million (ppm). The same model is then ran with 60 percent of present day continental weathering and erosion rates, which one might expect on an abiotic Earth (i.e. a lifeless Earth). After 4 billion years, the planet attains a steady state with continents covering ~5 percent of its surface and an upper mantle water concentration of 40 ppm.

Figure 3: Artist’s impression of an Earth-like world. In this case, it is a moon of a gas giant planet.

Figure 4: Artist’s impression of an Earth-like planet. Image credit: Adrian Thomassen.

These findings suggest that the difference between a life-filled and a lifeless Earth can showup as a significant difference in the extent of continental coverage. On a biotic world (i.e. a life-filled planet), the presence of life enhances the formation of continents and stabilizes plate tectonics. In contrast, on an abiotic world (i.e. a lifeless planet), continental coverage is expected to be less, and the occurrence of plate tectonics would be less likely. If future studies support these notions, the detection of large continental coverage and/or plate tectonics on Earth-like exoplanets could serve as a form of biosignature in the search for life beyond Earth. “If we find a planet somewhere in the universe with a continental coverage similar to the Earth, it may be a good place to search for life,” said lead author of the study, Dennis Höning, a planetary scientist at the German Aerospace Centre’s Institute of Planetary Research in Berlin.

D. Höning et al., “Biotic vs. abiotic Earth: A model for mantle hydration and continental coverage”, Planetary and Space Science 98 (2014) 5-13.