The geologic processes driving the primary fluxes of carbon between Earth’s deep and surface reservoirs have maintained the relatively narrow surface temperature range necessary for life to thrive during the last 3-4 billion years. These processes operate over deep time, cycling carbon among Earth’s shallow and deep reservoirs. In contrast, recent human activity has radically accelerated the rate of carbon release directly from geologic reservoir storage into the atmosphere. Thus, an appreciation of the entire vast, planetary system is fundamental to fully understanding the dramatic nature of the human carbon footprint.
In a review published in MRS Energy and Sustainability, Donald DePaolo (Lawrence Berkeley National Laboratory/University of California Berkeley, USA, and a member of DCO’s Deep Energy Scientific Steering Committee) meticulously interrogates the current literature to integrate what we know about the deep carbon cycle with how we interpret surface climate change .
In common vernacular “the carbon cycle” tends to refer to natural cycling of carbon through shallow Earth systems, including photosynthesis, respiration, and weathering. However, these relatively rapid cycles are only one piece of the puzzle.
The deep carbon cycle considers Earth’s largest carbon reservoirs in the core and mantle. Through volcanism and subduction, the deep Earth releases and recovers carbon from surface reservoirs. The key difference between the surface and deep carbon cycles is timing, with the latter operating on a time scale orders of magnitude slower than the former.
As noted by DePaolo (2015), the geologic carbon cycle regulates the amount of CO2 in the atmosphere and surface reservoirs over long times scales of millions of years and has kept Earth ‘habitable’ over most of its 4.5 billion year lifetime. “The key feature of the Earth is that carbon (from the mantle) released to the atmosphere by volcanic emissions can be returned to the deep Earth rather than retained in the atmosphere where it would accumulate to exceedingly high levels over geologic time,” according to DePaolo (2015). He highlights an important bottleneck. While the shallow and deep carbon cycles are intimately connected, the process of returning carbon to the deep Earth relies on one pathway: Carbon in the atmosphere has to make its way to the deep ocean, and from there into sediments which are ultimately subducted into the mantle.
Thus, predicting how our planet’s atmosphere will respond to ever-increasing concentrations of carbon depends in no small part upon a complete appreciation of the deep carbon cycle.
Image: Carbon reservoirs in Earth based on estimates in . Credit: Josh Wood