"One prominent idea for mitigating global climate change is to remove CO2 from the atmosphere by storing it in fluids in the natural environment; for example dissolved within sediments below the ocean floor or in oceanic crust. This carbon sequestration is popular because it would allow us to place carbon into semi-permanent (on human timescales) storage, ‘buying time’ to wean us from our dependence on carbon-based energy sources. Application of such a mitigation technique presumes knowledge of what will happen to carbon when it is dissolved in various environments. Studies of naturally produced excess dissolved CO2 are, however, equivocal; this lack of knowledge represents a huge deficit in our comprehension of the global carbon cycle and specifically the processes removing carbon from the surface of the planet over geological timescales.This proposal will resolve the sink for CO2 within marine sediments and oceanic crust. Beneath much of the ocean floor exists the ‘deep biosphere’, microbial populations living largely in the absence of oxygen, consuming organic carbon that has fallen to the sea floor, producing a large excess of dissolved inorganic carbon. This dissolved inorganic carbon can diffuse back to the ocean or can precipitate in situ as carbonate minerals. Previous attempts to quantify the flux of carbon through the deep biosphere focused mostly on studies of sulfur and carbon, and these studies cannot reveal the fate of the produced inorganic carbon. I propose a novel approach to constrain the fate of carbon through the study of the subsurface calcium cycle. Calcium is the element involved in precipitating carbon as in situ carbonate minerals and thus will directly provide the required mass balance to determine the fate of CO2 in the marine subsurface. This mass balance will be achieved through experiments, measurements, and numerical modeling, to achieve the primary objective of constraining the fate of carbon in submarine environments."