A Solid Earth Perspective on Earth’s Long Term Climate Trend and a New Proposed Method for CO2 Sequestration
Earth, Mars and Venus share similar planetary compositions and sizes—yet the atmospheres of the three planets differ drastically: effectively, Earth’s liquid water and water rock reaction processes have led to the natural sequestration of most of the CO2 into the mineral calcite (rock=limestone). Rock-ocean-atmosphere interactions have thus kept Earth in the “liquid water window,” making it habitable for us and life in general.
In this talk, I will go over basic background information about the solid Earth’s tectonic processes and geochemistry that likely control the long term (over 10-100 Myr) CO2 trends of Earth’s atmosphere. The chemical behavior of calcium in rocks is critical in this: Ca is a major constituent in common high temperature silicate minerals, feldspar and pyroxene. These minerals breakdown during weathering and alteration to make calcite. For 40+ years, weathering of continental crust has been viewed as the dominant process which draws down atmospheric CO2 and thus affects climate. However, the entire ocean crust, which covers ~66% of the planet, is made of calcium-rich basalt, making it the largest reservoir of Ca available for forming calcite. New ocean crust is constantly produced at mid-ocean ridge volcanos. These heat engines drive circulation of water-rich fluids that cause high temperature minerals to alter and release Ca. While “black smokers” are the examples many have likely heard about, I will make the case that there is a much more important, cryptic circulation of fluid through the ocean crust that leads to calcite formation and corresponding CO2 removal from the ocean (and thus from the atmosphere).
For 60 yrs, geologists have assumed that igneous (magma involving) processes must occur at > ~700°C. However, our laboratory experiments have challenged this, showing that igneous melts consisting of ~40% H2O (very thick solutions!) can exist at temperature as low as 330°C. Based on observed changes in ocean crust rocks during alteration, we can see that >60% of the Ca in the original basalt is made soluble for subsequent reactions to form calcite. The flux of Ca released from this process is 3 to >10x that of the continental weathering process. I will show that a proxy for Ca flux from weathering/alteration varies with rates of ocean crust production, providing evidence that this stealth fluid circulation process may well be the major control in regulating CO2 during the last 200 Myrs.
Finally, if this process really does control long term CO2, can we harness this process to sequester CO2 on human time scales? I proposed that by drilling into ocean crust near mid-ocean ridges (or Hawaii), we could cause increased circulation of this fluid, enhancing the natural process removing CO2 from ocean water or, potentially, sequestering concentrated CO2 sources. The questions of viability revolve around issues of permeability and reaction rate; the sources of heat (magma chambers) and rock (ocean crust) are limitless.
